Technical Paper: PIM International, Vol.3 No. 4 December 2009, pages 66-70, 3398 words

Authors: Steffen Krug [1] and Stefan Zachmann [2]

[1] Polymer-Chemie GmbH, Metal Powder Compound Division, Am Gefach, 55566 Bad Sobernheim, Germany
[2] Nabertherm GmbH, Bahnhofstr. 20, 28865 Lilienthal, Germany

Abstract

In industrial practice, stainless steel injection moulded parts are sintered under various sintering conditions. Common are molybdenum or graphite vacuum furnaces. Different sintering atmospheres and pressures are used. Based on a water soluble binder system 316L test bars are injection moulded and sintered under various sintering conditions (vacuum, nitrogen, hydrogen and argon). The main task of the study is to elaborate the impact of sintering atmospheres on the properties of Stainless Steel. To this purpose metallurgical, chemical and mechanical tests are carried out and discussed. Sintering tests are carried out in graphite and molybdenum furnaces. Debinding and sintering in a furnace with a retort as well as thermal binder removal and sintering in two different furnaces are evaluated. As shown by the tests carbon control during debinding and control of the sintering athmosphere has an important influence on finished part quality. So does the furnace technology used.

Introduction

Low carbon austenitic chromium-nickel-iron alloys with 17-19% chromium and 8-14% nickel are widespread due to their mechanical and technological advantages [1]. 316L with 16.5 – 18.5% chromium and 12-14% nickel is one of the most common materials for MIM-production. This is due to its excellent corrosion resistance, its high ductility, its high toughness and impact strength, even at lower temperatures and its good weldability [1]. Furthermore the material is characterised by its low yield point and its suitability to cold work hardening [1].

The metallurgy of chromium-nickel-iron alloys is rather complicated. The materials are considered corrosion resistant at chromium levels above 12-14% [1,2]. This is due to the passivation property of chromium. A thin protective surface film forms instantly under oxidising atmospheres. The stainless steel is passivated because an oxide film seals the bulk metal from oxygen diffusion and further corrosion [3]. At higher chromium concentrations the corrosion resistance increases and at lower concentrations it decreases. Therefore, the chromium has to be evenly distributed throughout the metal lattice. Total as well as local reduction of chromium will reduce the corrosion resistance.

If powder mixtures of master alloys and carbonyl iron are used alloying appears during sintering. In such powder concepts fine carbonyl iron powders are mixed with so called master alloys. The usually gas atomised master alloy is enriched with chromium and nickel atoms. During sintering the chromium and nickel atoms diffuse into the iron lattice. The chromium and nickel atoms take the atomic place of the iron atoms. This diffusion process takes place at sintering temperature and requires time. If the alloying process is stopped prematurely or if the master alloy carbonyl iron powder mixture is inhomogeneous in the first place the alloy will suffer liquation. In that case the chromium concentration may locally not reach 16.5%. It may even stay below 12% and be subject to corrosion. For the present study this effect is not relevant because gas atomised powders were used. Each powder particle consists of pre-alloyed 316L. Thus there is neither alloying nor liquation taking place.

Further sections of this paper include:

- Introduction
- Experimental details
- Results and discussion
  Sintering in a molybdenum furnace
  Sintering in a graphite furnace
  Residual binder and furnace technology
- Conclusions

Figures and Tables:

Fig. 1 Nabertherm VHT-08 sintering furnace

Fig. 2 Stress-strain behavior of 316L sintered in a molybdenum furnace at different sintering atmospheres

Fig. 3 In nitrogen and hydrogen sintered 316L test bars before and after pulling

Fig. 4 Optical micrographs of 316L sintered in hydrogen (left), argon (centre) or nitrogen (right) after etching

Fig. 5 Stress-strain behavior of 316L sintered in a VHT graphite furnace using nitrogen, argon, vacuum and an argon/hydrogen in comparison

Fig. 6 Schematic drawing of the sintering furnace

Table 1 Chemical composition of the 316L powder

Table 2 Mechanical and chemical results of sintered 316L samples at various sintering atmospheres