SCC stainless 3

[macmet]

I have a piece that is exhibiting cracks(some small, some large). I'm trying to come up with an initial cause of these failures before we send them off for further analysis, and I'm wondering if it's possible to eliminate SCC as a cause of these cracks without access to metallographic equipment?

The pieces were in service for 4-5 years, and remain in decent shape except for cracks forming across the corners. I was initially told they were in service for <1 year so I thought it might be casting defects, but now I'm not so sure because of the longer service time.

Enviro is 600-700 deg. C, chlorides present, SS/austenitic, small tensile forces present.

 

[EdStainless]

Any signs of pitting? Are the samples magnetic at all?
Do they see an temperature cycling?

Next to CSCC all of the other failure mechanisms have little chance. Mechanical or thermal fatigue? Doesn't sound likely.

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Corrosion, every where, all the time.
Manage it or it will manage you.

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[macmet]

They do see temperature cycling, the high and low temperature I do not have available at the moment, but typical pieces are exposed from 600~700->1000~1100 depending on the system and operators.

There is no evidence of pitting, but there is a region of porosity that looks like a casting defect. The holes are uniform in size, and there are two parrallel rows.

The material is not magnetic.

 

[stanweld]

How often is the temperature cycled?

 

[macmet]

Half of the year it runs at 12 hour intervals, 700-1100, while the other half would have 12 hr fluctuations from 900-1100.

Complete shutdown happens once a year, ideally.

 

[mcguire]

No SCC is possible at the operating temperatures cited. Depending on which specific alloy you have, however, other mechanisms could be in play. Why not provide the necessary specifics instead of making everyone guess what you problem is?

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[stanweld]

While thermal cycling is a highly probable cause of the cracking, you haven't provided information as to grade of stainless steel or the service environment; e.g., carburizing, oxidizing, etc.

 

[macmet]

Thanks for the responses, more information.

-The service environment is oxidizing
-They are made of astm a297-hh

what is the most important factor in the cycling that may lead to the cracking? Temperature range? Cooling/heating rate? Cycling times?

 

[macmet]

One other thing that might help, is the cracks are perpindicular to the direction of the forces. As I said before, the forces are small and I do not believe them to be excessive, even at these elevated temperatures.

 

[mcguire]

Since you state that the cracks are open, it implies that significant deformation has occurred. Since the applied stress is small, it seems likely that thermal fatigue is the culprit. The maximum thermal expansion strain involved is 0.1%. That translates to a lot of stress, more than enough to cause a failure. The key factor is the maximum temperature gradient during heating/cooling, so put a limit on the rate of both.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[macmet]

Is the direction of the cracks important? Would the direction of the cracks be controlled by forces alone? Could the casting shape be a factor? That may be too difficult to answer on here.

These pieces are about 1meter in length, and about .15 wide, with varying height(from .10m to .15). The forces act along the longer axis.

I'm fairly new at my position, and this is the first failure like this that I've seen. I'm just trying to learn from this case so I can understand and learn what to expect in the future.

 

[mcguire]

Analyze the direction of the thermal strains. On a flat plate thermal fatigue would result in craze cracking. Yours sounds like the cracks are all in one direction so the main thermal strains aren't simply between the surface and sub-surface, but in one major direction. That would be normal in a high aspect ratio member which has less constraint in the width and height directions.

If you can't find a way to control the cooling/heating rate, you'll need to redesign.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[macmet]

This thread has been dead for a bit, but I just got some information from our supplier.

I was just told by our supplier that HH tends to age harden which leads to the cracking. Can anyone tell me how common this problem is? We have had many of these components in service and never seen this time of problem before.

Also, they suggested upgrading to HK material. In the past I have conducted an experiment on various HK alloys, I found that HK had excessive spalling when there was cycling conditions. Has anyone seen similar or different results?

 

[dham]

Let me throw out a wild possibility, have you checked the grain size in the area of cracking to see if it could be reheat cracking?

 

[macmet]

No, I do not have access to those resources immediately but I can arrange for them to be done in the near future.

Would reheat cracking be apparent from large grains near the crack?

 

[EdStainless]

HH shouldn't harden unless you want it to. It is a sign of off balance chemistry. The alloy is designed for extended high temp service. You need to balance the chemistry differently depending on the service temp. The details are in many cast stainless ref.

The flaking/spalling in HK may be a chemistry issue. This alloy has better strength at temp and should be very stable. Of course higher strength would work against you if you are having thermal fatigue issues.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[macmet]

EdStainless,

Would cycling have no effect on the spalling of HK? Why does higher strength have a negative effect on thermal fatigue? I would have thought differently.

 

[rolledalloy]

Heat resistant alloy castings tend to crack under cyclical conditions due to thermal fatigue. This is for two reasons. They have a high carbon content near 0.4% and have a coarse grains structure since they are in the as cast condition. There are many castings used in the heat treat industry for carrier trays that have a grid pattern and cracks tend to be in the intersections of the grid. Going to HK would not help if this is the case. I am surprised that you see more scaling of HK than HH. With higher nickel and chromium it should scale less than HH does.

1100C is pushing either of them temperature wise it is at the limits that we suggest for 310 stainless which is the wrought equivalent to HK. For 309 or HH we suggest an upper limit of around 1025 to 1050C. Depending on the design if the design can be done with a fabricated wrought product you would get better ductility. The drawback is wrought alloys like 309 or 310 have lower creep rupture properties than their cast equivalents. Also they tend to be more expensive than castings.

Below is a link to a case history on some cast grids that cracked. You might want to take a look at it and see if it looks similar to your problem.

http://www.rolledalloys.com/trc/hrcasehist.aspx

select the case history titled RA330 grids.

 

[EdStainless]

High strength increases the stresses due to thermal cycling. Weaker material will just yield. With HH, if you are going to cycle it, you leave some residual ferrite. The ferrite is softer and it gives. This helps prevent thermal fatigue. The drawback is that if this version (type I) is used at high temp for long periods it will become brittle from carbide and/or sigma formation.

HK should be more stable. Less scale and it should be tight. If you are cycling though the higher strength of HK may not really help much. It may be a casting or design issue and the higher stresses will cause failures somewhere.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm