Abstract
This thesis studies controlling factors that govern transverse hydrogen cracking in highstrength multipass weld metal (WM). The experiments were concerned with heavyrestraint Y and UGroove multipass cracking tests of shieldedmetal arc (SMAW) and submergedarc (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 crackcontrolling factors. The objectives were: (i) the assessment of WM hydrogen cracking risk by defining the CrackNo Crack boundary conditions in terms of 'safe line' description giving the desired lowerbound estimates, and (ii) to derive predictive equations capable of giving reliable estimates of the required preheat/interpass temperature T_{0}/T_{i} for the avoidance of cracking.
Hydrogen cracking occurred predominantly in high strength weld metals of R_{p0.2} ≈ 580900 MPa. At intermediate strengths of R_{p0.2} ≈ 500550 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 SFSEN 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 Hcr corresponding to the CrackNo 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 T_{0}/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 buildup thickness a_{w} in the form of tanh expression and (iii) weld diffusible hydrogen HD in terms of a combined [ln / power law] expression was found descriptive.
Hydrogen cracking occurred predominantly in high strength weld metals of R_{p0.2} ≈ 580900 MPa. At intermediate strengths of R_{p0.2} ≈ 500550 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 SFSEN 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 Hcr corresponding to the CrackNo 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 T_{0}/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 buildup thickness a_{w} in the form of tanh expression and (iii) weld diffusible hydrogen HD in terms of a combined [ln / power law] expression was found descriptive.
Original language  English 

Qualification  Doctor Degree 
Awarding Institution 

Award date  15 Nov 2003 
Place of Publication  Oulu 
Publisher  
Print ISBNs  9514271807 
Electronic ISBNs  9514271815 
Publication status  Published  2003 
MoE publication type  G5 Doctoral dissertation (article) 
Keywords
 cold cracking
 hightstrength steels
 hydrogen cracking
 multipass welding
 weld metal cracking