Effect of deep cryogenic treatment on structure ...

Materials Science & Engineering A 684 (2017) 559–566

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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea

Effect of deep cryogenic treatment on structure-property relationship in an ultrahigh strength Mn-Si-Cr bainite/martensite multiphase rail steel ⁎

MARK

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Kaikai Wanga, Zhunli Tana, , Kaixuan Gub, , Bo Gaoa, Guhui Gaoa, R.D.K. Misrac, Bingzhe Baia a

Materials Science and Engineering Research Center, Bei Jing Jiaotong University, Bei Jing 100044, China CAS Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Beijing 100190, China c Laboratory for Excellence in Advanced Steel Research, Department of Metallurgical, Materials and Biomedical Engineering, University of Texas at El Paso, 500 W. University Avenue, El Paso, TX 79968-0520, USA b

A R T I C L E I N F O

A BS T RAC T

Keywords: Deep cryogenic treatment Bainite Microstructure Properties

The bainite/martensite (B/M) multiphase microstructure was studied in 0.22C–2.0Mn–1.0Si–0.8Cr–0.8(Mo +Ni) (wt%) bainitic steel subjected to deep cryogenic treatment (DCT) to elucidate the positive effect of DCT on structure and mechanical properties. The study indicates that DCT can improve mechanical properties and wear resistance. It reduces the content of blocky martensite/austenite (M/A) constituents by eliminating unstable retained austenite (RA). At the same time, RA is relatively more enriched in carbon after DCT, compared to the tempering process. During DCT, the brittle martensite is also avoided, since the enhanced recovery reduces the carbon concentration during tempering. Meanwhile, the contraction of unit cell at low temperature promotes the precipitation of fine dispersed carbides and contributes to wear resistance.

1. Introduction Deep cryogenic treatment (DCT) generally means that the material is held at a cryogenic temperature for a given soaking time and then heated to room temperature to improve the mechanical properties [1– 3]. DCT is an important supplementary process of conventional heat treatment to improve the microstructure and mechanical properties [1]. DCT is a cost effective process for improving the mechanical properties. A number of studies were carried out concerning the effect of DCT on the properties of cold-work tools, high-speed steels carburized steels, cast irons and stainless steels [4–9]. DCT can transform austenite into martensite, since martensite finish temperature (Mf) of existing austenite is below the room temperature. Li et al. [3] reported that RA can be transformed into martensite during DCT, which means that DCT can eliminate some unstable RA. It has also been reported that DCT can increase hardness and wear resistance by promoting the formation and homogenous distribution of carbides [10,11]. Under cryogenic temperature, contraction of martensite extrudes carbon atoms into defects to decrease internal stress, and those carbon atoms could provide conditions for carbide nucleation during tempering [12]. Under cryogenic temperature, contraction of martensite ejects carbon atoms to defects and decreases internal stress, and these carbon atoms can contribute to carbide nucleation during tempering [13–16].

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Bainite/martensite multiphase steels are considered potential candidates as next generation steels [17–21]. The excellent combination of strength and toughness of bainitic steels have led to wide sue as structural steels. Bainitic steels have complex microstructures depending on the alloying elements and heat treatment [22–26]. For example, B/M multiphase microstructure is composed of different types of bainite, martensite, retained austenite and carbides [17]. As a metastable phase, retained austenite (RA) exhibits film-like or blocky morphology in B/M multiphase steels, which possesses different stability [27–33]. The stability of RA is closely related to mechanical properties [31–33]. On the other hand, brittle martensite needs to be avoided, especially when strength is enhanced. As mentioned above, the role of DCT on RA and martensite can contribute to B/M multiphase bainitic steels. However, there were few studies on the effect of DCT on bainitic steels. The aim of this research is to study the microstructural evolution and mechanical property changes (including wear resistance) of bainitic steels during DCT, especially the variation of RA. 2. Experimental procedures The experimental bainitic rail steel has nominal chemical composition of 0.22C–2.0Mn–1.0Si–0.8Cr–0.8(Mo+Ni) (wt%). The cooling process of bainitic rail steels was detected by infrared detection gun.

Corresponding author. E-mail addresses: [email protected] (Z. Tan), [email protected] (K. Gu).

http://dx.doi.org/10.1016/j.msea.2016.12.100 Received 14 October 2016; Received in revised form 21 December 2016; Accepted 22 December 2016 Available online 23 December 2016 0921-5093/ © 2016 Elsevier B.V. All rights reserved.


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Fig. 1. Schematic diagram of experimental treatment process.

After austenitization and rolling on an industrial producing line, the cooling process of bainitic rails is shown in Fig. 1a. On cooling at 1 °C/s to a certain temperature (between 360 °C and 400 °C, marked as ‘BT’), it was held at BT for nearly 15 min. Next, the rail steels were cooled to room temperature at a cooling speed of 0.05 °C/s. After cooling to ambient temperature, the as-rolled rail steels were referred as Q, while the rail steels after tempering at 280 °C were referred as QT. Another set of rail steels were cooled to −196 °C at 1 °C/min using SLX-80 cryogenic system. After finish-soaking at −196 °C for 8 h, the rails were warmed up to room temperature (20 °C) at a speed of 1 °C/min and kept for 30 min, and were referred as QCT. Finally, the rail steels were tempered at 280 °C for 4 h. The sampling method of bainitic rail steels is shown in Fig. 1b. Standard tensile samples with a gage diameter of 5 mm and a gage length of 25 mm were used for tensile tests using a SUNS 5305 tensile testing machine (MTS Systems, China) based on GB/T 228.1-2010 standards. Two samples were tested at a crosshead speed of 10 mm/ min for each process and average tensile data was obtained. Impact tests were performed using standard Charpy U-notch specimens (10 mm×10 mm×55 mm, standard EN10045) using JB-30A impact tester device at 20 °C and −40 °C. Three specimens were used for each test. HRC was detected by TH320 Rockwell hardness tester. Microstructures were characterized by scanning electron microscopy (SEM, ZEISS EVO18, 20 kV) after polishing and etching in 2% nital solution. Different types of retained austenite and M/A constituents were characterized by transmission electron microscopy (TEM, FEI TECNAI G20, 200 kV). TEM studies were carried out on thin foils electropolished using a solution of 4% perchloric acid. The volume fraction of retained austenite (RA, vol%) and austenite lattice parameter aγ was measured by X-ray diffractometer (Rigaku Smartlab, CuKa radiation) at a step width of 0.01° and a counting time of 2 s/step using Φ10 mm×2 mm samples. The retained austenite fraction was calculated using a direct comparison method based on the integrated intensities of (200), (220) and (311) austenite peaks, and those of (200) and (211) of ferrite peaks. The precise austenite lattice aγ was obtained by Nelson-Riley extrapolation method [34]. The carbon concentration xC of retained austenite was estimated using Eq. (1) [35]:

a γ = 0.3556 + 0.00453xC + 0.000095xMn + 0.00056xAl

Fig. 2. Schematic diagram of the SLX-80 cryogenic system.

as cooling rate, soaking time and the temperature, all can be controlled accurately. Dilatometric measurements were carried out using Φ4 mm×10 mm cylinders on a Bähr D805L quenching device installed with quartz push-rods, to capture information about microstructural evolution during tempering. The temperature was monitored by a type-S thermocouple spot welded on the surface of the cylinder. Nitrogen (N2) was used as quenching medium. The microstructure evolution during tempering is reflected by length change △L (also related to volume change △V, Eq. (2) [37]):

∆L ∆V = L 0 3V0

(2)

where △L is the length change during tempering, L0 is the initial length before tempering and V0 is the initial volume. The wear tests were conducted using MMU-10G friction and wear test equipment of pin-on-disk type under dry sliding condition. The diameter of the disk was 43 mm and the thickness was 4 mm. The pin specimen from rail head (Fig. 1b) was machined into column with diameter of 4 mm and length of 15 mm. The tests were conducted at applied load of 200 N at fixed sliding velocity of 100 round/min. Each wear experiment was conducted for 1 h. During the test, friction coefficients were recorded in real time by the software. Before and after the test, the pins were soaked in petroleum ether and cleaned by ultrasonic cleaner for 5 min, and then weight loss values of pins were determined from weight differences by the precise electronic balance of BSA224S.

(1)

where aγ is the austenite lattice parameter (nm), xC, xMn and xAl are the concentrations of carbon, manganese and aluminum in austenite (wt%), respectively. The program controlled SLX-80 cryogenic system was used for the cryogenic treatment [36]. The schematic diagram and figure of SLX-80 is shown in Fig. 2. In this equipment, liquid nitrogen which is the coolant flows into the distributor to disperse uniformly and then vaporizes into nitrogen gas, and then the gas flows into the cryogenic tank in the role of fan rotation and exchanges heat with materials. After heat exchange with the material, the relatively hot nitrogen gas is discharged to the atmosphere from the exhaust port. Parameters, such

3. Results and discussion 3.1. Mechanical properties The mechanical properties of different samples are shown in Table 1. In general, bainitic steels with B/M multiphase have a good combination of strength and toughness. On tempering, tensile strength decreases and yield strength is increased. Also, toughness indicates 560


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It can be seen that lath and granular bainite coexists in Q sample. Film-like M/A constituents tend to be continuous narrow islands. After tempering, part of blocky M/A constituents are reduced, and the surface reveals characteristics of decomposition. Meanwhile film-like M/A constituents are discontinuous. These changes have been observed in a previous study [39]. DCT introduces a further change into microstructure. Blocky M/A constituents appear to split into several small parts. Compared to the previous fine characteristics, film-like M/ A constituents show the shape of a dispersed particle, which means that DCT has led to reduction in M/A (mainly RA), making the microstructure more stable. Thus, improvement in properties can be explained in terms of stabilization via DCT.

Table 1 Mechanical properties. Sample

Q QT QCT

Tensile Strength MPa

Yield Strength MPa

Elongation

Toughness

%

(U-notch) J

1383 ± 3 1346 ± 5 1389 ± 6

915 ± 5 997 ± 2 1035 ± 7

16.2 ± 0.7 16.8 ± 0.3 17.1 ± 0.4

40 ± 2 75 ± 4 78 ± 3

HRC

40.4 ± 0.2 39.6 ± 0.2 41.5 ± 0.1

significant improvement and hardness is decreased. Interestingly, a change in behavior occurs when DCT is implemented. Tensile and yield strength are increased together with higher elongation. In comparison to QT, the toughness of QCT is maintained in spite of increased strength. The increase in hardness is expected to contribute to superior wear resistance.

3.3. Effect of DCT on retained austenite To further illustrate the microstructural evolution, it is necessary to state specific changes in critical phases. The study of RA is important, because of its critical effect in properties. In previous study, the tempering eliminates part of unstable RA [38]. Compared to QT, RA volume is decreased further in QCT (Fig. 4a), which can be ascribed to martensite transformation of RA at liquid-nitrogen temperature and decomposition during tempering. Interestingly, RA is stable enough to exist after holding at −196 °C for 8 h, which confirms its superior stability (Fig. 4b). In contrast in Q and QT, the maximum austenitic diffraction peak in QCT is reduced but is still present. We discuss RA in some detail below it. Carbon content is a key factor that governs RA stability [29], and its higher stability can contribute to improve work hardening and elongation. Gaussian distribution of 2θ of peak in (200)ϒ (220)ϒ and (311)ϒ is presented in Fig. 5. Compared to the air-cooled sample, three austenitic peaks of QT indicate a tendency of shift to left, implying increased degree of carbon content in RA. Furthermore, QCT indicates an identical behavior in contrast to QT. It is confirmed that RA of QCT has highest carbon content among all the samples (Fig. 5d). Through DCT, existing RA has got higher degree of carbon-enrichment, which can provide

3.2. Microstructure Fig. 3 shows SEM micrographs of samples subjected to different treatment. Through appropriate alloy design chemical composition, proeutectoid ferrite and pearlite cannot be observed in bainite/ martensite multiphase microstructure. The microstructure comprises of martensite, bainite, retained austenite and carbides. Because of the nonuniform composition of carbon and alloying elements, continuous cooling process leads to the coexistence of austenite and martensite, which can be regarded as martensite/austenite (M/A) constituents. Two kinds of bainite (mainly granular and lath bainite) can be observed in the microstructure. Different kinds of M/A constituents and bainitic ferrite constitutes different types of bainite. Film-like M/A constituents with bainitic ferrite form lath bainite, while granular is comprised of blocky M/A and surrounding bainitic ferrite. Furthermore, properties depend on the nature of M/A constituent [38,39]. Blocky M/A constituents are believed to have an adverse effect on properties [38]. While film-like M/A constituents contribute to properties.

Fig. 3. SEM micrographs of different samples: (a) Q, (b) QT and (c) QCT. B M/A, blocky M/A; F M/A, film-like M/A.

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Fig. 4. Variation of RA volume (a) and X-ray diffraction pattern (b) with different treatment.

tempering can be attributed to the decomposition of RA (Fig. 6a). This means some unstable RA decomposes, which has been analyzed in previous study [38]. In contrast, situation changes on introducing DCT. It is worth noting that the increase in slope of △L/L0 is very sharp during early stage (within 0.2 h) and then decreases at longer holding time. The decrease in △L/L0 (sample contraction) can be attributed to the occurrence of carbide precipitation (recovery of martensite) [37]. The rapid expansion during early stage is caused by decomposition of RA, while recovery of martensite triggers contraction. Tempering behavior at identical temperature between the samples is described below. It can be inferred that martensite unit cells suffer a dramatic contraction during soaking at −196 °C, which provides driving force for carbon atoms to jump out [16]. Thus, compared to QT, carbon atoms can easily precipitate during tempering, which provides an explanation to recovery (Fig. 6b). Carbides in QCT sample

support to its enhanced stability. Therefore, it is understandable QCT can get outstanding mechanical properties. 3.4. Dilatometric and TEM It is suggested from SEM and XRD that recovery of martensite (possibly accompanied by ε -carbide and/or cementite precipitation) and phase transformation from retained austenite (e.g. martensite formation) may occur during tempering, thereby affecting the carboncontent in retained austenite. The recovery of martensite and phase transformation from retained austenite may lead to contraction and expansion of the samples as revealed by dilatometric tests, respectively [37]. Fig. 6 shows the relative length change (△L/L0) of QT and QCT samples during tempering at 280 °C. The increase in △L/L0 during

Fig. 5. 2θ value of austenitic X-ray diffraction peak: (a) (200)ϒ, (b) (220)ϒ, (c) (311)ϒ, and (d) changing carbon contents of RA in different samples.

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Fig. 6. Dilatometric analysis of samples (a) QT and (b) QCT during tempering.

Fig. 7. TEM micrographs of Q (a and b), QT (c and d) and QCT (e and f). Blocky M/A denoted in (a) and (b); filmy RA in (c) and (d); twinned martensite in (e) and (f). TM, twinned martensite; GB, prior austenite grain boundary.

are observed by TEM and brittle martensite after DCT treatment (with relatively low carbon concentration [40]) is avoided. In this circumstance, recovery of martensite can create conditions to achieve RA with

higher carbon content (Fig. 5). Some RA can be found in blocky M/A (Fig. 7a and b), which is observed by SEM (Fig. 3a). Blocky M/A constituents tend to be present

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at triple points. In the presence of applied load, RA in blocky M/A would be detrimental [22]. Unstable RA is removed during the tempering process and the remaining RA continues to exist after tempering (Fig. 7c and d). It can be inferred that tempering improves toughness by eliminating unstable RA. The situation is changed on introducing DCT before tempering. In support of the above discussion (Fig. 6b), carbide precipitation is observed (Fig. 7e), which is expected to contribute to wear resistance [41,42]. Another phenomenon is presence of twinned martensite (TM) in microstructure (Fig. 7e and f). Meanwhile, TM is believed to form from previous blocky M/A (Fig. 7f). DCT can transform RA into twinned martensite during soaking at sub-zero temperature below martensite start temperature (Ms) and RA with high carbon content can lead to twinned martensite (TM). Furthermore, because of recovery during tempering, martensite with lower carbon concentration can benefit mechanical properties. Meanwhile, it can also provide an explanation for further carbonenrichment in RA [43].

Fig. 8. Variations in mass loss in different samples at applied load of 200 N.

3.5. Effect of DCT on wear resistance Considering above microstructural evolution, the performance of

Fig. 9. Micrographs with different magnification of worn surface of specimens in group QT (a–c) and QCT (d–f).

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Acknowledgement

DCT on wear resistance is the focus below. The mass loss of QCT is 14.07 mg, while that of QT is 16.37 mg. Compared to QT, the loss mass is reduced by 10% in QCT samples. The results of wear characteristics indicate that DCT can to some degree enhance the wear resistance of B/ M multiphase steels (Fig. 8). In order to elucidate the mechanism of improvement in wear resistance, worn surfaces were studied by SEM (Fig. 9). During abrasive wear, ploughing is both observed along the sliding direction in both specimens. However, morphology and extent of ploughing are different. Ploughing in QT is more significant than QCT (Fig. 9). Another, characteristic of adhesive wear also need attention. Some fracture ridges and deformation lips are present on the worn surface, as a result of heavy plastic deformation and stress concentration during the wear process. Comparing with the worn surface of QCT, the ridges and lips in QT exhibit brittle character. Shell-like defects on the surface of QT weaken its wear resistance ability. In contrast, the worn surface of QCT is smoother than QT. Plastic deformation introduces significantly less change and only some cracks are observed on the surface of QCT. Based on the high linear speed of the dry sliding wear in our study, the delamination is the primary mechanism. The adhesive nodes are caused by the adhesion effect in the presence of applied load, which makes wear surface look like shear fracture. It also results in formation of wear debris, which aggravates the destruction from abrasive wear [37]. The main mechanism of wear is micro-cutting, which generates shear fracture, ploughing and cutting on the surface. Thus, higher hardness of QCT sample can benefit wear resistance to damage from abrasive particle. Meantime, spalling is found on the worn surface of QT, which is induced from deformation strengthening. While, the surface of QCT shows a homogeneous delamination during dry sliding. From the foregoing discussion, the DCT can contribute to the wear resistance of B/M multiphase steels. It may be noted that the rails experience three different types of service condition, straight-line rail, curve rail and turnout greatly. For example, curve rail or turnout would mainly experience sliding-rolling wear, while rolling wear would occur mostly in straight-line rail. Another condition, the sliding wear would be obvious even in straight-line rail when an emergency brake is applied. Thus, several types of wear would co-exist in an entire rail line. However, our previous study [44] showed that the studied rail steel in this paper was not sensitive to wear loss in mild wear, and relatively wear damage was most serious in severe sliding wear, and therefore, the severe sliding wear processing parameters were studied in this paper.

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4. Conclusions Deep cryogenic treatment significantly influences the mechanical properties and microstructure, and these effects are studied here. DCT is an attractive and promising approach to enhance the properties of bainite/martensite multiphase steels. The conclusions as followed can be obtained from the analysis of the experimental results: (1) DCT effectively improves the mechanical properties (including strength, elongation, toughness and hardness). A good combination of strength and ductility are obtained after deep cryogenic treatment and tempering at 280 °C (ultimate tensile strength: 1389 MPa; total elongation: 17.1%; U-notch impact toughness at 20 °C: 78 J/cm2). (2) DCT reduces the blocky M/A constituents by eliminating unstable RA, and improves the stability of RA with higher carbon-contents. The supersaturation of martensite in carbon-content is relieved on tempering after cryogenic treatment, which benefits the mechanical properties to avoid brittleness. (3) The recovery of martensite leads to precipitation of carbides and contributes to hardness and wear resistance.

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