4140 heat treating variation in rc

[jjimjjones]

We had 7/8" dia bar 4140h heat treated, 41-50 rc. the vendor could not get a consistent rc reading on the outside ,30-54. Why would this be. Also we machined some parts and got 41-54 rc readings. Why would this be? The vendor has also tried re-heat treating the bars with the same result on the o.d. when trying hardness test(varies 30-52). The ht process is vacuum / gas, not oil quench. We want to finish machine the bar stock at the blueprint rockwell call-out(41-50rc). We also want to hold the rc to with-in 5 points on the lower end, for consistant machining purposes.Could you suggest a fix to get this material in this range, preferably 42-47 rc. Why would the od be so varied?
Thanks
Jim

 

[metengr]

The variation in hardness for the 4140H material could be caused by several variables;

1. decarburization during heat treatment OR

2. method of hardness testing


I would have a complete metallurgical analysis performed of a heat treated bar to evaluate the specific cause of hardness variability.

Hardness testing alone will not provide you with the root cause of the problem. It could be that the method of hardness testing is the cause of variability.

 

[jjimjjones]

Could you go into more detail about decarburization & method of hardness testing and a solution. How thick would the decarb be ? How could the decarb be removed?
Have you seen this problem before? It has us scratching our heads. We though we just send the bars out for ht get them back certified to rc and machine. Why such a problem? I also have mill certs for the mat'l.

 

[funnelguy]

I'm no expert, but I have used vacuum HT facilities over the years. I think your problem lies with the quench rate. Decarb is a non-issue in a vacuum furnace. Has your vendor succesfully processed similar cross sections in the same material before? A friend of mine had problems processing 4340 in a vacuum furnace with nitrogen as the atmosphere and quench.

 

[metengr]

I really can't give you a solution until you find out what is causing the variability in hardness because it would be speculation on my part. I have seen where the furnace atmosphere was not purged correctly, resulting in local decarburization affects (removal of carbon from the surface of the part) that reduces hardness. Depending on exposure, the decarburization can be localized.

I have also seen where hardness testing was done incorrectly resulting in lower hardness values. The hardness testing should be performed by a qualified lab.

You need to work directly with your vendor or heat treater to resolve this problem. They should provide you with a metallurgical analysis report or at least request one as a customer. Also, I would verify who is performing the hardness testing and their test method.

 

[jjimjjones]

Is this material salvagable, How would we get the correct rc reading from each bar, 4 ft lengths? Would the decarb need to be removed?
Is there anyone who has ran across this problem? How was it resolved?
Thanks,

Jim

 

[TVP]

I concur with [blue]metengr[/blue] regarding hardness testing. This is frequently a problem, due to improper test set-up, poor calibration, etc. Decarburization definitely produces the type of problems you describe, as does inadequate quenching. Gas quenching can be quite tricky. You need to have a proper metallurgical analysis performed. The microstructure should be characterized in a location that measures low in hardness and in a location that measures high in hardness.

 

[metman]

Jim,
Decarb can be removed by machining which you have done.

Were hardness tests after machining performed by heat treater or others? If the hardness tests before and after heat treat were done by the same facility, it would point to localized decarb since the lower readings increased after machining.

The large variation in hardness readings after machining suggests the possibility of alloy segregation. This condition occurs during the steelmaking process and can be verified by metallurgical analysis which has already been suggested.

Jesus is THE life,
Leonard

 

[jjimjjones]

The rc testing was performed by the heat treater and our selves. Thank you to all. I will keep you posted.

Jim