What causes hydrogen-induced cracking during welding?

What causes hydrogen-induced cracking during welding?

• A. Hydrogen diffusion during cooling
• B. Thermal contraction during welding
• C. Oxidation of weld metal
• D. Intergranular corrosion of grain boundaries

Answer: (A)

Hydrogen diffusion during cooling explains hydrogen-induced cracking because hydrogen is picked up from sources like moisture, oils, fluxes, or coatings during welding. At the high temperatures of welding, hydrogen dissolves in the metal; as the weld and heat-affected zone cool, the metal’s capacity to hold hydrogen drops. The dissolved hydrogen then diffuses to regions of high tensile stress, especially along grain boundaries, inclusions, or other traps, and accumulates there. This accumulation weakens the metal locally and causes decohesion and crack propagation under the residual stresses created by welding. The result is a brittle, intergranular cracking pattern typical of hydrogen-assisted cracking, particularly in high-strength steels. Other mechanisms mentioned—thermal contraction, oxidation, or intergranular corrosion—describe different failure modes. Thermal contraction creates stresses from shrinkage but isn’t driven by hydrogen diffusion; oxidation is a surface reaction without the hydrogen embrittlement mechanism; intergranular corrosion is a corrosion process along grain boundaries, not specifically hydrogen diffusion and trapping during cooling.

Hydrogen diffusion during cooling explains hydrogen-induced cracking because hydrogen is picked up from sources like moisture, oils, fluxes, or coatings during welding. At the high temperatures of welding, hydrogen dissolves in the metal; as the weld and heat-affected zone cool, the metal’s capacity to hold hydrogen drops. The dissolved hydrogen then diffuses to regions of high tensile stress, especially along grain boundaries, inclusions, or other traps, and accumulates there. This accumulation weakens the metal locally and causes decohesion and crack propagation under the residual stresses created by welding. The result is a brittle, intergranular cracking pattern typical of hydrogen-assisted cracking, particularly in high-strength steels.

Other mechanisms mentioned—thermal contraction, oxidation, or intergranular corrosion—describe different failure modes. Thermal contraction creates stresses from shrinkage but isn’t driven by hydrogen diffusion; oxidation is a surface reaction without the hydrogen embrittlement mechanism; intergranular corrosion is a corrosion process along grain boundaries, not specifically hydrogen diffusion and trapping during cooling.