Microstructure Question

[metalman8357]

I'm wondering what the process history is for the part that I have (attached). The material is either 1018 or 1022, and the vendor (China) claims that the part was carburized, quenched and tempered but I am not seeing this upon looking at the microstructure. The surface hardness is 35HRC and the core is around 33HRC. I'm clearly seeing pearlite and I'm assuming the white is ferrite. The sample was etched in 2% nital for 30 seconds, and the mag is 1000x. If I'm seeing pearlite and ferrite, how is it possible that the hardness is so high without heat treat? Extensive cold-drawing?

 

[redpicker]

It's hard to tell from a photograph, but I wouldn't be surprised if what you are looking at is a mixture of martensite and peralite.

How deep is the carbuirzed case?

How deep is the location of the photomicrograph?

1020 doesn't have much hardenability. Even in the case with 1% C, you can get a fair amount of pearlite formation during the transfer from the furnace to the quench. I suspect that is what has happened here. Do you have a microhardness tester? Can you test the hardnesses of the light and dark regions? That may give you a clue.

rp

 

[mrfailure]

Do you even see a case under the microscope? You can confirm surface treatment was performed by extending rp's idea and using microhardness to measure a profile from the surface.

Aaron Tanzer

http://www.lehightesting.com

 

[metalman8357]

I performed micro hardness measurements and there was really no difference between the surface and the core in terms of hardness (0.002" to 0.012" for the case). They were right around 33-35HRC. This leads me to believe that no caburizing was performed. In addition, etching with nital did not reveal a darkened ring around the surface of the part, which is what I would expect had the part been case hardened. The micrograph that is shown was taken about 0.020" below the surface. The surface and the core look almost identical in terms of microstructure. It was right around 33-35HRC. I tried RP's idea of taking the micro hardness readings in the light and dark regions but I wasn't able to get a good reading since it was hard to measure the length of the indentation.

I'm starting to think that RP might be right. It looks like pearlite and martensite. Ferrite and pearlite would not explain the high hardness values seen.

 

[dbooker630]

Do you have a mill cert for the material that confirms 1018 or 1022? Maybe the manganese is on the high side, or there are some other residuals that influence hardenability.

 

[Guest102023]

The difference between 35HRC and 33HRC is measurement error; i.e., there is no demonstrable case hardening. 33HRC is practically impossible for 1018 steel.

One standard definition and measurement for case hardening is the depth to which 50HRC minimum is attained. That is found nowhere on your sample, although you may have a product-specific requirement.

The structure looks to me more like ferrite + pearlite in a medium carbon steel.

Buy American would be my solution.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[tamal1475]

I agree with brimstoner. It is a medium carbon steel. May be fast cooled from normalizing temperature hence showing higher hardness.