A Study of Root Cracking in High Strength Steel Welds (Report 1)

Effect of thermal restraint on cracking

Abstract

Weld cracking of high strength steels is one of the most impotant factors of weldability and a large number of test methods have been proposed to study cracking. Among those methods, the butt weld cracking test specimens of Lehigh restraint (U-groove) and Tekken (y--groove) types were selected for an extensive study of cracking properties of mild steels and electrodes for a range of tensile strength between 40 and 90 kg/mm2.
Effects of groove shape, welding heat input, preheating temperature, immediate postheating, coating type and strength of electrode and chemical composition of steel on cracking behavior and instant of crack initiation were investigated to search for a reasonable method of cracking test of high strength steels and electrodes.
Cracking behavior of high strength steels and electrodes is very differently affected by shape of groove in specimen, especially with Lehigh type symmetrical U-groove and Tekken type oblique y-groove. Therefore, two different types of groove should be used to differentiate cracking susceptibility of base metal from that of electrode ; that is, y-groove is more preferable to base metal testing, while U-groove or Y-groove to electrode. Since there are a number of cracks which do not appear upon the face of weld, cracking should better be examined in cross section or on root of weld rather than on face only.
The cold cracking in this investigation occurred more than three minutes after welding at temperatures below 90°C, regardless of steels, electrodes and preheating temperatures. Although the effect of preheating was remarkable in preventing cracking, a postheating of weld HAZ at 600°C immediately after welding within two minutese was found very effective in obtaining a crack free weld even without preheating.