Effect of Microstructure on Root Cracking in High Strengh Steel Welds

A Study of Root Cracking in High Strengh Steel Welds (Report 6)

Abstract

Effects of microstrure on root cracking of high strength steel welds were discussed in this report, mainly with the use of SH-CCT diagrams of test steels. Critical hardness values for root cracking were also discussed.
The main conclusions obtained in this study are as follows :
(1) A safety cooling process to prevent root cracking in HAZ should be chosen so that the ferrite and intermediate structure may appear more than about 40% for HT 60, intermediate structure more than about 25% for HT 70, and more than about 10% for HT 80 steels, respectively, when a joint of severe restraint is welded with a low hydrogen type electrode.
(2) For a weld metal, it is necessary to prevent root cracking in a severe restraint joint that the ferrite and intermediate structure precipitate more than in HAZ.
(3) A root crack in HAZ or weld metal is a mixed fracture of transcrystalline and intercrystalline types.
(4) There was found no relationship between root cracking tendencies and Mf temperatures.
(5) Retained austenite in HAZ was decomposed with the lapse of time above 250°C, but did not affect the root cracking phenomena.
(6) Critical hardness, above which a root crack will surely occur in a joint of severe restraint, were found to be as follows :
270 VHN (1 kg) for HAZ of mild steel weldment
365 VHN (1 kg) for HAZ of HT 50 weldment
400 VHN (1 kg) for HAZ of HT 60 weldment
405 VHN (1 kg) for HAZ of HT 70 weldment
415 VHN (1 kg) for HAZ of HT 80 weldment
290 VHN (1 kg) for weld metal of HT 50 and HT 60 weldments
330 VHN (1 kg) for weld metal of HT 70 and HT 80 weldments