Study of Reheat Cracking in Weldment (Part 1)

Abstract

It is generally said that residual stress existing in weldment must be removed for its unfavourable effects on brittle fracture, delayed cracking or stress corrosion cracking etc.. Postweld heat treatment is usually applied to remove the residual stress. Some problems are met when the heat treatment is applied, namely toughness decrement, reheat cracking especially in weldment of high tensile steel.
This report describes, concerning mainly with high tensile steel weldment, proposal of testing method to evaluate the cracking susceptibilities and some characteristics of the cracking.
The results obtained are summerized. as follows;
1) Reheat cracking susceptibilities can be evaluated quantitatively, easily and precisely by newly developed testing methods. Same crack is obtained as in the actual weldment.
2) This crack is typical grain boundary crack along prior austenite grain boundary. At the tip of crack, voids are observed and some of them coalesce to become a small crack. These voids exist on grain boundary which is perpendicular to stress axis and they are round type just like ones observed in creep phenomenon.
3) The behavior of the cracking is considerably affected by the existence of notch. When the weld reinforcement is removed, the crack can not be found at the surface but small cracks are observed at the interior of plate and so it is difficult to detect the crack with NDT. There is no effect of heating rate in generally used range on the susceptibility of cracks and cracks begin to grow at the temperature of about 550°C.
4) There is a tendency that cracking susceptibility decreases with higher preheat temperature or lower strength electrode.
5) Alloying elements, V, Cu, Cr, Mo, Nb and C are unfavourable for the reheat cracking. But only Mn is favourable. Quantitative effects of these elements on the cracking are obtained.