Paraequilibrium Carburization of Duplex and Ferritic Stainless Steels

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-μm 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-μm 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. This article is based on a presentation given at the “International Conference on Surface Hardening of Stainless Steels,” which occurred October 22–23, 2007 during the ASM Heat Treating Society Meeting in Cleveland, OH under the auspices of the ASM Heat Treating Society and TMS.

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Effects of Test Temperature and Loading Conditions on the Tensile Properties of a Zr-Based Bulk Metallic Glass

The effects of changes in test temperature (20 °C, 260 °C, 330 °C, and 380 °C), strain rate (10⁻⁵ to 10⁻¹ s⁻¹), and loading conditions (displacement control vs loading-rate control) on the tensile behavior of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 (LiquidMetal 1 (LM1)), a bulk metallic glass (BMG), have been determined. Significant effects of the test temperature, strain rate, and loading condition were observed on the strength, ductility/elongation, and mechanisms of failure (shear, ductile rupture, etc.). This material exhibited extensive elongation (i.e., >100 pct) prior to failure when tested near the glass transition temperature (T _g  ≈ 375 °C) at sufficiently low strain rates, while higher strain rates or lower test temperatures produced shear fracture at low elongation. The flow and fracture behavior was also significantly affected by the loading condition (i.e., displacement vs loading-rate control). The effective strain rate necessary to cause failure in shear without significant global flow was several orders of magnitude lower in loading-rate control than in displacement control. Samples exhibiting high elongation tested in displacement control gently and convexly drew to a near point (i.e., ductile rupture). Samples tested at the same temperature exhibiting high elongation in loading-rate control rapidly and concavely necked, followed by drawing to a constant diameter “wire” (i.e., ductile drawing), eventually failing by nearly pure ductile rupture. All samples that displayed significant elongation did so inhomogeneously, and were characterized by non-Newtonian global flow. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.

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Effect of thermomechanical treatment on microstructure and mechanical properties of AISI 301LN stainless steel

Abstract

A wide range of cold thickness reduction (10–80%) and subsequent annealing were carried out on AISI 301LN stainless steel. X-rays and Feritscope MP30 were used to identify the strain induced α′-martensite phase and its volume fraction respectively. The microstructure was observed by optical micrograph and scanning electron microscope. The results show that shear bands were present and strain induced α′-martensite nucleated at their intersections. The volume fraction of α′-martensite increased with the increased cold reduction by the continuous growth of embryos, which resulted in the increasing yield and tensile strength. The reversion of α′-martensite to austenite occurred after subsequent annealing. The grain size variation of austenite was related to the annealing regime. A good combination of strength and ductility can be obtained after annealing at 650°C for 30 min. The effect of grain size on yield strength conformed with the Hall–Petch relationship in the entire range of our analysis.     

Austenite Stability Effects on Tensile Behavior of Manganese-Enriched-Austenite Transformation-Induced Plasticity Steel

Manganese enrichment of austenite during prolonged intercritical annealing was used to produce a family of transformation-induced plasticity (TRIP) steels with varying retained austenite contents. Cold-rolled 0.1C-7.1Mn steel was annealed at incremental temperatures between 848 K and 948 K (575 °C and 675 °C) for 1 week to enrich austenite in manganese. The resulting microstructures are comprised of varying fractions of intercritical ferrite, martensite, and retained austenite. Tensile behavior is dependent on annealing temperature and ranged from a low strain-hardening “flat” curve to high strength and ductility conditions that display positive strain hardening over a range of strain levels. The mechanical stability of austenite was measured using in-situ neutron diffraction and was shown to depend significantly on annealing temperature. Variations in austenite stability between annealing conditions help explain the observed strain hardening behaviors.