AWS Certified Welding Inspector — FundamentalsWelding MetallurgyHard
A welding engineer is evaluating a new high-strength steel alloy for a critical application. The material exhibits a high susceptibility to hydrogen-induced cracking (HIC). Which of the following microstructural constituents significantly increases the steel's susceptibility to HIC?
- AMartensite
- BSpheroidized carbides
- CCoarse ferrite-pearlite
- DFine-grained austenite
Show answer & explanationAnswer & explanation
Correct answer: A. Martensite
Martensite is a very hard and brittle microstructure that is highly susceptible to hydrogen-induced cracking (HIC). Its body-centered tetragonal (BCT) crystal structure contains high internal stresses and numerous lattice defects, which act as traps for hydrogen. When sufficient hydrogen is present along with tensile stresses, cracks readily initiate and propagate through the brittle martensitic structure.
Why the other options are wrong
- B. Spheroidized carbides are associated with softened microstructures (e.g., in annealed steels) and do not increase HIC susceptibility; in fact, they generally improve ductility.
- C. Coarse ferrite-pearlite structures are generally ductile and less susceptible to HIC due to their lower strength and more accommodating microstructure.
- D. Fine-grained austenite is typically ductile and has a face-centered cubic (FCC) structure that can dissolve more hydrogen and is generally less prone to HIC than martensite.
Martensite and HIC
Martensite is a very hard and brittle microstructure formed by rapid cooling of steel, and its presence significantly increases the steel's susceptibility to hydrogen-induced cracking (HIC).
- Martensite is highly stressed and brittle.
- Its lattice structure provides traps for hydrogen.
- Low fracture toughness makes it prone to cracking under hydrogen and stress.
- Formation is controlled by cooling rate and alloying elements (hardenability).
Memory trick: Martensite's rigid grip holds hydrogen, leading to a sudden, brittle break.