Heat Treatment Queries

[alliedtrans]

Hello I am new to the forum and had a couple of questions:

1. What should be the case depth for gears which are fitted in the Corolla Car and Minivan gearbox as I am receiving complaints of broken gears. i used 20MnCr5 with a CD of .8 to 1mm. Should I used EN 353 and if so can someone please tell me the cycle I need to use.

2. I would like to manufacture the attached component and would like inputs as to what the procedure should be to achieve a hardness of 48 to 53 HRC with minimal distortion as straightening for this component is extremely difficult. Material can be EN 8D or 16MnCr5. I know companies using En 8D and using induction hardening with plug quenching.

Any help would be greatly appreciated.

Thanks
Ankur

 

[TVP]

1. 20MnCr5 with a case depth of 0.8 to 1.0 mm is commonly used for automotive gears. However, you need to understand many other issues that will affect gear life: gear geometry, mesh conditions, lubrication, surface roughness, and shot peening are all important considerations.

2. I think it is most common to use a material like EN8 (1035 SAE J403, C35 EN 10083-2) and induction hardening to achieve surface hardness of 48 to 53 HRC. I would not recommend using a 16MnCr5 and case carburizing for this part.

 

[alliedtrans]

Thanks a lot for your reply. Really appreciate it.

Should I use EN 353 for smaller gearboxes. I use 20MnCr5 for gears for trucks and it works fine but I am facing this issue for the first time as this gear was fitted in a smaller corolla gearbox.

Could you please let me know where I can get a design for the induction with plug quenching furnace from?

Thanks

 

[tbuelna]

alliedtrans,

Your 20MnCr5 steel alloy is a good choice for fine pitch carburized gears. I've attached a table giving some guidelines for effective carburized case depth (to Rc50) and hardenability versus tooth size. With a carburized automotive-size gear made of this material you should expect a core hardness of at least Rc36. I believe 20MnCr5 is roughly equivalent to AISI 4820.

Hope that helps.
Terry

 

[alliedtrans]

Thanks Terry for your answer. Was the first time I posted a query and really happy to see experts reply.

Should I increase the case depth to 1-1.2 mm for better results in service life?

Ankur

 

[tbuelna]

The guidelines provided for case depth will work very well. The case depths are optimized for specific tooth sections, and using a thicker case than recommended for a given tooth size is not beneficial. However, you should also remember that what really matters are the case properties after finish grinding, rather than just the case properties after heat treatment.

While you did not provide details of the duty cycle your gears are subject to, they would not likely benefit from a heavier case than the tables recommend. A heavier case would only be beneficial if your gears had a contact stress limitation, and this sort of problem is usually better addressed using other means, such as increased face width.

Good luck to you.
Terry