Abstract: The effect of weld matching, strain ageing and postweld heat treatment on the strngth of wlded structures are investigated and appraised directly by experiments made in accordance with the General Yielding Criterion at room temperature. The test results show that for A313 and A537 steels overmatching of weld metal increases the possibility of having a general yielding. This means that p oper matching of the weld metal with the parent metal may help tolerate larger weld defects or critical crack siz a_cr. Certain factors, such ass train ageing and heat treatment,an change the degre of weld matching and thus affect a_cr.