4130 dimensional change in heat treatment

[rwilcox]

We have a cast 4130 part that we quench and temper to RC 44-48. How much will the material grow or shrink in heat treatment? Is there a reference which tabulates growht and shrinkage of various steels in heat treatment? Thanks

 

[redpicker]

Yes, your part will grow or shrink during heat treatment.

There are many factors that will affect this behavior, so I know of no reference that would give such information. If there was one, I wouldn't trust it.

For a start, you can figure that any dimension may grow or shrink by as much as 1%. It will probably be less, but it could be more, depending. Like I said, you can start with 1% and adjust your process as necessary depending on actual results.

rp

 

[arunmrao]

rwilcox, you must be water quenching the casting , other than the shift in dimension,how are you compensating for scaling and distortion?

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[rwilcox]

What I am looking for is a general rule of thumb for the volumetric change of 4130 with a standard quench and temper process....

 

[redpicker]

Well, there is no volumetric change as a result of the quench and temper process. A volumetric change would result in a change in the density of the part (since the weight of the part does not change, assuming the heating is performed in a controlled atmosphere to prevent loss due to scaling). Since the density after heat treatment is the same as it was before heat treatment, the overall volume of the part is the same. (OK, this may not be 100% accurate, particularly with castings, but any changes due to a change in density are miniscule to the changes caused by distortion)

There are significant volumetric changes that occur during the heat treating process, however, and these changes will result in distortion. That is, the dimensions will change. In some directions, they will increase (grow), and in others, they will decrease (shrink), as indicated in my first response. Which dimensions will grow and which ones will shrink all depends on a myiad of factors that you will never be able to satisfactorly account for.

The only "rule of thumb" that I am aware of is that you will generally see less than +/- 1%. That is, if a pre-heat treat dimension is 1.000", after heat treatment you can expect it to be between 0.990 and 1.010". Now, I have seen considerably higher amounts of distortion, but these are generally special case instances for long bars with very high length to diameter ratios or rings with high daimeter to thickness ratios. But, as a general rule, +/- 1% is a good place to start.

rp

 

[rwilcox]

There is a definite volumetric change due to crystal structure changes in austenitization and martensite formation, amonng others. I have gotten best estimates of .0005 in/in to .002 in/in as a max. It varies depending on the particular alloy and teh tempering temperature.

Thanks

 

[redpicker]

While you are correct there are volumetric changes due to crystal structure changing from ferrite and carbide to austenite. You will also see volumetric changes when the crystal structure changes from austenite to martensite. Even more volumetric changes will occur on tempering, when the crystal structure changes from martensite to ferrite and carbide. In the end, however, prior to heat treatment your crystal structure is ferrite and carbide and after heat treatment, the crystal structure is ferrite and carbide, with the same volume fractions of ferrite and carbide in both cases. The only difference is the size and distribution of the carbides, not their volumes. If one dimension is longer after heat treatment, I guarantee you there is another dimension that gets smaller so the conservation of volume is maintained.

rp

 

[Metalguy]

At that hardness he isn't going to be tempering at a high temp, so he'll have a tempered martensite microstructure with relatively little change in volume. The volume when it's FCC doesn't matter.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein