Stress corrosion cracking and hydrogen embrittlement susceptibility studies on modified 9Cr-1Mo steel weldments in acidic and neutral media

Abstract

There is much scope for the development of new engineering materials for high temperature applications such as for tubing used in steam generation in fossil fuel and nuclear power plants and in petrochemical cracking units. The materials for such applications are often manufactured from mild steel and low alloy ferritic steels containing up to 9%Cr. Modified 9Cr-1Mo ferritic steel, prepared by incorporating vanadium and niobium, is one of the newer materials extensively used for high temperature applications. A study of weldments of this alloy and its susceptibility to stress corrosion cracking (SCC) and hydrogen embrittlement was carried out in acidic and neutral media. The anodic behaviour of modified 9Cr-1Mo under 90% proof stress was also examined. Mechanical properties of the material were assessed before and after SCC and hydrogen embrittlement tests. The SCC tests were made at various anodic potentials in 1M H₂SO₄ and showed that the welded alloy is not susceptible to SCC in all three critical zones. The alloy was, however, susceptible to SCC in chloride environments, namely a solution containing equal volumes of 1M NaCl and 1M MgCl₂. Fractographic analysis was carried out to determine the reasons for the failure of this alloy. The hydrogen embrittlement studies in 1M H₂SO₄ showed that the alloy is prone to delayed hydrogen cracking at more negative potentials. The fractographic examination using scanning electron microscopy revealed the presence of adsorbed hydrogen in microvoids which were responsible for the failure of the alloy. Other possible reasons for the failure of the alloy are also discussed.