Heating & welding high strength bolts 1

[dbags]

AISC 9th edition indicates that quenched and tempered high strength bolts should not be heated or welded. Can anyone explain this restriction? What is the affect on the high strength bolt properties? If heating is a problem, why can these bolts be hot-dip galvanized?

 

[Metalguy]

You can heat Q&T fasteners, just as you stay below the tempering temp. (and factor in the time if the temp. is close). This means you have to know what that temp. was. Welding always exceeds it, of course.

*IF* you exceed the tempering temp. the fastener will lose strength.

 

[boo1]

and becomes an un-tempered fastener.

 

[TVP]

The immediate answer to your question is that zinc melts around 420 C. Tempering is usually performed around this temperature, anywhere from ~ 370 to 675 C, so minimal or no property changes occur. Steel melts when welded, and this temperature is in the neighborhood of 1500 C. The subsequent heat transfer into the Heat Affected Zone means that the weld and the adjacent area have properties that in no way resemble those of the quenched and tempered martensitic structure that previously existed.

 

[Metalguy]

boo1,

Depends on how high the temp. is and how long it's held there. If the tempering temp. was 1,000 deg F., and the bolt is then heated to 1,200, it's still tempered martensite, just softer. Once you get a bit past 1,300 it's all over.

 

[rmp319]

dbags,

Where in the 9th edition is this information listed???

 

[dbags]

rpoche,
It is very well hidden. Table 1-C; page 4-4 in the lower footnote area. What do you think?

 

[rmp319]

dbags,

The reference made states "Anchor Bolt material...". This subject is quite confusing. Below is a reply from AISC posted on the AWS Forum on whether or not HSB's could be welded. This constantly comes up for field applications and I wish a definitive code source was available to clarify this matter.

Here's what the AISC had to say....

start quote [ John:

Sorry for the delay in responding...I discussed your question with an ex-fabricator in the office, and I now have the following comments:
1. If possible, make the bolt accesible with access holes.
2. If possible, try using a bolt that is weldable (such as A307). This assumes that you don't have to pretension the bolts, as A307 can't be pretensioned.
3. There are U-shaped clips that are weldable that you could use to "clamp" the bolt head- and weld the clip.
4. If all else fails, you may have to weld the A325. This is not something that is recommended, as you may be damaging the heat-treatment of the bolt by welding to it.

Hope this helps.

Keith Mueller, Ph.D.

AISC Steel Solutions Center ] end quote

Thanks for the response,

 

[CWIC]

HSB's can be welded provided the welding is performed in accordance with a WPS specifically for the material joined. This includes ASTM A 449 Q&T materials. Yes, you will lose some of the mechanical properties, but it has been performed successfully on a number of structures here in the West.

The note in the AISC ASD has been brought to my attention by others in the past. They disregarded this note when the testing agencies pull tester crushed itself under load while trying to demonstrate to my client that the HSB's could not be welded.

Filler metals, preheat and interpass temeratures are the key. No PWHT was performed, delayed cooling was. I have written several procedures and WPS's for damaged in-place anchors, all with satisfactory test results.

 

[dbags]

Is there specific material tests available to determine if the welding did compromise the a449 q&t bolt?

 

[metengr]

dbags;
Possibly. It depends on access to the bolt after it is installed in the field. If you can access the bolt head or threaded end, you could perform portable hardness testing (using an EquoTip or possibly a Microdur tester). Portable hardness testing in field applications should be conducted by a reputable metallurgical testing company.

The other possibility is to remove several of the welded bolts and have them hardness tested at a metallurgical engineering lab to determine if the heat from welding had changed the bulk properties of the bolt. I would select a handful that are representative of the welding performed in the field.

 

[unclesyd]

Even though it is not recommenced or a good practice we welded thousands of 2H nuts to B7 & B16 studs in the 1 1/2" to 2 1/4" dia range. In violation of another poor practice we used the welded nut end to tighten the fasteners on 2500 Class Flanges operating at up to 2000 psig @ 600°F. Some of these studs are 30 years old and get removed every 90 days or so.
I have sectioned several of the welded nuts and found no detrimental defects. The heat effected zone, as measured by Microhardness, was never over .200" wide.
The nuts were welded cold with a proprietary alloy rod, ALLSTATE 275.

 

[Metalguy]

The exposure to 600 deg F probably tempered the HAZs enough to avoid cracking, or the cracks stopped in the base metal.

When failures caused by such practices result in big accidents, millions of $$$$ change hands. Unfortunately lawyers manage to prevent the root causes from becoming public knowledge whenever they can, so newer engineers can't easily learn from the failures.

Too bad those who are responsible don't get to spent a few years in prison.

 

[unclesyd]

[/b]Metalguy[/b]
Sounds as though you are have been associated in some way with a litigation firm.

I can assure you that the welding of the nuts on studs as posted was a very deliberate and well researched decision by very experienced and competent people with no attempt to deceive anybody.
As stated we never had any fastener failures in this application the 30 years mentioned. The process of doing this goes back to 1960. Again when one of these fasteners becomes available there is comprehensive metallurgical examination and nothing has ever been found that would have a detrimental effect on the fasteners. As previously posted we have a complete Metallurgical Laboratory with an SEM and access to two others on site. Even further if one would look at the materials that the aforementioned fasteners are made from all are readily weldable by a competent welder. Welding on the outer/external face of a 2H nut has no bearing on the load carrying ability of the fastener as the load is essentially carried on the first few threads in the nut.

I agree that indiscriminate welding on every type fastener is not recommended or advisable but circumstances sometimes require engineers of all sorts to come up with engineered solutions, with peer review, that are foremostly safe and then reliable and not covered in a book.

 

[Metalguy]

Unclesyd,

I wasn't referring to your specific case-just the general practice of someone welding steels etc. that require special techniques, etc. and those techs. are not used. How many times have you seen cases where the wrong techs. were used, and you raised the question, only to be told "But we've ALWAYS done it that way". I had the chief engineer of a large steam turbine tell me that after I discovered they had welded 410/403 SS COLD and didn't use PWHT. The HAZ's were ~Rc45-as hard as most 410 will get. I was having the meeting with him (and his people) because we discovered a LOT of HAZ cracks. Oh yeah, he also told me it was all OK because they used 308L filler metal!

Had another case where some large (2 1/2") nuts were losing preload and falling off. Real problem was a too low preload spec. Our maint. dept. decided to fix this one w/o telling engineering about it. They were mostly concerned with someone getting hit by a falling nut-from ~20-30' high. Rather than finding a suitable preload, they simply cut the studs off flush with the end of the nuts and welded a bead right around the stud/nut thread interface. These were probably B7 or similar high-strength studs. The weld HAZ didn't cause problems, but they finally got engineering (me) involved after 2 of the welded nuts fell off anyway--but this time they weighed a little more, because they had a short piece of stud INSIDE them! Yup, classic FATIGUE fractures from low preload!

 

[unclesyd]

dbags,
Sorry to digress from your question.
You have gotten a pretty good explanation to why you don't want to weld or heat high strength fasteners above their tempering temperature. High strength fasteners are often heated under controlled conditions in the turbine and split case pump industry in order to achieve the proper preloads.

One thing not mentioned is that the galvanizer likes to keep his pots as cold as possible to keep the amount of dross, a 5% zinc 95% iron alloy, from forming. This usually keeps the temperature below the tempering temperature.

As far as welding or heating of AISC fasteners I don't recommend it at all and is a bad practice to get into. Having said that if you go out on most jobs of the shopping mall variety you will see a tremendous number of the fasteners tacked as a fixing measure. My stepson welds for a fairly large erection company and he is always telling me that the "Wheels" come by and told him to tack certain fasteners. He said he has never welded a fastener other than the fixing tack.

This may not be the place but here goes.
There is another problem that is cropping up lately due to the fastener shortage and the cost situation. That is the reuse of fasteners. This includes all grades of the structural fasteners. The ones that I laid my hands on were black A 325 that had been recovered and brought back as HDG. The other was some A490 that had been acid cleaned and were still smutted and had very slight pitting. I understand these were recovered as surplus off a job where they had been improperly stored with no lubricant or wax.
I have no idea how prevalent this is but it is something to be on the watch for.