Bent SS316L shaft 1

[cokeguy]

First of all, pardon my metalworking/metallurgy illiteracy, but here it goes: one of our fan´s 4-7/16" diameter by 90" long SS316L shafts was bent right at the middle (about 1 degree) because one of the fan blades broke off and the resulting inbalance "folded" the shaft at the middle, right where the fan side bearing is located. So now, the shaft´s end is off-centered about 1" with respect to its original position. We had to replace the shaft for the moment, but my question is, can this bent shaft be somehow straightened up, and if it can, how should we do it? Will it retain its original properties or will it be weakened up at the middle because of the bending/unbending operation? Also, if we reground it to a lesser diameter or cut it up to smaller pieces to make smaller shafts, ribs, etc.., should we take any special care with the bended area, avoid it, etc? Thanks in advance for your comments.

 

[EdStainless]

How long did it run bent?
If not long then I wouldn't worry too much. It isn't much of a bend.

You probably could have it straightened. It is done is presses with support blocks and dial indicators.

If you are going to machine it down you would either need to straighten it first, or cut out the bent section. It has to be real straight to grind.

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[metengr]

From what you described, the fan shaft took a permanent set (you exceed the yield strength in a local region) from excessive bending stresses caused by the imbalance in service. Frankly, with austenitic stainless steel shafts I prefer replacement versus attempting to heat straighten (from my own practical experience with pump shafts).

Regarding under sizing the shaft to remove the permanent bend, this is acceptable. Also, if you intend to use the material for other applications as you described this is also acceptable. The shaft material located in the bend area can be used it just received a little more strain in comparison to the remainder of the shaft material.

 

[EdStainless]

DO NOT HEAT STRAIGHTEN. That could reall screw it up.
Doing it cold in a press should work. Make sure that he guys that do the work know what they are doing. If they seem uncertian, find someone else.

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[cokeguy]

Thanks a lot guys. How about bent fan blades? When we replaced that particular shaft and impeller assembly, some of the blades on the new fan where bent slightly at one of their corners because a rib from the fan´s casing got loose and hit them a few times before we were able to stop the fan. We simply rebalanced the fan in order to start it up and reduce downtime, but I would like to know which is the correct way to straighten up these blades (about 20"x30", 1/4" thick SS316 plates) during the next maintenace shutdown. We cannot take the fan out, so should we heat up each bent section and try to bend with heavy duty wrenches, hammer it down without heating, or what? Again, please pardon my ignorance on these matters. The bent section is a triangle about 4"x4" at one of the corners of each blade.

 

[cokeguy]

I replied without reading EdStainless post (we posted at the same time I guess). I cannot move the fan to a press because it is far too costly and time consuming to dismount it. How could we do it on the spot, without a press, which tools would you recommend? If it is not possible, I would have to leave it as it is, but would sure like to give it a try.

 

[EdStainless]

For the blades, how about the biggest pipe wrench that you can find and just more or less bend them back into place.
On a fan this size it can't really hurt your performance. There is one potential bad situation. You did static balance the fan. If the blades are bent funny it may be out of balance when running because of slightly different pressure on the blades.

I was speaking of using a press to straighten the shaft.

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[cokeguy]

Thanks, we did balance the fan dynamically and tested it throughout its speed range before restarting. So, if necessary we´ll bring up the good old 36" stilson during the next shutdown to straighten up those blades, and rebalance if necessary.

What exactly is the problem if you heat up austenitic SS to make it easier to bend? Our welders and mechanics sometimes want to heat up parts to ease bending or straightening, but I haven´t let them do it yet, at least on critical parts.

 

[metengr]

I agree with edstainless regarding heat straightening. I can tell you from first hand experience that heat straightening austenitic stainless steel is too risky and in most cases will result in more harm than good. The corrosion resistance of austenitic stainless steels can be altered from exposure to local heating, and shaft movement is more difficult to predict because of the difference in thermal diffusivity between austenitic stainless and carbon steel.

I never had much success in cold pressing austenitic stainless steel shafts (3” or greater in diameter) because of residual stresses introduced during an equipment failure. We had mechanically pressed one pump shaft, determined it was running true, and it never ran the same after it was installed. We kept chasing vibration and could not balance the pump. When we replaced the shaft, it ran fine.

 

[EdStainless]

When you heat to straighten you can get contamination from the surface not being clean, you cna get grain boundary carbid formation, you will get localized grain growth, and maybe even a little bit of melting.

Straightening really takes skill. The key is to restraighten the entire shaft. Most people focus on the major bend, and they forget that it must be bent at every support location. You have to check the straightness by rotating and using dial indicators. You have to do it up and down the shaft at various locations and spans. It is a couple hours work, but a shaft hat size may well be worth it.

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[tc002]

it sounds like you don't have much confidence in your maintenance department and from what I gather there isn't much experience in fan and stainless technology either. When in doubt contact the fan vendor, tell him the problem, ask for assistance and a quote to fix it or get a comparison quote for a new replacement.
Working with a 90 inch stainless shaft and fan blades can't be done in a job shop with inexperienced people and pipe wrenches.

 

[cokeguy]

You´re right tc002, however, not necessarily are we going to do these things ourselves, we are not metalworkers, but these forums do give us a chance to enlighten ourselves a little bit in areas outside our confort zone, so we can confidently discuss our options with vendors or detect when something is being done or proposed incorrectly. Thanks everyone.

 

[robsalv]

I have doubts that the shaft could be bent back to true. You could try though... perhaps you could write the attempt off against your R+D budget?!?!

Regarding the fan blade, I can't see any issue with beating the bent tip back. You'd need to use SS friendly tools or risk damaging anticorrosion properties... but I suppose pickling could fix that. You could even heat it and bend it back... but you may reduce the corrosion protection properties - heating would be hard to control insitu.


Just back on the shaft again, putting the contamination issue to one side, if heat straightening was attempted below the carbide precipitation temperature, would detrimental properties still be induced?

 

[EdStainless]

I have never seen someone heat straighten in a manner that I would feel comfortable with on stainless. The local temps always get too high.

Cold bending the blades back into shape is no big deal. Just re-balance afterwards and you should be fine.
I wouldn't even wory about surface contamination. After all this was stiking a steel support brace.


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[VincePav]

First off this is a rescue and not a permanent solution, get a new shaft and new blades.

You can straighten the shaft if it has a gentle bend, but not if it is a 'crease' or 'buckle'.

Do not even consider hot bending, do it COLD.

Assuming it is gentle and given the size it is worth a try. Put it in a lathe between centres. Run a dial gauge down it and find the highest point and the points at which the shaft starts to bend. As a precaution use a Chloride free pen/marker. Set the shaft up in a hydraulic press between blocks, high point up and set the blocks at the positions where the bend starts. If the distance between the blocks is less than 10 diameters you will really struggle. Set up dial gauge to measure the imposed deflection from the press. Make sure you protect the shaft at the blocks and the press nose. Press the shaft until the dial gauge reads 0 then go on until it reads half the original bend, i.e. push it right through 0. Leave it for a few minutes and release the pressure. Check the dial gauge. Repeat this until you achieve a near 0 reading on the dial gauge. For a shaft this size a deflection of 0.010" would be accetable.

If the thing will not respond then certainly cut it up and make shafts from it, you will not have to worry about the material in the bent area other than it makes machining very awkward. You sould turn shafts between centres to avoid taking them out of the lathe to do one end and then the other.

The fan blades. Are they solid or hollow section? If solid then OK these can be heated and bent, after all you can weld 316L, as a previous respondant has mentioned just pickle and then don't forget to passivate. It is very easy to set up the right conditions for Stress Corrosion Cracking to get hold. Water vapour with 35ppm chlorides at 50 degree C will crack stainless nicely when stress is applied.

If you damage the blades in the process (splits, creases or cracks) then weld them and grind them smooth and use Dye Penetrant Inspection to check for surface breaking defects. Use TiG welding.

If the blades are hollow section these will be past their best, scrap them.

I would be more worried about why the original blade came off in the first place. What one has done the rest will probably follow.

 

[corus]

I'd be concerned about the fatigue life of a straightened shaft as you must be putting some residual stress back into the surface of the shaft. Instead of the stresses varying about a zero mean stress they might vary from some compressive value at a point on the surface shaft where cracks will originate and hence the likelihood of cracks developing may increase.

corus

 

[Cyclefrog]

Actually, most shops that do propeller repair for large yachts and commercial marine vessels also do shaft straightening. Not knowing where you are located, I can't suggest a specific shop, but it is a fairly common procedure.