7018 H4

Date 07-13-2007 11:28

H4 is an optional designator that indicates the deposited weld metal has less than 4 ml/100g of diffusible hydrogen. It is the lowest of the three dif H designators that AWS uses (H4, H8 and H16).

In laymans terms....a "tested" low hydrogen rod

By reddoggoose (**)

Date 07-13-2007 16:41 Edited 07-13-2007 16:44

Philthomas has offered very sound and very correct advice. The lower the H number, the lower the hydrogen in the rod. The lowest possible H-designation would be the best choice if there were concerns with possible hydrogen cracking. For hydrogen cracking to occurr three items must be present.
1. A microstructure susceptable to hydrogen cracking such as a martensitic structure.
2. A stress must be applied. This stress may only need to be stresses from shrinkage or distortion for cracking to occurr.
3. And of course you must have hydrogen.

Take away any of the three and you take away hydrogen cracks. In reality it is very difficult to remove one of these three things entirely so what we do is try to minimize all three. By removing hydrogen from the rod we lower available hydrogen to the weld puddle. By preheating we reduce the chances of a martensitic structure and help relieve shrinkage and distortion stresses. Kind of a long answer but that is the reason behind the H- designations

By medicinehawk (**)

Date 07-16-2007 08:56

Thanks, but I was in error as it actually has 7018 H4R or is ity HR4....crap, Anyway, it sounds like what you are saying is correct......so this is a good rod for pipe welding???
Thanks,
Hawk

By 803056

Date 07-16-2007 13:16

High strength low alloy steels are more prone to hydrogen induced cracking, thus it is beneficial to use low hydrogn electrodes. The lower the hydrogen, the better. You can use EXX18 with 16 ml/100 g of diffusible hydrogen, or lower hydrogen levels such as the EXX18-H8 or EXX8-H4 covered electrodes. It is good to calculate the carbon equivlency to determine if it is high enough to be an issue. The higher the CE, the more important it is to use LoHi electrodes that are properly conditioned before welding with them.

Other sources of hydrogn must be considered as well, root passes made with EXX10, paint, oils, organic materials, etc. Welding with E7018-H4 does little good if the joint isn't properly cleaned to remove organics containing hydrogen compounds. Even fumes from paint that is close enough to be volitize by the heat generated by welding can introduce hydrogen into the weld zone if they are drawn into the gas envlope around the weld puddle.

Best regards - Al

By medicinehawk (**)

Date 07-17-2007 09:01

So this a good thing. Low H number designation, now I see and of course this would be appropriate for pipe as well. Thanks guys. I used to be more up-to-date on this stuff but more lately I have been into the doing of fabrication then as to why. Great comments and again, thanks.
Hawk