Blueing in low carbon steel and it's effects?

[texasmetal]

I should start by saying I am not an engineer. I am an inspector charged with watching steel pipe manufacturing and plant applied FBE applications. But I have a question I can not get answered. No one, from seasoned industry friend's to reference books, have given me a solid answer.

When coating the steel pipe it is heated, usually in gas furnaces, from ~100 degrees to ~ 450 degrees fahrenheit in roughly 1 minute. It is moved using a wheeled conveyor system during the entire process. It's heated, then FBE is applied, then it is quenched using water to bring it to ~200 degrees or less.

From time to time there are issues that shut down the conveyor and pipe gets stuck in the furnace from anywhere between 1-2 minutes. The furnaces will be shut off but retain a large amount of heat. This will cause the pipe to come out with a heavy layer of bluing. Assuming that means the pipe was heated from anywhere between 500-600 degrees. When it begins moving a couple of minutes later it will then go through the same water quenching system as the coated pipe. What affect will this have on the hardness and ductility of the steel. These are things I know (at least think I know)

1. The time spent in the furnace can increase the oxide layer even of the temperature did not reach the related tempering color. How long does it take for this affect to happen? If its held at 450 degrees f for an extra minute could that cause the oxide layer to build enough to show the blueing affect? Or does it take longer?

2. After 500 degrees the hardness and ductility can change from its tested values. Could there be enough change to worry about this?

There are many company specifications that reject any steel that has the oxide layer build up. However there are also plenty that don't. No one I know seems to have any rhyme or reason for it.

Thank you for your help! Please excuse any spelling errors. This was sent with my phone.

 

[metengr]

Given your limited information on the your pipe material;

1. The time spent in the furnace can increase the oxide layer even of the temperature did not reach the related tempering color. How long does it take for this affect to happen? If its held at 450 degrees f for an extra minute could that cause the oxide layer to build enough to show the blueing affect? Or does it take longer?

The blue temper color you are observing is caused by exposure to temperature, in lieu of exposure time at lower temperature. So, the blue temper color would indicate exposure to a surface temperature of plain carbon steel at 550-600 deg F, and will form instantaneously in an air environment.

2. After 500 degrees the hardness and ductility can change from its tested values. Could there be enough change to worry about this?

There can be a change based on chemistry of the low carbon steel and prior forming. Examples include hardening from strain age embrittlement or hardening based on microalloying of your low carbon steel (similar to bake hardening). These are examples and indicate your material could behave in a similar fashion.

My suggestion would be to perform hardness testing on pieces that have the blue temper color and evaluate the change in hardness. No change in hardness means no significant change on ultimate tensile strength, and most likely no change in ductility.

 

[texasmetal]

At the coating facilities I don't have the equipment to perform a hardness test. That's why I'm looking for a somewhat ballpark answer if at all possible. Just to know what i can expect. I'm coating X52 psl2 pipe, but I do not have the MTRs for it.

 

[metengr]

texasmetal;
You can either send one of the suspect pipe sections to a reputable metallurgical lab for portable hardness testing or have the lab conduct portable hardness testing at your facility. It is not that expensive and would provide defendable test results related to your process.

 

[mrfailure]

The color is the result of light interference from oxide film formation on the surface. Besides elevated temperature metengr mentioned, it is possible that film had grown at a temperature below 500 due to the extra exposure time. It is an indicator that you need to test hardness following metengr's advice.

 

[EdStainless]

There is also the risk that the oxide will interfere with coating adhesion.
Pick a few samples and have some testing done.
Don't just do hardness, have tensile testing done, you want to see if you have lost ductility.

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Plymouth Tube

 

[texasmetal]

Thanks a ton guys. This confirmed my thoughts. I thought I was correct on the oxide layer thickening due to prolonged exposure to heat. Does anyone know how long this takes though? If its at 470 for an extra couple of minutes is that enough or does it take longer? I will still go with the testing on the pipe. It won't change what I do differently, I just want to know for my own knowledge.

 

[metengr]

I would think temperature will be more of a driver rather than exposure time. I have seen where mill furnace lines have suddenly stopped before the furnace trips and the temperature spiked before decreasing. I think this was more of the case rather than exposure time.