Critical Temperature For Heating Spring Steel

[MTUJeeper]

We have a coil spring made of 5160H alloy steel. In our shop we powder coat the spring, which it is held at 400*F for roughly 25 minutes. We have been experiencing some spring breakage and one thought was we used to paint the spring and bake it at 400*F for 5 minutes. There were some other changes made at this time as well, but one thought was that there could be a metallurgical process that is causing breakage. I have looked briefly for a phase diagram for this alloy but have not found anything. Would this temperature/time exposure cause any significant changes to the material properties? The spring sees a great deal of bending force as it is not used in a conventional linear fashion.

Thanks,
Christian

 

[TVP]

No, 25 minutes at 205 C (400 F) will not cause any metallurgical (microstructure) changes for this alloy. This is likely to be a hot wound/formed spring, which means that it is heated above 830 C, formed into the spring shape, then quenched into oil. Subsequent tempering is performed in the range of 400-500 C (750-930 F). Any temperatures below this tempering temperature will not cause microstructural changes that will change the strength. I think you already have the answer for your problem: large bending forces will cause large stress concentrations that significantly degrade spring durability.

 

[MTUJeeper]

I wound up finding a diagram for this alloy after posting, but I appreciate your reply. It is a coil spring on one of those animal shaped rockers at the park. We've been having the spring fail, the casting that holds down the spring fail, and also the bolts shearing off. From my digging it sounds like an installation issue, as we never had trouble with it before, but who knows. I appreciate your help.

 

[metengr]

We have been experiencing some spring breakage and one thought was we used to paint the spring and bake it at 400*F for 5 minutes. There were some other changes made at this time as well, but one thought was that there could be a metallurgical process that is causing breakage.

Have you had the spring failures subjected to a proper failure analysis by a metallurgical lab? I would start with this approach because otherwise you are simply guessing.

 

[MTUJeeper]

No we have not. I don't believe we have any of the broken springs either. I have only worked here for 3 weeks and this isn't my project, its just I have a better grasp on metal working than the girl who's project it is. We had a spring from a different supplier that we did not have any failures with. Supposedly this one is built to the same specs. One problem I see is that the hold down is on an active coil, so it can compress and when it rebounds the other direction it impacts the cast clamp that holds it down, eventually causing failure over time.

 

[metengr]

If I were you establish a relationship with a local metallurgical lab. You can count on having periodic failures and knowing the root cause of these will help you stay clear of repeat problems.

 

[HBSpring]

5160H is a recognized alloy for ASTM-A-125: Steel Springs.

At my place of employment we hot coil this alloy into springs,
they are quenched and then tempered. We aim for a hardness
of RC 42-48.

I know of many of these Rocking Horse applications where 'someone'
has tried to weld the Spring Steel to the Base Plate.

Not so good. Best Spring designs have the ends with a partially
open pitch. One end latches to the underside of the horse,
the other end latches into the base plate.

 

[MTUJeeper]

Ours are ground flat on both ends and clamped, no welding here, that would be a bad idea...lol.

 

[dbooker630]

The 5160 may also be quenched in polymer. This alloy is also used for stabilizer bars on police vehicles. When you get the spring back (hopefully), you or your selected test lab may want to check for any crimping marks that could be the source of a quench crack.