Reduction of Residual Compressive Stresses

[NickE]

Hi all...

Does anyone have any information or experience with room temp reductions of residual stresses. We mfg. small thin parts out of modified 1095 steel. some of these are shot blasted on both sides, immediately after the peening process the parts are checked for flatness. our customer has stated that 2 in 2000 parts are not meeting flatness spec upon installation (may be as long as 3 months after shipping from here.)

Any ideas? All I can figure is that the residual stresses are relaxing unevenly causing the parts to warp before assembly.

nick

 

[unclesyd]

If your shot-peening, compressive stresses are what you are after. If you do anything after peening you are defeating your purpose.
2 in 2000 is low rejection rate. Is this after your are his inspection?

If the part in question has a lot of work prior to the peening you could have combination of stresses that is causing the problem. You could stress relieve the part prior to peening.

Peening is also used as forming method on thin parts and close control is required to control the intensity which controls the distortion. Also the size of shot plays a big role on thin sections

If the 1095 is heat treated prior to peening you could be picking up some distortion from it. Double tempering would help. Changes in cross section can be hard to shot-peen uniformly.

Come back with a little more information on your 1095 part.

 

[NickE]

Edits and corrections.....

Part is not 1095 as i previously stated. Hardened and tempered Modified 420ss,.015" x Coil, w/ HRC50-55, Ultra-fine polished. Stamped in carbide dies using custom clearances. Part is finished to remove stress raisers, (polish lines, tooling marks) this finishing process imparts even compressive stresses to the whole surface. The parts are then selectively shot blasted to increase surface roughness in specific areas (both sides). parts are then inspected for surface defects (in the not-blasted area) then sorted for flatness using 50% of the customers tolerances. Part is completely flat (or supposed to be, its a reed valve{ ie. no forming})

Parts are packed in VCI paper in a stack and shipped, they may go through normal temperature variations in shipping. 2 of 2000 are failing fit function after unknown time on the shelf. (Assembly fails) at our customer.

the shot blast process is performed using a Silica Media.

nick