Heat treatment has weakened my crankcase? 4

[RGA1200]

Hi All,

I have a sand cast aluminium motorcycle crankcase (believed to be A356 T6) which had been 'ventilated' by an exploding component. I had the case welded and then found it had warped by 2-3mm and upset the alignment of bearing bores at the split line.

Through various channels including this site I decided on a process of precipitation heat treatment. I made steel mandrels which located on the bores for the three shafts, then baked it (in my kitchen oven!), initially to a heat of around 160C for 6 hours, unfortunately this achieved nothing. The second attempt was to a temp of 250C, gradulally bringing the temp up 25C per hour, holding for 24hrs, then reducing 25C per hour again. Temp monitoring was by 4 K-type thermocouples at various locations. My kitchen smelt pretty bad!

Happily the distortion come out perfectly and the case was dead flat again and the bearing bores were now all in line again. I point out that this case is very rare and no longer available. I seemed to have saved it.

I assembled the engine and ran for 500km. Upon removing the top end to adjust valve clearances, I found one loose head stud (out of 12), it had torqued up ok on assembly, but with a few heat cycles had stripped.

Is it possible that my heat treatment has softened the material? I understood that I had not gone high enough in temp to affect the temper. This engine had numerous stripped studs in other places, so this is possibly just a residual problem from earlier over-tensioning by previous owners. There is not much room for a time-sert, so you only have one shot at it. If the material is possibly weakened I need to address that before doing anything else.

Any thoughts would be most appreciated. Apologies for the long post.

Thanks in advance
Steve

 

[sidnoid9]

Hi Steve, I can't be much help regarding how you may or may not have change the characteristics of the base material. But based on what you have said about other stripped studs, I am taking the liberty to treat this as a pre-existing condition. VW aircooled engines use a similiar design and "pulled" head studs were common. In applications where there was no room for an insert known as a "case saver" we would use a self tapping stud, which was slightly larger than the stock diameter but would still pass thru the cylinder and head. These studs were made specific for this application. The self tapping end was a courser thread. We used these only as a last result to save an block from the junk pile. Perhaps you could do a search on vw self tapping head studs to see if there is one available with the proper od and length, or to just get ideas to design your own. Also try to see if you could adapt a tridair insert. (ms51830, ms51831 or similiar in metric)Be sure to use the proper thread lubricant when torquing unless your owners manual calls for a dry torque. Overtorquing by a previous owner could have been the root cause of thread pull out.

 

[Metaljon]

Your process of heat treatment is wrong. You can't recover the properties once you have welded and softened the material. To do this properly you would need to solution anneal the material first followed by ageing. For this alloy it requires a solution temperature of 535 deg C. The correct ageing temperature for this alloy is only 152 -157 deg C. You have now overaged it as well into more of a T5 condition. Consequently the tensile properties have been degraded by 40%. Sorry I think you have "over cooked" it.

 

[CoryPad]

Metaljon is correct - the welding has weakened the casting. If you are to recover the strength, you need to perform the solutionizing treatment followed by a quench back to around room temperature. Then, you perform the aging treatment.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[RGA1200]

Thank you all for your contributions. I based the second treatment on a quote of Aerospace Material Specification AMS2771 stating solution heat treating at a temp of 540C for six hours before quenching in hot water. After quenching age at 230C for 6-12 hours. I performed only the latter part (a correction on my earlier post, I went to 230, not 250C), as I figured most of the casting, with the exception of the small area around the weld, was still more or less at T6. I'm now not sure of whether that info was for the T6 condition, seems not. :-(

If I timesert the holes, I gain approximately 18% on shear area. But if I have brought the casting back to more like a T5, that will still be an inferior overall thread strength, correct? In general terms, this casting is enormously over-dimensioned, I don't think overall strength is an issue aside from the threads.

Does it seem feasible to be able to re-solutionize and age a fully machined casting without losing dimensional accuracy?

regards
Steve

 

[TVP]

No.