The influence of palladium on the hydrogen-assisted cracking resistance of PH 13-8 Mo stainless steel

143

110

Near-peak-aged AERMET 100 is susceptible to severe internal hydrogen embrittlement (IHE) at 23 ЊC, if a sufficient diffusible hydrogen content is present, compromising the high toughness of this ultrahigh-strength steel (UHSS). Evidence includes the threshold stress intensity for subcritical IHE (K TH ) as low as 10 pct of the plane-strain fracture toughness (K IC ) and a fracture-mode transition from microvoid coalescence to brittle transgranular (TG) cracking, apparently along martensite lath interfaces and cleavage planes. The K TH value decreases from a K IC value of 132 to 143 MPaΊm to 12 MPaΊm, and the amount of brittle TG fracture increases to nearly 100 pct as the concentration of diffusible H increases from essentially 0 to 8 wppm, with severe embrittlement in the 0 to 2 wppm H regime. The IHE is time dependent, as evidenced by increasing K TH values with increasing dK/dt and K-independent subcritical crack growth rates, and is attributed to diffusional H repartition from reversible trap sites to the stressed crack tip. The partition distance is ϳ1 m, consistent with the fine-scale microstructure of AERMET 100. The causes of the susceptibility of AERMET 100 to TG IHE are very high crack-tip stresses and a reservoir of mobile H trapped reversibly at (Fe,Cr,Mo) 2 C precipitates. These factors enable repartition of H to misoriented martensite lath interfaces and interstitial sites near cleavage planes, with each prone to decohesion along a connected path. Predissolved H also reduces the ductile fracture toughness of AERMET 100 at high loading rates, perhaps due to reduced void growth caused by H trapped strongly at undissolved metal carbides.

Section: The H-uptake-capacity Dependence Ofmentioning

confidence: 82%

Section: A Hydrogen Embrittlement Of Ultrahigh-strength Steelsmentioning

confidence: 99%

Internal hydrogen embrittlement of ultrahigh-strength AERMET 100 steel

Thomas

Scully

Gangloff

2003

143

110

“…[9] In contrast, a homogeneous distribution of irreversible or strong-reversible traps, that do not themselves constitute fracture-initiation sites and a connected crack path, could prevent H from segregating to susceptible interfaces prone to H-enhanced crack growth and, thus, alleviate IHE. [31,32,33] A basic understanding of the role of H trapping in solubility, diffusivity, and embrittlement requires information on the trap binding energy and trap distribution in the microstructure. This analysis is hindered by the multiple trapping states present in an UHSS microstructure.…”

Section: Ultrahigh-strength Steel (Uhss) Is Used Inmentioning

confidence: 99%

Hydrogen trap states in ultrahigh-strength AERMET 100 steel

Gangloff

Scully

2004

173

Hydrogen (H) trap states and binding energies were determined for AERMET 100 (Fe-13.4Co-11Ni-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 (D L ) associated with interstitial H was measured experimentally and verified through trapping theory to yield H-trap binding energies (E b ). Solid-solution elements in AER-MET 100 reduce D L by decreasing the pre-exponential diffusion coefficient, while the activation energy for migration is similar to that of pure iron. M 2 C precipitates are the major reversible trap states, with E b 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 E b 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 M 2 C traps to the crack tip under the influence of high hydrostatic tensile stress.

“…The function f is given by [11] The crack growth rate is plotted against the stress intensity [5] factor range in Figure 11. and…”

Section: B R ϭ 01 Testsmentioning

confidence: 99%

“…This aging treatment is typical of that used for critical aerospace toughness. The tensile yield strengths that have been reported for H1050 aging range from 1140 to 1360 MPa, [4,5,8,9,11] with applications needing a balance of strength, toughness, and good corrosion resistance. This is an overaged condition, [7,8] the wrought alloy tending to be a little higher than the as-cast.…”

Section: Introductionmentioning

confidence: 99%

Growth of small fatigue cracks in PH 13-8 Mo stainless steel

Patel

Neu

Pape

1999

The growth of small fatigue cracks in PH 13-8 Mo (H1050) stainless steel under constant amplitude loading at different mean stresses (R ϭ 0.1 and Ϫ1) under generally high cycle fatigue conditions was investigated. Small cracks were allowed to initiate naturally at the root of a single edge notch specimen and were monitored using a surface replicating technique. It was found that the initiation and growth of surface cracks up to 100 m encompassed 70 to 90 pct of the total fatigue life at stress amplitudes just above the fatigue limit. Cracks of length less than 100 m were subject to strong influences of the microstructure and exhibited stage I (shear-dominated) growth, which was manifested in oscillatory crack growth rates. The oscillations diminished as the crack transitioned to stage II growth. The higher stress ratio (R ϭ 0.1) resulted in a more rapid transition from stage I to stage II growth in comparison to R ϭ Ϫ1. After transitioning to stage II, the crack growth could be well characterized by conventional long crack tools even when the crack was still physically small. The small crack growth behavior is shown to be similar to that of a quenched and tempered AISI 4340 steel having a comparable strength.