Stronger alternative for 20MnCr5 is EN353 3

[namelessudhay]

Hi,

I am facing a gear root bending failure in one of our commercial application transmissions. Due to practical reasons, changing any dimension is impossible.

The gear is made of 20MnCr5, gas carburized. Couple of seniors are suggesting to go with EN353 instead. Two reasons:
1. EN353 is stronger.
2. It is easily available.

I am trying to understand how EN353 is stronger. I see the carbon percentage of both the materials to be closely same. However, these are some material composition differences I find:
EN353 has got:
1. Less manganese
2. More Phosphorus
3. Nickel present - 1 to 1.5%
4. Molybdenum present - 0.08 to 0.15%

I am not a metallurgy guy. I would request some experts here to explain these chemical differences help EN353 to become a stronger candidate. Thanks.

 

[EdStainless]

You left out that they similar Cr content.
When you carburize the 353 you are forming a high C alloy steel, very strong, tough, and deep hardening.
The Mo really improves hardenability (even at 0.17% C) and the Ni makes the alloy so that you can get uniform properties and good toughness in deep sections.
There are other alternatives, but as you say 353 is available and the processing won't be very different.
You carburize, oil quench, and then temper? You may need to look at temper temperatures.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[gearcutter]

A "stronger" material will not necessarily improve resistance to tooth bending failure.
The most important thing to consider is the type of loading, then figure out the best material to use.
Analyse the fracture surfaces of the failed components to determine the type of bending failure that has occurred.

I'd be very surprised if there isn't room for improvement of the gear tooth/root geometry.

 

[3DDave]

I'd look at root geometry first. Stress concentrations can overwhelm any material strength improvements.

You write you can't change dimensions, but I'll bet there are tolerance ranges.

 

[namelessudhay]

Thanks, Ed.
//You left out that they similar Cr content//
True I should have listed down the similarities as well.


//When you carburize the 353 you are forming a high C alloy steel, very strong, tough, and deep hardening.//
Isn't it the same when I gas carburize 20MnCr5 as well? I just make a low carbon alloy steel to a high carbon alloy steel, correct?


//The Mo really improves hardenability (even at 0.17% C) and the Ni makes the alloy so that you can get uniform properties and good toughness in deep sections.//
This is the exact piece of info. I wanted. Grateful to it! Will read further about use of Mo and Ni.

//You carburize, oil quench, and then temper? You may need to look at temper temperatures.//
I am not very sure of this. Since Heat Treatment (HT) team is in agreement with this proposal, I hope they have fair experience in the process.

Thanks again!

 

[tbuelna]

Gear tooth bending failures typically start with fractures at the root fillet surface, which would have a carburized case in your situation. It is important to have a good balance between case depth and core strength with any carburized gear. The optimum case depth on the tooth flank and root fillet surfaces after finish grinding needs to be carefully controlled and will depend on the gear tooth section.

To correct your tooth bending failure problem, anything you can do to reduce root fillet tensile stress levels or improve fatigue performance in the root fillet surface material will help. Even small tweaks to the root fillet surface profile can reduce tensile stress levels. Shot peening of the root fillet surfaces will improve tooth bending fatigue performance significantly, but will add cost. Anything you can do to reduce mesh transmission errors will usually reduce dynamic tooth loading and bending stress levels.

Some of these changes will require some engineering effort, but may not add cost to manufacture the gear.

Good luck to you.