Movement of C, Cr and Ni in SUS 27 and S45C Welded by Friction Welding and Mechanical Properties of the Joint

Abstract

Weld joints of austenitic steel and carbon steel are largely used when corrosion resistance and heat resistance are required, such as motor shaft, engine valve, etc. Dissimilar weld joint is attended with change of strength owing to the element diffusion at weld, or with difficulty of machining owing to increase of hardness. Then studies on change of strength and heat treatment after welding are required. When SUS27, austenitic stainless steel, and S45C, carbon steel, are welded by friction welding, movement of certain elements and mechanical properties of the joint are studied for the following cases:
1. as welded
2. exposed 1 hour at 700°C and then air cooled
3. exposed 100 hours at 700°C and then air cooled
4. exposed 1 hour at 850°C and then air cooled
with following results.
Study of element distribution, structure and hardness distribution at weld shows that there is a little element migration across the weld interface. It is almost caused by mechanical agitation of revolving friction during welding, and therefore elements are mixed at the weld interface. The width of the mixed zone is 15μ-40μ at center and 50μ-200μ at the part above 2mm far from center. As for the joint heat treated after welding, elements which have large atomic diameter such as Cr. Ni hardly migrate; while C is diffused from S45C to SUS27, producing decarburized zone on S45C side and carburized zone on SUS27 side. Above a certain length of heating time, carbide precipitates on SUS27 side near the weld interface.
The joints which have been heat treated for short time rupture at base metal of SUS27 except in the case of Charpy impact test. The decrease of Charpy strength, however, little differs from that of similar weld joint, and no influence on strength of dissimilar weld joint can be observed. On the other hand, the joints heat treated for long time rupture at weld in any mechanical tests which prove that the strength dedecreases. This is attended with coarse grain zone of ferrite and precipitation of carbide caused by progress of forming of decarburized zone on S45C side and carburized zone on SUS27 side. It is. therefore, due to the diffusion of C atoms because of difference of chemical composition, not to the fault of friction welding. Moreover, such heating time as causes a decrease of strength is so long, and there is no problem unless the joints are used at a high temperature.