huangyhg

stress relief fixtures
we are having large steel pistons for landing gear go out of round during baking or shot peen, is there any guidance on use of holding fixtures such as used in heat treating?
jim
shot peen always changes the part dimensions, if you're round going in and not afterwards you may be unevenly shotpeening. on very tight tols +/-.0005 dia or less, you may not be able to hold tols after a shot peen.
if it's in heat treat you can make big fixtures (they'll try to warp too, so be careful) but you might consider a light final machine after ht(depends on the nature of the ht). uneven heat up or cool down also will warp parts as thermal strain build ups and tries to relieve itself, but plain cylinders shouldn't be a problem. you might adjust the cycle, but i would check with your metallurgist first.
jim,
i suggest that you use sub-harmonic stress relief immediately before heat treating your large steel pistons. it is a mild process that itself does not cause any treatment distortion, plus, more importantly to your case, sub-harmonic processing relieves any thermally induced stress which if left unaddressed before heat treating, would cause treatment distortion following the heat treating. our own studies have shown a decrease in distortion through the heat treating process by as much as 70%. so in effect, the heat treating operation is a lot like machining, if the workpiece has stress in it before heat treating, then there will be distortion following the heat treating process. this sub-harmonic process is used frequently to help prevent distortion caused by heat treating.
by the way, sub-harmonic stress relief is already being used on landing components.
as far as using the sub-harmonic process to reduce the distortion that is caused from peening, that's different. i am not optimistic that you would gain anything in using the sub-harmonic procees for this aspect of your application. that's addressing "mechanically induced" stress which is outside of the capability of the sub-harmonic process to control.
best of luck
bti guy
this question has lots of missing information...
steel alloy/spec??
part size and general layout??
any "as-forged" surfaces left-over... or minimum material
removal requirement???
temper and "form" prior to machining [hardness/ftu #s {annealed for rough machining, etc]? forged block, die-forging, plate, bar, casting, etc?]?
heat treat spec, exact processing sequence & media; and final temper [hardness/ftu #s]?? decarburization requirement? ammount of material allowed for finish machining [presumes part heat treated in rough-machined state... then finish-machined].
stress relieved after finish-machining/deburing???
machined finish [asme b46.1 ra?] requirements??
shot-peening reqirements/specs/media??
corrosion protective finishes [platings]??
special manufacturing controls or sequence requirements (ie: sae arp1631)?
regards, wil taylor
if anyone needs to know something about dimensional stability of jet engine parts, during the manufacturing cycle (distortion from residual stress, machining, peening, heat treat, etc), i have some knowledge. (and a lot of gray hair to prove it)
seen some success using cold treatment in conjunction with heat treating of some steels to stabilize dimensions but that is subject to many of the questions wktaylor raised. u may not be able to use this method and meets specs. good reference is

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]