Behaviour of trace bismuth in welded joints of nodular cast iron and development of ZU238F covered electrodes

Abstract

The effect of trace bismuth in nodular cast iron welds on nodulisation, graphitisation and ferrite formation has been investigated. The results indicate that as trace Bi increases the graphite nodule count and nodulising stability increase, promoting the forming of ferrite, and effectively decreasing or even eliminating the cementite in the weld and partial melting region. Therefore, the crack resistance and machinability are improved greatly in repair welds. If the Bi content is 100–120ppm, a first or second grade nodulisation can be obtained with 80–95% ferrite and no cementite. The tensile strength ò_h is 540MPa and elongation δ is 5.7%. As a result, a new type of covered electrode, ZU238F, containing Bi with high crack resistance and good machinability has been developed. This type of electrode has been successfully used in repair welding of nodular cast iron.