Pergamon Scripta Materialia, Vol. 37, No. 11, pp. 181 l-1819.1997 Elswin Scieme Ltd PII zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED S1359-6462(97)00336-9 Copyright 0 1997 Acta Metallurgica Inc. Printed in the USA. All rights reserved 13594462197 $17.00 + .OO SEGREGiATION OF IMPURITIES ON GRAIN BOUNDARIES IN TESTS OF RESISTANCE TO “REHEAT AND UNDERCLAD” CRACKING V. Magula, D. Grman* and J. Patscheider* Welding Research Institute, Racianska 7 1,832 59 Bratislava, Slovakia *EMPA, ijberlandstrasse 129,860O Dubendorf, Switzerland (Received December 14, 1995) (Accepted August 12, 1997) Introduction “Reheat and underclad” cracks have been understood as a kind of intercrystalline failure occurring in the coarse-gratned section of the heat-affected zone in a welded joint during stress relief annealing. [l] An effect similar to annealing can also be produced by heat from a weld deposit affecting a previous deposit. [2] One of the factors affecting the “reheat and underclad” cracking [l] is the segregation of impurities on the grain boundaries, which has been the subject of about two decades of study. In the present work, segregation models as well as the effects exerted upon mechanical properties are dealt with. The kinetics of segregation in binary systems has been described by McLean [3] whose work, to- gether with that of Guttman, was later extended [4] to a multicomponent system, particularly to the segregation of metal (M) and impurity (I) in a Fe-M-I ternary system. The M-I segregation was ex- perimentally p:roved in low-alloy steels [5,6], where the principal segregated elements reported were phosphorus, tin, antimony, sulphur and arsenic. A further study [7] showed that the segregation of phosphorus in steels can also be affected by other elements. Under such conditions, the model of ter- nary system Fme-M-I cannot be employed. Also, the segregation of phosphorus’by austenization of ferritic steels [B] can be considered significant. The segregation of phosphorus during long annealing of a low-alloy 2.600.7Mo0.3V steel has been described in references 9 and 10. The effect of carbon on phosphorus segregation, i.e., the preferential segregation of carbon on the grain boundaries at the expense of phosphorus, has been proved. This effect is emphasized with increased annealing tempera- ture. The effect of long annealing on 2.7Cr0.6Mo0.3V steel has also been the subject of a study [ 1 l] in which not only phosphorus was found to segregate, but also sulphur, although to a lesser extent. Sul- phur segregation is thought to play a significant role at simultaneously occurring stresses [ 12,131. In addressing the problem of reheat and underclad cracks in low-alloy 3Cr0.7Mo0.3V and 2Cr0.6MoO. lV1.2Ni steels, the effect of chemical composition and impurities on cracking resistance was analyzed [ 141. It was found that cracking resistance is significantly affected by the total sulphur content. The linear dependence between the reduction of area (RA) by which steel resistance is evalu- ated [15] and sulphur content was determined. A critical value of O.OOSwt?~ was determined, that is, if 1811