Chromium Carbides in F44 Stainless Steel

[jimsan]

We have produced forgings in ASTM A182F44 material and subsequently heat treated @ 1150 C Water Quench

At some point during either the forging or heat treatment process we appear to have picked up carbon in the forging surface resulting in chromium carbides at the surface.

We did not use any graphite luricants during the forging process and we have forged this material successfully in the past without concern.

There does not appear to be any chromium carbide present in the original bar material used for forging.

Anyone have any thoughts?

 

[CoryPad]

At what temperature and in what atmosphere was the forging performed? In what atmosphere was the heat treatment performed?

Regards,

Cory

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[metengr]

Do you cast your own ingots for forging or do you purchase the ingots or billets and forge them into final shape? If you purchase the ingots or billets do you check the incoming material? It could be that the source of surface carburization is related to the supplier instead of your process?

 

[jimsan]

Forging was performed around 1180 C. The bar used as raw material appears to be free of chromium carbides.
The material is heated in a natural gas fired furnace for forging which is used regularly for this type of material. The atmosphere is slightly oxidising.

Heat treatment is carried out in a natural gas fired furnace - again slightly oxidising

Jimsan

 

[metengr]

Is your solution anneal heat treatment temperature the same as your forging temperature? Do you use the same furnace for heating to forge and perform the solution anneal heat treatment?

How do you actually confirm the oxidizing atmosphere?

Perhaps you overloaded the furnace with charge (this time) versus in the past (since this has not happened before) resulting in a change from oxidizing to a slightly reducing atmosphere over time?

You need to increase the flow of air (add compressed air) into the natural gas fired furnace to assure complete surface oxidation of the stainless steel.

 

[jimsan]

Hello metengr
We forge & heat treat in different furnaces. We haven't done any real checks on the furnace atmosphere at this stage.

Are you saying that the carbon pick up , if that is what it is . is from the atmosphere? If so what is the mechanism?

Jimsam

 

[metengr]

jimsan;
For natural gas fired furnaces you should monitor flame appearance for combustion and the atmosphere to assure adequate air flow for surface oxidation, prior to and during charging of parts. I have heard of situations where furnaces are overcharged, and once the oxygen is depleted due to poor ventilation you can change to a reducing atmosphere especially with moderate to poor combustion of natural gas. Unburned natural gas at elevated temperatures with no oxygen is a direct source of carbon.

 

[CoryPad]

Jimsan,

The mechanism for carburization/decarburization is atomic diffusion. The carbon sources are the steel itself plus the atmosphere. You must control the carbon potential of the atmosphere to match your steel. Your atmosphere uses natural gas, which has a variety of gases containing carbon. At high temperatures and high carbon potential, the carbon from the atmosphere is absorbed into the steel and diffuses some distance based upon time and temperature. The diffused carbon can react with chromium to form chromium carbide. You should review your heat treating practices to insure that your products have the required properties.

Regards,

Cory

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[jimsan]

Thanks you all
I believe that the heat treatment furnace atmosphere was adjusted with excess gas at some point for some reason and may well be the cause.

Regards

Jimsan

 

[BobUXL]

What's the carbon level in the material?

Bob

 

[jimsan]

Carbon is 0.015%
Chromium 20%
Nickel 18%
Molybdenum 6%

Jimsan