In this study, microstructures of a heat-affected zone (HAZ) of an SA 508 steel were identified by Mössbauer spectroscopy in conjunction with microscopic observations, and were correlated with fracture toughness. Specimens with the peak temperature raised to 1350 ЊC showed mostly martensite. With the peak temperature raised to 900 ЊC, the martensite fraction was reduced, while bainite or martensite islands were formed because of the slow cooling from the lower austenite region and the increase in the prior austenite grain size. As the martensite fraction present inside the HAZ increased, hardness and strength tended to increase, whereas fracture toughness decreased. The microstructures were not changed much from the base metal because of the minor tempering effect when it was raised to 650 ЊC or 700 ЊC. However, fracture toughness of the subcritical HAZ with the peak temperature raised to 650 ЊC to 700 ЊC was seriously reduced after postweld heat treatment (PWHT) because carbide particles were of primary importance in initiating voids. Thus, the most important microstructural factors affecting fracture toughness were the martensite fraction before PWHT and the carbide fraction after PWHT.

This study is concerned with the effects of microstructural parameters on the cracking phenomenon occurring during cold forging of two AISI 1010 steels that were fabricated by converter steel making and electric furnace steel making, respectively. This allowed a comparison between microstructures that contained a small or large amount of nitrogen. Detailed microstructural analyses of the cracked region showed that a number of adiabatic shear bands, along which cracks initiated and propagated, were formed in the top interior part of the cold-forged pulley. Dynamic torsional tests were conducted using a torsional Kolsky bar in order to investigate the dynamic deformation behavior during cold forging, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. From the dynamic shear stress-strain curves, the steel containing a considerable amount of nitrogen showed a smaller shear strain of 0.2 at the maximum shear stress point, after which the shear stress decreased rapidly prior to fracture, whereas the other steel containing a smaller amount of nitrogen showed relatively homogeneous shear deformation. This dynamic torsional behavior correlated well with the cracking and adiabatic shear banding behavior, together with the yield-point phenomenon occurring in the steel containing more nitrogen. Because the cracking occurring during cold forging was associated with the adiabatic shear banding and the yield-point phenomenon, the minimization of nitrogen and the fast cooling rate after hot rolling were suggested to prevent the cracking.