Effects of Nitrogen Content on Powder Metallurgical Properties of Water-atomized Chromirm Stainless Steel Powders

Abstract

An investigation was made on pressing and sintering behavior of water-atomized 13%Cr, 17%Cr and 23%Cr-steel powders having 0.25% or less of nitrogen.
The results were summarized as follows:
(1) Excess nitrogen was mostly retained in powders as a state of solid-solution on account of rapid solidi-fication obtained by quench-atomization of melt and the powder-compressibility was inclined to get lowered with a solid-solution hardening proportional to the amount of nitrogen plus carbon dissolved in particle matrix.
(2) Especially for nitrogen-rich 13%Cr steel powders, the powder compressibility was more deteriorated under the added influence of γ→α' phase transformation which occurred on cooling to room temperature during quench-atomization. On the other hand, the situation was appreciably improved by annealing the powders at the temperature around Al, because of the precipitation of nitride from α'.
(3) Nitrogen in powders (compacts) was found to affect the rate constant K (ΔL/L=K(T, D)⋅tⁿ) in relation to the isothermal transformation (γ→α) arising from denitrogenization during vacuum sintering for lower Cr steel powders.
Consequently, the same level of densification rates for all powders were obtained in the middle stage of sintering (approximately at the temperature above 1100°C), irrespectively of original amount of nitrogen.