This month we will focus on a very important PM finishing process – The steam treating of powder metal parts.
There are several benefits to steam treating powder metal parts. However, there are some important issues the designer must be aware of. But what exactly is the steam treating?
This treatment consists of a controlled oxidation of ferrous metal, producing a thin and hard oxide layer on the metal surface. For ferrous gearing, it provides a component with:
1. Increased corrosion and wear resistance
2. Increased surface hardness
3. Attractive surface finish
4. And in powder metal: seals the part porosity by plugging it with metal oxides.
In the steam-treat process, the part is usually oven-heated to at least 700°F (370°C), before it is brought into contact with dry steam. The process uses ambient air, not an inert gas environment. With the injection of dry steam, the gear or component continues to 1000°F (538°C). When done correctly, the part will not exhibit reddish rust color from the water vapor. Metallurgically, the iron in the part reacts with the steam developing magnetite (Fe3O4). It forms an oxide layer that is typically five to seven micrometers thick at the component surface. The oxides itself have a micro-hardness of approximately 50 Rockwell C. Steam-treated components should be blue to black in color. The part does not dimensionally change except for the surface oxide layer and the oxides that plug the porosity in PM material. Note that for internal and external geometry, oxides grow the surface five to seven micrometers on a side.
Steam treating is not considered a heat treatment because there are no structural changes occurring in the matrix. However, the magnetite oxide plugging the powder metal pores does create compressive stresses in the part making it more brittle. Depending on the process cycle, impact strength can be reduced 25% – 50% after treatment. On the other hand, after magnetite fills in porosity at the surface layer it leads to improved compressive strength. The process is most effective on parts with maximum carbon content between 0.5% – 0.8%.
Steam-treated parts are then often dipped in oil or a rust preventative, which enhances the blue/black appearance and increases the corrosion resistance even further. The treatment is not generally applied to hardened and tempered parts because exposure to the high-temperature steam can change a prior hardening precondition. Therefore, if mechanical strength is in doubt as a result of this process, have your PM supplier give you a few surrogate parts for validation testing. It should be noted that steam treatment is also a low-temperature/low-distortion process. Tooling changes are typically unnecessary to steam treat an untreated part design later on.
For all the benefits of PM steam treating, the reader should be aware that the magnetite surface layer can be an abrasive if the original surface finish has a certain level of initial roughness. If a gear profile with magnetite oxide comes in sliding and rolling contact with a softer, untreated gear profile, generating wear could be a cause for concern. On the other hand, two steam-treated surfaces may polish each other. One example occurred when a 6.3 g/cm3 density PM steam-treated pinion mated with a plastic gear. The pinion quickly wore out the plastic gear, but the same steam-treated gear at 7.0 g/cm3 did not wear out.
Here’s an interesting case study: Many years ago, I was working on a hydraulic pump and motor system for a hydrostatic geared transaxle. This transmission was to be used as a foot pedal-speed-controlled lawn and garden tractor. The hydrostatic portion consisted of a pressure ring that housed several piston slippers. By varying the amount of hydraulic pressure to the motor through a pintle valve, the speed could be regulated externally by the operator either by lever or pedal. The first prototype used a 6.7 g/cm3 density ring housing that was too quick to bleed off the hydraulic pump pressure. It became clear that the PM ring had to be either impregnated with resin or steam treated to stop the bleeding. Resin worked just fine, but steam treating the ring was far less expensive. The problem: We knew that if the piston slippers came into contact with the magnetite oxide layer during start-up when pressures were very low then the slipper surface would get scratched or potentially gouge. It was imperative that the surface finish remain flawless. I tried steam treating the rings and a number of slippers just to see what would happen.
The results were amazing. The steam-treated ring and the untreated slippers were an improvement. However, with steam treated ring and slippers, the results were superior. Overall transaxle efficiency increased 40% relative to the original prototype with untreated parts. It was a big enough boost to approve and launch into production. In Figure 1, the steam-treated part is on the left; the untreated part is on the right.
In the end, bleed-through and hydrostatic start-up damage of the motor and pump was a concern. The untreated slippers tended to have scuffed surfaces, which degraded the efficiency of the system. However, by using steam-treated surfaces of both slipper and ring, the system survived the first few operational start-ups in good condition. Subsequently, something interesting happened. Because of the high hydraulic oil pressure between the slippers and the ring, the oxide layer in the pump and motor ring became highly polished with a glass-like finish. Once the initial break-in ended, start-up damage and efficiency was never a problem again. In fact, system efficiency continued to increase as the magnetite surface of the ring continued to embed the oxides into a mirror-like surface.
Understanding the pros and cons of steam treating powder metal components allows us to leverage the valuable strengths of this important and economical process.