metals

Article Effect of Cryogenic Treatment on Microstructure and Wear Resistance of Carburized 20CrNi2MoV

Binzhou Li, Changsheng Li *, Yu Wang and Xin Jin

The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China; [email protected] (B.L.); [email protected] (Y.W.); [email protected] (X.J.) * Correspondence: [email protected]; Tel.: +86-024-8368-7749

 Received: 13 September 2018; Accepted: 5 October 2018; Published: 9 October 2018 

Abstract: This paper investigated the response of carburized 20CrNi2MoV steel to cryogenic treatment including microstructure and wear resistance. Two cryogenic treatment methods including cryogenic treatment at −80 ◦C (CT) and deep cryogenic treatment at −196 ◦C (DCT) as well as conventional heat treatment (CHT) were carried out after process. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometry (XRD) were employed for microstructure characterization. The wear resistance was investigated by ball-on-disc sliding wear test on a multi-functional tribometer. The results show that the wear resistance of the experimental steel has been improved by 17% due to CT and by 25.5% due to DCT when compared to CHT. This significant improvement in wear resistance after cryogenic treatment is attributed to the microstructural changes including the finer martensitic structure, the reduction of retained austenite and the development of fine and more numerous carbides. Among these factors, the precipitation of fine carbides plays a more prominent role in enhancing wear resistance.

Keywords: carburizing steel; wear resistance; microstructure; cryogenic treatment; retained austenite; carbide

1. Introduction Carburizing is a surface- treatment which is usually used to improve the performance of components such as bearings and ring gears which require a very hard surface to resist wear along with a tough interior to resist impact in service [1–4]. The achievement after carburizing process is obtaining the carbon content gradient, i.e., carburized layer. However, the heat treatment following the carburizing process is the key to obtaining perfect microstructure and excellent performance. The conventional heat treatment (CHT) for carburizing is and low temperature . Whereas the major disadvantage of CHT is the terrible microstructure of coarse plate martensite with terribly high content of retained austenite (RA) [5–7]. The RA is a soft phase in microstructure and may have an adverse effect on properties like hardness and wear resistance [8,9]. In addition, RA is unstable and is easy to transform into martensite as a result of thermal and mechanical stresses encountered during service [10]. The transformation process is associated with volume expansion, therefore it leads to dimensional variation and distortion of the components and even to failure [11,12]. The CHT process results in a variety of problems of low hardness, poor wear resistance, dimensional instability and premature failure of components. The service life of components made of carburizing steels mainly depends on their wear resistance. Therefore, substantial efforts have been devoted to enhancing the service life of the carburizing steel components, especially by improving their wear resistance property. Cryogenic treatment is one of the effective approaches to obtaining better wear resistance of the carburizing steels [13–15]. There are two types of cryogenic treatment [16]:

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“cryogenic treatment” (CT) at temperature about −80 ◦C, and “deep cryogenic treatment” (DCT) at temperature about −196 ◦C. From the published works [17–19], it is widely accepted that CT is sufficient to transform RA into martensite which is considered to be a major factor for improving wear resistance. However, other recent studies have [20,21] indicated that wear resistance could be further improved by DCT. Since the content of RA was no longer changed with the temperature of cryogenic treatment decreasing, there must be other reasons responsible for the improvement of wear resistance. Recent studies have shown that [22,23] the improvement of wear resistance after DCT could be due to the following two aspects: (i) the reduction of the retained austenite content; (ii) the increase of carbides content. In addition to these, the residual stress existing in the carburized layer still has an effect on the wear resistance. The residual compressive stress exists in the case microstructures of carburized samples as a result of transformation and temperature gradients induced by quenching. The magnitude and distribution of the residual stress therefore are complex functions of the temperature gradients induced by quenching. The cryogenic treatment would also cause the residual stress changes and thus cause the changes of wear resistance. However, the residual stresses as a function of case depth are routinely measured by x-ray diffraction which would lead to a relatively high data error. Ghidelli et al. [24] put forward a new method of detecting residual stress by nanoindentation technique which is a very effective way to detect the residual stress of carburized layer. The effect of DCT on wear resistance varies with the steels, and there is not a satisfactory explanation for the phenomena up to now. The purpose of this paper is to investigate the effect of cryogenic treatment on the microstructure and wear resistance of the carburized 20CrNi2MoV steel. The wear resistance of the experimental steel after CT and DCT has been investigated by sliding wear test. In addition, in order to explain the changes of wear resistance, microstructure and worn surface observations were carried out.

2. Materials and Experimental Procedure

2.1. Material Preparation The experimental steel was produced in a vacuum induction melting furnace (JZDL-ZG-0.1, Jinzhou, China). The ingot was forged into 150 × 80 × 50 mm3 billet. The chemical composition is listed in Table1. For carburizing process, the samples were cut from the billet in size of 60 × 60 × 20 mm3 and were subjected to surface grinding process before carburizing process.

Table 1. Chemical composition of the experimental steel.

Element C Si Mn Cr Ni Mo V P S wt.% 0.2 0.25 0.61 0.56 1.77 0.26 0.21 0.007 0.002

Carburizing process was performed in a continuous gas carburizing furnace (ZWZ, Wafangdian, China) which was composed of heating zone, carburizing zone, diffusion zone and equalization zone. The whole carburizing process would last for 27 h, consisting of 7 h in heating zone at 905 ◦C with 0.6% carbon potential, 6 h in carburizing zone at 935 ◦C with 1.3% carbon potential, 10 h in diffusion zone at 935 ◦C with 1.27% carbon potential, and 4 h in diffusion zone at 880 ◦C with 1.05% carbon potential. The process was designed to produce a case depth of ~2.0 mm. After carburizing, all samples were quenched immediately in oil.

2.2. Heat Treatment Process In this study, three types of heat treatment were carried out after carburizing process. All the samples of as-quenched condition after carburizing were re-austenitized at 880 ◦C for 45 min in a tube furnace under flowing argon atmosphere, quenched in oil and kept in oil for 30 s, and then air cooled to room temperature. Then, one sample as a reference, designated CHT, was directly heated to the tempering temperature 180 ◦C, held at the temperature for 2 h, and then air cooled to room temperature. Metals 2018, 8, 808 3 of 13

Metals 2018, 8, x FOR PEER REVIEW 3 of 13 Another sample, designated CT, was cooled from room temperature to −80 ◦C at a slow cooling rate ◦ warmingofMetals ~0.2 20C/s18 back, 8, forx FORto avoiding room PEER REVIEWtemperature thermal shocks, and thereafter kept at the heated temperature to 180 for°C 4for h, graduallythe same low warming-temperature3back of 13 to ◦ temperingroom temperature process and as thereafterCHT sample. heated The to DCT 180 C consisted for the same of slowly low-temperature cooling from tempering the as- processquenched as ◦ conditionCHTwarming sample. to back about The to DCT room −196 consisted temperature °C, holding of slowly and at this thereafter cooling temperature from heated the forto as-quenched 180 4 h°C and for thenthe condition same gradually low to- abouttemperature bringing−196 the C, tempering process as CHT sample. The DCT consisted of slowly cooling from the as-quenched sampleholding back at this to temperature room temperature, for 4 h and and then ther graduallyeafter heated bringing to 180 the °C sample for the back same to room low-temperature temperature, condition to about −196 °C, ◦ holding at this temperature for 4 h and then gradually bringing the temperingand thereafter process heated as CHT to 180 sample.C for The the thermal same low-temperature cycles including quenching, tempering cryogenic process as treatment CHT sample. and sample back to room temperature, and thereafter heated to 180 °C for the same low-temperature tTheempering thermal are cycles illustrated including in Figure quenching, 1. cryogenic treatment and tempering are illustrated in Figure1. tempering process as CHT sample. The thermal cycles including quenching, cryogenic treatment and tempering are illustrated in Figure 1.

Figure 1. Heat treatment process. Figure 1. Heat treatment process. 2.3. Sliding Wear Test and Hardness Test 2.3. Sliding Wear Test and Hardness Test 2.3.The Sliding sliding Wear wear Test testsand Hard wereness performed Test at room temperature on a multi-functional tribometer from The sliding wear tests were performed at room temperature on a multi-functional tribometer Rtec InstrumentsThe sliding (Rtec-MFTwear tests were 5000, performed San Jose, CA,at room USA). temperature The schematic on a multi diagram-functional of the experimentaltribometer from Rtec Instruments (Rtec-MFT 5000, San Jose, CA, USA). The schematic diagram of the apparatusfrom Rtec is Instrumentsshown in Figure (Rtec2-a.MFT The 5000, module San of Jose, ball-on-disc CA, USA). configuration The schematic was diagram applied of for the this experimental apparatus is shown in Figure 2a. The module of ball-on-disc configuration was applied test.experim The sizeental of apparatus the disc-shaped is shown samples in Figure was 2a. The 40 mm module in diameter of ball-on and-disc 10 configuration mm in height. was GCr15 applied balls for this test. The size of the disc-shaped samples was 40 mm in diameter and 10 mm in height. GCr15 withfor this 5 mm test. in The diameter size of the and disc hardness-shaped ofsamples 63 HRC was were 40 mm used in diameter as friction and pairs.10 mm Duringin height. the GCr15 sliding balls with 5 mm in diameter and hardness of 63 HRC were used as friction pairs. During the sliding tests,balls the with ball 5 mm friction in diameter pairs were and kepthardness stationary of 63 HRC while were the used discs as werefriction maintained pairs. During as rotary. the sliding In this tests, the ball friction pairs were kept stationary while the discs were maintained as rotary. In this study,tests, 20the N ball were friction selected pairs as were the loads kept tostationary produce while a contact the discs pressure were levelmaintained of ~1.36 as GPa.rotary. The In slidingthis study, 20 N were selected as the loads to produce a contact pressure level of ~1.36 GPa. The sliding speedstudy, was 20 N set were as 500 selected rpm andas the the loads sliding to produce distance a contact ranged pressure from 100 level to 1500of ~1.36 m. GPa. All theThe testssliding were speed was set as 500 rpm and the sliding distance ranged from 100 to 1500 m. All the tests were repeatedspeed was three set times.as 500 Afterrpm and tests, the the sliding samples distance were ranged carefully from cleaned 100 to 1 by500 alcohol m. All inthe the tests ultrasonic were repeatedrepeated three three times. times. After After tests, tests, the the samples samples were were carefully carefully cleaned cleaned by by alcohol alcohol in in the the ultrasonic ultrasonic cleaner and dried for the observation of worn surface morphology. The topography of worn surfaces cleanercleaner and and dried dried for for the the observation observation ofof worn surface morphology.morphology. The The topography topography of ofworn worn surfaces surfaces were characterized by the 3D-profilometer attached to Rtec Instruments (Rtec instruments, San Jose, werewere characterized characterized by by the the 3D 3D--profprofilometerilometer attached toto Rtec Rtec Instruments Instruments (Rtec (Rtec instruments, instruments, San San Jose, Jose, CA, USA), and the cross-sectional areas of the wear tracks were calculated at the same time, as California,California, USA) USA), and, and the the cross cross--sectionalsectional areas of thethe wearwear tracks tracks were were calculated calculated at atthe the same same time, time, as as shown in Figure2b. The microhardness was tested by FUTURE-TECH FM-700 Vickers durometer shownshown in in Figure Figure 2b 2b. .The The microhardness microhardness was testedtested byby FUTUREFUTURE-TECH-TECH FM FM-700-700 Vickers Vickers durome durometer ter (FUTURE-Tech, Kawasaki, Japan) with applied load of 1kgf. All the hardness values presented in this (FUTURE(FUTURE-Tech,-Tech, Kawasaki Kawasaki, ,Japan) Japan) with with applied load ofof 1kgf. 1kgf. All All the the hardness hardness values values presented presented in thisin this study were an average of three repeated tests. studystudy were were an an average average of of three three repeated repeated tests.

FigureFigure 2. 2Schematic. Schematic diagram diagram of ofwear wear experimental experimental apparatus apparatus (a) and(a) and schemes schemes of cross-sectional of cross-sectional profile Figuremeasurementprofile 2 .measurement Schematic of the wear diagram of the track wear of (b track). wear (b experimental). apparatus (a) and schemes of cross-sectional profile measurement of the wear track (b). 2.4. Microstructural Characterization 2.4. Microstructural Characterization

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Metals 2018, 8, x FOR PEER REVIEW 4 of 13 2.4. Microstructural Characterization TheThe carbon contentcontent distribution distribution of of carburized carburized case case was was analyzed analyzed by aby Shimadzu a Shimadzu PDA-5500 PDA- II5500 direct II directreading reading spectrograph spectrograph (Shimadzu, (Shimadzu, Kyoto, Japan).Kyoto, MicrostructuresJapan). Microstruct wereures observed were observed by a ZEISS by ULTRA-55 a ZEISS ULTRAscanning-55 electron scanning microscopy electron microscopy (ZEISS-SEM, (ZEISS Jena,-SEM Germany), Jena, after Germany) manual after grinding, manual mechanical grinding, mechanicalpolishing, and polishing, etched and by 4% etched nital by solution. 4% nital A solution. FEI Tecnai A G2FEI F20 Tecnai transmission G2 F20 transmission electron microscopy electron microscopy(FEI-TEM, Hillsborough, (FEI-TEM, Hillsborough, OR, USA) was O usedR, USA to observe) was the used fine to structure observe at the an acceleratedfine structure voltage at an of accelerated200 kV. To prepare voltage theof 200 thin kV. foils To for prepare TEM, firstly, the thin slices foils of for 500 TEM,µm thick firstly, were slices cut of from 500 samples μm thick by were wire cutcutting, from then samples thinned by towire 40 µcutting,m by manual then thinned grinding, to and40 μm punched by manual into standard grinding, TEM and discs punched of 3 mm into in standarddiameter. TEM Finally, discs the of foils 3 mm were in diameter. electropolished Finally, by the the foils solution were ofelectropolished 8% perchloric by acid the and solution 92% alcohol of 8% perchloricat a potential acid of and 32 V92% and alcohol temperature at a potential of −20 of◦C. 32 A V D/max2400 and temperature X-ray of diffractometry −20 °C. A D/max2400 (XRD) with X-ray Cu diffractometryKα radiation (Shimadzu, (XRD) with Kyoto, Cu Kα Japan) radiation was employed(Shimadzu, to Kyoto, identify Japan) the retained was employed austenite to content identify in the the retainedmicrostructure. austenite The content diffraction in the anglemicrostructure. ranged from The 30 diffraction to 120◦, with angle a 0.05 ranged◦ step from at a 30 speed to 120°, of 1 with◦/min. a 0.05°The worn step at surfaces a speed of of the 1°/min. samples The were worn also surfaces studied of bythe SEM. samples were also studied by SEM.

3.3. Results Results

3.1.3.1. Carbon Carbon Content Content Gradient Gradient and and Hardness Hardness Gradient Gradient TheThe carbon carbon content content gradient gradient of of samples samples after after carburizing carburizing as as a a function function of of distan distancece from from surface surface isis shown shown in in Figure Figure 33aa.. TheThe carboncarbon contentcontent ofof thethe surfacesurface layerlayer isis ~0.88~0.88 wt.%,wt.%, and at the casecase depthdepth isis ~~200200 μmµm fr fromom the surface, the highest the carbon content reaches is ~0.9~0.9 wt wt.%..%. The The total total case case depth depth is is ~~2.32.3 mm and and the the carbon carbon content gradient curve falls flat. flat.

FigureFigure 3 3.. Carbon content content gradient gradient of of samples samples after after carburizing carburizing process process ( (aa)) and and hardness hardness gradient gradient of of samplessamples under under different different heat treatments ( b).

TheThe hardnes hardnesss gradients ofof thethe samplessamples afterafter differentdifferent heat heat treatments treatments are are shown shown in in Figure Figure3b. 3b. It isIt isshown shown that that the the hardness hardness values values decrease decrease continuously continuously with with the increasethe increase of depth of depth from casefrom to case center to centerof all theof all specimens. the specimens. The variation The variation trend oftrend hardness of hardness is in keeping is in keeping with that with of thethat carbon of the content.carbon content.As demonstrated As demonstrated in Figure in3b, Figure cryogenic 3b, cryogenic treatment treatment markedly increasesmarkedly the increases hardness the of hardness the carburized of the carburizedlayer. The maximumlayer. The hardnessmaximum value hardness of CHT value sample of CHT is 812 sample HV while is 812 that HV of while CT sample that of hasCT reachedsample hasto 848 reached HV. The to 848 DCT HV. further The improvedDCT further hardness improved as compared hardness toas CTcompared and the to hardness CT and valuethe hardness reached valuethe maximum reached ofthe 886 maximum HV. The ofincrease 886 HV. of hardness The increase by cryogenic of hardness treatment by cryogenic could improve treatment the could wear improveresistance the to wear some resistance extent. to some extent.

3.3.2.2. Wear Wear Resistance Resistance TheThe wear wear performances performances of of samples samples suffered suffered from from CHT, CHT, CT CT and and DCT DCT were were compared compared by by the the measurementmeasurement of of wear wear volume volume and and wear wear rate. rate. Wear Wear rate rate was was expressed expressed as as the the wear wear volume volume loss loss per per unitunit sliding sliding distance: distance: Wr = WV/L (1) WWLrV (1) where, Wr was the wear rate (mm3/m), L was the sliding distance (m), and WV was the wear volume (mm3) which was calculated by the product of the cross-sectional area of wear track and its

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3 Metalswhere,Metals 20 201818W, ,8 r8, ,x wasx FOR FOR thePEER PEER wear REVIEW REVIEW rate (mm /m), L was the sliding distance (m), and WV was the55 wear of of 13 13 volume (mm3) which was calculated by the product of the cross-sectional area of wear track and circumference.itscircumference. circumference. Figure Figure Figure 4a 4a shows shows4a shows the the variation thevariation variation of of wear wear of wearvolume volume volume with with sliding withsliding sliding distance distance distance of of samples samples of samples under under differentunderdifferent different heatheat treatments.treatments. heat treatments. ItIt isis observedobserved It is observed thatthat thethe that wearwear the volumevolume wear volume ofof allall thethe of specimensspecimens all the specimens increasesincreases increases rapidlyrapidly withrapidlywith the the withincrease increase the increaseofof sliding sliding of distance. distance. sliding distance.The The wear wear Thevolume volume wear of of volume CT CT as as well well of CT as as asDCT DCT well sam sam asples DCTples is is samples less less than than is that that less ofthanof CHT CHT that samples samples of CHT at at samplesthe the same same at sliding sliding the same distance, distance, sliding and and distance, the the wear wear and volume volume the wear o off DCT DCT volume samples samples of DCT is is the the samples least. least. Figure Figure is the 4bleast.4b illustrates illustrates Figure the 4theb illustrateswear wear rate rate of of the the the wear CHT, CHT, rate CT CT and ofand the DCT DCT CHT, samples. samples. CT and The The DCT wear wear samples. rates rates of of all all The the the wear samples samples rates present present of all thethe trend trend samples of of firstly firstly present increas increas the trendiingng and and of then firstlythen slightly slightly increasing decreas decreas andinging then alongalong slightly with with sliding decreasingsliding distance distance along increasing. increasing. with sliding At At thedistancethe initial initial increasing.stage stage of of sliding sliding At the distance distance initial stageless less than than of sliding 400 400––500500 distance m, m, the the lesswearwear than of of samples samples 400–500 was was m, theseveresevere wear and and of resulted samplesresulted inwasin aa severerapidrapid rise andrise ofresulted of wear wear in rate. rate. a rapid This This rise process process of wear isis the rate. the breaking breaking This process--inin wear wear is the stage. stage. breaking-in With With the wearthe prolongation prolongation stage. With the of of slidingprolongationsliding distance, distance, of slidingthe the wear wear distance, rate rate started started the wear to to decline decline rate started and and the the to specimens specimens decline and enter enter theed specimensed the the stable stable entered wear wear stage. stage. the stable The The wearwear rate stage.rate at at the Thethe same same wear sliding sliding rate at distance distance the same of of sliding the the samples samples distance that that of suffered suffered the samples from from thatcryogeniccryogenic suffered treatment treatment from cryogenic is is much much lowertreatmentlower thanthan is thatthat much ofof CHTCHT lower samplessamples than that. . Furthermore,Furthermore, of CHT samples. thethe wearwear Furthermore, raterate ofof DCTDCT the samplessamples wear rate isis significantlysignificantly of DCT samples lowerlower is thansignificantlythan that that of of CT CT lower samples samples than. . that of CT samples.

FigureFigure 4 4.4. . WearWear volume volume ( (aa))) and andand wear wearwear rate raterate ( ((bbb))) vary varyvary with withwith sliding slidingsliding distance distancedistance of ofof samples samplessamples under underunder different differentdifferent heatheat treatment. treatment.

TheThe performance performance of of of wear wear wear resistance resistance resistance can can can be be beseen seen seen more more more intuitive intuitive intuitive according according according to to the the to wear thewear wear depth depth depth and and wearandwear wear areaarea obtained areaobtained obtained fromfrom fromthethe profileprofile the profile ofof thethe of wearwear the weartrack.track. track. FigureFigure Figure 5a5a showsshows5a shows thethe profileprofile the profile ofof thethe of wearwear the weartracktrack oftrackof samplessamples of samples underunder underdifferentdifferent different heat heat treatmenttreatment heat treatment afterafter slidingsliding after slidingforfor 15001500 for m.m. 1500 TheThe track m.track The depthsdepths track of of depths CHT,CHT, CT ofCT CHT, andand DCTCTDCT and samples samples DCT are are samples 13 13 μm, μm, are 11 11 μm 13μmµ and andm, 119.89.8 µμm, μm,m andrespectively. respectively. 9.8 µm, respectively.Meanwhile, Meanwhile, the the Meanwhile, track track width width the of of the trackthe samples samples width sufferedofsuffered the samples from from cryogenic cryogenic suffered treatment treatment from cryogenic becomesbecomes treatment narrow narrow compared becomescompared narrowto to that that of of compared the the CHT CHT tosamples.samples. that of The The the wear wear CHT −2 3 −2 3 volumessamples.volumes ofof The CHT,CHT, wear CTCT volumes andand DCTDCT of CHT,samples,samples, CT andareare 2.37 DCT2.37 ×× samples, 1010−−22 mmmm3 are3, , 1.961.96 2.37 ×× ×1010−10−22 mmmmmm33 andand, 1.96 1.761.76 × ×× 10 10−−22 mmmm33, , −2 3 −5 3 respectively,andrespectively,1.76 × 10 whilewhilemm thethe, corresponding respectively,corresponding while wearwear the ratesrates corresponding areare 1.571.57 ×× 1010−−55 wear mmmm33/m,/m, rates 1.31.3 are ×× 1010 1.57−−55 mmmm× 3103/m/m andandmm 1.171.17/m, ×× −5 3 −5 3 101.310−−55 × mmmm1033/m/mmm, , respectively,respectively,/m and 1.17 asas shown×shown10 ininmm FigureFigure/m, 5b respectively,5b. . TheThe wearwear as raterate shown hashas beenbeen in Figure considerablyconsiderably5b. The wear improvedimproved rate has byby 17%been17% due due considerably to to CT CT and and improvedby by 25.5% 25.5% due bydue 17% toto DCT DCT due when towhen CT compared andcompared by 25.5% to to CHT. CHT. due to DCT when compared to CHT.

FigureFigure 5 5.5. .Profile ProfileProfile of of wear wear track track ( (aa)) and and wear wear rate raterate ( ((bb))) after afterafter 1500 15001500 m m sliding. sliding.

ToTo understandunderstand thethe wearwear mechanismmechanism ofof thethe steelssteels underunder slidingsliding wear,wear, thethe topographytopography ofof wornworn surfacessurfaces ofof samplessamples werewere investigated.investigated. Firstly,Firstly, FigureFigure 66 showsshows thethe wearwear trackstracks featuresfeatures atat differentdifferent

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To understand the wear mechanism of the steels under sliding wear, the topography of worn surfaces of samples were investigated. Firstly, Figure6 shows the wear tracks features at different wearMetals stages 2018 of, 8, DCTx FOR sample.PEER REVIEW As shown in Figure6a, for a sliding distance of 100 m, many raised6 of 13burrs (spalls) and shallow grooves are observed corresponding to the two-dimensional morphology being featuredwear ofstages shallow of DCT grooves sample. andAs shown large in craters Figure around 6a, for a withsliding scattered distance of debris, 100 m, asmany shown raised in burrs Figure 6b. (spalls) and shallow grooves are observed corresponding to the two-dimensional morphology being With increasing the sliding distance to 500 m, the three-dimensional morphology of the worn surface featured of shallow grooves and large craters around with scattered debris, as shown in Figure 6b. reveals different depths of grooves, as shown in Figure6c. The two-dimensional morphology is With increasing the sliding distance to 500 m, the three-dimensional morphology of the worn surface characterizedreveals different by wide dep groovesths of grooves, of different as shown depths in and Figure a small 6c. Thenumber two- ofdim debris,ensional as morphology shown in Figure is 6d. Whencharacterized the sliding by distance wide grooves increases of different to 1000 depths m, the and three-dimensional a small number of debris, morphology as shown of in the Figure wear scar is a deep6d. When groove the sliding (Figure distance6e). While increases for the to two-dimensional1000 m, the three-dimensional morphology morphology of the surface, of the aswear shown in Figurescar is6 af, deep deep groove grooves (Figure and 6e). delaminations While for the weartwo-dimensional become dominant, morphology but of a the certain surface, amount as shown of large cratersin Figure exist in6f, somedeep grooves regions. and When delaminations sliding wearwear isbecome increased dominant, to 1500 but m,a certain the wear amount surface of large still has the characteristiccraters exist in ofsome groove regions. wear, When but sliding the groove wear is becomes increased narrow to 1500 m, and the the wear wear surfa trackce still becomes has the deep, as showncharacteristic in Figure of 6grooveg,h. Therefore, wear, but fromthe groove the observation becomes narrow of the and wear the surfaceswear trac atk becomes different deep, wear as stages, shown in Figure 6g,h. Therefore, from the observation of the wear surfaces at different wear stages, it can be seen that groove wear took the predominant position during sliding wear process, and the it can be seen that groove wear took the predominant position during sliding wear process, and the groove wear gradually narrows down with the prolongation of wear duration. groove wear gradually narrows down with the prolongation of wear duration.

FigureFigure 6. Three-dimensional 6. Three-dimensional morphology morphology of the wear wear scar scar of ofdeep deep cryogenic cryogenic treatment treatment (DCT (DCT)) samples samples underunder different different sliding sliding distances distances ( (aa)) 100 100 m, ( (cc)) 5 50000 m, m, (e () e 1000) 1000 m, m, (g) ( 1500g) 1500 m and m and (b,d,f (,bh,)d the,f,h ) the corresponding two-dimensional wear track morphology. corresponding two-dimensional wear track morphology.

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The variation of wear rate of the samples under different heat treatment is probably related to The variation of wear rate of the samples under different heat treatment is probably related to the different wear mode. Figure7 represents the SEM morphologies of the worn surfaces of CHT, the different wear mode. Figure 7 represents the SEM morphologies of the worn surfaces of CHT, CT CT and DCT samples after sliding 1500 m. Deep parallel grooves as well as delaminations scattered and DCT samples after sliding 1500 m. Deep parallel grooves as well as delaminations scattered around the grooves are the main morphologies of the worn surface for all the samples under identical around the grooves are the main morphologies of the worn surface for all the samples under identical wear conditions. The wear surface of CHT sample (Figure7a) exhibits very deep and wide grooves wear conditions. The wear surface of CHT sample (Figure 7a) exhibits very deep and wide grooves parallel to the sliding direction and large amount of delaminations and craters, while the worn surfaces parallel to the sliding direction and large amount of delaminations and craters, while the worn of CT (Figure7b) and DCT (Figure7c) appear much smoother. When comparing the feature of the surfaces of CT (Figure 7b) and DCT (Figure 7c) appear much smoother. When comparing the feature wornof the surfacesworn surfaces of CT of and CT DCT, and itDCT, can it be can found be found that the that groove the groove is narrower is narrower and shallower and shallower for DCT for DCTsamples. samples. This indicates This indicates that the that plastic the deformation plastic deformation degree of CT degree sample of isCT severe sample when is severecomparing when to comparingDCT sample. to DCT sample.

Figure 7 7.. MorphologyMorphology of of the the worn worn surface surface of ofexperimental experimental steel steel under under different different heat heat treatment: treatment: (a) (conventionala) conventional heat heat treatment treatment (CHT) (CHT),, (b) (cryogenicb) cryogenic treatment treatment (CT (CT)) and and (c) (DCT.c) DCT.

3.3.3.3. Microstructure The effect of cryogenic treatment on wear resistance resistance is is definitely definitely attributed attributed to to the the microstructure microstructure of the the experimental experimental steel. steel. Therefore, Therefore, microstructural microstructural examination examination was carried was carried out to revealout to the reveal changes the changesin microstructure in microstructure on account on account of cryogenic of cryogenic treatment. treatment. Firstly, theFirstly, SEM the micrographs SEM micrographs of CHT, of CT CHT, and CTDCT and specimens DCT specimens are shown are shown in Figure in Figure8. It is8. It observed is observed that that the the microstructures microstructures of of CHT, CHT, CT CT and and DCT specimens are all mainly composed of plate martensite with a large number of carbides in the matrix, but there are also obvious differences. The first first significant significant change is that the microstructure becomes finer finer after cryogenic treatment as compared to CHT sample. Furthermore, as the cryogenic treatment temperature decreases, the microstructure becomesbecomes refined.refined. It can be contributed to crystal crystal breaking caused by the plastic deformation resulting from the martensite transformation at low temperature [2 [255]. The other significant significant change is the distribution and volume fraction of carbides in the matrix. AsAs shownshown inin FigureFigure8 b,8b, the the carbides carbides in in CHT CHT sample sample are are mainly mainly 200–300 200–300µm μm in sizein size and and the thedistribution distribution is inhomogeneous, is inhomogeneous, while while the the small-size small-size carbides carbides are are hardly hardl observed.y observed. In In CT CT sample sample of ofFigure Figure8d, 8d, the the large-size large-size carbides carbides are are disappeared disappeared and and replaced replaced with with a certain a certain amount amount of small-size of small- particles.size particles. When When comparing comparing Figure Figure8d,f, 8d, thef, the carbides carbides in in the the DCT DCT sample sample become become more more refined refined andand more homogeneous. Meanwhile, the volume fraction of precipitated carbides markedly increases after cryogenic treatment.

Metals 2018, 8, 808 8 of 13 more homogeneous. Meanwhile, the volume fraction of precipitated carbides markedly increases after cryogenic treatment. Metals 2018, 8, x FOR PEER REVIEW 8 of 13

FigureFigure 8. SEM 8. SEM micrographs micrographs of samples of samples unde underr different different heat treatment: heat treatment: (a,b) CHT; (a,b )(c CHT;,d) CT ( cand,d) CT(e,f) and DCT. (e,f) DCT. In order to further investigate the fine microstructure, especially the carbide precipitation, the microstructuresIn order to of further CHT, CT investigate and DCT the samples fine microstructure, are observed by especially TEM, and theas illustrated carbide precipitation, in Figure 9. Thethe microstructuresmorphology of ofmartensite CHT, CT andis of DCTplate samples martensite are observedin all the bysamples. TEM, andIn CHT as illustrated samples, inthe Figure coarse9. Thetwinned morphology substructure of martensite can be clearly is of plate observed, martensite as demonstrated in all the samples. in Figure In CHT 9a, samples,which is the no longercoarse observedtwinned substructurein CT and DCT can samples. be clearly The observed, CHT samples as demonstrated exhibit a greater in Figure number9a, of which large- issize no carbides longer butobserved a smaller in CT number and DCT of small samples.-size Thecarbides. CHT For samples the CT exhibit sample, a greater the number number of oflarge large-size-size carbides carbides is significantlybut a smaller decreased number of while small-size small carbides.-size carbides For thein the CT microstructures sample, the number are observed of large-size to be carbides fine and is uniformlysignificantly distributed decreased. As while shown small-size in Figure carbides 9d, some in rod the-like microstructures carbides appear are in observed some areas, to be and fine some and fineuniformly spheroidally distributed. carbides As shownappear inin Figureother areas9d, some which rod-like have a carbides considerable appear dislocation in some areas, density. and While some comparingfine spheroidally the microstructure carbides appear of inDCT other with areas CT, which as shown have in a considerableFigure 9f, precipitation dislocation density.of carbides While in DCTcomparing samples the are microstructure further increased of DCT with with small CT, asspherical shown incarbide Figure particles9f, precipitation in size ofbetween carbides 10 inand DCT 40 nsamplesm in diameter are further being increased distributed with moresmall numerouslyspherical carbide and uniformlyparticles in in size the between matrix. 10Using and 40image nm analysis,in diameter the beingvolume distributed fraction of more carbides numerously in different and samples uniformly was in counted the matrix. and calculated. Using image The analysis, results showthe volume that the fraction volume of carbidesfraction inof differentcarbides samplesin CHT, was CT countedand DCT and sam calculated.ples is 9.8%, The 13.6% results and show 18.7%, that respectively. The volume fraction of the samples that suffered from cryogenic treatment is higher than that in the CHT ones. Furthermore, with the temperature of cryogenic treatment decreasing, the volume fraction of carbides further increases.

Metals 2018, 8, 808 9 of 13 the volume fraction of carbides in CHT, CT and DCT samples is 9.8%, 13.6% and 18.7%, respectively. The volume fraction of the samples that suffered from cryogenic treatment is higher than that in the CHT ones. Furthermore, with the temperature of cryogenic treatment decreasing, the volume fraction ofMetals carbides 2018, further8, x FOR PEER increases. REVIEW 9 of 13

FigureFigure 9. 9TEM. TEM micrographs micrographs of of samplessamples underunder different heat heat treatment: treatment: (a (,ab,b) CHT;) CHT; (c (,dc,)d CT) CT and and (e, (fe) ,DCT.f) DCT.

TheThe other other obvious obvious characteristic characteristic after cryogenic after cryogenic treatment treatment is the retained is the austenite retained transformation. austenite Thetransformation. XRD patterns The of XRD the samplespatterns of are the illustrated samples are in illustrated Figure 10 in. Figure It can 10. be It seen can thatbe seen the that peaks the of austenitepeaks of exhibit austenite a low exhibit intensity a low value intensity after value cryogenic after cryogenic treatment, treatment, which indicates which indicates that a that fraction a fraction of retained austenite transforms into martensite during the cryogenic treatment process, as highlighted in Figure 10.

Metals 2018, 8, 808 10 of 13 of retained austenite transforms into martensite during the cryogenic treatment process, as highlighted Metalsin Figure 2018, 108, x. FOR PEER REVIEW 10 of 13

FigureFigure 1 10.0. XRDXRD patterns patterns of of the the samples samples under under different different heat heat treatment. treatment.

TheThe relative relative proportions proportions of austenite of austenite and martensite and martensite were calculated were calculatedfrom the X-ray from diffractograms the X-ray diffractogramsby using the formula: by using the formula: hkl hkl Iγ /Rγ γ = hklhkl % hkl , (2) Ihkl/RhklIRγ + I γ /Rhkl γ γ M γ %γ hklhklhklhkl , (2) hkl hkl IRIRγ γ  M γ where Iγ and IM are the integrated intensities of the austenite and martensite peaks, respectively, hkl hkl and R h kland R are the relative intensity factors of the corresponding crystallographic planes. where γI and MI h kl are the integrated intensities of the austenite and martensite peaks, respectively, For simplicity,γ the lowerM angle peaks γ(111) and M/α(110) were neglected and only the peak M/α(200), α h kl γ hkl γ andM/ (211),Rγ and(220) RM and are (311)the relative were used intensity for these factors calculations. of the corresponding The corresponding crystallographic relative intensity planes. factors were adopted from those calculated by Dickson [26] for Fe-1%C alloys. The phase fraction of For simplicity, the lower angle peaks γ(111) and M/α(110) were neglected and only the peak M/α(200), each sample was calculated and is shown in Table2. As can be seen, the volume fraction of austenite is M/α(211), γ(220) and γ(311) were used for these calculations. The corresponding relative intensity decreased sharply from 11.8% to 5.6% when compared with CHT and CT samples, while the volume factors were adopted from those calculated by Dickson [26] for Fe-1%C alloys. The phase fraction of fraction of austenite decreased rarely from 5.6% to 4.8% when compared with CT and DCT samples. each sample was calculated and is shown in Table 2. As can be seen, the volume fraction of austenite It can be concluded that the retained austenite cannot transform into martensite completely even when is decreased sharply from 11.8% to 5.6% when compared with CHT and CT samples, while the cryogenic treatment or deep cryogenic treatment was carried out, corresponding to the existence of volume fraction of austenite decreased rarely from 5.6% to 4.8% when compared with CT and DCT austenite peaks of the CT and DCT samples as shown in Figure 10. samples. It can be concluded that the retained austenite cannot transform into martensite completely even when cryogenic treatment or deep cryogenic treatment was carried out, corresponding to the Table 2. Volume phase fraction of the samples under different heat treatment. existence of austenite peaks of the CT and DCT samples as shown in Figure 10. Volume Phase Fraction (%) Phase Table 2. Volume phase fractionQT of the samples QCTunder different heat QDCT treatment. Martensite 88.2 ±Volume0.4 Phase 94.4 Fraction± 0.3 (%) 95.2 ± 0.3 Phase Austenite 11.8 ±QT0.4 QCT 5.6 ± 0.3QDCT 4.8 ± 0.3 Martensite 88.2 ± 0.4 94.4 ± 0.3 95.2 ± 0.3 4. Discussion Austenite 11.8 ± 0.4 5.6 ± 0.3 4.8 ± 0.3 The results on microstructure evolution, wear test and worn surface morphology reveal that 4.the Discussion wear resistance mainly depends on the microstructure. In the discussion, two major questions are discussed.The results Firstly, on microstructure the microstructure evolution, evolution wear test is analyzed.and worn surface Secondly, morphology the relationship reveal betweenthat the wearmicrostructure resistance andmainly wear depends resistance on isthe discussed. microstructure. In the discussion, two major questions are discussed. Firstly, the microstructure evolution is analyzed. Secondly, the relationship between 4.1. Microstructure Evolution during Cryogenic Treatment microstructure and wear resistance is discussed. As stated above, the significant microstructure changes as a result of cryogenic treatment can 4be.1. summedMicrostructure up as Evolution follows: during firstly, Cryogenic the microstructure Treatment becomes more refined and dense; secondly, As stated above, the significant microstructure changes as a result of cryogenic treatment can be summed up as follows: firstly, the microstructure becomes more refined and dense; secondly, the volume fraction of retained austenite is decreased; thirdly, the precipitation of fine carbides is increased.

Metals 2018, 8, 808 11 of 13 the volume fraction of retained austenite is decreased; thirdly, the precipitation of fine carbides is increased. As is known, the characteristic of high carbon content in the carburized layer of the experimental steel would lower the characteristic temperatures of Ms and Mf. The temperature of Mf is far below the room temperature for the carburized layer. Therefore, the conventional heat treatment fails to transform a considerable amount of austenite into martensite and therefore leads to an unacceptable content of retained austenite for about 11.8%. As the temperature continuously decreases to below room temperature during cryogenic treatment, the transformation of retained austenite to martensite continues to take place. Because the specific volume of martensite is greater than that of austenite, martensitic transformation results in an increase in volume and is accompanied by the introduction of compressive stress in the retained austenite. As the sample is held at cryogenic temperature for a long enough time, the retained austenite gradually transforms into martensite. However, when a low amount of retained austenite remains in microstructure, the retained austenite would be surrounded by martensite. The martensite shrinks at cryogenic temperature and imposes an additional compression on the retained austenite. The compressive state of stress is expected to stabilize austenite and the retained austenite is difficult to further transform [27]. Therefore, while both CT and DCT reduce the volume fraction of retained austenite drastically, there is still a small amount of retained austenite. The volume fractions of retained austenite of CT and DCT samples are 5.3% and 4.6%, respectively. The mechanism of the precipitation of fine carbides by cryogenic treatment is explained as follows. Because of the volume contraction at cryogenic temperature, crystal lattice decreases and internal micro stresses are generated. These internal stresses are responsible for the formation of crystal defects and lead to an increase of dislocation density [25,28]. The carbon atoms would disable diffuse at cryogenic temperature, but they could be captured by the gliding dislocations and segregate nearby by dislocations [29]. Carbides cannot precipitate at cryogenic temperature; however, the segregated carbon and alloying atoms can form clusters that ultimately become the nuclei for the development of carbides during successive tempering [30]. Because the segregation carbon atoms clusters are random and homogeneous, the precipitated carbides are numerous and dispersed. In the CT process, the cryogenic temperature is not sufficient to make martensitic lattice shrinkage and therefore has less influence on the carbides’ precipitation. Therefore, the volume fraction of the precipitated carbides in the CT sample is less than that in the DCT sample [20].

4.2. Relationship between Wear Resistance and Microstructure The results of wear tests indicate that the wear resistance has been significantly improved due to cryogenic treatments. The wear rate of the specimen that suffered from CT treatment, compared to the specimen of CHT treatment, decreases by ~17%, in the same case of sliding 1500 m. From the conventional perspective, there is agreement that the major factor for improving wear resistance by cryogenic treatment is the decrease of the content of retained austenite and the increase of hardness. The XRD and hardness results have supported this perspective with the volume fraction of retained austenite decreasing sharply from 11.8% to 5.6% when compared with CHT and CT samples, along with the hardness of CT specimen markedly increasing from 812 to 848 HV as compared to the CHT sample. Therefore, the wear resistance is greatly improved after the CT process. Furthermore, the wear rate of the DCT sample is significantly lower than that of the CT sample; that is, the improvement of wear resistance after the DCT process is considerably higher than that suffered from the CT process. However, both CT and DCT almost reduce retained austenite completely and the volume fractions of retained austenite of CT and DCT samples are only 5.3% and 4.6%, respectively. Thus, there must be some other phenomena than the variation in retained austenite content responsible for the difference in their wear resistance. Based on the experimental measurement of the precipitation of carbides from SEM and TEM, the volume fraction of carbides in the CHT, CT and DCT samples is 9.8%, 13.6% and 18.7%, respectively. The volume fraction of carbides in the DCT sample is higher than in the CT sample. The increase in carbides’ precipitation also brings the increase Metals 2018, 8, 808 12 of 13 of hardness from 848 to 886 HV. Therefore, it is concluded that the improvement in wear resistance by DCT over CT could be due to the greater precipitation of fine carbides. The substantial precipitation of carbides in DCT samples can reduce the carbon and alloy contents in the matrix and therefore improves the toughness of the matrix. Therefore, the microstructure composed of more carbides and tougher matrix is certain to lead to the improvement of wear resistance. On the basis of the XRD analysis, it can be concluded that the retained austenite cannot transform into martensite completely even when deep cryogenic treatment is carried out. According to the SEM observation (Figure7) of the worn surface, the predominant wear mechanisms of samples were groove and delamination. The wear rate may be controlled by crack nucleation and propagation beneath the surface. Retained austenite may inhibit crack propagation either by changing the crack propagation direction or by absorbing energy. Therefore, a small amount of retained austenite in the microstructure is partly beneficial to the improvement of wear resistance.

5. Conclusions In this paper, the effect of cryogenic treatment on the microstructure and wear resistance of carburized 20CrNi2MoV steel was investigated. The microstructure evolution during cryogenic treatment and the relationship between wear resistance and microstructure were analyzed. The conclusions of the investigation are as follows:

(1) The significant microstructure changes as a result of cryogenic treatment are summed up as follows: the microstructure becomes more refined and dense; the volume fraction of retained austenite is decreased and the precipitation of fine carbides is increased. (2) The wear resistance is improved by 17% due to CT and by 25.5% due to DCT when compared to CHT. The significant improvement in wear resistance after cryogenic treatment is attributed to the microstructural changes. The precipitation of fine carbides plays a more prominent role in enhancing wear resistance.

Author Contributions: Both authors contributed equally to the work. Funding: This research was funded by National High-tech R&D Program (863 Program), grant number 2012AA03A503. Acknowledgments: The authors are grateful to the State Key Laboratory of Rolling and Automation for material preparation and mechanical properties testing. Conflicts of Interest: The authors declare no conflict of interest.

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