Heat treating A36 steel rings

[Limbowhale]

Can anyone give me some guidance/advice?
We make a ring gear 21" od x 19-1/2" id x 1-1/4" thk from HT 4340 (320-360bhn) which is about the hardest that our gearcutter can cut. We burn the rings from annealed plate then HT the rings and then we machine them and gerarcut them

We have a customer who has started making their own gears from A36 plate which they say they are heat treating to 56Rc and carburizing to .030 dp.

They claim their gear outlasts our gear by 2:1

I am not an engineer,but:
1) does this seem like a true statement?
2) the reason we use a through hardening alloy is because we can HT the material before we machine it and not have problems with warping.
3) They are apparently machining and gearcutting before HT and Carburizing - how can they do that without making potato chips out of flat parts?

We want to make the best gear we can for them.

 

[Metalmeister]

Limbowhale,

This guy is pulling your flipper. The maximum hardness of steel is essentially a function of the carbon content. A36 has a max carbon content of about 0.28%, which would produce a maximum theoretical hardness,assuming 99.9% martensite transformation, of about 52Rc. Since it is unlikely that his quench is that effective, I would venture a guess that he would be lucky to achieve an as quenched Rc hardness in the mid 40s. You are using a far better material for the reasons you indicated. your material is also much tougher and would resist chipping and fracture of the gear teeth.

 

[BobM3]

But his customer is carburizing the steel - doesn't that increase the carbon content near the surface?

 

[dbooker630]

Yes 4340 is better material than A36 in a head to head comparison. I don't know the application and what durability is required, but if the customer is to get the most out of carburizing it is probably with a CP of at least 1.10% in order to get the reported hardness and depth. They may not be telling you that they are also shot peening to improve fatigue life.

As far as the 4340, the noted Brinell is on the low side for a gearing application. I would look into induction hardening of the teeth of your gear and then challenge the customer again.

 

[Metalguy]

I'm surprised their carburized gear doesn't last FAR longer than yours. How does your fail--tooth wear? Shotpeening won't help there. You need surface hardness. You might try low-temp. nitriding the 4340, which might outlast their gear.

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling

 

[redpicker]

This thread reminds me of a problem a maintenance department had with a pinion from a hand hoist at a previous job. It seems this particular pinion failed after many years of service and the maintenance department was making new ones, which were lasting only a few weeks. They were using a 0.40-0.50 carbon Cr-Mo steel at 300-350 BHN. I suggested to the maintenance supervisor that he flame/induction harden the teeth (which were what was failing). He argued that it wasn't wear, that the teeth were breaking off and making them harder wasn't the answer.

I told him that fatigue was probably the base problem and that the hardening operation would stop the fatigue. I was convincing enough to get him to try it.

As far as I know, that pinion is still in service (although this was over 10 years ago, so it wouldn't surprise me if the entire hoist had been replaced). That maintenance supervisor was very surprised.

The bad side is that he started coming to me with all of his problems after that...

rp