Letter 31059

Heat treatment of Electroless Nickel plating - coating vs. substrate hardness  

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We are using Roller Dies of "High Carbon High Chromium" tool steel of very intricate shapes. The same is heat treated to 50-55 Rc hardness. We got it (High Phosphorous) Electroless nickel plated of 20~25 microns thickness . During the entire stage the distortion in geometries/ profile of the job remained within acceptable range. Further my platers have advised to heat treat the job at 400 deg. celsius for 1 Hour. This they say would increase the hardness to 66 ~70 Rc.

My questions:

1. Is the job allowed to air cool after one hour of heating.What is the exact procedure for heating the job.
2. Does the increase in hardness of coating also increase the wear resistance and life of job.
3. Will the heat treatment affect the hardness of the substrate material. Or it may cause distortion in the size of substrate material.If yes, then what temperature n time will be advisable.

Gaurav Garg
Manufacturer - N.Delhi, India

Gaurav, the one hour is long enough for this precipitation hardening reaction to take place. The reaction doesn't depend on cooling rate, so you want that sufficiently slow so as not to give you any thermal expansion gradients. Air cooling should be fine.

For question 2, well, it might. I can't tell from here, as I don't know what kind of service the parts will see, nor the environment the dies must endure. If a hardened nickel layer will help the dies resist problems, then it seems like a good idea.

Another reason for doing it is that when you get wear, you can strip off the coating and recoat, for far less money than you can rework a die.

For question 3, it depends on the tempering temperature of the particular grade of die steel you're using. ALL of the D grades (AISI designation) have tempering temperatures greater than the 400C called for by your plater. You can find out the tempering temperature used by your heat treater by asking them, can you not?

One problem you may see with the aging treatment of the EN, is that like all precipitation hardening reactions, there will be a slight shrinkage of the electroless nickel coating. This means that if you have ID's coated, the coating may want to peel off. I'd suggest trying it on only one die first, after getting the plater to agree to strip and replate if the coating flakes off.

Good luck!

Lee Gearhart metallurgist East Aurora, NY

Depending on why you EN coated this part will effect whether you want to heat treat it. If you coated the part for improved corrosion resistance, do not heat treat it. Heat treatment will severely reduce corrosion resistance.

If you wish to improve wear resistance then heat treatment may be desired. Heat treatment to maximum hardness will usually at least double wear resistance.

After heat treatment the part can be allowed to cool in the oven or in still air. The coating shrinks during heat treatment and will probably crack. If cooled in air the cracks will be wider than if the part is oven cooled. However, the hardness and wear resistance will be the same.

If your part is a Type D or Type H tool steel, heat treatment at 400C for one hour will probably have little effect on its hardness or stability.

Ron Duncan Palm International, Inc. - LaVergne, TN

Ed. note: It is our sad duty to alert the readers to the passing of Ron Duncan on Dec. 15, 2006. For those who would like to know more about him, a brief obituary opened Update No. 13 of our Metal Finishing Industry Update Podcast.

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