MIM 316

The main properties of stainless steel 316L are influenced by process parameter of sintering. The mechanical properties primarily depend on the sintering density and microstructure. However, residual carbon from incomplete debinding will hinder the sintering, high-temperature vacuum sintering will cause the evaporation of volatile elements, control hard and brittle sigma phase by cooling rate,

There is a characteristic of high temperature sintering, the final sintered properties depend on not only controllable factors, but also on factors difficult to control. For MIM 316L, the main factors are injection molding parameters – pressure, temperature, sintering condition – debinding, sintering atmosphere and temperature.

Ceramic Al₂O₃/SiC plates are applied as support for the debinded and sintered components, in order to prevent distortion. Comparing to metallic supports, these ceramic plates are much more stable during high-temperature sintering (1350℃). The vacuum sintering is at an absolute pressure of approximately 1Pa. The sintering process for stainless steel is either super-solidus liquid-phase sintering (SLPS) or liquid-phase sintering (LPS). From thermo-dynamic calculations, most sintering is in the solid-state, as the liquid starts to form only above 1390℃.

The stainless steel chemistry specification (UNS SD31603) is vary broad, its extreme properties may vary in a significant range. 316L is nominally classified as a fully austenitic alloy. But this depends on the absolute levels of Cr, Ni, Mo, and Cr/Ni ratio, delta ferritic phase are still exist in the final micro-structure.

In medical applications, especially in the vicinity of high-powered magnets (like: MRI scanners), it is essential that metal components must be non-magnetic with no residual delta ferrite. Ni content with 10-14% will promote the formation of austenite and ensure the fully austenitic microstructure after sintering.

The theoretical density of SS 316L is 8.0g/cm³, while the average sintered density can achieve 7.81g/cm³, or 97.6%. In vacuum sintering, the achieved density is relatively high, because pore shrinkage is not hindered by internal gas pressure. While in reductive hydrogen atmosphere, the experimental density is 0.1% lower.

The tensile strength of sintered stainless steel 316L can reach up to 587.0 MPa. However, it is evident that prealloyed feedstock provide higher tensile strength than master alloy. The main reasons of this difference are the residual carbon from carbonyl iron powder, or the contamination with element (Si, C, H, N) diffusion.

It is obvious that different powder types and sintering parameters have significant effect on tensile strength, while negligible effect on final hardness. The following picture are optical micrographs in both polished and etched conditions. It is obvious that master alloy has a finer grain size than pre-alloy, this results in the lower sintering temperature and less porosity.

The small additives of B and Cu will enhance densification, it will increase the final sintering density and improve the mechanical properties.

Zr is a stabilizing element in ferritic stainless steels, it can improve oxidation resistance at high temperature. It can inhibit the formation of Silicon oxide, its crystal grain refinement effect will suspect grains refinement in MIM 316L. In The MIM 316L sintering, the over growth of Si oxide in the grain boundary will inhibit densification, this leads to the degradation of mechanical properties and hardness. Once add Zr additive, the finely dispersed silicon oxide will act as the pinning agent. This will prevent the overgrowth of crystal grains, and refine the micro-structure of 316L parts.

All in all, the addition of Zr will contribute to resfinement of powder structure and inhibit the formation Si oxide on powder surface. Finally, improve the sintering characteristics and mechanical properties in MIM 316L sintering.

In MIM 316L process, tensile ductility and axial fatigue strength will decrease as the content of delta ferrite increase. Otherwise, delta ferrite will also activate densificaiton, this will compensate these effects. The amount of delta ferrite increase with sintering temperature (over 1300℃), while decrease with the carbon content. In reason of carbon element stabilizes the austenite phase in stainless steel.

Early 316L sintering experience indicates that H₂ atmosphere is the optimum choice for low carbon stainless steel. As this will ensure the reduction of silicone and chromium oxides, which are stable even at high temperatures. Furthermore, there is no quality difference between sintered 316L parts in H₂ and vacuum atmosphere. The following picture is micro-structure of phase distribution of 316L with low Ni content (10.4%). It is consisting of austenitic matrix with delta ferrite island (red arrows)

Once sinter 316L parts with high Ni content (13.5%) in H₂ and vacuum atmosphere, the micro-structures exhibit a wholly austenitic as in the following picture. However, there are large number of visible pores, this results in the low theoretical density value of 95%.

Sintering in N₂ results in nitrogen absorption, this will reduce density and improve yield strength. The fine powder with greatest surface area can absorb most nitrogen in sintering, this results in the highest strength with loss of some ductility.

For 316L parts sintered in N₂ containing atmosphere, the visible phase is only austenite in micro-structure, even for the 316L with Low Ni (10.4%),(which is easy to form delta ferrite phase in sintering. Please check the micro-graph of 316L (Low Ni) in N₂ sintering atmosphere.

Sintering in N2 atmosphere provides the highest hardness value, as particle size of metal powder decreases, hardness increase. Nitrogen sintering also enhances stress by almost 100 MPa comparing to hydrogen sintering. Elongation results are influenced significantly by sintering atmosphere: H₂ sintering achieve 70% elongation, while vacuum at 43% and N₂ at 33% in average. The carbon content levels in vacuum and N₂ are lower than H₂, this suggests that carbon is the primary reductant for surface oxygen rather than hydrogen.