High Temperature Cracking in the Weld Heat Affected Zone of High Strength Ferritic Steels

Abstract

A study has been made of the effects of steel composition and welding parameters on hot cracking in the heat affected zone of high strength ferritic steels. In addition, the hot ductility test has been studied to determine whether it is capable of distinguishing unambiguously between crack sensitive and crack resistant steels. This has involved parallel weld cracking and hot ductility tests on low alloy steels to specifications SAE 4130, EN 24 and ASTM A387B. Both composition and welding parameters were shown to have a marked influence on cracking severity. Consideration of composition confirmed the detrimental effects of carbon, sulphur and phosphorus. High levels (>0.030% wt) of either sulphur or phosphorus could cause significant. cracking; phosphorus being the more detrimental. For a constant steel composition, cracking was found to increase with i) Increasing Heat Input/unit length; (ii) Increasing depth of finger penetration; (iii) Application of external restraint; (iv) Increasing distance along the weld bead. Metallographic and fractographic examination of welded specimens has shown that cracking occurs only within a discrete zone in the high temperature region of the H.A.Z., and is associated with the modification of MnS type inclusions. Furthermore, in the high sulphur steels, this crack susceptible zone is brittle at ambient temperatures due to extensive sulphide films at the grain boundaries. It was found that the H.A.Z. hot cracking severity of a steel could be related quantitatively to the zero ductility range, measured during the cooling portion of a simulated weld thermal cycle, providing the peak temperature was equivalent to the nil-strength temperature or above. The greatest sensitivity was achieved with a peak temperature of 12°C below the bulk melting temperature.