4340 hardening processes

[thinksmall]

My company uses rings (1.6" OD x 1.4" ID x 0.5" W) made from 4340 HTSR as targets for an inductive type proximity sensor. The rings are machined from HTSR bar (HRC ~35). We are experiencing problems with electrical runout. Our theory is that non-uniform structures generated during the quench and temper of the HTSR is the source of the electrical runout. We have tried making the rings from 4340 Annealed bar (HRC ~25) and observed an improvement in the electrical runout.

The problem with using annealed material is that one of the axial surfaces of the ring is also used as a landing bushing and we need to have a hard surface that will wear well. We are concerned that annealed material in an untreated state will be too soft. Strength is not an issue, it is only an issue of hardness.

We have tried to normalize the annealed 4340 (1700F for 1 hour and air cool). This seems to only slightly improve the hardness. Do we need to machine away a small layer to properly measure the hardness after normalizing?

What are some other methods (besides normalizing) to treat an annealed material to achieve a surface hardness of ~35HRC while maintaining a uniform structure in the bulk of the material?

 

[salmon2]

I wanted to say carburizing but it is usually for low carbon steel. A normaling should give you plenty hardness for this chemistry. Something is not right of your normalizing. The hold period seems very long.

 

[thinksmall]

What reference can I check to find out what the hold period should be for the normalizing? Will the hold period depend of the size/mass of the parts?

 

[salmon2]

Yes, it depends primarily on size and also final properties interested. You check some hardenabily data or Jominy bar test for this chemistry. Your size is quite small given this chem. What I can think of right now is Timken's metallurgist practical handbook which is available on their website.

 

[israelkk]

Magnetic properties of 4340 and low alloy steel are not guarantied.
You may experience large deviations of magnetic properties for every new batch of material.

 

[unclesyd]

You might want to demagnetize them in the hardened condition prior to use. May not work because at times we've had trouble demagnetizing larger parts made from 4340. It's still worth a try.

 

[arunmrao]

Will case hardened 8620 exhibit consistent magnetic properties? I have no experience for such applications.

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