Oxidation Behavior at 1173 K of Modified P/M Stainless Steel 316 L by Addition of Cr, Ni, and Cr with Ni

Specimens of P/M 316 L stainless steel and modified P/M 316 L stainless steel with various amounts of Cr, Ni, and Cr along with Ni additions were compressed by a 500 MPa-hydraulic press for a duration of 30 s and then sintered at 1573 K in a hydrogen atmosphere for 2.7 ks. The oxidation resistance tests of those specimens were carried out at 1173 K in air for 360 ks. The 5 wt.-% Cr-added specimen resulted in the best oxidation resistance with the lowest oxidation rate constant of 1.54 × 10-7 kg2 × m-4 × s-1. The oxidation products which formed inside the pores and on the surfaces of all specimens were analyzed using scanning electron microscope/energy dispersive spectroscopic (SEM/EDS) and X-ray diffraction (XRD) analysis techniques. The results show that the oxides forming on both inside pores and surfaces were identified as Cr2O3, Fe2O3, (Fe0.6Cr0.4)2O3, NiFe2O4 and NiCr2O4 in all tested conditions. The measured hardness of the modified specimens was in the range of 87-92 HRB, compared to the hardness of the 316 L specimen, which was 93 HRB. Increasing the amount of powder added provided a slightly lower hardness value due to increased porosity.

Effects of Ni Additions on the High Temperature Expansion, Melting and Oxidation Behaviors of Cobalt-Based Superalloys

Nickel is often added to cobalt-based superalloys to stabilize their austenitic structure. In this work the effects of Ni on several high temperature properties of a chromium-rich cobalt-based alloy reinforced by high fraction of TaC carbides are investigated. Different thermal analysis techniques are used: differential scanning calorimetry (DSC), thermo-mechanical analysis (TMA) and thermogravimetry (TG). Results show that the progressive addition of nickel did not induce great modifications of microstructure, refractoriness or thermal expansion. However, minor beneficial effects were noted, including reduction of the melting temperature range and slight decrease in thermal expansion coefficient. The most important improvement induced by Ni addition concerns the hot oxidation behavior. In this way, introducing several tens wt % Ni in this type of cobalt-based alloy may be recommended.