Hydrogen Environment Assisted Cracking of Modern Ultra-High Strength Martensitic Steels
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RODUCTION
ULTRA-HIGH strength steel (UHSS) alloys, based on precipitation-strengthened martensite of 1550 to 1700 MPa yield strength, are tough but limited by stress corrosion cracking (SCC) at ambient temperature.[1–3] Subcritical crack propagation is intergranular (IG) in impure-older steels,[4] and occurs above a fracture mechanics threshold (KTH) at a rate (da/dt), which increases with increasing mode I stress intensity factor (K).[5] The dominant mechanism is hydrogen environment assisted cracking (HEAC).[1,5] Atomic hydrogen (H) is produced on the crack tip surface and diffuses as an interstitial into the fracture process zone (FPZ) ahead of the tip to promote damage through interaction of interface-bond decohesion[6] and localized plasticity.[7,8] Crack growth rate is governed by time-dependent surface reaction and H diffusion in the FPZ, and reaches a diffusion limited Stage II rate (da/dtII) as K GREGER L. PIOSZAK and RICHARD P. GANGLOFF are with the Center for Electrochemical Science and Engineering, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904-4745. Contact e-mail: [email protected] Manuscript submitted October 22, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
rises.[5,9] The enriched H concentration in the FPZ; dictated by stress state, chemical variables (e.g., crack tip H overpotential and temperature), and H trapping; is central to predicting KTH and da/dtII.[1,5,10–13] The effectiveness of H in promoting decohesion is governed by: (a) high crack tip tensile and hydrostatic stress,[1,12,14] (b) impurity segregation to interfaces,[4,15,16] (c) localized dislocation plasticity,[7,8] and (d) H mobility.[9] Computational materials engineering is being used to efficiently develop new UHSS compositions and processing conditions,[17–20] building on pioneering work with Co-bearing steels.[21,22] Three aspects of alloy design should promote HEAC resistance: (1) precipitation of coherent, nano-scale carbides, or intermetallics, which reversibly trap hydrogen to reduce the effective diffusivity of hydrogen (DH-eff) and thus da/dtII,[9] (2) reduced impurity content for increased intergranular cohesion leading to improved KTH and da/dt,[15,16] and (3) rare earth addition to further reduce free-mobile impurity concentration.[2] Moreover, thin-film austenite precipitated at martensite interfaces during aging was divergently reported to: (a) reduce DH-eff to reduce da/ dtII,[23] or transform to H-supersaturated brittle martensite during crack tip straining to promote HEAC.[24] The impact of these factors is illustrated by the improved H
cracking resistance of AerMet100 (0.2C-13.4Co11.1Ni-3.0Cr-1.2Mo); by wt pct and designated AM100,[25,26] with nano-scale (Mo,Cr)2C and austenite precipitates in unrecrystallized martensite,[27] compared to low purity 300M (0.4C-1.8Ni-0.8Cr-0.4Mo; wt pct) with large (Fe,Cr)3C and austenite.[23,26,28,29] In spite of these improvements, AM100 remains susceptible to HEAC, but crack growth kinetics are significantly reduced and the path
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