Welding of high strength grade 8.8 steel bolts 1

[icebloom]

Ok now I know that welding of high strength bolts is rarely if ever permitted. Grade 8.8 (830MPa ultimate tensile strength) is the common bolt grade we use. Does anyone know of any good texts/industry guides/academic references that explain in detail what goes on when you weld high strength bolts?

What I always see in general terms is that welding high strength fasteners removes the heat treatment and embrittles the metal.

The following doc talks about how even tack welding of the bolts can embrittle them.

http://www.nationalprecast.com.au/wp-content/uploads/2015/10/specs-of-welding-high-tensile-bolts.pdf

I'm familiar with the docs in thread725-500924 also.

I'm not trying to find a way to actually weld them, just an authoritative or definitive explanation of what is actually occuring when you weld high tensile fasteners. There may be situations where you are welding near high strength bolts which may experience some localised heat transfer. Are there limits for this, can they take a certain low temperature before strength degradation occurs? Would be interested if anyone has managed to find more detailed info on these issues.

 

[r6155]

Can someone send us an example of the need for welding on bolt 8.8 ?
Thanks
Regards

 

[kingnero]

Here's a common assembly, rails with multiple bolts and locator pins (in hard steel), used on airplanes.
The ISO view is a detail, and doesn't show the entire part.

 

[r6155]

@ kingnero
Thank you !

Regards

 

[dgeesaman]

Can someone send us an example of the need for welding on bolt 8.8 ?
Thanks
Regards

In my experience, it's a site maintenance manager who managed to keep a badly designed (or overtly abused) machine in operation longer by welding together some fasteners. Then they apply that requirement to all machinery.

Also in my experience, it's impossible to predict what will happen when you weld on a bolt. Will it crack? Will the bolt lose preload? Will the bolt gain preload and stretch? Will the base components distort and lose connection quality? Nobody knows because it's unpredictable.

David

 

[kingnero]

Welding on these alloys has been widely tested and documented. I wouldn't say it's unpredictable. I would say it's generally a bad idea, as we can predict fairly accurately what's going to happen.

 

[gtaw]

I'm not sure, but you might want to take a look at the "Farm Code" for a welding project like this. Do an on line search for the Farm Code 2002.

Best regards - Al

 

[TugboatEng]

Welding to 8.8 fasteners together maybe a bad idea but welding an 8.8 fastener to some A36 plate may give better results as the A36 has such low carbon content it may provide a better microstructure when mixed with the 8.8.

 

[EdStainless]

But the HAZ or the 8.8 will still have zero ductility.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Tmoose]

"Grade 8.8 bolts are heat treated by quenching and tempering in accordance with the DIN 933 grade 8.8 standard.

This process involves:
- Heating the steel to around 870°C, where it transforms into a γ-austenite crystalline structure
- Rapidly cooling the steel, which transforms the austenite into a martensite crystalline structure
- Reheating the steel to a tempering temperature of 425°C​

https://www.researchgate.net/public...pering-solid-line-and-isothermal-bainitic.jpg

The required times at each stage are generally MANY minutes or even hours.

I wonder how hot the various locations within your bolts will get during welding, how long they will stay hot, how rapidly they will cool, and if rapidly cooled ( quenched) how much time they will subsequently spend at 425°C / 800°F .

And like EdStainless said, where in the bolt and the weld joint rapid quenching will very likely create hard brittle steel badly in need of tempering to produce mechanical properties similar to 8.8, or at least not dangerous.

And there may also be issues with residual stresses that, depending on the loads the bolt and welds have to endure, might contribute to early failure.

https://media.springernature.com/lw...6-y/MediaObjects/40194_2019_816_Fig1_HTML.png

Maybe melt a small spot on the bolt head with a TIG torch and let it cool.
Then Test with a file to see how hard the bolt steel gets when rapidly quenched.

 

[icebloom]

"Grade 8.8 bolts are heat treated

Thanks for the references Tmoose I will have a look