7018 vs 8018

xwrench · Thu Sep 04, 2014 12:02 pm

Greetings all. Years ago I worked as a heavy equipment mechanic and welder working mostly on caterpillar equipment. Most of the repairs required gouging out the crack with an air arc and then using multiple passes to weld it in. The company I worked for insisted I use 8018 on the repairs. I also did repairs on crushing equipment and was told to use 7018 for that. What would be the reason for this?

Braehill · Thu Sep 04, 2014 12:09 pm

X,
If I had to guess (which I do) I would say it was just because the price difference. I can't think of a reason why you couldn't use 8018 or even 10018 anywhere you can use 7018. Unless they wanted the welds to be able to flex more on the crushing equipment, but I think that's stretching it a bit.

Len

WTFH · Thu Sep 04, 2014 4:52 pm

The electrode should match the composition of the metal being welded. My guess is Cat were the ones saying use 8018 because of the material they used to fabricate the equipment.

Bill Beauregard · Thu Sep 04, 2014 7:58 pm

7018 is more ductile than 8018. Imagine a crack in the edge of a bucket. If you don't preheat, the bucket may sit there pretty stable without much internal tension. Open up that crack, and weld it, the now hot metal you introduced to fill the gap while very hot is just the size it needs to be to fill the gap. As it cools, it shrinks. By the time it is fully cool it is too small to fill the gap. The bucket is too strong to pull it to a new shape. Something has to give, the bucket breaks, usually beside the new weld, or the new weld stretches. According to the Lincoln book, Metals And How To Weld Them, stretching a small amount makes metals stronger. While a 70,000 PSI filler would stretch ever so slightly relieving stress, and strengthening the filler, 80,000 PSI rated filler might be so resistant to stretching it would crack.

Thu Sep 04, 2014 8:03 pm

Bill Beauregard wrote:7018 is more ductile than 8018. Imagine a crack in the edge of a bucket. If you don't preheat, the bucket may sit there pretty stable without much internal tension. Open up that crack, and weld it, the now hot metal you introduced to fill the gap while very hot is just the size it needs to be to fill the gap. As it cools, it shrinks. By the time it is fully cool it is too small to fill the gap. The bucket is too strong to pull it to a new shape. Something has to give, the bucket breaks, usually beside the new weld, or the new weld stretches. According to the Lincoln book, Metals And How To Weld Them, stretching a small amount makes metals stronger. While a 70,000 PSI filler would stretch ever so slightly relieving stress, and strengthening the filler, 80,000 PSI rated filler might be so resistant to stretching it would crack.

That's a GREAT simplification of ductility versus hardness.

Thank you.

Steve S