Paraequilibrium Carburization of Duplex and Ferritic Stainless Steels

Abstract

AISI 301 and E-BRITE stainless steels were subjected to low-temperature (743 K) carburization experiments using a commercial technology developed for carburization of 316 austenitic stainless steels. The AISI 301 steel contained~40 vol pct ferrite before carburization but had a fully austenitic hardened case,~20-lm thick, and a surface carbon concentration of~8 at. pct after treatment; this ''colossal'' paraequilibrium carbon supersaturation caused an increase in lattice parameter of~3 pct. The E-BRITE also developed a hardened case, 12-to 18-lm thick, but underwent a more modest (~0.3 pct) increase in lattice parameter; the surface carbon concentration was~10 at. pct. While the hardened case on the AISI 301 stainless steel appeared to be single-phase austenite, evidence for carbide formation was apparent in X-ray diffractometer (XRD) scans of the E-BRITE. Paraequilibrium phase diagrams were calculated for both AISI 301 and E-BRITE stainless steels using a CALPHAD compound energy-based interstitial solid solution model. In the low-temperature regime of interest, and based upon measured paraequilibrium carbon solubilities, more negative Cr-carbon interaction parameters for austenite than those in the current CALPHAD data base may be appropriate. A sensitivity analysis involving Cr-carbon interaction parameters for ferrite found a strong dependence of carbon solubility on relatively small changes in the magnitude of these parameters.

The “Expanded” Phases in the Low-Temperature Treated Stainless Steels: A Review

“…structure of metal atoms, while the maximum C content is ~19 at.% [59]. In expanded ferrite, obtained in ferritic stainless steels, a maximum N content of ~24 at.% was observed [60], and the reported maximum C content was ~10 at.% [61]. Similar values were detected in expanded martensite, with a maximum N content of ~22 at.% [62] and a maximum C content of ~10 at.% [63].…”

Low temperature plasma carburizing of AISI 316L austenitic stainless steel and AISI F51 duplex stainless steel

“…On the other hand, case hardening of austenitic stainless steels (ASS) by nitriding can be achieved by intensive precipitation of chromium nitrides (CrN, Cr 2 N) in the diffusion zone during nitriding at temperatures around 500 o C, increasing hardness up to 1400 HV, but decreasing the corrosion resistance [Larisch, 1999], Czerwiec, 2000], [Liang, 2000]. However, when low temperature nitriding is used, close to 400 o C, the hardening mechanism changes from nitride precipitation to lattice distortion of the FCC austenitic phase, leading to formation of expanded austenite, with hardness close to 1400 HV and no loss of corrosion resistance [Fewell, 2000], Borgioli, 2005], [Mingolo, 2006]. The hardening mechanism is related to high compressive residual stresses arisen from lattice distortion.…”

“…This behavior was attributed to the higher content of chromium in the ferritic phase [Christiansen, 2005]. [Michal, 2009], on the other hand, found that after low temperature plasma carburizing an AISI 301 stainless steel, containing approximately 40% of ferrite, the ferrite peaks disappeared and the austenite peaks have shifted to smaller 2q angles, indicating an expansion of ~3 % on the lattice parameter of austenite.…”

Low-Temperature Carburization of AL-6XN Enabled by Provisional Passivation

“…"Low," in this context, means low enough to suppress carbide precipitation by immobilizing Cr, but still high enough to enable carbon diffusion into technically useful depths within industrially feasible processing times. Idealizing these nonequilibrium conditions as a state with no metal atom mobility at all, the system is then confined to a trajectory leading to a "carbon paraequilibrium", rather than conventional thermodynamic (or "ortho") equilibrium [3]. With this approach, treatment times of order 0.1 Ms (1 day) can infuse carbon to an average depth z ≈ 10 µm.…”