I have not had the occasion to weld these powdered metals myself, so anybody that has had some experience must please help if I am missing the boat.

The reason that I asked about why you want to go the powder metallurgy route is that it is only really cost effective under certain circumstances. These typically being:

1)Where a near net shape part is produced that needs almost no extra finishing.
2)Alloys are "designed" that can not actually exist in the moulten state. (Typically the moulten metals are not miscible.)

Actually the powder metallurgy route is not a hell of a cheap route. Almost always these parts are made where a great number of parts will be used, where the added benefit of the "no extra finishing" is of major importance.

If the parts your company are making need to be close tolerance, then they will most likely need to be final machined following welding, so I don't actually see the benefit of the PM route. What I am trying to say here is just that the "engineers" must make sure they have done their sums right if they recon that PM will be cheaper. If the parts can be "rough and ready" without close tolerances, then forging would usually be cheaper.

When welding this type of material, there are a number of "issues". These are:

1)The method whereby the PM part was made. Basically there are two broad categories of fabrication method. The first is to just press the part and then sinter it. The second is to also add a lower melting point metal into the mix which then tends to "braze" the PM part together during the sintering cycle. Just make sure that your components do not have the added low melting point bit. The 99.7%Fe almost certainly will not have, but I am not sure about the 97.4%Fe components. If this is present, then your whole problem grows.
2)These PM parts typically have densities of 95 - 99% those of "ordinary" parts. This means that you have quite a bit of gas in the parent metal itself. (Up to 5%.) This can only mean all kinds of trouble when welding. It is clear that porosity will necessarily be an issue that you need to worry about.
3)When you weld these parts, you will melt the PM base material and this will be included in the weld metal. (No surprise, but just remember that any impurities introduced through the PM process will end up in your weld.)

If the material is a 0.3% C steel, then the metallurgy of the part and weld should not pose too big a problem. With 0.6% C, you will need to follow the typical procedures for high C steels. In particular the Hydrogen issues become very important because the base metal has so many spots where H can accumulate. (All the little pores.)

As I have said, I have not welded this myself, but here is the approach that I would take as a start:

1)Get a filler and gas combination that gives you a weld pool that is as fluid as possible. In addition, use high heat inputs. This will again allow the weld pool to stay fluid for a long time. (If welding GMAW, use spray transfer if possible.) I would suggest you try FCAW, as the flux covering will allow slower cooling. All this is to try and give the gas bubbles enough time to rise to the surface of the weld pool.
2)If you are welding components with C content in excess of 0.4%, then go the whole hog of pre-heating and PWHT. You stated that the components would be load bearing, so a brittle weld is bad news. With a 0.6% C content you will definatelly end up with a brittle weld. You therefore will need the PWHT.

Just another thought. If you are replacing 1010 - 1026 grade steels, why the high C content of the PM parts? I would try to stick to low C contents.

Hope this gives you a place to start.

Regards
Niekie Jooste