Reheat Cracking?

[SMF1964]

Does anyone have suggestions regarding the possibility of reheat cracking of bolts. I have two that failed by overload when they were torqued, but both have a small (10% of the cross-sectional area) semi-circular region of initiation that is intergranular, oxidized (magnetite, rather than standard "rust" and non-branching.

The bolts are carbon steel, at least 0.31% carbon, with no other elements specified, but a quenched and tempered structure, specified hardness of RC 39-45 (measured hardness via 500g Vickers converts to RC 43-45). The threads are rolled (based on defects in the thread crowns that could only come from rolling and not machining of the threads) which was likely done prior to final heat treatment.

We've ruled out stress-corrosion cracking in service, based on the observed magnetite on the crack faces and a service temperature of only 175°F would indicate the cracks were present prior to some much higher temperature (such as during hot rolling of the threads, to then oxidize during the final quench & temper). Is is possible to get reheat cracking of bolts? I've usually only seen it associated with welding of boiler tubes, lugs and other highly constrained circumstances. Other possibilities include the range of embrittlement mechanisms documents in the ASM books.

Your thoughts (collectively)?

 

[CoryPad]

The heat sources during bolt manufacture and use capable of producing dark oxide like we see here are:

1) hot rolling of raw material
2) austenitization and tempering during heat treatment of bolt
3) end use

Thread rolling, as stated by metengr, is done near ambient temperature and isn't a possibility.

If your end use did not produce high enough temperatures, then it is either from the raw material or during heat treatment of the bolt. There seems to be enough evidence to prefer heat treatment rather than raw material. When you say there are no other cracks - have you checked on a longitudinal section of the part using a metallograph? Or, are you basing it on visual/stereomicroscope observation of the whole part? I suggest using a metallograph to view the sectioned part.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SMF1964]

CoryPad - "No other cracks" was determined from the longitidinal metallographic sections made through both bolts' fracture surfaces and include only 3 additional threads (and corresponding thread roots).

The end use temperatures are only 175°F - definitely below 212°F else the cooling water jacket would boil and burst. I agree that the cracking was probably present during the final tempering process given the oxide. When it cracked prior to that is an academic exercise and one, in my technical opinion, worth persuing on our part. However, it doesn't have as much importance to the station engineers since they paid for sound bolts and got pre-cracked bolts instead.

 

[Carburize]

What was the finish condition of the threads before the fasteners went into service?
From the image it looks like the oxide coating on the thread and on the cracked area are similar - is that true?

 

[SMF1964]

No specific information as to the finish condition.

 

[redpicker]

This is a very interesting thread and the photos contribute a good deal. I would like to offer the following, even though it may go against some apparent conclusions already made.

1) Grain Size. From the looks of the intergranular SEM micrograph, I would estimate a fine grain. This is difficult, but it would look like the diameter of a "average grain" to be somewhere around 10-20 microns which would put the ASTM GS at around 10 to 8. If the micron marker is accurate, a GS of 6 would only give you 4 to 6 grains in the whole field (6 is still considered fine).

2) If it is 1541 Modified with Cr at 43-45 HRC, intergranular fracture is not unexpected. This material would be rather brittle at this hardness. While you state "tempering not less than 650 F", I would be surprised if the tempering temp was over 800 F. It would not surprise me to see intergranular fatigue cracks is this material, particularly if corrosion fatigue is involved. Note that obvious signs of corrosion is not needed for corrosion fatigue.

3) I am not convinced of the conclusion that the cracks intitiated by a quench crack. The discoloration of the fracture surface suggests temper scale, but it doesn't have to be. It could be corrosion and/or lower temperature scale that formed in service. Even at 175 deg F, you can get such scale in the right environment. I don't think you would get the "beach marks" you see in the optical fractograph with a quench crack. The shape of the discolored region sure looks like a fatigue crack and not any quench crack. In particular, the step in the center of the discolored region makes me suspicious. It would interesting to see if there are any other cracks on this bolt.

I could be wrong, but I don't think it is a quench crack. The crack may not have existed prior to service. If so, the question as to when it cracked is not just academic.

ab

 

[SMF1964]

redpicker: Hmmm. Two metallographic sections through the bolts (two bolts, two fracture surfaces, two longitudinal metallographic sections) showed three adjacent threads/thread roots adjacent to the actual fracture surface(s). No other cracking was observed in either metallographic section. The scale bar is accurate, to the limits of the instrument - Amray 1830 SEM, digital image captured using PGT IMIX-PC system - although no attempt was made to verify calibration.

 

[Carburize]

The reason for my question was that if the threads were bright finished as-rolled then it might be possible to make a judgement on the scale that is on the threads now (if any) as a measure of the potential for oxidation in service and then make a comparison with the oxide on the fracture surface. As I noted in my post of January 11 - fatigue looks like a possible mechanism.

 

[redpicker]

Etch the longitudinal section with aqueous picric acid with dishwashing detergent added for 10 to 15 minutes. This should reveal grain size (might need a light polish after etching).

From the optical fractrograph, it would seem the existing crack was only 1/8" long. I am not sure of a NDT method that would easily find such cracks. Maybe someone who was good with flourescent PT? Two longitudinal sections may not catch such a short crack. The "needle in a haystack" problem.

ab

 

[SMF1964]

redpicker: what strength of aqueous picric. I have 1.2% strength in the lab. This etch brings out prior austenite grain boundaries?

 

[TVP]

Until redpicker posted today, I had not actually seen the SEM image of the intergranular fracture. I'm not sure how I missed the link, but I concur with the grain size analysis-- definitely fine grain with an average austenitic grain size of 10-20 micrometers.

The reason I ruled out fatigue was because of the response from SMF1964 on 11 Jan 06 stating no evidence of fatigue, only intergranular fracture and microvoid coalescence. I agree that the dark areas at the bottom of the image appear more like fatigue beach marks that have been stained by corrosion, or by the separate mechanism of corrosion fatigue, but appearances can be deceptive.

 

[redpicker]

For the aqueous picric, I use a saturated solution. It took me decades (literally) to figure out how to do it as the literature is not very clear on the issue. Solid Picric Acid and water as much picric as will dissolve. It dissolves slowly, so be patient. I usually use 100 ml of water in a 250 ml beaker and add a couple of small scoups of Picric. Add a squirt or two (maybe 2 ml?) of hand dishwashing liquid (I use Palmalove). Stir well. It isn't as easy to use as nital, but it works. Wear rubber gloves since Picric Acid can be absorbed through the skin. Funny thing, the older this stuff gets, the better it works. Clear yellow solutions take a lot of working with to reveal the grain boundries. Once it gets a deep green/black color, it is great.

TVP (and others),
I had already made up my mind that this was a quench crack. It was the "appearances can be deceptive" that made me want to take another view of the whole thing. I am still not sure that this isn't a quench crack. But, I do have enough doubt to raise the question.

Does the intergranular fracture exist in the discolored region?

Is there a identifiable corrodant (ie, chlorine) present in a EDS scan of the discolored region?

I still don't know, but it sure is interesting.

ab

 

[SMF1964]

The discolored region was almost all intergranular with some (small) amounts of microvoid coalescence. What I suspect is creating the deceptive "beachmarks" is actually variations in the % amount of microvoid coalescence present. (Under the SEM, the 'beachmarks' are actually bands of subtly different contrast at low magnification, but moving in to higher magnification the amount of microvoid coalescense comes into focus).

EDS analysis of the intergranular surface showed nothing like a specific corrodent (like Cl, etc). There was occasions of zinc and moly or sulfur (can't differentiate the two on EDS and if the amount is just a surface layer, boosting the keV to 30kV to get the higher Moly peak doesn't work as the beam just blasts right through the surface layer). I thought about moly disulfide as a thread lubricant, but was told that this COULD NEVER happen. Since it came from a nuclear plant, I guess you have to believe them (?). The zinc could be explained from the ZDDP additive in the lube oil that this bolt was exposed to following the failure, as could the sulfur (if it is sulfur). The amounts were exceedingly small (quantitatively less than 0.5%).

regarding Picric acid: solid (moist, whatever you want ot call it) picric is banned from our laboratory because of what might have been an over-reaction incident under a predecessor about 10 years ago. Nothing went boom, even when the bomb squad tried to detonate the bottle, but the end result was a re-shuffling of laboratory management and a ban.

 

[redpicker]

Too bad about the ban on picric acid. I've gotten so that I'll almost always use the aqueous solution I mentioned (after examining with nital and repolishing) when I'm looking at a puzzler. I've been told that in the state of Oklahoma you have to have an explosives license just to possess it in solid form. I've been told it's explosive for 30 years now, but I've never been able to make it boom (and I've foolishly tried). I am sure it will, but I haven't been able to do it. I do keep a wet ball of paper towels in the jar, however. I also make sure my boss knows about it, but I don't advertize it to our HSE group.

The more I look at the optical fractograph, the more I believe it to be a service induced crack. It looks like there are two cracks with a step between them. This can happen if the thread root is more of a "U" than a "V" and you get a crack growing from each radius at the base of the "U". As they grow, they eventually joint, forming the step. I'd also postulate that the one on the left started first. When the two joined, the one on the right grew rapidly to "catch up". This is the reason for the lighter bands in the beach marks of the crack on the right. The darkness of the discoloration being indicative of the crack growth rate. But, this is all just wild speculation. Worth what you paid for it.

If you can get the SEM time, maybe examine the thread roots for additional cracks. If this is the result of an initiation and growth mechanism, and there are two cracks here, there are bound to be others, they would just be rather small. Guestimating from the photo, this was maybe 1/8" long and 1/16" deep.

rp

 

[Carburize]

+ 1 for redpicker.

 

[SMF1964]

redpicker: Since I am the only one with access to the SEM, I guess I have the SEM time available. I'll give it a run.

carburize: I didn't realize the judges were keeping score. ;-) Will my being a professional engineer create problems with the amateur status of some of the other participants in this thread? I wouldn't want the IOC to come down on them and spoil their chances to participate in the Winter Engineering Olympics this year. I hear the cryogenic Charpy events are going to be close.

 

[kclim]

What about hydrogen cracking? Some of the prereqs appear to be there - well, the high tensile strength is there. As for a source of hydrogen, perhaps the zinc you're finding is some sort of galvanised layer?

As for NDT, fluorescent magnetic particle inspection should be able to pick up fine cracking. More sensitive than dye penetrant (assuming your bolt is magnetic).

Did you perform a nital etch on your metallographic bolt section (wasn't clear from the messages)? Nital (without picral) ought to be good enough. The images should tell you quite a bit of information.

As for distinguishing Mo and S under EDS, my understanding is that Mo has a few additional peaks - you should be able to tell when you have sulfur exclusively, or moly and sulfur (just not any other combination). I don't really trust SEM EDS for compositional analysis - If you have time and money, send the bolts to a lab that can analyse them via an atomic emission (typically ICP-AES) technique.

 

[SMF1964]

kclim:

In-service hydrogen embrittlement: It was considered, but we kept falling back on the heat tinting of the intergranular region that would occur at temperatures that were well above the service temperatures of the bolts. That kept telling us that the intergranular crack must have formed prior to the final tempering process, since that was the only elevated temperatures we could find in the pedigree of these bolts. This would then rule out in-service causes.

The surface of the bolts did not show galvanizing. No zinc was detected via SEM/EDS and the purchase specification does not specify a protective coating beyond the three oxide finishes specified in the ASTM spec.

The two broken bolts were not tested via NDT, but the remaining 6 bolts were examined using fluorescent MT and found no evidence of cracks.

I did etch with nital (3%) and it produced a nice martensitic microstructure but the prior austentite grain boundaries were not very clear. I did try FeCl3 in water with a few drops of HCl (it was in the ASTM E403(?) spec for metallography as an austenite etch for martensitic steels) but that wasn't much better.

Regarding Mo & S: In theory, the two major L lines for moly are of a different peak height ratio than the two major K-lines for sulfur. In reality, a small amount of molybendum in the sample produces a wide single peak that is indestinguishable from sulfur. As far as compositional analysis goes, we've had very good agreement with standards using the algorythims that PGT supplied with the system. For "Certified Chemical Analysis", we use either ICP or OES, but for initial screening, we can get a high confidence level in the SEM/EDS results.


Others: regarding corrosion-fatigue: can you get corrosion fatigue that runs intergranularly?

 

[redpicker]

Any possibility for H2S exposure? Probably not, if it was, you would not be wondering if that was a Sulfur K line; the black discoloration would be FeS and you would have such a strong S line that you would probably see the K-beta peak, too.

At that hardness with (supposedly 1541), I would not be that surprised at intergranular corrosion fatigue. It may or may not be IG, it would depend on many variables.

Does a transverse micro show any branching of the crack? If so, you may be able to get an EDX scan from a branch, presumeably filled with scale. I don't know, but maybe worth a shot. My guess is you find out what is discoloring the surface and you'll find out what happened.

rp (or ab, I guess I'm confused)

 

[CoryPad]

On January 12 you stated "No specific information as to the finish condition." Now you mention an oxide finish? That certainly seems a likely candidate for when the darkening occurred. So, cracking preceded the oxide finishing process. That means raw material, forming, or heat treating.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SMF1964]

"Oxide Finish" is what is quoted in the ASTM specification. What our particular bolts' specified finish was: "no specific information." So whether these bolts had the "bright uncoated", "thermal black oxide" or "chemical black oxide" was not specified. However, I agree that an oxide surface finish would produce a time frame as to when the darkening appeared - which again points to cracking during the fabrication process.