Class 60 grey iron 1

[teb1]

I am looking for fatigue data on Class 60 grey iron.

Would anyone in this group have a recent source for this information.

Specifically an S-N curve for the notched and un-notched condition.

Thanks in advance.

teb1

 

[Goahead]

I could not find the fatigue data you look for.
But the authors of the following article refer to a large database that could probably be obtained by contacting them.
The abstract of the article is found at

http://www.ingentaconnect.com/content/bsc/ffe/2005/00000028/F0020001/art00010

that seems to deal with a similar problem but it is not clear if it provides actual data, although it mentions a large database from current literature.

Available also from

http://www.blackwell-synergy.com/li...695.2004.00862.x/abs/;jsessionid=hL1OveaC6Phg

http://www.welding-advisers.com/

 

[teb1]

Goahead,

Thanks for the reference. I'll explore it further.

teb1

 

[Carburize]

Can't find anything either but did come across a reference that indicated that there is a lot of information on fatigue properties of cast irons in "Metal Fatigue" edited by J.A. Pope and published by Chapman and Hall 1959.

 

[Carburize]

A second reference that might give some guidance is "Atlas of Fatigue Curves" edited by H.E.Boyer and published by ASM. Available from their book store on the web site at

http://www.asm-intl.org

 

[mcguire]

Gray iron has the same fatigue properties notched and un-notched because the graphite flakes act as notches all the time.

 

[unclesyd]

Checkout the URL given in my reply in:
thread367-89643


There is a significant difference in the values for a notched or unnotched GI at your strength level. There is a interesting set of S-N curves for mechanical rolling the notches in high strength GI.

 

[teb1]

I am trying to determine why a piece of equipment developed a crack. I have done extensive FEA on this piece and strain gaged it for model verification. The stress are not that high! In the past I have repaired similar pieces of equipment where there was a documented shock load. In this particular case there was no "documented" shock load, so I am having to prove that the cause of the crack was not "normal" operating stresses. The appropriate S-N curves for this class cast iron would be most helpful.

I do sincerely appreciate the suggestions and help.

Thanks,

Tommy

 

[metengr]

Have you considered that the cause of cracking could be related to residual tensile stress in the grey iron casting?

It might be possible that the grey iron casting had contained residual tensile stress from fabrication that after placed in service cracked from the additive effects of residual and service stresses. Material exposed to tensile stresses will either deform or crack, depending on the ductility of the material. Grey cast iron does not exhibit much ductility.

 

[teb1]

Metengr,

This piece was in service for approximately 20 yrs. That equates to approximately 170 million stress cycles.

I believe that manufacturing practice for this very large casting is to oven stress relieve prior to machining.

Would you still consider residual stresses a factor with the above knowledge?

Thanks

Tommy

 

[unclesyd]

Can you come back with a little more information about the location and description of the crack. Are there any casting features like a boss, bolt hole, gate, etc. associated with the crack. Any signs that it may be preexist crack covered with "foundryman's putty" that may have grown in service.

If you can find a library that has the booklet I mentioned it has a great deal of Information on the endurance of grey iron. A point to make is that continual under stressing grey iron actually increases the endurance limit.

 

[mcguire]

Good information, Unclesyd. I stand corrected.

 

[teb1]

Unclesyd,

The vast majority of the cracking occurs outside the bolt circle of this very large mating flange. There is some fretting on the mating flange face. The casting appears to have been QA'd very thoroughly before it was sent to the machine shop and after machining. There was no "hidden" defects.

Investigating other sources (ASM), they list a fatigue strength of 24,000psi. The stresses in the area where the cracking occurred are below 6000 psi even with the stress concentration effects of the bolt holes. But like I said the majority of the cracking occurs outside of the bolt circle diameter. But interestingly enough the crack does cross the bolt circle and ends uo on the inside of the bolts on the inside of the hub. Picture the plane of the crack passing through the bolt hole at about the mid point of the bolt hole length.

 

[unclesyd]

You have what I call "Radial Flange Cracking". We had it occur on several large flanges of GI, SS, and CS. Our GI cracking occurred on 2 of 4 fairly large steam, 25#, driven low pressure (8#) air blowers acting as a inlet boosters on Nitric Acid Unit Air Compressors. The flanges on the casing were about 21/2" thick. We were able to weld repair all and had only 2 or 3 break again. We left some on the other two machine alone, they never progress to the opposite side of the bolt hole. The repairs were accomplished under the supervision of a Cronatron represenetive using their electrodes by our welders using their cold CI welding procedure

Both the SS (12' dia 3"tk) and the CS, (30' dia 31/2"tk) , crossed the bolt hole and progressed toward the center of the flange. This occurs unless the outside cracking is repaired in a timely manner. To repair this type of cracking in SS and CS requires an entirely different procedure.

 

[teb1]

Unclesyd,

With all due respect, it doesn't sound like your repair efforts were too successful. You state you had 2 or 3 break and you had 2 or 3 break again. Welding of grey iron is never a good idea. You can recast it, but you can never achieve a weld simialr to what you would get welding a similar joint made from steel. I know, I've heard every story about cast iron welding with SMAW. You can do it, but the mechanical properties won't be for squat! I have been the cast iron repair business for over 20 years, if there was a good way to do it, it would be in our repetoir of tricks.

I'm sorry if I failed to adequately describe the cracking in this piece. The cracks initiated (in my opinion) from a point on a diameter outside the bolt circle and progressed axialy (parallel with bore of the bolt holes) to a diameter smaller than the bolt circle.

Anyway, the part that I am working with is not a steam header. This particular piece is approximately 7 inches thick at the bolt circle. It is just one part of a rotating piece of equipment.

Thanks for the imput,

Tommy

 

[unclesyd]

teb1,

The ones that broke back out were just several in over 100 repairs and were easily fixed the next outage.

I agree that CI welding is problematic at best and is very dependent on the starting material and welder. As you well know the weld metal being much stronger that the base metal constraint is always a problem. I have always considered CI welding with SMAW to be a painting process rather than a welding process.

Back to your cracking, If I'm getting the description right will the crack eventually break to the outside?

I have values of 29,500 for the endurance limit of GCI Class 60. This is from two different books.

PS,
There is way to positively insure that a GCI weld will work. Think 1348?F.