I'm beginning to understand the whole thing.
C1018 is a standard for threaded bars, rods and the like. The material is cold drawn mild (i.e., low carbon) steel. The cold draw operation increases the material hardness and decreases ductility. After having been cold drawn, the threads are cut in a lathe.
Then the piece is case hardened and quenched. As a matter of fact, low carbon steel will never reach 40 HRC, even if cold drawn, and that's the reason for case hardening: have the stud surface reach a superficial hardness of 40 HRC. I said superficial, i.e., the hardness is limited to a very thin layer in the stud surface. Mac himself says that the hardened thickness is limited to 0,0005 to 0,002 inches. The stud inside is much softer, its hardness being approx. 120 Brinell, as one of the references posted above states. Why such a low hardened thickness? Because the surface hardness purpose is just to resist the rubbing produced by the nut. Apparently, in this application the nut has to be tightened and loosened frequently.
Then, the stud is welded onto the plate. If the whole stud body had a 40 HRC hardness, welding would be impossible, or at least extremely difficult. But only a very thin superficial layer is that hard, and this layer is easily molten down during the first seconds of welding. So, welding can proceed with no difficulty.
Now, after all of these treatments, the stud broke down when the nut was tightened. In my opinion, the fracture looks like a fragile one, not ductile. This would indicate that the fault is not on the material, but on the treatments the stud was subjected to.
Possibility n° 1. As Allan said, the threads bottom might have been cut sharply instead of leaving a radius.
Possibility n° 2. A mistake in case hardening and quenching. The temperatures and/or times were different than those stated by Mac, producing a thicker hardened (and therefore more fragile) layer that didn't resist the stress posed by the nut tightening. Initially a very little crack, the failure propagated to the whole stud section.
Possibility n° 3. A mistake in the cold drawing operation. The bar was drawn into a smaller dye than the regular one, thus increasing the material hardness and fragility.
Possibility n° 4: ?
Giovanni S. Crisi
Possibility n° 4: ? spatter or a stray arc strike?
Giovanni, what is 'Fragile' fracture?
46,
before answering your question, I'll have to grab the dear old Marks & Baumeister, which is available in Mackenzie's library, and see how are some Portuguese words translated into English.
What's the translation of "encruamento" and "estricao"?
Unless someone, possibly Al or Henry, are ready to explain you what the fragile fracture is.
Giovanni S. Crisi
Giovanni, I think frágil is what you are trying to say which translates into Brittle in English.
You're perfectly right!
Giovanni S. Crisi
>Apparently, in this application the nut has to be tightened and loosened frequently.
When speaking with Adam on the phone the other day, he mentioned that this bolted connection will be in service for 30 plus years and they do not want to have to go back and tighten the connection and to his understanding that was the reason he was told that they case hardened the threaded bar.
I suppose they didn't want to gall these threads when tightening the nut?
Wonder if it is possible to double nut this connection so they don't have to worry about the connection loosening over time(30+ years)?
Wondering if the radius at the root of the threads are too sharp as noted by Allan caused this to fail?
Yes, the nut is ideally only tightened once. John you're correct in the reason for the case hardening being to prevent the nut from galling. If that does happen there is an enormous amount of work that has to be done to repair the stud. You are also correct in us using a jam nut to prevent any loosening.
Giovanni, thank you for your help in this problem. Your explanation has helped out tremendously. I am leaning towards explanation number 2. Possibly a combination of 1 and 2. After reviewing Allan's post and your own it is noted that there is discoloration. I am guessing that this indicates the starting point of the failure, wich is on the same side and thread in both failures. Most likely from a small crack due too an over hardening??? I have attached some more pictures that may help some other more experienced eyes form an opinion.
Attachment:
Image13.jpg - There appears to be small cracks along the edge?? (0B)
Mac,
Maybe in the future, the machinist can radius those root transitions vs leaving them square as shown in your picture.
Mac,
1)Is the C1018 mandatory for the material or can something else be substituted?
2)Also can anti-seize be used on the threads of the nut to prevent galling vs case hardening?
3)Does the nut run up on the threads by hand all the way easily?
4)Is there any slop in the threads?
5)Is it trying to gall while hand tightening?
just a few more questions....
Ha no worries on the questions John, I always appreciate the help.
1) I'm not sure about the material, the spec was written in 94. If the person who made the decision on material is still around they may not even remember the reasoning for the C1018. We will need to look into the history a little more.
2) I don't believe that any kind of ant-seize will be allowed because of the contamination issues.
3) Yes the nut usually runs up and down the stud by hand. Except when we get some spatter on them .
4) There is very little, if any slop in the threads.
5) Usually doesn't gall when hand tightening.
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