Development of Reaction Stress and Weld Cracking under Restraint

On weld cracking, especially on the cold cracking during cooling and after cooled to room temperature, intensity of restraint of weld joints will be one of the important factors. Research was made to investigate the relationship between the intensity of restraint and weld cracking under controlled contraction. In the previous report, a new test method named "Rigid Restraint Weld Cracking Test (RRC-test)" was developed for the first-pass weld cracking as related to external restraint of a weld joint, in this method the length of a specimen (or the restraining gauge length l) is kept constant during cooling (See Fig. 1). Critical intensity of restraint Kcr for weld cracking was obtained by this method. In this report, effect of plate thickness and welding heat input on the development of reaction stresses and the critcal intensity of restraint were investigated for the weldments of a mild steel (M) and a high strength steel of 80 kg/mm2 tensile strength level (H 8). The results obtained are as follows. 1) In the RRC-test, reaction force increases with cooling and the tendency of its development is almost similiar to the contraction process of weld joints during cooling, as related to the restraining gauge length, plate thickness and welding heat input (Figs. 2-4). 2) The magnitude of final reaction stress in weld (σw)t=∞, in the range that the intensity of restraint K is small and plate thickness h is larger than critical plate thickness hcr is proportionate to K independently of heat input and plate thickness (Eq. 8). The proportionate factor m is (3.9-5.0) × 10-2 in steel weldments (Eq. 9). As K-value increases, (σw)t=∞ approaches a constant value which is almost equal to the tensile strength of the weld metal. 3) The critical intensity of restraint Kcr for given materials and welding conditions can be obtained from the RRC-test results. Kcr of the mild steel weldments is independent of plate thickness and welding heat input in the case of h>hcr (Figs. 14 and 15). Kcr of the high strength steel weldments decreases with increase of plate thickness in the case of a constant heat input (Fig. 17). 4) The intensity of restraint had been measured for weld joints of actual structures such as ships as well as some weld cracking test specimens by one of the authors and others. These values were compared with the Kcr obtained. The posibilities of cracking in those weld joints were discussed (Fig. 18).