Study on the Overlay Welding of Austenitic Stainless Steel with Rectangular Electrode

Effect of Stress-relief on the Bending Ductility of Weld Metal

Abstract

Recently the production of thick wall pressure vessels with corrosion resistant stainless steel clading for atomic and chemical application has been incresing rapidly. Some large sized pressure vessel have been repeated stress-relief heat treatment up to 50 hrs. As a result of heat treatment, the hardened layer is formed due to carbon migration from low alloy steel (base metal) to stainless steel, which may cause cracking in the bend test. Such problem takes place not only in the rectangular electrode process but also in other overlay arc welding processes.
From the observation on the netallurgical study, the boundary layer formed during welding has width of about 20-45μ (martensite structure), but in this case no cracking is found in the bend test. When the boundary layer is received stress-relif annealing, the hardened layer is formed near the bond line of the weld metal by carbide precipitation. If the meximum hardness of the hardened layer exceeds Hv400, the bend ductility of metal decreases.
According to the examinations of heat treatment condition for various austenitic stainless steels, it is reasonable that the process of 309 stainless steel for the 1st layer and 308 or 347 stainless steel for the 2nd layer is preferable to a single-pass welding of 308 or 347 stainless steel.
In any case, since higher annealing temperature may cause problemes, so the anneling should be performed at temperature below 625°C.