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Modeling hardness variation in core developed steel via FEA

Modeling hardness variation in core developed steel via FEA

Modeling hardness variation in core developed steel via FEA

(OP)
Folks;

We have a situation in our hands, whereby a batch of parts have been processed that have incorrect metallurgical properties. And it is my attempt to use FEA to model this discrepancy.

The part is a hollow tube, machined from a solid bar of stainless steel. As part of manufacturing process, this part is cored eveloped to approx. 30 HRC prior to be induction hardened to approx. 51 HRC. This process is well controlled and repeatable. However, their is an inspection step prior to core developing or rather machining the bar. And it is to check the impurities in the material. What our metalurgical inspector does is cuts a cross section of incoming bar and checks hardness as well as impurities such are voids, inclusions etc. If he had done his job, this batch of wrong parts would have been avoided. Because what he overlooked and now a problem, is the fact that their are strands of hard material buried uniformly within rest of the material ( see attached link, light color represents strands, dark core-developed material). The cut sections at cross-section of the bar would look like dispersion of spots all over the cross-sectional area that represent these local hard material strands. Another cut section along the length of the tube (shown in picture) would show these same spots running along the length of tube. If you put two cross-sections together it gives an appearance of chopped fibres in composite material.

Now the problem is when the part was designed, uniform and homogeneous material was assumed. Now with this non-homogeneous material configuration, the question is "what is the effect on yield and/ or fatigure life?".

Their is no literature that I am aware of answering this issue, so were thinking of what if we simulate this hardness variation within a material as composite material. I am thinking about chopped fibres, aligned lengthwise but disperesed in an epoxy. In this case, spoxy would be steel material with uniform-homogeneous material property, while the fibres would represent hardened strands. Next step would be to measure quantatively to come up with the volume of strand in given volum of total material. This can be done by taking some representative area and measuring  the lengths of strands and their approx. cross-sectional areas.

Once this representative element is developed, we conduct simple pull-test to measure tensile strength and use that as ultimate strength of the material for the part.

It sounds like pretty involved solution process, but not sure if this right approach. Please share you thoughts on this. Thanks!
 

RE: Modeling hardness variation in core developed steel via FEA

bsdhr:

First let me say I am neither a composites or fatigue specialist so take my comments with a fairly big grain of salt......

I think your approach should give you a decent handle on the change in yield stress and elastic moduli. However, I would be much more skeptical as to the fatigue results....

If yielding is not important (as I suspect it isn't in a batch part) then you can verify the changes by a simple tensile test and then use the new values to check deflections etc. for the modified properties. However, for fatigue properties I would go to testing to determine the new life/properties as I would not trust the results from a FEM analysis to give modified properties......

I would also consider the cost of doing all of this work, the criticality of the part, possible liability of failure etc. and depending these factors might just through away the batch of parts and have a new batch made.....

Ed.R.

RE: Modeling hardness variation in core developed steel via FEA

(OP)
Ed. R;

You are right, simple test are requried for correlation. I am planning to conduct simple pull-test made from specimen material. This approach should be satisfactory to evaluate strength of the part.

For fatigue, unless we make another sample and run it thru life testing, the above approach is non-feasible, as you cited. It's hard to model the bond between the hard and soft material, and that is where the fatigue crack is suspected to intiate at.

RE: Modeling hardness variation in core developed steel via FEA

That photo looks like machining marks, not a polished specimen of stainless under a microscope. I'll have to take your word for it that an end section reveals these strings to be of small and uniform cross section. Has this been put under an SEM with EDACS to determine what the inclusions are? Has anyone run a spectrographic analysis to determine the composition of the steel itself?

If this is any kind of critical structure and you have this much evidence of poor quality control of the source material why are you even considering going on with it?

You well know that in FEA you must know the material properties before you can construct a valid model. Just where you will get the material properties for the matrix or the inclusions remains to be seen.

TOP
CSWP, BSSE
www.engtran.com  www.niswug.org

"Node news is good news."

RE: Modeling hardness variation in core developed steel via FEA

I have seen this sort of micro-mechanical analysis done on particle reinforced metals. It was a PhD project and took about a year or more for two people. In your case it will be harder because your inclusions are not spherical or uniformly distributed.

I think you are on to a loser here, the material modeling, analysis and fatigue testing will take longer and cost more than re-making the parts, unless the parts are extraordinarily expensive.

gwolf.
 

RE: Modeling hardness variation in core developed steel via FEA

You can do the FEA with no problem assuming it to be homogenous material.  The only material properties that you need to put into a (linear) FE analysis are the YM and poisson ratio. The stress results will be fine, because none of the heat or other treatments will have any effect on the Young's modulus.

Having found the stress results - or just using the ones you originally had, if it was done before - you are still left with the question of what effect the inclusions will have on the fatigue life. This is a different question, which I believe can only be properly answered by testing.

RE: Modeling hardness variation in core developed steel via FEA

ToneyL

He is trying to get the fatigue properties from the FEA. So the OP's intention is TO model the inclusions. But this is a dead end because he doesn't have the material properties, the time or the ability to verify with a test. Like gwolf said, this is a PhD thesis, not a viable thing to do in production.  

TOP
CSWP, BSSE
www.engtran.com  www.niswug.org

"Node news is good news."

RE: Modeling hardness variation in core developed steel via FEA

I came across FEA software that is intended to derive material properties from micro mechanical considerations. It is primarily used for research but it might be of some interest.

OOF

TOP
CSWP, BSSE
www.engtran.com  www.niswug.org

"Node news is good news."

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