Hardness Test for a PM part

A Rockwell or other test (Sharpy) tells you the surface hardness. There are tables to compare different types, some use balls and some used diamond points. If your material is not surface hardened, and I would suggest a sintered part is not, then any test should do! You must assume the material is homogoneous, and has no internal flaws.

Hello Rick,

The real question you should ask: What do I want to check or test?

In the paint business these are the most frequent tests use in the industry, but there is a lot more depends what kind of data you are looking for.

Take a look at these for a start:

Impact direct/indirect on steel: ASTM D2794

Pencil Hardness: ASTM D3363

Hardness shore Durometer: ASTM D2240

Surface Hardness: ASTM D4366 (Konig Pendulum hardness)

Hope this will help otherwise let us know clearly what you are looking for. A test is good only if you set the parameter (pass/fail and is relevant to your business) and want to compare similar coatings.

I think he is not talking about powder coating, which is what I first thought as well. He actually never says coating anywhere.

I think he is talking about a powder mixture that gets sintered to a piece or component.

In this scenario you need to always ask why do I need to test. What do I want to get to know about my product.

Some hardness testers are specially for the surface and will only tell you how hard your product is to lets say 20 or 30 microns deep whereas others can tell you the hardness over a larger range. Some are very picky about inconsistencies and will throw wildly varying figures at you if your product is not totally homogeneous but sometimes that is exactly what you want to know.

I am sure that if you googled well targeted for the specific needs to you or your customer, you will get articles outlining how to make your choices but I think you will not end up doing just one test.

Regards,

Case491

Hi Case491,

You are absolutely right and that's what I was asking him to clarify at the beginning of my answer.

I am guessing he's talking about the same part in a previous thread: how to be sure it's a powder coating part and if there was a simple test to prove it.

Same as you are assuming it's not powder paint and asking also for clarification I just giving him a starting point to look around until he's more specific about what he's looking for...

I agree with you he will have to perform more than 1 test probably. If it's compressed powder he can check in the United States Pharmaceopeia book (U.S.P. yes a book... and a lot easier to consult than an internet search) for a hardness test for pills or tablet. If its a powder compressed in some part as you thing, probably a Durometer test is maybe the next best thing to do?

Hope he's going to get back to clarify

kind regards,

Hi madmax,

Not all powder sintered components remain soft. Don't forget that ceramics are mostly sintered and they can be the next hardest thing after diamonds.

They are doing tests with sintered ceramic engine components while the brittle nature is still a problem, the hardness is never considered a limiting factor.

Hello all,

It is my fault for not being clear on my question. This is the problem I have exactly.

I am a technician in a lab. I have been given a blender that includes gears (sprockets) outside the housing motor. These gears are cracking and my customer suspects the cracks are caused by the gear not having a high enough hardness value. He has asked me to tell him the (surface) hardness of the material. I know for a fact that the gears are made using a powder metal (PM) process (comact, sintered and coined).

Can I test the hardness using a Rockwell Hardness Tester or is there a preferred method of testing hardness on a PM part I have been told that I must use an Apparant Hardness Test becasue these are PM parts. Which method is correct for testing the hardness for this particular part?

Thanks you to everyone who has responded thus far. I appreciate the input.

Hi Rick,

Good to see you are still hangin in there, lots of times we just see posts disappear as if we smell or something.

I cannot truly decide what environment you are working with or in which your part is working.

As the first post suggested a sharpy test could do but I see lots of refferences to vickers as well for pm parts. Some times these parts are also surface hardened which would make base hardness test a bit more difficult.

Please also consider that if the parts breaks and cracks, the hardness may be too high

In this case the thing to do would be to try and get a softer core and surface harden the running surfaces or induction harden the crown of the gears.

Your best bet is to find a local hardness lab and go there with your parts and just ask the questions. They will have to see the part before they can quote you and if you don't like the prices you can go it alone thereafter. You will have learned something in the meantime.

It does sound more a lab thing than just technical cookery lessons to me.

Hi Rick

Let me clarify one thing first - cracking of P/M gears most likely will not have anything to do with to low hardness. If any that would be to high hardness (parts not tempered after heat treating or sinterhardening), which at some shock load conditions my cause parts to crack (I am assuming that happens when gears are subject to load). If on the other hand parts are cracking, while being installed on shafts you will have to take a look at shaft/id size. Now, it is possible that hardness testing is being suggested not to check actual hardness value but to determine material strength/part density.

What I would suggest to do is perform superficial hardness test (using T or N scale) and density check - to see if parts are at density they should be. I do not have enough information to suggest anything else - but it is always possible that wrong material have been used or something went wrong during sintering for example parts get decarburized or not fully sintered, and that will manifest as lower than specified hardness and other mechanical properties (strength, ductility etc.).

Hi Rick, If the component you have is sintered you can use thr HRB scale on a rockwell tester or T scale on a green compact (un sintered). You can assume that if it has been compacted to a green density around 6.5g/cm3 then the hardness would be approx 45-70HRb. Other contributing factors come into play as was stated before the sintering conditions greatly impact the hardness and tensile strength. My first thoughts would be that if it is below a green density of 6.5g/cm3 then it would not withstand to much torque. If you have some of these components you could try to copper infiltrate them on a continuous belt furnace reaching 1120c this can treat any minor subsurface defects caused by ejection process during compacting and excessive porosity during the compacting stage.

Thanks Andrew for your input.

The part I am testing is a gear component from a motor assembly. Therefore it is fully compacted, sintered and coined. The part is also hardened. I know this because I have done a microstructure of the specimman.

I now know that apparent hardness is the best way to go for this particular part.

I have never heard of a T scale. I will look into that scale. Maybe that is an alternative way for me to check hardnesses. I have a Rockwell Hardness Tester at work.

Rick

Hi Rick the T scale we have used in the past to measure green compacts only as a guide as accuracy is not good.

You mentioned that the part was hardened do you mean steam treated if so this could be the cause of the failure, as it does make the component brittle if penetration is deeper than a few microns.
If this is not steam treated I would be intersted in the process that you used and if you have the spec on the type of powder used and any additives eg, copper, manganese, titanium etc.

Andrew,

Generally, the vendor is to supply parts with a tempered martensite structure. Case depth is only a few mircons, just like you mentioned in your comment.

Therefore, I can't be sure of if it is steam treated. The manufacturer of the part is in China! Though steam treatment is a typical treatment for this type of part, a gear in an appliance.

Based on my limited knowleged of powder metal (I worked for two years as a process engineer in PM) I believe the process used is a simple compact, sinter, coin, heat treatment due to the low cost of the part. I have do not have a mircrostructure to confirm my assumptions.

In P/M industry there is a tendency to use T and N scales (also called superficial hardness testing) because they will give you better "feel" of actual matrix hardness.

Remember you are testing something very similar to sponge. When you use full load of 150kg for RB and RC you are getting sort of average between pores and material. Once you drop load to 45, 30 or 15 kg you will start to get closer to actual material hardness.

It still will be several points lower than microhardness (in case of hardened parts even by 15-20 points) but it is prefered method of checking hardeness because of convience and speed (no or little preparation necessary).

When you are using superficial testing remember to use correct penetrator and convert reading. Some hardness testers do have separate scales (manual ones) but some require user to convert reading in conversion tables. On some full scale testers there is also a minor load which needs to be removed before doing a superficial testing
- just follow the manual.

I had a look today on our hardness tester and we are using and have used for many years a 1/16 ball penetrator with 100kg load using the HRb scale. this is used on all our sintered components with density range from 5.8 - 7.1 g/cm3

Andrew

Do not use it on hardened parts - you might "flatten" the ball. You will need to find or buy "Brale" diamond penetrator. Ball penetrator will let you test only up to 100RB, and it is equivalent of about 20RC (this were "C" scale starts). It is OK to use on non hardened parts and parts which are not subject to sinterhardening.

I am guessing that you are using manual hardness tester - if so you can add weights to 150 kg on weight hanger. If you decide to go with superficial testing you set of weight should allow you to set either 15, 30 or 45 kg for 15N, 30N or 45N scales.do not forget to remove minor load for superficial testing (on Wilson TT it is a weight in pot on top of tester seating i t-slot, so it could be dis-enagaged). There is also matter of scale - my advice RTFM, if you do not have one check it on manufacturer web site.