Odd crack formation in mild steel 4

[deco0404]

Guys,

I have this crack formation, and was wondering if anyone has seen anything comparable.
I havn't had a chance to test yet, but am pretty sure its fatigue. I just havn't seen this pattern before.

It is inside a cyclone, towards the top, the cyclone is anchored approx 1.5Metres below the cracking allowing virtually no movement.The steel is C-Mn (mn0.3%). It has been in service approx 25years. Heat is not really a factor, even though it operates at quite a high temperature, there is almost a foot of refractory on the fire side

any opinions would be great...

Declan

 

[stanweld]

Looks heat related to me. I believe that the refractory is not doing its job anymore. Have you done any thermal imaging during operation?

 

[unclesyd]

I concur with stanweld that is locks heat related,
It also looks like the refractory may have broke out at the anchors. One will have to see the fracture surface to better understand what's going on.
I would get a refractory expert in if you are going to attempt any repairs. Some of the older refractories will not bond to patching material very well.

Have you inspected around the vessel supports for any cracking?

 

[Jmoore1]

Whats the service material? caustic? H2S ? salts? and what is operating temp?

 

[chicopee]

If it were heat related, wouldn't there be discoloration of the surface?

 

[deco0404]

Guys,

thanks for your input here, I have attached some grain structure photos ( done in a bit of a rush so not great quality).There appears to be very little pearlite present. I am not an expert on these things, but am guessing that adds a considerable amount of weight to the thermal fatigue argument....

This is part of a solid fuel boiler, so there should be no environmental effects.

I have noticed that on the hot side the hardness is approx 120Hv, whilst on the cold side it is closer to 150Hv. The thickness of the plate is only 6mm. Am I the only one that thinks this odd?

 

[deco0404]

Forgot to mention, the refractory was still in place when we got in here........

 

[stanweld]

It appears that a very low carbon steel was used and it looks like there may be oxide in the intergranular fracture. There also
appears to be bulging of the shell in the area of cracking. The wrong material may have been used or specified.

 

[deco0404]

..........and second photo....structure x100

 

[metengr]

This really looks like stress corrosion cracking from exposure to concentrated caustic solution. What kind of cyclone application? Are you referring to a Power boiler cyclone furance or FB boiler primary cyclone or what?

 

[Ron]

My take on this is "oil canning fatigue". Not a technical term, but appears to be flat plate out of plane buckling, likely due to thermal issues, but fatigue nonetheless.

 

[deco0404]

The boiler is a solid fuel steam boiler. Not for power generation. I think it knocks out approx 40Bar steam.

I have attached a free-hand drawing which may give a better idea of how this is coming about.....

Ron, there is some slight buckling around the cracked areas.

Meteng, I agree it looks like caustic cracking, but its not.

Stanweld, it is a low alloy steel. We did a quick PMI test and got Fe + 0.3% Mn. ( cant measure the C, but am sure its quite low). Like I said it was built 30 years ago, so its kind of difficult to say it is the wrong steel having lasted that long!


I realise some of the details given by me are a little vague, but I got called in on Friday afternoon, so only had a little while to go through things.

thanks again for all your help guys

Declan

 

[EdStainless]

I would suggest that his may be thermal fatigue. Given the flow in the cyclone this area may heat and cool quickly. Combined with it being too restrained leads to distortion and tearing.
Don't try to make it heavier or stiffer, rather allow it to move a bit more.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[unclesyd]

I'm try to find some metallogrphs of thermal fatigue and stress rupture of CS in refractory lined vessel.
The picture post show some characteristics of thermal fatigue, surface braking, normal to surface, oxide wedge. The only problem is the crack appears to be intergranular instead of trans granular mode expected with low cycle thermal fatigue.
If the OP could post a metallograph with a light nital etch we might be able to see if the initiation site the cracks starts out as transgranular and morphs into intergranular.

A little information on the operating condition especially temperature and cycle time.

 

[Guest102023]

The cracks are intergranular, so simple fatigue is ruled out. The many cracks initiated without benefit of geometric stress raisers. There is some corrosion in the cracks that indicates they are connected to the surface (although not evident in the sectioning plane). The micro is a little underetched, but appears to show a very low carbon steel.

Possibilities therefore include SCC (caustic as mentioned, since it goes IG). Creep-fatigue is possible, but you would need to find some suitable source for the cyclic strains imposed to justify that. However, I don't see microvoids or triple-point microcracks near the crack, and the micros just look visually more like SCC than anything else.

More info still needed. Looks like the mess has been cleaned up, but maybe you can swab nearby material and test for dried caustic. Or even break open up a larger crack and hunt there. Look for possible spheroidization in the microstructure to gauge what temperature it might have seen.

 

[unclesyd]

brimstoner,

I was thinking all the lines of low cycle thermal fatigue mainly due to oxide wedge in the cracks buu now I'm leaning towards a stress rupture mechanism.

deco0404,

Can we have the operating conditions?

Where are cracks originating, inside or outside?

 

[Guest102023]

The more I look at the micro the more it reminds me of several past F/As I have done that were IG SCC in carbon steel, and I am now leaning heavily in that direction.
[Send me a sample; I am accepting new clients]

 

[deco0404]

A few more pieces of the jigsaw guys....

operating temperature inside cyclone 800c, engineers reckon 150c max on steel

Boiler runs 24/7 for approx 300 days

Service is ash, gas and sand. They assure me they have no difficulty with corrosion anywhere down the system....

I'm not sure if the ash is corrosive or not, but I did just find out that some of the refractory had broken away, and a section of the shell needed replacement. The locations of the cracking is a high stress location, the material is susceptible to cracking, so am assuming with the breakdown of refractory something got in behind it and on to the steel surface, thus creating all the correct conditions for formation of SCC!

The cracking is internal surface only. Average crack depth on the samples I have is 1.5mm, nominal wt is 6mm. Hardness on internal is avg 120Hv, external surface 150Hv (not insulated, clad etc.....fully exposed to the environment)

If you guys need any further info let me know and I will get it tomorrow. I am leaning towards the SCC myself, but being a keen amateur at this kind of thing, dont want to make a complete fool of myself...

Declan

 

[TVP]

My $0.02...

Looks more like SCC than thermally-induced fatigue cracking. Multiple, branched cracks is usually SCC.

 

[metengr]

TVP

I would figure you're up to at least $1 worth of advice, and not $0.02,