Material Selection help 2

[kantipacific]

Dear Group,
I just joined the group and am looking for your in material selection (cost and performance balance).

Application: Planetery gear train.

Can you please suggest which case is best suitable for gears:
1) 40Cr (AISI 5140) alloy steel gears with HRC 45-50
2) AISI 4140 Cr-Mo alloy steel gear with HRC 46-48
3) SCM415 (AISI 4115 or OLD SCM21)Cr-Mo steel case hardened to HRC 55-58
4) AISI 4340 Ni-Cr-Mo steel HRC 46-48 (I think this might be a overkill?)

Thank you

 

[TVP]

It all depends on the stresses (contact and bending) and lifetime expected. What are the dimensions of the gears?

 

[kantipacific]

Thank you for the response TVP.

Its 3 stage planetary reducer(300:1)
1) Sun: 0.709", Planet: 1.595"
2) Sun: 0.709", Planet: 1.595"
3) Sun: 0.827", Planet: 2.067"
Input max torque = 115 in-lb RPM = 900
Life expectancy of max. 10,000,000 rpm (K=1.4).

I am interested in knowing which steel is more suitable and which steel is more economical. a)Case hardened steel b) Thru hardened

 

[EdStainless]

There are factors other than hardness to deal with. You need to look at the contact stresses involved. It may be that case hardening of the larger parts would be a good option.
You also need to think about toughness and internal fatigue. How clean is the microstructure, how good of a surface finish do you need, and what impact loads might you need to deal with.

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[kantipacific]

Thank you EdStainless.
Can anyone suggest 4140 steel hardenablity specs and consequencial impact on the toughness.
I am looking for two ranges HRC 45-48 and HRC 52-56
Thank you

 

[arunmrao]

Recently I supplied a few (100 no.s)small gear castings in AISI 1040 grade. The application of this gear is in a locomotive exhaust ventilator, which opens and closes as per the temperature conditions in the engine.

The surprising note was the castings were used in normalised conditions without any heattreatment like hardening and tempering or surface hardening after machining.

When this issue was brought up by me, I was advised that the practice in US loco industry is to use 1040 gears without heattreatment for this application.

 

[Metalguy]

kantipacific,

4140 is usually hardened by oil quenching from 1550 F and then tempering at ~750 F for your first hardness requirement, or 500 F for your second. You can expect pretty low impact values for either treatment.

Also, when 4140 is near 2" or more in thickness it may not harden very well. Check the Cr, Mo and Mn amounts. If they're on the low side of the spec. thick sections won't harden well at all.