Predictive model for the prevention of weld metal hydrogen cracking in high-strength multipass welds

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>This thesis studies controlling factors that govern transverse hydrogen cracking in high-strength multipass weld metal (WM). The experiments were concerned with heavy-restraint Y- and U-Groove multipass cracking tests of shielded-metal arc (SMAW) and submerged-arc (SAW) weld metals. Results of tensile tests, hardness surveys, weld residual stress measurements and microstructural investigations are discussed. The analytical phase comprised numerical calculations for analysing the interactions between crack-controlling factors. The objectives were: (i) the assessment of WM hydrogen cracking risk by defining the Crack-No Crack boundary conditions in terms of ‘safe line’ description giving the desired lower-bound estimates, and (ii) to derive predictive equations capable of giving reliable estimates of the required preheat/interpass temperature T₀/T_i for the avoidance of cracking.</p><p>Hydrogen cracking occurred predominantly in high strength weld metals of R_p0.2 ≈ 580–900 MPa. At intermediate strengths of R_p0.2 ≈ 500–550 MPa, cracking took place in the cases where the holding time from welding to NDT inspection was prolonged to 7 days. Low strength WMs of R_p0.2 ≤ 480 MPa did not exhibit cracking under any conditions examined. Cracking occurrence was, above all, governed by WM tensile strength, weld diffusible hydrogen and weld residual stresses amounting to the yield strength. The appearance of cracking vanished when transferring from 40 to 6 mm thick welds. The implications of the holding time were more significant than anticipated previously. A period of 16 hrs in accordance with SFS-EN 1011 appeared much too short for thick multipass welds. Interpass time and heat input showed no measurable effect on cracking sensitivity, hence being of secondary importance. Equations were derived to assess the weld critical hydrogen content H_cr corresponding to the Crack-No Crack conditions as a function of either weld metal P_cm, yield strength R_p0.2 or weld metal maximum hardness HV_5(max). For the calculation of safe T₀/T_i estimates, a formula incorporating: (i) WM strength as a linear function of either weld carbon equivalent CET or weld HV_5(max), (ii) weld build-up thickness a_w in the form of <em>tanh</em> expression and (iii) weld diffusible hydrogen H_D in terms of a combined [<em>ln / power law</em>] expression was found descriptive.</p></div>