What qualities do you mean? Chemical composition? Hardness? Tensile strength? Fatigue strength? If you have a current application that is an investment cast component (8620 or 4140) and wish to replace it with a powder metal component, then you need to provide more details on this application in order to receive a meaningful response:
The Investment cast part was replaced by P/M part (FL4605-100HT). Soon thereafter, we discovered part broke very easily when tools jammed. The breakage is near a corner. I would be looking for toughness qualites and a similar hardness. The parts are not used in extreme environmental conditions. Dimensions between the parts remained nearly identical between material change. Also, an increase in surface area would be ideal, however, clearnance in the tool would prohibit appreciable area gains.
Forces are tensile and produce a bending moment on a shape similar to one below (arrows indicate force direction)
Thanks for the information. FL-4605-100HT is inferior to your castings in terms of fracture toughness. Can you provide additional details on the following:
1. Density of the PM part?
2. Process used for PM? Was it press, pre-sinter, re-press, and final sinter?
3. Are the castings heat treated?
What I can tell you right now is that the PM part will be extremely notch sensitive, due to its very low toughness. The lack of toughness is due to the porosity, and the poor microstructure that develops in these materials. In order to achieve the desired macrohardness/tensile strength, the microstructure is essentially as-quenched martensite with a microhardness that converts to ~ 60 HRC. The combination of hard martensite and porosity produces a macrohardness level more like 29 HRC.
So, what to do? There are a number of things to investigate, depending on your answers to the above questions. Increasing the density will improve toughness somewhat. However, this still produces an extremely brittle part. The low-alloy steel powders that contain Ni, Mo, and Cu will provide higher fracture toughness for a given strength level (FLNC-4405). A change in geometry may be necessary, such as including a generous radius.
I beleive they are pressed, repressed and then sintered. I believe (due to lack of specific numbers from vendor)that the density of the material is about 90-94% of the density of a cast part-I dont' know if this correlates to porosity in any way. Also, the part is heat treated to about 55 RHC...(after the porosity is accounted for)
I feel that a change in material may be called for here. Also, I have experience with an air hardening P/M on another part we produce...would this be an avenue to consider?
You did answer the question about whether or not the original casting was heat treated (quench & temper, normalized, etc.)-- can you provide an answer?
Next, there is a big difference between 90% density and 94% density, in terms of strength, fracture toughness, etc. In order to maximize the properties of the PM part, you should investigate making the minimum density = 94%. This should be achieved by pressing, pre-sintering, re-pressing, and final sintering. The part should feature as large of a radius as possible in the critical area.
If I understand correctly, a Rockwell hardness test using the Brale indenter and 150 kgf (Rockwell C scale parameters) on this part gives a hardnes number of 55 HRC. Are you sure the 55 HRC is not converted from microhardness (Vickers or Knoop)? If it is a true HRC reading, then the hardness is WAY too high. PM parts have terrible fracture toughness as it is, and 55 HRC macrohardness is essentially untempered martensite. Have you had any metallographic analysis performed to confirm that the martensite is tempered? I would recommend doing so if you have not.
As I mentioned previously, look into the Ni-Mo-Cu alloys for improved toughness. Personally, I really dislike working with PM parts, for this very reason. You may want to consider a parallel investigation for converting the part back to a casting or some other form (fineblanking, etc.). Good luck.
The shape size and distribution of porosiy in the P/M compact affects the mechanical strenghth. This in turn depend on the shape of metal powder and their size distribution. A round shaped grain gives a better compact than an angular one. It is due to this low density in P/M component it has failed. If you can add some extra stiffeners in the compnent by redesigning you can convert it successfully into a p/m component.
The investment cast part was heat treated.
Also, the hardness of about 55 was a microharndess as they come out at about 30 on the RHC scale (per testing at our facility).
