When should you preheat? 2

Hello,

I am currently designing a drive shaft, which will spin at a relatively low rpm (approx. 200 rpm). The drive shaft is basically composed of 3 components. Two flanges both ASTM A572 grade 50 and a 3 1/4" OD, 1/2" wall mechanical tubing, AISI 1026 (UNS G10260).

I am concerned about cracking in the weld or adjacent to the weld. Would preheating, post heating help reduce the risk of cracking? Should I be aware of any potential problems?

Any feedback on the subject would be greatly appreciated.


Thanks
J. Roth

 

[welding1]

You apply preheat to reduce the cooling rate of the weld between 800°C to 500°C.
Basically keeps the weld hardness down and reduces the risk of hydrogen cracking.

You will find a very good article on hydrogen cracking at

http://www.twi.co.uk,

you will have to register which I think is free for all. Enter job knowledge in the search bar and its guide Number 73, Title:- Job knowledge for welders: Defects / hydrogen cracks in steels - identification. (Connect January 2000) 6%

Describes the characteristic features and principal causes of hydrogen cracks in ferritic steels

If you have a weld subject to fatigue you will need a good root run suggest TIG (GTAW).

I have written an article on the fatigue of welded joints at

http://www.gowelding.com/

 

[JLAU]

Second the input re hydrogen cracking.
However, fatigue as mentioned could also be a problem.
Hence, post weld stress relieving would help minimize the stresses in the weld, which together with a high-quality weld will improve the fatigue strength.

JLAU

 

[EngineerDave]

Interesting, this is very similar to a project I am working on.

How can you distinguish hydrogen embrittlement cracking from cracking due to other means?

 

[intimidator3]

One way to distinguish between hydrogen embrittlement from other sources of failure is to determine where the weld failed.
If it lost integrity anywhere other than at the toe of the weld,such as in the effective throat,this MAY be a clue.
I stress the word may, because metal incompatability, will not "mix" properly,and will cause cracking.
We ran into this trying to TIG weld A572 to a 304s 16 ga.sheet,using un-compatable filler metal.We changed to a different filler metal and our prob was solved.
A good source of info on this subject is the above thread site,gowelding.com.

 

[stanweld]

To J.Roth:

If you use a low hydrogen welding process you should not need to preheat when welding the materials and thicknesses described. Hydrogen cracking should not pose a problem. Residual welding stress can be reduced somewhat by the use of preheat and maintaining an interpass temp between 450 F and 650 F

Fatigue failure will be your problem in service. The residual welding stresses are additive to the imposed mechanical stress in operation. The residual stress at the weld toe can be nearly = the yield strength of the material and the strength of the weld in fatigue is expected to be about 60% of that of the base metal, assuming a full penetration, smoothly contoured butt joint, without PWHT or some other method of stress relief. If your weld is a fillet its strength in fatigue is expected to be << 25% of that of the base material.