Factors Affecting on Shrinkage Stresses of Weld Under Restraint

Abstract

Weld cracking is influenced by both reaction stress during cooling and properties of weld metal and heat-affected zone. The problems of weld cracking, therefore, should be explored not only from metallurgical standpoint but also from standpoint of reaction stress. Researches on effect of reaction stresss during cooling or restraint on weld cracking are now undertaken by the authors, especially for first-pass-weld-cracking of restrained welds of some mild and high strength steels. At earlier stage of the research project, informations on what factors influence to reaction stress under restraint were wanted for reasonable research planning. The present paper was prepared as preliminary informations for the subsequent test planning.
In a restrainning system shown in Fig. 1 shrinkage force will be produced by hindered contraction or extension of first-pass weld metal during cooling. Analytical investigation was conducted on the extension of weld metal. The anaiysis is based on the assumptions that behavior of mother plate (B) and restraining members (C) is elastic during cooling, however, in the weld metal will be produced some plastic deformations under the stress-strain relation as shown in Fig. 3. The extension (λ_w) of weld metal at a given temperature below about 300°C is obtained by
p_wh_w/ph⋅λ_wⁿ+λ_w=S
where ph and p_wh_w are rigidity or force per unit weld length required for unit extension of mother plates and weld metal respectively, and S is free contraction between restrained length (l) of mother plate. Relations of λ_w versus rigidity ratio p_wh_w/ph are shown in Fig. 4 for a few values of S. Larger extension of weld metal will be produced under smaller rigidity ratio and larger free contraction.
Free contraction S will be influenced by heat input and overall cooling behavior of mother plate of length l. Under a given heat input the overall cooling rate or heat loss from the mother plate of length l will be increased as the decrease of heat capacity or length (l) and thickness (h) of mother plate.
It will be concluded, therefore, that under a given heat input reaction stress of weld metal at a given temperature is influenced by the following factors ;
1) restraint coefficient (p), which contains restrained length (l) and ratio of sectional area of mother plate to restraining members.
2) thickness (h) of the mother plate.
3) restrained length (l) of the mother plate.
4) rigidity of weld metal (p_wh_w).
Figs. 8 and 9 show some calculated examples of the effect of l and h on the extension of weld metal.