[From an e-mail sent to Dr. Peener]
I need to resolve some doubts. Could you help me?
We produce parabolic leaf springs and we have determined through many years and by many fatigue tests that with an arc height of the Almen C strips of 0,20 ± 0,04 mm, the results in fatigue of our springs are good and fulfill with our customers expectations, obtaining about 1200 Mpa of compressive residual stresses at 0,15 mm of depth.
To achieve this, we use a 4 turbines machine (37 KW, 380V, 2500 rpm, 500/135 diameter , 8 blades with a normal intensity of 75/25 Amp. full/empty.
This gives in theory a shot speed of 87 m/seg that I can't change because 2500 rpm is an immovable value.
The shots we are using are 1,19 mm and they are automatically removed when arrive to 0,60 mm.
The conveyor velocity is normally 4 m/ min.
The coverage on the leaves it seems very good, perhaps 98 %???
But when we make saturation curves with 1,2,4 and 8 times of exposure and following the 10% rule, the Almen C intensity obtained is about 0,35 mm which is very high and probably not very good for the surface of the spring. Of course we only pass 1 time the springs through the machine and in these conditions we are obtaining on the strips 0,20 ± 0,04 mm.
Questions:
If 0,35 mm is the real Almen C intensity of the process ( in saturation) and perhaps 0,20 mm is only the arc height of the Almen C strips, what is the intensity on the leaf springs?.
If we don't saturate the leaf springs, the process is not correct?
If the springs were passed through the machine 4 times in order to saturate or we reduce the conveyor velocity until perhaps 1m/min ( of course this is absolutely impossible under the productivity point of view) what would be the benefit in terms of quality? Excessive coverage may create surface damage, where is the equilibrium?
Are we wasting shots and reducing life of machine components by using extremely high Almen intensity without necessity ?.
How to do to reduce the Almen C intensity of the process from 0,35 to 0,24 or maximum 0,26 if we can't reduce rpm of turbines neither shot characteristics ( hardness, size, etc) ?. Reducing the shot flow could affect negatively to coverage, perhaps reducing shot flow and conveyor velocity as much as possible ?.
Your only good option is you could reduce the size of the shots. The smaller shot size has less energy (still at 87m/sec) and therefore the peening intensity will be reduced. However, you will have many more shots/kg of media and therefore you will be creating many more dents/minute. You should then experiment with conveyor velocity and shot flow rate. You might want to run the conveyor at its highest speed (highest production rate) and then experiment with shot flow rate. You want the lowest flow rate that just gives you 100% coverage at the high conveyor speed. You might be surprised that the shot flow rate could be quite low. You'll have to experiment.
Where is the mistake, perhaps in the design of the turbines (rpm should be easily controlled) or the shot size is too big and needs a lot of denting time ?
Thank you for your help
