Unique Behavior of PH13-8Mo Stainless Steel in Additive Manufactured or Highly Cold-Worked Wrought Conditions

PH13-8Mo is a precipitation-hardened martensitic stainless steel that gains strength through formation of nanoscale NiAl precipitates. With highly cold-worked small diameter rod, uniquely high hardness was achieved, above HRC 50, during age hardening at 950°F (510°C), presumably due to enhanced precipitation on the prior dislocation network. In this condition, the hardness/strength (UTS) relationship began to deviate from the expected trend observed in previous studies of lower-strength wrought PH13-8Mo bar. Interestingly, additive manufactured PH13-8Mo shows similar deviation in the H950 aged condition -- with lower UTS values than would be expected from the hardness measurements. In this case, the fine scale AM microstructure and cellular substructure could play a similar role in terms of enhanced precipitation. AM material was also found to be more prone to austenite reversion during age hardening treatments ≥1100°F (593°C) compared to wrought PH13-8Mo. Reasons for this behavior are unclear but could also be related to the fine substructure which may provide more nucleation sites for austenite formation compared to coarser wrought microstructures.

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