Material Selection Advice needed

[Dougt115]

Hey All,

I have two parts one 440A and the other A2 that slide against each other in a unit. Direct contact, no lubrication. Surface area is .02 in^2 and the load is 2-4 lbs. One of the units came back badly worn at the interface with both parts having a flaky appearance to them. The larger part rotates and the smaller part is a locking key that falls into place when the key and the groove align. The A2 key has lost about .05 inches in length and the rotating part, 440A, has lost about .01 on the radius. While still able to rotate the key can no longer perform its function as the teeth have been ground down.

Analysis shows that while the loads are not large both surfaces chip as they wear and do not smooth out. The chipped surfaces are acting like a sandpaper so the chipping continues. I would like one of them to hold the surface finish so that the chipping never starts. To achieve this I would like to change one of the parts to a material with better wear characteristic.

Looking into the material, both have great hardness but poor wear. What I am looking at is changing the 440A to either 1095 or 5160 or other alternates you might suggest. It is the larger part and can perform with more wear/deformation than the other part and it is easier to machine.

What is your take on this?

 

[btrueblood]

You might look at gall-resistant materials like Nitronic alloys or Gall-Tough, if a stainless steel is required. L-type and S-type tool steels are tougher than the A- and D-type tool steels.

 

[tbuelna]

The simple P/A bearing stress does not seem large on its own. But from your description of the kinematics in this mechanism (...the smaller part is a locking key that falls into place when the key and the groove align...), it sounds like there may be excessive dynamic/impact forces to consider. You might have better results by modifying the geometry of your parts and motion of your mechanism to reduce the impact force that is causing fracture/spalling of the component surfaces.

 

[EdStainless]

Mating 440C and Ntronic 40 would be a good option, the 440C is very hard, and the N40 is very resistant to wear and galling, but softer.

Of if you want to stay with your 440A you could nitride the wear surface.

You could make the smaller part from the harder material, since you want it to wear less.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Dougt115]

Good ideas and I am looking at the geometry to find a solution. I wish I could supply a picture but it is not allowed, intellectual property issues.

There is no impact force on the key where it has worn out, the point. More of a constant pressure as it slides. The material of both parts must be hard because when the key engages the sides of it are what keep the part from rotating. Forces can reach 600lbs and the surface area is only .02 in^2 for a pressure of 30,0000psi. The non key parts are also structural members of the main assemble and must support the main loading. These parts are fine now it is just the key that is the issue and its wear.

1095, 5160 and 440C can all meet the structural demands but which if any is best for the wear?

 

[EdStainless]

For good wear performance you what two different materials with two different hardness values.
You have to choose which will wear the most, you cannot prevent both from wearing.
That is a very high contact stress.
You need the key to be significantly harder than the disc.
What makes this worse is that you are wanting friction, which causes more wear.
I think that you are going to need to start looking at specialty coatings.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

Dougt115-

In your OP you used the term "chipping" to describe the failure mode of your parts. To me this implies the surfaces suffered spalling. Spalling is where pieces of the surface fracture and pop out. Spalls are usually the result of subsurface shear fractures produced by excessive contact stress levels propagating up to the local surface. Spalling is different than adhesive type failures such as galling or scuffing. With adhesive type failures such as galling or scuffing, you will see smearing and a transfer of metal from one surface to the other, rather than the surface pitting typical with spalling.

Either type of failure will produce metallic debris and a rougher surface that will accelerate abrasive wear of the contact interfaces. But the best approach to resolve each problem is a bit different. Spalling is resolved by reducing the contact stress levels and/or increasing the material subsurface shear strength at the contact location. Galling/scuffing can be resolved by several means. As others suggested, you can use different materials and coatings that will inhibit the tendency of the contacting surfaces to mechanically bond. Modifying the surface textures will help. Also, if possible modifying the mechanism kinematics to promote more of a rolling contact versus sliding contact will be beneficial.

The one thing you can do to that won't cost much would be to optimize the geometry of every contact surface in your design. Even tiny tweaks to the contact surface profiles can reduce stress levels significantly.

 

[Ramkiyengar]

Hi Dougt115, I read the string of suggestions, Some have suggested surface coating as a possible solution. I do research on surface modification using a technique called Electrospark deposition. I have successfully reduced wear in some industrial components. Since the wear areas mentioned in your OP (What is it? Opening Presentation?) stated 0.02 square inches for the key, this technique is well suited for such small areas. If you have not already solved the issue of wear, and would like to find a PRACTICAL solution, then I would like to work with you and do some surface modification of one or two A2 (key) parts.