Rock Drills- Material Advise

I've built many different types of rock drills, pilots and augers over the years. Larger diameters, up to 96". Core-barrels too. Usually the construction is 1045 steel for the barrel/chisel edges, and Bauer (sp?) pockets welded in place. According to independent testing, I've been told by people a few links removed from the source that the Bauer pockets are 300Rc. I believe they are confusing Rockwell with Vickers.

In any event, I'd like to try to make some rock auger edges out of a harder steel than 4140. The current iterations are 4140HT, machined while annealed. Hardened and stress relieved to 35Rc. My heat treater said that it's very difficult to get a narrow/reasonable hardness range for 1045; 4140 on the other hand can be HT to within 2Rc. My chem background tells me this is due to the increased Chromium content of 4140 and the questionable consistency of 1045.

If I want an extremely hard yet durable rock edge into which to machine pockets for bullet teeth, what material should I use? I don't trust the current 35Rc 4140 versions. My thoughts:

1: Machine of 4140 and HT to 50Rc

2: Try AR400 and hate life all the while machining it and then HT to 50Rc

3: Change processes and sinter them like the Bauer pockets are.

4: Use a different material and machine?

Thanks for any advise or expertise.

QT: [I've built many different types of rock drills, pilots and augers over the years.]
Can't imagine anyone having more experience than you.
I would think 38/40C good but I don't know anything about them.

L6 tool steel still has very good toughness when hardened and tempered to the Rc 55-57 range. Machining is no more difficult than O1 tool steel and it heat treats predictably. Price is reasonable as tool steels go.

S7 probably has a bit higher shock and wear resistance than L6. I've never used it though so I can't say anything about price or machinability.

Thank you for the response. I have turned S7 for die pins, rivet die blocks etc and it was awful to machine. Machining the 4140A was sweet, just like 1045. S7 I can't imagine being a good time, or I'd have to replace drill points after 3 pockets (average # of pockets is about 95).

If I machine the L6 annealed and it cut like 4140A, that's worth a shot so long as it outperforms 4140 @ 50Rc. The main issue I'm having is the pockets where the teeth go actually pulling/ovalling out. I suspect case-hardening it not possible for this application as it requires consistent strength throughout and the hardened surface/shock absorbing middle doesn't have much advantage here.

The pockets from cat are cheaper than you can make them.

For a road reclaimer.

Build up the edges with hard face rod. You can get rod for severe abrasion that is orders of magnitude more resistant than any base material you can still fabricate. This way you retain the toughness of the base steel. If it is just the pockets that are going out of shape, make a pocket bar that can be welded to the flighting. The pocket bar can also be made from an alloy for greater strength than a material that can be fabricated.

Use steel that readily weldable then hardface it. An alternative might be replaceable hard edges cast from an appropriate wear resistant alloy.

gbent said:

If it is just the pockets that are going out of shape, make a pocket bar that can be welded to the flighting. The pocket bar can also be made from an alloy for greater strength than a material that can be fabricated.

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This is what's happening. I'm making the pocket bars and they are welded to the flights but the bores are being pulled oblong by the teeth. It's not the abrasion resistance of the steel so much as the low strength. If I hard face 4140HT, I'll lose the HT and gain little by the abrasion resistance alone.

Since you are welding to the flights, I would probably try 4340 and shoot for about 45Rc to start. Adjust up or down depending on if it cracks or elongates.

**I have no experience with rock drills.

OK, I think I'm beginning to understand. Your material has to be weldable. Your material has to either be heat treated after welding or the hole portion has to be kept below the tempering temperature during welding.

What if you keep your current processes, but make the holes in the pocket bar larger and bush them? You could start with your 4340 at Rc 50, but S7 may be a better choice. Even if you only have room for a thin wall bushing I think the harder bushing and greater surface area will prolong the life of the hole. I think I would try for a heavy press, say .001 to .002/inch.

I don't think you will like AR400. It is a mystery metal. About the only controlled property is the 400 Brinell hardness. Even if you get lucky on the machinability of the first batch, the next batch may be different.

I don't know anything at all about your field or the application. But am somewhat curious about your experience with S7. For small parts, I prefer it, if it is affordable for the app. I always thought the normalized condition (as received in drill rod and small bar/flats) to machine about ideally. About like W steels. Maybe I just have not used big enough pieces with big enough pockets to cut out? I do like machining 4140 prehardened 28-30 Rc. I use that for bigger stuff than S7 where the hardness is acceptable as is.

smt

You are never going to get the pocket to stay until you use forged blanks like cat does.

Why reinvent the wheel?

I made some long thin push pins out of S7 that experience a High shear load bout a year ago.
Haven't heard back anything negative from the customer yet. Made me a believer out of that material! It kinda sucks machining it, but better than some materials...

Heavey Metal said:

You are never going to get the pocket to stay until you use forged blanks like cat does.

Why reinvent the wheel?

Click to expand...

I thought the same thing about a year ago and we tried welding the pockets directly to the edge of the flight. The issue was a single pocket would wear and it takes just as long to gouge out a single pocket as replacing the entire edge. There is no clearance on the cutting edge to accommodate any type of reinforcing bar that would effectively weld all the pockets together. So far they've liked replacing the entire edge, but some gnarly rock has destroyed the previous versions.

Here's a photo of a generic Chinese auger with pocket bars welded in two sections.

Status

Thank you for all replies. Based on the above I'm going to nix the AR possibility and HT 4140 to 50Rc. We'll see how that holds up on the next run. I believe the best next course of action should the solid 4140 fail is to follow gbent's suggestion and make the base edge from 4140 and press in S7 or L6 sleeves.

Rstewart- yes, S7 is an excellent material for the physical properties in service but machinability (as you've found) is not one of them!