Titanium carbides as the cause of knife‐line corrosion Available to Purchase

ALLOYING with titanium transfers the steel into the passive state, lowering the critical passivation current density and, above all, increases the resistance to inter‐granular corrosion of welded joints in the critical temperature range, but a higher Ti content lowers the stability of the passive state and increases sensitivity to pitting corrosion by comparison with unstabilised steels. Adequate stabilisation ensures resistance to inter‐granular corrosion under the influence of critical temperatures, but stabilised steels are sensitive to inter‐granular corrosion in the overheated zone adjacent to the deposited metal of a welded joint, frequently to knife‐line corrosion even without the dual heating action of overheating to the solidus temperature and heating to critical temperatures during the application of a second weld layer or bead. Knife‐line attack is the result of heating the vicinity of a weld bead to temperatures close to the melting point, causing extensive dissolution of the Ti carbides, grain growth and an increase in ferrite content. During the subsequent action of critical temperatures (500–950°C) precipitation of Cr carbides, breakdown of ferrite, sigma‐phase formation, etc. occur which, jointly with the action of stress peaks in this zone, promote the tendency to knife‐line attack.