The Effect of Soaking on Segregation and Primary-Carbide ... - MDPI

metals Article

The Effect of Soaking on Segregation and Primary-Carbide Dissolution in Ingot-Cast Bearing Steel Fareed Ashraf Khan Division of Processes, Department of Materials Science and Engineering, KTH Royal Institute of Technology, Brinellvägen 23, 100 44 Stockholm, Sweden; [email protected]; Tel.: +46-73-775-2789 Received: 18 September 2018; Accepted: 2 October 2018; Published: 6 October 2018







Abstract: In this work, segregation in the cast and hot worked structure, as well as the effects of soaking on macro and micro segregation, in hypereutectoid bearing steel produced by ingot casting were studied. Samples were selected from ingots that where either as cast or soaked for twenty hours. Two similar bearing steel grades were used for this investigation. For the as cast ingot, samples were selected from both A-segregation channel regions and the matrix region. Samples were also selected from hot-worked bars originating from ingots that had been soaked for four hours or twenty hours. Micro and macro examinations of the microstructures were conducted and compared. In addition, a segregation analysis of the substitutional solute elements was performed using EDX equipment mounted on a Scanning Electron Microscope (SEM). EMPA mapping of the composition pattern in the bulk, as well as the carbides, was conducted. Precipitation of M3 C, M2 C, and M6 C was observed. The carbides at A-segregation channels were found to have a different morphology to those precipitated in the bulk matrix. After soaking at 1200 ◦ C for 4 h, all the primary carbides are dissolved. Keywords: Keywords: cast structure; segregation; carbides; soaking time; ingot casting; ball bearing steel

1. Introduction The steel used for the fabrication of ball bearings needs to endure severe cyclic and static loads to operate reliably in hostile environments. Bearings produced from this material include parts such as balls, bars, tubes, and rings. The steel used for bearings undergoes very large reductions in the plastic range, as it is formed from ingots into billets or blooms. Soaking is applied prior to hot working to reduce segregation that occurs as a result of solidification. Two types of steel are used for producing most bearings: soft core steel with a hard surface, produced through case hardening or induction hardening, and steel hardened throughout the body due to the presence of martensitic or bainitic structures [1]. The bearing steel market has been historically dominated by steel possessing a 0.8–1.1 wt% C content and less than 3 wt% solute content [2]. The focus of this work is on two bearing steel grades, 100CrMnMoSi8-4-6 and 100CrMnSi6-4. Th100CrMnMoSi8-4-6 has high hardenability and is used for larger sections. Bearing steel is usually cast using two methods: ingot casting and continuous bloom casting. The ingots cast by uphill casting undergo large forging or rolling reductions to achieve the required final quality. After solidification, the ingots are removed from the mold and sent to a soaking pit, where they are kept at a suitable temperature, known as the soaking temperature, for a certain time, known as the soaking time, before any initial rolling processes [3].

Metals 2018, 8, 800; doi:10.3390/met8100800

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Metals 2018, 8, types x FOR PEER REVIEW 2 of Several of segregation take place in an ingot, as shown schematically in Figure 1, all of18 which are due to the same fundamental phenomena. The basic reason for the occurrence of these types of segregation is theconvection natural convection due to thermal density gradients, density in differences the oftypes segregation is the natural due to thermal gradients, differences the liquid,inand liquid, and solidification shrinkage during solidification [4–6]. solidification shrinkage during solidification [4–6].

Figure Figure1.1. AA rough rough illustration illustration ofofthe thecrystal crystalstructure structureand andmacro macrosegregation segregationoccurring occurringduring during solidification of large ingots. The dashes delineate the extents of the different crystal solidification of large ingots. The dashes delineate the extents of the different crystalzones. zones.

In the positive segregation areas, the solute concentration is higher than the mean concentration of In the positive segregation areas, the solute concentration is higher than the mean concentration solute, and in the negative segregation areas the solute concentration is less than the mean concentration of solute, and in the negative segregation areas the solute concentration is less than the mean of solute. The negative segregation area at the bottom part of the ingot is caused by sedimentation of concentration of solute. The negative segregation area at the bottom part of the ingot is caused by free crystals. sedimentation of free crystals. A-segregation is characterized by pencil-like channels, which contain a very high solute A-segregation is characterized by pencil-like channels, which contain a very high solute concentration and are caused by upward flow of liquid that has a lower density than the bulk concentration and are caused by upward flow of liquid that has a lower density than the bulk liquid. liquid. V-segregation is formed due to thermal contraction during solidification [4]. In addition V-segregation is formed due to thermal contraction during solidification [4]. In addition to these, to these, interdendritic segregation is also present, which adversely affects the mechanical properties interdendritic segregation is also present, which adversely affects the mechanical properties of the of the material. material. Interdendritic segregation or micro segregation is a result of the uneven distribution of alloying Interdendritic segregation or micro segregation is a result of the uneven distribution of alloying elements over short distances. If the partition constant between solid and liquid is less than 1, the solute elements over short distances. If the partition constant between solid and liquid is less than 1, the elements become enriched in the remaining liquid. Also, if the diffusion rates of these elements are solute elements become enriched in the remaining liquid. Also, if the diffusion rates of these elements low in the solid state, the last solidified liquid present between the dendrite arms becomes even more are low in the solid state, the last solidified liquid present between the dendrite arms becomes even enriched with solute elements [4]. more enriched with solute elements [4]. In ball bearing steel, the alloying elements Cr and Mo will segregate to the interdendritic areas. In ball bearing steel, the alloying elements Cr and Mo will segregate to the interdendritic areas. The elements Cr and Mo have a strong affinity for carbon [1], and the melt finally precipitates primary The elements Cr and Mo have a strong affinity for carbon [1], and the melt finally precipitates primary carbides. This may result in the formation of large eutectic carbides. Some of these carbides become carbides. This may result in the formation of large eutectic carbides. Some of these carbides become fragmented during mechanical working, but the particles remain coarse. These coarse carbide particles fragmented during mechanical working, but the particles remain coarse. These coarse carbide can serve as fatigue initiation sites and adversely affect the formability [1]. particles can serve as fatigue initiation sites and adversely affect the formability [1]. To counter the segregation and primary carbide precipitation caused during solidification, To counter the segregation and primary carbide precipitation caused during solidification, the the ingots usually undergo soaking. Soaking or homogenization treatment has the same objective as a ingots usually undergo soaking. Soaking or homogenization treatment has the same objective as a solution treatment and is carried out at a typical rolling temperature of 1200 ◦ C, to homogenize the solution treatment and is carried out at a typical rolling temperature of 1200 °C, to homogenize the substitutional alloying elements in the austenitic region, as well as to make the distributions of other substitutional alloying elements in the austenitic region, as well as to make the distributions of other alloying elements more uniform [7]. The homogenization process is isothermal, and the diffusion has alloying elements more uniform [7]. The homogenization process is isothermal, and the diffusion has both short- and long-range controlling factors. both short- and long-range controlling factors. The diffusion process follows the Hume-Rothery rules [8] and occurs in the FCC-austenitic single phase field. However, this can also happen in the multiphase regime. The degree of homogenization achieved during soaking depends on several factors, including the soaking

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The diffusion process follows the Hume-Rothery rules [8] and occurs in the FCC-austenitic single phase field. However, this can also happen in the multiphase regime. The degree of homogenization achieved during soaking depends on several factors, including the soaking temperature and time, dendritic arm spacing, the diffusion coefficient of the diffusing element, and any chemical inhomogeneity [4]. Beswick [9] observed that for hypereutectoid steel, Cr influences carbide stability, and the diffusion of carbon is reduced at the carbide–matrix interface due to the slow diffusion of Cr. Beswick also observed growth of (FeCr)3 C phase during soaking and concluded that, for an extended soaking time, the matrix becomes diluted of chromium, which reduces the hardenability of the matrix. In addition, when we relate the carbide dissolution during soaking to the Cr composition in the carbide and matrix, an equilibrium balance is observed between the composition of the chromium and carbon in the matrix, which is independent of the soaking condition. Kim et al. [10] concluded that the soaking time and temperature are dependent on the diameter of the largest carbide particle precipitated and calculated that the soaking time is dependent on Cr diffusivity. Reger et al. [11] summarized that the carbon concentration gradient acts as the driving force for solid state diffusion. Additionally, as the carbon composition difference decreases, the driving force decreases, and the homogenization process stops when the concentration difference is approximately 0.05–0.1 wt% carbon. Elements possessing a low diffusion coefficient react very sluggishly to the homogenization effect induced by hot working [12]. Fisher et al. [13] observed that, for a 300-µm dendrite arm spacing, to reduce the substitutional element segregation by 50%, a total of 35 h of soaking time is required at a soaking temperature of 1250 ◦ C. In the present work, the solidification structure and segregations in ingot cast ball bearing steel, as well as the structure and segregations in hot worked and soaked material, were investigated. Different etching techniques, as well as SEM EDX (Hitachi, Japan) measurements, were used to determine different types of primary carbide precipitations. Initially, calculations were carried out using the computational thermodynamic and kinetic tool, Thermo-Calc (2017b), to obtain guidance regarding the solidification mode with primary carbide precipitations and solid-state phase transformations [13–15]. 2. Calculation of the Solidification Evolution by Means of Thermo-Calc Thermo-Calc was used to model and predict the solidification evolution of the investigated alloy1, with composition presented in Table 1. First, equilibrium conditions were assumed with infinite diffusion rate. Figure 2a shows calculated fraction of phase versus temperature for all phases. The phase transformations with descending temperature can be expressed as follows: •

L → γ Solidification of austenite,

•

L → γ + MnS Manganese sulphide,

• • • •

γ → M7 C3 M7 C3 γ → M7 C3 + M3 C γ + M7 C3 → M23 C6 + M3 C γ + M23 C6 → α + M3 C

γ, starts at 1445 ◦ C. MnS, starts to precipitate at 1325 ◦ C. Full solidification occurs at 1317 ◦ C. Carbide starts to precipitate at 915 ◦ C. Cementite, M3 C, starts to precipitate at 885 ◦ C. M23 C6 carbide starts to precipitate at 793 ◦ C. Ferrite, α, starts to precipitate at 745 ◦ C.

It can be noted that the Thermo-Calc calculations show no primary carbides precipitating during solidification at equilibrium conditions. Table 1. Composition of the alloys investigated, in wt%. Alloy

C

Si

Mn

P

S

Cr

Ni

Mo

Cu

Aloy1 Alloy2

0.98 0.94

0.52 0.63

0.97 1.03

0.012 0.009

0.005 0.004

1.89 1.94

0.14 0.17

0.56 0.04

0.128 0.144

Output from the Thermo-Calc calculations also gives concentrations of Cr and Mo in the secondary carbides. This information is helpful when determining the type of carbide present in the investigated

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alloy. carbide composition varies with temperature. The compositions in Table 2 were Metals The 2018, 8, x FOR PEER REVIEW 4 ofevaluated 18 at the maximum temperature in the temperature interval where the carbide exists.

(a)

(b)

(c)

Figure 2. 2. (a) Mole fraction of all versus temperature for thefor steel Figure (a) Phase Phaseequilibrium. equilibrium. Mole fraction of phases all phases versus temperature thegrade steel grade investigated; (b) Scheil equation. Weight fraction of all phases versus temperature for the steel grade investigated; (b) Scheil equation. Weight fraction of all phases versus temperature for the steel grade investigated; (c) Mol fraction of all versus temperature for thefor composition investigated; (c)phase phaseequilibrium. equilibrium. Mol fraction ofphases all phases versus temperature the composition inγγ in in contact contact with in withcarbide. carbide. Table 2. Concentration of Cr and Mo in secondary carbides from Thermo-Calc calculations at maximum temperature with conditions.carbides from Thermo-Calc calculations at maximum Table 2. Concentration of Crequilibrium and Mo in secondary temperature with equilibrium conditions. Type of Carbide Cr wt% Mo wt% Temperature, °C M7Carbide C3 24.3 917 Type of Cr wt% 10.0Mo wt% Temperature, ◦ C M23 C6 13.6 9.8 793 M7 C3 24.3 10.0 917 M C 12.7 1.7 885 M233C6 13.6 9.8 793 M3 C 12.7 1.7 885 From the Scheil equation,

From the Scheil equation,

𝐶𝑠 = 𝑘 ∙ 𝐶0 ∙ (1 − 𝑓𝑠 )𝑘−1

(1) k −1

in which 𝐶𝑠 is the concentration in the solid, the Cs = 𝐶k0·Cis0 ·( 1 −mean f s ) concentration, and 𝑘 is the partition (1) coefficient between solid and liquid; it is possible to calculate the micro-segregation of Cr and Mo in inγ-phase whichunder Cs is the theassumption concentration the solid, C0solid. is the mean concentration, and k is the partition of noin diffusion in the coefficient between solid liquid;toitcalculate is possible to calculate the micro-segregation of Crwith and Mo in Thermo-Calc has alsoand the option solidification according to the Scheil equation γ-phase under assumption ofequilibrium no diffusion in the solid. the choice of nothe solid diffusion or conditions for each element. The partition coefficient Thermo-Calc alsowill the option calculatebut solidification according to the Scheil equation with between solid andhas liquid not be to a constant will vary in accordance with equilibrium conditions. conditions hold for all precipitated during solidification until a the choice ofThe no determined solid diffusion or equilibrium conditions forphases each element. The partition coefficient singular solid pointand is reached allbe thea phases solidify at equilibrium. between liquid where will not constant but will vary in accordance with equilibrium conditions. The calculations were done theprecipitated conditions of no back diffusion for all alloying The determined conditions hold with for all phases during solidification until aelements singular point except for carbon, which was assumed to diffuse faster and be in equilibrium. Figure 2b shows the is reached where all the phases solidify at equilibrium. results from calculations of weight fraction of all phases versus temperature. The solidification The calculations were done with the conditions of no back diffusion for all alloying elements sequence can be expressed as follows:

except for carbon, which was assumed to diffuse faster and be in equilibrium. Figure 2b shows  results 𝐿 → 𝛾from Solidification of austenite, γ, starts at 1445 °C.phases versus temperature. The solidification the calculations of weight fraction of all  𝐿 → can 𝛾 + be MnS(s) Manganese sulphide, s tarts to precipitate at 1311 °C when the sequence expressed as follows: •  •  •

𝛾

fraction of 𝛾, 𝑓𝑠 = 0.939. L → γ Solidification of austenite, γ, starts at 1445 ◦ C. 𝛾 ◦ C when the 𝐿 → 𝛾 + MnS + M6 C M6C starts to precipitate as 1148 °C, when 𝑓𝑠 = 0.990atand Manganese sulphide, starts to precipitate 1311fraction 𝑀𝑛𝑆 (s) L → γ + MnS γ of MnS, 𝑓𝑠 = 0.00018. fraction of γ, f s = 0.939. 𝛾 γ ◦ 𝑓 = 0.990, L → 𝛾 + MnS + M6 C + M7 C3 M7C3 starts M to precipitate at 1130 °C, in which 𝑓𝑠MnS and = 6 C starts to precipitate as 1148 C,𝑠when f s = 0.990 L → γ + MnS + M C 6 MnS 0.00018. fraction of MnS, f = 0.00018. s

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M7 C3 starts to precipitate at 1130 ◦ C, in which f s = 0.990, f sMnS = 0.00018. KSI_Carbide starts to precipitate at 1081 ◦ C, when CCr = 5.7% and CMo = 14% in liquid. γ

•

L → γ + MnS + M6 C + M7 C3

•

L → γ + MnS + M7 C3 + KSI _Carbide

Scheil-equation calculations show that primary precipitation of carbides occurs during solidification, but at a low temperature. It is worth mentioning that the results indicate that liquid is present at temperatures as low as 1081 ◦ C, which is not to be expected in reality. Due to the steep concentration gradients in the solid, the back diffusion will disable such high liquid concentration peaks. The solidus temperature is therefore expected to be higher than 1081 ◦ C. Most probably, no primary carbides will precipitate for this alloy if back diffusion calculations are taken into account. Table 3 summarizes the concentration of Cr and Mo in primary carbides. In comparison with the compositions given in Table 1, the Mo content is higher for the Scheil calculation. Table 3. Concentration of Cr and Mo in primary carbides from Thermo-Calc calculations with the Scheil equation. Carbides

Cr wt%

Mo wt%

Temperature, ◦ C

M6 C M7 C3 KSI_Carbide

2.82 26.2 8.2

51.9 18.9 39.8

1148 1130 1081

The alloy concentration in the γ phase, at the point where primary carbides start to precipitate, is higher than the concentration from the heat analysis due to the micro segregation as shown in Table 4. During cooling, that material will undergo phase transformations that differ from the case displayed in Figure 2a and about which it is of interest to know more. Thermo-Calc was therefore used to calculate the amount of each phase versus temperature for this higher concentration at equilibrium, and the results are displayed in Figure 2c. Table 4. Alloy composition in γ at the start of the precipitation of primary carbides from the Scheil equation. Values in wt%. Mo

Cr

Mn

Si

Cu

Ni

S

P

3.6

4.04

2.13

0.52

0.13

019

0.001

0.15

The solidification sequence of equilibrium calculation with the input composition given in Table 3 can be expressed as follows: •

L→γ

•

L → γ + MnS

• • •

γ → M6 C + MnS γ → M7 C3 + M6 C γ + M7 C3 + M6 C → M23 C6 + KSI _Carbide

Solidification of γ starts at 1420 ◦ C. MnS starts to precipitate at 1212 ◦ C. Full solidification occurs at 1152 ◦ C. M6 C carbide start to precipitate at 1146 ◦ C. M7 C3 starts to precipitate at 1058 ◦ C. M23 C6 and KSI_Carbide starts to precipitate at 825 ◦ C.

Concentrations of Cr and Mo in the carbides are given in Table 5. Table 5. Concentration of Cr and Mo in carbides. Equilibrium Thermo-Calc calculations performed with alloy compositions given in Table 3. Carbides

Cr wt%

Mo wt%

Temperature, ◦ C

M6 C M7 C3 KSI _Carbide M23 C6

3.36 29.6 8.1 23.7

52.3 25.7 38.4 14

1146 1058 825 825

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3. Experimental Work Two similar bearing steel grades were used in this investigation, which are commercially available through hardening bearing steels. These are low-alloyed carbon steels that have a hypereutectoid composition; the compositions are given in Table 1. The alloy1 was in as cast and soaked form, and alloy2 was in soaked and hot rolled form. The experimentally determined liquidus temperature for alloy1, i.e., the temperature at which solidification starts is 1460 ◦ C. The alloys were melted in an electric arc furnace, treated in a ladle furnace, vacuum degassed and then cast into 4.2-ton ingots by uphill casting. Once solidified, the ingots were removed from the molds and soaked at 1200 ◦ C in the soaking pits to homogenize the segregated areas. For alloy1 samples were taken from as cast ingot and ingot soaked for 20 h. Cross section samples were cut off from a position of half the ingot height at the central part of the ingot. Sample designations, soaking times, and hot working conditions are presented in Table 6. For alloy2, the soaking times were 4 h and 20 h, respectively. The ingots were then hot worked to size 147X147 mm, corresponding to an elongation factor of 16. Cross section samples from alloy2 were cut at a position corresponding to half of the ingot height. Table 6. Designation of samples investigated. Sample Designation

Alloy

Soaking Time

Hot Working

0SB 0SA SOA4h SOB4h SO20h HW4h HW20h

Alloy1 Alloy1 Alloy1 Alloy1 Alloy1 Alloy2 Alloy2

No/As Cast No/As Cast 4h 4h 20 4h 20 h

No No No No No Yes—Rolled Yes—Rolled

After grinding and cleaning, the cross section samples were macro-etched in a solution of 38% HCl and 12% H2 SO4 in water. After meticulous visual observations of the macro-etched samples, several representative areas were selected, and micro-samples were cut from these selected areas. From the as cast macro-sample, micro-samples were selected from the bulk, 0SB, and from the areas containing A-segregation channels, 0SA. Caution was taken to cut the samples from similar areas over the cross section to enable a comparison. The samples marked SOA20h were taken from the ingots, which were soaked for 20 h, and the samples marked as HW4h and HW20h were taken from ingots, which were soaked for 4 h and 20 h, respectively, and then hot rolled. In addition, in order to carefully study the soaking effect on the cast structure in A-segregates and the bulk, soaking was performed on micro samples, 0SA, and OSB in a lab furnace at 1200 ◦ C for 4 h. The soaked sample has the designation SOA4h and SOB4h, respectively, in Table 6. The micro-samples were progressively ground. After grinding and cleaning, the samples were polished using a 3-micron and then a 1-micron diamond suspension polish. For optical microscopy and SEM imaging, the samples were etched with 2% Nital. To differentiate between different groups of carbides, selective etching was conducted. Samples were etched with Groesbeck’s reagent [16,17]. The MC carbides are not attacked and appear in pink color; M2 C are colored dark brown; M6 C are outlined and colored blue or yellow; M7 C3 are faintly attacked and colored; and M3 C is not affected. To verify the results, the samples were also etched with Murakami reagent, which gives similar etching effects [16]. After conducting the microscopic observations, specific areas of interest were marked for EDX and EMPA analysis equipment mounted on Hitachi SEM. The samples were then re-polished, as the analysis of the etched sample does not provide the true results, because the etchant depletes or reacts

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with certain phases and changes the composition. After re-polishing, quantitative and qualitative The secondary arm spacing was measured in the and in A-segregated composition analyses were performed for the matrix andbulk the carbide precipitation. areas. Hardness testing was performed in order to distinguish between matrix and carbide in interdendritic regions. The secondary arm spacing was measured in the bulk and in A-segregated areas. Hardness testing The fractioninof primary carbides between in interdendritic regions in in theinterdendritic bulk of as cast samples wasarea performed order to distinguish matrix and carbide regions. Thewas area evaluated the Leica QWin (V 3.5.1 2008, Switzerland). fraction ofusing primary carbides in Stadard interdendritic regions in the bulk of as cast samples was evaluated using the Leica QWin Stadard (V 3.5.1 2008, Switzerland). 4. Results 4. Results 4.1. Macrosegregation 4.1. Macrosegregation Randomly orientated dendrites were seen after the macro-etching on the unsoaked and soaked Randomly were seenThese after the macro-etching on the unsoaked and soaked samples, as seenorientated in Figuredendrites 3a,b respectively. randomly orientated dendritic structures are samples,representative as seen in Figure respectively. TheseThe randomly orientated dendritic structures are typically of an3a,b equiaxed crystal zone. areas exhibiting darker shades correspond typically representative of an equiaxed crystal zone. The areas exhibiting darker shades correspond to regions with high segregation [14]. The dendritic structure is not clearly visible in the soakedto regionsdue withtohigh segregation [14].of The dendritic structure is not clearly in the soaked sample sample the homogenization solute elements. The rounded darkvisible areas marked by arrows in due to 3a therepresent homogenization of solutechannels. elements.The Thediameters rounded dark marked by spots arrows in3Figure 3a Figure A-segregation of theareas A-segregation are mm on represent A-segregation channels. The diameters of the A-segregation spots are 3 mm on average. average.

(a)

(b)

Figure3.3.(a) (a)Cross Crosssection sectionofofmacro macroetched etchedas ascast castunsoaked unsoakedsample sampleofofalloy1. alloy1.A-segregation A-segregationchannels channels Figure are marked marked with with arrows. arrows. Equiaxed Equiaxedcrystals crystalsare arevisible. visible.(b) (b)Cross Crosssection sectionofofmacro macroetched, etched, as-cast as-cast are soaked sample for 20 h, alloy1. Dendritic structure is not clearly visible due to homogenization soaked sample for 20 h, alloy1. Dendritic structure is not clearly visible due to homogenization ofof soluteelements. elements. solute

4.2. Microstructure in the Bulk of as Cast Sample 4.2. Microstructure in the Bulk of as Cast Sample The microstructure in the bulk can be seen in Figure 4a. The etching has revealed the dendrite The microstructure in the bulk can be seen in Figure 4a. The etching has revealed the dendrite structure. The secondary arm spacing was measured as 270 µm ± 41 µm. Precipitation of a white structure. The secondary arm spacing was measured as 270 μm ± 41 μm. Precipitation of a white phase of primary carbides occurs at certain interdendritic regions and is marked by arrows. The area phase of primary carbides occurs at certain interdendritic regions and is marked by arrows. The area fraction of primary carbide was measured to be f sCarbide = 0.0011. This value can be compared to fraction of primary carbide was measured to be 𝑓𝑠Carbide = 0.0011. This value can be compared to the the Scheil calculations that indicate a fraction of liquid, f L = 0.01, at the start precipitation of M6 C at Scheil◦ calculations that indicate a fraction of liquid, 𝑓𝐿 = 0.01, at the start precipitation of M6C at 1148 C. Assuming that all remaining liquid would solidify to primary carbide, there is a big difference 1148 °C. Assuming that all remaining liquid would solidify to primary carbide, there is a big between the calculated and measured results. difference between the calculated and measured results. Figure 4b shows a SEM back scattered micrograph of the bulk matrix of the unsoaked as cast Figure 4b shows a SEM back scattered micrograph of the bulk matrix of the unsoaked as cast sample. A gray phase of carbide exists in the interdendritic region with precipitation of MnS and a sample. A gray phase of carbide exists in the interdendritic region with precipitation of MnS and a white phase. The matrix has a lamellar structure. white phase. The matrix has a lamellar structure. The SEM images exhibited in Figure 5a,b were recorded with back-scattered electron (BSE) The SEM images exhibited in Figure 5a,b were recorded with back-scattered electron (BSE ) imaging and show the carbides precipitated in the bulk matrix of unsoaked as cast sample. imaging and show the carbides precipitated in the bulk matrix of unsoaked as cast sample. The The composition of Mo and Cr in the carbide at the points marked A, B, C, and D was selected composition of Mo and Cr in the carbide at the points marked A, B, C, and D was selected for chemical for chemical analysis. In Figure 5a, analysis of the light gray carbide compositions at locations A analysis. In Figure 5a, analysis of the light gray carbide compositions at locations A and B in the and B in the matrix is in the range of 5–8% Cr and 3–4% Mo. The gray border carbide, marked by C, matrix is in the range of 5–8% Cr and 3–4% Mo. The gray border carbide, marked by C, surrounding the light silver carbide, shows compositions of approximately 12.4% Cr and 7.5% Mo. The light silver carbide in the center, marked by D, has as high values as 20.5%Cr and 53.5% Mo.

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surrounding the light silver carbide, shows compositions of approximately 12.4% Cr and 7.5%8 of Mo. Metals 2018, 8, x FOR PEER REVIEW 18 The light silver carbide in the center, marked by D, has as high values as 20.5%Cr and 53.5% Mo.

(a)

(b)

(a) (b) Figure 4. (a) LOM micrograph of as cast sample 0SB. Carbide precipitation marked by arrows can be seen in interdendritic regions. Some ferritic crystalline growth can be seen in the center of dendrites. Figure 4. (a) LOM micrograph of as cast sample 0SB. Carbide precipitation marked by arrows can be (b) SEM back scattered micrograph of as cast sample 0SB, showing the distribution of carbides and seen in interdendritic regions. Some Some ferritic ferritic crystalline crystalline growth growth can can be be seen seen in in the the center center of of dendrites. dendrites. MnS along the interdendritic region. Some lamellar precipitate can be observed in the pearlitic matrix. (b) SEM back scattered micrograph of as cast sample 0SB, showing the distribution of carbides and MnS along the interdendritic region. Some lamellar precipitate can be observed in the pearlitic matrix.

(a)

(b)

(a) analysis (b) Figure sample. Figure5.5.(a) (a)Composition Composition analysis of of aacarbide carbidein inthe thebulk bulkmatrix matrixof ofan anunsoaked unsoaked sample. The The mass mass percentage at area by Cr, 3.1 (B) 4.3 12.43 7.5 Mo; percentage composition atanalysis area marked marked by (A) (A) 5.5 5.5 Cr, bulk 3.1 Mo; Mo; (B) 7.8 7.8 Cr,unsoaked 4.3 Mo; Mo; (C) (C) 12.43 Cr, Cr, 7.5 Mo; Figure 5. (a)composition Composition of a carbide in the matrix of Cr, an sample. The mass and (D) 20.5 53.5 Mo. (b) EDS analysis of a carbide in the bulk matrix of an unsoaked sample. and (D) Cr, 53.5 Mo. (b) EDS analysis of a carbide in the bulk matrix of an unsoaked sample. The percentage composition at area marked by (A) 5.5 Cr, 3.1 Mo; (B) 7.8 Cr, 4.3 Mo; (C) 12.43 Cr, 7.5 Mo; The mass percentage composition at area marked by (A) 5.3 Cr, 51.53 Mo; (B) 29.7 S, 61.7 Mn; (C) 22.7 mass percentage composition at area marked by (A) 5.3 Cr, 51.53 Mo; (B) 29.7 S, 61.7 Mn; (C) 22.7 Cr, and (D) 20.5 Cr, 53.5 Mo. (b) EDS analysis of a carbide in the bulk matrix of an unsoaked sample. The Cr, 50.1 Mo; and (D) 7.7 Cr, 5.8 Mo. 50.1 Mo; and (D) 7.7 Cr, 5.8 Mo. mass percentage composition at area marked by (A) 5.3 Cr, 51.53 Mo; (B) 29.7 S, 61.7 Mn; (C) 22.7 Cr, 50.1 Mo; and (D) 7.7 Cr, 5.8 Mo. In In Figure Figure 5b, 5b, the the white white carbide, carbide, marked marked A, A, contains contains 5.3% 5.3% Cr Crand and51.5% 51.5%Mo. Mo. The The light light silver silver carbide, marked C, contains 27.7% Cr and 50.1% Mo. The gray border carbide, surrounding the light carbide, marked C, contains 27.7% Cr and 50.1% Mo. The gray border carbide, surrounding the light In Figure 5b, the white carbide, marked A, contains 5.3% Cr and 51.5% Mo. The light silver silver carbide, marked D, contains 7.7% Cr and 5.8% Mo. In the last solidified region MnS, type III silver carbide, marked D, contains 7.7% Cr and 5.8% Mo. In the last solidified region MnS, type III carbide, marked C, contains 27.7% Cr and 50.1% Mo. The gray border carbide, surrounding the light has precipitated, which is marked by B and contains high concentrations of Mn and S. The micro has precipitated, whichD,iscontains marked 7.7% by B Cr and contains high of Mn andMnS, S. The micro silver carbide, marked and 5.8% Mo. Inconcentrations the last solidified region type III segregations of Cr and Mo in the bulk matrix of unsoaked as cast samples were mapped by EMPA. segregations of Cr and Mo in the bulk matrix of unsoaked as cast samples were mapped by EMPA. has precipitated, which is marked by B and contains high concentrations of Mn and S. The micro The presented in Figure 6. The concentrations in the the interdendritic areasareas are quite high with Theresults resultsare are presented 6. matrix The concentrations are by quite high segregations of Cr and Mo in in Figure the bulk of unsoaked in as castinterdendritic samples were mapped EMPA. spots of very high concentrations, which seem to originate from primary carbides. In the center of the with spots of very high concentrations, which seem to originate from primary carbides. In the center The results are presented in Figure 6. The concentrations in the interdendritic areas are quite high dendrites, the concentration is moreiseven. of thespots dendrites, concentration more even. with of verythe high concentrations, which seem to originate from primary carbides. In the center The map also shows that the elements are map also that the elements are not not evenly evenly distributed distributed along along the the interdendritic interdendritic regions regions of theThe dendrites, theshows concentration is more even. but vary substantially. The segregation index, defined as the concentration in the interdendritic but The varymap substantially. The segregation index, defined as the concentration in the interdendritic also shows that the elements are not evenly distributed along the interdendritic regions matrix divided by in center of isisroughly = 2.4 for for Cr, Cr, matrix divided bythe theconcentration concentration inthe the centerdefined ofthe thedendrites, dendrites, roughly3.4%/1.4% 3.4%/1.4% = 2.4 but vary substantially. The segregation index, as the concentration in the interdendritic and 4.5 for for Mo. Mo. The The micro micro segregation segregation is higher higher for for Mo Mo than than for for Cr. Cr. and 1.8%/0.4% 1.8%/0.4% == 4.5 matrix divided by the concentration in the center ofisthe dendrites, is roughly 3.4%/1.4% = 2.4 for Cr, and 1.8%/0.4% = 4.5 for Mo. The micro segregation is higher for Mo than for Cr.

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Figure showing the distribution of Mo and Cr ininthe the bulk matrix. Figure6.6.EMPA EMPAreport reportofofsample sample0SB, 0SB,showing showingthe thedistribution distributionof ofMo Moand andCr Crin thebulk bulkmatrix. matrix. Uneven Unevendistribution distributionofofalloying alloyingelements elementsalong alongthe theinterdendritic interdendriticregion regionwas wasobserved, observed,with withhigh high concentration concentrationspots. spots.

By ititwas was possible totoreveal reveal more details about the primary carbides and especially Bycolor coloretching, etching,it waspossible possibleto revealmore moredetails detailsabout aboutthe theprimary primarycarbides carbidesand andespecially especially By color etching, the white carbides, as seen in Figure 7a,b. According to [16,18], M C is colored brown, M C is outlined 22C 66C thewhite whitecarbides, carbides,asasseen seenininFigure Figure7a,b. 7a,b.According Accordingtoto[16,18], [16,18],MM 2Cisiscolored coloredbrown, brown,MM 6Cisisoutlined outlined the and colored blue, and M C is faintly colored. Cementite is not affected by the etch reagent. 7 3 and colored blue, and M 7 C 3 is faintly colored. Cementite is not affected by the etch reagent. and colored blue, and M7C3 is faintly colored. Cementite is not affected by the etch reagent. Both Figure 7a,b show eutecticstructure structure of carbides with C as brown phase M as 2as 6C BothFigure Figure7a,b 7a,bshow showaaaeutectic eutectic structure carbides with 2C asbrown brown phase andand 6C blue Both ofof carbides with MM2M C phase and MM6C asas blue blue phase appearing, especially atborders the borders between the brown white phase. These carbides phase appearing, especially the borders between thebrown brown andand white phase. These carbides are phase appearing, especially atatthe between the and white phase. These carbides are are precipitated in a matrix of uncolored white phase in contrast to the matrix, which is shifting in precipitatedininaamatrix matrixofofuncolored uncoloredwhite whitephase phaseinincontrast contrasttotothe thematrix, matrix,which whichisisshifting shiftingininaaa precipitated slightly gray tone. This white phase seems to correspond to the gray phase in Figures 4b and slightlygray graytone. tone.This Thiswhite whitephase phaseseems seemstotocorrespond correspondtotothe thegray grayphase phaseininFigures Figures4b 4band and5a. 5a. slightly 5a.

(a) (a)

(b) (b)

Figure Figure7. (a)Matrix Matrixofofthe thecolored coloredetched etchedsample sample0SB. 0SB.M 2Cisiscolored coloredbrown, brown,M 6Cisisoutlined outlinedand and 7.7.(a) the colored etched sample 0SB. MM colored brown, MM 22C 66C colored isisfaintly faintly coloredblue, blue,and andM 7C faintlycolored. colored.(b) (b)Matrix Matrixofofthe thecolored coloredetched etchedsample sample0SB. 0SB.M 2Cisiscolored colored blue, and MM 0SB. MM 77C 3 3is 22C brown, C isisfaintly faintly colored. brown,MM 6Cisisoutlined outlinedand andcolored coloredblue, blue,and andMM 7C faintlycolored. colored. 333is 6C 7C

Hardness were carried outout in order to tryto to betweenbetween different carbides. Hardnessmeasurements measurements were carried out order todistinguish trytotodistinguish distinguish betweendifferent different Hardness measurements were carried ininorder try In the center of the dendrite, the hardness was about 210 HV. In the primary carbide precipitation, carbides.InInthe thecenter centerofofthe thedendrite, dendrite,the thehardness hardnesswas wasabout about210 210HV. HV.InInthe theprimary primarycarbide carbide carbides. the hardness varied between 900–1200 HV, depending on the amount of gray carbon phase and precipitation, the hardness varied between 900–1200 HV, depending on the amount of gray carbon precipitation, the hardness varied between 900–1200 HV, depending on the amount of gray carbon silver/white phase present. One conclusion is that the white phase in Figure 7a,b is carbide and not phaseand andsilver/white silver/whitephase phasepresent. present.One Oneconclusion conclusionisisthat thatthe thewhite whitephase phaseininFigure Figure7a,b 7a,bisiscarbide carbide phase retained austenite. andnot notretained retainedaustenite. austenite. and An EDS analysis SEM was carried out totoevaluate evaluate how the Cr and Mo concentration decreases AnEDS EDSanalysis analysisin SEMwas wascarried carriedout outto evaluatehow howthe theCr Crand andMo Moconcentration concentrationdecreases decreases An ininSEM from the center of the dendrite to the interdendritic region. The results are presented in Figure fromthe thecenter centerofofthe thedendrite dendritetotothe theinterdendritic interdendriticregion. region.The Theresults resultsare arepresented presentedininFigure Figure888as from asas dotted lines. Every dot isaaameasurement measurementpoint. point. the same diagram, the results from calculating dottedlines. lines.Every Everydot dotisis measurement point. the same diagram, the results from calculating the dotted InInIn the same diagram, the results from calculating the 𝛾/𝐿

𝛾/𝐿 micro-segregationprofiles profilesby bythe theScheil Scheilequation, equation,Equation Equation(1), (1),are areplotted plottedasassolid solidlines, lines,with with𝑘𝑘𝐶𝑟 micro-segregation 𝐶𝑟 ==

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0.58micro-segregation and 𝑘𝑀𝑜 = 0.31. The k-values evaluated from the Thermo-Calc calculation the stage the profiles bywere the Scheil equation, Equation (1), areScheil plotted as solid at lines, with γ/L γ/L 𝛾/𝐿 when the first M 6C was precipitated. There is good agreement between the measured and calculated k = 0.58 and k = 0.31. The k-values were evaluated from the Thermo-Calc Scheil calculation 0.58 The k-values were evaluated from the Thermo-Calc Scheil calculation at the stageat Crand 𝑘𝑀𝑜 = 0.31.Mo concentrations until 𝑓𝑠precipitated. ~0.93. The precipitated. measurement values >0.93 originate mainly from the gray 𝑠 agreement the stage when the first M6 C was There is for good𝑓between the measured and when the first M6C was There is good agreement the between measured and calculated carbide. calculated concentrations until f ~0.93. The measurement values for f > 0.93 originate mainly from s s concentrations until 𝑓 ~0.93. The measurement values for 𝑓 >0.93 originate mainly from the gray 𝑠

𝑠

the gray carbide. carbide.

Figure 8. Comparison between calculated and experimental values of Mo and Cr segregation along the dendrite. It can be seen that precipitation of carbides starts earlier than predicted by the Scheil Figure 8. 8. Comparison between calculated and experimental valuesvalues of Mo of andMo Cr and segregation along Figure Comparison between calculated and experimental Cr segregation calculation. the dendrite. It can beItseen precipitation of carbides starts earlier predicted by the Scheil along the dendrite. can that be seen that precipitation of carbides startsthan earlier than predicted by the calculation. Scheil calculation.

4.3. Microstructure in A-Segregates, as Cast Samples 4.3. Microstructure in A-Segregates, as Cast Samples 4.3. Microstructure in A-Segregates, Cast Samples Figure 9a shows the dendriteasstructure in the A-segregated channels. The secondary arm spacing Figurecompared 9a showsto thethat dendrite structure inleft thein A-segregated channels. The secondary spacing is is smaller in the bulk to the the figure. In the A-segregated area, arm the secondary Figure 9a shows the dendrite structure in the A-segregated channels. The secondary arm spacing smaller compared to that in the bulk to the left in the figure. In the A-segregated area, the secondary arm spacing was measured as 183 μm ± 29 μm, compared to the average value of 270 μm in the bulk. is smaller compared to that in the bulk to the left in the figure. In the A-segregated area, the secondary arm spacing was measured ascarbide 183 µmnetwork ± 29 µm, compared to in thethe average value of 270 µm in The the bulk. Additionally, large primary was observed A-Segregation channel. area arm spacing was measured as 183 μm ± 29 μm, compared to the average value of 270 μm in the bulk. Carbidewas observed in the A-Segregation channel. The area Additionally, large in primary carbide network fraction of carbide this region was 𝑓 = 0.0027, i.e., twice that in the bulk. Figure 9b shows 𝑠 Additionally, large primary carbide network was observed in the A-Segregation channel. The area fraction ofimage carbide in this region was f sCarbide =carbides 0.0027, i.e., twice that in the Figure 9b shows the SEM of interdendritic regions and with indications ofbulk. point aidthe of Carbide fraction of carbide in this region was 𝑓𝑠 = 0.0027, i.e., twice that in the bulk. analyses Figure 9bby shows SEM image ofby interdendritic regions and carbides with indications of point analyses by aid of EDS, EDS, marked A, B, C, and D. The gray border carbide, marked by A and darker carbide phase, the SEM image of interdendritic regions and carbides with indications of point analyses by aid of marked by B, C, andcomposition D. The gray border carbide, marked by A and darker carbide phase, markedC,by B, marked byA, B,by has 10–12% Cr andcarbide, 5–6% Mo. The white phase, marked has EDS, marked A,the B, C, and D. The gray border marked by Acarbide and darker carbide phase, has the composition 10–12% Cr and 5–6% Mo. The white carbide phase, marked C, has the composition the composition 2.8% Cr and 49% Mo. Secondary plate-like carbide the matrix, marked has marked by B, has the composition 10–12% Cr and 5–6% Mo. The whiteincarbide phase, markedbyC,D,has 2.8% Cr and 49% 9% Mo.CrSecondary plate-like carbide in the matrix, marked by D,within has the composition 9% the composition and 4% Mo. Presence of type III MnS was also noted the vicinity of the the composition 2.8% Cr and 49% Mo. Secondary plate-like carbide in the matrix, marked by D, has Cr and 4% Mo. Presence of type III MnS was also noted within the vicinity of the carbides. carbides. the composition 9% Cr and 4% Mo. Presence of type III MnS was also noted within the vicinity of the carbides.

(a)

(b)

(a) (b) Figure 9. (a) A-segregation A-segregation channel; large large primary primary carbide carbide network network precipitation precipitation at the the tip of the channel; at dendrite in in the the interdendritic interdendriticregion regionwas wasobserved, observed,along along with precipitation of type III MnS. with thethe precipitation of type III MnS. (b) Figure 9. (a) A-segregation channel; large primary carbide network precipitation at the tip of the (b) Composition analysis of the carbide in the bulkmatrix matrixofofan anunsoaked unsoakedsample. sample. The Themass mass percentage Composition analysis of the carbide in the bulk dendrite in the interdendritic region was observed, along with the precipitation of type III MnS. (b) composition at area marked by (A) 10.52 Cr, 6.07 Mo; (B) 11.83 Cr, 5.87 Mo; (C) 2.84 Cr, 49.00 Mo; and Composition analysis of the carbide in the bulk matrix of an unsoaked sample. The mass percentage (D) 9.01 Cr, Cr, 4.04 4.04 Mo. Mo. composition at area marked by (A) 10.52 Cr, 6.07 Mo; (B) 11.83 Cr, 5.87 Mo; (C) 2.84 Cr, 49.00 Mo; and (D) 9.01 Cr, 4.04 Mo.

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TheEMPA EMPAmap mapgiven givenininFigure Figure1010 shows composition distribution of Mo Crthe in Athe The shows thethe composition distribution of Mo andand Cr in A-segregate channel. The interdendritic segregated regions are wider compared to those in the bulk segregate channel. The interdendritic segregated regions are wider compared to those in the bulk inin Figure6.6.There Thereare aresome somesmall smallareas areasatatthe thecenter centerthat thathave haveaacomparatively comparativelyvery veryhigh highconcentration concentration Figure of these elements. The segregation index in this case is roughly 4%/1.6% = 2.5 for Cr and 4%/0.5%==88 of these elements. The segregation index in this case is roughly 4%/1.6% = 2.5 for Cr and 4%/0.5% for Mo. The micro segregation is higher in the A-segregated channel for Mo but the same forCr Crinin for Mo. The micro segregation is higher in the A-segregated channel for Mo but the same for comparisonwith withthe thesegregation segregationindex indexin inthe thebulk. bulk. comparison

Figure10. 10.EMPA EMPAreport reportofofsample sampleOSA, OSA,showing showingthe thedistribution distributionofofMo Moand andCr Crin inan anA-segregate A-segregate Figure channel.High Highconcentration concentrationareas areasininthe themiddle middleofofthe thecarbide carbidecan canbe beseen. seen. channel.

Examplesofofcolor coloretching etchingofofthe thecarbides carbidesininthe theA-segregated A-segregatedinterdendritic interdendriticareas areasare areshown shownin in Examples Figure 11a,b. A eutectic precipitation of white carbon phase and brown M C dominates. Hardness Figure 11a,b. A eutectic precipitation of white carbon phase and brown M 222C dominates. Hardness measurementsgave gavevalues valuesininthe therange rangeofof500–800 500–800HV, HV,with withsome somevalues valuesup uptoto1140 1140HV. HV.The Thelower lower measurements hardness of the carbides in A-segregate could be attributed to larger areas of dark gray carbides. hardness of the carbides in A-segregate could be attributed to larger areas of dark gray carbides.

(a)

(b)

Figure11. 11.(a) (a)Color Coloretching etchingofofprimary primarycarbides carbidesininA-segregate. A-segregate.The Theeutectic eutecticconsists consistsmainly mainlyofofM M22C, Figure 2 C, browncolor, color,and andwhite whitecarbide carbidephase. phase.Some SomeM M66C C exists at the border. (b) Color etching of primary brown exists at the border. (b) Color etching of primary 6 carbidesininan anA-segregation A-segregationchannel channelat atlower lowermagnification. magnification. carbides

4.4. Microstructure in as Cast-Soaked Samples 4.4. Microstructure in as Cast-Soaked Samples No primary carbides in bulk were seen in the interdendritic regions of the sample SO20h soaked No primary carbides in bulk were seen in the interdendritic regions of the sample SO20h soaked for 20 h; see Figure 12a. The contrast between the center of the dendrite and the interdendritic region for 20 h; see Figure 12a. The contrast between the center of the dendrite and the interdendritic region has decreased, which shows that the composition difference of alloying elements has decreased in has decreased, which shows that the composition difference of alloying elements has decreased in these areas compared to the unsoaked sample in Figure 4a. these areas compared to the unsoaked sample in Figure 4a. To enable a comparison with the unsoaked condition, an EDS analysis in SEM was carried out to evaluate how the Cr and Mo concentration decreases from the interdendritic region to the center

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the unsoaked condition, an EDS analysis in SEM was carried12out to of 18 of the dendrite after soaking. The results are presented in Figure 12b. An evaluation of the segregation evaluate how the Cr and Mo concentration decreases from the interdendritic region to the center of the index gives 2.6%/1.5% = 1.7 The for Cr and are 1.25%/0.4% = in 3.1Figure for Mo. InAn comparison with unsoaked of the dendrite after soaking. results presented 12b. evaluation thethe segregation dendrite after soaking. The results are presented in Figure 12b. An evaluation of theofsegregation index samples, the2.6%/1.5% index has =decreased mainly due to decreased concentrations in the interdendritic areas. index and 1.25%/0.4% = 3.1 In comparison the unsoaked gives gives 2.6%/1.5% = 1.7 for1.7 Crfor andCr 1.25%/0.4% = 3.1 for Mo.for InMo. comparison with thewith unsoaked samples, In the micro-sample, SOB4h,mainly soakeddue for 4tohours, all primary carbides in were in the bulk. samples, has decreased decreased concentrations thedissolved interdendritic the indexthe hasindex decreased mainly due to decreased concentrations in the interdendritic areas. areas. In the micro-sample SOA4h, a few tiny carbides remained in the A-segregates. Figure 13a shows the In Inthe themicro-sample, micro-sample,SOB4h, SOB4h,soaked soakedfor for44hours, hours,all allprimary primarycarbides carbideswere weredissolved dissolvedininthe thebulk. bulk. soaked structure with a dendritic appearance, grain boundary cracks can also Figure be seen. It seems that In Inthe themicro-sample micro-sampleSOA4h, SOA4h,aafew fewtiny tinycarbides carbidesremained remainedin inthe theA-segregates. A-segregates. Figure13a 13ashows showsthe the the grain boundary cracks have developed due to the soaking. One example of remaining carbide is soaked structure with a dendritic appearance, grain boundary cracks can also be seen. It seems that soaked structure dendritic appearance, grain boundary cracks can also be seen. It seems that the shown in Figure 13b. The results indicate that only 4 h of soaking are needed to dissolve all primary the grain boundary cracks developed due the soaking. One example of remaining carbide is grain boundary cracks havehave developed due to thetosoaking. One example of remaining carbide is shown carbides the bulk, but results longer soakingthat times are4 needed to dissolve primary carbides the Ashown in in Figure 13b. The h of soaking are needed to dissolve allin primary in Figure 13b. The results indicateindicate that only 4 honly of soaking are needed to dissolve all primary carbides segregates, withbulk, thicker carbides. carbides in the butsoaking longer times soaking aretoneeded to primary dissolve carbides primary in carbides in the Ain the bulk, but longer aretimes needed dissolve the A-segregates, segregates, with thicker carbides. with thicker carbides.

(a)

(b)

(a) (b) visible in LOM. Less Figure 12. (a) Microstructure of the sample SO20h. No primary carbides are intense solute segregation. (b) SEM picture of sample SO20h, exhibiting secondary carbide Figure12. 12. (a)Microstructure Microstructure the sample SO20h. Noprimary primary carbides arevisible visible LOM. Less Figure (a) ofofthe sample SO20h. No carbides are ininLOM. Less precipitated along the inter-crystalline region. It also shows the pattern of EDS analysis. intense solute SEM picture of sample SO20h, exhibiting secondarysecondary carbide precipitated intense solute segregation. segregation.(b)(b) SEM picture of sample SO20h, exhibiting carbide along the inter-crystalline region. It also showsIt the analysis. precipitated along the inter-crystalline region. alsopattern shows of theEDS pattern of EDS analysis.

(a)

(b)

(b) aa dendrite Figure 13. (a) (a) Micro Micro(a) structure in in sample sample SOA4h SOA4h from from the the bulk bulk region, region, showing showing dendrite structure structure Figure structure with no primary primary carbides, carbides, as well as grain boundary cracks in the inter-granular regions. (b) (b) SEM SEM with Figure 13. (a) Micro structure in sample SOA4h from the bulk region, showing a dendrite structure photo showing a remaining primary carbide in the A-segregated area. photo showing with no primary carbides, as well as grain boundary cracks in the inter-granular regions. (b) SEM a remaining primaryMaterial carbide in the A-segregated area. 4.5. photo Resultsshowing on Soaked and Hot Worked

4.5. Results on Soaked and Hot Worked Material A banded structure was observed in the hot worked samples. These bands were aligned in the 4.5. Results on Soaked and Hot Material A banded structure wasWorked observed in the hot worked samples. These bands were aligned in the hot working direction. The segregation bands were very distinctive in the samples heat treated for hot working direction. The segregation bands were very distinctive in the samples heat treatedinfor A banded structure was observed in theinhot worked aligned the4h 4 h (HW4h) and then deformed, as shown Figure 14a, samples. whereas These for thebands ingot were heat treated for 20 h (HW4h) and then deformed, as shownbands in Figure whereas for the ingot heat treated for 20 hfor and hot working direction. The segregation were14a, very distinctive in the samples heat treated 4 then deformed, the bands are weaker with low levels of contrast between the bands, as shown in h (HW4h) and then deformed, as shown in Figure 14a, whereas for the ingot heat treated for 20 h and Figure 14b. A high of segregation of low the alloying the darker areas ofasthese bands then deformed, the degree bands are weaker with levels ofelements contrast in between the bands, shown in Figure 14b. A high degree of segregation of the alloying elements in the darker areas of these bands

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of 18 are weaker with low levels of contrast between the bands, as13shown 13 of 18 in Figure 14b. A high degree of segregation of the alloying elements in the darker areas of these was noted. The fraction of secondary carbides was also high in these dark bands, and some of the bands was noted. The fraction of secondary carbides was alsoinhigh thesebands, dark bands, andof some was noted. The fraction of secondary carbides was also high theseindark and some the carbides were observed to be surrounding the elongated MnS. The average distance between the of the carbides were observed to be surrounding the elongated MnS. The average distance between carbides were observed to be surrounding the elongated MnS. The average distance between the darker bands is about 230 μm in Figure 14. This distance can be compared with the recalculated the darker bands is about µm Figure Thisdistance distancecan canbe be compared with with the the recalculated recalculated darker bands is arm about 230230 μm in in Figure 14.14.after This average dendrite spacing of around 70 μm hot working withcompared an elongation factor of 16. This average dendrite dendritearm armspacing spacingofofaround around 70 µm after hot working with an elongation factor of 16. average 70 μm after hot working with an elongation factor of to 16.the This shows that it is only the most segregated interdendritic areas in the cast structure that give rise This!shows that it is only the most segregated interdendritic areas in the cast structure that give rise shows that it isanalyses only theby most segregated interdendritic in the cast structure that give rise The to the bands. Linear EDS were carried out acrossareas the banded structure in Figure 14a,b. to the bands. Linear analyses bywere EDS carried were carried out across the banded structure in Figure 14a,b. bands. Linear analyses by EDS out across the banded structure in Figure 14a,b. The results are presented in Figure 15a,b. The results show variations in concentrations with high peaks The results are presented in Figure 15a,b. The results show variations in concentrations with high results arepresence presented Figure 15a,b. The results show variations in concentrations with high peaks due to the of in carbides. peaks to the presence of carbides. due todue the presence of carbides.

(a)

(b) (a) (b) Figure 14. (a) LOM image of the banded structure of sample HW4h. The band lines are sharp and narrower. contrast. (b)ofLOM image ofstructure the banded structure of sample HW20h. Band image the banded of sample HW4h. The band sharpare and Figure 14.High (a) LOM The lines are lines blunt and wider. Low contrast. narrower. High contrast. (b) LOM image of the banded structure of sample HW20h. Band lines are blunt and wider. Low contrast.

(a)

(b)

(a) ofofsample (b) thethe Figure15. 15.(a) (a)Line Lineanalysis analysis sampleHW4h, HW4h,showing showingthe thedistribution distribution along bands. Line Figure ofof CrCr along bands. (b)(b) Line analysissample sampleHW20h, HW20h,showing showingthe thedistribution distributionofofCrCralong alongthe thebands. bands. analysis Figure 15. (a) Line analysis of sample HW4h, showing the distribution of Cr along the bands. (b) Line analysis sample showing the distribution of Cr along the bands.between dark and light band However, it isHW20h, interesting to note that the average differences

However, it is interesting to note that the average differences between dark and light band decreases with withsoaking soakingtime. time. decreases However, it is interesting to note that the average differences between dark and light band Figure 16 16shows showsmicrographs micrographsofofboth bothdark darkand andlight lightbands bandsininsample sample HW4h, which was soaked Figure HW4h, which was soaked decreases with soaking time. for 4 h and deformed, and sample HW20h, which was soaked for 20 h and deformed. Both the samples for h and deformed, and sample HW20h, which was soaked for 20 h and deformed. Both the Figure 16 shows micrographs of both dark and light bands in sample HW4h, which was soaked are equally Figure 16a shows16a theshows dark band with high element concentration, samples are strained. equally strained. Figure the dark band alloying with high alloying element for 4 h and deformed, and sample HW20h, which was soaked for 20 h and deformed. Both the with the structure consisting of completely spheroidized carbides within the ferrite within matrix. the No ferrite lamellar concentration, with the structure consisting of completely spheroidized carbides samples are equally strained. Figure 16a shows the dark band with high alloying element cementite visible. cementite Figure 16bisshows lighter with lower composition of alloying elements. matrix. Noislamellar visible.the Figure 16bband shows thealighter band with a lower composition concentration, with the structure consisting of completely spheroidized carbides within the ferrite of The of sample has a mixture spheroidized carbides and lamellar Thealloying sample elements. has a mixture spheroidized carbidesofand lamellar pearlite scattered around.pearlite Fraction matrix. No lamellar cementite is visible. Figure 16b shows the lighter band with a lower composition scattered pearlite phase is high;carbides in someare areas, are matrix. seen of pearlitearound. phase isFraction high; inofsome areas, secondary seen secondary nestled in carbides the pearlitic of alloying elements. The sample has a mixture of spheroidized carbides and lamellar pearlite nestled in the pearlitic matrix. Figure 16c exhibits a darker band within the sample that was soaked scattered around. Fraction of pearlite phase is high; in some areas, secondary carbides are seen for 20 h and deformed, yielding large spheroidized regions with comparatively finer carbides that nestled in the pearlitic matrix. Figure 16c exhibits a darker band within the sample that was soaked are mixed with a small pearlitic region containing a pearlite lamella. Figure 16d shows a lighter area for 20 h and deformed, yielding large spheroidized regions with comparatively finer carbides that are mixed with a small pearlitic region containing a pearlite lamella. Figure 16d shows a lighter area

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Figure 16c exhibits a darker band within the sample that was soaked for 20 h and deformed, yielding large spheroidized regions with comparatively finer carbides that are mixed with a small pearlitic Metals 8, x FOR PEER REVIEW 14 of 18 region2018, containing a pearlite lamella. Figure 16d shows a lighter area in the banded structure, which exhibits a structure similar to that in Figure 16b, but with a region containing a higher fraction of in the banded structure, which exhibits structure similar that in Figure 16b, but with a region lamellar pearlite. It is obvious that the asoaking time and to concentration differences influence the containing a higher fraction of lamellar pearlite. It is obvious that the soaking time and concentration matrix structure. differences influence the matrix structure.

(a)

(b)

(c)

(d)

Figure 16. Micrographs of of the the hot (a) sample Figure 16. Micrographs hot worked worked samples samples etched etched with with Nital: Nital: (a) sample HW4h, HW4h, exhibits exhibits aa dark band region; (b) sample HW4h, shows a lighter region; (c) sample HW20h, exhibits dark band region; (b) sample HW4h, shows a lighter region; (c) sample HW20h, exhibits aa darker darker region; and region; and (d) (d) sample sample HW20h, HW20h, showing showing lighter lighter region, region, of of the the banded banded structure. structure.

5. Discussion 5. Discussion In Figure 4a, it can be seen that the primary carbides precipitate only at certain interdendritic areas. In Figure 4a, it can be seen that the primary carbides precipitate only at certain interdendritic Consequently, the alloy concentration varies from one area to another. However, the Scheil equation areas. Consequently, the alloy concentration varies from one area to another. However, the Scheil predicts a continuous precipitation of primary carbides in interdendritic regions. The measured area equation predicts a continuous precipitation of primary carbides in interdendritic regions. The fraction of carbide was f carbide = 0.0011. The Scheil calculation shows that carbide M6 C starts to measured area fraction ofs carbide was 𝑓𝑠carbide = 0.0011. The Scheil calculation shows that carbide precipitate at f L = 0.01. Assuming that all of the remaining liquid solidifies to carbide, the total primary M6C starts to precipitate at 𝑓𝐿 = 0.01. Assuming that all of the remaining liquid solidifies to carbide, precipitation of carbide should be 10 times that of the measured fraction. It is obvious that the Scheil the total primary precipitation of carbide should be 10 times that of the measured fraction. It is equation does not predict the amount of primary carbide precipitation for the bearing steel studied. obvious that the Scheil equation does not predict the amount of primary carbide precipitation for the The average distance between the segregation lines in the hot worked samples in Figure 14 bearing steel studied. is, on average, 230 µm. This corresponds to a distance of 920 µm in the as cast samples, which The average distance between the segregation lines in the hot worked samples in Figure 14 is, can be compared with the average measured secondary arm spacing of 270 µm. From this, it clear on average, 230 μm. This corresponds to a distance of 920 μm in the as cast samples, which can be that the concentrations in the interdendritic areas vary from region to region, producing peaks of compared with the average measured secondary arm spacing of 270 μm. From this, it clear that the high concentration that cause precipitation of primary carbides. A hypothesis to explain this is concentrations in the interdendritic areas vary from region to region, producing peaks of high concentration that cause precipitation of primary carbides. A hypothesis to explain this is that the concentration peaks arise due to redistribution of enriched liquid by fluid flow caused by solidification shrinkage. The measured micro segregation levels of Cr and Mo in as cast samples are presented in Table 7. Soaking for 20 h decreases the Cr concentration significantly in interdendritic areas, but not in the

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that the concentration peaks arise due to redistribution of enriched liquid by fluid flow caused by solidification shrinkage. The measured micro segregation levels of Cr and Mo in as cast samples are presented in Table 7. Soaking for 20 h decreases the Cr concentration significantly in interdendritic areas, but not in the dendrite center. The explanation is that the concentration gradient is steeper closer interdendritic areas and hence the back diffusion rate will be higher. In A-segregate channels, the mean compositions of Cr and Mo are higher compared to those in the bulk. This explains why the fraction of interdendritic carbides is higher in A-segregation channels, f sCarbide = 0.0027, in comparison with the bulk, f sCarbide = 0.0011. Table 7. Mean segregation before and after soaking. Sample/Region 0SB Unsoaked–Bulk SO20h-Soaked 20 h 0SA A-Segregation Channel

Alloying Element

Mean of Dendrite Center

Mean of Interdendritic Area

Cr% Mo% Cr% Cr% Mo%

1.5 0.4 1.6 1.90 0.43

2.85 1.2 2.3 3.6 1.93

A summary of the compositions in dark and light areas in the hot worked samples is shown in Table 8. As expected, longer soaking time decreases the segregation of Cr, and the measured values are in agreement with those in Table 7. Table 8. Composition analysis of the dark and light areas in the soaked and deformed samples. Sample No. HW4h-Soaked for 4 h & then deformed HW20h-Soaked for 20 h & then deformed

Area Analysed

Cr wt%

Dark Area Light Area Dark Area Light Area

2.23 1.48 1.97 1.67

In Table 9, a summary of different appearance of carbides with measured compositions is presented. The phases light gray, gray, and dark gray have more or less the same composition range. In comparison with the different carbide compositions, calculated by Thermo-Calc, the closest carbide type is cementite, M3 C in Table 2. The result from the color etching also supports that these phases are M3 C carbides. It could also be possible that the gray and dark gray phases are M23 C6 . The white phase in the bulk samples corresponds to M6 C given in Table 5 with high content of Mo. The measured composition of the light silver phase in the bulk sample could be a mix of different carbides, for example, M7 C3 and M6 C. Figures 7 and 11b which shows the color etched phases, reveals that the white carbide in Figure 4b is M2 C (brown color), with M6 C, blue colored, appearing at the border of cementite. This also indicates that M6C precipitated during cooling, and not during solidification. Primary carbides in A-segregated areas shown in Figure 11 seem to be a cooperated precipitation of cementite and M2 C. This difference in carbide appearance in comparison to the bulk could be attributed to higher level of the carbide forming elements Cr, Mo, and C. Table 9. Summary of appearance of carbides and corresponding measured compositions. Shade of the Carbide Light gray phase in matrix Gray phase Dark gray phase Light silver White

Composition of Cr. wt% Bulk A-Segr 5.5–9.0 7.7–12.4 20.5–22.7 5.3

9 10.5 11.8 2.8

Composition of Mo. wt% Bulk A-Segr 3.1–4.4 5.8–7.5 50.1–53.5 51.5

4.4 6.1 5.9 49

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To get get an an idea idea of of the the dissolution dissolution time time of of the the primary primary carbides, carbides, aa simple simple model model was was developed developed To with the following assumptions: with the following assumptions: 1. 1. 2. 2. 3. 3. 4. 4.

Diffusion Diffusionof ofCr Crin inγγisisthe therate ratedetermining determiningstep stepfor forcarbide carbidedissolution. dissolution. The density of the carbide and the γ phases is the same. The density of the carbide and the γ phases is the same. The Thephase phaseboundary boundarybetween betweendissolving dissolvingcarbide carbideand andγγdoes doesnot notmove. move. The concentration gradient in γ is assumed to be a straight line, as shown in Figures 17 and 18a. The concentration gradient in γ is assumed to be a straight line, as shown in Figures 17 and 18a.

Figure17. 17.Schematic Schematicshowing, showing,diffusion diffusiondistance, distance,x,x,carbide carbidethickness, thickness,w, w,and andhow howthe themass massbalance, balance, Figure Equation(2), (2),isis calculated. calculated. Equation

Amass massbalance balance for for Cr Cr between between carbide carbide and and γγ gives gives A   Carbide/𝛾 𝛾/Carbide 𝛾/Carbide 0 0 (𝐶𝐶𝑟 (CCarbide/γ − 𝐶𝐶𝑟− C γ/Carbide ) ∙ 𝑤 =)·𝑥 ∙= (𝐶x𝐶𝑟· C γ/Carbide − 𝐶𝐶𝑟 ) CCr w − Cr Cr Cr

√ 𝑥 = 0.477 ∙ √4𝐷𝑡 x = 0.477 · 4Dt

(2) (2) (3)

(3)

carbide/ 𝛾 γ/carbide in which C𝐶carbide/γ Cr-concentration in the carbide, 𝑤 thickness is the thickness of dissolved 𝐶𝑟 in𝛾/𝑐𝑎𝑟𝑏𝑖𝑑𝑒 which isisCr-concentration in the carbide, w is the of dissolved carbide, Ccarbide, Cr Cr 0 𝐶is𝐶𝑟the Cr-concentration is the Cr-concentration in γwith in contact with 𝐶𝐶𝑟 is the concentration the center in γ in contact carbide, C0 carbide, is the concentration in the center ofinthe dendrite,

Cr

of dendrite, D constant is the diffusion constant of Cr, and 16 t isindicates time; Figure indicates EquationThe (2) D the is the diffusion of Cr, and t is time; Figure how 16 Equation (2)how is calculated.  √ is calculated. The diffusion distance, x, is determined from the erf solution of Fick’s second law at diffusion distance, x, is determined from the erf solution of Fick’s second law at erf x/ 4Dt = 0.5. erf(𝑥/√4𝐷𝑡) = 0.5. Input data to Equations (2) and (3) are given in Tables 10 and 11 for calculations of concentration Input data to Equations (2) and (3) are given in Table 10 for calculations of concentration gradients and dissolved carbide thickness. gradients and dissolved carbide thickness. Table 10. Input data to Equations (2) and (3), with a note on where the values are chosen from. Table 10. Input data to Equations (2) and (3), with a note on where the values are chosen from. Cr, wt% Note Carbide/γ

Carbide/𝛾 Cr 𝐶C 𝐶𝑟γ/Carbide C𝛾/Carbide 𝐶𝐶𝑟Cr 0 CCr 0

𝐶𝐶𝑟

Cr, wt% 10 10 4 4 1.5 1.5

Note Table 9 Table 9 Table 3 Table 3 Table 7 Table 7

Table 11. Gives the diffusion constant of Cr at different temperature [19]. Table 11. Gives the diffusion constant of Cr at different temperature [19]. Temp. ◦ C

Temp. °C 1150 1150 1200 1200 1250 1250

D, (cm2 /s) [19]

D, (cm2/s) [19] 1.2110 ×−10 10−10 1.21 2.8 × 10−10 2.8 10−10 −10 6.0 × 10 6.0 10−10

Figure 18b of calculated carbide thickness versusversus soaking time fortime threefor different Figure 18b shows showsthe theresults results of calculated carbide thickness soaking three soaking temperatures. From Figures 4b and 9b, it is possible to estimate the thickness of the primary different soaking temperatures. From Figures 4b and 9b, it is possible to estimate the thickness of the carbides carbides to be about 12 µm. The indicate half the carbide thickness, the results that primary to be about 12circles μm. The circles indicate half the carbideand thickness, andindicate the results ◦ ◦ the primary are dissolved before 4 hoursbefore soaking at the temperatures C and 1250 C but indicate that carbides the primary carbides are dissolved 4 hours soaking at the1200 temperatures 1200 °C and 1250 °C but not for the lower temperature 1150 °C. This is in agreement with the measurement results of carbides on the soaked samples.

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not for the temperature Metals 2018, 8, xlower FOR PEER REVIEW

1150 ◦ C. This is in agreement with the measurement results of carbides 17 of 18

on the soaked samples. The SOA4h showed showed a Thedetailed detailedmapping mappingof ofthe theremaining remaining primary primary carbides carbides in the soaked sample SOA4h afew fewprimary primarycarbides carbidesleft leftininthe theA-segregated A-segregatedarea. area. indicates that there exist areas in the ingot It It indicates that there exist areas in the ingot that that segregation the interdendritic the requirement havehave quitequite highhigh segregation levelslevels in the in interdendritic regions.regions. To meetTo themeet requirement of havingof no having no primary carbides in the final product by setting a sufficient soaking time and temperature primary carbides in the final product by setting a sufficient soaking time and temperature will not be will not be an easy task. For example, soaking time of 10 hmost would, most probably, an easy task. For example, selecting selecting a soakinga time of 10 h would, probably, dissolvedissolve all of the all of the carbides. However, it would be very difficult to prove this from a practical point of carbides. However, it would be very difficult to prove this from a practical point of view, since view, a huge since a huge number of micro-samples needfrom to bedifferent taken from different positions inevaluation, the ingot number of micro-samples would need would to be taken positions in the ingot for for in aorder to have a reliable statistical basis. in evaluation, order to have reliable statistical basis.

(a)

(b)

Figure18. 18.(a) (a)Cr Cr concentration in thethe primary carbide for different soaking times. Figure inγγversus versusdistance distancefrom from primary carbide for different soaking The diffusion distance, x, in Equation (3) is marked out; (b)out; dissolved primary carbidecarbide versus versus soaking times. The diffusion distance, x, in Equation (3) is marked (b) dissolved primary time at time different soaking soaking temperatures. The vertical indicates half the thicknesss, and horizontal soaking at different temperatures. The axis vertical axis indicates half the thicknesss, and line gives line the corresponding time for complete horizontal gives the corresponding time for dissolution. complete dissolution.

6. Conclusions 6. Conclusions The main conclusions, as regards the experimental work, can be summarized as follows: The main conclusions, as regards the experimental work, can be summarized as follows: • • • • • •

Duringsolidification, solidification,primary primarycarbides carbidesprecipitate precipitateininthe theinterdendritic interdendriticregions regionsfor forthe thebearing bearing During grade,100CrMnMoSi8-4-6. 100CrMnMoSi8-4-6. grade, Theanalysis analysisshows showsthat thatthe themain mainpart partofofthe theprimary primarycarbides carbides contains~10 ~10wt% wt%Cr Crand and5 5wt% wt% The contains Moand andisisprobably probablycementite. cementite. Mo The reveals thatthat M2CM carbide precipitates in cooperation with cementite during Thecolor coloretching etching reveals precipitates in cooperation with cementite 2 C carbide solidification. during solidification. A-segregated A-segregatedchannels channelscontain containabout aboutdouble doublethe theamount amountofofprimary primarycarbides carbidescompared comparedtotothe the bulk bulkdue duetotothe thehigher higherconcentrations concentrationsofofcarbide carbidestabilizing stabilizingelements elementsCr Crand andMo. Mo. ◦ C,but The Theprimary primarycarbides carbidesininthe thebulk bulkare aredissolved dissolvedafter after44hhofofsoaking soakingatat1200 1200°C, butlonger longersoaking soaking time is needed to dissolve all the carbides in the A-segregate region. time is needed to dissolve all the carbides in the A-segregate region. ItItisisthe themost mostinterdendritic interdendriticsegregated segregatedareas areasininthe thebulk bulkthat thatgive giverise risetotoaabanding bandingstructure structureinin the hot worked material. the hot worked material.

However, it is also worth highlighting the reliability, or otherwise, of Thermo-Calc in providing However, it is also worth highlighting the reliability, or otherwise, of Thermo-Calc in providing information on the solidification characteristics of this particular alloy: information on the solidification characteristics of this particular alloy:  It was found that Thermo-Calc underestimated the liquidus temperature of this alloy by around • 15It °C. was found that Thermo-Calc underestimated the liquidus temperature of this alloy by around ◦ 15  It wasC.limited in its ability to predict, at realistic temperatures, the formation of primary carbides • that It was limited in itsexperimentally. ability to predict, at realistic temperatures, the formation of primary carbides were observed that were observed experimentally. Funding: Jernkontoret, Hugo Carlsson foundation Acknowledgment: I would like to thank my distinguished and learned supervisors Professor Hasse Fredriksson Docent Michael Vynnycky, and Professor Bo Rogberg for their valuable advises, critical comments, and guidance. The valuable comments given by Sven-Olof Ericsson at Ovako Steel were also very appreciable. Financial and

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Funding: Jernkontoret, Hugo Carlsson foundation Acknowledgments: I would like to thank my distinguished and learned supervisors Professor Hasse Fredriksson Docent Michael Vynnycky, and Professor Bo Rogberg for their valuable advises, critical comments, and guidance. The valuable comments given by Sven-Olof Ericsson at Ovako Steel were also very appreciable. Financial and material support provided by Ovako steel and Jernkontoret, Hugo Carlsson foundation, was of immense importance in completing this work. Conflicts of Interest: There are no conflicts of interest.

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13. 14.

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16. 17. 18.

19.

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