Plastic Deformation Behavior and Processing Maps of 35CrMo steel

Plastic Deformation Behavior and Processing Mapsof 35CrMo SteelZheng-bing Xiao,Yuan-chun Huang,and Yu Liu(Submitted October 21,2015;in revised form December 27,2015;published online February 16,2016)Hot deformation behavior of 35CrMo steel was investigated by compression tests in the temperature range of 850to 1150°C and strain rate range of 0.01to 20s 21on a Gleeble-3810thermal simulator.According to processing maps constructed based on the experimental data and using the principle of dynamic materials modeling (DMM),when the strain is 0.8,three safe regions with comparatively high efficiency of power dissipation were identified:(850to 920)°C/(0.01to 0.02)s 21,(850to 900)°C/(10to 20)s 21,and (1050to 1150)°C/(0.01to 1)s 21.And the domain of (920to 1150)°C/(2.7to 20)s 21is within the instability range,whose efficiency of power dissipation is around 0.05.The deformed optical microstructure indicated that the combination of low deformation temperature (850°C)and a relatively high strain rate (20s 21)resulted in the smallest dynamic recrystallized grains,but coarser grains were obtained when a much higher strain rate was employed (50s 21).A lower strain rate or a higher temperature will accelerate the growth of grains,and both high temperature and high strain rate can cause microcracks in the deformed steel.Integration of the processing map into the optical microstructure identified the region of (850to 900)°C/(10to 20)s 21as the ideal condition for the hot deformation of 35CrMo steel.Keywords35CrMo steel,hot workability,microstructure,pro-cessing map1.IntroductionMaterial flow behavior is complex and is an important issue for hot forming processes (Ref 1–3).The microstructure evolution of a part during its deformation is significantly affected by the processing parameters such as temperature,strain rate,and the resulting strain.The mechanical properties of the finally formed part are heavily dependent on the microstructure.Understanding the flow behavior and kinetics of metallurgical transformations is of great importance for mate-rial processing designers.The concept of processing map was first put forward by Prasad et al.(Ref 4)based on the dynamic materials modeling (DMM),which aims to correlate the constitutive behavior with flow instability,hot workability and microstructure evolution.It has been used extensively to separate the safe domains with optimized deformation temperature and strain rate from the unsafe zones,for the hot working of metals such as steels,aluminum alloys,magnesium alloys,and super alloys (Ref 5–11).Wen et al.and Zhang et al.have optimized the hot working parameters for two kinds of Ni-based super alloys by using theDMM,which agreed well with experimental results (Ref 8,12).Through the analysis of the data obtained from hot compression tests using DMM,Aneta et al.have plotted the processing maps and hardness maps of the Ti-3Al-8V-6Cr-4Zr-4Mo alloy and ascertained the influence of recrystallization and dynamic recovery on the strength of the alloy (Ref 9).Using the processing maps,Zhou et al.have characterized the workability of a SiCp/AZ91enforced magnesium matrix composite and identified the optimum working temperature range as (270to 370)°C with a strain rate of (0.001to 0.01)s À1,where the dissipation efficiency was found to be as high as 38%.The characteristics of the predicted microstructures agreed well with the optical microscopy observations (Ref 6).Moreover,the processing maps were revealed for AZ31magnesium alloy (Ref 13),Al-6.2Zn-0.70Mg-0.3Mn-0.17Zr alloy (Ref 14),plain eutectoid steels (Ref 15),Nickel-based corrosion resistant alloy (Ref 16),etc.These works serve to highlight the significance of processing maps in predicting the optimal parameters to be used as guidelines for plastic forming.Due to its excellent mechanical properties,especially the optimized combination of strength,wear resistance,and toughness,35CrMo steel has been widely used in bearings,gears,transmission shafts,etc.In the past,considerable attention has been focused on the microstructure evolution and flow behavior of 42CrMo steel (Ref 17–25).However,for 35CrMo steel,more efforts were concentrated on the surface treatment procedures such as nitrocarburizing and post-oxida-tion or the influence of the inclusion size on the fatigue behavior (Ref 26–28).Despite the fact that that there is only a minor difference in composition between 42CrMo and 35CrMo (C levels,Mn and Cr contents),their mechanical properties are significantly different.For 42CrMo,the higher C content contributes to more carbide,and finer grains are obtained after quenching.Both r b (‡1080MPa for 42CrMo and ‡985MPa for 35CrMo)and r s (‡930MPa for 42CrMo and ‡835MPa for 35CrMo)values of 42CrMo are much higher than those of 35CrMo (Ref 29).Therefore,the conclu-Zheng-bing Xiao ,School of Mechanical and Electrical Engineering,Central South University,Changsha 410083,China and Collaborative Innovation Center of Advanced Nonferrous Structural Materials and Manufacturing,Changsha 410012,China;Yuan-chun Huang ,School of Mechanical and Electrical Engineering,Central South University,Changsha 410083,China and Light Alloy Research Institute,Central South University,Changsha 410012,China;and Yu Liu ,Light Alloy Research Institute,Central South University,Changsha 410012,China.Contact e-mails:xiaozb@,science@,and 249120777@.JMEPEG (2016)25:1219–1227ÓASM International DOI:10.1007/s11665-016-1933-71059-9495/$19.00sions obtained for42CrMo are not applicable for35CrMo.To the best of our knowledge,there is little literature available on the workability characteristics and processing maps of35CrMo steel and these aspects need to be investigated further.In the present study,flow stress,hot deformation behaviors, and microstructures of35CrMo steel were studied using isothermal compression tests at different temperatures and under different strain rates.Simultaneously,the processing maps under different strains were plotted based on the DMM, and the optical microstructures in the feasible and dangerous zones were obtained to verify the maps and determine the appropriate working parameters for35CrMo steel.2.Materials and Experimental DetailsThe chemical compositions(wt.%)of the35CrMo studied are as follows:0.344C-0.21Si-0.56Mn-0.018P-0.005S-0.95Cr-0.19Mo-0.0032Al-(bal.)Fe.Cylindrical specimens with a diam-eter of10mm and a height of12mm were machined from the cast ingot.Frictions between the specimens and the upsetting dies were minimized by using a graphite foil with the thickness of0.1and15mm in diameter between them.As shown in Fig.1,the specimens were heated to1200°C at a heating rate of10°C/s and kept at this temperature for 6min,then cooled to the deformation temperature at a rate of 10°C/s and held for2min to eliminate the thermal gradient before loading.Hot compression tests were carried out on a Gleeble-3810thermo-simulation machine atfive different strain rates(0.01,0.1,1,10and20sÀ1)under four different temperatures(850,950,1050,and1150°C),with60% reduction in height and a true strain of0.85.After the hot compression,specimens were quenched in water immediately to preserve the hot deformation microstructure.Then,the deformed samples were sliced along the axial section,polished, and etched in the solution consisting of picric acid(2.5g)+H2O(50ml)+HCl(1ml)+detergent(2g)at60 to80°C for4to10min before microstructural analysis. During the compression tests,the true stress,strain,and temperature were automatically controlled and recorded using a computer.3.Results and Discussion3.1True Stress-Strain CurvesTypical true stress-strain curves obtained from the hot deformation tests are shown in Fig.2.Obviously,the true stress is susceptible to the deformation temperature,the strain rate, and the true strain.For afixed true strain,theflow stress decreases markedly with increase in temperature or reduction in strain rate.For the investigated steel,it is found that,in the initial stage, all theflow curves show a sharp increasing trend;after reaching the peak stresses,the curves begin to separate into three types. Thefirst type drops to a fairly constant value(0.01sÀ1),the second remains almost at the peak stress(0.1and1sÀ1),and the third falls gradually without reaching a plateau(10and 20sÀ1).It is well known that the characteristics of the true stress-strain curves represent the competition between work hardening and dynamic recovery/recrystallization(Ref30,31).In the beginning,when the true strain is small,the instant generationFig.2Typical true stress-strain curves of the studied35CrMo steel at0.01sÀ1(a)and850°C(b)and multiplication of dislocations result in the work hardening effect playing the dominant role,but the dislocation climbing, cross-slip,and sliding caused by the dynamic recovery process are too weak to relieve the work hardening effect,and as a result,theflow stress increases rapidly.When the true strain reaches the critical strain e c,for thefirst type of curve,the combined effect of the softening of the dynamic recovery process and dynamic recrystallization becomes strong enough to counteract the effect of work hardening,leading to a steady-state dynamic equilibrium(Ref32–34).For the second type,it is assumed that only the dynamic recovery process is operative, even though it is not possible to judge solely from the true strain-stress curve,whether or not dynamic recrystallization occurred(Ref35).For the third type,it is believedthat Fig.3Relationships between theflow stress and strain rate under strains of(a)0.1;(b)0.2;(c)0.3;(d)0.4;(e)0.5;(f)0.6;(g)0.7;(h)0.8 (symbols for the experimental results;solid line for thefitting curves)extensive dynamic recrystallization contributes to the continual reduction offlow stress,which was proven by microscopic examination,as described in section3.4.3.2Theory of Processing MapsAccording to the DMM initially proposed by Prasad et al. (Ref4),a power dissipater can be used to characterize the hot deformation process in materials.Generally,the power dissi-pation P representing the total energy absorbed by the object during the hot deformation can be divided into two parts,the G content and the J co-content,where G stands for the power dissipated in the plastic deformation and J the power absorbed during the microstructure transformation(Ref36).The total power consumed by the deformed object can be defined asP¼r_e¼GþJ¼Z_e0r d_eþZ r_e d r:ðEq1Þ_e in the equation is the strain rate in sÀ1,and r is theflow stress in MPa.For the object compressed at a constant temperature and strain,r can be calculated asr¼K_e m;ðEq2Þwhere K is a material constant;m is the strain rate sensitivity coefficient(at constant T and e)and can be represented as fol-lows:m¼dJ¼dðln rÞe;T:ðEq3ÞThe value of the J can be calculated by substituting the value of m in the equation shown below:J¼Z r_e d r¼mmþ1r_e:ðEq4ÞThe efficiency of power dissipation,g,for a non-ideal linear dissipation process is determined as follows:g¼JJ max¼2mmþ1:ðEq5ÞUsually,a higher value of g indicates a better processability.The following equation,proposed by Prasad et al.(Ref4, 37),can be used in hot deformation to determine the instability regions on the power dissipation maps:nð_eÞ¼@ln mmþ1@ln_eþm0:ðEq6ÞA cubic spline function is adopted tofit the ln r and ln_e curves,so that the value of m for a certain strain and temperature can be expressed as m¼AþB ln_eþC ln_e2; where A,B,C,andD are constants.Thus,Eq6can be rewritten asTable1Coefficients for strain rate sensitivity m vs.ln_e,T=850to1150°C,and e=0.01to20s21Strain T(°C)m/ln_e relation0.1850m¼0:07239þ0:02368ln_eþ0:00792ln_e2950m¼0:10936þ0:0002563ln_eþ0:0005859ln_e21050m¼0:10902À0:000059ln_e0:000393ln_e21150m¼0:14615À0:02138ln_eÀ0:0081ln_e20.2850m¼0:05869þ0:02144ln_eþ0:00999ln_e2950m¼0:12731À0:02244ln_eÀ0:00741ln_e21050m¼0:10524À0:01142ln_eþ0:003271ln_e21150m¼0:15582À0:03456ln_eÀ0:00543ln_e20.3850m¼0:04768þ0:02464ln_eþ0:01359ln_e2950m¼0:13449À0:02536ln_eÀ0:00627ln_e21050m¼0:13801À0:01476ln_eÀ0:00146ln_e21150m¼0:20559À0:0421ln_eÀ0:01164ln_e20.4850m¼0:0406þ0:023ln_eþ0:01584ln_e2950m¼0:15059À0:02694ln_eÀ0:00876ln_e21050m¼0:15387À0:01884ln_eÀ0:00414ln_e21150m¼0:19933À0:03824ln_eÀ0:00777ln_e20.5850m¼0:03938þ0:01576ln_eþ0:01491ln_e2950m¼0:15449À0:03218ln_eÀ0:01074ln_e21050m¼0:15308À0:02464ln_eÀ0:00588ln_e21150m¼0:19298À0:04642ln_eÀ0:008161ln_e2 0.6850m¼0:0274þ0:01936ln_eþ0:01716ln_e2950m¼0:13587À0:02718ln_eÀ0:00711ln_e21050m¼0:13599À0:0222ln_eÀ0:00345ln_e21150m¼0:16298À0:04478ln_eÀ0:00312ln_e20.7850m¼0:02334þ0:01828ln_eþ0:01665ln_e2950m¼0:12065À0:02788ln_eÀ0:00537ln_e21050m¼0:12392À0:02876ln_eÀ0:00474ln_e21150m¼0:14375À0:0562ln_eÀ0:003451ln_e20.8850m¼0:01517þ0:01517ln_eþ0:0159ln_e2950m¼0:10062À0:03218ln_eÀ0:00426ln_e21050m¼0:10777À0:03892ln_eÀ0:00666ln_e21150m¼0:11555À0:0691ln_eÀ0:00351ln_e2nð_eÞ¼@ln mmþ1@ln eþm¼Bþ2C ln_e1þAþB ln_eþC ln_e2ðÞþAþB ln_eþC ln_e20:ðEq7ÞHere,the instability index n is a dimensionless parameter;avalue of n£0indicatesflow instability,and that the domain isnot‘‘safe’’for hot working.3.3Processing Maps for the35CrMo SteelThe relationships between ln r and ln_e under differentconditions are illustrated in Fig.3;all the curves havebeenFig.4Hot processing maps of tested35CrMo steel at different true strains.(a)0.1;(b)0.2;(c)0.3;(d)0.4;(e)0.5;(f)0.6;(g)0.7;(h)0.8fitted by cubic splines,referring to the equations in Table1.By increasing the strain rate to more than10sÀ1(20sÀ1in this study),the stress values of the compressed samples(except for the one deformed at850°C)did not increase in the same way as for lower strains but was equal to or even lower than that for 10sÀ1,especially at high strains.It is believed that massive nucleation and dynamic recrystallization after the peak stress at the strain rate of20sÀ1was responsible for the phenomenon, and this is demonstrated by the optical microstructure obser-vation results as described in section3.4.The processing map can be constructed by superimposing the instability map for index n onto the power dissipation map with index g,which can be used to optimize the processing parameters.According to the criterion given by Eq7,the material will exhibit instability if nð_eÞbecomes negative.The processing maps under different strains are shown in Fig.4.The contour numbers in thefigures represent the efficiency of power dissipation,g.The gray areas are the domains with negative g, and thosefilled by slash are the instability regions with negative n.It is easy to infer from Fig.4that all the areas with negative values of g are covered by instability regions with negative n values,which is in accordance with the conclusion that a negative value for power dissipation efficiency indicates the formation of microstructure defects(Ref38).As the true strain increases,the instability region is seen to expand,which is in accordance with the results reported in other studies(Ref39–41).It can be seen that the instability region appears in all the figures,irrespective of the value of the true strain.During the processing of steel,such areas should be avoided to prevent the occurrence of instability phenomena such asflow localization, adiabatic shear band,and cracking,which will significantly hamper the mechanical properties of the processed material.Apart from the dangerous areas,the left part in the processing maps is the zone that is‘‘safe’’for processing. Usually,the higher the value of power dissipation efficiency in the stable regions,the more is the fraction of energy consumed for microstructure transformations,such as dynamic recrystal-lization and dynamic recovery,which are,in turn,more favorable for materials processing.Therefore,the regions with high power dissipation efficiencies in the stable region are ideal for the hot deformation.As marked in thefigures,there are three domains with comparatively high g values in the‘‘safe’’zones as the true strain increases to0.8,specifically,(850to900)°C/(10to 20)sÀ1(marked as A),(850to920)°C/(0.01to0.02)sÀ1(-marked as B)and(1050to1150)°C/(0.01to1)sÀ1(marked as C).However,the area with(920to1150)°C/(2.7to20)sÀ1 (marked as D)lies in the instability region.For the samples deformed at low temperatures with high strain rate(marked as A),more deformation energies are stored in the deformed blocks,for the dislocation multiplication,and effective pinning of the dislocation movement for the second phases in the matrix.Meanwhile,the elongated and broken grains and sub-grains will act as nuclei for dynamic recrystal-lization and,when combined with the power dissipated in plastic deformation,would lead to a temperature rise and afford little incubation time for recrystallization.All these factors promote dynamic recrystallization,resulting in a high power dissipation efficiency,which is in accordance with theflow curves in Fig.2(b)and the optical microstructure in Fig.4(b). However,as the deformation temperature becomes higher (marked as D),more energy is consumed for the vigorous dislocation mobility,annihilation and the relaxation of the lattice distortion,and the energy for microstructural evolution accounts for a lower fraction of the total.In this situation,a low strain rate(marked as B and C)would be favorable to the growth of grains and has a higher efficiency of power dissipation.3.4Microstructure ObservationFigure5(a)shows the optical microstructure of the sample before deformation,the deformed microstructures of the steel at different zones marked as A,B,C,and D in Fig.4,and other conditions are illustrated in Fig.5(b)to(h),and the grain sizes estimated from the images are shown in Table2.The figures and the measured grain sizes for all the deformed samples show dynamic recrystallization,and that the grain sizes are more sensitive to the deformation temperature than to the strain rate in the studied range of process parameters.Thefinal grain sizes of the samples deformed at1150°C are much higher than those deformed at850°C,for the strain rate of20 or0.01sÀ1.By raising the temperature,the mobility of the grain boundaries is increased,as also the annihilation of dislocation,which favors the grain growth,and due to the lesser number of nuclei for dynamic recrystallization,the grain size is increased.Microcracks,both transgranular and along the grain boundaries were found in the sample deformed at1150°C/ 20sÀ1,as illustrated in Fig.5(d),which belongs to the instability region in the processing map(Fig.4h).It is well known that dynamic recrystallization starts at the grain boundaries with a high dislocation density and a high stored energy.Raising the strain rate would increase work hardening rate,which is beneficial for the recrystallization,for more energy,and dislocations are introduced(Ref42).At a low temperature(850°C),raising the strain rate,on one hand,will be disadvantageous for dislocation annihilation,which in-creases the dislocation density and the energy stored,so that the number of nuclei for dynamic recrystallization rises sharply.On the other hand,the time for the growth of dynamically recrystallized grains is reduced.As a result,a high strain rate (20sÀ1)leads tofiner grains,as shown in Fig.5(b).However, by continuing to increase the strain rate to50sÀ1,the grains become bigger.Table2Grain sizes of the photos presented in Fig.5Fig.a b c d e f g h Deformation condition Temperature,°C (8508501150115085010501050)Strain rate,sÀ1…200.01200.01501020 Grain size,l m273.57.2911.8629.3953.3313.7728.4523.731150°C/20sÀ1,(e)1150°C/0.01sÀ1,(f)850°C/50sÀ1,(g)1050°C/10sÀ1,and(h)1050°C/20sÀ1Following the investigations by Mejı´a et al.(Ref2), Ebrahimi et al.(Ref43),and Luton et al.(Ref32),unlike the flow curves with several oscillations,recrystallization cycles would begin before the end of a previous one for theflow curve with a single peak(just as the curves obtained in this investigation),and the material showing such a behavior is, therefore,always in a partly recrystallized state after thefirst pared to the strain rate of50sÀ1,the time for the hot deformation is increased for those deformed at the strain rate of 20sÀ1,given that the newly formed nuclei in the subsequent cycles have a greater chance to grow into grains,or even more subsequent cycles of recrystallization,and thus refine the grains.For the strain rate of50sÀ1,there is little time for the newly formed nuclei to grow into recrystallized grains or less cycles of recrystallization,and hence,numerous tiny nuclei are found along the grain boundaries as depicted in Fig.5(f).The efficiency of power dissipation was calculated to be0.1527, which is much smaller than that for850°C/20sÀ1.By lowering the strain rate,the energy and dislocation caused by work hardening decrease,while the time for the growth of grains increases substantially,and therefore,the grains in Fig.5(c)become coarser.Under the condition of 1150°C/0.01sÀ1,both the high temperature and low strain rate allow the grains to steadily grow to the largest observed size as shown in Fig.5(e).As for the condition of1150°C/20sÀ1, though the high strain rate could possibly increase the number of nuclei sharply,the high temperature will quicken the annihilation of dislocations and the atomic mobility to reduce the area with a high dislocation density.Hence,very little energy is consumed by the microstructure evolution,conform-ing to the processing map of Fig.4(h),which shows that the sample deformed at1150°C/20sÀ1has only a power dissipa-tion of0.05.In summary,a comparatively higher strain rate, 20sÀ1in this study,along with a lower deformation temper-ature will result infiner recrystallized grains for35CrMo steel.Additionally,as can be seen from Fig.5(g)and(h),the grain size becomes smaller when the strain rate increases from10to 20sÀ1,and the same phenomenon is also observed in the samples deformed at950and1150°C.Apparently,the volume fraction of dynamic recrystallization increased with increase in strain rate,and the corresponding increase of the stress was compensated by the resultant softening effect,as shown in Fig.3.Based on the processing maps,in combination with optical microstructures,the most suitable processing condition for 35CrMo is concluded to be(850to900)°C/(10to20)sÀ1.In order to avoidflow instability,the material should not be forged under the condition of(920to1150)°C/(2.7to20)sÀ1.4.ConclusionsThe hot deformation behavior of35CrMo steel has been investigated at a temperature range of850to1150°C and strain rate range of0.01to20sÀ1.The following conclusions can be drawn from this research:1.The true stress-strain curves of35CrMo steel are sensi-tive to the deformation temperature and strain rate and can be classified into three types.The true stress in-creases with decrease in deformation temperature or in-crease in strain rate when the strain rate is below10sÀ1,after which the true stress increases or even decreases with further increase in strain rate depending on the deformation temperature.2.According to the processing maps constructed,there arethree domains with a relatively high efficiency of power dissipation when thefinal strain is0.8,which are benefi-cial for the hot processing,namely,(850to920)°C/(0.01 to0.02)sÀ1,(850to900)°C/(10to20)sÀ1,and(1050 to1150)°C/(0.01to1)sÀ1.The instability domains are mainly located in the(920to1150)°C/(2.7to20)sÀ1 zone,the areas of both the instability domains and the feasible domains mentioned above increased with increasing true strain.3.Optical microstructure observations indicated that dy-namic recrystallization is easy to occur in35CrMo steel during hot deformation,and conditions of high strain rate (20sÀ1)along with low deformation temperature (850°C)will sharply decrease the grain size,for the driving force and energy introduced by much higher work hardening rate,and also shorter time at compara-tively high strain rate.However,by increasing the strain rate to50sÀ1,the time for the newly formed nuclei to grow during subsequent recrystallization cycles was re-duced,which resulted in coarser grain sizes and much lower efficiency of power dissipation.Lowering the strain rate or/and increasing the temperature(850°C/0.01sÀ1 and1150°C/0.01sÀ1)will lead to coarser grains.Condi-tions with both high temperature and high strain rate (1150°C/20sÀ1)will not only reduce the number of nu-clei but also significantly prevent the growth of grains, resulting in a small number of dynamic recrystallized grains,and microcracks along the grain boundaries,cor-responding to a5%efficiency of power dissipation.Fi-nally,the condition(850to900)°C/(10to20)sÀ1is found to be ideal for the hot deformation of35CrMo, whereas the condition(920to1150)°C/(2.7to20)sÀ1 corresponds toflow instability.AcknowledgmentsThe authors are grateful for thefinancial support from the National Program on Key Basic Research Project of China(No. 2014CB046702)and to Wang Zi in 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