MECHANISM FOR FORMATION OF THE RESIDUAL-STRESSES FROM QUENCHING

On the basis of the notion of 'strain shift induced by quenching' a co ntinuum model is proposed to provide mechanistic insight into the form ation of the residual stresses from quenching. An equation has been de rived stating that for a homogeneous quenched body the residual elasti c strain of each elemental volume is equal to the difference in the am ount of plastic strain generated by the alternating odd and even strai n shifts. Despite the widespread view, plastic yielding is neither nec essary nor sufficient for the existence of residual stresses after que nching. The proposed model successfully rationalized the results from the computer simulations and the residual stress patterns after oil an d water quenching of decarburized and non-decarburized cylindrical ste el specimens. If the net plastic strain (the difference in the amount of plastic strain during the first and second strain shift) created in the thermal contraction region is greater than the net plastic strain created in the transformation region the axial residual stress at the surface of a quenched cylindrical specimen is compressive. The magnit ude of the transformation stresses generated is inversely proportional to the width of the transformation temperature interval. It is establ ished that the thermal stresses during quenching raise the martensite start temperature of the surface. An equation and a numerical model ha ve been derived for determining the shift of the martensite start temp erature caused by thermal stresses of large magnitude.