Gary M. Michal

Colossal carbon supersaturation in austenitic stainless steels carburized at low temperature

A novel, low-temperature (470 °C) gas-phase carburization treatment, developed by the Swagelok Company, increases the surface hardness of 316 austenitic stainless steels from ≈200 to ≈1000 HV25 and improves the corrosion resistance. While normally the precipitation of carbides restricts the carbon concentration in the austenite of 316 steels to <0.015 at%, the Swagelok treatment generates a colossal supersaturation of up to 12 at% carbon in solid solution. Only upon extended treatment, does carbide precipitation eventually occur, but the colossal carbon supersaturation of the austenite is maintained. Unusual for austenitic stainless steels, the precipitates are Hagg carbide (M5C2).

Transformation to Ni5Al3 in a 63.0 At. Pct Ni-Al alloy

Microstructures in a 63 at. pct Ni-Al alloy, produced by a powder metallurgy process, have been investigated in detail in as-quenched and aged (823 to 923 K) conditions. The parent L10 martensite plus B2 NiAl microstructure in the as-quenched state transformed nearly completely to the orthorhombic Ni5Al3 phase upon aging at 823 K for 720 hours. The volume fraction of Ni5Al3 formed as a function of aging time at 823 K was observed to obey cellular reaction kinetic behavior. The specimens aged at 823 K for short times indicated that nucleation of the Ni5Al3 phase occurred preferentially at grain boundaries. Transmission electron microscopy (TEM) observations of short-time aged specimens revealed a complex microstructure consisting of shortrange ordered domains of Ni2Al in a matrix of 7R martensite, in addition to new variants of 3R martensite and Ni5Al3. Aging at 873 and 923 K for 720 hours produced a stable two-phase microstructure consisting of NiAl and Ni5Al3. A quantitative phase analysis was carried out to calculate the (NiAl + Ni5Al3)/Ni5Al3 phase boundary locations. The measured lattice parameters of Ni5Al3 formed at 823, 873, and 923 K indicated an increase in tetragonality of the phase with increasing nickel content.

Carburization-induced passivity of 316 L austenitic stainless steel

A low-temperature (450-500°C) gas-phase process for introducing substantial amounts of carbon, without carbide formation, into 316L austenitic stainless steel has been developed. This process, termed low-temperature colossal supersaturation (LTCSS), provides surface carbon concentration as high as 14 atom % and dramatically improves the localized corrosion resistance of 316L austenitic stainless steel in ambient temperature seawater. In particular, the LTCSS-treated steel increases the seawater breakdown potential by more than 600 mV. This result is remarkable, as traditional carburization methods have historically decreased the corrosion resistance of stainless steels.

Preferred orientations in extruded nickel and iron aluminides

Preferred orientations in both powder-extruded and cast and extruded binary NiAl (≃45 at. pet Al), FeAl (≃40 at. pet Al), and Ni3Al (≃24 at. pet Al) have been characterized by plotting inverse pole figures. The preferred orientation, [111], was observed along the extrusion direction in both powder-extruded and cast and extruded NiAl. Powder-extruded FeAl also exhibited [111] as the preferred orientation in the as-extruded condition. However, [110] was observed to be the preferred orientation in the cast and extruded FeAl and was replaced by a [211] orientation preference upon annealing. Annealing did not change the preferred orientations in NiAl or in powder-extruded FeAl. In contrast to the B2 NiAl and FeAl alloys, the Ll2 Ni3Al alloy exhibited nearly random orientations with only a minor preference for a [111] orientation in the as-extruded condition.

Production and processing of Cu-Cr-Nb alloys

A new Cu-based alloy possessing high strength, high conductivity, and good stability at elevated temperatures was recently produced. This paper details the melting of the master alloys, production of rapidly solidified ribbon, and processing of the ribbon to sheet by hot pressing and hot rolling.

Wear maps for low temperature carburised 316L austenitic stainless steel sliding against alumina

Abstract Wear maps illustrating the tribological benefits imparted to interstitially hardened austenitic stainless steels carburised at low temperatures and sliding against alumina were developed using pin-on-disk wear tests. Low temperature carburisation enables the use of austenitic stainless steel grade 316L under applied loads and sliding speeds that would usually be considered far too severe for this alloy. Comparisons of wear volume, wear debris and wear scars produced by sliding under various speeds and applied loads showed that low temperature carburisation of 316L stainless steel extends the mild wear regime and prevents the severe wear under harsh test conditions observed in non-treated grades of this steel.

Carburization-Enhanced Passivity of PH13-8 Mo: A Precipitation-Hardened Martensitic Stainless Steel

Interstitial hardening of the martensitic stainless steel PH13-8 Mo has been achieved by low temperature gas-phase carburization. After treatment, hardness is increased to a depth of =50 μm, with a surface hardness that is twice the core hardness and a corresponding improvement in pin-on-disk wear resistance. Pitting potential is increased by ≈ 0.5 V in 0.6 M NaCl. Elemental analysis and X-ray diffraction suggest the formation of a thin (= 2 μm) carbidic surface layer that is both wear and corrosion resistant.

Pinning of Austenite Grain Boundaries by

The growth behavior of austenite grains in the presence of A1N precipitates varies with the temperature and time of anneal. To study this behavior, two iron alloys, (in weight percent) a 0.1 carbon base chemistry with 0.03A1/0.01N and 0.09A1/0.04N, respectively, were annealed between 1000 °C and 1200 °C for times of up to 180 minutes. Using optical microscopy, as many as 1000 austenite grains per heat-treatment condition were measured. Conditions of sup- pressed, abnormal, and uniform grain growth were observed. Using an extraction replica tech- nique, the size, shape, and distribution of the A1N particles were determined using transmission electron microscopy (TEM). The largest grain boundary curvatures calculated, using the Hellman- Hillert pinning model, were in close agreement with independent calculations of curvatures using the grain size data. The largest grains in the lognormal size distribution of austenite grains were found to be the ones with the potential to grow to abnormally large sizes.

The kinetics of carbide precipitation in silicon-aluminum steels

The kinetics of carbide precipitation in a fully processed 2.3 wt Pct silicon, 0.66 wt Pct aluminum electrical steel with carbon contents of 0.005 to 0.016 wt Pct were investigated over the temperature range from 150 to 760 °C and times from 30 seconds to 240 hours. The size, morphology, and distribution of the carbide phases, as functions of aging time and temperature, were determined by optical and transmission electron microscopy. The 1.5T core loss was also evaluated and correlated with the changes in precipitation. Distinct C curves were observed for the formation of grain-boundary cementite at temperatures above 350 °C and a transition carbide ({100}α habit plane) at temperatures below 350 °C. Grain-boundary cementite had a relatively small effect on core loss. The large increases in core loss that accompanied transition carbide precipitation peaked at specific aging temperatures depending on the carbon content of the steel. Once a transition carbide dispersion was initially established at a given aging temperature, particle coarsening and core loss changes were generally insensitive to aging time. The influence of a combined addition of silicon and aluminum on the solubility of cementite and the transition carbide in iron was estimated and discussed.

Numerical Simulations of Carbon and Nitrogen Composition-Depth Profiles in Nitrocarburized Austenitic Stainless Steels

Unusual composition-depth profiles have been observed after low-temperature nitrocarburization of austenitic stainless steels. When nitridation is performed after carburization, the carbon concentration in the nitrogen diffusion zone is reduced from ≈10 to ≈2 at. pct. Conversely, the carbon concentration in advance of the nitrogen diffusion zone is as high as 10 at. pct. This has been called a “push” effect of nitrogen on carbon, but this concept is non-physical. The profiles can be better understood from conventional thermodynamic principles, recognizing that (1) diffusion always occurs in response to gradients in chemical potentials and (2) the diffusivity of interstitial solutes in austenite is strongly concentration dependent, increasing dramatically with higher solute concentrations. Parameters from the CALPHAD literature quantitatively indicate that interstitial nitrogen and carbon in austenitic stainless steel mutually increase their chemical potentials. Based on these data, we have conducted numerical simulations of composition-depth profiles that correctly account for the chemical potential gradients and the concentration dependence of the diffusion coefficients for nitrogen and carbon. The simulations predict the “push” effect observed on nitridation after carburization, as well as the corresponding composition-depth profiles for other scenarios, e.g., carburization followed by nitridation or simultaneous nitridation and carburization (nitrocarburization).