materials
Article Mechanism of the Microstructural Evolution of 18Cr2Ni4WA Steel during Vacuum Low-Pressure Carburizing Heat Treatment and Its Effect on Case Hardness
Bin Wang 1,* , Yanping He 1, Ye Liu 1, Yong Tian 1,* , Jinglin You 2 , Zhaodong Wang 1 and Guodong Wang 1
1 State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China; [email protected] (Y.H.); [email protected] (Y.L.); [email protected] (Z.W.); [email protected] (G.W.) 2 State Key Laboratory of Advanced Special Steel, Shanghai Key Laboratory of Advanced Ferrometallurgy, School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China; jlyou@staff.shu.edu.cn * Correspondence: [email protected] (B.W.); [email protected] (Y.T.)
Received: 21 April 2020; Accepted: 18 May 2020; Published: 20 May 2020
Abstract: In this study, vacuum low-pressure carburizing heat treatments were carried out on 18Cr2Ni4WA case-carburized alloy steel. The evolution and phase transformation mechanism of the microstructure of the carburized layer during low-temperature tempering and its effect on the surface hardness were studied. The results showed that the carburized layer of the 18Cr2Ni4WA steel was composed of a large quantity of martensite and retained austenite. The type of martensite matrix changed from acicular martensite to lath martensite from the surface to the core. The hardness of the carburized layer gradually decreased as the carbon content decreased. A thermodynamic model was used to show that the low-carbon retained austenite was easier to transform into martensite at lower temperatures, since the high-carbon retained austenite was more thermally stable than the low-carbon retained austenite. The mechanical stability—not the thermal stability—of the retained austenite in the carburized layer dominated after carburizing and quenching, and cryogenic treatment had a limited effect on promoting the martensite formation. During low-temperature tempering, the solid-solution carbon content of the martensite decreased, the compressive stress on the retained austenite was reduced and the mechanical stability of the retained austenite decreased. Therefore, during cooling after low-temperature tempering, the low-carbon retained austenite transformed into martensite, whereas the high-carbon retained austenite still remained in the microstructure. The changes in the martensite matrix hardness had a far greater effect than the transformation of the retained austenite to martensite on the case hardness of the carburized layer.
Keywords: vacuum low-pressure carburizing; 18Cr2Ni4WA steel; low-temperature tempering; retained austenite; case hardness
1. Introduction The 18Cr2Ni4WA steel is a low-carbon alloy steel with a good hardenability. After carburizing, a high-strength surface and high-toughness core can be achieved simultaneously. This steel is widely used under high-speed and heavy-duty conditions for the main load-bearing parts of aviation engines, gas turbines and other large machines, such as crankshafts, gear teeth, adapter plates and flanges [1–3]. Carburizing this material results in a high surface carbon content, a martensite
Materials 2020, 13, 2352; doi:10.3390/ma13102352 www.mdpi.com/journal/materials Materials 2020, 13, 2352 2 of 17 matrix with a high strength and hardness and a large quantity of retained austenite, which can be obtained during quenching or even under air cooling conditions [4]. Low-temperature tempering is subsequently used to reduce or eliminate internal stresses in the quenched microstructure to improve the dimensional accuracy and strength of the workpieces. Over the 150 ◦C–250 ◦C range generally used for low-temperature tempering, the migration of carbon atoms in the carburized layer over short distances results in a series of microstructural changes. The microstructural evolution that occurs during carburizing heat treatments ultimately determines the hardness and gradient distribution of the carburized layer, which directly affects the service performance under surface wear and fatigue [5–8]. Tempering is generally considered to reduce the stability of the retained austenite, which promotes the transformation to martensite during subsequent cooling to increase the surface hardness [9,10]. Shi [11] proposed that the retained austenite in 18Cr2Ni4W steel could transform into secondary martensite during tempering and cooling. Chen et al. [12] studied the tempering of GCr15 bearing steel and found that the thermal stability of austenite depended on the combined effects of the carbon distribution in austenite and carbide precipitation. Nam et al. [13] found that alloying elements did not affect the decomposition of the retained austenite in induction hardened-and-tempered steels. Wu et al. [14] conducted a detailed study on microstructural evolution and the effects of alloying elements during the tempering of martensite. Feng et al. [15] proposed that during low- and medium-temperature tempering, carbon discharge from martensite to nearby austenite increased the carbon content and thermal stability of the retained austenite. Chen et al. [16] found that the low-temperature cryogenic treatment of 40CrNiMoA steel promoted the transformation of retained austenite to martensite and the precipitation and uniform distribution of carbides. The martensite substructure was also refined, and the steel hardness was improved. Therefore, an in-depth study of the mechanism of the microstructural phase transformation that occurs during a carburizing heat treatment can provide important guidance to control carburization and optimize the material performance. In this study, vacuum low-pressure carburization was carried out on 18Cr2Ni4WA steel. Multistage pulse carburization was used to precisely control the depth of the carburized layer and the carbon content and prevent intergranular oxidation, thereby forming a uniform and stable carbon gradient distribution. A thermodynamic model was used to calculate the driving force for the austenite phase transformation and decomposition. These results were used to study the migration and diffusion behavior of carbon in the martensite in the carburized layer microstructure of 18Cr2Ni4WA steel, the conditions for the transformation of retained austenite and the subsequent effects on the hardness of the carburized layer. This study can provide a theoretical reference for investigating the microstructural evolution of case-carburized steels during carburizing and tempering heat treatments.
2. Experimental Materials and Methods of Vacuum Low-Pressure Carburization Th experiments were performed on 18Cr2Ni4WA steel, a heavy-duty gear steel (Fushun Special Steel, Fushun, China). The steel was processed into a φ22 mm bar sample after vacuum induction melting, cold drawing and annealing. The main chemical components of 18Cr2Ni4WA steel are shown in Table1.
Table 1. Elemental content (mass fraction) of the 18Cr2Ni4WA steel.
Element C Si Mn Cr Ni W Content 0.2 0.33 0.5 1.4 4.3 1.0
A schematic of the vacuum low-pressure carburizing process is shown in Figure1. The sample was placed in a DBVC-433 vacuum carburizing device developed by Northeastern University (Shenyang, China) for two-stage heating at a rate of 10 ◦C/min. The temperature was held at 800 ◦C for a period of time, after which the austenitizing temperature was held at 950 ◦C for 1 h; then, the sample was carburized. The experiment was carried out using the “carburizing + diffusing” multistage pulse method. The carburizing medium was C2H2 gas introduced with a flow rate of 10 L/min and a Materials 2020, 13, x FOR PEER REVIEW 3 of 19 Materials 20202020,, 1313,, 2352x FOR PEER REVIEW 3 of 1719 multistage pulse method. The carburizing medium was C2H2 gas introduced with a flow rate of 10 L/minmultistage and apulse carburizing method. pressure The carburizing of 300 Pa. medium To prevent was Cthe2H cementite2 gas introduced from affecting with a flow the evolutionrate of 10 andcarburizingL/min hardness and a pressure carburizingof the matrix of 300 pressure microstructure, Pa. To preventof 300 Pa. the the To cementite carbon prevent content from the cementite a onffecting the surface the from evolution ofaffecting the carburized and the hardness evolution layer of wastheand matrix sethardness to 0.7%. microstructure, of Duringthe matrix the the microstructure,diffusing carbon pulse, content thethe on carbon gas the in surface thecontent furnace of on the thewas carburized surface pumped of layer theout carburizedand was the set furnace to 0.7%.layer wasDuringwas filledset theto with 0.7%. diff nitrogenusing During pulse, atthe a pressure thediffusing gas in of pulse, the 70 furnacePa. the The gas “carburizing was in the pumped furnace + outdiffusing” was and pumped the cycle furnace out was and wasrepeated the filled furnace eight with times.nitrogenwas filled The at withtotal a pressure nitrogencarburizing of 70at a Pa. timepressure The was “carburizing of12 70h forPa. a The carburizing+ di “carburizingffusing” depth cycle + diffusing” wasof 1.3 repeated mm. cycle eight was times. repeated The eight total carburizingtimes. The total time carburizing was 12 h for time a carburizing was 12 h for depth a carburizing of 1.3 mm. depth of 1.3 mm.
T/ oC Heating Carburizing + Diffusing Quenching o Heating Carburizing + Diffusing Quenching PT// PaC (~3 h) (~12 h) (~1 h) P/ Pa (~3 h) (~12 h) (~1 h) T 950 T 950 800 850
800 850
iffusing iffusing
D D
Carburizing Carburizing
uenching
iffusing iffusing
Q
D D
Carburizing Carburizing uenching
300Pa Q P 300Pa P 70Pa 70Pa
t/ h t/ h
Figure 1. SchematicSchematic diagram diagram of of the the vacuum vacuum low low-pressure-pressure carburizing process. Figure 1. Schematic diagram of the vacuum low-pressure carburizing process. The post-carburizing heat treatment is shown in Figure2. After carburizing, the furnace was The post-carburizing heat treatment is shown in Figure 2. After carburizing, the furnace was cooled to 850 C, and the temperature was held for 0.5 h. Vacuum oil quenching was performed at an cooledThe to pos850t ◦-°C,carburizing and the temperature heat treatment was is held shown for 0.5in Figureh. Vacuum 2. After oil quenchingcarburizing, was the performed furnace was at oil quenching rate of approximately 75 C/s. The temperature of the oil was 25 C and the quenching ancooled oil quenchingto 850 °C, and rate the of temperature approximately ◦was 75 held °C/s. for The 0.5 temperatureh. Vacuum oil of quenching the oil◦ was was 25 performed °C and the at time was approximately 15 min. Subsequently, low-temperature tempering was conducted at 160 C quenchingan oil quenching time was rate approximately of approximately 15 75 min. °C/s. Subsequently, The temperature low - oftemperature the oil was tempering 25 °C and was the◦ (process 1) for 2 h. A cryogenic treatment (process 2) was performed as a supplemental experiment conductedquenching at time 160 was°C (process approximately 1) for 2 h.15 A min. cryogenic Subsequently, treatment low (process-temperature 2) was performed tempering aswas a prior to the low-temperature tempering: the temperature was maintained at 73 C for 2 h using a supplementaconducted atl experiment 160 °C (process prior 1)to the for low 2 h.- temperatureA cryogenic tempe treatmentring: the (process temperature 2)− was◦ was performed maintained as a mixed dry ice/ethanol solution as the cooling medium. atsupplementa -73 °C for 2l hexperiment using a mixed prior dry to theice/ethanol low-temperature solution as tempe the coolingring: the medium. temperature was maintained at -73 °C for 2 h using a mixed dry ice/ethanol solution as the cooling medium. (a) (b) 950 oC 13 h o (a) 950 C 13 h (b) 950Austenitizing oC 13 h o C and
950Austenitizing C 13 h o Austenitizing C
/ / o o
and C Carburizingand 850 C 0.5 h o
/ / Austenitizing C Carburizing o / o
o and 850 C 0.5 h Carburizing 850Quenching C 0.5 h / / Carburizing 850Quenching oC 0.5 h Quenching Quenching 800 oC 0.5 h o 800 C 0.5 h 800Stage oC holding 0.5 h Stageo holding Temperature Stage holding
Temperature Temperature 800 C 0.5 h
Stage holding Temperature Temperature Temperature
160 oC 2 h o 160 C 2 h 160Tempering oC 2 h 160Tempering oC 2 h Tempering Tempering Time / h o Time / h -73 C 2 h Time / h Cryogenic-73 oC treatment 2 h Time / h Cryogenic treatment Figure 2. Schematic diagram of the carburizing heat treatment process of the 18Cr2Ni4WA steel: Figure(a) process 2. Schematic 1: quenching diagram+ tempering; of the carburizing (b) process heat 2: quenchingtreatment process+ cryogenic of the treatment 18Cr2Ni4WA+ tempering. steel: (a) processFigure 2.1: Schematicquenching diagram + tempering; of the (b carburizing) process 2: heatquenching treatment + cryogenic process treatmentof the 18Cr2Ni4WA + tempering. steel : (a) Afterprocess the 1: entirequenching treatment + tempering; was completed, (b) process the 2: quenching microstructure + cryogenic and properties treatment of+ tempering. the carburized steel wereAfter analyzed. the entire Following treatment erosion was by completed, 4% nitric acidthe microstructure in ethanol, the and microstructure properties of the carburized steellayer After were was observedthe analyzed. entire using treatment Following a BX53M was erosion completed, optical by microscope 4% the nitric microstructure acid (Olympus, in ethanol, and Tokyo, properties the Japan) microstructure of and the ancarburized Ultra of the 55 steel were analyzed. Following erosion by 4% nitric acid in ethanol, the microstructure of the
Materials 2020, 13, 2352 4 of 17 scanning electron microscope (SEM, Zeiss, Germany). The microhardness of the material was measured every 0.1 mm along the direction of the carburized layer by using an FM-700 Vickers hardness tester (Future-Tech, Kawasaki, Japan). The microstructure was observed via transmission electron microscopy (TEM) using a TECNAI-G2 20F (FEI, Hillsboro, OR, USA). To prepare the TEM sample, mechanical polishing was used to reduce the sample thickness to 50 µm, which was followed by twin-jet polishing. The electrolyte solution was 10% (by volume) perchloric acid in ethanol, the polishing temperature was 25 C, the polishing − ◦ voltage was 38 V and the current was 55 mA. The X-ray diffraction (XRD) sample size was 10 mm 10 mm 5 mm. Twin-jet polishing was × × followed by electrolytic polishing using the following parameters: a voltage of 20 V, a current of 1.1–1.2 A and an electrolytic polishing time of 20 s. The XRD was performed using a D8 Advance X-ray diffractometer (Bruker, Leipzig, Germany) with a diffraction angle of 40–100◦ and a speed of 2◦/min. A standard round bar specimen with a length of 100 mm was also carburized to determine the carbon content distribution of the carburized layer. Iron chips were obtained from the carburized specimen by cutting layer by layer with a 0.1 mm layer depth. Then, the carbon content of the iron chips obtained from each layer was measured using a CS230 carbon–sulfur analyzer (LECO, St Joseph, MI, USA).
3. Results: Microstructure and Performance after Carburizing and Quenching Figures3 and4 show the optical micrographs and SEM images of the microstructure after carburizing and quenching, respectively. The surface microstructure of the carburized layer was composed of small acicular martensite particles, and no apparent carbide precipitation was observed. The subsurface layer was composed of a large quantity of acicular martensite and a small quantity of lath martensite. As the subsurface had a lower carbon content than the surface layer, there were large, relatively coarse needle-like martensite structures. Further reduction in the carbon content in the core microstructure resulted in the formation of lath martensite. The carburized layer had a martensite matrix with a fine, uniform microstructure and high hardness. Figures5 and6 are the optical micrographs and SEM images of the microstructure after low-temperature tempering, respectively. The matrix microstructure gradually transformed from high-carbon acicular martensite in the surface layer to low-carbon lath martensite in the core. No distinct carbide precipitation or change in the microstructure type was observed after quenching, and the substructure appeared slightly large. Materials 2020, 13, 2352 5 of 17
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Figure 3. Optical micrographsFigure 3. Optical micrographs of the microstructureof the microstructure after after carburizing carburizing and quenching and: (a) quenching: the surface; (a) the surface; (b) the subsurface; and (c) the core. (b) the subsurface;Materials and 2020, 13 (c, )x FOR the PEER core. REVIEW 6 of 19
Figure 4. SEM imagesFigure of4. SEM the images microstructures of the microstructures after after carburizing and quenching and quenching:: (a) the surface; (b (a) the) the surface; (b) the subsurface; and (c) the core. subsurface; and (c) the core. Figures 5 and 6 are the optical micrographs and SEM images of the microstructure after low- temperature tempering, respectively. The matrix microstructure gradually transformed from high- carbon acicular martensite in the surface layer to low-carbon lath martensite in the core. No distinct carbide precipitation or change in the microstructure type was observed after quenching, and the substructure appeared slightly large.
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Figure 5. Optical micrographsFigure 5. Optical micrographs of the microstructuresof the microstructures after after low-temperature low-temperature tempering: (a) tempering:the surface; (a) the surface; (b) the subsurface; and (c) the core. (b) the subsurface;Materials and 2020, 13 (c, )x FOR the PEER core. REVIEW 8 of 19
Figure 6. SEM imagesFigure of6. SEM the images microstructures of the microstructures after after low-temperaturelow-temperature tempering tempering:: (a) the surface; (b (a) the) the surface; (b) the subsurface; and (c) the core. subsurface; and (c) the core. The carbon content distribution and the hardness gradient of the carburized layer of the 18Cr2Ni4WA steel after quenching are shown in Figure 7. According to the Chinese national standard GB/T 9450-2005 [17], the hardened layer produced by carburizing and quenching was defined as the vertical distance from the surface to a depth with a Vickers hardness of 550 HV. Therefore, the depth of the carburized layer after carburizing was 1.35 mm, which corresponds to 96% of the pre-set carburized layer depth. After quenching, the surface of the carburized layer had a 0.67% carbon content. The measurements of the carburized layer were consistent with the pre-set values. Vacuum low-pressure carburization resulted in a good carbon content distribution in the carburized layer. The carbon content of the carburized layer decreased gradually and evenly, resulting in a uniform hardness gradient of the carburized layer. The hardness gradient curve shows that the carbon content gradually decreased with increasing depths in the carburized layer and the high-carbon acicular martensite in the surface layer was
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transformed into low-carbon lath martensite in the core. The overall hardness of the layer was The carbon content distribution and the hardness gradient of the carburized layer of the gradually reduced, with small fluctuations in the hardness. After carburizing and quenching, the 18Cr2Ni4WA steel after quenching are shown in Figure7. According to the Chinese national surface hardness of the carburized layer of the sample was 687 HV and the hardness of the core at a standard GB/T 9450-2005 [17], the hardened layer produced by carburizing and quenching was defined 2.0 mm depth was 519 HV. The maximum surface hardness of the carburized and quenched + low- as the vertical distance from the surface to a depth with a Vickers hardness of 550 HV. Therefore, temperature-tempered samples was 620 HV, and the hardness of the core at a 2.0 mm depth was 485 the depth of the carburized layer after carburizing was 1.35 mm, which corresponds to 96% of the HV. The hardness gradient shows that the hardness of the carburized layer was relatively high after pre-set carburized layer depth. After quenching, the surface of the carburized layer had a 0.67% quenching. However, the surface hardness of the carburized layer was reduced by 67 HV and the carbon content. The measurements of the carburized layer were consistent with the pre-set values. hardness of the core was reduced by 34 HV after low-temperature tempering. After low-temperature Vacuum low-pressure carburization resulted in a good carbon content distribution in the carburized tempering, the overall hardness of the carburized layer decreased considerably. To explain this result, layer. The carbon content of the carburized layer decreased gradually and evenly, resulting in a the transformation process of martensite and the retained austenite in the carburized layer and the uniform hardness gradient of the carburized layer. diffusion behavior of carbon atoms during low-temperature tempering are analyzed in depth below.
After quenching 700 After low-temperature tempering 0.66
650 0.60
600 0.54
550 0.48
Carbon content / %
MicrohardnessHV /
500 0.42
0.36 450 0.0 0.4 0.8 1.2 1.6 2.0 2.4 Depth of carburized layer / mm
Figure 7. Carbon content distribution and the hardness gradient of the carburized layer. Figure 7. Carbon content distribution and the hardness gradient of the carburized layer. The hardness gradient curve shows that the carbon content gradually decreased with increasing depths4. Discussion in the carburized layer and the high-carbon acicular martensite in the surface layer was transformed into low-carbon lath martensite in the core. The overall hardness of the layer was gradually4.1. Effect of reduced, Low-Temperature with small Tempering fluctuations on the inMicrostructure the hardness. and C Afterase H carburizingardness and quenching, the surfaceFigure hardness8 shows ofthe the SEM carburized images of layer the ofmorphology the sample of was the 687 carburized HV and thelayer hardness at a 0.3 ofmm the depth core atafter a 2.0 quenching mm depth andwas 519 low HV.-temperature The maximum tempering. surface The hardness quenched of the microstructurecarburized and quenchedwas mainly+ low-temperature-temperedcomposed of acicular and lath samples martensite was 620 + retained HV, and austenite. the hardness There of thewas core a quantity at a 2.0 of mm block depth-shaped was 485retained HV. The austenite hardness between gradient the shows martensite that the matrices. hardness After of the low carburized-temperature layer tempering, was relatively the high main aftermicrostructure quenching. remained However, as the martensite surface hardness + retained of the austenite. carburized Thus, layer compared was reduced to the by carburizing 67 HV and theand hardnessquenching of, thethecore low- wastemperature reduced bytempering 34 HV after resulted low-temperature in the disappearance tempering. of After block low-temperature-shaped retained tempering,austenite. A the few overall fine carbide hardnesss were of the present carburized between layer the decreased martensite considerably. needles and To the explain laths this after result, low- thetemperature transformation tempering process. of martensite and the retained austenite in the carburized layer and the diffusion behavior of carbon atoms during low-temperature tempering are analyzed in depth below.
4. Discussion
4.1. Effect of Low-Temperature Tempering on the Microstructure and Case Hardness Figure8 shows the SEM images of the morphology of the carburized layer at a 0.3 mm depth after quenching and low-temperature tempering. The quenched microstructure was mainly composed of acicular and lath martensite + retained austenite. There was a quantity of block-shaped retained austenite between the martensite matrices. After low-temperature tempering, the main microstructure remained as martensite + retained austenite. Thus, compared to the carburizing and quenching, the low-temperature tempering resulted in the disappearance of block-shaped retained Figure 8. SEM images of the morphology of the carburized layer at a 0.3 mm depth for process 1: (a) after quenching; and (b) after the low-temperature tempering.
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transformed into low-carbon lath martensite in the core. The overall hardness of the layer was gradually reduced, with small fluctuations in the hardness. After carburizing and quenching, the surface hardness of the carburized layer of the sample was 687 HV and the hardness of the core at a 2.0 mm depth was 519 HV. The maximum surface hardness of the carburized and quenched + low- temperature-tempered samples was 620 HV, and the hardness of the core at a 2.0 mm depth was 485 HV. The hardness gradient shows that the hardness of the carburized layer was relatively high after quenching. However, the surface hardness of the carburized layer was reduced by 67 HV and the hardness of the core was reduced by 34 HV after low-temperature tempering. After low-temperature tempering, the overall hardness of the carburized layer decreased considerably. To explain this result, the transformation process of martensite and the retained austenite in the carburized layer and the diffusion behavior of carbon atoms during low-temperature tempering are analyzed in depth below.
After quenching 700 After low-temperature tempering 0.66
650 0.60
600 0.54
550 0.48
Carbon content / %
MicrohardnessHV /
500 0.42
0.36 450 0.0 0.4 0.8 1.2 1.6 2.0 2.4 Depth of carburized layer / mm
Figure 7. Carbon content distribution and the hardness gradient of the carburized layer.
4. Discussion
4.1. Effect of Low-Temperature Tempering on the Microstructure and Case Hardness Figure 8 shows the SEM images of the morphology of the carburized layer at a 0.3 mm depth after quenching and low-temperature tempering. The quenched microstructure was mainly composed of acicular and lath martensite + retained austenite. There was a quantity of block-shaped Materials 2020, 13, 2352 8 of 17 retained austenite between the martensite matrices. After low-temperature tempering, the main microstructure remained as martensite + retained austenite. Thus, compared to the carburizing and austenite.quenching, Athe few low fine-temperature carbides weretempering present resulted between in the the disappearance martensite needles of block and-shaped the laths retained after low-temperatureaustenite. A few fine tempering. carbides were present between the martensite needles and the laths after low- temperature tempering.
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Figure 9 shows the XRD results obtained for the microstructure of the carburized layer at a 0.3 mm depth after carburizing/quenching and quenching + low-temperature tempering. Using the standard Powder Diffraction File card as a reference, the matrices of the carburized layer of the sample at a 0.3 mm depth before and after low-temperature tempering were determined to contain both martensite and austenite. The strongest XRD peak was assigned to the martensite (110) crystal plane and no carbide peaks were observed. The low carbide content and volume preventedFigure the 8. SEMSEM detection images images of of the thecarbide morphology morphology XRD of peaks. of the the carburized carburized The results layer layer at were ata 0.3 a0.3 mm consistent mm depth depth for with forprocess process the 1: ( SEMa 1:) morphological(aftera) after quenching; quenching;results. and Compared and (b) after (b) after the with thelow the low-temperature-temperature XRD peak tempering. obtained tempering. for the quenched microstructure, the XRD peak of the martensite (110) crystal plane after low-temperature tempering shifted to higher angles Figureand was9 shows well separated the XRD resultsfrom the obtained XRD peak for theof the microstructure austenite (111) of the crystal carburized plane, which layer at therefore a 0.3 mm appeareddepth after distinct. carburizing /quenching and quenching + low-temperature tempering.
after quenching after low-temperature tempering
(110) α (110)
(111) γ (111)
(311) γ (311)
(002) α (002) α (220)
(200) γ (200)
(121) α (121)
(220) γ (220)
Intensity
40 50 60 70 80 90 100 2θ / (o)
Figure 9. XRD pattern of the carburized layer at a 0.3 mm depth for process 1. Figure 9. XRD pattern of the carburized layer at a 0.3 mm depth for process 1. Using the standard Powder Diffraction File card as a reference, the matrices of the carburized layerThe of XRD the sample data were at a 0.3processed mm depth using before Equation and after (1) to low-temperature calculate the volume tempering fraction were of determined retained austeniteto contain [18 both]: martensite and austenite. The strongest XRD peak was assigned to the martensite (110) crystal plane and no carbide peaks were observed. The low carbide content and volume prevented 1.4I the detection of carbide XRD peaks. The resultsV = were consistent with the SEM morphological results.(1) Compared with the XRD peak obtained for the quenchedII+1.4 microstructure, the XRD peak of the martensite (110) crystal plane after low-temperature tempering shifted to higher angles and was well separated where V is the volume fraction of the retained austenite; I is the average integrated intensity of from the XRD peak of the austenite (111) crystal plane, which therefore appeared distinct. the XRDThe peaks XRD of data the austenite were processed (200), (220) using and Equation (311) crystal (1) to planes; calculate and the I volume is the average fraction integrated of retained intensityaustenite of [the18]: XRD peaks of the martensite (200) and (211) crystal planes. Equation (2) was used to calculate the carbon content1.4Iγ of the retained austenite [19,20]: Vγ = (1) Iα + 1.4Iγ a 3.556 C = (2) where Vγ is the volume fraction of the retained 0.0453 austenite; Iγ is the average integrated intensity of the XRD peaks of the austenite (200), (220) and (311) crystal planes; and Iα is the average integrated whereintensity C ofis thethe XRDcarbon peaks content of the of martensitethe retained (200) austenite and (211) and crystal a is planes.the lattice constant of the face- centered cubic (FCC) structure. The average value of the austenite (200), (220) and (311) lattice constants was used to calculate using Equation (3):
2 2 2 a d h k l (3) where h, k, l is a diffraction peak crystal plane index and d is the interplanar spacing, which can be calculated from the Bragg diffraction equation given below:
2d sin = (4)
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Equation (2) was used to calculate the carbon content of the retained austenite [19,20]:
aγ 3.556 C = − (2) γ 0.0453 where Cγ is the carbon content of the retained austenite and aγ is the lattice constant of the face-centered cubic (FCC) structure. The average value of the austenite (200), (220) and (311) lattice constants was used to calculate aγ using Equation (3): p 2 2 2 aγ = d h + k + l (3) where h, k, l is a diffraction peak crystal plane index and d is the interplanar spacing, which can be calculated from the Bragg diffraction equation given below:
2d sin θ = λ (4) where θ is the diffraction angle and the wavelength of Cu irradiation λ is taken to be 1.54056 Å. After low-temperature tempering, the strongest martensite peak shifted to higher angles. Equation (4) shows that the crystal plane spacing d decreases as the diffraction peak angle θ increases. Equation (3) shows that the lattice constant a decreases as the interplanar spacing d decreases. The decrease in the lattice constant reflected a reduction in the solid-solution carbon content of martensite. The hardness is positively correlated with the carbon content in the martensite matrix. The higher the carbon content, the more severe the martensite lattice distortion is. Less distortion of the martensite lattice reduced the martensite contribution to the hardness, resulting in an overall decrease in the hardness of the carburized layer after the low-temperature tempering. The decrease in the solid-solution carbon content of the martensite in the matrix reflected the diffusion of carbon atoms in martensite during the low-temperature tempering, which may have enabled the formation of fine carbides, which precipitated or partitioned into the retained austenite [21]. Figure 10 shows the TEM images of the morphology of the surface microstructure before and after the low-temperature tempering. The high-carbon content of the carburized layer resulted in the formation of high-carbon twin-type martensite in the carburized layer after quenching. After low-temperature tempering, the solid-solution carbon content in the martensite decreased, resulting in less twin-type martensite and a higher proportion of low-carbon lath martensite, which was consistent with the martensite XRD peak shifting to a higher angle. No significant carbide precipitates were found in the low-temperature tempered carburized layer, indicating that the carbon atoms in the large quantity of martensite were diffused into the retained austenite in solid-solution form [22,23]. Equations (1) and (2) were used to calculate the diffraction peak intensities. After carburizing and quenching, the retained austenite content was 24.91% and the carbon content in the retained austenite was 0.66%. After low-temperature tempering, the retained austenite content was reduced to 17.66% and the carbon content of the retained austenite increased to 0.74%. The volume expansion that occurs upon the transformation of austenite to martensite subjects the untransformed retained austenite to compressive stress, thereby improving the mechanical stability of the retained austenite. Low-temperature tempering reduced the extent of martensite lattice distortion, thereby lowering the compressive stress on the retained austenite. The corresponding reduction in the mechanical stability of the retained austenite resulted in some of the retained austenite transforming into martensite [18,21]. At the same time, some of the carbon atoms in the martensite diffused into the retained austenite, thereby raising the carbon content and enhancing the thermal stability of the retained austenite. As a result, some high-carbon retained austenite remained stable after low-temperature tempering. Materials 2020, 13, x FOR PEER REVIEW 11 of 19 where θ is the diffraction angle and the wavelength of Cu irradiation λ is taken to be 1.54056 Å . After low-temperature tempering, the strongest martensite peak shifted to higher angles. Equation (4) shows that the crystal plane spacing d decreases as the diffraction peak angle θ increases. Equation (3) shows that the lattice constant a decreases as the interplanar spacing d decreases. The decrease in the lattice constant reflected a reduction in the solid-solution carbon content of martensite. The hardness is positively correlated with the carbon content in the martensite matrix. The higher the carbon content, the more severe the martensite lattice distortion is. Less distortion of the martensite lattice reduced the martensite contribution to the hardness, resulting in an overall decrease in the hardness of the carburized layer after the low-temperature tempering. The decrease in the solid-solution carbon content of the martensite in the matrix reflected the diffusion of carbon atoms in martensite during the low-temperature tempering, which may have enabled the formation of fine carbides, which precipitated or partitioned into the retained austenite [21]. Figure 10 shows the TEM images of the morphology of the surface microstructure before and after the low-temperature tempering. The high-carbon content of the carburized layer resulted in the formation of high-carbon twin-type martensite in the carburized layer after quenching. After low- temperature tempering, the solid-solution carbon content in the martensite decreased, resulting in less twin-type martensite and a higher proportion of low-carbon lath martensite, which was consistent with the martensite XRD peak shifting to a higher angle. No significant carbide precipitates were found in the low-temperature tempered carburized layer, indicating that the carbon atoms in theMaterials large2020 quantity, 13, 2352 of martensite were diffused into the retained austenite in solid-solution 10 form of 17 [22,23].
FigureFigure 10. TEMTEM images images of of the the morphology morphology of the of martensite the martensite in the carburized in the carburized layer: (a ) layer after: quenching; (a) after quenching;and (b) after and the (b low-temperature) after the low-temperature tempering. tempering. 4.2. Driving Force for Phase Transformation from Retained Austenite into Martensite Equations (1) and (2) were used to calculate the diffraction peak intensities. After carburizing and quenching,The thermal the stability retained of the austenite austenite content in the carburizedwas 24.91% layer and mainly the carbon depends content on the in carbon the retained content austeniteand the temperature. was 0.66%. Aft Theer drivinglow-temperature force for the tempering, phase transformation the retained ofaustenite austenite content into martensite was reduced was tocalculated 17.66% and using the the carbon iron–carbon content phaseof the transformationretained austenite KRC increased model proposed to 0.74%. by The Kaufman volume et expansion al. [24,25]. thatIn this occurs section, upon the the thermal transformation stability of of the austenite retained to austenite martensite with subjects different the carbon untrans contentsformed is discussedretained austenitefrom a thermodynamic to compressive perspective. stress, thereby improving the mechanical stability of the retained austenite. γ α Low-temperatureThe driving tempering force ∆G reduced→ for the the extent phase of transformationmartensite lattice of distortion, austenite thereby to martensite lowering (i.e.,the compressivea supersaturated stress ferrite)on the retained with the au samestenite. composition The corresponding can be described reduction as follows: in the mechanical stability of the retained austenite resulted in some of the retained austenite transforming into martensite aα aα γ α γ α C Fe [18,21]. At the same time,∆G → some= ( of1 thex ) ∆ carbonG → + atomsRT[x inln the martensite+ (1 x ) ln diffuse] d into the retained(5) − γ Fe γ γ − γ γ austenite, thereby raising the carbon content and enhancingaC the thermal a stabilityFe of the retained
γ α where ∆G → represents the change in the partial molar free energy of pure iron under a γ α Fe → phase transformation and is a function of temperature (the values presented in Mogutnov et al. [26,27] γ α were used for calculation purposes); aC is the activity of carbon in austenite; aC is the activity of γ α carbon in martensite; aFe is the activity of iron in austenite; and aFe is the activity of iron in martensite. The activities of carbon and iron atoms in austenite and ferrite can be described as follows:
xs xγ ∆Hγ ∆Sγ T γ = + − ln aC ln , (6) 1 zγxγ RT −
where z is the interstitial coordination number, z = 14 12 exp( wγ/RT); wγ is the interaction γ γ − − energy of a pair of adjacent carbon atoms in austenite; xγ is the mole fraction of carbon in austenite; xs ∆Hγ and ∆Sγ are the changes in the partial molar enthalpy and the partial molar non-configurational entropy of carbon in austenite, respectively; R is the ideal gas constant (8.31 J/(mol K)); and T is the · absolute temperature. Shiflet et al. [28] performed numerous experiments to obtain wγ = 8054 J/mol, xs ∆Hγ= 38573 J/mol, and ∆S = 13.48 J/(mol K): γ · xs α xα ∆Hα ∆Sα T ln aC = ln + − (7) 3 zαxα RT − Materials 2020, 13, 2352 11 of 17
In the equation above, the coordination number of an interstitial site zα is zα = 12 8 exp( wα/RT); xs − − xα is the mole fraction of carbon in ferrite; and wα, ∆Hα, and ∆Sα are the interaction energy, the change in the partial molar enthalpy and the change in the partial molar non-configurational entropy of carbon in ferrite, respectively. Shiflet et al. [28] reported the following values: wα = 8373 J/mol, xs ∆Hα = 112206 J/mol, and ∆S = 51.46 J/(mol K). α · The chemical potentials of iron and carbon in austenite satisfy the Gibbs–Duhem equation + = γ (x1d ln a1 x2d ln a2 0). Thus, aFe can be obtained by integrating Equation (6):
Z x γ xγ 1 1 zγxγ γ = ( γ ) = ( − ) ln aFe d ln aC ln (8) Materials 2020, 13, x FOR PEER REVIEW− 1 xγ zγ 1 1 xγ 13 of 19 0 − − − α Similarly, aFe can be obtained byx the definite integration of the Gibbs–Duhem equation: x3 3 z x lnaFe d (ln a C ) ln[ ] . (9) Z0xα 1x z 3 3(1 x ) α xα α 3 3 zαxα ln aFe = d(ln aC) = ln[ − ]. (9) − 1 xα zα 3 3(1 xα) The activities determined by Equations0 − (6–9) can be −substituted− into Equation (5) to obtain the driving force for the decomposition of austenite to martensite within the KRC model [24,25]: The activities determined by Equations (6)–(9) can be substituted into Equation (5) to obtain the RT driving force forG the decomposition of[( austenite z 1)(3 to martensite z x )ln(3 within z x the ) KRC model [24,25]: (zz 3)( 1) γ α RT ∆G → = [(zγ 1)(3 zαxγ) ln(3 zαxγ) (zα 3)(zγ 1) − − − (10) (z 3)(1 z x )ln(1 − z x− ) ( z 3 z )(1 x )ln(1 x ) (zα 3 )( 1 zγxγ) ln ( 1 zγx γ) + (z α 3zγ )(1 xγ) ln (1 xγ) (10) − − − − γ α − − −xs xs ( )( ) ] + ( ) → + [ ( xs) xs] 3 zγ 1 1 xγ ln 3 1 xγ ∆GFe xγ ∆Hα ∆Hγ ∆Sα ∆Sγ T −3(z − 1)(1− x )ln 3] (1− x ) GFe x [ H− H− ( S− S ) T ] FigureFigure 11 11 shows shows the the driving driving force force for for the the phase phase transformation transformation of of austenite austenite into into martensite martensite as as a a functionfunction of of temperature temperature and and the carbon the carbon content. content. For a fixed For a carbon fixed content, carbon the content, driving the force driving gradually force increasedgradually with increased a decreasing with temperature. a decreasing At temperature. a fixed temperature, At a fixed the driving temperature, force gradually the driving decreased force withgradually an increasing decreased carbon with content. an increasing In brief, carbon low-carbon content and. In low-temperature brief, low-carbon conditions and low- favoredtemperature the transformationconditions favored of retained the transformation austenite into of martensite. retained austenite into martensite.
-1 -6000
mol -173 °C
J -73 °C -5000 27 °C
-4000 127 °C 160 °C 227°C -3000 327 °C -2000
-1000
Driving force for austenite to martensite / 0.0 0.2 0.4 0.6 0.8 Carbon Content/%
Figure 11. Driving force for the phase transformation of austenite into martensite. Figure 11. Driving force for the phase transformation of austenite into martensite. The thermodynamic calculation results showed that the driving force for the phase transformation into martensite was higher for austenite with a low-carbon content than for austenite with a high-carbon The thermodynamic calculation results showed that the driving force for the phase content. As carbon is an austenite-forming element, a high content of solid-solution carbon leads transformation into martensite was higher for austenite with a low-carbon content than for austenite to a more stable austenite and a low driving force for the transformation into martensite. Therefore, with a high-carbon content. As carbon is an austenite-forming element, a high content of solid- solution carbon leads to a more stable austenite and a low driving force for the transformation into martensite. Therefore, after low-temperature tempering, some of the low-carbon retained austenite is likely to transform into martensite, whereas high-carbon retained austenite is more thermally stable than low-carbon retained austenite and is retained at room temperature. Compared with that of low-carbon retained austenite, the calculated driving force for the phase transformation of high-carbon retained austenite only changed slightly with temperature, that is, the high-carbon retained austenite was less affected by temperature. As the temperature decreases, the increase in the carbon content of the carburized layer microstructure inhibits the transformation of the retained austenite into martensite. Ishida [29] developed an empirical equation for determining the starting temperature Ms of the austenite into martensite transformation, as shown in Equation (11):
Ms (°C, wt%) = 545-330C + 2Al + 7Co-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si + 13Ti + 4V + 0W (11)
Materials 2020, 13, 2352 12 of 17 after low-temperature tempering, some of the low-carbon retained austenite is likely to transform into martensite, whereas high-carbon retained austenite is more thermally stable than low-carbon retained austenite and is retained at room temperature. Compared with that of low-carbon retained austenite, the calculated driving force for the phase transformation of high-carbon retained austenite only changed slightly with temperature, that is, the high-carbon retained austenite was less affected by temperature. As the temperature decreases, the increase in the carbon content of the carburized layer microstructure inhibits the transformation of the retained austenite into martensite. Ishida [29] developed an empirical equation for determining the starting temperature Ms of the austenite into martensite transformation, as shown in Equation (11):
Ms (◦C, wt%) = 545-330C + 2Al + 7Co-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si + 13Ti + 4V + 0W (11) Materials 2020, 13, x FOR PEER REVIEW 14 of 19 Using Equation (11), the Ms of the 18Cr2Ni4WA steel core microstructure with 0.2% carbon was Using Equation (11), the Ms of the 18Cr2Ni4WA steel core microstructure with 0.2% carbon was 389.7 C; and the Ms of the outermost surface of the 18Cr2Ni4WA steel carburized layer with 0.67% 389.7 °C;◦ and the Ms of the outermost surface of the 18Cr2Ni4WA steel carburized layer with 0.67% carbon was approximately 234.6 C, corresponding to a difference of 155.1 C. The solid-solution carbon was approximately 234.6 °C,◦ corresponding to a difference of 155.1 °C.◦ The solid-solution carbon content in the matrix increases with the carbon content in the layer. Thus, the Ms transformation carbon content in the matrix increases with the carbon content in the layer. Thus, the Ms temperature is correspondingly lower, and the retained austenite has a strong thermal stability and is transformation temperature is correspondingly lower, and the retained austenite has a strong thermal not likely to transform into martensite. stability and is not likely to transform into martensite. 4.3. Effect of Cryogenic Treatments on the Microstructure and Case Hardness 4.3. Effect of Cryogenic Treatments on the Microstructure and Case Hardness The mechanical and thermal stability of the retained austenite was further investigated by introducingThe mechanic a 73al C and cryogenic thermal treatment stability of between the retained quenching austenite and waslow-temperature further investigated tempering, by − ◦ introducingas shown in a Figure −73 °C2b. cryogenic treatment between quenching and low-temperature tempering, as shownFigure in Figure 12 shows 2b. the SEM images of the structural morphology of the carburized layer at a 0.3 mmFigure depth 12 shows after the SEM cryogenic images treatment. of the structural The morphology morphology of of the the carburized carburized layer layer after at a the0.3 mmcryogenic depth treatmentafter the cryogenic did not obviously treatment. change The morphology from the quenched of the carburized microstructure layer shownafter the in cryogenic Figure8a, treatmentand the block-shaped did not obviously and lath-shaped change from austenite the quenched structures microstructure were distributed shown in thein Figure martensite 8a, and matrix. the blockAfter- low-temperatureshaped and lath- tempering,shaped aus thetenite carbon structures content were of the distributed retained austenite in the decreasedmartensite and matrix. needle-like After lowand-temperature lath-like martensite tempering, structures the carbon were content clearly of observable. the retained There austenite was a decreased negligible and quantity needle of-like fine andcarbides lath- inlike the martensite microstructure. structures were clearly observable. There was a negligible quantity of fine carbides in the microstructure.
FigureFigure 12. 12. SEMSEM images images of ofthe the morphology morphology of the of thecarburized carburized layer layer at a at0.3a mm 0.3 depth mm depth for process for process 2: (a) after2: (a )the after quenching the quenching + cry+ogeniccryogenic treatment; treatment; and and(b) after(b) after the thequenching quenching + cry+ cryogenicogenic treatment treatment + +tempering.tempering.
FigureFigure 1313 shows the the hardness hardness gradient gradient of of the the carburized carburized layer layer under under the the cryogenic cryogenic treatment. treatment. After cryogenic treatment at 73 C, the maximum hardness value of the surface of the carburized layer After cryogenic treatment at− −73◦ °C, the maximum hardness value of the surface of the carburized layerwas 689 was HV 689 and HV the and hardness the hardness of the coreof the was core 545 was HV. 545 After HV. low-temperature After low-temperature tempering, tempering, the surface the surface hardness of the sample carburized layer was 603 HV, the maximum hardness was 621 HV at a 0.2 mm depth, and the core hardness was 443 HV. After cryogenic treatment, the hardness of the carburized layer did not change and the hardness of the carburized layer decreased considerably after low-temperature tempering.
Materials 2020, 13, 2352 13 of 17
hardness of the sample carburized layer was 603 HV, the maximum hardness was 621 HV at a Materials0.2 mm 2020 depth,, 13, x FOR and PEER the REVIEW core hardness was 443 HV. After cryogenic treatment, the hardness15 ofof 19 the carburizedMaterials 2020 layer, 13, x didFOR notPEER change REVIEW and the hardness of the carburized layer decreased considerably15 after of 19 low-temperature tempering. 750 After quenching 750 After cryogenic treatment After quenching 700 After low-temperature tempering After cryogenic treatment 700 After low-temperature tempering 650 650 600
600
550 550 MicrohardnessHV / 500 MicrohardnessHV / 500 450 450 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 Depth of carburized layer / mm Depth of carburized layer / mm FigureFigure 13. 13. HardnessHardness gradient gradient of ofthe the carburized carburized layer layer for for process process 2. 2. Figure 13. Hardness gradient of the carburized layer for process 2. TheThe XRD XRD results results obtained obtained for forthe themicrostructure microstructure of the of carburized the carburized layer layer at a 0.3 at amm 0.3 depth mm depth are shownare shown The in FigureXRD in Figure results 14. Compared14 obtained. Compared for to the the to microstructure the XRD XRD results results afterof the after quenching,carburized quenching, layer the the cryogenic at cryogenic a 0.3 mm treatment treatmentdepth are moderatelymoderatelyshown in increased increasedFigure 14.the the Comparedpeak peak intensity intensity to theof of the the XRD strongest results peakpeak after correspondingcorrespondi quenching,ng theto to (110) (110) cryogenic martensite martensite treatment in in the themicrostructuremoderately microstructure increased and and did the did not peak considerably not considerablyintensity changeof the change strongest the di ff theraction peak diffraction angle.correspondi Thus,angle.ng the Thus, to cryogenic (110) the martensite cryogenic treatment in treatmentpromotedthe microstructure promoted the transformation the and transformation did not of someconsiderably retained of some change austenite retained the into austenite diffraction martensite. in to angle. martensite. After Thus, low-temperature the After cryogenic low- temperaturetempering,treatment thetempering,promoted diffraction the the peaks transformation diffraction of martensite peaks of of (110) some martensite in retained the microstructure (110) austenite in the in shiftedmicrostructureto martensite. to higher shifted angles After lowtoand - higherthetemperature distinct angles diand tempering,ffraction the distinct peak the diffraction of diffraction the austenite peak peaks of (111) theof martensite crystalaustenite plane (111) (110) indicated crystal in the plane thatmicrostructure theindicated carbon that shifted atoms the in to carbonmartensitehigher atoms angles di inff usedmartensiteand the into distinct the diffused retained diffraction into austenite. the peak retained of the austenite. austenite (111) crystal plane indicated that the carbon atoms in martensite diffused into the retained austenite.
After quenching After cryogenic treatment After quenching After low-temperature tempering After cryogenic treatment After low-temperature tempering
(110) α (110)
(111) γ (111)
(311) γ (311)
(110) α (110)
(002) α (002)
(200) γ (200)
(220) α (220)
(220) γ (220)
(121) α (121)
(111) γ (111)
(311) γ (311)
(002) α (002)
(200) γ (200)
(220) α (220)
(220) γ (220)
(121) α (121)
Intensity
Intensity
40 50 60 70 80 90 100 40 50 60 2θ / (o70) 80 90 100 o 2θ / ( ) Figure 14. XRD patterns of the carburized layer at a 0.3 mm depth for process 2. Figure 14. XRD patterns of the carburized layer at a 0.3 mm depth for process 2. Figure 14. XRD patterns of the carburized layer at a 0.3 mm depth for process 2. The volume fraction and carbon content of the retained austenite were calculated by substituting the XRDThe data volume into fraction Equations and (carbon1) and content(2). The of resultsthe retained obtained austenite for thewere different calculated treatments by substituting are comparedthe XRD in dataFigures into 15 Equations and 16. (1) and (2). The results obtained for the different treatments are comparedThe retained in Figures austenite 15 and content 16. was reduced from 24.91% to 22.29% by the cryogenic treatment and furtherThe retainedreduced toaustenite 16.37% contentby the low was-temperature reduced from tempering. 24.91% to Thus, 22.29% the bycryogenic the cryogenic treatment treatment had noand apparent further effectreduced on to the 16.37% martensitic by the low transform-temperatureation tempering. and much Thus, more the retained cryogenic austenite treatment was had no apparent effect on the martensitic transformation and much more retained austenite was
Materials 2020, 13, x FOR PEER REVIEW 16 of 19 transformed by low-temperature tempering than by the cryogenic treatment. In addition to the Materialstemperature, 2020, 13 other, x FOR factors,PEER REVIEW such as stress, affected the transformation of the retained austenite.16 of 19A large quantity of martensite was generated during the quenching process (after carburizing) and the transformedresulting volume by low expansion-temperature generate temperingd a compressive than by stress the cryogenicon the untransformed treatment. Inretained addition austenite, to the temperature,suppressing further other factors, transformation. such as stress,A higher affect drivinged the force transformation for the phase of tra thensformation retained austenite. is required A largeto overcome quantity this of mcompressiveartensite wa stresss generated to transform during thethe retainedquenching austenite process in(afterto martensite carburizing) during and the resultingcryogenic volume treatment. expansion Therefore, generate evend a ifcompressive the temperature stress on reaches the untransformed the martensitic retained transformation austenite, suppressingtemperature, further the compressive transformation. stress Ain higherhibits the driving complete force transformation for the phase tra ofnsformation the retained is austenite. required toThe overcome thermodynamic this compressive calculation stress results to transform in Figure the 11 retained show that austenite the driving into martensite force for duringthe phase the cryogenictransformation treatment. of the retained Therefore, austenite even ifwith the 0.6% temperature carbon, increased reaches thefrom martensitic −3139 J/mol transformation to −3697 J/mol temperature,when the temperature the compressive dropped stress from in hibits room temperaturethe complete (27transformation °C) to −73 °C. of the The retained magnitude austenite. of the Theincrease thermodynamic in the driving calculation force was results not large in Figure and there 11 show was no that clear the promotion driving force of the for martensiticthe phase transformation ofdue the to retained the temperature austenite drop.with 0.6% As a carbon relatively, increased low quant fromity − 3139of the J/mol retained to −3697 austenite J/mol whentransformed the temperature (as shown in dropped Figure 15) from, the room volume temperature fraction of (27 martensite °C) to −73 did °C. not The change magnitude substantially, of the Materials 2020, 13, 2352 14 of 17 increaseand the inhardness the driving did not force significantly was not large increase. and there The washardness no clear of the promotion carburized of the layer martensitic was not transformationconsiderably improved due to the after temperature the cryogenic drop. treatment As a relatively, as shown low in quantFigureity 13 of. the retained austenite transformedThe volume (as shown fraction in Figure and carbon 15), the content volume of the fraction retained of martensite austenite were did not calculated changeby substantially, substituting andthe XRD the hardness data into Equations did not35 significantly (1) and (2). The increase. results The obtained hardness for the of di thefferent carburized treatments layer are compared was not After quenching considerablyin Figures 15 improved and 16. after the cryogenic treatment, as After shown cryogenic in treatment Figure 13. 30 After low-temperature tempering 35 24.91 24.91 25 After quenching After cryogenic22.29 treatment 30 After low-temperature tempering 20 17.66 24.91 24.91 16.37
25 15 22.29
20 10 17.66 16.37
15 5
Without cryogenic treatment cryogenic Without Volume fraction of residual austenite /% residual of fraction Volume 10 0 Process 1 Process 2 5
Without cryogenic treatment cryogenic Without
Volume fraction of residual austenite /% residual of fraction Volume 0 Figure 15. Volume fraction of the Processretained 1 austenite in the carburizedProcess 2 layer at a 0.3 mm depth.
Figure 15. Volume fraction of the retained austenite in the carburized layer at a 0.3 mm depth.
Figure 15. Volume fraction of the retained austenite in the carburized layer at a 0.3 mm depth. After quenching After cryogenic treatment 0.8 After low-temperature tempering
0.74 After quenching After cryogenic treatment 0.71 0.8 After low-temperature tempering 0.7 0.67 0.66 0.66
0.74 0.71 0.7 0.6 0.67 0.66 0.66
Without cryogenic treatment cryogenic Without
Carbon content in retained austenite /% retained in content Carbon 0.6 0.5 Process 1 Process 2
Without cryogenic treatment cryogenic Without Figure 16. Carbonaustenite /% retained in content Carbon content of the retained austenite in the carburized layer at a 0.3 mm depth. Figure 16. Carbon content0.5 of the retained austenite in the carburized layer at a 0.3 mm depth. The retained austenite content wasProcess reduced 1 from 24.91% toProcess 22.29% 2 by the cryogenic treatment and further reduced to 16.37% by the low-temperature tempering. Thus, the cryogenic treatment had no apparentFigure effect 16. on Carbon the martensitic content of transformationthe retained austenite and muchin the carburized more retained layer austeniteat a 0.3 mm was depth. transformed by low-temperature tempering than by the cryogenic treatment. In addition to the temperature, other factors, such as stress, affected the transformation of the retained austenite. A large quantity of martensite was generated during the quenching process (after carburizing) and the resulting volume expansion generated a compressive stress on the untransformed retained austenite, suppressing further transformation. A higher driving force for the phase transformation is required to overcome this compressive stress to transform the retained austenite into martensite during the cryogenic treatment. Therefore, even if the temperature reaches the martensitic transformation temperature, the compressive stress inhibits the complete transformation of the retained austenite. The thermodynamic calculation results in Figure 11 show that the driving force for the phase transformation of the retained austenite with 0.6% carbon, increased from 3139 J/mol to 3697 J/mol when the temperature dropped from − − room temperature (27 C) to 73 C. The magnitude of the increase in the driving force was not large ◦ − ◦ and there was no clear promotion of the martensitic transformation due to the temperature drop. As a relatively low quantity of the retained austenite transformed (as shown in Figure 15), the volume Materials 2020, 13, 2352 15 of 17 fraction of martensite did not change substantially, and the hardness did not significantly increase. The hardness of the carburized layer was not considerably improved after the cryogenic treatment, as shown in Figure 13. The cryogenic treatment did not have a dramatic effect on the carbon content in the retained austenite, which increased from 0.66% to 0.67%. However, the low-temperature tempering significantly increased the carbon content in the retained austenite to 0.71%, indicating a considerable diffusion of carbon from the martensite into the retained austenite during tempering. The content of solid-solution carbon in martensite was reduced, the volume distortion was lessened and the martensite contribution to the hardness was significantly reduced. At the same time, the compressive stress on the retained austenite was reduced and the mechanical stability of the retained austenite decreased. Thus, as discussed in Section 4.2, some of the retained austenite with a low-carbon content transformed into martensite during the cooling period after tempering, since the high-carbon retained austenite was more thermally stable than the low-carbon retained austenite. Thus, the retained austenite with a high-carbon content was measured in the microstructure after tempering, as shown in Figure 16. Therefore, the mechanical stability—not the thermal stability—of the retained austenite in the carburized layer dominated and the low-temperature tempering effectively promoted the transformation of the retained austenite. The changes in the martensite matrix hardness had a far greater effect than the transformation of the retained austenite into martensite on the microhardness of the carburized layer.
5. Conclusions 1. After carburizing and quenching, the carbon content of the carburized layer decreased gradually and evenly as the depth of the carburized layer increased. The matrix microstructure changed from high-carbon acicular martensite in the surface layer into low-carbon lath martensite in the core and the microhardness gradually decreased. 2. Calculating the driving force for the phase transformation of austenite showed that low-carbon and low-temperature conditions favored the transformation of retained austenite to martensite. Therefore, some low-carbon retained austenite was more likely to undergo martensitic transformation after low-temperature tempering. 3. During low-temperature tempering, the solid-solution carbon content of martensite decreased, the compressive stress on the retained austenite was reduced and the mechanical stability of the retained austenite decreased. Low-temperature tempering, rather than the cryogenic treatment, effectively promoted the transformation of the retained austenite.
Author Contributions: Conceptualization, B.W. and Y.T.; methodology, Y.H. and Y.L.; software, Y.T.; validation, B.W., Y.T. and Z.W.; formal analysis, Y.H.; investigation, Y.L. and Y.H.; resources, B.W.; data curation, Y.L.; writing—original draft preparation, B.W. and Y.H.; writing—review and editing, Z.W. and G.W.; visualization, Y.H.; supervision, Z.W. and G.W.; project administration, Z.W. and J.Y.; funding acquisition, J.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was financially supported by the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2017YFB0305301), Open Project of the State Key Laboratory of Advanced Special Steel, Shanghai Key Laboratory of Advanced Ferrometallurgy, Shanghai University (SKLASS 2019-02), and Science and Technology Research Projects of CNPC (No. 2019B-4014 and No.2018E-2101). Conflicts of Interest: The authors declare no conflict of interest.
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