Effect of Al on Hardenability and Microstructure of 42CrMo Bolt Steel
LU Chaoran1, XU Le1(), SHI Chao2, LIU Jinde3, JIANG Weibin4, WANG Maoqiu1
1 Central Iron & Steel Research Institute, Beijing 100081, China 2 Inner Mongolia North Heavy Industries Group Co. Ltd. , Baotou 014033, China 3 Ningxia Tiandi Benniu Industrial Group Co. Ltd. , Shizuishan 753001, China 4 Jianlong Beiman Special Steel Co. Ltd. , Qiqihaer 161041, China
Cite this article:
LU Chaoran, XU Le, SHI Chao, LIU Jinde, JIANG Weibin, WANG Maoqiu. Effect of Al on Hardenability and Microstructure of 42CrMo Bolt Steel. Acta Metall Sin, 2020, 56(10): 1324-1334.
42CrMo steel has a good combination of strength and toughness after quenching and tempering treatment, which make it an ideal candidate material for high strength bolt. Nevertheless, with the increase of bolt diameter in wind power field, the hardenability of 42CrMo steel is inadequate to manufacture the high strength bolt with diameter over 36 mm. Recent study indicates that Al addition is an economical and effective way to affect the phase transformation product during quenching process. In order to improve the hardenability of 42CrMo bolt steel, the effect of Al on the hardenability of 42CrMo was investigated by Jominy test and cross section hardness distribution test. OM and SEM were used to analyze the morphology of the grain size; chemical phase analysis test was used to detect the precipitation in Al addition steels; the isothermal transformation diagram (TTT curve) was measured to study the phase transformation of the steels; the three dimensional atom probe (3DAP) was used to analyze the Al distribution in matrix; the tensile and impact toughness properties of Al addition steels were also examined. It was found that the hardenability of 42CrMo bolt steel could be improved significantly by Al-Ti and Al-B addition, the hardness was increased by 6 HRC at the position of 25 mm from quenched end, the center hardness in diameter of 42, 48 and 56 mm was increased by 7, 10 and 14 HRC, respectively. The improvement of hardenability for Al-Ti addition steel can be attributed to the increasing dissolved Al content in the matrix because of the Ti addition, which suppresses the formation of bainite during the quenching process. The hardenability of Al-B addition steel is better than that of Al-Ti addition steel, which can be ascribed to the dissolved Al and B inhibiting the phase transformation of ferrite and pearlite. Moreover, Al can play an important role in increasing dissolved B content by means of AlN formation, in which the dissolved Al dispersive distribution in matrix is favorable to improve the hardenability of 42CrMo steel. Meanwhile, the tensile strength and Charpy V-notch impact energy at -40 ℃ of Al addition steels are adequate to manufacture grade 12.9 high strength bolt.
Table 1 Chemical compositions of the experimental steel
Fig.1 Hardenability curves of the tested steels by Jominy method
Fig.2 Cross-sectional hardness curves of the tested steels with the diameters of 42 mm (a), 48 mm (b) and 56 mm (c) (R—radius of cross section)
Fig.3 Grain morphologies of the tested steels of 1# (a), 2# (b) and 3# (c)
Fig.4 Microstructures of the tested steels of 1# (a, b), 2# (c, d) and 3# (e, f) at 25 mm (a, c, e) and 35 mm (b, d, f) distances from the quenched end
Fig.5 Comparisons of cross-sectional core structures of 1# (a~c), 2# (d~f) and 3# (g~i) steels with the diameters of 42 mm (a, d, g), 48 mm (b, e, h) and 56 mm (c, f, i)
Fig.6 Tensile strength and low temperature (-40 ℃) impact properties of the tested steels at different tempering temperatures (Rm—tensile strength, KV2—Charpy V-notch impact energy)
Fig.7 Isothermal transformation curves of 42CrMo steel undercooled austenite (F—ferrite, B—bainite, P—pearlite, Ac1—start temperature of austenite formation during heating, Ac3—finish temperature of austenite formation during heating, Ms—martensite start temperature)
Fig.8 XRD spectra of the 2# (a) and 3# (b) steels
Fig.9 SEM images (a, c, e) and EDS analyses (b, d, f) of Ti(C, N) (a, b), AlN (c, d), BN (e, f) precipitated in 2# steel (a, b) and 3# steel (c~f)
Fig.10 Phase analysis results of the tested steels
Fig.11 Needle tip samples with grain boundary (a) and three-dimensional spatial distribution of alloy element atoms (b), metal element content at the needle tip (c) measured by 3DAP Color online
Fig.12 Element distributions of 2# steel in 3D space Color online
Fig.13 Distribution of elements at grain boundaries in 3D space Color online (a) isoconcentration surface with 0.4%Mo (atomic fraction)(b) content of each element in the diameter 5 nm×40 nm micro-region across the grain boundary
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