Hot Ductility of Stainless Steels during Weld Thermal Cycle (Report 2)

Abstract

In this report, the continuation of authors' first report, various types of austenitic, martensitic and ferritic stainless steels were studied by RPI hot ductility test, the effects of working method upon the hot ductility were investigated about wrought round bars and rolled plates of austenitic stainless steels, and the effects of austenite grain size on the hot ductility were also studied. The following results were obtained.
(1) It was found that the hot ductility during the cooling portion of thermal cycle is, in general, higher in rolled plates than in wrought round bars.
In the 347 type stainless steels, the effect of working method on the hot ductility is remarkable, and it appears that this is one of the characteristic of these 347 type stainless steels. It was also found that the variation of hot ductility depending on the change is large in this steel.
(2) The hot ductility measured at 1200°C to 1300°C during the cooling portion was the best in a 16-8-2 Cr-Ni-Mo alloy among various austenite stainless steels, followed by 316 type, 304L type, 304 type and 347 type in that order, the lowest value being given by 310 type. It was found by metallographic observation that the decrease of hot ductility of these austenitic stainless steels was due to the embrittlement caused by the partial liquation of grain boundary din heating up to 1340°C.
(3) The behaviours of ferritic stainless steel 430 type and martensite stainless steel 410 type were quite different from that of austenitic ones.
These steels did not show the embrittlement caused by liquation of grain boundary which occurred in austenitic stainless steel, since their melting points were much higher. Therefore, the decrease of hot ductility during cooling portion may be attributable to the growth of grains. In general, these steels showed good hot ductility, and it appears that the weld cracking of these steels may not be hot cracking but low temperature cracking due to martensitic transformation.
(4) It was found by the RPI hot ductility test that the weldability of some austenitic stainless steels could be improved by the refinement of grain size and the steels with large grains inclined to suffer from more weld cracking.