Abstract
Abstract Ni-free austenitic steels alloyed with Cr and Mn are an alternative to conventional Ni-containing steels. Nitrogen alloying of these steel grades is beneficial for several reasons such as increased strength and corrosion resistance. Low solubility in liquid and δ-ferrite restricts the maximal N-content that can be achieved via conventional metallurgy. Higher contents can be alloyed by powder-metallurgical (PM) production via gas–solid interaction. The performance of sintered parts is determined by appropriate sintering parameters. Three major PM-processing routes, hot isostatic pressing, supersolidus liquid phase sintering (SLPS), and solid-state sintering, were performed to study the influence of PM-processing route and N-content on densification, fracture, and mechanical properties. Sintering routes are designed with the assistance of thermodynamic calculations, differential thermal analysis, and residual gas analysis. Fracture surfaces were studied by X-ray photoelectron spectroscopy, secondary electron microscopy, and energy dispersive X-ray spectroscopy. Tensile tests and X-ray diffraction were performed to study mechanical properties and austenite stability. This study demonstrates that SLPS process reaches high densification of the high-Mn-containing powder material while the desired N-contents were successfully alloyed via gas–solid interaction. Produced specimens show tensile strengths >1000 MPa combined with strain to fracture of 60 pct and thus overcome the other tested production routes as well as conventional stainless austenitic or martensitic grades.
Highlights
STAINLESS steels are indispensable in industry and daily life
This volume is determined to 0.7 pct in case of the HIPed specimen and 1.2 pct for the supersolidus liquid phase sintering (SLPS) specimen
The SSS0 specimen was measured to inhibit a significantly higher volume of pores and inclusions reaching about 12 pct, whereas this amount was slightly improved in the case of solid-state sintering (SSS) + 700 (11 pct), but deteriorated significantly if a dwelling step at 1423 K (1150 °C) was applied (20 pct)
Summary
STAINLESS steels are indispensable in industry and daily life. The production of 35.4 million tons of stainless steels in 2012 underlines their importance.[1]. Because precipitation starts at lower temperatures, a lower critical cooling rate is required to obtain a precipitate-free material leading to higher producible cross sections.[15,17] This higher nitrogen content of 0.825 mass pct can be obtained if a powder-metallurgical (PM) production route is applied. Nitrogen may degas and the high vapor pressure of Mn leads to its evaporation sublimation at temperatures above 973 K (700 °C) in high vacuum.[22,23,24] Another problem is the enclosure of the surface oxides in the developing inter-particle necks resulting in a decreased performance of sintered steel as they act as crack initiation sites. This study investigates the influence of PM-processing route on the mechanical properties of sintered N-containing Ni-free austenitic stainless steel. The resistance against d-ferrite and a-martensite is proven by means of X-ray diffraction (XRD) of highly strengthened area
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