Improvement of Weld Fusion Zone Toughness by Fine TiN

Abstract

A. marked improvement in toughness oj weld Jusion zone in large heat input welding processes such as one-side submerged arc welding, electrogas welding, and electroslag welding has been accomplished through reJining oj Jusion zone microstructure by the aid oj Jine TiN jJl'ecijJitates.The Jollowing results have been obtained :(1) Fine TiN precipitates, particularly those smaller than ca.0.051" are 4!ective in jJl'eventing austenite grain coarsening.Austellite grain diameter d r at J 400C can be expressed bywhere, r: mean diameter oJTiN remaining undissolved at J 400 C J: volume Jraction oj the undissolved TiN smaller than 0.0511 at J 400 ' C K: 0.12 (experimental determination).(2) The Jine TiN precipitates are also 4fective in stimulating Jerrite Jormation .At jJlaces where there are Jine TiN precip itates congregated in a high concentration prior to cooling transJormation, the austenite transforms into a fine structure.(3) As the result, the diameter oj brittle fracture, revealed by the (100) etch Pits as to be in the same orientation, becomes smaller.Thu s toughness oj Jusion zone in large heat input welding is greatly imjJroved.(4) Using these two speciJic ~f!ects oj Jine TiN precijJitates, Jusion z one toughness oj commercial 40, 50 and 60 kg/mm' class steels has been improved Jor a wide range oj welding conditions covering small to large heat input.