Abstract
Fabrication of process plant, pipework etc in ferritic-austenitic steels commonly entails fusion welding. The weld thermal cycle can significantly influence material corrosion behaviour and hence service performance. The paper reviews the situation, with emphasis on arc welding as most commonly employed by industry.
An outline is given of the major metallurgical changes due to welding which take place in the heat affected zone in base steel and in the fused weld metal. The weld thermal cycle experienced alters the ferrite/austenite structure from that in the parent material, and can induce intermetallic precipitation. Nitrogen may also be lost from the weld metal. These changes affect corrosion resistance, and must be controlled to achieve optimum service properties. The consequences of surface oxidation in the weld area and of local residual stresses are also considered, and it is pointed out that resistance to stress corrosion cracking in chloride or sour, H2S media is dependent on ferrite/austenite balance.
The main factors in formulating a welding procedure are described. Depending on the material composition and joint heat sink, arc energy should be held between minimum and maximum levels to promote adequate austenite formation in the weld area without inducing intermetallic formation. Nitrogen loss should be minimised, and adequate filler should be added: slight overalloying of the consumable is preferred, provided that intermetallic precipitation is avoided.
