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)?

 

[Flesh]

What about the possibility it is a "quench crack"? These typically are intergranular, non-branching, and have a tenacious oxidation layer.

 

[Carburize]

Can you possibly upload some images to a site like

http://flypicture.com/

or similar
and post a link?

 

[SMF1964]

I was thinking that the bolts are too small? They are only 1/2" in diameter and 1.25" in length. But it does explain a lot of what I'm seeing. Thanks.

 

[metengr]

SMF1964;
What is the diameter of the bolts? Have you ruled out possible quench cracks versus reheat cracks? Normally, for reheat cracking the mechanism involves the precipitation of coherent vanadium carbides or precipitates within the grains that upon exposure to thermal (welding or PWHT) results in fissures along the weaker strength grain boundaries. I really don't see this type of mechanism occurring with this grade of bolt material.

 

[SMF1964]

As I said before, the fracture surface is only 1/2" across.

The fracture is intergranular in the tinted region and a mixed mode intergranular/microvoid coalescence as you move away from that. The relative amount of microvoid coalescence increases to 100% by the time you get to the far side, although the narrow band/facet on what is the left side of the picture is 100% microvoid coalescence over its whole surface.


http://flypicture.com/bin/?id=rtn8l6ra

 

[Carburize]

I was thinking quench crack as well when I asked for the image but having looked at it I don't think it is.
Is there any evidence on the fracture surface of fatigue crack growth? I could convince myself that there is a fatigue striation in the center of the left hand part of the darkened areas!

 

[SMF1964]

SEM examination of the darkened region showed only intergranular (or intergranular with oxidation), no fatigue. I agree, there appears to be visual "beachmarks" but under the electron microscope, these 'beachmarks' apparently represent different amounts of microvoid coalescence among the intergranular.

 

[swall]

Is the cracking confined to the thread that failed during torqueing or are cracks present in other threads? Also, the material selection is out of whack for a 1/2" bolt where a hardness of Rc 35-45 is desired.

 

[SMF1964]

I only have the bolt head and the threads leading down to the fracture surface, but no other thread roots show cracks.

Bolt material (ASTM A574) allows for a hardness range of RC 39-45, but puts no requirements on chemistry other than 0.31% Carbon minimum and "enough Cr, Mo, Ni, V to ensure strength properties are met after oil quenching and tempering". Elsewhere is requires tempering not lower than 650°F. SEM/EDS analysis shows 1.3-1.8Mn, 0.4Cr, no measurable Ni, Mo or V. Carbon content is unknown at this time.

 

[metengr]

One other suggesiton, check for any evidence of local decarburization along the intergranular crack paths using the optical microscope. If you see a narrow band of decarb that follows the crack paths, these cracks were present in the original bar prior to the quench and temper heat treatment. If there is no decarb, the cracks had formed either during quenching or after heat treatment. I believe the threads would have been rolled at ambient temperature using a lubricant.

 

[Flesh]

If the oxidation hasn't masked all of the intergranular fracture: is there any mirco-ductility present on the grain facets? Presence of grain-boundary seperation?

 

[SMF1964]

Metengr: no decarb of the intergranular crack.
Flesh: I'm not clear on what you mean by "micro-ductility". I including a link to an SEM image of the intergranular portion of one of the bolts.

http://flypicture.com/bin/?id=rtn8l6vb

 

[Carburize]

I know we are not answering your question - but this is a great exchange of information particularly being able to view fractographs!

 

[TVP]

SMF1964,

Is the dark area with the largest diameter (out of focus relative to the fracture surface) the underhead surface of the bolt? Is this surface highly corroded?

Based on the EDS analysis I would guess SAE 1541 with some residual Cr. IFI-140/ASTM F 2282 allow 0.20 Cr maximum. This would allow for sufficient hardenability in a 1/2 SHCS, especially if it was produced to a coarse grain practice.

 

[metengr]

SMF1964;
After further review of the macrophotograph showing the oxidized, thumbnail shaped pre-existing cracks on the fracture surface and information obtained from "The Physical Metallurgy Handbook" by Anil Sinha, my vote is quench cracks.

Some additional information related to Quench Cracking from The Physical Metallurgy Handbook...
" Quench cracking is mostly intergranular, and its formation may be related to some of the factors that cause intergranular fracture in overheated and burned steels. Important considerations to cracking, apart from stress, in heat treatment are (1) part design (2) steel grades (3) part defects (4) heat treating practice (5) tempering practice ". Further on in this Chapter on Quench Cracking, Sihna describes each of the 5 items above in detail.

Hope this helps

 

[SMF1964]

TVP: The darker 'ring' around the fracture surface is actually the next thread. The underside of the cap screw head is only visible as a bright band in the lower left corner (the reflection of the camera lights). The fracture occurred a bit less than 1" below the bolt head, where the cap screw threads into the body of the engine block (there is a 3/4" 'gasket' piece and a 1/8" thick cover. Thus, the high stress point would be where the bolt enters the block.

There is not a significant amount of corrosion of the bolt.

 

[deanc]

http://www.tribology-abc.com/sub9.htm

Take a look.

 

[TVP]

SMF1964,

I looked through several references, including the ASM Handbook Volumes 4, 11, 12 and the Handbook of Case Histories in Failure Analysis (also from ASM), and I agree with metengr that the likely root cause is quench cracks. The description and image that you provided correspond well with the textbook definition of quench cracking, although the physical size of the defects are a little small. I would definitely investigate the quenching conditions, as well as the alloy being used. Coarse grain practice promotes quench cracking vs. fine grain steel, and since this alloy appears to have minimal hardenability, it was probably produced to a coarse grain practice. You should change to a fine grain, alloy steel such as one of the grades listed in Table S1.1 of ASTM A 574M.

 

[SMF1964]

I'm not sure how to discuss the grain size issue with the supplier - the ASTM spec requires that all alloys in this spec be made with fine grain practice. However, "fine grain practice" does not guarentee a "fine grain size". Furthermore, these bolts come from the engine component manufacturer, who gets them from a bolt supplier who gets them from a bolt distributor who gets them from a bolt manufacturer, who... You get the idea.

I appreciate all the assistence in this. It has been extremely educational. Thanks again.