How to resist vibration imprint in steel body 2

sammer003 · Mar 1, 2013

Hi guys,
I have a nitrided, hardened, steel puck that sits and is held in a milled steel (4145 HTSR) pocket. The pocket of the body is gradually showing an imprint of the bottom of the puck, due to vibration and contact on the top of the puck. So basically, the contact is on the top of the puck, and the bottom of the puck (the puck bottom has a couple of grooves) is leaving an imprint in the pocket. This also results in the puck not being to size anymore, since it now sits about .030" lower (because of the imprint) after time, even .080", and that would be the maximum it would be. After that, the pocket is damaged, and is discarded.

We want to increase the life of the pocket without changing the puck. Material would be the best.
1. Would a higher material charpy impact test help? Currently we show about 55-60 ft-lbs charpy impact test.
2. We thought of using a sacrificaial 'disc' underneath, about .020-.030 thick. Haven't tried it yet.
3. We must have the puck nitrided and hardened, to resist the constant pounding on the top of the puck. Otherwise, it doesn't last.
4. Would gas nitriding the pockets help?
5. Would hardening the pockets help? Ideas?

Thanks.

MikeHalloran · Mar 1, 2013

Replaceable liners as you propose are a common feature of tooling, and need not be super hard to be effective. You just make a bunch with simple geometry, and throw them away as the puck brinells into them.

OR, you could make the pocket of material with a higher Brinell hardness.
Since it's not suffering brittle fracture, higher Charpy values wouldn't help.

Mike Halloran
Pembroke Pines, FL, USA

TVP · Mar 1, 2013

1. Good advice from Mike.
2. No, increasing toughness (higher Charpy) will not help.
3. Hardening the pocket would likely help. 4145 HTSR has hardness below 30 HRC, I believe. You could surface harden using flame or induction, up to 50-57 HRC, for a depth of ~ 1 to 2 mm.
4. Alternatively, make the existing pocket larger and make some inserts that can be pressed into the pocket. These inserts could be tool steel like D2 @ 58-62 HRC or similar.
5. Lots of other options include case carburizing or nitriding the pocket or inserts. Bearing steel like 52100 @ 60+ HRC?

Tmoose · Mar 2, 2013

Sounds like there is some motion going on. Can things be clamped harder to stop the motion?

I have no idea what your application is tooling, multiple installations, or what, but generally fighting steel on steel contact wear with hardness when motion is involved only gains 2 or 3X. Lubrication can gain 100X.
I'm thinking dry film MoS2?

How is the upper end of the puck doing? How is it secured?
If the upper joint could be a rubber biscuit allowing transverse motion via internal shear maybe the motion in the pocket would stop?

hydtools · Mar 2, 2013

As has been said, the pocket needs to be harder than the puck. Are the grooves in the puck by design or result of machining? Can the grooves be eliminated or very much reduced? The pocket needs higher surface hardness and a stronger core to support the hard surface and resist deforming/yielding due the puck loading. Have had good experience with EN30B or 9328 case hardened. It develops a good core for the case support.

Ted

MikeHalloran · Mar 2, 2013

We here have absolutely no idea what the puck does, or what hits it, or how hard, or how fast.

It's entirely possible that the puck's interests would be better served by a more resilient support, like rigid polyurethane. ... but there's no way for us to know that.

Mike Halloran
Pembroke Pines, FL, USA

tbuelna · Mar 2, 2013

Mike Halloran's suggestion is excellent. Making the parts harder doesn't usually help a whole lot. From the description, it sounds like there is a fretting condition. And with contact fretting, there are only three ways to eliminate the condition. The first way is to reduce the dynamic contact stress at the interface to extremely low levels (ie. below the fretting limit for the materials involved, which is typically around 5ksi for heat treated alloy steels). The second way is to increase the contact preload force such that there will never be any possibility of relative motion at the contact interface. The third way, as Mike Halloran suggests, is to add a washer or shim between the parts that is made of a non-metallic material having enough mechanical strength to support sustained loads without creeping, while also being resilient enough to prevent abrasive wear of the mating parts.

Hope that helps.
Terry

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