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The reinforcement role of deep cryogenic treatment on the strength and toughness of alloy structural steel
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The reinforcement role of deep cryogenic treatment on the strength and toughness of alloy structural steel Zeju Weng a, b, Kaixuan Gu a, *, Kaikai Wang a, Xuanzhi Liu a, b, Junjie Wang a, b a b
CAS Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Beijing, 100190, China University of Chinese Academy of Sciences, Beijing, 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Deep cryogenic treatment Inter-critical quenching 30CrMnSi alloy structural steel Ferrite refinement Mechanical properties
Deep cryogenic treatment (DCT) is believed to have superior performance in coordination with traditional heat treatment. In present work, the effect of DCT combined with inter-critical quenching treatment on the me chanical properties and microstructure of 30CrMnSi alloy structural steel was investigated. Before DCT employed, quenching treatment was conducted at different austenitizing temperatures (780 � C, 820 � C and 900 � C) to obtain diverse initial states of microstructure. The influence of different sequence between DCT and tempering was also studied. The results showed that compared to quenching-tempering (QT) treatment, the impact toughness of 30CrMnSi alloy austenitized at all the three temperatures was improved by conducting DCT after quenching and tempering (QTC). The strength and toughness were concurrently improved by quenchingtempering-cryogenic (QTC) treatment at the austenitizing temperature of 820 � C. Microstructural character izations were carried out by optical microscopy (OM), scanning electron microscopy (SEM), transmission elec tron microscopy (TEM), X-ray diffraction (XRD) and electron backscattered diffraction (EBSD). Strip ferrite and lath martensite were refined, as well as high-angle grain boundaries increased after 820-QTC. Deep cryogenic treatment has also promoted the dispersed precipitation of stable carbides in the boundaries of ferrite and martensite, through constricting the martensite lattice and increasing the nucleation energy under ultra-low temperature.
1. Introduction Deep cryogenic treatment (DCT) is the process of subjecting mate rials at ultra-low temperature (generally below 100 � C) for certain time to optimize the service performance through changing the micro structure irreversibly. It has been well proved that deep cryogenic treatment is beneficial to the mechanical properties [1], wear resistance [2] and dimensional stability [3] of ferrous metals, including tool steels [4], carburized steels [5], alloy structural steels [6] and stainless steels [7]. Benefiting from the progress of cryogenic technology and test methods, the application of deep cryogenic treatment has been extended to nonferrous metals and compound materials [8–11]. The mechanisms of deep cryogenic treatment on steels are discussed as well in the re searches. It has been confirmed that the transformation of retained austenite into martensite and the precipitation of ultra-fine carbides are the main two reasons that induce the improvement of performance [12–15]. The process of deep cryogenic treatment is the critical factor that
dictates the ultimate effects. It is well known that deep cryogenic treatment must be integrated with traditional heat treatment process routes. As a complementary process of conventional heat treatment, deep cryogenic treatment can remit the defects in microstructure after quenching, and further homogenize and stabilize the microstructure. Most existing studies on deep cryogenic treatment of steels have mainly focused on its combination with quenching and tempering. Many re searchers have claimed that deep cryogenic treatment should be per formed directly after quenching and succeeded by progressive tempering in order to promote the transformation of retained austenite as completely as possible [16,17]. However, an ordinary Quenching-Cryogenic-Tempering treatment is not able to satisfy the performance demands of different kinds of steels. For some steels, due to the low volume fraction of retained austenite after quenching, conduc tion of cryogenic treatment after quenching and prior to tempering (QCT) can achieve little benefit compared to the conduction of which after quenching and tempering (QTC) [18]. Preciado [19] also revealed that low temperature tempering prior to deep cryogenic treatment
* Corresponding author. E-mail address: [email protected] (K. Gu). https://doi.org/10.1016/j.msea.2019.138698 Received 20 September 2019; Received in revised form 14 November 2019; Accepted 15 November 2019 Available online 16 November 2019 0921-5093/© 2019 Published by Elsevier B.V.
Please cite this article as: Zeju Weng, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2019.138698
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increased the wear resistance and hardness of carburized steel. Furthermore, different kinds of heat treatments are usually necessary for various steels to obtain optimal microstructure and properties. Studies on the combination of DCT with novel heat treatment schemes have attracted a lot of attentions in recent years. For example, the elimination of blocky retained austenite and more interfaces caused by Quenching-Partitioning-Cryogenic-Tempering treatment (QPCT) would make contribution to the toughness with strength increase slightly in low carbon steel [20]. Yan [21] et al. proposed a novel process con sisting of deep cryogenic treatment and inter-critical annealing (DCT þ IA), whose results showed that superior tensile strength and total elon gation of new medium-Mn steel were obtained. The conduction of deep cryogenic treatment combined with the newly developed quenching, lamellarizing and tempering treatment of 9%Ni improved both the room temperature impact toughness and cryogenic ductility, as a result of increasing the volume fraction of reversed austenite [16]. The low alloy structural 30CrMnSi steel is widely used in manufacturing important components including sprocket wheels, shafts, clutches, etc. that are subjected to high speed and load conditions [22]. Inter-critical quenching has been considered as an effective method for the toughening of hypoeutectoid steels [23]. This process indicates that steel is heated to preserve at two-phase region between the temperature of Ac1 and Ac3 for thermal insulation for a certain time, and then quenched to room temperature. However, even though inter-critical quenching can elevate the toughness of steel in some extent, the decline of strength is unavoidable. Reports showed that there were lath martensite, dispersed ferrite and retained austenite remained in the microstructure of 30CrMnSi after inter-critical quenching treatment [24]. Many studies have utilized the deep cryogenic treatment on the complete austenitizing steel, while research about deep cryogenic treatment conducted on the alloy structural steel with coexistence of ferrite and martensite after inter-critical quenching treatment is rare relatively. As mentioned above, a significant number of studies have shown the good effects and wide applications of deep cryogenic treatment. How ever, deep cryogenic treatment must cooperate with traditional heat treatment for optimal treating effects. Nowadays some advanced heat treatments, including quenching-partitioning-cryogenic-tempering treatment (QPCT) and deep cryogenic treatment combined with intercritical annealing (DCT þ IA), have brought excellent effects on the mechanical properties of steels. However, studies on the deep cryogenic treatment combined with inter-critical quenching, an advanced heat treatment that benefits the toughness of hypoeutectoid steels, are still inadequate. Therefore, the present work is devoted to investigate the effects of deep cryogenic treatment on the properties and microstructure of 30CrMnSi alloy structural steel. Deep cryogenic treatment is combined with different austenitization temperatures. Moreover, the combination of deep cryogenic treatment and heat treatment was also studied by exchanging the sequence of DCT and tempering. Various microstructural characterization methods were adopted to reveal the potential mechanisms.
employed for heat treatment (quenching and tempering process). Three austenitizing temperatures of 780 � C, 820 � C and 900 � C were adopted to obtain different microstructure. A program controlled SLX-80 cryogenic system was used for carrying out the deep cryogenic treatment [25]. Specimens were cooled down to 196 � C with the cooling rate of 2 � C/min, and then soaked in liquid nitrogen for 12 h. After that, speci mens were taken out and recovered to room temperature in the atmo sphere. Deep cryogenic treatment was performed by combining with quenching and tempering in different order. In all groups of treatment classified by quenching (austenization) temperature, tempering (560 � C � 2 h) employed after quenching directly, deep cryogenic treatment conducted before and after tempering were named as QT, QTC and QCT, respectively. The detail of experimental processes overall scheme is presented in Fig. 1. After that, specimens were cut along forging direc tion by turning and wire electric discharge machining. The hardness measurement was conducted in SHBRV-187.5 Digital Brinell Rockwell & Vickers Hardness Tester with an error range of �2%. The loading force for the test was 200 N and the duration time was 10 s. Five points were tested for each sample and the average values were recorded. Impact toughness was tested by a JB-30A impact test device with standard Charpy U-notch (CUN) specimens at room temperature, according to the standard of GB/T 229-2007. Three specimens were used for each process, and the average values were calculated as the final result. The MTS-SANS CMT5000 Electronic Universal Tensile Testing Machine was employed for testing the tensile strength, yield strength and elongation of the experimental steels at room temperature. Cylin drical specimens with a gage diameter of 5 mm and a gage length of 25 mm (standard GB/T 228-2002) were used and the strain rate during the process of tensile was 0.001/s. Three samples were tested for each state and average values were obtained. For the purpose of investigating the changes in microstructure, specimens were ground in abrasive papers with meshes from 400 to 2000, and then conducted mechanical polishing. After that, the polished surfaces were etched in a 6% nital solution for 10 s. Optical Microscope (OM) (Olympus BX53 M) and Scanning Electron Microscope (SEM) (Hitachi SU1510) were used for detecting the microstructure morphology. The phase composition of experimental material was characterized via X-ray Diffractometer (XRD) (Rigaku Smartlab, Cu Kα radiation) at a step of 0.02� . Transmission Electron Microscope (TEM) (JEM-2100) was employed for further microstructural examination with lamellar specimens ground to 50 μm and then subjected to two jet thinning method. Specimens after TEM test were also adopted to conduct EBSD measurement with a step size of 0.1 μm using the PHI 710 Scanning Auger Nano-probe. The proportions of grain boundaries and volume fraction of phases were calculated by EBSD quantitative analysis. 3. Results and discussion 3.1. Mechanical properties The microhardness of samples treated by different processes with austenitizing temperature at 780 � C, 820 � C and 900 � C are given in Fig. 2(a). After austenization at 780 � C and 900 � C, QTC and QCT have little effect on the microhardness, and the value decreases slightly after QTC. However, the microhardness of specimens treated by 820-QTC is increased by approximately 8%, compared to the samples treated by 820-QT. Fig. 2(b) and (c ~ e) present the changes in impact toughness and tensile strength of samples treated by different processes, respectively. It can be seen that impact toughness is enhanced by QTC at all austeni tizing temperatures, while the effect of QCT is not as good as QTC. There are some interesting changes occur in the tensile strength. At the aus tenitizing temperature of 780 � C, the tensile strength increases slightly after being treated by QCT, while no abvious change induced by QTC. Tensile strength and impact toughness of samples treated by 820-QCT
2. Experimental details The chemical composition of raw material (30CrMnSi) employed in this work is shown in Table 1. For the purpose of homogenization treatment, the experimental steel was heated to 900 � C for 1 h and then water quenching followed. The DC-B15/13 type high temperature resistance furnace was Table 1 Chemical composition of experimental material (wt. %). C
Si
Mn
Cr
S
P
Fe
0.30
1.08
0.97
1.02
0.032
0.032
Bal.
2
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Fig. 1. Schematic diagram of experimental processes.
are practically identical to those of specimens treated by 820-QT. Con cerns are raised that both impact toughness and tensile strength are improved by 820-QTC, with the increase of 5% and 6% respectively. After austenizatized at 900 � C, tensile strength of QTC and QCT samples are both lower than that of QT samples. It can be also seen that QTC and QCT in all austenization temperature almost have no positive effect on the elongation except for 900-QCT. The above results indicate that inter-critical quenching combined with deep cryogenic treatment has potential to further optimize the performance of 30CrMnSi steel. However, the effects differ with the prior austenization temperature as well as the sequence between tempering and deep cryogenic treatment. Compared to other processes in this work, deep cryogenic treatment following with tempering at the austenization of 820 � C (820-QTC), which can improve the hardness, toughness and strength simultaneously without sacrificing too much ductility, is more beneficial in such cases that two-phase region quenching is used for enhancing the strength and toughness of 30CrMnSi alloy structural steel.
transforms into martensite. After tempering at 520 � C, martensite de composes and ferrite experiences recovery and recrystallization [27]. In order to determine the distribution of different phases in the micro structure, microhardness with the applied load of 20 N is performed in different regions of samples treated by 780-QT. It can be seen from Fig. 4 that the hardness of the white region is lower than the dark region, which indicates that white region mainly consists of ferrite and the dark region mainly consists of tempered sorbite in the OM micrographs. With the supplement of deep cryogenic treatment (QTC and QCT), the dif ference from OM micrographs is that higher volume fraction of precip itated carbides distributes in ferrite in 780-QTC sample, and ferrite in 780-QCT sample is dispersed more homogeneously with smaller size. With the increase of austenitizing temperature into 820 � C, the vol ume fraction of austenite increases while that of ferrite decreases during austenitization. As shown in the optical micrographs, the microstructure of 820-QT sample also consists of ferrite and tempered sorbite. After 820-QTC treatment, the ferrite is refined and disperses among the martensite laths. The 820-QCT treatment also has the effect of refine ment in ferrite. Complete austenitization occurs at 900 � C, therefore the micro structure consists of tempered sorbite and precipitated carbides after quenching and tempering. There is no obvious difference in the OM micrographs of samples treated by 900-QTC and 900-QCT. The microstructure of 30CrMnSi steel treated by different processes is also detected by SEM, and the detail feature of ferrite and precipitated carbides are stated clearly in the micrographs (see Fig. 5). It can be seen that two kinds of ferrite differ on morphology. Ferrite in 780-QT and 820-QT samples is mainly distributed in the shape of long strip, which is call layered δ-ferrite in Ref. [28]. However, most ferrite in 780-QTC and 820-QTC samples is distributed in the shape of acicular with relative small size, respectively. Higher volume content of precipitated carbides is observed within the ferrite of sample treated by 780-QTC and
3.2. Microstructure The main distinction in microstructure after quenching at different temperatures is the contents and morphology of ferrite and martensite. Because of the good hardenability of 30CrMnSi steel, the retained austenite content after quenching is also very low. When subsequent treatments are conducted, the microstructure changes in ferrite and martensite may be the main factor that determine the treating efficiency. The optical micrographs of 30CrMnSi steel treated by different processes are shown in Fig. 3. The AC3 temperature of 30CrMnSi steel is 846 � C [26], therefore some ferrite is retained when holding at inter-critical temperatures of 780 � C. After quenching into water, eutectoid precipitation of this ferrite occurs first, and austenite 3
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Fig. 2. Mechanical properties of samples treated by different processes. (a) hardness, (b) impact toughness, (c ~ e) tensile properties.
dispersed in the phase interface of sample treated by 780-QCT. With the increase of austenitization temperature, the volume frac tion of ferrite in the microstructure of 30CrMnSi steel is decreased. The tendency of refinement in ferrite can also be observed in 820-QTC and 820-QCT samples compared to 820-QT, while the change of precipitates in the matrix is not so obvious. There is also no significant change in the microstructure of samples austenitized at 900 � C. It can be inferred that ferrite in the microstructure of 30CrMnSi alloy has response to deep cryogenic treatment. During the process of tempering, martensite decomposes into the mechanical mixture of ferrite and carbides. However, the recovery and recrystallization of eutectoid ferrite occurs in supersaturated state dur ing the tempering. The solubility of carbon in ferrite is very low, the lattice shrinkage at cryogenic temperature could further reduces the solubility. Thus, DCT after tempering might promote carbides precipi tation through the effect of lattice extrusion in the ferrite under cryo genic environment. The precipitation of carbides in ferrite has the effects of releasing internal stress and improving the impact toughness. However, DCT after quenching and prior to tempering further in creases the distortion energy of metastable martensite and eutectoid
ferrite by lattice contraction, which could provide higher driving force for the decomposition of martensite and recovery of ferrite. As a result, higher volume fraction of carbides exists in the matrix of martensite and refined ferrite is obtained. Therefore, the strength of 30CrMnSi steel is improved by the treatment of QCT while the plasticity is sacrificed to a small extent. X-ray diffraction (XRD) has been employed to examine the phase and crystal structure changes in 820-QT and 820-QTC specimens. It is clear that the diffraction peaks are almost classified as martensite, while no diffraction peaks of austenite can be detected, as shown in Fig. 6(a). It can be further confirmed that the volume fraction of retained austenite is so low. Comparing the (110)M, (200)M and (211)M diffraction peaks of 820-QT and 820-QTC specimens, we can see that diffraction peaks of 820-QTC sample have obvious deviation to the right side, as shown in Fig. 6(b ~ d). According to the Bragg Equation [29] shown below, this deviation is mainly caused by the reduction of lattice parameter of martensite. Therefore, the addition of deep cryogenic treatment may promote the lattice contraction of martensite, which is beneficial to the improvement of strength and toughness.
4
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Fig. 3. OM graphs of samples treated by different processes.
boundaries, which makes contribution to the increase of high-angle grain boundaries (HAGB). More HAGB can make contribution to impede the crack propagation with the accumulation of dislocation in the boundaries, as a result, the tensile strength and toughness would be enhanced. In order to further investigate the mechanism of improvement in strength and toughness by 820-QTC, TEM is employed to investigate the microstructural changes in 820-QT and 820-QTC specimens. Fig. 8(a) exhibits the existence of body-centered cubic (bcc) ferrite in the microstructure of sample treated by 820-QT, which has been confirmed by the selected area electron diffraction. It can be also seen that lath martensite exists alongside the blocky ferrite due to the martensite transformation, and partial ferrite still maintains typical lath structure, as Fig. 8(b) and (c) shown respectively. There are a lot of large blocky ferrite with straight boundaries distributed in the microstructure of samples treated by 820-QT. The existence of that ferrite is not beneficial for the properties of steel because of the decrease in strength and the increase in risk of stress concentration. The microscopic morphologies of 820-QTC specimen in TEM are presented in Fig. 8(d ~ f). It can be seen from Fig. 8(d) that blocky ferrite decreases with the addition of deep cryogenic treatment and in terweaves with lath martensite in approximately parallel. It can be inferred that deep cryogenic treatment after QT would further refine the ferrite grains through lattice contraction under ultra-low temperature. Consequently, higher volume fraction of thin lamellar ferrite grains is detected in the microstructure of 820-QTC specimen. The refinement of undissolved ferrite in hypoeutectoid steel can reduce the risk of stress concentration and restrain the crack propagation, which eventually improves the toughness of material. In addition, the precipitation of carbides can be easily found at the grain boundary of martensite and ferrite in both 820-QT and 820-QTC specimens. It is obvious that higher content of ultrafine carbides is
Fig. 4. Micro-hardness test in the different regions of (a) ferrite and (b) tempered sorbite.
2d sin θ ¼ λ
(1)
.pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d¼a h2 þ k2 þ l2
(2)
where d is the interplanar spacing (nm), θ is the reflection angle between the incident ray and lattice plane (� ), λ is the wavelength of incident ray (nm), a is the lattice parameter (nm). The method of EBSD is adopted to detect the phase distribution of specimens treated by 820-QT and 820-QTC as shown in Fig. 7(a) and (b), respectively. It can be seen that ferrite and lath martensite in the microstructure of QTC sample are finer than that of QT sample, which is also discovered through SEM (Fig. 5). In addition, as presented in Fig. 7 (c), sample treated by QTC has higher fraction of high-angle grain boundaries (>15� ) when compared to QT. Combined with the obser vations from microstructure morphology, it can be inferred that the refinement of ferrite and martensite after QTC could form new 5
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Fig. 5. SEM graphs of samples treated by different processes. S Ferrite: strip ferrite, B Ferrite: blocky ferrite, A Ferrite: acicular ferrite.
Fig. 6. XRD patterns of samples treated by 820-QT and 820-QTC. (a) all diffraction peaks, (b ~ d) reflection of (110)M, (200)M and (211)M, respectively.
distributed in the microstructure of 820-QTC sample as shown in Fig. 8 (e). Fig. 8(f) shows an interesting note that fine acicular ε 0 -carbides can be found in the ferrite matrix, which is in accordance with Gavriljuk’s
report [15] that orthorhombic ε 0 -carbides would be precipitated by implementing deep cryogenic treatment and is rare in the QT specimen. These results reflect that deep cryogenic treatment can promote the 6
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strength are enhanced by QTC and QCT, respectively. Compared to QT treatment, both impact toughness and tensile strength of 30CrMnSi steel can be improved by QTC at the austenitization temperature of 820 � C, while micro-hardness is increased slightly and a little expense is taken in ductility. After complete austenitizing at 900 � C, neither QTC nor QCT has obvious effect on the tensile strength, while QCT can increase the ductility of 30CrMnSi steel significantly. 2) The addition of deep cryogenic treatment may have some effects on the morphology of ferrite. Higher volume fraction of acicular ferrite can be obtained both by the treatment of QTC and QCT at the aus tenitizing temperature of 780 � C. Precipitation of carbides in the matrix of ferrite is induced by the treatment of QTC, while more precipitates in the tempered sorbite and grain boundaries are induced by the treatment of QCT. 3) At the austenitizing temperature of 820 � C, the conduction of deep cryogenic treatment after quenching and tempering also has the ef fect on the refinement in ferrite. The martensite laths also have the tendency of refinement. The precipitation of carbides can be attrib uted to the lattice contraction of martensite and the increase of nucleation energy under cryogenic temperature. The refinement of ferrite and martensite, as well as the further dispersed precipitation of stable carbide particles, make contribution to the simultaneous enhancement of toughness and strength.
Fig. 7. EBSD microstructural graphs of samples treated by (a) 820-QT and (b) 820-QTC; (c) proportion of high-angle (>15� ) grain boundaries (HAGB) calculated by EBSD.
Data availability statement All data generated or analyzed during this study are included in this article.
precipitation of carbide particles and make the carbides become more stable, which could be attributed to the increase of nucleation energy in the grain boundary of martensite and ferrite caused by deep cryogenic treatment.
Author contribution statement Zeju Weng: Conceptualization, Methodology, Investigation, Data Curation, Writing - Original Draft. Kaixuan Gu: Conceptualization, Methodology, Investigation, Writing - Review & Editing, Supervision, Project administration. Kaikai Wang: Conceptualization, Investigation, Writing - Review & Editing. Xuanzhi Liu: Software, Visualization. Junjie Wang: Resources, Supervision, Project administration.
4. Conclusions In the present work, the effects of inter-critical quenching combined with deep cryogenic treatment on the properties and microstructure of 30CrMnSi alloy structural steel were studied. The main conclusions are shown in the below. 1) Deep cryogenic treatment can be well combined with inter-critical quenching treatment, however, the treatment efficiency has close contacts with the austenitizing temperature, as well as the execution order between deep cryogenic treatment and tempering. At the austenitizing temperature of 780 � C, impact toughness and tensile
Declaration of competing interest The authors declare that they have no known competing financial
Fig. 8. TEM graphs of samples treated by (a ~ c) 820-QT and (d ~ f) 820-QTC. F: ferrite, M: martensite. 7
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interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (grant number 51805521) and Youth Innovation Promotion Association of CAS (grant number 2016021). References [1] A. Bensely, D. Senthilkumar, D.M. Lal, G. Nagarajan, A. Rajadurai, Effect of cryogenic treatment on tensile behavior of case carburized steel-815M17, Mater. Char. 58 (2007) 485–491. [2] K.K. Wang, Z.L. Tan, K.X. Gu, B. Gao, G.H. Gao, R.D.K. Misra, B.Z. Bai, Effect of deep cryogenic treatment on structure-property relationship in an ultrahigh strength Mn-Si-Cr bainite/martensite multiphase rail steel, Mater. Sci. Eng. A 684 (2017) 559–566. [3] D. Senthilkumar, I. Rajendran, M. Pellizzari, Juha Siiriainen, Influence of shallow and deep cryogenic treatment on the residual state of stress of 4140 steel, J. Mater. Process. Technol. 211 (2011) 396–401. [4] S. Zhirafar, A. Rezaeian, M. Pugh, Effect of cryogenic treatment on the mechanical properties of 4340 steel, J. Mater. Process. Technol. 186 (2007) 298–303. [5] P. Baldissera, C. Delprete, Effects of deep cryogenic treatment on static mechanical properties of 18NiCrMo5 carburized steel, Mater. Des. 30 (2009) 1435–1440. [6] S. Ramesh, B. Bhuvaneshwari, G.S. Palani, D.M. Lal, K. Mondal, R.K. Gupta, Enhancing the corrosion resistance performance of structural steel via a novel deep cryogenic treatment process, Vacuum 159 (2019) 468–475. [7] P.J. Singh, S.L. Mannan, T. Jayakumar, D.R.G. Achar, Fatigue life extension of notches in AISI 304L weldments using deep cryogenic treatment, Eng. Fail. Anal. 12 (2005) 263–271. [8] K.X. Gu, B. Zhao, Z.J. Weng, K.K. Wang, H.K. Cai, J.J. Wang, Microstructure evolution in metastable β titanium alloy subjected to deep cryogenic treatment, Mater. Sci. Eng. A 723 (2018) 157–164. [9] K. Amini, A. Akhbarizadeh, S. Javadpour, Investigating the effect of quench environment and deep cryogenic treatment on the wear behavior of AZ91, Mater. Des. 54 (2014) 154–160. [10] G.H. Kumar, H. Mohit, R. Purohit, Effect of deep cryogenic treatment on composite material for automotive ac system, Mater. Today: Proceedings 4 (2017) 3501–3505. [11] Y. Jiang, D. Chen, Effect of cryogenic treatment on WC-Co cemented carbides, Mater. Sci. Eng. A 528 (2011) 1735–1739. [12] F.J. da Silva, S.D. Franco, A.R. Machado, E.O. Ezugwu, A.M. Souza Jr., Performance of cryogenically treated HSS tools, Wear 261 (2006) 674–685. [13] A.I. Tyshchenko, W. Theisen, A. Oppenkowski, S. Siebert, O.N. Razumov, A. P. Skoblik, V.A. Sirosh, YuN. Petrov, V.G. Gavriljuk, Low-temperature martensitic transformation and deep cryogenic treatment of a tool steel, Mater. Sci. Eng. A 527 (2010) 7027–7039.
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Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
The reinforcement role of deep cryogenic treatment on the strength and toughness of alloy structural steel Zeju Weng a, b, Kaixuan Gu a, *, Kaikai Wang a, Xuanzhi Liu a, b, Junjie Wang a, b a b
CAS Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Beijing, 100190, China University of Chinese Academy of Sciences, Beijing, 100049, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Deep cryogenic treatment Inter-critical quenching 30CrMnSi alloy structural steel Ferrite refinement Mechanical properties
Deep cryogenic treatment (DCT) is believed to have superior performance in coordination with traditional heat treatment. In present work, the effect of DCT combined with inter-critical quenching treatment on the me chanical properties and microstructure of 30CrMnSi alloy structural steel was investigated. Before DCT employed, quenching treatment was conducted at different austenitizing temperatures (780 � C, 820 � C and 900 � C) to obtain diverse initial states of microstructure. The influence of different sequence between DCT and tempering was also studied. The results showed that compared to quenching-tempering (QT) treatment, the impact toughness of 30CrMnSi alloy austenitized at all the three temperatures was improved by conducting DCT after quenching and tempering (QTC). The strength and toughness were concurrently improved by quenchingtempering-cryogenic (QTC) treatment at the austenitizing temperature of 820 � C. Microstructural character izations were carried out by optical microscopy (OM), scanning electron microscopy (SEM), transmission elec tron microscopy (TEM), X-ray diffraction (XRD) and electron backscattered diffraction (EBSD). Strip ferrite and lath martensite were refined, as well as high-angle grain boundaries increased after 820-QTC. Deep cryogenic treatment has also promoted the dispersed precipitation of stable carbides in the boundaries of ferrite and martensite, through constricting the martensite lattice and increasing the nucleation energy under ultra-low temperature.
1. Introduction Deep cryogenic treatment (DCT) is the process of subjecting mate rials at ultra-low temperature (generally below 100 � C) for certain time to optimize the service performance through changing the micro structure irreversibly. It has been well proved that deep cryogenic treatment is beneficial to the mechanical properties [1], wear resistance [2] and dimensional stability [3] of ferrous metals, including tool steels [4], carburized steels [5], alloy structural steels [6] and stainless steels [7]. Benefiting from the progress of cryogenic technology and test methods, the application of deep cryogenic treatment has been extended to nonferrous metals and compound materials [8–11]. The mechanisms of deep cryogenic treatment on steels are discussed as well in the re searches. It has been confirmed that the transformation of retained austenite into martensite and the precipitation of ultra-fine carbides are the main two reasons that induce the improvement of performance [12–15]. The process of deep cryogenic treatment is the critical factor that
dictates the ultimate effects. It is well known that deep cryogenic treatment must be integrated with traditional heat treatment process routes. As a complementary process of conventional heat treatment, deep cryogenic treatment can remit the defects in microstructure after quenching, and further homogenize and stabilize the microstructure. Most existing studies on deep cryogenic treatment of steels have mainly focused on its combination with quenching and tempering. Many re searchers have claimed that deep cryogenic treatment should be per formed directly after quenching and succeeded by progressive tempering in order to promote the transformation of retained austenite as completely as possible [16,17]. However, an ordinary Quenching-Cryogenic-Tempering treatment is not able to satisfy the performance demands of different kinds of steels. For some steels, due to the low volume fraction of retained austenite after quenching, conduc tion of cryogenic treatment after quenching and prior to tempering (QCT) can achieve little benefit compared to the conduction of which after quenching and tempering (QTC) [18]. Preciado [19] also revealed that low temperature tempering prior to deep cryogenic treatment
* Corresponding author. E-mail address: [email protected] (K. Gu). https://doi.org/10.1016/j.msea.2019.138698 Received 20 September 2019; Received in revised form 14 November 2019; Accepted 15 November 2019 Available online 16 November 2019 0921-5093/© 2019 Published by Elsevier B.V.
Please cite this article as: Zeju Weng, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2019.138698
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increased the wear resistance and hardness of carburized steel. Furthermore, different kinds of heat treatments are usually necessary for various steels to obtain optimal microstructure and properties. Studies on the combination of DCT with novel heat treatment schemes have attracted a lot of attentions in recent years. For example, the elimination of blocky retained austenite and more interfaces caused by Quenching-Partitioning-Cryogenic-Tempering treatment (QPCT) would make contribution to the toughness with strength increase slightly in low carbon steel [20]. Yan [21] et al. proposed a novel process con sisting of deep cryogenic treatment and inter-critical annealing (DCT þ IA), whose results showed that superior tensile strength and total elon gation of new medium-Mn steel were obtained. The conduction of deep cryogenic treatment combined with the newly developed quenching, lamellarizing and tempering treatment of 9%Ni improved both the room temperature impact toughness and cryogenic ductility, as a result of increasing the volume fraction of reversed austenite [16]. The low alloy structural 30CrMnSi steel is widely used in manufacturing important components including sprocket wheels, shafts, clutches, etc. that are subjected to high speed and load conditions [22]. Inter-critical quenching has been considered as an effective method for the toughening of hypoeutectoid steels [23]. This process indicates that steel is heated to preserve at two-phase region between the temperature of Ac1 and Ac3 for thermal insulation for a certain time, and then quenched to room temperature. However, even though inter-critical quenching can elevate the toughness of steel in some extent, the decline of strength is unavoidable. Reports showed that there were lath martensite, dispersed ferrite and retained austenite remained in the microstructure of 30CrMnSi after inter-critical quenching treatment [24]. Many studies have utilized the deep cryogenic treatment on the complete austenitizing steel, while research about deep cryogenic treatment conducted on the alloy structural steel with coexistence of ferrite and martensite after inter-critical quenching treatment is rare relatively. As mentioned above, a significant number of studies have shown the good effects and wide applications of deep cryogenic treatment. How ever, deep cryogenic treatment must cooperate with traditional heat treatment for optimal treating effects. Nowadays some advanced heat treatments, including quenching-partitioning-cryogenic-tempering treatment (QPCT) and deep cryogenic treatment combined with intercritical annealing (DCT þ IA), have brought excellent effects on the mechanical properties of steels. However, studies on the deep cryogenic treatment combined with inter-critical quenching, an advanced heat treatment that benefits the toughness of hypoeutectoid steels, are still inadequate. Therefore, the present work is devoted to investigate the effects of deep cryogenic treatment on the properties and microstructure of 30CrMnSi alloy structural steel. Deep cryogenic treatment is combined with different austenitization temperatures. Moreover, the combination of deep cryogenic treatment and heat treatment was also studied by exchanging the sequence of DCT and tempering. Various microstructural characterization methods were adopted to reveal the potential mechanisms.
employed for heat treatment (quenching and tempering process). Three austenitizing temperatures of 780 � C, 820 � C and 900 � C were adopted to obtain different microstructure. A program controlled SLX-80 cryogenic system was used for carrying out the deep cryogenic treatment [25]. Specimens were cooled down to 196 � C with the cooling rate of 2 � C/min, and then soaked in liquid nitrogen for 12 h. After that, speci mens were taken out and recovered to room temperature in the atmo sphere. Deep cryogenic treatment was performed by combining with quenching and tempering in different order. In all groups of treatment classified by quenching (austenization) temperature, tempering (560 � C � 2 h) employed after quenching directly, deep cryogenic treatment conducted before and after tempering were named as QT, QTC and QCT, respectively. The detail of experimental processes overall scheme is presented in Fig. 1. After that, specimens were cut along forging direc tion by turning and wire electric discharge machining. The hardness measurement was conducted in SHBRV-187.5 Digital Brinell Rockwell & Vickers Hardness Tester with an error range of �2%. The loading force for the test was 200 N and the duration time was 10 s. Five points were tested for each sample and the average values were recorded. Impact toughness was tested by a JB-30A impact test device with standard Charpy U-notch (CUN) specimens at room temperature, according to the standard of GB/T 229-2007. Three specimens were used for each process, and the average values were calculated as the final result. The MTS-SANS CMT5000 Electronic Universal Tensile Testing Machine was employed for testing the tensile strength, yield strength and elongation of the experimental steels at room temperature. Cylin drical specimens with a gage diameter of 5 mm and a gage length of 25 mm (standard GB/T 228-2002) were used and the strain rate during the process of tensile was 0.001/s. Three samples were tested for each state and average values were obtained. For the purpose of investigating the changes in microstructure, specimens were ground in abrasive papers with meshes from 400 to 2000, and then conducted mechanical polishing. After that, the polished surfaces were etched in a 6% nital solution for 10 s. Optical Microscope (OM) (Olympus BX53 M) and Scanning Electron Microscope (SEM) (Hitachi SU1510) were used for detecting the microstructure morphology. The phase composition of experimental material was characterized via X-ray Diffractometer (XRD) (Rigaku Smartlab, Cu Kα radiation) at a step of 0.02� . Transmission Electron Microscope (TEM) (JEM-2100) was employed for further microstructural examination with lamellar specimens ground to 50 μm and then subjected to two jet thinning method. Specimens after TEM test were also adopted to conduct EBSD measurement with a step size of 0.1 μm using the PHI 710 Scanning Auger Nano-probe. The proportions of grain boundaries and volume fraction of phases were calculated by EBSD quantitative analysis. 3. Results and discussion 3.1. Mechanical properties The microhardness of samples treated by different processes with austenitizing temperature at 780 � C, 820 � C and 900 � C are given in Fig. 2(a). After austenization at 780 � C and 900 � C, QTC and QCT have little effect on the microhardness, and the value decreases slightly after QTC. However, the microhardness of specimens treated by 820-QTC is increased by approximately 8%, compared to the samples treated by 820-QT. Fig. 2(b) and (c ~ e) present the changes in impact toughness and tensile strength of samples treated by different processes, respectively. It can be seen that impact toughness is enhanced by QTC at all austeni tizing temperatures, while the effect of QCT is not as good as QTC. There are some interesting changes occur in the tensile strength. At the aus tenitizing temperature of 780 � C, the tensile strength increases slightly after being treated by QCT, while no abvious change induced by QTC. Tensile strength and impact toughness of samples treated by 820-QCT
2. Experimental details The chemical composition of raw material (30CrMnSi) employed in this work is shown in Table 1. For the purpose of homogenization treatment, the experimental steel was heated to 900 � C for 1 h and then water quenching followed. The DC-B15/13 type high temperature resistance furnace was Table 1 Chemical composition of experimental material (wt. %). C
Si
Mn
Cr
S
P
Fe
0.30
1.08
0.97
1.02
0.032
0.032
Bal.
2
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Fig. 1. Schematic diagram of experimental processes.
are practically identical to those of specimens treated by 820-QT. Con cerns are raised that both impact toughness and tensile strength are improved by 820-QTC, with the increase of 5% and 6% respectively. After austenizatized at 900 � C, tensile strength of QTC and QCT samples are both lower than that of QT samples. It can be also seen that QTC and QCT in all austenization temperature almost have no positive effect on the elongation except for 900-QCT. The above results indicate that inter-critical quenching combined with deep cryogenic treatment has potential to further optimize the performance of 30CrMnSi steel. However, the effects differ with the prior austenization temperature as well as the sequence between tempering and deep cryogenic treatment. Compared to other processes in this work, deep cryogenic treatment following with tempering at the austenization of 820 � C (820-QTC), which can improve the hardness, toughness and strength simultaneously without sacrificing too much ductility, is more beneficial in such cases that two-phase region quenching is used for enhancing the strength and toughness of 30CrMnSi alloy structural steel.
transforms into martensite. After tempering at 520 � C, martensite de composes and ferrite experiences recovery and recrystallization [27]. In order to determine the distribution of different phases in the micro structure, microhardness with the applied load of 20 N is performed in different regions of samples treated by 780-QT. It can be seen from Fig. 4 that the hardness of the white region is lower than the dark region, which indicates that white region mainly consists of ferrite and the dark region mainly consists of tempered sorbite in the OM micrographs. With the supplement of deep cryogenic treatment (QTC and QCT), the dif ference from OM micrographs is that higher volume fraction of precip itated carbides distributes in ferrite in 780-QTC sample, and ferrite in 780-QCT sample is dispersed more homogeneously with smaller size. With the increase of austenitizing temperature into 820 � C, the vol ume fraction of austenite increases while that of ferrite decreases during austenitization. As shown in the optical micrographs, the microstructure of 820-QT sample also consists of ferrite and tempered sorbite. After 820-QTC treatment, the ferrite is refined and disperses among the martensite laths. The 820-QCT treatment also has the effect of refine ment in ferrite. Complete austenitization occurs at 900 � C, therefore the micro structure consists of tempered sorbite and precipitated carbides after quenching and tempering. There is no obvious difference in the OM micrographs of samples treated by 900-QTC and 900-QCT. The microstructure of 30CrMnSi steel treated by different processes is also detected by SEM, and the detail feature of ferrite and precipitated carbides are stated clearly in the micrographs (see Fig. 5). It can be seen that two kinds of ferrite differ on morphology. Ferrite in 780-QT and 820-QT samples is mainly distributed in the shape of long strip, which is call layered δ-ferrite in Ref. [28]. However, most ferrite in 780-QTC and 820-QTC samples is distributed in the shape of acicular with relative small size, respectively. Higher volume content of precipitated carbides is observed within the ferrite of sample treated by 780-QTC and
3.2. Microstructure The main distinction in microstructure after quenching at different temperatures is the contents and morphology of ferrite and martensite. Because of the good hardenability of 30CrMnSi steel, the retained austenite content after quenching is also very low. When subsequent treatments are conducted, the microstructure changes in ferrite and martensite may be the main factor that determine the treating efficiency. The optical micrographs of 30CrMnSi steel treated by different processes are shown in Fig. 3. The AC3 temperature of 30CrMnSi steel is 846 � C [26], therefore some ferrite is retained when holding at inter-critical temperatures of 780 � C. After quenching into water, eutectoid precipitation of this ferrite occurs first, and austenite 3
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Fig. 2. Mechanical properties of samples treated by different processes. (a) hardness, (b) impact toughness, (c ~ e) tensile properties.
dispersed in the phase interface of sample treated by 780-QCT. With the increase of austenitization temperature, the volume frac tion of ferrite in the microstructure of 30CrMnSi steel is decreased. The tendency of refinement in ferrite can also be observed in 820-QTC and 820-QCT samples compared to 820-QT, while the change of precipitates in the matrix is not so obvious. There is also no significant change in the microstructure of samples austenitized at 900 � C. It can be inferred that ferrite in the microstructure of 30CrMnSi alloy has response to deep cryogenic treatment. During the process of tempering, martensite decomposes into the mechanical mixture of ferrite and carbides. However, the recovery and recrystallization of eutectoid ferrite occurs in supersaturated state dur ing the tempering. The solubility of carbon in ferrite is very low, the lattice shrinkage at cryogenic temperature could further reduces the solubility. Thus, DCT after tempering might promote carbides precipi tation through the effect of lattice extrusion in the ferrite under cryo genic environment. The precipitation of carbides in ferrite has the effects of releasing internal stress and improving the impact toughness. However, DCT after quenching and prior to tempering further in creases the distortion energy of metastable martensite and eutectoid
ferrite by lattice contraction, which could provide higher driving force for the decomposition of martensite and recovery of ferrite. As a result, higher volume fraction of carbides exists in the matrix of martensite and refined ferrite is obtained. Therefore, the strength of 30CrMnSi steel is improved by the treatment of QCT while the plasticity is sacrificed to a small extent. X-ray diffraction (XRD) has been employed to examine the phase and crystal structure changes in 820-QT and 820-QTC specimens. It is clear that the diffraction peaks are almost classified as martensite, while no diffraction peaks of austenite can be detected, as shown in Fig. 6(a). It can be further confirmed that the volume fraction of retained austenite is so low. Comparing the (110)M, (200)M and (211)M diffraction peaks of 820-QT and 820-QTC specimens, we can see that diffraction peaks of 820-QTC sample have obvious deviation to the right side, as shown in Fig. 6(b ~ d). According to the Bragg Equation [29] shown below, this deviation is mainly caused by the reduction of lattice parameter of martensite. Therefore, the addition of deep cryogenic treatment may promote the lattice contraction of martensite, which is beneficial to the improvement of strength and toughness.
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Fig. 3. OM graphs of samples treated by different processes.
boundaries, which makes contribution to the increase of high-angle grain boundaries (HAGB). More HAGB can make contribution to impede the crack propagation with the accumulation of dislocation in the boundaries, as a result, the tensile strength and toughness would be enhanced. In order to further investigate the mechanism of improvement in strength and toughness by 820-QTC, TEM is employed to investigate the microstructural changes in 820-QT and 820-QTC specimens. Fig. 8(a) exhibits the existence of body-centered cubic (bcc) ferrite in the microstructure of sample treated by 820-QT, which has been confirmed by the selected area electron diffraction. It can be also seen that lath martensite exists alongside the blocky ferrite due to the martensite transformation, and partial ferrite still maintains typical lath structure, as Fig. 8(b) and (c) shown respectively. There are a lot of large blocky ferrite with straight boundaries distributed in the microstructure of samples treated by 820-QT. The existence of that ferrite is not beneficial for the properties of steel because of the decrease in strength and the increase in risk of stress concentration. The microscopic morphologies of 820-QTC specimen in TEM are presented in Fig. 8(d ~ f). It can be seen from Fig. 8(d) that blocky ferrite decreases with the addition of deep cryogenic treatment and in terweaves with lath martensite in approximately parallel. It can be inferred that deep cryogenic treatment after QT would further refine the ferrite grains through lattice contraction under ultra-low temperature. Consequently, higher volume fraction of thin lamellar ferrite grains is detected in the microstructure of 820-QTC specimen. The refinement of undissolved ferrite in hypoeutectoid steel can reduce the risk of stress concentration and restrain the crack propagation, which eventually improves the toughness of material. In addition, the precipitation of carbides can be easily found at the grain boundary of martensite and ferrite in both 820-QT and 820-QTC specimens. It is obvious that higher content of ultrafine carbides is
Fig. 4. Micro-hardness test in the different regions of (a) ferrite and (b) tempered sorbite.
2d sin θ ¼ λ
(1)
.pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d¼a h2 þ k2 þ l2
(2)
where d is the interplanar spacing (nm), θ is the reflection angle between the incident ray and lattice plane (� ), λ is the wavelength of incident ray (nm), a is the lattice parameter (nm). The method of EBSD is adopted to detect the phase distribution of specimens treated by 820-QT and 820-QTC as shown in Fig. 7(a) and (b), respectively. It can be seen that ferrite and lath martensite in the microstructure of QTC sample are finer than that of QT sample, which is also discovered through SEM (Fig. 5). In addition, as presented in Fig. 7 (c), sample treated by QTC has higher fraction of high-angle grain boundaries (>15� ) when compared to QT. Combined with the obser vations from microstructure morphology, it can be inferred that the refinement of ferrite and martensite after QTC could form new 5
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Fig. 5. SEM graphs of samples treated by different processes. S Ferrite: strip ferrite, B Ferrite: blocky ferrite, A Ferrite: acicular ferrite.
Fig. 6. XRD patterns of samples treated by 820-QT and 820-QTC. (a) all diffraction peaks, (b ~ d) reflection of (110)M, (200)M and (211)M, respectively.
distributed in the microstructure of 820-QTC sample as shown in Fig. 8 (e). Fig. 8(f) shows an interesting note that fine acicular ε 0 -carbides can be found in the ferrite matrix, which is in accordance with Gavriljuk’s
report [15] that orthorhombic ε 0 -carbides would be precipitated by implementing deep cryogenic treatment and is rare in the QT specimen. These results reflect that deep cryogenic treatment can promote the 6
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strength are enhanced by QTC and QCT, respectively. Compared to QT treatment, both impact toughness and tensile strength of 30CrMnSi steel can be improved by QTC at the austenitization temperature of 820 � C, while micro-hardness is increased slightly and a little expense is taken in ductility. After complete austenitizing at 900 � C, neither QTC nor QCT has obvious effect on the tensile strength, while QCT can increase the ductility of 30CrMnSi steel significantly. 2) The addition of deep cryogenic treatment may have some effects on the morphology of ferrite. Higher volume fraction of acicular ferrite can be obtained both by the treatment of QTC and QCT at the aus tenitizing temperature of 780 � C. Precipitation of carbides in the matrix of ferrite is induced by the treatment of QTC, while more precipitates in the tempered sorbite and grain boundaries are induced by the treatment of QCT. 3) At the austenitizing temperature of 820 � C, the conduction of deep cryogenic treatment after quenching and tempering also has the ef fect on the refinement in ferrite. The martensite laths also have the tendency of refinement. The precipitation of carbides can be attrib uted to the lattice contraction of martensite and the increase of nucleation energy under cryogenic temperature. The refinement of ferrite and martensite, as well as the further dispersed precipitation of stable carbide particles, make contribution to the simultaneous enhancement of toughness and strength.
Fig. 7. EBSD microstructural graphs of samples treated by (a) 820-QT and (b) 820-QTC; (c) proportion of high-angle (>15� ) grain boundaries (HAGB) calculated by EBSD.
Data availability statement All data generated or analyzed during this study are included in this article.
precipitation of carbide particles and make the carbides become more stable, which could be attributed to the increase of nucleation energy in the grain boundary of martensite and ferrite caused by deep cryogenic treatment.
Author contribution statement Zeju Weng: Conceptualization, Methodology, Investigation, Data Curation, Writing - Original Draft. Kaixuan Gu: Conceptualization, Methodology, Investigation, Writing - Review & Editing, Supervision, Project administration. Kaikai Wang: Conceptualization, Investigation, Writing - Review & Editing. Xuanzhi Liu: Software, Visualization. Junjie Wang: Resources, Supervision, Project administration.
4. Conclusions In the present work, the effects of inter-critical quenching combined with deep cryogenic treatment on the properties and microstructure of 30CrMnSi alloy structural steel were studied. The main conclusions are shown in the below. 1) Deep cryogenic treatment can be well combined with inter-critical quenching treatment, however, the treatment efficiency has close contacts with the austenitizing temperature, as well as the execution order between deep cryogenic treatment and tempering. At the austenitizing temperature of 780 � C, impact toughness and tensile
Declaration of competing interest The authors declare that they have no known competing financial
Fig. 8. TEM graphs of samples treated by (a ~ c) 820-QT and (d ~ f) 820-QTC. F: ferrite, M: martensite. 7
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interests or personal relationships that could have appeared to influence the work reported in this paper.
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