Possible Causes of Failure

[mtiplady]

Hi all,

Just wondered if anyone could point me in the right direction as to where I might find some answers on this question.

I need to suggest some possible causes for the failure of a carburized alloy steel gear, which was meant to have a surface hardness of RHC60, but, after an in-service inspection, it registered a hardness value of RHC30. I'm an electrical engineer and have absolutely no clue where to start on this.

Many thanks,

Regards,

Matt

 

[salmon2]

It is easy to get a lower hardness reading if the part is not supported firm enough. But 60 vs 30 seems big. It seems to me HRC60 is as quenched hardness, but HRC30, if it is real, seems really soft.

What is process history of the part? What is the base chemistry? What is the failure? Is HRC30 from undamaged surface or from layer lower than carburization?

Sending the part to a metallurgical lab should help locate/solve your problem.

 

[mtiplady]

Salmon2, thanks for your reply. Unfortunately, the question originates from an assignment question, but I'm completely stumped. The question implies that during a routine maintenance inspection, the teeth on a carburized alloy steel gear wheel are found to have worn away. As previously stated, the gear wheel was meant to have a RHC of 60, but was found to have an RHC of 30? If I'm honest, I have no idea where to start with any possible reasons why the dramatic reduction?

 

[salmon2]

OK, it seems the HRC30 is measured from core, not the carburization layer. Remember by carburization process, you will get a hard/brittle carburization and tough/soft core. So it is like either the part got worn out too fast or the carburization layer is too thin. The thickness depends on the duration of carburization.

 

[stanweld]

Either the gear was not hardened after carburizing, the gear was not carburized to an appropriate depth or the gear was simply run past its service life date (worn out), I would suspect.

 

[mtiplady]

Salmon2 and Stanweld, thank you both for your replies. I still have absolutely no idea about this question. I will use your suggestions though and scour the internet some more to try and help find an answer.

 

[mtiplady]

Salmon2 & Stanweld, just a further point. You've mention that the part could have got worn out too quick and is being read from the inner material and not the carburized layer, which is why the RHC value is being read as 30. Why could this happen? And what possible solutions are there to prevent it from happening again?

 

[stanweld]

The carburized layer is a surface layer only and does not extent into the substrate beyond the thickness specified.

 

[TVP]

If you are going to be responsible for this project, you need to educate yourself about gear materials, heat treatments, etc. The following are just a few links that I obtained quickly by typing case depth hardness gear into Google:

http://pt.rexnord.com/customer_support/Technical_Information/tech5.asp

http://www.baysidemotion.com/web/te...4ed91d8cde95ab5285256ca900594c5b?OpenDocument

http://www.geartechnology.com/issues/0308/krantz.htm

 

[mtiplady]

TVP, thanks for your reply. It was some helpful links I was after as I will need to show evidence/proof that I actually understand the possible causes of the materials failure, and right now, I don't.

 

[metengr]

I would send the gear to a metallurgical laboratory for proper failure analysis. One can speculate all day long, and time is precious.

 

[metengr]

,,, excellent suggestions above, however, you need to be realistic here, a materials engineer/metallurgist one is not going to make overnight. Responsible project engineer or not, you need to have a proper failure analysis to at least get to the root cause and prevent re-occurrence. This is the prudent course of action.

 

[Robertmet]

With the remains of the gear you could determine if the gear was carburized and heat treated properly .The tooth tip and root should be undamaged.This would involve microscope and hardness examination. 30HRc core and 60HRc case would be fairly normal.Another possibility would be lubrication.Incorrect or contaminated lube .I've investigated failures due to contamination by sand and even flies !!! Anything can happen in failure analysis that's what makes it fun !!

 

[SMF1964]

A metallurgical analysis of the gear is advised (my 2 cents - which doesn't buy much with the weakening dollar). In a previous employment life, I was consistently asked about components where the written specification would provide a minimum Rockwell hardness level, with the understanding that the component was carburized. Thus, the hardness measurement would be more of an average of the very hard surface layer and the softer underlying core. If the material was adequately carburized, the Rockwell hardness would come in ok.

The purchasing group would regularly get proposals from vendors to supply an alternative alloy that would meet the minimum hardness requirement but since it was not carburized would not have the hardened surface layer and, consequently, would be totally unsuitable for our application. Since they were significantly cheaper than the carburized parts, we would end up with a sample to test to see if they could replace this expensive part with this less expensive one.

 

[tomwalz]

I like to start with a list of all the reasons something could fail, failure mechanisms, then move onto contributing causes.

If this is real world then the sky is the limit. If this is limited to the condition of hardness then you cans till get a good, long list.

In the question as presented, we don’t know if there was excessive wear, pitting, chipping, cracking, deformation or anything else.

The part could have failed because it was hit with a forklift and the hardness was incidental.

In my career as a consultant there have been several times when I have told a group not to do something and seen a few faces registering surprise. You could almost hear them thinking “Shucks, I better quit doing that.”

Here is my list as to why tungsten carbide might “wear out.” It doesn’t feel completely right to me but it is very handy. Do something like this for you project.

Theoretical considerations
1. Wear – the grains and the binder just plain wear down
2. Macrofracture – big chunks break off or the whole part breaks
3. Microfracture – edge chipping
4. Crack Initiation – How hard it is to start a crack
5. Crack propagation - how fast and how far the crack runs once started
6. Individual grains breaking
7. Individual grains pulling out.
8. Chemical leaching that will dissolve the binder and let the grains fall out.
9. Rubbing can also generate an electrical potential that will accelerate grain loss
10. Part deformation - If there is too much binder the part can deform.
11. Friction Welding between the carbide and the material being cut
12. Physical Adhesion – the grains get physically pulled out. Think of sharp edges of the grains getting pulled by wood fibers.
13. Chemical adhesion – think of the grains as getting glued to the material being cut such as MDF, fibreboard, etc.
14. Metal fatigue – The metal binder gets bent and fatigues like bending a piece of steel or other metal
15. Heat – adds to the whole thing especially as a saw goes in and out of a cut. The outside gets hotter faster than the inside. As the outside grows rapidly with the heat the inside doesn’t grow as fast and this creates stress that tends to cause flaking (spalling) on the outside.
16. Compression / Tension Cycling - in interrupted cuts the carbide rapidly goes though this cycle. There is good evidence that most damage is done as the carbide tip leaves the cut and pressure is released.
17. Tribology – as the tip moves though the material it is an acid environment and the heat and friction of the cutting create a combination of forces.


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[tripleZ]

Wrought gear or P/M gear? Many have gone to P/M due to the cost savings. You can create P/M alloys, and you can carburize P/M parts. Did the teeth wear down or were they broken off? If they were broken off, you added a whole new group of issues ot the equation.

I tend to agree with Metengr...if you have no background with this one, find a metallurgical lab and pay the consultant's fee. We have metallurgists on staff but still will send stuff out for a second opinion. They'll be able to identify the material, the correct way of mounting, etc.

 

[desertfox]

Hi tips6

Can you post a picture or photograph of the failed gear.
Often looking at the surface where it failed can give some clue's as to failure.
In addition if you can provide any evidence of the load the gear see's might also be helpful.

regards

desertfox

 

[plasgears]

I had the surprise of finding out that a low hardness reading on heat treated gears at a PM source was due to the following:
- gears were induction heat treated;
- operator 2d shift collected his heated gears and dumped them as a grouop later;
- supervision got their a** kicked.