Toughness of tempered bainitic microstructures in a 4150 steel alloy

Often high residual stresses and quench cracking problems are associated with the heat treatment of high alloy steels. To alleviate these problems, a slower cooling rate with the formation of bainite resulting in a slack-quenched structure may be prefered. The toughness of these steels in the as-quenched condition may be severely degraded with the presence of upper bainite. To investigate the possiblity of increasing the toughness of these structures, different bainitic morphologies were tempered in a 4150 alloy steel. The toughness of two lower bainite morphologies was compared to that of upper bainite. After tempering at 600 C for 48 hours, the charpy upper shelf values for upper bainite approached those of similarly tempered lower bainite. However, the charpy transition temperature of upper bainite was higher than that of lower bainite regardless of the tempering procedure. No major change in toughness or morphology of the bainitic structures was observed until tempering temperatures approached 600 C. The increase in toughness observed for these high tempering temperatures was attributed to the decrease in the carbide aspect ratio and the subsequent increase in carbide spacings as the microstructure began to coarsen.