There are instances where employing the preheat temperature for preventing root cracking as determined from single-pass welding cracking test results for the continuous multi-pass welding errs on the side of safety.
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) Nearly the same critical thermal factor [(ΣDδt) cr] for root cracking applies to both continuous multi-pass welding and in single-pass welding. (ΣDδt) cr is given by the following weldment cracking parameter (Cw)
(ΣDδt) cr=7.94×10-4Cw-0.123
Cw=0.32HVmax.+30.5 log H0+F1 × σw
where, F1 × σw is a local stress parameter for root cracking. (3) Critical cooling time to 100°C [(t100) cr] for root cracking is given as :
(t100) cr=exp [1.22 {ln1/α (7.94×10-4Cw-0.123) +9.79}]
α= 1 : Single-pass welding
2 : Continuous multi-pass welding
