FOR INTERNAL USE ONLY
Hot or cold cracking in the weld metal or HAZ depends on the same fundamental factors as in the base metal, i.e. hydrogen content, microstructure and residual stress. In practice the controlling variables are usually strength, hydrogen content, restraint, stress concentrations, and heat input.
Figure 38
Minimizing Heat Input by Multi-Pass Welding
In single pass welds and root runs of multiple pass welds the root pass may provide a stress concentration which can lead to longitudinal cracks in the weld metal. High dilution of the root run (high heat input) can often result in a harder weld bead more likely to crack (this is commonly seen in such applications as pipeline welding). Figure 40 illustrates the physical appearance of hydrogen cracking in welds.
Figure 39 Schematic View of Typical Weld Cracks
Figure 40 Cracking Caused By Lack Of Fusion in Weld
In Figure 41 the crack has initiated at the root of the weld where a lack of fusion can be seen. The crack has then traveled through the HAZ mainly in the coarse grained region. In heavy multiple – pass welds cracking will generally be transverse to the weld direction, sometimes running through the weld itself since the maximum cooling rate is along the weld axis. Many HSLA steels in critical repair situations where PWHT is impracticable are welded using a filler metal of good toughness and ductility and in such cases the HAZ may be more crack sensitive. The risk of hydrogen-induced cold cracking in the weld can be minimized by: ♦ Reducing hydrogen pick-up (low hydrogen flux chemistries) ♦ Maintaining a low carbon content ♦ Avoiding excessive restraint ♦ Control of welding procedures (preheat; heat input; PWHT etc.) ♦ Developing a non-sensitive weld microstructure WELDING AND COATING METALLURGY2
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