10. Preheat Temperature for Preventing Root Cracking in Continuous Multi Pass Welding

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It is experientially well known that in the continuous multi-pass welding, root cracking does not easily occur like it does in one-pass welding. The authors devised a preheat temperature selection procedure and successfully developed a handy formula to prevent the root cracking in continuous multi-pass welding. Parameters employed in the preheat temperature selection procedure and the formula thereby obtained are summarized as follows: (1) The susceptibility to weld root cracking in continuous multi-pass welding is less than in single-pass welding, mainly because of decrease in hydrogen at the root due to post weld heating effect. This effect is expressed by thermal factor (ΣDΔt) which is determined by thermal cycle. (2) The same critical thermal factor [(ΣDΔt) cr] for root cracking can be applied to both continuous multi-pass welding and single-pass welding. (ΣDΔt) cr is given by the following weld cracking parameter (C_W) (ΣDΔt)cr=7.94×10^<-4>C_W-0.123 C_W=0.32Hv_<max>+30.5 1ogH_0+F_1×σ_W where, F_1×σ_W is a local stress parameter for root cracking. (3) Critical cooling time to 100℃[(t_<100>)cr] for root cracking is given as: [numerical formula] [table]
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