Distinguish between hydrogen cracking and solidification cracking and how to mitigate each.

• A. They are the same phenomenon.
• B. Hydrogen cracking occurs during solidification; solidification cracking occurs due to hydrogen gas.
• C. Hydrogen cracking occurs only in stainless steel.
• D. Hydrogen cracking occurs in the weld metal/HAZ due to hydrogen absorption during cooling; solidification cracking occurs during solidification of the weld metal in susceptible alloys; mitigate with proper preheat, low-hydrogen electrodes, faster cooling control, and correct alloy selection.

Answer: (D)

The main idea is that two different cracking mechanisms affect welds in different ways and require distinct controls: hydrogen cracking and solidification cracking.

Hydrogen cracking happens in the weld metal and heat-affected zone as hydrogen generated during welding diffuses into the cooling metal and gets trapped by internal stresses. The crack forms because hydrogen embrittles the newly solidified metal, especially in high-strength steels or welds where hydrogen sources are present. To mitigate this, use preheating to slow the cooling rate so hydrogen has time to diffuse out, employ low-hydrogen filler metals, ensure low moisture and clean welding conditions to minimize hydrogen generation, and select alloys less susceptible to hydrogen cracking. Controlling the cooling rate—typically toward slower cooling—is about giving hydrogen a chance to escape rather than getting trapped.

Solidification cracking occurs while the weld metal is still solidifying. It arises in susceptible alloys when the solidification range is broad and there are segregants at grain boundaries, so tensile stresses during solidification cause cracks in the partially solidified weld. Mitigation focuses on reducing these stresses and the tendency to segregate: choose alloys with a more favorable (narrow) solidification range, control heat input to manage cooling and solidification rates, preheat to reduce thermal gradients, and use appropriate filler metals and welding practices that minimize harmful impurities and stress.

Together, the statement reflects that hydrogen cracking is related to hydrogen absorption during cooling in the weld region, while solidification cracking is tied to the behavior of the weld metal as it solidifies, and both can be mitigated by proper preheating, careful control of hydrogen sources and cooling, and appropriate alloy selection.