annealing and re-hardening ASTM A532-93 Class III Type A high chrome abrasion resistant cast iron 1

[Tmoose]

A machine shop has reported that some castings we sent for machining are harder than our drawing specifies.
" These parts are harder than the drawing states. We are having an issue machining the parts. The parts measured are measuring 59, & 60 Rockwell C. We anticipated 500 HB, which is 51 to 52 Rockwell C. "

They are suggesting to soften the parts (to HRC 40) for machining, and to then re-harden them.
" Can we make this part softer? Then we would harden it after machining? "

Attached is an image of 2 similar parts we have made before.
For The shorter, simpler part we called for an as cast hardness of 500 HB.
The longer, more complicated part calls for the part to be annealed to 400 HB.
I guess we specified the current parts to be as cast 500 HB.

We have approached the foundry to answer questions about softening and heat treating options.

I foresee problems with distortion and perhaps even cracking of the finish machined parts.
(The castings already are distorted enough to require dimensional compromises).

My question is, can high chrome cast iron be "heat treated" to re-harden to 500 HB after annealing or even tempering to 400?

Some of the curves in the "Abrasion Resistant Cast Iron Handbook" suggest some high chrome irons can end up with vastly different hardnesses even after tempering to the same temperature, depending on retained austenite.

 

[EdStainless]

We used to stress relief anneal cylinders that we cast, we made sure to stay sub-critical.
We were casting our own and kept retained austenite very low.
You need to check on the current structure, and perhaps do some trials.
Re-hardening is not advised as parts break easily.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[arunmrao]

TMoose,
I regularly anneal the castings for machining to about 300-320 BHN and then later harden and temper to 500-550BHN. There is no issue involved. What is the part under consideration. I have done slurry pump parts and grinding rolls.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[metengr]

arunmrao is an expert regarding castings.

 

[Tmoose]

Hi arunmrao,

attached is an image of the part in question, already cast and HRC 60-ish, and needing machining of several features.
About 4 feet ( 1200 mm ) long.
About 2 feet ( 600 mm) diameter.
A little less than 1 inch in thickness ( 24 mm)
A 1.38 inch thick ( 25 mm) thick flange on one end.
Four 16 mm holes drilled in the body about 12 inches ( 300 mm ) from the large end with no flange.

 

[arunmrao]

As a first step, I would temper this casting to drop the hardness to 450-500BHN , before attempting annealing. Also, if distortion is a concern, provide some supports.

Are you using cubic boron nitride tips or carbide tips for machining?

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[1gibson]

If the issue is that they take longer to machine or they go through tooling faster, can you throw more money at the machine shop to just work through it? Worth a shot for a few parts, for 50 parts that approach isn't going to be realistic.

 

[Tmoose]

Hi arunmrao,

The shop has replied says they are using cubic boron nitride tooling.
So now we are waiting to hear from the foundry about how to get the hardness down to machinable levels.
And we should try to confirm the actual material.
(The foundry has replied they are very surprised the parts can be as hard as the machine shop reports)

If we can positively identify the material ( unlikely, I know) I'm interested if you have an idea of the annealing/quenching/tempering/?? recommended after the first tempering to 450-500BHN process you mentioned.

If distortion is a possibility ( and it sounds like it is ) then re-hardening is going to be mighty risky, so I'd much prefer machining to be the last step.

regards,



Dan T