determination of reduced stresses due to extreme heating

[mmart]

Hi all..I'm trying to locate a good source of information and formulae for calculating the stresses on mild steel ofter being heated with a torch. The component is a clevis that had a frozen pin that had to be pressed out due to corrosion on the clevis.

Any information is considered helpful.

Regards and thanks in advance

 

[IceStationZebra]

I am not sure what you are getting at? Do you want to know what the residual stress is? What is your concern? Generally speaking heating steel past a certain point relieves stress, but that depends on the material, cooling time, stresses present when cooling, etc.

ISZ

 

[MrMyers]

Generally speaking the stresses are a function of many things:

Rate of heating, start/end temperatures
Thickness of material
Material's thermal expansion/conductivity/etc
Constraint and geometry

There are many interdependencies, FEA may be the way forwards. Otherwise aim for minimising stresses by trying to heat slowly, uniformly and relaxing any restraint counteracting the natural expansion direction.

Hope that helps

 

[mmart]

The heating had already taken place courtesy of a blow torch. Not exactly sure of the material type other than some form of mild steel. I'm taking a ballpark number of 50kips for yield of the material. when the heat was taken off the material, it was allowed to cool naturally without any quenching. It's hard to say how long the material was under the torch but i do know it didn't get get to the point of glowing. Material thickness is 1" with a trapazoidal geometry.

Does this help?

 

[rb1957]

it'd be nice to know if the original steel piece was tempered (high strength) or just normalised or annealled.

if it was normalised/annealled i don't think you can affect the material properties by heating it the way it was done.

if it is high strength steel there is a chance (i think) that you've reduced the properties locally.

i think the key issue is not the stresses induced by heating but the effect this has on material properties, and how well the piece was supported when the bearing was removed (and how carefully this was done, ie not with a sledge).

 

[mmart]

bearing was removed with an arbor press. The piece is either normalized or annealed. I'm only saying this because I know it didn't take much at all to hone out the inner surfaces. The pin is another story. That is a case hardened piece. The other half of the clevis was re-bored for a bushing made of oilite bronze. I would have liked to do the same for the heated piece but I'm concerned about shear tearout and available material surrounding the pin hole.

 

[MiketheEngineer]

I think you are worried about the capacity of the clevis and/or pin.

Just replace BOTH and you can sleep tonight.

 

[rb1957]

repair the hole in the clevis ... sounds like it was "mashed" getting the bearing out. if you don't know th e load on the clevis lugs, you can't do much of a calc. i'd fit a freeze fit bushing, preferreably 1/16" wall, 1/32" if the geometry won't take it ... freeze fit helps reduce the effect of the bush on the lug. i'd be surprised if the lug was particularly critical ... be careful about repositioning the bearing ... you have to get it back to the same place it was in before !

replace the pin, not many choices there !

 

[mmart]

Yeah, I am worried about the capacity..I've been working the problem using the design capacity of the actuator (20,000lbs) and using a double shear condition for the clevis and pin. I'm thinking that anybody with half a brain would design with a safety factor of at least 1.5 so I'm fairly certain that the clevis isn't seeing those kinds of loads in operation. Replacing the clevis and pin is not an option at this point being that the system has to get on a boat next week. We did freeze fit an oilite bushing on the actuator side of the clevis assembly but I don't think the other half of it doesn't have enough meat for a .032 wall thickness. The good part of this is that there are failsafes on the system so I'm not dealing with a single point of failure.

 

[monkeydog]

Hmm,

"worried about capacity ... design capacity of the actuator (20,000lbs) ... certain that the clevis isn't seeing those kinds of loads in operation ... not an option at this point being that the system has to get on a boat next week ... The good part of this is that there are failsafes on the system so I'm not dealing with a single point of failure."

mmart, would you stand under it with 20,000 lbs on it?

 

[mmart]

It's not something you can stand under..or would want to if you could. And it's a rotating load, not vertical. One end of the system is hinged on a rotating bearing, the other end (actuator side) is pushed into position much like a dump truck bed. 2 actuators run this system and are fitted with torque limiters and brakes if the feedback runs into an out of bound condition. So, if one side croaks, it's designed to stop. I'm just trying to make sure that if any damage does occur, it's minimized.

 

[Tmoose]

Might be worth doing a hardness test at various locations . And magnafluxed, too, not that I'd expect a crack from heating or beating, I mean pressing.

20,000 lbs on a lug that you don't want to reduce by boring 2/32. Hmmm.

Why was oilite chosen?
These guys say The best bronze based ones aren't good much beyond 10,000 psi static load

http://www.oilitebearings.com/materials

The holes should be finished at the same time, in the same setup to maintain alignment so the pin can be assembled. Align-bore, align-reamed, or honed.

If there are oncerns over future corrosion or galling I bet the right anti-seize or assembly paste would work nicely, no machine work required

 

[mmart]

20,000 is the maximum rating of the actuator itself, not the loads it's pushing. In fact, it'll be something much less as there are 2 of these actuators in tandem. Oilite was used because it was readily available in our machine shop, it has excellent self lubricating qualities, and when freeze pressed into the lug, we could finish the honing/ reaming to get a clearence of within .0003 on the pin. I have all the calculated stress and load values for the clevis/ lug/ pin assembly if you want to see them, but that'll have to wait until wednesday. Full load analysis on the entire system has to wait until i get firm values for mass, inertias, etc, etc

 

[desertfox]

Hi mmart

Assuming your loads etc are okay,I agree with earlier posts that if its just mild steel then the heat shouldn't be a problem, however your problem began with corrosion and I am wondering how the steel and bronze will sit in terms of
corrosion?.

regards

desertfox

 

[mmart]

desertfox, I think it'll be okay. The pin is anodized and since the oilite gets its lubrication qualities from the trace leads in it, that should work out. We'll find out today when the guys get it put back together and it gets tested.