Reaction of Unalloyed and Cr-Mo Alloyed Steels with Nitrogen from the Sintering Atmosphere

Abstract

Nitrogen is usually regarded as an inert sintering atmosphere for PM steels; however, this cannot be taken for granted in particular for steels alloyed with nitride forming elements. Among those elements, chromium has become more and more important as an alloying element in sintered low alloy structural steels in the last decade due to the moderate alloying cost and the excellent mechanical properties obtainable, in particular when sinter hardening is applied. The high affinity of Cr to oxygen and the possible ways to overcome related problems have been the subject of numerous studies, while the fact that chromium is also a fairly strong nitride forming element has largely been neglected at least for low alloy steel grades, although frequently used materials like steels from Cr and Cr-Mo prealloyed powders are commonly sintered in atmospheres consisting mainly of nitrogen. In the present study, nitrogen pickup during sintering at different temperatures and for varying times has been studied for Cr-Mo prealloyed steel grades as well as for unalloyed carbon steel. Also the effect of the cooling rate and its influence on the properties, of the microstructure and the composition have been investigated. It showed that the main nitrogen uptake occurs not during isothermal sintering but rather during cooling. It could be demonstrated that a critical temperature range exists within which the investigated CrM-based steel is particularly sensitive to nitrogen pickup.

Microstructural coarsening during supersolidus liquid phase sintering of alpha brass

“…Optical microscopy (model, Olympus PMG3) was employed for investigating the metallographic cross-sections. Grain and pore intercepts were measured and analysed according to equation (3), and the number of pores were measured using image analysis software (ImageJ 1.42q) from optical microscopy images…”

“…2 Furthermore, increasing the sintering temperature and time enhances grain growth while the number of grains decreases. 3,4 In SLPS, the liquid provides a capillary force and high atomic transport rate, so the high diffusivity leads to progressive microstructural coarsening. Numerous researchers have studied the problem of solid–liquid coarsening.…”

Reactions between ferrous powder compacts and atmospheres during sintering – an overview

“…This means that deoxidation does not start before 900-950°C and is not even finished at 1300°C, the critical temperatures depending on the chromium content, as also shown in [46]. The consequence for the mechanical properties is a pronounced effect of the sintering temperature: below the first degassing peak, there is virtually no formation of metallic sintering contacts [47], and even at temperatures above this threshold, higher temperatures have a strongly beneficial effect on both monotonic and cyclic mechanical properties [47,48]. Molybdenum as an alloying element (also contained in these steel grades) hardly affects deoxidation, Mo alloyed steels being comparable to Fe-C [49].…”

Iron Carbon Masteralloy as Sintering Activator for Low Alloy Powder Metallurgy Steels

“…Reducing these oxides is only possible at high sintering temperatures, with carbon acting as the main reducing agent even if sintering in H2 containing atmospheres. There are several studies that investigated this issue [12,13].…”