Refined Bainite Microstructure and Mechanical Properties of a High‐Strength Low‐Carbon Bainitic Steel Treated by Austempering Below and Above M_S

Abstract

Four isothermal heat treatment schedules are designed to study the effects of transformation temperature below and above martensite starting temperature (M S ) on bainitic transformation kinetics, microstructure, and mechanical properties of a low-carbon bainitic steel. The results indicate that the product of tensile strength and elongation (PSE) of tested steel do not show any improvement when the samples are austempered at a temperature below M S . Finer bainite microstructure can be observed when the samples are austempered at a temperature below M S , but the highest PSE is obtained in the sample austempered at a temperature above M S (400 C). Lower PSE of the samples transformed below M S is mainly attributed to less bainite and retained austenite (RA) amount and more amount of athermal martensite, as well as the existence of carbide. In addition, with the decrease of the isothermal transformation temperature, the PSE first increases and then decreases. Different from austempering above M S , when the sample is austempered below M S , PSE decreases with the decrease of transformation temperature. Moreover, whether the treatment below M S can improve mechanical property of bainitic steels depends on the composition of steels and transformation temperature.

Transformation Behavior of Bainite during Two-step Isothermal Process in an Ultrafine Bainite Steel

“…3(c) is much larger than that during the isothermal period (after point A). According to the previous result reported in Ref., 30) it is reasonable to infer that the rapid increase in dilatation before austempering is caused by the formation of preformed athermal martensite (AM). Therefore, the zero point (point Z) in Fig.…”

Effect of Heat Input and Undermatched Filler Wire on the Microstructure and Mechanical Properties of Dissimilar S700MC/S960QC High-Strength Steels

“…One reason for the softening was the choice of filler wire and heat input values, which had an impact on the chemical and mechanical characteristics of the weld joints. The hardness, YS, and UTS increased as the temperature dropped with the increasing cooling rate [37]. This phenomenon was due to the parameters, such as the alloy element compositions of both materials and the filler wire.…”

“…The data were recorded in weight %, which presents the percentages of C transforms, Si, Cr, Mn, Fe, Ni, and Mo. Figure 12a,b shows the proportions of the weights of the micro-elements of the alloys obtained [37]. An absence of Ni and Mo in the microstructure of the S960QC (Figure 12a), a decrease in the weight % of Cr (0.1%), and an increase of Mn (1.8%) can be seen.…”

Effect of two-step ausforming on bainite transformation and retained austenite in a medium-carbon bainitic steel

“…In addition, the BF amount of non-deformed specimen ( figure 8(a)) is smaller than deformed specimens. To quantitatively compare the fractions of BF in different specimens, a statistical method described in [24] was used. Figure 9 presents an example (specimen D) to illustrate the method.…”