How can you prevent hydrogen delayed cracking in weldings.?
Answer:
*Preheating is required to avoid hydrogen cracking .
Hydrogen cracking may also be called cold cracking or delayed cracking. The principal distinguishing feature of this type of crack is that it occurs in ferritic steels, most often immediately on welding or a short time after welding.
*Causes
There are three factors which combine to cause cracking:
hydrogen generated by the welding process
a hard brittle structure which is susceptible to cracking
tensile stresses acting on the welded joint .
*It may be too conservative to employ the critical preheating temperature obtained from root-pass cracking tests for the necessary preheating temperature. In this study, the critical preheating temperatures for preventing cracking in the root-pass welds and multi-run welds have been compared for steels including mild steel, pressure vessel steel and high strength steel. The cracks investigated in these cracking tests include root crack, toe crack and under-bead crack. The findings in this study are as follows : 1) The cold cracking susceptibility of carbon manganese steels and low alloy steels with carbon more than 0.16% may be better evaluated by C_<eq> (WES) than P_<CM>. For steels with carbon less than 0.16%, P_<CM> may be a proper parameter rather than C_<eq>. 2) The critical preheating temperature for preventing root cracking in multi-run welds is less than that for root-pass welds by 50 to 75℃, mainly because of a decrease in hydrogen accumulation at the root by a post heating effect of subsequent passes. 3) The critical preheating temperature for preventing toe and under-bead cracking in restrained multirun butt weldments is lower than that for multi-run root cracking, especially in a lower C_<eq> range. 4) It may be practical to select the preheating and inter-pass temperature based on the toe cracking prevention preheating temperature rather than that for root cracking in root-pass welds.
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