Transformation Induced Plasticity effect (TRIP effect) for dummies, please? 1

[jmarkus]

Hi,

My background is mechanical engineering - but I don't have a technically detailed background in metallurgy. I've been trying to understand the phenomenon known as the TRIP effect (transformation induced plasticity effect) which is sometimes observed in hot stamping or press hardening operations. (I'm not talking about TRIP steel itself).

Here is what I think I understand:

When a 22MnB5 steel part is heated to austenitizing temperature it undergoes thermal expansion. If the part were left to cool at a slow rate (i.e. not quenched, not transformed into martensite) it would contract again back to its original volume. In a hot stamping operation, the steel part is cooled at a sufficiently high rate that the austenite transforms into martensite. Martensite is BCT (whereas austenite is FCC) so the transformation to martensite results in a volumetric expansion of the steel. Because of this, once the part is quenched it (approximately) stays at its 'expanded' volume and so the final part is larger than the starting part before it was heated in a furnace.

What I have read in the literature is that if there are sufficient stresses present (perhaps due to forming strains in the hot stamping operation) the TRIP effect will cause the austenite to transform into martensite (which at this point happens regardless of the cooling rate????). These means that an ?additional? percent of material is transformed to martensite than would have been if the material was simply quenched at the proper rate. Because of this parts which undergo high strains in the hot stamping operation would come out larger than parts which don't experience high strains.

I don't know if the above is correct.

What I am trying to understand is if I can predict (roughly - not precisely) whether the starting blank needs to be undersized or on-sized for a given hot stamping operation/configuration once I have an understanding of the above.

I hope some smart (and straight talking) metallurgists can help me wrap my head around this.

Thanks,
Jeff

 

[EdStainless]

Thermal expansion has noting to do with this, it is just a side effect.
You need to understand transformation (TTT) diagrams to get how the process works.
You use steel chemistries and cooling rates such that you start with a mixed structure of ferrite, bainite, and austenite. None of these phases are very hard so the material is very formable.
During the forming process you use the combined thermal and work input to cause the austenite to convert to martensite. The marteniste then is what contributes the high strengths.

The sizes of blanks are more related to the forming process and margin needed for clamping and such.
The sizing of tooling for warm or hot stamping operations mush involve compensation for thermal expansion.

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P.E. Metallurgy, Plymouth Tube

 

[jmarkus]

Ed,

Are you saying that the TRIP effect and the transformation of austenite to martensite under quench (following the TTT diagram at the right cooling rate) are the same phenomena? I thought that the TRIP effect is another mechanism that can transform austenite to martensite due to applied strains?

Thanks,
Jeff

 

[EdStainless]

TRIP steel are not quenched to form martensite, the composition and initial heat treatment for a lot of retained austenite and no martensite. Then using forming strain you get the Aus to transform, in some grades at room temp to Martensite.

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P.E. Metallurgy, Plymouth Tube

 

[jmarkus]

Ed,

As I said in my original email - I am not talking about TRIP steels, I am talking about the TRIP effect. Here is a paper that discusses some of this (I've found others as well), but it is a little above my head, so I was looking for some clarification on the points in my original post.

Thanks,
Jeff

 

[MagBen]

To simply put, your concept is correct.

Martensite transformation in austenitics occurs in two ways, one is on cooling, the other is strain induced. The combined amount of martensite formation determines how much expansion you will get after cooling to RT. However, you missed another factor, coefficient of thermal expansion (CTE), that could take a leading role depending on the starting temperature of transform. austenitic is denser than martensite, and so martensite has a smaller CTE, which means martensite will contract less when material is cooling down.

Both the transformation itself and the decreased CTE contribute the total size increase at RT. normally the size increase from martensite transformation itself is less than .001 inch/inch (1000ppm), whereas size increase from the decreased CTE can be as high as 4ppm/F. If the transformation temperature is 375F, the size increase from the latter will be 4 x (375-75)= 1200 ppm, larger than the former factor.

 

[jmarkus]

MagBen,

I think that finally makes sense to me! Here is my interpretation of events, please let me know if it is correct:

So, if you heat the 22MnB5 steel, it expands by its CTE - this is the size that needs to fit in the tool.
When you form the part, if the strains are high enough some of the austenite will transform into martensite.
As you cool the part (at the critical cooling rate), the majority of the remaining austenite will transform into martensite AND it will contract based on its CTE of the phase that it is at (mostly martensite). Since martensite has a smaller CTE, it will contract less than the original 22MnB5 steel AND it already occupies more volume due to the BCT crystal structure so it will end up at a given size.

If there were not sufficient strains to induce the TRIP effect, the percent volume of martensite will be smaller in the final product and the part will both contract a little more, and occupy a little less volume. In this manner a strained part (during hot stamping) will end up larger than an unstrained part.

Did I miss anything (or mis-state anything)?

Thanks,
Jeff

 

[MagBen]

No star no more comment! just pulling your legs
you got the points!
By the way, martensite formation is a function of strain level (cold work) and chemistry composition.

 

[jmarkus]

You got your star, now. I just wanted to make sure that I had it straight first!

Thanks!
Jeff