Odd crack formation in mild steel
Odd crack formation in mild steel
(OP)
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
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





RE: Odd crack formation in mild steel
RE: Odd crack formation in mild steel
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?
RE: Odd crack formation in mild steel
RE: Odd crack formation in mild steel
RE: Odd crack formation in mild steel
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?
RE: Odd crack formation in mild steel
RE: Odd crack formation in mild steel
appears to be bulging of the shell in the area of cracking. The wrong material may have been used or specified.
RE: Odd crack formation in mild steel
RE: Odd crack formation in mild steel
RE: Odd crack formation in mild steel
RE: Odd crack formation in mild steel
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
RE: Odd crack formation in mild steel
Don't try to make it heavier or stiffer, rather allow it to move a bit more.
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube
RE: Odd crack formation in mild steel
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.
RE: Odd crack formation in mild 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.
RE: Odd crack formation in mild steel
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?
RE: Odd crack formation in mild steel
[Send me a sample; I am accepting new clients] ;)
RE: Odd crack formation in mild steel
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
RE: Odd crack formation in mild steel
Looks more like SCC than thermally-induced fatigue cracking. Multiple, branched cracks is usually SCC.
RE: Odd crack formation in mild steel
I would figure you're up to at least $1 worth of advice, and not $0.02,