Effect of Austempering-Partitioning on the Bainitic Transformation and Mechanical Properties of a High-Carbon Steel

doi:10.1007/s40195-015-0239-3

Abstract

Abstract:

The 1.1C-1.5Si-1.1Mn-1.4Cr-0.5Mo-0.6Al-0.6Co (in wt%) steel was treated, respectively, by isothermal austempering process and newly developed austempering-partitioning-tempering process (A-P-T). After austempering at 250, 280 and 300 °C for 38, 20 and 10 h, respectively, the sample microstructures were composed of bainitic ferrite plates and film-like retained austenite with thicknesses between 60 and 150 nm. The highest tensile strength of 2003 MPa and hardness value of 53.9 HRC were obtained for the steel after austempering at 250 °C for 38 h, resulting from the combining effect of super-saturated martensite decarburization and stabilization of bainitic formation. After A-P-T treating (heated at 300 °C for 8 h following water cooling, and then heated at 300 °C for 2 h following air cooling), bamboo leaf-like martensite, primary and secondary bainites and retained austenite were observed. The thickness of the secondary bainitic ferrite plates formed during partitioning is much smaller than that of the primary bainite formed during 300 °C austempering. Samples subjected to A-P-T treatment showed improvement in ductility compared to that subjected to austempering.

Key words: High-carbon alloy steel, Low-temperature bainite, Microstructure, Mechanical properties, Austempering-partitioning-tempering

Bo Han, Lei Chen, Su-Jun Wu. Effect of Austempering-Partitioning on the Bainitic Transformation and Mechanical Properties of a High-Carbon Steel[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(5): 614-618.

Figures/Tables 7

Fig. 1 a Calculated TTT diagram for initiation of bainitic transformation determining; b measured differential thermal analysis (DTA) diagram for phase transformation temperature determining

Fig. 2 Optical images of high-carbon alloy steel after austempered at different temperatures for different time: a 300 °C, 4 h; b 300 °C, 10 h; c 280 °C, 10 h; d 280 °C, 20 h; e 250 °C, 20 h; f 250 °C, 38 h

Fig. 3 XRD patterns of the high-carbon steel austempered at different temperatures for different time: a 300 °C, 10 h; b 280 °C, 20 h; c 250 °C, 38 h

Fig. 4 TEM images of the high-carbon steel austempered at 300 °C for 10 h a, 280 °C for 20 h b, 250 °C for 38 h c, and the corresponding SAED pattern d of bainitic ferrite plates and retained austenite

Table 1 Mechanical properties of high-carbon alloy steel under various austempered treatments

Treatment	Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)	Hardness (HRC)
300 °C for 10 h	1142.9	1467.7	4.6	47.2
280 °C for 20 h	1470.5	1623.7	5.1	49.5
250 °C for 38 h	1677.0	2003.1	3.2	53.9

Fig. 5 Optical images of high-carbon steel after austempering-partitioning-tempering treatment: a austempered at 300 °C for 4 h and partitioned at 300 °C for 2 h; b austempered at 300 °C for 6 h and partitioned at 300 °C for 2 h; c austempered at 300 °C for 8 h and partitioned at 300 °C for 2 h

Fig. 6 TEM image of high-carbon steel after austempered at 300 °C for 8 h and partitioned at 300 °C for 2 h a and the corresponding SAED pattern b

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