MIM 17-4

During sintering, the initial particle bonding occurs with surface diffusion, smaller particle size with more surface area will increase initial bonding and strengthening. Surface diffusion bonding will replace the particle adhesion by the backbone polymer as binder pyrolysis occurs. As sintering bonding grow between metal particles, grain boundary diffusion requires neck growth to form grain boundaries between particles. Once grain boundary form, the grain size has inverse effect on the sintering rate, it has the main role in sintering when shrinkage starts. However, longer holding time or higher peak temperatures can offset the effect of initial particle size.

The normal sintering atmosphere option in stainless steel including: hydrogen, nitrogen, hydrogen-nitrogen, argon-hydrogen and vacuum.

Hydrogen or vacuum are the most common sintering atmospheres for stainless steel 17-4 PH. If add carbon, there is significant oxide reduction. Without carbon additive of graphite, both chromium and silicon can retain oxygen. High temperature and higher hydrogen content result in oxide reduction, while a low dew point will ensure this process. After sintering , the cold surface oxidize will form a passive layer of iron, chromium and silicon, which provides corrosion resistance.

Sintering in hydrogen is sensible for carbon and oxygen removal, while vacuum sintering results in evaporation of chromium and copper at high temperature.

In the following table, we can find the comparative properties of stainless steel 17-4 PH in different atmospheres (Hydrogen, Vacuum, Dissociated ammonia).

Nitrogen is a potent austenite stabilizer, it affects the martensite formation and final properties of stainless steel. Nitrogen is soluble in austenite, and forms Cr₂N precipitates in cooling process. This precipitate will rob the grain boundary area of chromium, and result in rapid corrosion on localized area. Therefore, we need to avoid nitrogen in sintering atmosphere, and cooling parts at rapid rate of 200℃/min below 900℃. In order to suppress chromium nitride formation in stainless steel 17-4 PH.

In addition, graphite vacuum furnaces are successful in vacuum sintering, graphite can create low partial pressure of carbon monoxide in vacuum furnace. It can reduce chromium oxide to chromium at 1300℃ with low CO partial pressure of 0.001

The normal sintering cycles involve binder burnout and impurity removal procedures by heating and holding at different temperatures. Optimal temperature for final polymer removal is near 600℃, while near 1000℃ for impurity reduction.

The sintering shrinkage of stainless steel 17-4PH starts at near 900℃ in dilatometry testing, the peak shrinkage rate is near 1250℃, but full-density (98%) requires peak temperature of 1300℃. However, these high temperatures will evaporate copper and chromium, and result in potential loss of heat treatment and corrosion resistance.

In addition, higher temperatures are instrumental in delta-ferrite formation, this phase will assist the final densification by faster diffusion. From the following micro-structure graph of MIM 17-4PH in sintering, we can notice the white delta-ferrite colonies, and the dark regions of circular oxide pores.

Carbon is another austenite stabilizer during sintering process. Low retained carbon content levels below 0.1 wt.% in 17-4 will provide higher strength and hardness. The alternation of carbon level will impact phases, densification and mechanical properties of stainless steel 17-4 PH.

The carbon level depends on various factors in MIM process, including: atomized powder, binder residuals, debinding cycle, sintering atmosphere, initial oxygen, peak sintering temperature. The initial oxygen level is the main factor for final carbon level, as carbon react with oxygen in sintering. Such as, there is 0.03% carbon loss in hydrogen sintering at 1343℃ for 17-4 gas atomized powders. Moreover, carbon contamination also arises from binder residuals, especially at lower sintering temperatures.

The retained carbon levels affect the tensile strength of stainless steel 17-4 in hydrogen and vacuum sintering. No matter in normal sintering of 1300 to 1360℃, or H900, H1100 heat treatment. From the following picture, once carbon concentration reaches to higher levels, there is notable loss of tensile strength.

Holding time at the peak temperature in sintering are in the range of 60 to 120 minutes. From the following graph of sintered density versus time and temperature. It indicates a progessive density from 96.3% to 98.3%, as temperature increase from 1240℃ to 1360℃. With 60 minutes holding, the sintering compacts reach full density (98%). In this process, porosity is eliminated, and pore annihilation rate is declined. The main reasons are the exhaustion of porosity and the loss of grain boundaries.