To answer your question regarding grain structure, yes we can control that in certain materials. For example, we manufacture M2 cam rings that go into diesel fuel pumps that have a certain microstructural requirement...basically there are grain size requirements, an allowable amount of carbide free areas, etc. The M2 that we manufacture has a very tight carbon spec in the incoming powder, plus some special additives to enable us to get a slightly wider sintering window. To sinter these parts, we must run them in a vacuum furnace pumped down to about 300 microns during the sintering phase. There are strict pre-heat, pre-sinter, and sintering times and temperatures required to hit the microstructure properly. After sintering, each load has small lobes pulled from throughout the build that are mounted, polished, and viewed to ensure that the load is conforming. There are other controls, but that's the basic idea.
How is the P/M process different than casting? Well, because we can use very fine graphite throughout our powder blends, and because we control the temperatures (heat & cool) and partial pressures in the sintering process very tightly, we can achieve a more uniform microstructure with a finer dispersion of smaller carbides. T15 tool steel is a perfect example. Wrought T15 has large carbides spaced throughout the matrix. It's great for wear, but grinding wheels like to pull those carbides out (as do other applications) which can drive the formation of surface cracks. For PMT15, we can get a finer carbide dispersion than wrought. Because the sinter window there is less than say M2, we will undersinter the product, leave minimal porosity, then HIP the product to get the optimum microstructure and ensure full densification. This gives you a lot of little wear resistant carbides that are less likely to induce surface cracks during machining operations. Crucible and Bohler-Uddeholm have some good information on their respective websites reqarding P/M steels vs. wrought alloys.
There are some cases where we can get better properties than wrought. Take 410L. Wrought 410L is soft and is not very wear resistant. If I high temp sinter 410L in a mixture of hydrogen and nitrogen, I can basically put nitrides all along the grain boundaries. Now, my part isn't as corrosion resistant as the 410L, but I'll beat it for wear resistance everyday. Wrought 410 will be in the HRB range if measuring surface hardness (particle and macro). My PM grade 410 might have a macro hardness of 45 HRB, but the micro hardness of the individual particles will be in the HRC scale. The difference is comparing impact to wear resistance. You're basically finding the right material for the application.
In most cases a part forged from wrought is going to have better fracture properties under extreme circumstances (probably like you see running a race car). The average vehicle, with a properly designed and manufactured P/M part that doesn't see the extremes, may well suffice for everyday applications. It just depends on your usage I guess. For example, a forged SS 434 exhaust flange is going to have better corrosion resistance than a high temp sintered exhaust flange. It has to do with how the P/M flange is manufactured. However, that corrosion resistance in the PM flange may be fine for everyday driving...just don't put the same component in a ship going to sea!
Honestly it's all about material selection. Personally, the race application was probably not the one to put a PM part into. It's unfortunate because that's how certain industries get a bad rap.
lease see FAQ731-376 for tips on how to make the best use of Eng-Tips.