Cold Cracking Susceptibility of Steel Welds and the Development of a Preheat-Free 980MPa Grade Welding Wire

Cold cracking in a welded joint is a serious problem for the reliability of steel structures, and the assessment of its susceptibility is very important. To avoid cold cracking, preheat is a very effective way, and hence the methods to assess cold cracking susceptibility are generally those that predict the minimum preheat temperature required to avoid cracking. But there are several differences among them. For example, BS5135 uses CE(IIW) that has 1/6 as the Mn coefficient, while AWS D1.1 uses Pcm that has 1/20 as the Mn coefficient, which might produce the opposite evaluation. The paper discusses this difference from a viewpoint of hardenability. Another difference is the effect of diffusible hydrogen (Hd). There are two way of describing its effect, linear effect and logarithmic effect. AWD D1.1 uses SI=12Pcm +log(Hd), which means the log effect. The difference between log and linear effects becomes serious when Hd becomes low. This paper considers Hd effect from a viewpoint of hydrogen diffusion.

On the other hand, preheat temperature tends to increase as the strength of steel and/or weld metal increases. However, preheating itself is a hard work and the reduction of preheat temperature or even no preheating is desired for high strength steels. The Japanese national project is now developing a 980MPa grade welding wire that can be used without preheating. The key concept is to introduce retained austenite in order to trap hydrogen. The Cr-Ni prototype wire made by the project has 980MPa grade strength, and no cracks in the y groove weld cracking test were found when Hd was 1.5ml/100g, which is described in this paper.