Hydrogen trap states in ultrahigh-strength AERMET 100 steel

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1/30/04

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Hydrogen Trap States in Ultrahigh-Strength AERMET 100 Steel DAOMING LI, RICHARD P. GANGLOFF, and JOHN R. SCULLY Hydrogen (H) trap states and binding energies were determined for AERMET 100 (Fe-13.4Co11Ni-3Cr-1.2Mo-0.2C), an ultrahigh-strength steel using thermal desorption methods. Three major H desorption peaks were identified in the precipitation-hardened microstructure, associated with three distinct metallurgical trap states, and apparent activation energies for desorption were determined for each. The lattice diffusivity (DL ) associated with interstitial H was measured experimentally and verified through trapping theory to yield H-trap binding energies (Eb). Solid-solution elements in AERMET 100 reduce DL by decreasing the pre-exponential diffusion coefficient, while the activation energy for migration is similar to that of pure iron. M2C precipitates are the major reversible trap states, with Eb of 11.4 to 11.6 kJ/mol and confirmed by heat treatment that eliminated these precipitates and the associated H-desorption peak. A strong trap state with Eb of 61.3 to 62.2 kJ/mol is likely associated with martensite interfaces, austenite grain boundaries, and mixed dislocation cores. Undissolved metal carbides and highly misoriented grain boundaries trap H with a binding energy of 89.1 to 89.9 kJ/mol. Severe transgranular hydrogen embrittlement in peak-aged AERMET 100 at a low threshold-stress intensity is due to H repartitioning from a high density of homogeneously distributed and reversible M2C traps to the crack tip under the influence of high hydrostatic tensile stress.

I. INTRODUCTION

ULTRAHIGH-STRENGTH steel (UHSS) is used in high-performance aerospace applications that require high tensile strength and fracture toughness.[1,2] A secondary-hardening UHSS, AERMET* 100, was developed to provide plane-strain *AERMET is a trademark of Carpenter Technology Corporation, Wyomissing, PA.

fracture toughness (KIC) in excess of 120 MPa 1m doubling that of older steels such as AISI 4340 and 300M, each at a constant yield strength (YS) of 1750 MPa.[3,4] This strength is produced by a homogeneous distribution of nanoscale coherent M2C alloy carbides in a highly dislocated Fe-Ni lath martensite matrix.[5,6] The high KIC value of AERMET 100 is achieved by advanced melting to minimize sulfur plus phosphorus and inclusion contents, austenitization to control the undissolved carbides and grain size, and aging to optimize austenite precipitates along martensite lath interfaces.[2,4,7,8] Ultrahigh-strength steels are susceptible to severe internal hydrogen embrittlement (IHE) as well as hydrogen environment embrittlement (HEE), and AERMET 100 is no exception.[9] Several studies demonstrated subcritical HEE at apparent threshold stress-intensity levels as low as 20 to 30 MPa 1m when the microstructure, optimized for a high KIC (130 MPa 1m), was stressed in neutral chloride near the free corrosion potential.[10,11,12] An UHSS is often electroplated for corrosion resistance, introducing codeposited hyd