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metals Article

Effect of Quenching Conditions on the Microstructure and Mechanical Properties of 51CrV4 Spring Steel Lin Zhang 1, * , Dehai Gong 2 , Yunchao Li 1 , Xiaojun Wang 2 , Xixi Ren 1 and Engang Wang 1, * 1 2

*

Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Wenhua Road, Shenyang 110819, China; [email protected] (Y.L.); [email protected] (X.R.) CRRC Guiyang Co., LTD., Dulaying Road, Guiyang 550017, China; [email protected] (D.G.); [email protected] (X.W.) Correspondence: [email protected] (L.Z.); [email protected] (E.W.); Tel.: +86-24-8368-3985 (L.Z.); +86-24-8368-1739 (E.W.)

Received: 28 November 2018; Accepted: 11 December 2018; Published: 12 December 2018







Abstract: 51CrV4 steel is extensively used in large-size damping springs for trains and vehicles. Quenching conditions play an important role in performance enhancement. The present work investigated the effects of various oil-bath temperatures and out-of-oil temperatures on the microstructure and the mechanical properties of this steel. The morphological examination focused on both the quenched martensite and the tempered troostite. Tensile and hardness tests were carried out to evaluate the mechanical properties. The results showed that a coarsening of the martensite occurred at a high oil-bath temperature. In addition, the size and fraction of bainite islands also increased with the increase of oil-bath temperature. In contrast, the carbide size and the intercarbide spacing both increased with the increase of oil-bath temperature. Thus, the tensile strength and the hardness both decreased with increasing oil-bath temperature in accordance with the Hall-Petch relationship. Correspondingly, the ductility increased as the oil-bath temperature increased. At a relatively high out-of-oil temperature, the martensite underwent an auto-tempering process, which led to the precipitation of many tiny carbide particles in the as-quenched martensite laths. This auto-tempering effect enhanced the width of large-sized carbides and reduced their length in the final microstructure. The intercarbide spacings increased with increasing out-of-oil temperature. As the oil-bath temperature increased, the tensile strength and hardness decreased, and the ductility increased. The fracture morphology was examined to explain the results of mechanical properties. Keywords: 51CrV4; spring steel; quenching; martensite; carbides; strength; hardness

1. Introduction The development of high-speed railway trains imposes rigid requirements on the mechanical properties of coil springs used in freight car bogies. Over the last decades, considerable efforts have been made to develop high-performance spring steels. Most coil springs for railway applications are made of quenched and tempered high-strength steels. Elements such as chromium, manganese, and silicon are added to these steels [1]. 51CrV4 steel has high strength and fatigue performance due to the addition of Cr and V, which is extensively used in large-size damping springs for trains and vehicles [2]. The recommended properties of spring steel include high ductility and toughness at operating temperatures and good hardenability that provides required mechanical properties even at large dimensions [3]. One way to improve the mechanical properties of spring steel could be achieved through the control of alloy composition [4,5]. Thermodynamic calculation was used to identify the effects of element change on the phase fraction and transformation temperature in the soaking Metals 2018, 8, 1056; doi:10.3390/met8121056

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and tempering process [6]. Thermodynamic calculation can be performed using software such as Thermo-Calc. However, composition adjustment means to change the standard of spring steels, which lack feasibility for spring manufacturers. Steels with a similar chemical composition may behave differently due to various mechanical properties as a consequence of their manufacturing route. The mechanical properties can be improved by effective heat treatment [7,8] and thermomechanical treatment [1,9]. Quenching before tempering is a key technique to enhance the mechanical properties of spring steels, in which the final structure is significantly affected by the processing parameters during heat treatment. For this reason, the heat treatment parameters of spring steel are controlled thoroughly by the producers. Large efforts have been made in recent years to investigate the effects of heat treatment parameters during soaking and tempering process [8,10–12], such as the time and temperature of soaking and tempering. The emphasis in the research of spring steels has been focused on increasing the strength while maintaining good ductility [6,13]. One way of improving steel properties is refining its microstructure [14,15], which means to reduce the ferrite grain size, the size of carbides, and the intercarbide spacing. The mechanical behavior of steel is influenced by the inclusions and precipitates, which act as stress raisers [16]. Large inclusions are most harmful to mechanical properties. Fine precipitation can be achieved through microalloying and effective heat treatment [17]. This leads to the enhancement of hardness and strength. Retained austenite is also an important structural component in the spring steel, whose stability is affected by the processing parameters during heat treatment, such as the soaking temperature and the cooling rate [18]. The present work performed soaking, quenching, and tempering on 51CrV4 steel since this kind of heat treatment is a conventional manufacturing route to improve its mechanical properties. Previous researchers have studied the effects of various parameters during heat treatment on the microstructure and mechanical properties of 51CrV4 steels [3,19–21]. However, very few researches have focused on the detailed quenching conditions such as the oil-bath temperature and the out-of-oil workpiece temperature, which is essential to obtain the required strength and ductility. In this work, the heat treatment experiments were carried out on 51CrV4 steel under various oil-bath temperatures and out-of-oil temperatures. The quenched martensite and tempered troostite were examined. The microstructure of the steel was analyzed by scanning electron microscopy. Effects of the quenching parameters on the mechanical properties were evaluated in terms of tensile strength, elongation, hardness, and fracture. The relationship between the microstructure and the mechanical properties was discussed. The purpose of this work is to obtain a technical reference to control the mechanical properties using the oil-bath temperature and out-of-oil temperature. 2. Materials and Methods 51CrV4 spring steels have been processed by soaking, quenching, and tempering. Table 1 shows the chemical composition of the commercial 51CrV4 spring steel used in this work, which was identified by chemical analysis. The original specimens were cut from a hot deformed steel bar. The size of the cylinder specimen was Ø20 mm × 300 mm. This work performed the heat treatment according to a technology route usually used to fabricate a train damping spring. First, the steel bars were heated to 930 ◦ C and isothermally held for 30 min to form austenite with a uniform distribution of the alloying elements. During this soaking process, nitrogen gas was injected into the furnace to protect the steel bars from oxidization. Afterward, the steel bars were quenched into a stirred oil bath which was set at various oil-bath temperatures (20 ◦ C, 50 ◦ C, and 80 ◦ C). The cooling rates were estimated to be 34 ◦ C/s, 30 ◦ C/s, and 25 ◦ C/s respectively. The surface temperature of the workpiece was measured during the quenching process, and the workpieces were taken out of the oil bath at various temperatures (60 ◦ C, 90 ◦ C, and 120 ◦ C), which was defined as the out-of-oil temperature in this paper. After quenching, the workpieces were cooled in the air to room temperature (about 15 ◦ C). Next, the specimens were tempered at 450 ◦ C for 90 min. After tempering, the samples were water quenched.

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Table 1. Chemical composition of the as-received 51CrV4 steel (mass percent, %). C

Si

Mn

S

P

Cr

Ni

Cu

V

Al

Mo

Fe

0.546

0.229

0.951

0.004

0.014

1.08

0.051

0.106

0.168

0.017

0.006

balance

Each bar was subsequently sectioned and etched to reveal the microstructure. The microstructure was observed using an Ultra Plus field emission scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany). Image-Pro Plus software (Version 6.0, Media Cybernetics, Inc., Rockville, MD, USA) was used to analyze the size and the area fraction of phases. The phases were identified by the X-ray diffraction analysis using a Philips X0 Pert Pro MPD diffractometer (PANalytical Co., Almelo, The Netherlands). In order to evaluate the mechanical properties of the samples, tensile testing and hardness testing were performed. Hardness measurements were taken from a polished but unetched sample, using a Rockwell apparatus (Shenyang Kejing Auto-instrument Co., LTD, Shenyang, China), with a load of 150 kg and loading time of 5 s. Tensile testing was performed in a Shimadzu AG-X 100 kN testing machine (Shimadzu Corp., Kyoto, Japan) in accordance with the requirements and recommendations of the ISO 6892-1:2011 [22]. The round bone-shaped tensile testing specimens were prepared by lathe. The tensile testing specimen had an overall length of 150 mm, a gauge length of 50 mm, and a diameter of 6 mm. An extensometer was set during test to measure the strain precisely. The strain rate was about 2.3 × 10−5 ·s−1 . 3. Results and Discussion Figure 1a shows the microstructure of as-received commercial 51CrV4 steel used in this work. The microstructure of the as-received steel appeared to be spheroidite, in which many spherical carbides were distributed dispersedly in the ferrite grains. The XRD patterns of the as-received, quenched, and tempered specimens were analyzed. The XRD result of as-received steel showed a main phase of α-Fe (Figure 1b), which is the matrix of the microstructure shown in Figure 1a. The carbides had a small X-ray reflection and showed no peaks in the XRD result. Figure 1b–d shows the XRD patterns of specimens which were quenched at various oil-bath temperatures and out-of-oil temperatures. The quenched specimens showed a pattern of martensite, which appeared to have peak positions similar to the α-Fe phase. The pattern of quenched specimens in Figure 1b,c included the peaks of γ-Fe phase, indicating that retained austenite existed in the two specimens quenched at an out-of-oil temperature of 90 ◦ C. In the two patterns of the tempered specimens that corresponded to the quenched specimens respectively, the lower peaks of γ-Fe phase indicated that the fraction of retained austenite decreased (Figure 1b,c). The patterns of 80 ◦ C oil-bath temperature had a higher (200) peak of γ-Fe phase compared with that of 50 ◦ C oil-bath temperature (Figure 1b,c), indicating that high oil-bath temperature enhances the fraction of retained austenite. On the contrary, the high out-of-oil temperature reduced the fraction of retained austenite. The quenched and tempered specimens at 120 ◦ C out-of-oil temperature appeared to have no γ-Fe peaks in the pattern (Figure 1d).

to the quenched specimens respectively, the lower peaks of γ-Fe phase indicated that the fraction of retained austenite decreased (Figure 1b,c). The patterns of 80 °C oil-bath temperature had a higher (200) peak of γ-Fe phase compared with that of 50 °C oil-bath temperature (Figure 1b,c), indicating that high oil-bath temperature enhances the fraction of retained austenite. On the contrary, the high Metals 2018, 8,temperature 1056 4 of 16 out-of-oil reduced the fraction of retained austenite. The quenched and tempered specimens at 120 °C out-of-oil temperature appeared to have no γ-Fe peaks in the pattern (Figure 1d). o

α(110)

α'-M(220)

α(220)

α(211) γ(311)

α'-M(211)

α(200) α'-M(200) α(200)

α(220)

quenched

γ(200)

γ(111)

α'-M(110)

tempered

α(110)

intensity (a.u.)

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α(211)

α(110)

α'-M(200)

intensity (a.u.)

tempered

quenched

quenched 20

o

50 C oil-bath temperature o 120 C out-of-oil temperature

α'-M(220)

α'-M(211)

α'-M(200)

γ(200)

α(220)

α(200)

γ(200)

α(211)

α(110)

tempered

(d)

α'-M(110)

o

80 C oil-bath temperature o 90 C out-of-oil temperature

α'-M(110)

intensity (a.u.)

(c)

40

50

60

70

2 θ / (degree)

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100

20

30

40

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80

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100

2θ / (degree)

Figure 51CrV4 steel barbar and X-ray diffraction (XRD) patterns of asFigure 1. 1. Microstructure Microstructureofofas-received as-received 51CrV4 steel and X-ray diffraction (XRD) patterns of received, quenched, andand tempered specimens: as-received, quenched, tempered specimens:(a)(a)microstructure microstructureofofas-received as-receivedsteel; steel; (b) (b) XRD XRD ◦ C oil-bath patterns quenched and tempered specimens at 50 temperature and patternsof ofthe theas-received as-receivedsteel, steel, quenched and tempered specimens at°C 50oil-bath temperature ◦ C out-of-oil ◦ C oil-bath 90 °C90 out-of-oil temperature; (c) XRD of specimens at 80 °C oil-bath temperature and 90and °C and temperature; (c) patterns XRD patterns of specimens at 80 temperature ◦ ◦ out-of-oil temperature; (d) XRD patterns of specimens at 50 °C oil-bath temperature and 120 °C out90 C out-of-oil temperature; (d) XRD patterns of specimens at 50 C oil-bath temperature and 120 ◦ C of-oil temperature. out-of-oil temperature.

Figure 2a,c,e 2a,c,e presents presents the the microstructure microstructure of of steel steel bars bars after after quenching quenching at at various various oil-bath oil-bath Figure ◦ C, 50 ◦ C, and 80 ◦ C respectively (The out-of-oil temperature was 90 ◦ C). In the temperatures of of 20 20 °C, temperatures 50 °C, and 80 °C respectively (The out-of-oil temperature was 90 °C). In the as-quenched condition, condition, the the microstructure microstructure consisted consisted of of untempered untempered martensite, martensite, bainite, bainite, and and retained retained as-quenched austenite. The Theretained retained austenite austenitewas wasidentified identifiedby by the the XRD XRD results, results, whereas whereas itit was was not not profound profound in in austenite. the structure structure of of Figure Figure 2. The The needle-like needle-like martensite martensite lath lath was was the the main main component component of of the the quenched quenched the structure, accompanied accompanied by by aa few few large-sized large-sized martensite martensite laths. laths. The The martensite martensite laths laths were were apparently apparently structure, coarser under under higher higher oil-bath oil-bath temperature. temperature. The The athermal athermal transformation transformation from from austenite austenite to to martensite martensite coarser is dependent only on the amount of undercooling below the M temperature (∆T). The volume fraction is dependent only on the amount of undercooling below the Ms temperature (ΔT). The volume s of martensite increases increases with increasing ∆T [23]. The of martensite nuclei also increases fraction of martensite with increasing ΔTnumber [23]. The number of martensite nuclei with also the increase of undercooling to transformation The final grain offinal the martensite be increases with the increase ofprior undercooling prior to[24]. transformation [24].size The grain size will of the finer as the number of nuclei is larger. Quenching at a lower oil-bath temperature tends to achieve martensite will be finer as the number of nuclei is larger. Quenching at a lower oil-bath temperature higherto undercooling. Therefore, martensite tends to be finer as the oil-bath temperature tends achieve higher undercooling. Therefore, martensite tends to be finer as decreases. the oil-bath temperature decreases.

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Figure ◦ C (a), Figure 2. 2. Microstructure Microstructure of of the the steel steel bars: bars: Specimens Specimens quenched quenched at at oil-bath oil-bath temperatures temperatures of of 20 20 °C (a), ◦ ◦ 50 °C, (c) and 80 °C (e) respectively; (b), (d), and (f) are the tempered microstructures corresponding 50 C, (c) and 80 C (e) respectively; (b), (d), and (f) are the tempered microstructures corresponding to view of of a bainite island in to specimens specimens of of (a), (a),(c), (c),(e) (e)respectively; respectively;(g) (g)shows showsa ahigher-magnification higher-magnification view a bainite island the area outlined byby a rectangle in the area outlined a rectangleinin(a); (a);(h) (h)shows showsaamorphology morphologyof of carbide carbide precipitation precipitation of of higher higher magnification same specimen specimen of of (d). (d). magnification in in the the same

The quenched lens-shaped bainite islands, as shown in Figure 2a,c,e,g. There quenchedstructure structurecontained contained lens-shaped bainite islands, as shown in Figure 2a,c,e,g. were carbide particles precipitated in the bainite islands (Figure 2g). A similar morphology has been There were carbide particles precipitated in the bainite islands (Figure 2g). A similar morphology analyzed by Sencic by et al. to beetconsidered as bainite grains [3]. Gao et al. this kind of has been analyzed Sencic al. to be considered as bainite grains [3].also Gaoobserved et al. also observed islands in of theislands martensite and described it asdescribed bainite islands [25]. The size and ofand the this kind in thematrix martensite matrix and it as bainite islands [25].fraction The size bainite increased withincreased the increase of the oil-bath temperature. various oil-bath temperatures, fractionislands of the bainite islands with increase of oil-bathAt temperature. At various oil-bath the cooling rate of quenching was different. Thus, the diffusion distance of carbon atoms was varied and the γ → α transition velocity was also changed. Besides, bainite formation is much slower than martensite because it requires diffusion. Therefore, the low oil-bath temperature prevented carbon

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temperatures, the cooling rate of quenching was different. Thus, the diffusion distance of carbon atoms was varied and the γ → α transition velocity was also changed. Besides, bainite formation is much slower than martensite because it requires diffusion. Therefore, the low oil-bath temperature prevented carbon atom diffusion, delaying the bainite transformation and reducing the size of bainite islands. Taken together, these results suggest that the formation of martensite and bainite islands were both influenced by the oil-bath temperature. In the tempering process, the martensite and retained austenite decomposed, leading to the precipitation of carbides. The formation of fairly fine carbides was shown in Figure 2b,d,f. The tempered structure appeared to be troostite, in which the prior martensite laths were replaced by ferrite grains. It can be seen that the microstructure of all the three specimens contained long carbides. The long carbides were accompanied by a certain number of short carbide particles (Figure 2h). The carbides were located mainly at prior interlath boundaries, although some carbide particles distributed inside the prior martensite lath. The length and width of the carbides both increased with the increase of oil-bath temperatures. Correspondingly, the intercarbide spacings also increased. At oil-bath temperatures of 20, 50, and 80 ◦ C, the average intercarbide spacings were measured to be 0.184, 0.212, and 0.242 µm respectively. Carbides can easily nucleate at dislocations due to a stronger enrichment of carbon. In the tempering process of a quenched steel bar, the carbides appeared to precipitate along the grain boundaries and the interphase boundaries between martensite and other phases [26–28]. Therefore, small martensite laths with more boundaries tend to prepare more locations for the nucleation of carbides. Jang et al. found that the carbides precipitated along the surfaces of the martensite phase, and the carbides have smaller size and were more widely dispersed due to the smaller width of martensite phase [29]. As discussed above, the size of martensite laths decreased with the decrease of oil-bath temperature in this work, thus the number of precipitated carbides tended to increase due to more interfaces or boundaries in the microstructure. Correspondingly, the carbides tended to have a smaller size. On the contrary, the number of precipitated carbides decreased and the carbides tended to have a larger size with the increase of oil-bath temperature. The bainite islands were also observed in the tempered microstructure (Figure 2d), which has a similar lens-shaped morphology to those in the quenched sample. The temperature of workpieces decreased with an increase in submerged time during the quenching process. The surface temperatures of the steel bars were measured when they were taken out of the oil bath (out-of-oil temperature) to identify its effects on the microstructures. Figure 3a,c,e present the microstructures of steel bars quenching at various out-of-oil temperatures of 60 ◦ C, 90 ◦ C, and 120 ◦ C respectively (The oil-bath temperature was 50 ◦ C). Martensite was the main phase in the microstructures of all the three conditions. Both large-sized martensite laths and slim needle-like martensite existed in the microstructure. There was no significant discrepancy in the size of martensite laths in the specimens at various out-of-oil temperatures. Whereas, a high out-of-oil temperature of 120 ◦ C enhanced the chance of auto-tempering in the microstructure, which led to the precipitation of many tiny carbide particles in the as-quenched martensite crystals (Figure 3h). In contrast, the carbide precipitation was not observed in the specimen at a low out-of-oil temperature such as 60 ◦ C (Figure 3g), indicating that the auto-tempering was inhibited. The auto-tempering was more profound in the large-sized martensite lath. Morsdorf et al. found that the coarse martensite lath has a higher density of carbide particles compared to the thinner laths, which is due to the high degree of auto-tempering and lower dislocation density in the coarse laths [30].

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Figure quenched at at various out-of-oil temperatures of Figure 3. 3. Microstructure Microstructureofofthe thesteel steelbars: bars:Specimens Specimens quenched various out-of-oil temperatures 60 °C ◦(a), 90 90 °C◦ (b), and 120 °C◦ C(c)(c)respectively; of 60 C (a), C (b), and 120 respectively;(b), (b),(d), (d),and and(f) (f)are are the the tempered tempered microstructures microstructures corresponding the specimens of (c), (a),(e)(c), (e) respectively; (g) show and the (h) martensite show the morphology martensite corresponding totothe specimens of (a), respectively; (g) and (h) morphology of higher magnification in the specimens of (a) and (e), respectively. of higher magnification in the specimens of (a) and (e), respectively.

Figure 3b,d,f 3b,d,fshows showsthe thetempered temperedmicrostructures microstructurescorresponding corresponding quenched specimens Figure toto thethe quenched specimens in in Figure 3a,c,e respectively. The microstructurecontained containedlong longcarbides carbidesaccompanied accompaniedby by aa certain certain Figure 3a,c,e respectively. The microstructure number of of short short carbide carbide particles. particles. The The carbide carbide distribution distribution appeared appeared to to be be different different in in the the tempered tempered number microstructures at various out-of-oil out-of-oil temperatures. temperatures. There There were were carbide-depleted carbide-depleted regions regions (large (large black black microstructures regions) in in the microstructure where the distribution of carbide was sparser than other regions. The The regions) area of the carbide-depleted regions appeared to decrease with the increase of out-of-oil temperature. area of the carbide-depleted regions appeared to decrease with the increase of out-of-oil temperature. These carbide-depleted regions might correspond to the large-sized martensite laths in the quenched specimens. As discussed above, carbides prefer to precipitate along the lath boundary, and only a few carbide particles tended to precipitate inside the large-sized martensite lath at a low out-of-oil

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These carbide-depleted regions might correspond to the large-sized martensite laths in the quenched specimens. As discussed above, carbides prefer to precipitate along the lath boundary, and only a temperature. The martensite underwent an auto-tempering process at a relatively high out-of-oil few carbide particles tended to precipitate inside the large-sized martensite lath at a low out-of-oil temperature, and carbide particles could nucleate inside the large martensite laths during the autotemperature. The martensite underwent an auto-tempering process at a relatively high out-of-oil tempering process (Figure 3h). This provided an opportunity for them to grow into larger carbides temperature, and carbide particles could nucleate inside the large martensite laths during the in the subsequent principal tempering process, leading to the increase of final number density of auto-tempering process (Figure 3h). This provided an opportunity for them to grow into larger carbides, especially in the carbide-depleted region. Therefore, the carbide-depleted regions were not carbides in the subsequent principal tempering process, leading to the increase of final number density profound in the specimen at a high out-of-oil temperature such as 120 °C (Figure 3f), and the of carbides, especially in the carbide-depleted region. Therefore, the carbide-depleted regions were distribution of carbides appeared to be more homogeneous than that at a lower out-of-oil not profound in the specimen at a high out-of-oil temperature such as 120 ◦ C (Figure 3f), and the temperature. However, the carbide size increased with the increase of out-of-oil temperature. distribution of carbides appeared to be more homogeneous than that at a lower out-of-oil temperature. Correspondingly, the intercarbide spacings also increased. At out-of-oil temperatures of 60, 90, and However, the carbide size increased with the increase of out-of-oil temperature. Correspondingly, the 120 °C, the average widths of the intercarbide spacing were measured to be 0.177, 0.222, and 0.231 intercarbide spacings also increased. At out-of-oil temperatures of 60, 90, and 120 ◦ C, the average μm respectively. widths of the intercarbide spacing were measured to be 0.177, 0.222, and 0.231 µm respectively. Figure 4 shows the size distribution of tempered carbides in the steel bars at various out-of-oil Figure 4 shows the size distribution of tempered carbides in the steel bars at various out-of-oil temperatures (The statistical area in the microstructure was 600 m22). The number density of small temperatures (The statistical area in the microstructure was 600 m ). The number density of small carbides (with a length smaller than 0.4 μm) increased considerably as the out-of-oil temperature carbides (with a length smaller than 0.4 µm) increased considerably as the out-of-oil temperature increased from 60 °C to a higher degree of 90 °C or 120 °C. This was caused by the auto-tempering increased from 60 ◦ C to a higher degree of 90 ◦ C or 120 ◦ C. This was caused by the auto-tempering effect discussed above. The lowest number density of small carbides was observed in the specimen effect discussed above. The lowest number density of small carbides was observed in the specimen at out-of-oil temperature of 60 °C, when the auto-tempering has not taken effect. A relatively high at out-of-oil temperature of 60 ◦ C, when the auto-tempering has not taken effect. A relatively high out-of-oil temperature tended to increase the width of large-sized carbides (with a width larger than out-of-oil temperature tended to increase the width of large-sized carbides (with a width larger than 0.24 μm). However, the length of large-sized carbides (with a length larger than 1.2 μm) tended to 0.24 µm). However, the length of large-sized carbides (with a length larger than 1.2 µm) tended to decrease with the increase of out-of-oil temperature. This effect indicates that the fraction of long decrease with the increase of out-of-oil temperature. This effect indicates that the fraction of long carbides decreased, and the particle-shaped carbides tended to have a higher fraction as the out-ofcarbides decreased, and the particle-shaped carbides tended to have a higher fraction as the out-of-oil oil temperature increased. As discussed above, the carbides nucleated inside large martensite laths temperature increased. As discussed above, the carbides nucleated inside large martensite laths as tiny as tiny particles during an auto-tempering process, which reduced their chance to grow into long particles during an auto-tempering process, process, which reduced their chance of to grow into long carbides carbides in the later principal tempering thus the fraction particle-shaped carbidesin the later principal tempering process, thus the fraction of particle-shaped carbides increased. increased. 1200

o

(a)

60 C out-of-oil temperature o 90 C out-of-oil temperature o 120 C out-of-oil temperature

1000

500

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400

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800

600

300

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200

0 0.0

0

0.2

0.4

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Carbide length (μm)

1.4

1.6

1.8

0.04

0.08

0.12

0.16

0.20

0.24

0.28

0.32

0.36

Carbide width (μm)

Figure Figure4.4.The Thecarbide carbidelength length(a) (a)and andwidth width(b) (b)distribution distributionininthe thetempered temperedsteel steelbars barsquenched quenchedatat ◦ ◦ ◦ various variousout-of-oil out-of-oiltemperatures temperaturesof of60 60°C, C,90 90°C,C,and and120 120°CCrespectively. respectively.

Thephase phasediagram diagramofof51CrV4 51CrV4 steel was shown in Figure 5, which calculated based on The steel was shown in Figure 5, which waswas calculated based on the the Thermo-Calc software. The composition of 51CrV4 to the eutectoid point. Thermo-Calc software. The composition of 51CrV4 steelsteel was was closeclose to the eutectoid point. The The A1 A1 temperature about There were several kindsofofprecipitations precipitationsformed formedatat various various temperature was was about 743 743 °C. ◦ C. There were several kinds temperatures,including includingthe theMnS, MnS,the thecementite, cementite,the theM M7C (carbideofofFe Feand andCr), Cr),the thegraphite, graphite,the the temperatures, 3 3(carbide 7C M2P P (phosphide of Fe and Mn), and the M C (carbide of Fe and Cr). At the tempering temperature M (phosphide of Fe and Mn), and the M 3 C 2 (carbide of Fe and Cr). At the tempering temperature 2 3 2 thiswork, work,the theM M7C carbideswere wereprecipitated. precipitated.Moreover, Moreover,MnS MnSexisted existedas asinclusions inclusionsatatboth both ofofthis 3-type 7C 3 -typecarbides soaking and tempering is predicted to precipitate belowbelow 600 ◦ C600 in the soaking temperingprocess. process.Graphite Graphite is predicted to precipitate °C phase in thediagram. phase However,However, graphite graphite was not observed in the microstructure of this work. diagram. was not observed in the microstructure of this work.

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Figure5.5. 5.Phase Phasediagram diagramof of51CrV4 51CrV4 spring spring steel (calculated based on the Thermo-Calc software). Figure steel (calculated (calculatedbased basedon onthe theThermo-Calc Thermo-Calcsoftware). software). Figure Phase diagram of 51CrV4 spring

Figure phaseschanging changingwith withtemperature, temperature,which which was Figure6 66shows showsthe themole molefraction fraction of of precipitated precipitated phases phases was Figure shows the mole fraction of precipitated changing with temperature, which was calculated using Thermo-Calc software. There are are several several kinds kindsof ofcarbide carbideprecipitations precipitationsoccurring occurring calculated using Thermo-Calc software. calculated◦ using Thermo-Calc software. There are several kinds of carbide precipitations occurring below and the the M M777C -typecarbides. carbides.A significant below740 740 C, °C,including includingthe thecementite, cementite,the theM M333C C222-type, -type, and and the CC33-type carbides. AAsignificant significant 3-type below 740 °C, including the cementite, the M C -type, ◦ amount range of of 590–720 590–720°C, C,whereas whereasit rapidlydecreases decreases amountofof ofcementite cementiteisis isstable stablein inthe the temperature temperature range range of 590–720 °C, whereas ititrapidly rapidly decreases amount cementite stable in the temperature ◦°C. with decreasing temperature below 590 The cementite is completely replaced by M 3C 2 -type and with decreasing temperature below 590 C. The cementite completely replaced by M C and 2 -type with decreasing temperature below 590 °C. The cementite is completely replaced by M3C3 2-type and M 7 C 3 -type carbides. M 7 C 3 carbide reaches its largest fraction and remains stable in the temperature MM carbides. M carbide reaches its largest fraction and remains stable in the temperature 7C 3 -type 7C 3-type carbides. M77C33 carbide reaches its largest fraction and remains stable in the temperature ◦ C,°C, range of 420–480 which covers the tempering tempering temperature ofwork, this work, work, indicating that the range of 420–480 which covers the tempering temperature of this indicating that the carbides range of 420–480 °C, which covers the temperature of this indicating that the carbides observed in Figures 2 and 3 were mostly M 7C 3-type. The amount of M 7C3 carbide is gradually observed in Figures 2 and 3 were mostly M C -type. The amount of M C carbide is gradually reduced 7 3 7 3of M7C3 carbide is gradually carbides observed in Figures 2 and 3 were mostly M7C3-type. The amount ◦ C. 420 °C. reduced andby replaced by M M33C C22 carbides carbides below and replaced M3 C2 carbides below 420 reduced and replaced by below 420 °C. 0.10 0.10

Cementite Cementite MC M33C22 MC M77C33 MnS MnS

Fraction Fractionofofphases phases

0.08 0.08

0.06 0.06

0.04 0.04

0.02 0.02

0.00 0.00 200 200

400 400

600 600

o

Temperature ( o C) Temperature ( C)

800 800

1000 1000

Figure varioustemperatures temperatures(calculated (calculated based Figure6.6. 6.The Themole molefraction fractionof ofprecipitated precipitated phases at various various temperatures (calculated based onon thethe Figure The mole fraction of precipitated phases at based on the Thermo-Calc software) Thermo-Calc software) Thermo-Calc software)

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As indicated by the phase diagram (Figure 5), the MnS inclusions could exist in a large As indicated by the phase diagram (Figure 5), the MnS inclusions could exist in a large temperature temperature range below 1400 °C. Figure 7a,b shows the morphology of several MnS inclusion range below 1400 ◦ C. Figure 7a,b shows the morphology of several MnS inclusion particles, which was particles, which was identified by EDS elemental mapping and point analysis. Figure 7c shows the identified by EDS elemental mapping and point analysis. Figure 7c shows the corresponding EDS corresponding EDS elemental mapping images of the inclusions in Figure 7a, which indicate that the elemental mapping images of the inclusions in Figure 7a, which indicate that the main elements of main elements of the inclusions were Mn and S. Figure 7d shows the EDS point analysis of point 1 in the inclusions were Mn and S. Figure 7d shows the EDS point analysis of point 1 in the inclusion of the inclusion of Figure 7b. The composition appeared to be 40% Mn and 42% S, indicating that the Figure 7b. The composition appeared to be 40% Mn and 42% S, indicating that the inclusion was MnS. inclusion was MnS.

Figure 7. MnS Inclusion particles: (a,b) SEM image of two inclusion particles; (c) EDS elemental Figure 7. MnS Inclusion particles: (a,b) SEM image of two inclusion particles; (c) EDS elemental mapping around the inclusions of (a); (d) EDS point analysis of point 1. mapping around the inclusions of (a); (d) EDS point analysis of point 1.

The influenced by the intercarbide intercarbide The mechanical mechanical properties properties of of tempered tempered steel steel bars bars were were directly directly influenced by the spacing. The troostite belongs to the pearlite-type structure. The strength of the pearlite-type spacing. The troostite belongs to the pearlite-type structure. The strength of the pearlite-type structure carbide) hashas beenbeen reported to follow a Hall-Petch type relationship with respect to structure (ferrite (ferrite+ + carbide) reported to follow a Hall-Petch type relationship with the intercarbide spacing [31–34]. Both yield strength and hardness follow a Hall-Petch relationship respect to the intercarbide spacing [31–34]. Both yield strength and hardness follow a Hall-Petch [33,35,36]: relationship [33,35,36]: σyσ ==σσ0 ++ kks λλ−−1/1/2 2 y

0

s

Hrc = H0 + kh λ−1/2

(1) (1) (2)

−1 / 2

H rc = H 0 + k h λ (2) where λ is the intercarbide spacing, σy is the yield strength of the steel, σ0 is the internal frictional strength of the ferrite with the infinite path, strength and ks isofa the dislocation constant during the yield steel, σ0 locking is the internal frictional where λ is intercarbide spacing,mean σy is free yielding.of Hrc is thewith Rockwell hardness, is the hardness ferrite with the infinite mean free strength ferrite the infinite meanH0free path, and ks of is athe dislocation locking constant during path, and k is related to the dislocation locking constant during hardness measurement. h yielding. Hrc is the Rockwell hardness, H0 is the hardness of the ferrite with the infinite mean free intercarbide spacings increased with the increase of oil-bath temperature, the tensile path,Since and kthe h is related to the dislocation locking constant during hardness measurement. strength should decrease with the increase of oil-bath temperature according to the Sinceand thehardness intercarbide spacings increased with the increase of oil-bath temperature, the tensile Hall-Petch relationship. Figure 8a shows the ultimate tensile strength (UTS) and yield strength strength and hardness should decrease with the increase of oil-bath temperature according to(YS) the changing with oil-bath temperature, in which the stress and strain were engineering As(YS) the Hall-Petch relationship. Figure 8a shows the ultimate tensile strength (UTS) and yieldvalues. strength ◦ oil-bath temperature increased from 20 to 80 C, average UTS decreased from 1599values. to 1549AsMPa, changing with oil-bath temperature, in which thethe stress and strain were engineering the and the average YS decreased from 1451 to 1398 MPa. Correspondingly, the percentages ofMPa, area oil-bath temperature increased from 20 to 80 °C, the average UTS decreased from 1599 to 1549 reduction and elongation at break both1451 increased with increasing oil-bath temperature, indicating that and the average YS decreased from to 1398 MPa. Correspondingly, the percentages of area a high oil-bath temperature benefits the enhancement of ductility. An extensometer was set during reduction and elongation at break both increased with increasing oil-bath temperature, indicating

that a high oil-bath temperature benefits the enhancement of ductility. An extensometer was set

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Yield strength,RP0.2(MPa) / Reduction of area, Z(%)

Elongation at break,A(%) / Utimate Strength,Rm(Mpa)

the tensile to measure the strain precisely. The elastic modulus was calculated from the from slopesthe of during the test tensile test to measure the strain precisely. The elastic modulus was calculated elastic part of the stress-strain curves. The elasticity modulus, which is an important parameter relating slopes of elastic part of the stress-strain curves. The elasticity modulus, which is an important to the spring behavior, also decreased withalso increasing oil-bath as shown in Figure 8b. parameter relating to the spring behavior, decreased withtemperature, increasing oil-bath temperature, as Figure 8c shows the hardness results tested along a diameter in a cross section of the tempered steel shown in Figure 8b. Figure 8c shows the hardness results tested along a diameter in a cross section bar.the The hardnesssteel value showed a small discrepancy between center andbetween surface, indicating that of tempered bar. The hardness value showed a smallthe discrepancy the center and the steel indicating bar was fully during quenching process. hardness ofprocess. tempered workpieces surface, thathardened the steel bar wasthe fully hardened duringThe the quenching The hardness decreased with the increase of oil-bath temperature. As discussed in Figure 1, the fraction of austenite of tempered workpieces decreased with the increase of oil-bath temperature. As discussed in Figure ◦ ◦ was higher in samples at 80 C oil-bath temperature than that at 50 C, which also contribute to °C, the 1, the fraction of austenite was higher in samples at 80 °C oil-bath temperature than that at 50 decrease of strength and hardness. which also contribute to the decrease of strength and hardness.

240 220 200 180 160 10

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Oil-bath temperature 20 C o Oil-bath temperature 50 C o Oil-bath temperature 80 C

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47 46 45 44 43 42 41 40 -12 -10

-8

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0

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Distance from center(mm)

Figure Figure 8. 8. Mechanical Mechanical properties properties of of the the steel steel bars bars at at various various oil-bath oil-bath temperatures: temperatures: (a) (a) ultimate ultimate tensile tensile strength, yield strength, elongation at break, and reduction of area (The stress strain are strength, yield strength, elongation at break, and reduction of area (The stress and strain and are engineering engineering value); (b) elasticity (c) rockwell hardness (HRC) samples. of tempered samples. value); (b) elasticity modulus; (c)modulus; rockwell hardness (HRC) of tempered

As discussed above, the the intercarbide intercarbide spacings spacings decreased decreased with with the increase of out-of-oil temperature. According to the Hall-Petch relation, the tensile strength and hardness should decrease with increasing oil-bath temperature. Figure Figure 9a shows the ultimate tensile strength (UTS) and yield strength (YS) changing with out-of-oil temperature, temperature, in which the stress and strain were engineering ◦ C,°C, values. As the theout-of-oil out-of-oiltemperature temperatureincreased increased from to 120 average decreased from 60 60 to 120 the the average UTSUTS decreased fromfrom 1615 1615 to MPa, 1513 and MPa, the average YS decreased 1461 to 1404 MPa. Correspondingly, the to 1513 theand average YS decreased from 1461from to 1404 MPa. Correspondingly, the percentages percentages of area reduction andatelongation break both increased with of theout-of-oil increase temperature, of out-of-oil of area reduction and elongation break bothatincreased with the increase temperature, indicating that a high out-of-oil resulted temperature in the enhancement of ductility. indicating that a high out-of-oil temperature in theresulted enhancement of ductility. The elasticity The elasticity decreased withofthe increasetemperature, of out-of-oil temperature, as shown modulus also modulus decreasedalso with the increase out-of-oil as shown in Figure 9b. in Figure 9b.

(a)

1600

Rm

1700 1600

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Out-of-oil temperature 60 C o Out-of-oil temperature 90 C o Out-of-oil temperature 120 C

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47 46 45 44 43 42 41 40 -12

-10

-8

-6

-4

-2

0

2

4

6

8

10

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Distance from center(mm)

Figure Figure 9. 9. Mechanical Mechanical properties properties of of the the steel steel bars bars at atvarious variousout-of-oil out-of-oil temperatures: temperatures: (a) (a) ultimate ultimate tensile tensile strength, yieldstrength, strength, elongation at break, and reduction area (The strain are strength, yield elongation at break, and reduction of area of (The stress andstress strain and are engineering engineering value); (b) elasticity (c) rockwell hardness (HRC) of tempered samples. value); (b) elasticity modulus; (c)modulus; rockwell hardness (HRC) of tempered samples.

The auto-tempering auto-temperingwas wasa possible a possible reason to cause the change of mechanical properties at The reason to cause the change of mechanical properties at a high a high out-of-oil temperature. The auto-tempering process generated carbide particles the out-of-oil temperature. The auto-tempering process generated carbide particles inside theinside carbidecarbide-depleted a high out-of-oil temperature and reduced the volume fraction of depleted region atregion a highatout-of-oil temperature (Figure(Figure 3), and3), reduced the volume fraction of such such areas the final microstructure after subsequent principal tempering.Besides, Besides,the theauto-tempering auto-tempering areas in theinfinal microstructure after subsequent principal tempering. reduced the thelength lengthofoflarge-sized large-sized carbides and enhanced their width at same the same (Figure 4). reduced carbides and enhanced their width at the time time (Figure 4). This This coarsening behavior the carbides at high out-of-oil temperature contributed a relative drop coarsening behavior of theofcarbides at high out-of-oil temperature contributed to atorelative drop of of tensile strength. tensile strength. As the temperature increased, the hardness decreased due to the increase of intercarbide As theout-of-oil out-of-oil temperature increased, the hardness decreased due to the increase of ◦ C out-of-oil spacing of carbides 9c). In the tempered specimen quenched at 60 temperature, intercarbide spacing(Figure of carbides (Figure 9c). In the tempered specimen quenched at 60 °C out-of-oil most of the hardness were in the rangewere of 46~47 HRC; when out-of-oil changed temperature, most ofresults the hardness results in the range of the 46~47 HRC; temperature when the out-of-oil to 120 ◦ C, most of thetohardness results were in the results range were of 44~45 HRC, and the decrement temperature changed 120 °C, most of the hardness in the range of 44~45 HRC, andwas the about 4.3%.was about 4.3%. decrement The yield yield strength strength and and hardness hardness of the steel samples with various intercarbide intercarbide spacings spacings are The shown in in Figure Figure 10. Two Two dashed lines are plotted as functions of the the inverse inverse of of the the square square root root of of the the shown − 1/2 −1/2 intercarbide intercarbide spacing spacing (λ ) in ) inFigure Figure10a,b 10a,brespectively. respectively.From Fromthe theplot, plot,intercepts intercepts and and slopes slopes were were calculated, calculated, and the following empirical relationships were obtained: 2 = 973 + 206λ−−1/1/2 σσ y y= 973 + 206λ

(3) (3)

−1/2 HHrcrc== 34.5 34.5+ + 5.1λ 5.1λ − 1 / 2

(4) (4)

This implies that the yield strength and the hardness both decreased with increasing intercarbide in accordance with the Hall-Petch relationship. Both high oil-bath temperature and high out-of-oil

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49

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Hardness (HRC)

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Intercarbide spacing (μm)

(a)

0.24

44 0.16

0.18

0.20

0.22

0.24

Intercarbide spacing (μm)

(b)

Figure Figure 10. 10. Yield Yield strength strength and and hardness hardness of of the the steel steel bars bars with with various various intercarbide intercarbide spacings: spacings: (a) (a) yield yield strength; strength; (b) (b) hardness. hardness.

This implies yield strength and hardnessproperties, both decreased intercarbide According tothat the the above discussions on the mechanical bothwith highincreasing oil-bath temperature in accordance with the Hall-Petch relationship. Both high oil-bath temperature and high out-of-oil and high out-of-oil temperature benefited the enhancement of the ductility, which correspondingly temperature resultedand in an increase in intercarbide which led to the decrease of strength reduced the strength hardness. These effects werespacing, further proved by the fracture analysis. Figure and hardness. 11 shows the SEM microfractographs of the steel bars investigated. The fractures were uneven and According the above discussions high oil-bath temperature and showed randomtofluctuations. Fractures on of mechanical all the fourproperties, conditionsboth were composed by dimple and high out-of-oil temperature the enhancement of the ductility, which reduced quasi-cleavage morphology,benefited which indicated the experimental steel had highcorrespondingly toughness. Many deep the strength hardness. further proved by thearound fracturelarge analysis. Figure 11 shows dimples wereand observed, andThese manyeffects smallwere dimples were distributed dimples. There were the SEMexisting microfractographs of conditions, the steel bars investigated. fractures uneven and showed pockets in all the four which containedThe features that were resemble localized quasirandom fluctuations. all fracture the four conditions were composed by dimple and 80 quasi-cleavage cleavage. Figure 11a,bFractures presentsofthe morphology of the specimens at 20 and °C oil-bath morphology, which indicated the experimental steel had high toughness. Many deep dimples temperatures, respectively. The specimen at 80 °C oil-bath temperature had a larger dimple were size, observed, and small dimples distributed around large11c,d dimples. There pockets existing indicating thatmany this specimen hadwere a higher ductility. Figure shows thewere tensile fractures of in all the four conditions, which contained features that resemble localized quasi-cleavage. Figure 11a,b tempered 51CrV4 spring steel bars at various out-of-oil temperatures. At 60 °C out-of-oil temperature, presents the fracture morphology of thefracture specimens at 20 andand 80 ◦ C oil-bath temperatures, respectively. the dimples appearing in the tensile were small uniformly distributed, most of the ◦ The specimen at 80less C oil-bath temperature had a larger temperature, dimple size, indicating that this specimen had dimples had sizes than 1 μm. At 120 °C out-of-oil many large dimples with sizes a higher ductility. Figure 11c,d shows the tensile of tempered 51CrV4 steel bars at larger than 1 μm were observed. Therefore, the fractures fracture was more ductile at aspring higher out-of-oil various out-of-oil temperatures. At 60 ◦ C out-of-oil temperature, the dimples appearing in the tensile temperature. The fracture morphology could explain the mechanical property results presented in ◦C fracture8were uniformly distributed, most the dimples had sizes less than µm. At 120 Figures and small 9. It isand noted that a fine inclusion wasofobserved in Figure 11d, and the1 inclusion was out-of-oil temperature, many large dimples with sizes larger than 1 µm were observed. Therefore, MnS compound according to the EDS analysis. the fracture was more ductile at a higher out-of-oil temperature. The fracture morphology could explain the mechanical property results presented in Figures 8 and 9. It is noted that a fine inclusion was observed in Figure 11d, and the inclusion was MnS compound according to the EDS analysis.

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Figure 11. Fracture morphology of tempered steel bars: specimens quenched at oil-bath temperature Figure 11. Fracture morphology of tempered steel bars: specimens quenched at oil-bath temperature of 20 °C (a) and 80 °C (b), respectively; specimens quenched at out-of-oil temperature of 60 °C (c) and of 20 ◦ C (a) and 80 ◦ C (b), respectively; specimens quenched at out-of-oil temperature of 60 ◦ C (c) and 120 ◦°C (d), respectively. 120 C (d), respectively.

The byby the number and distribution of The size size of of the thedimples dimpleson onthe thefracture fracturesurface surfacewas wasgoverned governed the number and distribution microvoids that are nucleated at regions of localized strain discontinuity, such as that associated with of microvoids that are nucleated at regions of localized strain discontinuity, such as that associated grain grain boundaries or second phase particles [37]. The[37]. microvoids grew to a large ledsize to large with boundaries or second phase particles The microvoids grew size to aand large and dimples when the nucleation sites were few and widely spaced. As discussed above, the average led to large dimples when the nucleation sites were few and widely spaced. As discussed above, intercarbide spacing increased with both increase of oil-bath temperature the average intercarbide spacing increased withthe both the increase of oil-bath temperatureand and out-of-oil out-of-oil temperature. The large intercarbide spacing caused a small number density of nucleation and temperature. The large intercarbide spacing caused a small number density of nucleation sites sites and resulted in large large dimple dimple size. size. resulted in 4. 4. Conclusions Conclusions The work performed performed soaking, soaking, quenching, quenching, and and tempering tempering on on 51CrV4 51CrV4 steel. steel. The The present present work The effects effects of of various oil-bath temperatures and out-of-oil temperatures on the quenching microstructure, various oil-bath temperatures and out-of-oil temperatures on the quenching microstructure, tempering tempering microstructure, and the mechanical were investigated, main conclusions microstructure, and the mechanical propertiesproperties were investigated, the mainthe conclusions can be can be summarized as follows: summarized as follows: The of 51CrV4 51CrV4 steel The as-quenched as-quenched microstructure microstructure of steel consisted consisted of of needle-like needle-like martensite, martensite, large-sized large-sized martensite laths, lens-shaped bainite islands, and retained austenite. The tempered microstructure martensite laths, lens-shaped bainite islands, and retained austenite. The tempered microstructure contained longcarbides, carbides,accompanied accompanied a certain number of carbide particles. According the contained long byby a certain number of carbide particles. According to the to phase phase diagram calculated based on Thermo-Calc software, M7C3carbides -type carbides were precipitated diagram calculated based on Thermo-Calc software, the M7the C3 -type were precipitated at the at the tempering temperature of this work. tempering temperature of this work. A lower oil-bath oil-bath temperature A lower temperature tended tended to to achieve achieve higher higher cooling cooling rate rate and and higher higher undercooling, undercooling, which led led to to the the finer finer size size of of martensite. martensite. A A coarsening coarsening of of the the martensite martensite occurred occurred at at aa high high oil-bath oil-bath which temperature. In addition, the size and fraction of the bainite islands increased with the increase of oiltemperature. In addition, the size and fraction of the bainite islands increased with the increase bath temperature. In theIn tempered specimens which have been quenched oil-bath of oil-bath temperature. the tempered specimens which have been quenchedatataa high high oil-bath temperature, the number of precipitated carbides decreased and the carbides tended to have temperature, the number of precipitated carbides decreased and the carbides tended to have aa larger larger size. Sincethe theintercarbide intercarbide spacings increased increasing oil-bath temperature, thestrength tensile size. Since spacings increased withwith increasing oil-bath temperature, the tensile strength and hardness both decreased with increasing oil-bath temperature in accordance with the and hardness both decreased with increasing oil-bath temperature in accordance with the Hall-Petch Hall-Petch relationship. Correspondingly, the ductility increased with the increase of oil-bath relationship. Correspondingly, the ductility increased with the increase of oil-bath temperature. temperature.

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A high out-of-oil temperature enhanced the chance of auto-tempering, which led to the precipitation of tiny carbide particles in the as-quenched martensite laths. This auto-tempering effect reduced the length of large-sized carbides and enhanced their width in the final tempered microstructure. The intercarbide spacings between carbides increased with the increase of out-of-oil temperature. The tensile strength and hardness decreased with increasing oil-bath temperature, and the ductility increased correspondingly. The fracture of 51CrV4 steel was composed by dimple and quasi-cleavage morphology. Both a higher oil-bath temperature and a higher out-of-oil temperature resulted in larger dimples, which can explain the enhancement of ductility. Author Contributions: Methodology, L.Z. and E.W.; Investigation, L.Z. and Y.L.; Formal Analysis, X.R. and D.G.; Data Curation, X.W. and D.G.; Writing—Original Draft Preparation, L.Z. Funding: This research was funded by the National Natural Science Foundation of China, grant number 51674083, the project of heat treatment optimization and product quality control of springs used in urban rail train (funded by the CRRC Guiyang Co., LTD.), and the Programme of Introducing Talents of Discipline to Universities (the 111 Project of China), grant number B07015. Conflicts of Interest: The authors declare no conflict of interest.

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Ardehali, B.A.; Ponge, D.; Raabe, D. Refinement of grain boundary carbides in a Si-Cr spring steel by thermomechanical treatment. Mater. Sci. Eng. A 2006, 426, 194–201. [CrossRef] Wang, Z.; Liu, X.; Xie, F.; Lai, C.; Li, H.; Zhang, Q. Dynamic recrystallization behavior and critical strain of 51CrV4 high-strength spring steel during hot deformation. JOM 2018, 70, 2385–2391. [CrossRef] Sencic, B.; Leskovsek, V. Fracture toughness of the vacuum-heat-treated spring steel 51CrV4. Mater. Tehnol. 2011, 45, 67–73. Nam, W.J.; Lee, C.S.; Ban, D.Y. Effects of alloy additions and tempering temperature on the sag resistance of Si-Cr spring steels. Mater. Sci. Eng. A 2000, 289, 8–17. [CrossRef] Zhang, C.L.; Liu, Y.Z.; Jiang, C.; Xiao, J.F. Effects of Niobium and Vanadium on Hydrogen-induced Delayed Fracture in High Strength Spring Steel. J. Iron Steel Res. Int. 2011, 18, 49–53. [CrossRef] Podgornik, B.; Tehovnik, F.; Burja, J.; Seni, B. Effect of modifying the chemical composition on the properties of spring steel. Metall. Mater. Trans. A 2018, 49, 3283–3292. [CrossRef] Ostash, O.P.; Chepil, R.V.; Markashova, L.I.; Hrybovs’ka, V.I.; Kulyk, V.V.; Berdnikova, O.M. Influence of the modes of heat treatment on the durability of springs made of 65G steel. Mater. Sci. 2018, 53, 684–690. [CrossRef] Liu, S.Y.; Liu, D.Y.; Liu, S.C. Effect of heat treatment on fatigue resistance of spring steel 60Si2CrVAT. Met. Sci. Heat Treat. 2010, 52, 57–60. Ai, J.H.; Zhao, T.C.; Gao, H.J.; Hu, Y.H.; Xie, X.S. Effect of controlled rolling and cooling on the microstructure and mechanical properties of 60Si2MnA spring steel rod. J. Mater. Process. Technol. 2005, 160, 390–395. [CrossRef] Harada, Y.; Mori, K. Effect of processing temperature on warm shot peening of spring steel. J. Mater. Process. Technol. 2005, 162, 498–503. [CrossRef] Anil, K.V.; Karthikeyan, M.K.; Gupta, R.K.; Ramkumar, P.; Uday, P.M. Heat treatment studies on 50CrV4 spring steel. Mater. Sci. Forum 2015, 830, 139–142. [CrossRef] Xu, L.; Chen, L.; Sun, W. Effects of soaking and tempering temperature on microstructure and mechanical properties of 65Si2MnWE spring steel. Vacuum 2018, 154, 322–332. [CrossRef] Nie, B.; Zhang, Z.; Zhao, Z.; Zhong, Q. Very high cycle fatigue behavior of shot-peened 3Cr13 high strength spring steel. Mater. Design 2013, 50, 503–508. [CrossRef] Podgornik, B.; Leskovek, V.; Godec, M.; Sencic, B. Microstructure refinement and its effect on properties of spring steel. Mater. Sci. Eng. A 2014, 599, 81–86. [CrossRef] Luo, Z.F.; Liang, Y.L.; Long, S.L.; Jiang, Y.; Wu, Z.L. Effects of ultra-refine grain and micro-nano twins on mechanical properties of 51CrV4 spring steel. Mater. Sci. Eng. A 2017, 690, 225–232. [CrossRef]

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