Material and heat treat recommendations 1

[markarnold]

We manufacture a series of pneumatically driven rack and pinion actuators. When the series was originally designed, the engineer chose 1144 stressproof as the material for both the rack and pinion gear. He also specified flame hardening .030" to .060" deep to 50-55 Rockwell C scale of the tooth flanks. This design has been trouble free for almost 10 years until recently. On our highest volume size we are breaking the teeth of pinion at the root of the teeth. I suspect a heat treat method. We've used the same supplier for several years with no problems. Their claim is there has been no change in material or heat treat. We did find after cutting a pinion gear in half that the hardness extends throughout the tooth and just below the root. Below the root it transitions very quickly to 30-35 Rockwell C. The gears haven't fatigued as we are seeing failures upon initial startup testing at our facility. The supplier's heat treater claims that the 1144 material does not case harden very well and that the heat treat goes much deeper. I am considering change the gear to an 8620 alloy that is case hardened via gas carburization, but I am both concerned about the lower tensile strength and the fact that they had not failed before. I would like some suggestions as to where I should proceed next.

 

[diamondjim]

Did the fillet in the root of the pinion
change. Is it a full fillet radius?
Did the cleanliness spec of the steel for
the pinion change? Did they change their
inductor design? Seems strange that all of
a sudden you start running into problems.
Did they change the source of their steel.
I assume the pinions were hobbed with a full
fillet radius. Did the core of the material
specification change ie did they go from
H&T steel to normalized steel?

 

[markarnold]

Diamondjim, thanks for the reply.
1. The fillet did not change.
2. It is a full radius fillet.
3. Our specification on the steel did not change. Our supplier supplied certification of material which we had independently verified.
4. The heat treater for the supplier claims to not have changed any of their methods.
5. The supplier claims no material change.

My concern is also the sudden problem. We have changed absolutely nothing on our end and have verified all dimensionally fits as well. Is there any way to post a picture on these forums? If not, maybe I can post a link to some items that may be useful.

 

[scarecrow55]

I would get a met lab check it out, give them an old one to compare. Like DiamondJim says, something has changed.

 

[tbuelna]

markarnold,

Your supplier's heat treater is correct, 1144 is not a good choice for flame hardening. 1144 obtains its properties through a cold working process, as opposed to thermal treatment. It is relatively free machining and stable, and is meant to be used "as machined", with no further heat treat.

I assume you chose to flame harden the part due to its shape/complexity. If you're design/processing only permits to locally flame harden the gear tooth area, a better material choice would be 4150.

Although carburizing steels like 8620 or 9310 would produce a very high quality gear, I would only use them as a last resort. They are fairly expensive. Also, the carburizing process requires putting the entire part in a furnace and then quenching it, likely causing significant distortion.

Good luck.
Terry

 

[Carburize]

A major problem with flame hardening is the shape of the hardened zone. I have seen many failures by root bending fatigue where the load surface is hardened but the root is not. This gives a bending strength no better than that of the base steel and some would actually argue that because there is a "metallurgical notch" in the area of the root at the intersection of the hard and soft material that the tooth bending strength is actually degraded by the hardening process. Get a sample of broken tooth and polish and etch it to look at the hardening pattern.