12" working railroad

[rkcarguy]

Better picture of the sprockets for the rubber tracks. These were laser cut from 3/4" plate.

[Steggy]

Sprockets arrived, specs say 1040 steel.

1040 isn't mild steel. Did you order sprockets with hardened teeth?

[Harold_V]

Sprockets arrived, specs say 1040 steel.

1040 isn't mild steel. Did you order sprockets with hardened teeth?

Good observation. 1040 is considered medium carbon steel. Assuming 1040 was used (not 1018 or 1020) I wonder if the point of cut is hard. Cutting action is so fast that the metal is cooled rapidly as the cut advances, which should leave the surface hard. A swipe with a file might be very revealing. The hardened depth is likely too shallow for a Rockwell C test.

H

[rkcarguy]

Just to clarify, I was referring to the #530 sprockets I'm going to use to drive the tracks. The specs from Bike Master show 1040 steel and they appear to be machined. The Struck dozer has a chain drive on each side that goes down to sprockets on hubs that spin on a fixed axle and drive the tracks on each side.
The rubber track sprockets I drew up in CAD and were laser cut from A572-G50 plate.

[Steggy]

Sprockets arrived, specs say 1040 steel.

1040 isn't mild steel. Did you order sprockets with hardened teeth?

Good observation. 1040 is considered medium carbon steel. Assuming 1040 was used (not 1018 or 1020) I wonder if the point of cut is hard. Cutting action is so fast that the metal is cooled rapidly as the cut advances, which should leave the surface hard. A swipe with a file might be very revealing. The hardened depth is likely too shallow for a Rockwell C test.

H

It's unlikely a thermal cutting process (flame or laser) would be able to sufficiently harden the material to give it good wearing properties. In a very simplified explanation, steel that is to be hardened has to be kept above the eutectoid (aka transformation) temperature (~1350° F for most steels) long enough for the material to become austenitic. If the steel is cooled before austenitic transformation occurs it will not harden. Flame-cutting will not raise the local temperature above the eutectoid point long enough for austenite to form—laser cutting even less so, which means little-to-no hardening will occur. Also, the rate of cooling is a factor in the degree of hardness attained. Air-cooling will do little to the steel's metallurgical properties.

The alternative, carburizing (case hardening), while not requiring that the steel be heated beyond transformation, requires that carbon be infused from a secondary source. That isn't going to happen while teeth are being cut.

Speaking of tooth-cutting, solid sprockets (meaning ones with integral hubs) are usually hobbed from machined blanks. Small-diameter plate sprockets are usually stamped if the material thickness allows it. Otherwise, plate sprockets are cut and the teeth undergo secondary machining to get the correct profile.

[Harold_V]

While I agree in principle with your comments, I have had enough negative experiences with flame cut materials to understand that the HAZ of steels is often hard, too hard to machine in some cases. It is that condition that gave me a glimmer of hope that the teeth may resist wear.

I attribute the hardness to the self quenching of the material, as it is slow to be heated and quickly cools the cut. It does not rely on air quench, which would be far too slow for any appreciable hardening, assuming the entire piece was heated to a critical temperature. In flame and laser cutting, that is rarely the case, if ever.

Your thoughts on this phenomenon?

I understand pack hardening. It's slow business, adding carbon depth of about .010" per hour of exposure.

H

[Steggy]

While I agree in principle with your comments, I have had enough negative experiences with flame cut materials to understand that the HAZ of steels is often hard, too hard to machine in some cases. It is that condition that gave me a glimmer of hope that the teeth may resist wear.

I attribute the hardness to the self quenching of the material, as it is slow to be heated and quickly cools the cut. It does not rely on air quench, which would be far too slow for any appreciable hardening, assuming the entire piece was heated to a critical temperature. In flame and laser cutting, that is rarely the case, if ever.

Your thoughts on this phenomenon?

I understand pack hardening. It's slow business, adding carbon depth of about .010" per hour of exposure.

H

My comment should have been worded "Flame-cutting will not raise the local temperature above the eutectoid point long enough for significant austenite to form..." Sometimes what I am thinking fails to make it to the page.

The cut gets hot enough to cause transformation, but doesn't stay in the austenitic state long enough for significant carbon diffusion to occur. Ergo any hardening that occurs during cool-down is very localized in nature. Quenching is mostly to air, due to the huge temperature gradient.

[Harold_V]

That's the point I was making, although I tend to not agree with the air cooling part. Sure, the absorbed heat is dissipated by air, but the overheated area is cooled by the heat travelling to the cooler area adjacent to the cut. The part self quenches due to the huge temperature differential. Doesn't really matter how it cools so long as it's fast enough to lock in the hardened condition, no matter its depth. That's the nature of the carbon cycle, at least as I understand it.

There will be minor hardening, assuming there's a reasonable amount of carbon present, albeit shallow in depth. It's obvious when a cut is taken on flame cut materials, and can be to advantage in this case.

I've also machined a fair amount of flame cut material that was subjected to an annealing process after flame cutting. There's a world of difference in how it machines.

H

[rkcarguy]

I noticed that when I was deburring the laser cut track sprockets that the trailing edge (down side of plate on the laser table) is a little harder than the top edge. Obviously more heat builds here. Generally though, the whole part doesn't get hot enough to do much, larger pieces with few cuts are only warm to the touch after a few minutes.
The OEM parts are typically castings that drive the tracks, but I have seen plate cuttings as well. I think the sprockets will last as long as the tracks do and they were about $35 each in plate and cut time so not a big expense.

[rkcarguy]

Sort of doing a mockup here without axles, just to place the channels so the tracks are as far forward as possible without interfering with the fenders/footwells.
2nd picture is the adjustable motor plate brackets, made these in stainless so they don't rust as they are adjusted.

[Harold_V]

I noticed that when I was deburring the laser cut track sprockets that the trailing edge (down side of plate on the laser table) is a little harder than the top edge. Obviously more heat builds here. Generally though, the whole part doesn't get hot enough to do much, larger pieces with few cuts are only warm to the touch after a few minutes.

That's pretty much in keeping with oxy/acet cutting. In spite of the fact that the material is literally burned away, and there is little free carbon, there's almost always a little hardening at the cut. There's more than enough heat to achieve the critical temperature. If there wasn't, the cut wouldn't be a success.

For wear characteristics, the slightly hardened area can't be all bad, even if it's relatively shallow, and it often is. It should add longevity to the sprockets, which, in turn, should allow the tracks to live longer. A win/win situation as I see it.

Interesting project you've undertaken. Sure is nice to have the laser cutter to help in your endeavors.
Continued success!

H

[rkcarguy]

I would case harden (carbon pack) them if I still had access to the forge at technical college, but I think for this application the A572-G50 will work well. Can look into AR plate in the future if wear seems to be premature.