SUPERPLASTICITY OF A WATER-QUENCHED AND TEMPERED 40Cr STEEL

ACTA METALLURGICA SINICA (ENGLISH LETTERS)Vol. 17 No.5 pp 682-688 October 2004SUPERPLASTICITY OF A WATER-QUENCHED ANDTEMPERED 40Cr STEELX.J. Xu, G.L. Liu, L.J Shi, X.N. Cheng and L. CaiSchool of Mechanical Engineering, Jiangsu University, Jiangsu 212013, ChinaManuscript recived 11 March 2003; in revised form 19 November 2003The superplastic deformation characeristics, of commercial 40Cr (ie, 5140) steel that waswater-quenched only I times and subsequent high-temperarure tempered, were investigated.The results showed that the 40Cr steel has a fine grain of 10- 15μm at room temperature,and exhibits a tensile elongation of 304%，a true flow stress of 89.3MPa and a strain ratesensitivity m-value of 0.227 at the initial strain rate of 1.0xI0-~s~'and at the temperature of750°C. The final fracure is caused by the development of neck. The experimental result ofelongation is in good agreement with the theoretically predicated value according to theanalytical expression ep=(-)"exp(n. +me)-1 (where ey, m, f, n, and e is respectively e-longation, average strain rate sensitivity, initial geometric defect, average strain hardeningsensitivity at constant deformation velocity and average true strain). The fracture surface isintergraular, and superplastic deformation induces an equiaxed and grown grain. Decreas-ing strain rate increases tensile elongation and strain rate senstivity m-value. The primarysuperplastic deformation mechanism is thought to be atomdifusion-controlled grain bound-ary sliding.KEY WORDS superplasticity, 40Cr ste, quenching mediun, quenching times,microstructure1. IntroductionSuperplasticity possesses many good characteristics such as high elongation, low flow stress andhigh atom diffusion ability, which make it show good application in metal forming and diffusionbonding of complex shape structure.Up to now，the superplasticity of steel has been more investigated, however the pretreatment inorder to obtain fine grain and then superplasticity was often carried out by circular quenchingof typically 3 times and subsequent high-temperature tempering, and the initial strain rateexhibiting superplasticity was lower than 1.0x10-5s18, which is not desirable for a manufacturingprocess.This paper firstly report that the 40Ct steel, which was water-quenched only 1 times and subsequenthigh-temperature tempered, can exhibits a good superplasticity with the tensile elongation of more than300% at a higher initial strain rate of 1.0x10*s'. In addition, the superplastic deformation characteristicswere also investigated.中国煤化工MYHCNMHG6832. ExperimentalThe chemical composition of 40Cr steel is 0.41C-0.3Si-0.65Mn-0.9SCr (wt%). The pretreatmentfor superplasticity includes water quenching and subsequent tempering. The quenching temperature andtime are respectively 840°C and 40min. The tempering temperature and time are respectively 650C and 2hoursThe superplastic tensile specimens with a 4-mm gauge width, a 10mm gauge lengths and 2mmgauge thickness are directly machined from the 40Cr steel. Prior to testing，the specimen is held at thespecified test temperature for about 5min to establish thermal equilibrium. The strain rate sensitivitym-value is measured by Backofen method at the true strain of 0.26. In order to take the repeatability intoaccount, the test results were obtained from the average of three readings. The tensile experiment iscarried out at different constant deformation velocity. The microstructures are examined by scanningelectron microscopy (SEM).3. Results and Discussion3.I MicrostructureThe fine grain is vital to cause superplasticity in metal materials. Fig.l shows that the 40Cr steel hasa tempered sorbite microstructure with the grain size of 10-15 μ m at room temperature.Fig.I Microstructure of the 40Cr steel3.2 Superplastic mechanical behaviorsTrue flow stress (@) and true strain rate (E) in a superplastic material can be expressed by theempirical equationofσ = ki "e"，where k is a constant, m is the strain rate sensitivity andn is the strainhardening sensitivity. In order to suppress neck formation and lead to high elongation, it is necessary tofind the optimum superplastic conditions under which the m-value and n-value is higher.Fig.2 shows the variation of true flow stress (a), elongation (b) and strain rate sensitivity m-value(c) with temperature of the 40Cr steel, respectively. At the temperatures ranging from 650 to 850°C andat the initial strain rates of 1.0x103 and 1.0x103s', the 40Cr steel exhibits a true flow stress of 74.67-284.16MPa, a tensile elongation of 102%-304% and a strain rate senstivityt m-value of0.10-0.227. Amaximum tensile elongation of 304% is obtaned at the initial strain of 1.0x10^s't and at the temperatureof 750C, where the true flow stress is 89.3MPa and the m-value is 0.227.中国煤化工MHCNMHG684向) True strain=0.26o' r(@) Temperature= 750°C星-- 1.0x0's'True strain= 0.26密10°180080050 90010'10Initialstrain rate. s'Temperature, °C400-■-10x10's '(b) Temperature= 750°c--1.0x10's"，300200, 200-600 650 700 75800 850 900Temperature, °cInitial strain rate, s'3-(C)(c) Temperature =750c--- 1.0x10's'True strain=0.26.2-1+°goo 650 700 75010Temperature,°CInitial strain rate, s"Fig.2 Variation of true flow stress (a), clongationFig.3 Variation of true flow stress (a), elongation(b) and m. value (C) with temperature of the(b) and m-value (Cc) with strain rate of the40Cr seel.Fig.3 shows the variation of true flow stress, elongation and strain rate sensitivity m-value withinitial strain rate of the 40Cr steel at the temperature of 750^C. In the strain rates ranging from 1.0x103to 1.0x10-2s, the true flow stress decreases, and the elongation and the strain rate sensitivitym-value both increase with decreasing strain rate.Fig4 shows the variation of m-value and true flow stress with true strain of the 40Cr steel at the .initial strain rate of 1.0xl0's1 and at the temperature of 750°C. The mvalue varies lttle with increasing中国煤化工YH ;CNMHG685strain, and the average m-value is about 0.227. The true flow stress - true strain curve belongs to hard-soft type, and the average strain hardening sensitivity at constant deformation velocity, n , is about -0.124.Fig.5 shows the comparison of the untested and deformed 40Cr steel at the initial strain rate of 1.0x10-s' and at the temperature of750"C. It is apparent that the final fracture is caused by the development ofneck..3-100-""*' . .....2-E50-/Temperature = 750°CInitial strain rate = 1.0x10's'0100.20.4 0.60.80.0 0.20.40.6 0.81.0 1.2 1.41.6True strainFig.4 Variation of m-value (国) and true flow stress (b) with true stain of the 40Cr stel.Fig.5 A comparison of undeformed and deformed 40Cr steel (initial strain rate=1.0x 10*s',temperature=750C, elongation -304%).Ref.[9] give a analytical expression for the correlation between the elongation ef, the strain ratesensitivity m and the strain hardening sensitivity at constant strain rate ng for geormetric defectsf as fllows-(G[exp(n2)-1(1)Ref.[10] gives a analytical expression for the correlation between the strain hardening sensitivity atconstant strain rate ng，the strain hardening sensitivity at constant deformation velocity n and the truestrain 8 as follows中国煤化工YHCNMHG686(2)ng=n, +m。εCombining Eqs.(1) and (2), we havees =exp(n, +me)-1(3)In the present investigation, the f-value is measured to be 0.001-0.003. Substituting the averagef-value of 0.002, the average m-value of 0.227, the average nv value of -0.124 and the average ε -valueof 0.707 into Eq.(3), the calculated theoretical value of elongation ey is about 300%，which is in goodagreement with the experimental result of 304%，confirming the final fracture of the superplasticdeformation is caused by the development of neck.3.3 Microstructure after superplastic deformationFig.6 shows the fracture surface of the 40Cr steel pulled at the initial stain rate of 1.0x10*s' and atthe temperature of 750C. The fracture surface is intergranular， ilustrating that the strength of grainboundary is weaker than that of grain interior and that grain boundary sliding has occurred during su-perplastic deformation.Fig.6 SEM fractograph of the 40Cr seel (nitial strain rate=1.0x10's", temperature=750C,elongation=304%).Fig.7 shows the microstructure of griped zone (Fig.7a) and gauge-deformed zone (Fig.7b) of the40Cr steel pulled at the initial stain rate of 1.0x10's' and at the temperature of 750C. Comparing themicrostructure s of griped zone and gauge-deformed zone, a significant strain-induced grain growth (thegrain size is about 2-3μm )，which can lead to a higher n-value and therefore can be beneficial tosuperplastic deformation!", was observed. In addition, it is also very obvious that an equiaxed grain wascaused by superplastic deformation. These results suggest that atom difusion has taken place duringsuperplastic deformation.中国煤化工MHCNMHG687a)Fig.7 A comparison of microstructures of griped zone and gauge -deformed zone afer superplastic deformation (nitial strain rate=1.0x10*s', temperature=750 C, elongation-304%): (a) gripedzone; (b) gauge-deformed zone.It has been well known that 40Cr steel is a medium carbon and low alloy steel.The quenchingmedium is usually oil. This paper firstly uses water with stronger cooling role as quenching medium of40Cr steel, which is responsible for the good superplasticity.The reason that decreasing strain rate increases tensile elongation (Fig.3b) is because atom has moretime for difusion and result in a higher m-value (Fig.3c). Just think if deformed at a moderately lowerstrain rate than 1.0x10*s ', a higher m-value and n-value were obtained, it is very possible that the40Cr steel will exhibit a higher elongation than 304%.From above mechanical behavior, microstructure and following discussion, it can be seenthat the superplastic deformation mechanisms of the 40Cr steel can be considered to beatom-diffusion-controlled grain boundary sliding.4. ConclusionsSuperplastic characteristics, of commercial 40Cr steel that was water- quenched only 1 times andsubsequent high-temperature tempered, were investigated. The following results were obtained:(1) The 40Cr steel has a fine grain of 10-15μum at room temperature， and exhibits an tensileelongation of 304%，a true flow stress of 89.3MPa and a strain rate sensitivity m-value of 0.227 at theinitial strain rate of 1.0x10's 'and at the temperature of 750C.(2) The final fracture is caused by the development of neck，and the experimental result ofelongation is in good agreement with the theoretically predicated value according to the analyticalexpression e, =(一)”exp( n。+me)-1 (whereef m，f，n, and 8 is respectively elongation,average strain rate sensitivity，initial geometric defect, average strain hardening sensitivity at constantdeformation velocity and average true strain).(3) The fracture surface is intergranular， and superplastic deformation induces an equiaxed andgrown grain.(4) Decreasing strain rate increases tensile elongation and strain rate sensitivity m-value. Theprimary superplastic deformation mechanism is thought to be atom-diffusion-controlled grainboundary sliding.中国煤化工MHCNMHG688REFEF ENCES1 HS. Shi, XF Wu,JG Zhang and D.S Sun, Chinese Joumal of Materials Research 162)(2002)214(n Chinese),2 M. Aramaki, K. Higashida and R. Onodera, Meallurgical and Materials Transactions A: Physical Metallurgyand Materials Science 30(5) (1999) 1185.3 M.M. Moshksar and R.E. Marzban, Joumal of Materials Processing Technology 83(1-3) (1998)115.4 G.H. Li, S.Z. Zong, Y.F. Qiu and H.W. Wu, Jourmal of Nanjing University of Science and Technology 26(4)(2002) 397 (in Chinese).5 M. Wang, lron and Steel 33(9) (1998) 49 (in Chinese).6 K.K. Zhang, Y.L. Yang, C.S. Wang and H.K. Li, J. Mater. Sci Technol. 17(1) (2001) 189.7 J.B. Wen and Y.S. Yang, Joumal of Materials Science and Technology 17(1) (2001) 175.8 Y.L. Yang and z. Li, lron and Steel 30(7) (1995)41 (in Chinese).9 J.S. Lin and B. Baudelet, Materials Science and Eninering 84 (1986) 157.10 Y.Q. Song, J.T. 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