Study of Predictive Equations of Thermal Factor and Hardness Relatated with Weld Cold Cracking

Abstract

This report deals with the relationship of welding conditions, such as cooling times, chemical compositions, vs. the maximum hardness at the neighbourhood of bond line in the heat affected zone and the thermal factor, which is connected with the residual diffusible hydrogen content accepted welding thermal cycle, being main factors for cold cracking. Relationships of thermal factor and hardness vs. cooling times and chemical compositions are not able to be shown as function formulae. Therefore, the regression equations are derived by the stagewise regression analysis from many data of thermal cycles and CCT diagrams containing hardness data. The results are summarized as following :
(1) The predicted value (∑DΔt)₁₀₀of thermal factor is connected with cooling times by following formula.
(∑DΔt)₁₀₀= (4.2t₂₀₀+2.73t₁₅₀-13) × 10^-5(cm²)
tθ; cooling time from melting temperature to θ°C at the neighbourhood of bond line (sec)
(2) The relationship of predicted value of hardness Hv vs. cooling time and chemical compositions is estimated by following formula.
τ≤τM Hv=812C+293
τ>τM Hv=(992C-230P_v+250) exp {-3t₈₀₀₅₀₀/(310)P_v}+188P_v+80
where τ = t ₈₀₀₅₀₀; cooling time from 800°C to 500°C at the bond line log τM= 2.5P_v-1.21
P_v=C + Mn/3+Ni/8+ Cr/12+ Mo/2+Cu/5
The unit of chemical compositions is weight %.