Welding Fasteners: Effects & Properties Changes

[CruiserFJ62]

What are the effects when welding takes place on a fastener. If the head of a bolt or nut is welded to a plate how does it change the properties of the hardware?

 

[FredGarvin]

It will lose any of the previous heat treating. The material the fastener is made of will, essentially be in its annealed state. Granted, it all depends on how large the fastener is and how long heat is applied and to what extent it propagates. However, it is safe to say that if it is in a structural member, stay away from welding on it if at all possible.

 

[CruiserFJ62]

Does anyone know of a table shows annealed vs tempered&quenched steel properties? What is the decrease in yeild stress on a grade 8 bolt that is annealed due to the heat from welding?

 

[turbo]

Does anyone know of a table shows annealed vs tempered&quenched steel properties? What is the decrease in yeild stress on a grade 8 bolt that is annealed due to the heat from welding?

You are posing a problem that is not possible to resolve with charts and tables. How much heat was applied during welding? What type of welder was used, and what is the operating temperature of the arc? How much of the bolt's surface was in contact with underlying metal, and what is the thermal conductivity of that metal? You can pile on complication on complication very easily with a bit of thought.

 

[CruiserFJ62]

You are posing a problem that is not possible to resolve with charts and tables. How much heat was applied during welding? What type of welder was used, and what is the operating temperature of the arc? How much of the bolt's surface was in contact with underlying metal, and what is the thermal conductivity of that metal? You can pile on complication on complication very easily with a bit of thought.

I'm not really trying to get an exact answer but an understanding of what the worst case would be if the material was in its annealed state?

 

[turbo]

I'm not really trying to get an exact answer but an understanding of what the worst case would be if the material was in its annealed state?

If the bolt is in its annealed state, it will have no more strength than a common fastener, and possibly less, since you might have created discontinuities in its internal structure that would make it prone to sudden complete failure, instead of just stretching/distortion that you might have gotten with a common fastener.

 

[CruiserFJ62]

If the bolt is in its annealed state, it will have no more strength than a common fastener, and possibly less, since you might have created discontinuities in its internal structure that would make it prone to sudden complete failure, instead of just stretching/distortion that you might have gotten with a common fastener.

What are the effects when the nut is welded to a structure and the bolt is threaded into the welded nut?

 

[FredGarvin]

The effect will be the same. However, it is a worse scenario in that nuts of an equivalent grade are, by design, meant to be stronger than the mating bolt. That way the failure mechanism is inclined to be bolt breakage and not thread stripping. If you alter the nut, that will no longer be true so you would stand a very good chance of stripping the threads in the nut out.

 

[Mech_Engineer]

If you really need a weld-on fastener, you should use one that is specifically designed for the application. These ones at McMaster-Carr have tabs and/or plates for welding the fastener in place.

http://www.mcmaster.com/#weld-studs/=4z0qak

 

[nvn]

CruiserFJ62: If the nut is partially annealed by the heating process, I think it could be classified as an ISO property class 5 nut, which is tensile ultimate strength Stu = 520 MPa, and tensile yield strength Sty = 420 MPa ... barring caveats mentioned by turbo-1 in post 6. But if the nut is fully annealed, such as the one listed by Mech_Engineer, it appears it is classified as ISO property class 4, which is Stu = 420 MPa, Sty = 340 MPa. As FredGarvin and turbo-1 pointed out, applying heat exceeding 50 C below the tempering temperature of the heat treatment process will alter the mechanical properties of high-strength fasteners, such as ISO property class 10.9 or 8.8 fasteners (or SAE grade 8 or 5). You pay a penalty for destroying the heat treatment (and certification) of high-strength fasteners.