Environment-Assisted Cracking in Custom 465 Stainless Steel

Abstract

The influence of cold work and aging on the environment-assisted cracking (EAC) behavior and mechanical properties of Custom 465 stainless steel (SS) was studied. Four sets of specimens were made and tested. All specimens were initially solution annealed, rapidly cooled, and refrigerated (SAR condition). The first specimen set was steel in the SAR condition. The second specimen set was aged to the H1000 condition. The third specimen set was 60 pct cold worked, and the fourth specimen set was 60 pct cold worked and aged at temperatures ranging from 755 K to 825 K (482°C to 552°C) for 4 hours in air. The specimens were subsequently subjected to EAC and mechanical testing. The EAC testing was conducted, using the rising step load (RSL) technique, in aqueous solutions of NaCl of pH 7.3 with concentrations ranging from 0.0035 to 3.5 pct at room temperature. The microstructure, dislocation substructure, and crack paths, resulting from the cold work, aging, or subsequent EAC testing, were examined by optical microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The aging of the cold-worked specimens induced carbide precipitation within the martensite lath, but not at the lath or packet boundaries. In the aged specimens, as aging temperature rose, the threshold stress intensity for EAC (K IEAC ) , elongation, and fracture toughness increased, but the strength and hardness decreased. The K IEAC also decreased with increasing yield strength and NaCl concentration. In the SAR and H1000 specimens, the EAC propagated along the prior austenite grain boundary, while in the cold-worked and cold-worked and aged specimens, the EAC propagated along the martensite lath, and its packet and prior austenite grain boundaries. The controlling mechanism for the observed EAC was identified to be hydrogen embrittlement.

The interaction of corrosion fatigue and stress-corrosion cracking in a precipitation-hardened martensitic stainless steel

“…First, given the enhanced susceptibility of the H900 condition compared to H950 (as shown for Custom 465 (ref. 21 ) and for a similar precipitation-hardened martensitic steel, Ferrium PH48S 28,48 ), the K ISCC values reported for the H900 condition can be considered lower bounds for the H950 used in the current study. Second, even at the most aggressive polarizations on the more susceptible H900 condition, the K ISCC of ≈10 MPa√m is still greater than K max of ≈5 MPa√m, where IG was observed in Figs.…”

“…Furthermore, prior fracture mechanics testing of the current lot of Custom 465 in the H900 condition (higher strength and more susceptible to IG-SCC 21 ) at polarizations between −500 and −250 mV SCE in 0.6 M NaCl show a lower bound K ISCC value of 47 MPa√m with some specimens demonstrating immunity. 28 This value fell to ≈10 MPa√m for cathodic polarizations <−500 mV SCE and at an anodic polarization of −150 mV SCE (open circuit poential (OCP) was measured at −225 mV SCE ).…”

“…As changes in bulk chloride concentration, heat treatment, and applied electrochemical potential can critically influence the K ISCC , it is important to evaluate literature K ISCC values to ensure that comparisons to the current data are justified. Vasudevan and colleagues 21 used rising step load data to demonstrate that varying chloride concentration from 0.0006 to 0.6 M will substantially alter the IG-SCC susceptibility of Custom 465 (60% cold worked then aged for 4 h at 551°C). This finding is consistent with the lack of IG-SCC observed below 0.6 M NaCl in Fig.…”

“…Specifically, researchers have demonstrated aqueous chloride environments can: induce localized corrosion damage [9][10][11][12] causing a severe reduction in overall fatigue life, 10,[13][14][15][16][17][18][19][20][21] accelerate fatigue crack-growth rates, 13,16,[21][22][23][24][25][26][27] and enhance susceptibility to stress-corrosion cracking (SCC). 21,28 Enhancement of the environmental cracking kinetics is widely attributed to hydrogen embrittlement (HE). 24 Recent research on Custom 465, a precipitation-hardened martensitic stainless-steel based on the Fe-Cr-Ni system, quantified and analyzed the corrosion and H-uptake behavior, 29,30 the pit-to-fatigue crack transition, the short-crack growth and long-crack growth behavior, 13 and the stress-corrosion cracking behavior.…”

“…24 Recent research on Custom 465, a precipitation-hardened martensitic stainless-steel based on the Fe-Cr-Ni system, quantified and analyzed the corrosion and H-uptake behavior, 29,30 the pit-to-fatigue crack transition, the short-crack growth and long-crack growth behavior, 13 and the stress-corrosion cracking behavior. 21,28 Critically, a time-dependent intergranular (IG) fracture process was observed at stress intensity ranges (ΔK) below the threshold value (ΔK TH ) during fracture mechanics-based fatigue testing in high [Cl − ] environments. 13 Detailed examination of this behavior was outside of the scope of the prior effort but serves as the motivation for the current study.…”