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Making gear rings that stay flat in heat treat 1

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Limbowhale

Mechanical
Jun 24, 2008
10
First the problem:
We make a ring gear that is not durable enough for our customer application. They have made their own gear which they claim lasts longer. We want to get the business back which means we have to make a gear that is equal to or better than theirs but have to be price competitive.

The facts:
The gear is 21" od x 19-1/2" id x 1-1/4" thk with external gear teeth. We make it from through hardened 4140 HT 320BHN forgings. The process we use is to machine the hardened ring and gearcut it. The problem is that at less than 320BHN the gear is not durable enough

In the distant past,we did make ring gears from carbon steels had problems with warpage when they were quenched so we reverted to buying the material that is through hardened to a hardness that is just below what the gearcutter can handle. This was OK for almost all our customers but in this application they are wearing our gears out in one year.

Our customer makes his gear from A36 stl which they carburize and quench to 52RC. They claim their gear outlasts ours 2:1 and they buy it at the same price.

As an experiment, we carb & quenched 2 of our 4140 gears on the gear teeth only (stop-off paint on everything but the teeth) and the gears did stay flat and the teeth did get a case so I assume they will be much better than the A36 gears.

My fear is that just because 2 gears done as an experiment by the heat treater stayed flat is no guarantte that if I send 24 gears they will all be flat

Our local heat treater cannot flame harden the gear due to the size and the cost to tool for it.

His quench press is too small. I am hesitant to send the gears out of the area because of time and cost considerations

Does anyone have any ideas?
 
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It appears that tooth surface hardness is the key characteristic here. I would not carburize 4140, instead through harden as you are doing, followed by induction hardening the teeth.

If your customer is using A36 they are using a far less expensive material than your 4140, as there are no significant alloying elements in A36 other than manganese to account for. A lot of the price difference may be only in the material.
 
Limbowhale--you have a whole laundry list of stuff here. There are many ways to make this gear and many material and heat treat options. You need to first get focused on what properties the gear needs in the application and how much the customer is willing to pay. Then you can shop for a material and process you can do in house or possibly with some subcontracting of the stuff that can't get screwed up.
 
Thanks Swall for replying. I know its a laundry list but had to get most of the info onto the thread to reduce the amount of questions that may have come from those who could offer help.

Our bottom line is we want to make a more durable gear that is manufacturable in our area with a minuimum of sending the parts out of town or out of state for processing.

I can carburize locally. Other treatments such as flame hardening and induction hardening require sending the parts out of town which is less desirable from a cost and control standpoint.

We can make the gear from the 4140 with carburized teeth and sell it to them for for about 200 more than they pay now but carbing 4140 may cause us to lose parts at the HT step

We can make the gear from 1040 or 8620 in about the same price range but again we stand to lose parts in HT due to warpage.

Just looking for ideas from those who are experienced.
 
Is the gear configuration such that they would stack flat and thus lend themselves to temper straightening after carburize? What is the minimum tooth hardness that is required to meet durability requirements?
 
Limbowhale,

After reading your first and second posts, I think that the situation breaks down into two different categories:

1. Keep everything the same except for one thing that might increase hardness/performance and therefore meet your customer's requirements

2. Make relatively significant changes to the material, process, sub-suppliers, etc. to optimize for cost and performance.

I think you will find that most of us on this particular Metallurgy forum are very knowledgeable about the engineering aspects as well as the other things like unwieldy supply base/logistics, potential quality issues with new suppliers, difficulties surrounding new product/process validation, keeping customers happy while still trying to make a buck, etc. While we recognize that you want Option 1 to work, it may not, and therefore you should be aware of what Option 2 entails.

My thoughts are as follows:

1. Induction hardening 4140 will almost assuredly give you the increased performance that you need. This should be able to be implemented with only a moderate amount of effort with a reputable induction heat treater.

2. 4140 is an incredibly expensive option to use in 2008 when the hardness is only 320 HB. AMM shows June prices for 4140 cold finished bars to $1820/ton compared to $1370/ton for 1018, with almost all of that difference due to the addition of ~ 0.25% Mo. Changing to 1018 and case hardening (carburizing) would almost assuredly give you a substantial cost improvement.

3. You manufacturing process may not be optimal to process case carburized gears. Some type of grinding after case carburizing is typical to achieve final dimensional tolerances and to correct for heat treat distortion.

I would seriously evaluate a case carburized gear and work on optimizing the process from raw material through machining and heat treating to obtain the necessary performance at a lower cost. Large gears can be heat treated using vacuum carburizing and high pressure gas quenching to minimize distortion. swall made a good suggestion with temper straightening. Other options can be structured around your existing machining processes or one that may integrate post-hardening machining (grinding, etc.).
 
Swall, Thanks again for the advice. To answer the questions the minimum tooth hardness is likely about 52RC

The gear profile is flat which would allow for "temper straightening".

I will check with our vendors to see if they can do it.

TVP, thank you so much for your comments. I am very interested in pursuing option 2. Our 4140 burned ring costs 290.If we were to go with 1018 it would be 190 while 1040 plate rings would cost 252.

I do wonder about what the actual savings would be if I save 100 on material but add a grinding step...not to mention the additional lead time to get it through that process. Of course if the gear is superior in durability it may be worthwhile

I am curious about the process and your take on it.
Currently we stress relieve and through harden the burned ring and then we machine it. We rough the first side, turn it over and rough the second side, then we drill it to fit a turning fixture and then we do the finish turning trying to remove material equally from both sides. After that it goes out for gearcut and then on to the heat treater who applies stop off paint to the machined surfaces except for the gear teeth. He then case hardens the teeth to about 54rc
 
Limbowhale--Your desired hardness will be somewhat problematic for temper straightening. If you made the gear from a carburizing grade of steel (or even A36) and carburized the teeth, your max tempering temperature would be limited to around 450F to avoid losing hardness in the teeth. At 450F, it will take a long time for temper straightening to be effective. If you temper straighten at 800F, you will be looking at a tooth hardness below Rc 50. But it would be worth a try.
 
mfgengear,
No, we are a small manufacturing company in Seattle. We designed and manufactured lots of different gears over the years but we use outside sources for gearcutting, heat treating and grinding.
Our various ring gears have a thin profile which tends to distort when heat treating. It gets expensive when you have invested material, labor, and gearcutting and then lose it all because the gear warps in heat treat.

Our production qty's are rather low 12-20 per year.

We started out making them from 1040 plate years ago and when we started losing them due to heat treat warpage we switched to 4140. We tried cast steel rings but had problems with porosity. We tried 8620 had the same warpage problems. Went back to through hardened 4140 which was OK for years but now we have an application requiring more durability of the teeth. All we know is that the customer is making his own gears from A36 plate and that the teeth are 52 RC.
At present we are considering dbooker630's suggestion of induction hardening the teeth on our 4140 gear which would likely add about $100 per gear wehn you consider both process and freight costs.

Based on all the input from forum members it would appear the best solution is to go forward with the 4140 gear we currently make but adding the induction hardening step.

I wish we could figure out a way to just duplicate what the customer is doing..i.e. make them from A36 plate and case harden the teeth. We just aren't smart enough to figure out a process which would reduce or eliminate the loss of parts at the final heat treat step.
 
Limbowhale,

Another possibility to try would be to switch materials from 4140 to 1541 and induction harden the teeth.

1541 does not necessarily have to be prehardened before IH if the durability is sound you may be able to eliminate the flatness concern after furnace HT.

After induction harden, polymer quench and a 325F temper you should have a final hardness of at least 50HRC.
 
Limbowhale--I would suggest you take a look at some flame hardening equipment and consider flame hardening the teeth in house. The equipment is not that expensive, does not take up a lot of floor space and does not require a high level of operational skill or maintenance.
 
Are the gears that "outlast" yours wearing out or breaking or ?? What about the mating gear? Continous service or occasional use , like a starter ring gear?
 
The company I work for quenches steel rails up to 40 feet in length. Without the proper tooling they would warp and deform.

Occassionally we get slight warps and bends, but most of the time they will straighten out in the tempering oven.

Here's a video of the quench tooling:

 
Tmoose and mscooling,
Tmoose their knockoff part is wearing out. The application is commercial fishing. The gear is part of a device used to pull in the purse seine nets on fishing boats and is driven by a smaller pinion gear mounted to a hydraulic motor.The gearset is in the open, not inside a gearbox or in an oilbath. The pinion is 8620 carburinzed and quenched to 58-62RC. This particualr application is very rough. The customer operates in 90+ heat, about 5' off the salt water. There is saltwater, mud, debris raining down onto the block from the net as it is hauled in. The gear life could be increased if they would just grease the gearing periodically but they do not. They have replaced our control panel which allowed a slow run up to speed with a control valve that is full speed off-on. They know they could change things to make it better but claim they cannot control their operators. Their solution was simple - they made a cheaper gear that lasts longer than ours
 
Limbo and all, I've read this thread as an educational experience, like most threads on this forum.

But this:

"I know its a laundry list but had to get most of the info onto the thread to reduce the amount of questions that may have come from those who could offer help."

is worth a star.
 
btrueblood,
Thanks for the kind words, I too mostly lurk in the shadows learning and this forum is a godsend for getting varied opinions and advice for problems that seem beyond our capability to solve.

I am very grateful to all those who took the time to make suggestions and we are pursuing those ideas with our vendors. At present we are looking at continuing to make our gear from 4140 which we will both through harden before machining and induction harden the teeth after gearcutting which should give us a better gear but unfortunately it won't give us a gear we can sell competitively against their knockoff made from A36.
If someone out there could suggest a process for case hardening a similar low carbon steel that would result in a high probability of flat gears after quench we would love to give it a try.
Again to all of you on this forum thanks for the help
 
Low cost process:
1. Use 1018 steel
2. Rough machine
3. Case harden (either carburize or carbonitride)
4. Finish (grind, hone, etc.)

Another option may be to nitride instead of carburize/carbonitride if the surface hardening is essentially for wear resistance only, and not for tooth bending resistance. With nitriding, and specifically ferritic nitrocarburizing (Kolene process), you could finish the gear and just nitride it for ~ 2 hours and the dimensional change will be negligible since the process is performed below the transformation temperature. A typical Kolene process (now called Nitromet) runs at 540-590 C. 4140 works great with the process, but even a low carbon steel like 1018 will achieve some amount of hardening. The nitride layer will also offer some corrosion resistance, especially if it is oxidized after nitriding. Look at the various information available on the Nitromet website:

 
TVP,
Thanks a million for the input.

It sounds like the nitriding process could be the solution if it will limit the distortion. I wonder what kind of hardness we could expect?

I am going to start looking for a vendor who could do this for us.

What would you change about this process for 1018?
1) purchase rings burned from 1018 plate
2) stress relieve (or would normalize or anneal be better?)
3) machine entire gear ring
4) gearcut
5) nitride or carb and quench


 
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