Effect of Liquid Nitrogen Chill on SCC of Ultra-High Strength Steel

The objective of this work is to test the hypothesized beneficial effect of cryogenic processing on transgranular stress corrosion cracking (SCC) of modern martensitic ultra-high strength steel. The mechanistic justification is elimination of thin-film austenite along martensite lath interfaces, which is hydrogen laden from environmental exposure and transforms to brittle martensite under crack tip strain.   Experiments yielded two dramatically different classes of SCC behavior for fatigue precracked single edge notch (SEN) specimens stressed in 3.5% NaCl solution near the free corrosion potential.  One-half of eight replicate experiments with the LN₂ chilled condition evidenced immunity to SCC, while four experiments yielded significant transgranular SCC identical to that observed for commercially processed martensitic UHSS.  While the deleterious role of residual austenite is supported and justified theoretically, there is no direct evidence for LN₂-dependent change in the typically low amounts of this phase.  An explanation for the bimodal SCC response based on heterogeneous CrS dissolution and occluded crack chemistry change is supported by measured-high Cr enrichment in the corrosion product present only on fast SCC surfaces; however, CrS is not evidenced in the microstructure.  The technological impact of near-immune SCC response for ultra-high strength steel justifies continued research on the role of residual austenite.