Carbon content of retained austenite in quenched steels

Abstract

Much effort has recently been directed towards the determination of the carbon content of retained austenite, i.e. x _γ, in directly quenched, or quenched and tempered low alloy steels. The results indicate that x _γ may be significantly higher than the average carbon content <mml:math><mml:mrow><mml:mover><mml:mi>x</mml:mi><mml:mi>¯</mml:mi></mml:mover></mml:mrow></mml:math> of the alloy concerned. The interpretation of such data is difficult, but the present paper is concerned with the possibility that some redistribution of carbon may occur during the transformation of austenite to what is generally referred to as ‘lath martensite’ in low alloy steels; the transformation may not be truly diffusionless and could perhaps involve the displacive growth of only partially supersaturated laths of martensite. The carbon content of such laths during growth, i.e. <mml:math><mml:mrow><mml:msubsup><mml:mi>x</mml:mi><mml:mi>α</mml:mi><mml:mtext>I</mml:mtext></mml:msubsup></mml:mrow></mml:math>, may be less than <mml:math><mml:mrow><mml:mover><mml:mi>x</mml:mi><mml:mi>¯</mml:mi></mml:mover></mml:mrow></mml:math>, although greater than the paraequilibrium level <mml:math><mml:mrow><mml:msubsup><mml:mi>x</mml:mi><mml:mi>α</mml:mi><mml:mi>αγ</mml:mi></mml:msubsup></mml:mrow></mml:math> given by the common tangent construction of Fig. 1, the remainder being pushed ahead of the transformation interface, giving rise to a diffusion controlled growth rate under steady state conditions.