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Welding of high strength grade 8.8 steel bolts 1

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icebloom

icebloom

Structural
Feb 15, 2024
49
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.


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.
 
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Hi icebloom

I don’t know of any papers but there may well be some.
My limited understanding is that if you weld or subject an 8.8 bolt to very high heat, then it loses its high strength so an 8.8 bolt has a minimum yield of 640Mpa in its off the shelf state but once it’s been subjected to high heat the minimum 640Mpa no longer applies.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein
 
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+ typically the high carbon and chromium content gives ideal circumstances for the creation of martensite after welding (due to rapid cooling). You're bound to end up with hydrogen cracking.
 
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What is the loading of the bolts in your assembly? What service life is required?

What is the welding these bolts will be subjected to ?

Tack welding heads to a substantial main component?
Welding the bolt head to a component to create a stud?
Something else altogether ?

Depending on your industry's required codes, and product, a few empirical tests could help answer a lot of questions.

What would happen if the head spontaneously snaps off the welded bolt in service?
The name plate will be secured by the other 3 bolts?
The bolt head will fall into the gear train of the helicopter gearbox with the Prsident on board at 1000 feet?
 
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Think about it this way, if you had solid material of the same alloy/properties would you weld it without PWHT?
I think not.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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My understanding was the main issue was pertaining to the heat treatment (as you and others make note of).

I try to design out the need for welding the nuts, some of the ways are:
- Using a tapped hole in the plate, instead of a nut.
- Using steel stabs to locate the nut and stop it spinning.
- Using a proprietary product (like this Link)
 
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Thanks for the replies guys.

Kingnero I will have a further look around about the hydrogen cracking you mention, and the formation of martensite.

Tmoose as per my post I am not actually trying to weld the bolts, just understand more the reasoning about what actually happens to the material properties and the reasons behind it. This is an academic pursuit, not an attempt to circumvent code rules that I'm sure are in place for good reasons.

 
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In a high hardenability steel like this you will form untempered martensite if you weld.
This material has nil ductility and high propensity to absorb hydrogen.
One common hydrogen source is corrosion.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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It would be great to have a white paper
On the effect of welding high strength bolts. Hydrogen embrittlement is a new one for me from welding .
However never had issues if post bake
Is accomplished to remove hydrogen..
 
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I frequently see and personally have welded 8.8 and 10.9 fasteners along with wrenches and sockets. It's a very common practice for tool making or as studs for wire hangers and such. These welds are typically to A36 steel which has very low carbon which may help reduce brittleness.

Would I sell a product made like this, maybe. Would I use it for my own purposes while understanding the limitations and risks? Absolutely.

A while ago I needed to make a Z shaped wrench. Had a bar of 4130 handy. I welded sockets to each end of it with presumably wet 7018, one for a 1/2 square drive and one for a 14mm hex key. The 4130 bar cracked longitudinally along its length. It still worked even with the crack but I was surprised to see the 4130 crack and not the welds to the sockets
 
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So I have done some hacks with welding.
I compensated by using thicker material.
To increase the strength. I modified a lot of tools for ease of assembly or almost impossible access. I use to make personal gadgets. For my home mechanic shop.
 
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For tool making I use a lot of silicon bronze. For T handle wrenches I use 1144 steel rod and whatever the sockets are made from. Silicon bronze joins the metals without too much care for metallurgy. I made a few are of T handle wrenches for my friends with this combo.
 
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I had access to a bunch of remnants
Left over high strength alloy.
Bunch of soc head screws.
In my younger days I would throw a
Old 6 cyl head on the bridge port mill
Reface the surface.
 
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We used to make a lot of things out of 4135 since we had a lot of it around.
We always tempered after any welding.
Leaving any untempered martensite is asking for issues later.
It has zero ductility.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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My rule: NEVER perform any type of welding on bolts of any material.

Regards
 
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A 300 or duplex SS you can leave unannealed if you don't really care about corrosion resistance.
400 series alloys will need to be either annealed or tempered depending on if they are ferritic or martensitic.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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I know, I often spec stainless (A2/A4) if welding of nuts is required (and if there are no problems with compatibility/galv.corr). I just asked because saying "never weld on bolts of any kind" is neither realistic nor correct in my line of activities.
 
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A2/A4-70/80 are austinitic stainless steels and are relatively insensitive to heat treatment. OP specifically asked about 8.8 bolts which are medium carbon steel and very sensitive to heat treatment.
 
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Some 30x stainless fasteners are strain-hardened in the thread rolling process; welding on them reduces their strength from the spec. value to something less. And as Ed says, you change their corrosion resistance as well.
 
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