Our intent on shot peening is to introduce some compressive stress and also make the end of the valve hard enough to withstand the hitting force of rockers.
This is requirement of one of our customer ‘Mercedez Benz’. The drawing merely mentions ‘end of the valve to be shot peened’. It does not mention anything about the requirement after shot peening. We derived our own internal specification from our trials, that hardness measured on a Vickers scale at 10Kg load will increase by about 50 counts for a martensitic valve alloy and by about 100 counts for an Austenitic valve alloy. (I would not know if this is right, but to start with we decided on this).
One more requirement from another of our customer is to ‘cold compact’ or ‘work harden’ the seat area. Here the customer print calls for a seat hardness of 400 HV10.
Regarding your question about failures-fatigue failure at tappet end has been sporadic and very few. But we have seen quite a few cases of fatigue failure at seat.
But our primary objective in this case has been not for fatigue failures but to look for an opportunity to replace ‘Hard facing’ cobalt alloys in the seat area. May be the improvement in fatigue that we will get will be a bonus add-on!!
