Hot take: skip the preheat on your next repair weld

I know everyone says you gotta preheat everything over 1 inch thick. But I was working on a big tank repair last month in Hammond and my torch died. Said screw it and welded a 1.5 inch carbon steel patch cold. Used a 7018 rod and ran it hot at 140 amps. That weld passed the dye pen test and the inspector didn't even blink. Been doing it on smaller jobs ever since with no cracks. Any other guys ever try this on purpose or am I just getting lucky?

3 comments

3 Comments

rubyk261mo ago

I can't go along with skipping preheat on 1.5 inch material. You might have gotten lucky on that tank repair in Hammond, but dye pen only checks for surface cracks, not what's happening deeper in the weld or heat affected zone. Carbon steel that thick has enough restraint to crack from hydrogen and fast cooling, even with 7018 rods. I've seen guys do what you're describing and end up with cracks showing up weeks later when the tank cycles through temperature changes.

the_wesley1mo ago

The 7/64" 7018 rods we ran on that Hammond job were low hydrogen out of a fresh oven, but I still wouldn't call it a smart move without preheat. @rubyk26, have you ever had to deal with a delayed crack showing up on a tank that was dye pen clean at the time of the weld? I'm asking because I've heard stories about them popping up after a few thermal cycles, but I've never actually seen it happen firsthand. Your point about hydrogen in the HAZ makes sense though, especially with 1.5 inch plate that has enough restraint to pull stress back into the weld zone. I'm just trying to figure out if my luck was genuine or if I was working with material that had some residual heat from the cutting process.

phoenixw111mo ago

Man @rubyk26 is right on the money with this one. I've been around long enough to see the same thing play out more than once. Hydrogen cracking is a sneaky bastard, it'll wait until you think everything is fine and then show up when that tank sees its first real temperature swing. Dye pen is a good quick check but it's not looking deep enough for the kind of trouble you can get with thick carbon plate and no preheat. I've got a buddy who swore by the same shortcut and ended up chasing a crack through three grind outs before he finally preheated the whole thing.