Carbon content increased in heat treat? 2

[cywork]

Hi all!

I have to admit the question I am about to ask is probably a seemingly dumb question in the metallurgy engineering world. I do have some explanations for it but would like to get some feedback on those if possible. I am not a metallurgist so I am not familiar of the details in regards to heat treating steel.

Question: There is no way that carbon content can be "added" to the material during heat treat is that correct? The material in question is 440C stainless.

Reason for the question:
Carbon content was 1.01% when the material is in its raw state (none heat treated drawn wire) but increased to 1.1% post-heat treat. Now the material tested is not exactly the same (as in we didn't take the same batch before heat treat for testing, process it and then perform another test) but they did come from the same heat load.

My thoughts on this:
1) Is it possible that carbon is introduced to material in the furnace during heat treat? We make balls and post-heat treat they get grinded down so I believe any surface carb may have been removed already and would probably not alter the material in the core.
2) Test method in obtaining chemical composition may be different each time?
3) Because we purchase bulk wire, is it possible that different parts of the coils of wire may have slightly different carbon content?

Again, I'm not an expert in this so I hope someone can help me demystify the increase in carbon content.

Thanks!

 

[Maui]

A 10% variation in carbon content would not be considered normal variation in a given heat of steel of this type, which brings into question the test method used to determine the carbon content. How are you measuring the carbon content of the wire? Are you performing wet chemistry, using an X-ray diffraction set-up, or running a sample of the wire in your SEM with an EDAX scanner?

Maui

 

[TVP]

How exactly was the C content measured? On the surface of a cleaned wire specimen and a cleaned ball? In theory, yes, the parts could have more surface carbon after furnace heat treating. Test method variation is also a possibility. As Maui stated, a 10% variation is excessive, so item 3 from your list is doubtful.

 

[stanweld]

Carbon can be induced into the surface when using a carburizing furnace atmosphere; however, there is too little info to determine if this was the case. Was the wire cold drawn and if so was a hydrocarbon lubricant used? As per Maui, the methods used in carbon analysis is more likely the cause of the differences observed.

 

[tbuelna]

cywork,

Carbon can be either added or removed from the surface of a steel alloy during a heat treatment. It depends upon the alloy, temperature, and atmosphere present. 440C is not a carburizing (case hardening) alloy, it is a thru hardening alloy. So there's no reason it should have gone thru a carburizing process. Standard 440C should have a carbon content between 0.95% and 1.20%

Carburizing is the intentional introduction of carbon into the surface of a steel alloy part in order to permit that surface to be case hardened.

Decarburization is a condition (usually unintentional) that can result when a steel alloy part is heated above a certain temperature in an atmosphere that has oxygen present. The surface of the part can lose carbon thru oxidation, resulting in a surface condition that is metallurgically inferior. This surface must usually be removed after heat treat.

Hope that helps.
Terry

 

[PeterCharles]

Years ago we did our own heat treatment in a gas fired furnace. 95% of this was either carburising or carbo-nitriding to give a case hardened surface. Occasionally some of the components were just through hardened. While half expecting a slightly de-carburised surface we found a slightly case hardened surface. It was put down at the time to carbon having impregnated the brickwork and coming out while we were attempting to maintain a neutral atmosphere.

 

[Guest102023]

I would suspect the testing is inconsistent. And not the method, either, because no matter how high the tech it always comes down to the nut behind the wheel.

 

[ornerynorsk]

Are your tests surface tests? If so it may be a migration of carbon toward the surface in the normal process of decarburisation if being heat treated in an ambient atmosphere oven. Just a thought.