carburised vs carbonitrided - which has a better wear resistance 2

[Jen08]

For the same hardness and depth, does carbonitrided case has a better wear resistance than a carburised case?
Involved in a project where we need to double the life of a bush. At present the bush wears out in one year. We need it to last two years. It's mating/sliding contact with a roller.

 

[TVP]

For the same hardness and depth, the wear resistance will be similar. Often the surface hardness will be higher for carbonitriding, with a somewhat lower CHD/ECD. For optimization of rolling contact resistance, you need to evaluate the following:
[ul]
[li]surface friction and lubrication[/li]
[li]non-metallic inclusion content[/li]
[li]residual stresses[/li]
[li]microstructure including retained austenite[/li]
[/ul]

Ceramics are also used in certain applications like rolling contact bearing (ball bearings) because they can improve rolling contact resistance in certain environments (high temperature, poor lubrication).

 

[Jen08]

Steel bush (case hardened) and steel roller ( thru hardened). Ceramic is not an option. One of the project objectives is improving wear with same or reduced cost.

They are a sub component in a farm machinery. We know there is no regular lubrication regime.

The solution being discussed around the table is to increase the case hardness and depth of the bush and hardness of the roller.
I can see the improvement if case hardness of bush is increased, keeping the roller hardness unchanged, thus making it sacrificial.
I can't see where is the improvement comes from if hardness of both contacting/sliding surfaces are increased?

 

[TVP]

All things being equal, two very hard surfaces in contact will have better wear resistance because there will be less deformation and debris generation. This is how internal combustion engine parts are designed such as cams, followers, etc. However, this is heavily influenced by the factors I mentioned previously. If this roller/bush contact is not lubricated, and is regularly exposed to contamination (dirt, rocks, etc.), then it will be difficult to achieve a 2x improvement in life, especially for no cost increase.

 

[EdStainless]

Raising hardness might get you another 25% in life, but double will require a different solution.
And it will cost something.
There are some PVD coatings that work very well with lack of lubrication.
And don't rule out ceramics, they are not that expensive and the wear can be fantastic.

making both harder will reduce wear, but you need to make sure that the substrate provides enough support to prevent compressive load failure of hte wear surface. You may need stronger material if you go harder at the surface.

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Plymouth Tube

 

[Tmoose]

Can you share a picture?
Of the arrangement and also the worn parts

 

[tbuelna]

Your description of the contact condition is a bit vague. Can you elaborate on what "mating/sliding" contact between the roller and bushing means? I'm assuming the contact is between the roller OD and bushing ID, but is the relative sliding in the axial or circumferential direction? Also, can you provide details of the mating surface geometries at the contact location? How conformal is the contact between the roller and bushing? What is the exact requirement for wear limits in your parts? All of these details matter. For example, if the roller rotates and the bushing is fixed with respect to the direction of load, then wear on the roller surface will be distributed and wear on the bushing surface will be localized. So there might some benefit with this example to slightly reducing the roller surface hardness so that the wear rates between the roller and bushing are more balanced.

Regarding carburized versus carbonitride surfaces, the primary difference is that carburizing can produce a thicker case, but the outer surface of a carbonitrided part will be a bit harder. However, you must also consider that if the case hardened surface is finish ground, the final carburized part surface might be harder than the carbonitrided surface if more than a minimal amount of stock is removed. The hardness of a carbonitrided case drops off much more quickly through its thickness than a carburized case, so you need to careful about how much stock is removed if the carbonitrided part is finish ground.

Lastly, if you currently have an unacceptable wear rate with parts that have a very high surface hardness, I don't think the best approach is slightly harder surfaces. To get the level of service life increase you desire (2X), you need to look at things like optimizing the relative surface geometries/textures, or applying surface coatings to reduce contact fretting.

 

[Jen08]

Hi all, they are components in a roller conveyor chain.

TVP: we know the environment these components are working in - regularly exposed to rocks, dirts etc and during downtime outdoor weather.

EDStainless: yes we are aware of the compressive failure. The proposed solution is to change from low carbon steel 10B21 to an alloy steel 5120! (which didn't go well with the objective "cost reduction")

Ttbuelna: The contact condition :bush is fixed, roller rotating. Wear is Bush OD and Roller ID. You are right regarding the wear pattern- we see wear on roller OD and ID and the wear on the bushing OD is localised. No finish ground on carbonitrided surface.

 

[Jen08]

Further to above. I have been told by a team member that a material with the same hardness but with different microstructures will give you a different wear resistance.
He claimed 5120 carbonitrided case will have "better" wear resistance than carburised 5120 case because of the presence of micro hard carbonitrides or nitrides in the microstructures.
In 10B21, there is no nitride forming element, so there will no nitrides/carbonitrides in the case??

 

[EdStainless]

Or you could look at a nitriding steel, with Al in it.
That is what it would take to a different enough structure to really help.

Your other issue is corrosion, and that will be a bigger issue.

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Plymouth Tube

 

[CoryPad]

It is possible to have different microstructures at same hardness level, and some microstructures can be better than others. There really is no big difference between carburized and carbonitrided 5120 steels - either will provide good results if done correctly. Iron will form carbonitrides during carbonitriding, you don't need other elements. Nitrides are formed by nitriding, a completely different process.

 

[metengr]

I think you need to spend a little time reading/researching some information to arm yourself for discussions with your "team" members;

http://books.google.com/books?id=eI...carburizing vs carbonitriding steels?&f=false