Material selection for vehicle suspension springs 2

[Tads3]

Our company is changing the supplier for the springs used in our vehicle suspension.

The drawings for the vehicle specify DIN17221 50CrV4 for the front springs and DIN17221 55Cr3 for the rears. The new supplier would like to use BS970 735A51 chrome vanadium steel for both front and rear springs.

The steel favoured by our new supplier would seem to be a good match for the front springs (composition and properties are almost identical) but a poor match for the rears.

Unfortunately as the vehicle specifications are old and inherited from a previous manufacturer we are unable to say why a different steel spec was required front to rear. Stress levels experienced by both springs are similar as is the design.

Any ideas?

 

[swall]

Many times, material selection for springs is based on availability of a certain alloy in the wire size needed. Also, with suspension springs, both cold coiled (from heat treated spring wire) and hot coiled springs are used.In the U.S., SAE 5160 tends to be the alloy of choice for hot coiled suspension springs and SAE 9254 tends to be the rule for cold coiled springs.So in this case, material choice goes with the process choice.

 

[Tads3]

I don't think material availabilty is the reason since the vehicle and the drawings have been around for 20+ years and the previous manufacturer was pretty good at noting changes due to things like stock availability. Also both front and rear springs are hot-rolled.

Is there difference between a chrome and a chrome vanadium steel that would account for one being more suited to use on the front compared to the rear? The vehicle is a 4x4 load carrier with a fairly constant front axle load and a variable rear axle load compensated for by air spring assistance.

 

[Metalguy]

For your purpose, V tends to keep the grain size smaller (beneficial). Are you having the springs shot-peened? If so, make sure the ID is peened very well, since most fatigue fractures start on the ID.

 

[swall]

Tads3--Both of these steels will work just fine and both can be hot coiled. The only advantage I can see for using the CrV vs the Cr would be if one of the springs was heavier section bar(wire).It would be advantageous to use CrV on a heavier section becaused of the greater hardenability it posesses compared to the straight Cr steel.Also, you can get higher post temper hardness out of the CrV if both materials were tempered at the same temp. Only disadvantage I can see with the CrV is that it is more expensive than the straight Cr steel.Also, if you are tapering bars prior to coiling, the machinability on the CrV is not as good as the Cr steel.

 

[Tads3]

Thanks for your help guys. I've awarded swall the star as I think he's found the crucial difference. Bar diameter is sligh tly larger front than rear. I suspect this was the reason for the need to step up to CrV on the fronts.