metals Article
Effect of the Parameters of Semi-Solid Processing on the Elimination of Sharp-Edged Primary Chromium Carbides from Tool Steel Hana Jirková 1, * , Kateˇrina Rubešová 1 , Pavel Konopík 2 1 2
*
and Kateˇrina Opatová 1
Faculty of Mechanical Engineering, University of West Bohemia, RTI—Regional Technological Institute, Univerzitní 22, CZ-306 14 Pilsen, Czech Republic;
[email protected] (K.R.);
[email protected] (K.O.) ˚ COMTES FHT a.s., Prumyslová 995, CZ-334 41 Dobˇrany, Czech Republic;
[email protected] Correspondence:
[email protected]; Tel.: +420-377-63-8782
Received: 26 July 2018; Accepted: 7 September 2018; Published: 12 September 2018
Abstract: Although conventional tool steels have been heat treated on a routine basis for decades, the search continues for new ways to eliminate their troublesome sharp-edged primary chromium carbides, which impair toughness. One of the available techniques is semi-solid processing, which involves partial melting of the workpiece. The structure after semi-solid processing consisted of a austenite and carbide-austenite network. The network can be broken up and its fragments distributed uniformly by subsequent forming with appropriate parameters. In this experimental study, X210Cr12 tool steel was heated to a semi-solid state, and after cooling to a solid state, worked in a hydraulic press. Suitable soaking temperatures were sought within an interval between 1200 ◦ C and 1280 ◦ C. The workpieces were quenched from the forming temperature in water or oil. In order to improve formability and reduce hardness, tempering was tested as well. Additional experimental regimes included conventional quenching and tempering. Once the appropriate parameters were chosen, the elimination of primary chromium carbides was successful. The resultant microstructures were fine and consisted of M-A constituent with a size of approximately 1 µm, and very fine Fe3 C and Cr7 C3 carbides. The hardness was in excess of 800 HV10. They were examined using optical and scanning electron microscopes. The carbides were characterized on transparent foils in a transmission electron microscope. Mechanical characteristics were determined in micro-tensile tests. Keywords: semi-solid processing; tool steel; primary chromium carbides; micro-tensile test
1. Introduction The properties of tool steels are governed by the amount, size, distribution, and shape of carbides [1–3]. Being hard particles, carbides have an effect on mechanical properties, wear, corrosion and creep resistance [2]. Depending on their size and morphology, they often play an important role in the initiation of failure under load [3]. Sharp-edged chromium carbides in particular reduce the toughness of the material. Primary carbides M7 C3 , which are typical of hypereutectoid steels, are produced by eutectic reaction during cooling [4]. They form directly from the melt and are difficult to eliminate by conventional process routes. For this reason, semi-solid processing appears to be an appropriate method for eliminating these carbides and producing fine-grained structures with fine carbide precipitates. Semi-solid processing (SSP) entails partial melting. The presence of a liquid phase causes the material to exhibit a thixotropic behavior. Complex-shaped parts can thus be made using a single forming operation. Difficult-to-form materials can be shaped with low forming forces [5–9]. Semi-solid processing of materials with high solidus temperature, such as steels, is complicated by the high
Metals 2018, 8, 713; doi:10.3390/met8090713
www.mdpi.com/journal/metals
Metals 2018, 8, 713
2 of 15
temperatures involved, by severe thermal loads on dies, and by a susceptibility to high-temperature oxidation [10]. Their resultant microstructures only after semi-solid processing consist of polyhedral austenite grains embedded in a ledeburite network [11,12]. The grain size and the morphology of the ledeburite network can be controlled by cooling [13]. Some studies dealt with behavior of the semi-solid structure after heat treatment and describe the stages of the austenite decomposition [11,13]. It is known that the austenite is stable up to annealing at 500 ◦ C. At this temperature the formation of fine pearlite was observed [7,11]. Also, the influence of deformation in the semi-solid state was investigated [11]. The austenite remained stable even after 100% deformation in the semi-solid state [11]. The research also described the behavior of the semi-solid structure during cold deformation [14,15]. It was found that during compression test, cracks are formed within the eutectic network and the structure shows frequent twin bands within austenitic grains, deformation twins and ε-martensite [14]. But the possibility to extremely refine the grain size, as well as very fine carbides precipitates during cooling, still remains unanswered. One of the possibility is the intensive forming in different axes after cooling back from semi-solid to solid area. This paper describes a combination of passage through semi-solid state with subsequent forming on a hydraulic press, which could lead to interesting structures. These structures may exhibit higher toughness due to the removal of sharp-edged primary carbides and good wear resistance. Thanks to the high soaking temperature the sharp-edged chromium carbides are dissolved and the ledeburite network resulting from the presence of liquid fraction is broken up and dispersed in the dynamically recrystallized matrix. To obtain the required structures, it is necessary to describe the influence of the individual processing parameters. The most important parameters are soaking temperature, cooling rate, and subsequent heat treatment. 2. Experimental Programme The microstructure produced by semi-solid processing greatly depends on the soaking temperature, which dictates how primary chromium carbides dissolve and what the liquid fraction will be. Both liquid fraction and cooling rate strongly influence the process by which ledeburite network forms. However, the network can be eliminated by mechanical working during cooling. The deformation breaks up the network, disperses the fragments, and refines austenite grains thanks to recrystallization. For this reason, the effects on microstructural evolution need to be identified with respect not only to the soaking conditions but also to cooling from the forming temperature to room temperature. To make the experimental programme comprehensive, conventional treatment comprising quenching and tempering was included. The purpose of this experimental programme was to produce a fine-grained microstructure with fine secondary carbide precipitates, free of primary chromium carbides, in X210Cr12 tool steel. 2.1. Experimental Material X210Cr12 steel is a typical representative of materials with sharp-edged chromium carbides. It is a hypereutectoid chromium steel with high hardenability (Table 1). It has a very high compressive strength and offers good cutting performance and wear resistance. It was developed for use in punching machines and presses, mainly for heavy-duty punches and highly-complex progressive and combination tools [16,17]. Products from this steel include general-purpose forming tools as well as extrusion tools and tools for cold shearing. In addition, it serves as a material for durable small dies for shaping plastic and powder materials, glass, porcelain, and ceramics. It is produced by a conventional metallurgical process which causes carbide banding and therefore poor toughness. Table 1. Chemical composition (wt. %) and fractions of individual phases (vol. %) of X210Cr12 steel. C
Cr
Mn
Si
Ni
P
S
HV10 (–)
1.8
11
0.2
0.2
0.5 max
0.03 max
0.01 max
220
BCC-Fe Cr7 C3 70
6
Fe3 C 24
Metals 2018, 8, 713 x FOR PEER REVIEW
of15 15 33of
The material was supplied in the form of annealed bars. The as-received microstructure The material supplied in the form of annealed bars. The microstructure consisted consisted of large was sharp-edged primary chromium carbides and as-received very fine cementite embedded in a of large sharp-edged primary chromium carbides and very fine cementite embedded in a ferritic ferritic matrix (Figure 1a). The hardness was 220 HV10. Using X-ray diffraction analysis,matrix 24% (Figure 1a). The was 220 analysis, 24% cementitePrevious and 6% cementite and 6%hardness Cr7C3 carbides byHV10. volumeUsing were X-ray founddiffraction in the as-received microstructure. Cr C carbides by volume were found in the as-received microstructure. Previous investigations 7 3 investigations into semi-solid processing of steels showed that sharp-edged chromium carbides into can semi-solid processing of steelsmatrix showed that sharp-edged chromium carbides can [7,12] be dissolved into be dissolved into an austenitic and then reprecipitated as a carbide network . After semian austenitic matrix then reprecipitated carbide network [7,12]. After semi-solidinprocessing, solid processing, theand microstructure consistsas ofapolyhedral austenite grains embedded a lamellar the microstructure consists of polyhedral austenite grains embedded in a lamellar network ledeburite network (Figure 1b). The amount of austenite was determined as 96%. ledeburite Since the presence (Figure The amount of austenite was determined as 96%. presence of such is of such 1b). networks is undesirable as concerns toughness, they Since need the to be eliminated by networks mechanical undesirable as concerns toughness, they need to be eliminated by mechanical working during cooling. working during cooling.
carbides and fine cementite Figure 1. (a) As-received condition of the the X210Cr12 X210Cr12 steel: steel: primary primary Cr Cr77C33 carbides in a ferritic matrix, light microscope; (b) semi-solid processed microstructure, soaking soaking temperature: temperature: 1240 ◦°C, C, polygonal austenite grains, ledeburite network, light microscope.
2.2. Determination of Semi-Solid Processing Parameters 2.2. Determination of Semi-Solid Processing Parameters The key to designing an entire process route is determining the relevant phase transformation The key to designing an entire process route is determining the relevant phase transformation temperatures. In this case, they were calculated using JMatPro software [18], using Thermo-Calc temperatures. In this case, they were calculated using JMatPro software [18], using Thermo-Calc software [19] (both using Thermotech databases), and determined by DSC analysis (Differential software [19] (both using Thermotech databases), and determined by DSC analysis (Differential Scanning Calorimetry) (Table 2). According to the calculation with JMatPro, melting begins at 1225 ◦ C Scanning Calorimetry) (Table 2). According to the◦ calculation with JMatPro, melting begins at 1225 and 100% of liquid fraction is obtained at 1373 C (Figure 2). A liquid fraction vs. temperature °C and 100% of liquid fraction is obtained at 1373 °C (Figure 2). A liquid fraction vs. temperature diagram shows that chromium carbides dissolve at 1255 ◦ C. This is in agreement with the curve which diagram shows that chromium carbides dissolve at 1255 °C. This is in agreement with the curve characterizes the presence of M C carbides. The calculation with JMatPro as well as Thermo-Calc which characterizes the presence7 of3 M7C3 carbides. The calculation with JMatPro as well as Thermowas based on an equilibrium state. Its results suggest that melting starts at 1245 ◦ C. This makes a Calc was based ◦on an equilibrium state. Its results suggest that melting starts at 1245 °C. This makes difference of 20 C to the previous calculation (Figure 2). The liquidus temperatures calculated using a difference of 20 °C to the previous calculation (Figure 2). The liquidus temperatures calculated using both software tools were almost identical: 1376 ◦ C. both software tools were almost identical: 1376 °C. Table 2. Chemical composition (wt. %) and phase transformation temperatures of X210Cr12 steel. Table 2. Chemical composition (wt. %) and phase transformation temperatures of X210Cr12 steel. JMatPro Thermo-Calc DSC JMatPro Thermo-Calc DSC ◦ ◦ ◦ ◦ ◦ ◦ ( C) ( C) Tsolidus T liquidus T solidus ( C) (°C)T liquidus ( C) TTliquidus T solidus liquidus solidus Tsolidus Tliquidus (°C) TT solidus (°C) (°C) (°C)( C)Tliquidus (°C) ( C) 1225 1225
1373 1373
1245 1245
1376 1376
1237 1237
1416 1416
The solidus and and liquidus liquidus temperatures temperatures were also determined determined by by experiment, experiment, using using differential differential The solidus were also scanning calorimetry (DSC) with a DSC PT 1600 instrument from the company Linseis (Robbinsville, scanning calorimetry (DSC) with a DSC PT 1600 instrument from the company Linseis (Robbinsville, NJ, NJ, USA). For purpose, this purpose, specimens 170inmg in weight were placed in an alumina and USA). For this specimens of 170ofmg weight were placed in an alumina cruciblecrucible and heated ◦ heated in an argon atmosphere to 1490 °C at a rate of 20 K/min. The solidus temperature was found in an argon atmosphere to 1490 C at a rate of 20 K/min. The solidus temperature was found to be to be◦1237 °C, i.e., between the values calculated the JMatPro and Thermo-Calc programs. The 1237 C, i.e., between the values calculated with thewith JMatPro and Thermo-Calc programs. The liquidus ◦ C.1416 liquidus temperature wastofound to be The parameters the experimental programme temperature was found be 1416 The°C. parameters used inused the in experimental programme were were based on these results and on previous trials with this material [7,12,13]. based on these results and on previous trials with this material [7,12,13].
Metals 2018, 8, 713
4 of 15
Metals 2018, 8, x FOR PEER REVIEW Metals 2018, 8, x FOR PEER REVIEW
4 of 15 4 of 15
Figure 2. Dependence of phase fractions on temperature, as calculated with JMatPro and Thermo-Calc. Figure 2. Dependence of phase fractions on temperature, as calculated with JMatPro and Thermo-Calc. Figure 2. Dependence of phase fractions on temperature, as calculated with JMatPro and Thermo-Calc.
2.3.2.3. Semi-Solid Processing andand Subsequent Thermomechanical Treatment Semi-Solid Processing Subsequent Thermomechanical Treatment 2.3. Semi-Solid Processing and Subsequent Thermomechanical Treatment TheThe experimental specimens werewere 20 mm in diameter and 45 mm45inmm height.height. Since they had to behad experimental specimens in diameter and The experimental specimens were 20 mm20 in mm diameter and 45 mm in height.inSince theySince had tothey be transferred between a furnace, a quenching bath, and a hydraulic press while in a partially melted to be transferred between a furnace, a quenching bath, and a hydraulic press while in a partially transferred between a furnace, a quenching bath, and a hydraulic press while in a partially melted condition, they werethey enclosed in a container made of S235JR low-carbon steel. TheThe solidus meltedthey condition, wereinenclosed in a container of S235JR low-carbon solidus condition, were enclosed a container made ofmade S235JR low-carbon steel. steel. The solidus ◦ temperature of this steel was above 1400 °C. S235JR exhibits good ductility and low deformation temperature of steel this steel was above C. S235JR exhibits ductility and deformation low deformation temperature of this was above 1400 1400 °C. S235JR exhibits goodgood ductility and low resistance. TheThe container hadhad a diameter of 30 mm, a wall thickness of 5ofmm, andand a length of 55 55 mm resistance. container a diameter of 30 mm, a wall thickness 5 mm, a length resistance. The container had a diameter of 30 mm, a wall thickness of 5 mm, and a length of 55 of mm mm (Figure 3a).3a). Soaking was performed in a furnace with no atmosphere. Water was chosen (Figure Soaking was performed a furnace noprotective protective atmosphere. Water was chosen as (Figure 3a). Soaking was performed in a in furnace withwith no protective atmosphere. Water was chosen as the quenchant. The subsequent forming was carried out in a hydraulic press with flat dies (Figure quenchant. The subsequent forming was carried out a hydraulic presswith withflat flatdies dies(Figure (Figure 3b). as thethe quenchant. The subsequent forming was carried out inin a hydraulic press 3b). Itwas wasfollowed followedbybyquenching quenchingininwater waterororoil, oil,and, and,ininsome some cases, tempering a furnace. cases, tempering inin furnace. 3b). ItItwas followed by quenching in water or oil, and, in some cases, tempering in aafurnace.
Figure 3. (a) Container made of a low-carbon steel and a specimen of the X210Cr12 steel; (b) forging FigureFigure 3. (a) Container mademade of a low-carbon steel steel and aand specimen of the steel; (b) (b) forging 3. (a) Container of a low-carbon a specimen of X210Cr12 the X210Cr12 steel; forging in in a hydraulic press. in a hydraulic press. a hydraulic press.
TheThe crucial parameter of semi-solid processing is thethe soaking temperature, which governs the crucial parameter of semi-solid processing soaking temperature, governs The crucial parameter of semi-solid processing is theissoaking temperature, whichwhich governs the the liquid fraction. The experimental programme therefore mainly focused on finding the appropriate fraction. The experimental programme therefore mainly focused on finding the appropriate liquidliquid fraction. The experimental programme therefore mainly focused on finding the appropriate soaking temperature and describing the influence of cooling rate and subsequent tempering (Table 3 soaking temperature and describing the influence cooling rate and tempering subsequent tempering soaking temperature and describing the influence of coolingof rate and subsequent (Table 3 and(Table Figure 4a) Furthermore, the effects of the conventional quenching and tempering on and Furthermore, Figure 4a) Furthermore, of the conventional quenching and tempering and Figure3 4a) the effectsthe of effects the conventional quenching and tempering on on microstructural evolution and mechanical properties were also investigated (Figure 4b). microstructural evolution and mechanical properties were also investigated (Figure 4b). microstructural evolution and mechanical properties were also investigated (Figure 4b). Table 3. Parameters of heat treatment andand hardness values. 3. Parameters oftreatment heat treatment hardness values. TableTable 3. Parameters of heat and hardness values. Time at Temp. Forging Temp. Time at Number of Cooling Tempering Temp. (°C)/ HV10 Regime Soaking Soaking Temp. (°C)Time at Temp. Forging Temp. Time at at Number of Cooling TemperingTempering Temp. (°C)/ HV10 Time Forging Time Number Cooling (min)at (°C) Temp. (min) Def. Stepsof Medium Time (min) (–) (–) Regime Regime Soaking Temp. (°C) Temp. HV10 ◦ C) (min) (min) (°C) (◦ C) Temp. (min) Def.Def. Steps Medium Time (min) (–) Temp. ( Temp. Temp. Temp. (min) Steps Medium 1 1200 15 1080 1.5 4 Water - (min) 848 (◦ C)/Time 1 1200 15 1080 1.5 4 Water 848 2 1220 15 1080 1.5 4 Water 803 1 1080 2 1220 1200 15 15 1080 1.51.5 4 4 Water Water - 803848 3 1240 15 1080 1.5 5 Water 864 2 1080 3 1240 1220 15 15 1080 1.51.5 5 4 Water Water - 864803 4 1280 15 1080 1.5 5 Water 866 3 1080 4 1280 1240 15 15 1080 1.51.5 5 5 Water Water - 866864 5 1240 15 1080 1.5 55 Oil 80 °C -778 4 1280 15 1080 1.5 Water 5 1240 15 1080 1.5 5 Oil 80 °C 778866 ◦ 6 1240 15 1080 1.5 5 Oil 80 °C 250/60 677 5 1080 6 1240 1240 15 15 1080 1.51.5 5 5 Oil 80 Oil °C 80 ◦ C 250/60 677778 7 9601240 20 -5 OilOil 80 °C 250/60 535 6 15 1080 1.580 C 250/60 677 7 960 20 Oil 80 °C 250/60 535 7 960 20 Oil 80 ◦ C 250/60 535
The first step involved heating in a furnace to a specified temperature. The first soaking The first step involved heating in a furnace to a specified temperature. The first soaking temperature chosen for investigation was 1200 °C, which is below the solidus temperature. Other temperature chosen for investigation was 1200 °C, which is below the solidus temperature. Other
Metals 2018, 8, 713
5 of 15
step involved MetalsThe 2018,first 8, x FOR PEER REVIEW heating in a furnace to a specified temperature. The first soaking 5 of 15 ◦ C, which is below the solidus temperature. temperature chosen for investigation was 1200 Metals 2018, 8, x FOR PEER REVIEW 5 of 15 temperatures tested were and 1280and °C (Figure A 15-min wassoaking followedwas by Other temperatures tested1220, were1240, 1220, 1240, 1280 ◦ C4a). (Figure 4a). soaking A 15-min temperatures tested were 1220, 1240, and 1280 °C ordinary (Figure A 15-min soaking was followed cooling inbywater to in 1080 °C. is◦ C. anThis ordinary forging4a). temperature for X210Cr12 steel. by After followed cooling water toThis 1080 is an forging temperature for X210Cr12 steel. ◦ C.The cooling in water to 1080 °C. the Thisspecimen is specimen an ordinary forging for X210Cr12 steel. After temperature homogenization, waswas placed in in atemperature furnace atat1080 °C. After temperature homogenization, the placed a furnace 1080 Thenext next step step was temperature homogenization, the specimen was placed in a furnace at 1080 °C. The next step was forging using four or five blows in a hydraulic press. The first height reduction was 30 mm. open-die forging mm. open-die forging using five for blows in blow. ablow. hydraulic press. The first reduction 30 mm. The strain rate rate of 0.56 0.56 was used The wereheight rotated The strain of ss−−11four wasor used for each each The specimens for eachwas deformation The rate of 0.56 s−1 was used for each blow. The specimens were rotated each deformation tostrain minimalize the deformation deformation inhomogeneity mechanism plastic step to minimalize the inhomogeneity (Figure 5). Because of thefor mechanism of plastic step to minimalize the deformation inhomogeneity (Figure 5). Because of the mechanism of plastic all deformation, the highest value of deformation was located in the specimen center, and therefore deformation, the highest value of deformation was located in the specimen center, and therefore all from this this area. analyses were done from area. All forming steps were performed at one time without reheating analyses were done from this area. All forming steps were performed at one time without reheating between the individual stages. After forging, the workpiece was quenched in water (regimes 1 to 4). between the individual stages. After forging, the workpiece was quenched in water (regimes 1 to 4). programme (i.e., (i.e., regime regime 5), the effects of the post-forming cooling In the next part of the experimental programme In the next part of the experimental programme (i.e., regime 5), the effects of the post-forming cooling were explored explored while while water water was was replaced replaced with with oil oil as as the the quenchant. quenchant. The The impact impact of of subsequent subsequent rate were rate were explored while water was replaced with oil as the quenchant. The impact of subsequent in which which semi-solid semi-solid processing processing tempering on microstructural evolution was examined in regime 6, in tempering on microstructural evolution was examined in regime 6, in which semi-solid processing ◦C were followed by tempering at 250 °C for 60 min. The last regime was carried for and forging followed by tempering at 250 for 60 min. The last regime was carried for and forging were followed by tempering at 250 °C for 60 min. The last regime was carried outout forout thethe ◦ purpose of comparison. In this regime, conventional quenching from 960 °C was followed thepurpose purposeofofcomparison. comparison. this regime, conventional quenching from C was followed In In this regime, conventional quenching from 960960 °C was followed by by ◦ tempering atat 250 (regime 7), (Figure 4b). tempering at 250 C°Cfor 4b). tempering 250°C for60 60min min(regime (regime 7), 7), (Figure (Figure 4b).
Figure 4. (a)Semi-solid Semi-solidprocessing processing routes, regimes 1–6; (b) quenching and tempering route, regime 7. Figure Figure 4. 4. (a) (a) Semi-solid processing routes, routes, regimes 1–6; 1–6; (b) (b) quenching quenching and and tempering tempering route, route, regime regime 7. 7.
◦ C. Figure5.5.Scheme Schemeof of the the forging forging process Figure process at atthe thetemperature temperatureofof1080 1080°C. Figure 5. Scheme of the forging process at the temperature of 1080 °C.
2.4.2.4. Methods ofofEvaluation Methods Evaluation 2.4. Methods of Evaluation Themicrostructures microstructures of of the the processed processed specimens specimens were techniques. The were examined examinedusing usingseveral several techniques. Specimen centerswere werechosen chosen because they had been deformation, thethe The microstructures of the processed specimens were to examined using several being techniques. Specimen centers because they been subjected subjected tothe thelargest largest deformation, being centers thecharacteristic characteristic diagonal cross of The included light microscopy Specimen centers were chosen because they had been subjected to the largest deformation, being the centers ofof the diagonal cross pattern of strain. strain. Thetechniques techniques included light microscopy (LM) and scanning electrondiagonal microscopy (SEM). Light microscopy was description primary centers of scanning the characteristic cross pattern of strain. The techniques included lightofmicroscopy (LM) and electron microscopy (SEM). Light microscopy wasused usedfor for description of primary chromium carbides distribution. Scanning electron microscopy was suitable for observation thethe (LM) and scanning microscopy (SEM). Lightmicroscopy microscopy was used for description of of primary chromium carbideselectron distribution. Scanning electron suitable for observation of fine recrystallized structure and precipitates. precipitates. The VEGA 3 SEM chromium carbidesstructure distribution. Scanning electron microscopyused was suitable for observation the fine recrystallized and The instruments instruments usedwere werea aTescan Tescan VEGA 3ofSEM (Tescan, Brno, Czech Republic) and Zeiss Crossbeam 340-47-44 (Zeiss, Oberkochen, Germany) for fine recrystallized structure andand precipitates. The instruments were a Tescan VEGA 3 SEM (Tescan, Brno, Czech Republic) Zeiss Crossbeam 340-47-44 used (Zeiss, Oberkochen, Germany) for detail analysis. 3% nital (ethanoland HNO 3) was used for etching. (Tescan, Brno, Czech Republic) Zeiss Crossbeam 340-47-44 (Zeiss, Oberkochen, Germany) for detail analysis. 3% nital (ethanol ++ HNO ) was used for etching. 3 The amount of retained austenite was measured by X-ray diffraction analysis. Phase fractions detailThe analysis. 3%ofnital (ethanol + HNO 3) was used forby etching. amount retained austenite was measured X-ray diffraction analysis. Phase fractions were determined using an AXS Bruker D8 Discover automatic powder diffractometer (Bruker, amount of retained austenite wasD8 measured X-ray diffraction Phase fractions were The determined using an AXS Bruker Discoverbyautomatic powderanalysis. diffractometer (Bruker, Karlsruhe, Germany) with a HI-STAR position-sensitive area detector and a cobalt X-ray source (λKα were determined using an aAXS Bruker D8 Discover automatic powder Karlsruhe, Germany) with HI-STAR position-sensitive area detector anddiffractometer a cobalt X-ray(Bruker, source = 0.1790307 nm). The instrument comprises polycapillary optics for focusing the primary X-ray beam Karlsruhe, Germany) with a HI-STAR position-sensitive area detector and a cobalt X-ray source (λKα into a circular spot with a diameter of 0.5 mm. Measurements were taken at the centers of = 0.1790307 nm). The instrument comprises polycapillary optics for focusing the primary X-ray beam metallographic sections at diffraction angles in the interval of 25 ÷ 110°2ϑ. into a circular spot with a diameter of 0.5 mm. Measurements were taken at the centers of metallographic sections at diffraction angles in the interval of 25 ÷ 110°2ϑ.
Metals 2018, 8, 713
6 of 15
(λKα = 0.1790307 nm). The instrument comprises polycapillary optics for focusing the primary X-ray beam into a circular spot with a diameter of 0.5 mm. Measurements were taken at the centers of metallographic sections at diffraction angles in the interval of 25 ÷ 110◦ 2ϑ. Transmission electron microscopy (TEM) was employed to determine the exact types of carbides and microstructure constituents. Thin foils were prepared by grinding and polishing to 100 µm thickness. Foils of 3 mm diameter were punched and thinned in their central region to approximately 15 µm in a Gatan dimple grinder. They were then ion-polished using the Gatan PIPS II system (PIPS II, Gatan, Inc., Pleasanton, CA, USA) to a thickness suitable for TEM observation (
