Effects of Tempering Temperature on Microstructure and Mechanical Properties of a Mn-Cr Type Bainitic Forging Steel

doi:10.11900/0412.1961.2020.00139

Acta Metall Sin

2020, Vol. 56

Issue (11): 1441-1451 DOI: 10.11900/0412.1961.2020.00139

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Effects of Tempering Temperature on Microstructure and Mechanical Properties of a Mn-Cr Type Bainitic Forging Steel

WANG Zhanhua, HUI Weijun(

), XIE Zhiqi, ZHANG Yongjian, ZHAO Xiaoli

School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China

Cite this article:

WANG Zhanhua, HUI Weijun, XIE Zhiqi, ZHANG Yongjian, ZHAO Xiaoli. Effects of Tempering Temperature on Microstructure and Mechanical Properties of a Mn-Cr Type Bainitic Forging Steel. Acta Metall Sin, 2020, 56(11): 1441-1451.

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Abstract

With continuous demands for cost reduction and environmental protection, bainitic forging steels, which have notably higher strength and toughness combination than ferritic-pearlitic forging steels, have been developed and gained an increasingly applications in a variety of critical automotive parts. In order to optimize the microstructure and properties of bainitic forging steel, the influences of tempering temperature ranging from 200 ℃ to 500 ℃ on the microstructure and mechanical properties of a Mn-Cr type bainitic forging steel were investigated based on microstructural observations and mechanical property tests. The results show that the microstructure in the as-forged condition of the tested steel is a mixture of lower lath-bainite and granular bainite. With the increase of tempering temperature (T_temp), the microstructure began to recover and the large blocky martensite/austenite (M/A) constituents decomposed granularly with the precipitation of fine cementites. Further increasing T_temp to 500 ℃, the blocky M/A constituents decomposed completely and the cementites were spheroidized. Consequently, the ultimate tensile strength (UTS) decreases gradually from 1418 MPa of the as-forged specimen to 1094 MPa of the specimen tempered at 500 ℃ with increasing T_temp, while the yield strength (YS) increases gradually with increasing T_temp at first, reaching a peak at 400 ℃, and then decreases with further increasing T_temp. As a result, the yield strength ratio (YS/UTS) increases continuously from 0.73 in the as-forged state to 0.93 of the specimen tempered at 500 ℃. Unlike those of the strengths, the impact energy increases at T_temp of 200 ℃ at first, then it decreases and reaches a valley at 400 ℃, and finally it increases notably again at T_temp of 500 ℃, an increase of about 27% than that of the as-forged one. It is concluded that suitable tempering treatment after forging can obtain better strength and toughness balance of the tested bainitic forging steel, and thus help to expand its application range.

Key words: bainitic forging steel tempering temperature mechanical property microstructure

Received: 30 April 2020

ZTFLH:

TG142.1

Fund: National Key Research and Development Program of China(2016YFB0300100)

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	Zhanhua WANG
	Weijun HUI
	Zhiqi XIE
	Yongjian ZHANG
	Xiaoli ZHAO

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https://www.ams.org.cn/EN/10.11900/0412.1961.2020.00139 OR https://www.ams.org.cn/EN/Y2020/V56/I11/1441

Fig.1 OM (a) and SEM (b) images of microstructure of the tested steel in the as-forged condition (LLB—lower lath-bainite, GB—granular bainite, PAGB—prior austenite grain boundary)

Fig.2 SEM images of microstructures of the as-forged steel tempered at different temperatures (M/A—martensite/austenite, TM—tempered martensite)
(a) 200 ℃ (b) 300 ℃ (c) 400 ℃ (d) 500 ℃

Fig.3 TEM images of microstructures of the tested steel under different conditions
(a, b) as-forged (Fig.3a shows the bainitic lath morphology with high dislocation density and lath-like cementites, while Fig.3b shows the MC carbides within bainitic ferrite and the inset shows their corresponding selected area electron diffraction pattern)
(c~f) tempered at 200 ℃ (c), 400 ℃ (d) and 500 ℃ (e, f), respectively

Fig.4 EBSD image quality (IQ) map (a), phase maps (b~e) and misorientation distribution (f) of the tested steel under different conditions (The red phase is austenite while the white phase is ferrite, and the blue lines are low-angle boundaries with misorientation angles between 2° and 15° while the black lines are high-angle boundaries with misorientation angles over 15°)
Color online
(a, b) as-forged (c) 200 ℃ (d) 400 ℃ (e) 500 ℃ (f) misorientation distribution

Fig.5 Variations of tensile properties with tempering temperature (R_m—ultimate tensile strength, R_p0.2—yield strength, A—total elongation, Z—reduction of area)
(a) strength (b) ductility (c) yield strength ratio

Fig.6 Variations of absorbed energy and impact load with displacement obtained in the instrumental impact tests of the tested steel under different conditions (P_y—yield load, P_m—maximum impact load, P_f—brittle fracture start load, P_a—fracture arrested load, E₁—elastic deformation energy, E₂—plastic deformation energy, E₃—crack propagation energy, E₄—brittle fracture energy, E₅—post-brittle fracture energy)
(a) as-forged (b) 200 ℃ (c) 400 ℃ (d) 500 ℃

Fig.7 Impact absorbed energy of the tested steel under different conditions
(a) impact energy at different stages (W_t represents the total energy)
(b) the total energy vs tempering temperature

Fig.8 Fracture morphologies of the impact specimens under different conditions (The arrows indicate ductile tear zones and dimples)
(a) as-forged (b) 200 ℃ (c) 400 ℃ (d) 500 ℃

Fig.9 Variations of nanoindentation hardness of different microstructures with tempering temperature of the tested steel

Fig.10 SEM images of the cross-sectional area of the Charpy impact specimens
(a, b) as-forged (Fig.10a shows cracks initiated at the interfaces of M/A and matrix while Fig.10b shows cracks initiated within fractured M/A)(c, d) 400 ℃ (Fig.10d represents high-magnification image of the rectangular region in Fig.10c)

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