拓展延伸Microstructural evolution in 42CrMo steel during
compression at elevated temperatures
猎豹Y .C.Lin ⁎,Ming-Song Chen,Jue Zhong
School of Mechanical and Electrical Engineering,Central South University,Changsha 410083,China
Received 24August 2007;accepted 15November 2007
Available online 22November 2007
Abstract
In order to study the workability and to optimize the processing parameters for 42CrMo steel hot forming,the hot compressive tests of 42CrMo steel were performed in the temperature range of (850–1150)°C at strain rates of (0.01–50)s −1and deformation degrees of (10–60)%on Gleeble-1500thermo-simulation machine.The effects of processing parameters,including the strain rate,forming temperature and deformation degree,on the microstructures of 42CrMo steel were investigated by metallurgical analysis.The results show that the average grain size of the deformed 4
金枪鱼的家常做法2CrMo steel increas with the forming temperature and decreas with the deformation degree and strain rate.In a word,the grain size of hot compressive 42CrMo steel is dependent on the forming temperature,strain and strain rate,also on Zener –Hollomon parameter.©2007Elvier B.V .All rights rerved.
Keywords:Metals and alloys;Microstructure;42CrMo steel;Processing parameters;Grain size
1.Introduction
In hot deformation,microstructure evolution occurs and it has an important influence on mechanical properties of the product.In order to improve the quality of products,understandings of the relationship between thermomechanical parameters and micro-structure of metals and alloys under hot deformation condition is of great importance for designers of metal forming process (hot rolling,forging and extrusion)becau of its effective role on the kinetics of metallurgical transformation [1–5].
42CrMo (American grade:AISI 4140)is one of the repre-ntative medium carbon and low alloy steel.Due to its good balance of strength,toughness and wear resistance,42CrMo high-strength steel is widely ud for many general purpo parts including automotive crankshaft,rams,spindles etc.In t
he past,many investigations have been carried out on the behaviors of 42CrMo steel [6–11].Lin et al.[7]established the flow stress constitutive equations of the work hardening-dynamical recovery period and dynamical recrystallization period.Kim et al.[8]investigate the effect of deformation mode on constitutive relation by hot torsion and compression tests.Kim and Yoo [10]esta-blished the quantitative relationships between the flow stress and the volume fraction of dynamic recrystallization (DRX)as a function of processing variables such as strain rate,temperature,and strain for AISI type 4140medium carbon steel,by means of torsion tests.Lin et al.[11]derived a revid constitutive equation incorporating the effects of forming temperature,strain rate and deformation degree by compensation of strain and strain rate.Despite large amount of efforts invested into the behaviors of 42CrMo steel,the effects of hot forming processing parameters,including the strain rate,forming temperature and deformation degree,on the microstructures of hot deformed 42CrMo steel need to be further investigated to study the workability and establish the optimum hot forming processing parameters.The objective of the prent work is to investigate the microstructure evolution inside 42CrMo steel during hot forming processing by metallurgical analysis.2.Experiments
The material ud in this investigation was the commercial 42CrMo high-strength steel,and its chemi
cal composition (wt.%)is 0.450C –0.280Si –0.960Cr –0.630Mn –0.190Mo –0.016P –0.012S –0.014Cu –(bal.)Fe.Cylindrical specimens were machined with a diameter of 10mm and a height of 12mm.In order to
Available online at
Materials Letters 62(2008)2132–
2135
/locate/matlet
⁎Corresponding author.Tel.:+8607318877915.E-mail address:yclin@mail. (Y .C.Lin).
0167-577X/$-e front matter ©2007Elvier B.V .All rights rerved.doi:10.1016/j.matlet.2007.11.032
minimize the frictions between the specimens and die during hot deformation,the flat ends of the specimen were recesd to a depth of0.1mm deep to entrap the lubricant of graphite mixed with machine oil.The hot compression tests were performed on Gleeble-1500thermo-simulation machine in the four different temperatures (850°C,950°C,1050°C and1150°C),four different strain rate (0.01s−1,0.1s−1,1s−1,10s−1and50s−1)and four different deformation degree(a reduction of10%,20%,30%,40%and60% in specimen height).As shown in Fig.1,the specimens were heated to1200°C at a heating rate of10°C/s,held for5min and cooled at 10°C/s to the forming temperature,held for1min to eliminate thermal gradients,Then,some specimens were compresd at the lected constant temperature and rapidly quenched with water to retain the recrystallized austenitic microstructures;others were directly and rapidly quenched with water to retain the austeniti
c microstructures at elevated temperatures.The former means the deformed microstructures,while the later means the undeformed microstructures.The specimens were ctioned along the long-itudinal compression axis.Then,the ctions were polished and etched in an abluent solution of saturated picric acid.The optical microstructures in the center region of the ction plane were examined.The average single-circle-intercept grain sizes were measured using the method described in the ASTM standards. 3.Results and discussion
3.1.Effect of the strain rate on the grain size
The effect of strain rate on microstructures of42CrMo steel was investigated at a deformation degree of60%and temperature of1050°C. Fig.2(a)–(e)shows the optical deformed microstructures of42CrMo steel with strain rates of0.01s−1,0.1s−1,1s−1,10s−1and50s−1,respectively. It can be easily found that the higher the strain rate,the finer the grains,
and Fig.2.Optical deformed microstructures of42CrMo steel under1050°C with strain rates of(a)0.01s−1;(b)0.1s−1;(c)1s−1;(d)10s−1;(e)50s−1
.
Fig.1.Experimental procedure for hot compression test.
2133
Y.C.Lin et al./Materials Letters62(2008)2132–2135
基带是什么the average grain sizes were measured as 133.93μm,78.58μm,45.52μm,30.53μm and 20.77μm for the strain rates of 0.01s −1,0.1s −1,1s −1,10s −1and 50s −1,respectively.With the increa of strain rate,dynamic recovery rate decreas [12].Meanwhile,the dislocation generation rate,the dislocation density and nucleation sites increa in the deformed microstructure.Then,for the ca of high strain rate,there are
more
Fig.3.Optical deformed microstructures of 42CrMo steel under strain rate of 0.1s −1and temperatures of (a)850°C;(b)950°C;(c)1050°C;(d)1150
°C.
Fig.4.Optical microstructures of 42CrMo steel with deformation degree of (a)0(underformed);(b)10%;(c)20%;(d)30%;(e)40%;(f)60%,under strain rate of 0.1s −1and temperature of 1050°C.
2134Y.C.Lin et al./Materials Letters 62(2008)2132–2135
deformation energies stored in the deformed42CrMo steel.Therefore,it is popular understood that more substructures can be generated in the initial grains when strain rate is higher,which will produce more nuclei per unit volume of the grains.This mechanism can make the grain finer when strain rate is higher.When the strain rate is decread,the dynamic recovery rate increas and the dynamic recovery proceeds adequately or the recrys-tallization occurs during deformation.However,there is not sufficient deformation energy for complete recrystallization,and then the recrystalli-zation degree is small.So,the grain sizes under lower strain rate are larger than tho under higher strain rate.Therefore,the effect of the strain rate on the microstructures of42CrMo steel is significant.Additionally,it should be noted that a higher temperature can provide more deformation energy to enhance the dynamic recrystallization although strain rate is lower.
3.2.Effect of forming temperature on the grain size
Also,the effect of forming temperature on the microstructures of 42CrMo steel was investigated at a deformation degree of60%and strain rate of0.1s−1.Fig.3(a)–(d)illustrates the optical deformed microstructures of42CrMo steel at temperatures of850°C,950°C,1050°C and1150°C, respectively.It is obvious that the average grain size increas with the increa of the forming temperature.The average grain sizes were measured as22.97μm,55.79μm,78.58μm and100.01μm for the deformation temperatures of850°C,950°C,1050°C and1150°C,respectively.At a higher forming temperature,grain growth takes place during deformation and the equiaxed grain structures are obtained.The higher the forming temperature,the larger the dynamic recrystallization degree.However,at the relatively low forming temperature(850°C),the elongated grains with rrations developed in the grain boundaries were obrved,indicating that the dynamic recrystallization occurs in evidence during the hot compression deformation,and the small grains located along the grain boundaries of larger grains are typical of“necklace”type dynamic recrystallization[13]. On the other hand,the“necklace”grains indicate the dynamic recrys-tallization is not complete.Therefore,the effect of forming temperature on the microstructures of42CrMo steel is also significant.
进步的阶梯From the above analysis,it is obvious that the dynamic recrys-tallization grain size of the hot deforme
d42CrMo steel is very nsitive to the forming temperature and strain rate.Usually,Zener–Hollomon parameter[14]is ud to describe the effects of the forming tem-perature and strain rate on behaviors of metals and alloys.The grain size of the hot deformed42CrMo steel is also dependent on the Zener–Hollomon ,decreasing of Z leads to more adequate proceeding of dynamic recrystallization.
3.3.Effect of the deformation degree on the grain size缓解英语
In this part,the effect of deformation ,a reduction in specimen height,on the microstructures of42CrMo steel was investigated under the strain rate of0.1s−1and forming temperature of1050°C.The optical microstructures of42CrMo steel with deformation degree of0 (underformed),10%,20%,30%,40%and60%are shown in Fig.4(a)–(f), respectively.It can be en that the microstructure before test has an equiaxed grain structure with an average grain size of about155μm.The average grain sizes for the deformed structures were measured as 130.61μm,100.02μm,88.46μm,82.35μm and78.58μm for the deformation degree of10%,20%,30%,40%and60%cas,respectively. The higher the deformation degree,the finer the grains.When the deformation degree is large,the dislocation generation rate,the dislocation density and deformation energy stored in the deformed structures all increa.So,for the larger deformation
degree,the recrystallized grains nucleate more easily,which may result in that the grains become finer and the grain boundary area per unit volume increas.Hence,during large strain deformation of metals and alloys the structural changes are characterized by the formation of strain-induced dislocation subboundaries, such as subgrain walls,shearbands,microbands and den dislocation walls,resulting in continuous subdivision of coar grains into misoriented fine domains[15].It is popular understood that the dynamic recrystalliza-tion will occur when the strain is over the critical strain.However,the average grain size of deformed specimens with10%deformation degree decreas,compared to that of the unformed specimens.This is becau the static recrystallization is ine vitable,although the specimens after compression were rapidly quenched with water.
4.Conclusions
In this study,the effects of the strain rate,forming temperature and deformation degree on the grain size of42CrMo steel were investigated by metallurgical analysis.It has been found that the grain size of the hot deformed42CrMo steel is dependent on the forming temperature,strain and strain rate,also on Zener–Hollomon parameter.The average grain size of hot deformed 42CrMo steel increas with forming temperature and decreas with deformation degree and strain ,the decrea of Zener–Hollomon parameter leads to more adequate proceeding of dynamic recrystallization.Therefo
re,to obtain uniform fine-grain microstructure in hot forming process,it is necessary to increa deformation degree and decrea forming temperature at an appropriate strain rate.Meanwhile,it should be emphasized that the forming temperature should be adequate for the dynamic recrystallization,becau the dynamic recrystallization will not occur or cannot be finished when the forming temperature is relatively low.
Acknowledgements
This work was supported by973Program(grant no. 2006CB705401),China Postdoctoral Science Foundation(grant no.20070410302),the Science Rearch Foundation Program of Central South University,and the Postdoctoral Science Founda-tion of Central South University.
References
[1]B.Eghbali,Mater.Lett.61(2007)4006–4010.
[2]X.G.Fan,D.M.Jiang,Q.C.Meng,L.Zhong,Mater.Lett.60(2006)
空调病有哪些症状1475–1479.
[3]J.T.Liu,H.B.Chang,R.H.Wu,T.Y.Hsu,X.Y.Ruan,Mater.Charact.
45(2000)175–186.
[4]J.Huang,Z.Xu,Mater.Lett.60(2006)1854–1858.
[5]S.Bruschi,S.Poggio,F.Quadrini,M.E.Tata,Mater.Lett.58(2004)
3622–3629.
[6]H.Holzapfel,V.Schulze,O.Vöhringer,E.Macherauch,Mater.Sci.Eng.A
248(1998)9–18.
[7]Y.C.Lin,M.S.Chen,J.Zhong,Mech.Res.Commun.(in press)DOI:
10.hrescom.2007.10.002.
[8]S.I.Kim,Y.Lee,S.M.Byon,J.Mater.Process.Technol.140(2003)84–89.
[9]F.Sarioglu,Mater.Sci.Eng.A315(2001)98–102.小米6重量
[10]S.I.Kim,Y.C.Yoo,Mater.Sci.Technol.18(2002)160–164.
[11]Y.C.Lin,M.S.Chen,J.Zhong,Comput.Mater.Sci.(in press)DOI:
10.atsci.2007.08.011.
[12]Y.C.Lin,M.S.Chen,J.Zhong,Comput.Mater.Sci.(submitted for
publication).
[13]C.M.Sellars,Czech J.Phys.35(1985)239–242.
[14]C.M.Sellars,Acta Met.14(1966)1136–1138.
[15]M.Richert,H.P.Stowe,J.Richert,R.Pippan,C.Motz,Mater.Sci.Eng.A
301(2001)237–243.
2135
Y.C.Lin et al./Materials Letters62(2008)2132–2135
