Stress Relieving of 304SS Mix Blade

[cxreilly]

My first time posting a thread on this site...

We have a SS mix blade weldment that through a couple of iterations is comprised of a 17-4PH SS Shaft, a 304 SS female coupling and (3) 304 SS Sheetmetal mix blades. After sending these blade weldments out to one of our customers, it was discovered that one of the blades was forming micro-cracks from the welding operation. The blade that is showing the cracks is welded to the coupling. We have since started stress relieving the blade weldments by heat treating them at 850°F (450°C) for 3 hours and then allowing them to air cool, based on our steel suppliers reccomendations. Our customer went out and got their own expert to give his opinion and said that we should be heating the blade to 1850-1900°F (1010-1040°C) and also that we should probably get rid of the 17-4 shaft. Any suggestions? I will be glad to provide anymore info that might be pertinent.

 

[metengr]

Several questions;

Did the cracks occur in service or were they noticed by the customer before being placed into service?

Where exactly (location) are the cracks occurring between the welded 304 SS blades and the 304 SS coupling?

What was the filler metal and welding process for the blades and coupling?

 

[cxreilly]

They were noticed after being run for a approximately 6 months of service. The blades do not turn continuously in one direction but are driven by a recipricating motor that turns them 3/4 of a turn one direction, then 3/4 turn back. At the end of each rotation, the blade comes to a hard stop, which is another issue, but that is also being worked on.

The cracks are occuring right at the edge of the weld bead on the blade side, which as far as I am concerned is in the heat affected zone. Since the blade is rectangular (see below as I answered this part last), the cracks are showing up in the middle of the 1-3/4" dimension.

We are using 308L filler material for all the welds. The coupling is machined so that it is 1-1/4" in diameter. The wall thickness of the coupling at this point is 7/16". The blade is 14 GA, about 4-1/2" long and 1-3/4" wide with bent tabs on each side. The blade sits on the collar tilted or angled at 20° off of the vertical. The center hole in the blade is undersized and we are milling that center hole to the correct size with the blade angled at 20°. This helps to insure that that there is a good contact surface between the two pieces and the fit is tight and hopefully allows our welder to not have to put a terrible amount of heat into the part.

 

[metengr]

Ok. From your description, it would appear that the cracks emanating from the toe of the fillet welds (joining the blade to the coupling) have initiated and propagated from mechanical fatigue. Since this would appear to be low cycle fatigue, you have several options;

1. Reduce the cyclic bending stress on the blades by stiffening them.
2. Contour the weld toes to eliminate undercut – by blend grinding.
3. Shot peen the welds and surrounding blade base material which could help to increase fatigue strength.

 

[metengr]

One other question that may or may not make a difference in your solution to the problem - what material are you mixing?

 

[cxreilly]

Virgin polycarbonite pellets (hard) and a color additive (softer pellets or powder).

 

[cxreilly]

Thanks very much for your help.

 

[unclesyd]

Do you have the Chloride content of the color concentrate. I've seen extremely high levels of Chloride in some color concentrates.

 

[cxreilly]

No, I don't. Our customer would have that, but I don't think that this is the issue because we have other customers who run the same material in larger machines, and hence larger blades that are also of the same material and we have not had these sort of problems. Those machines have been running for years.