Heat Treating In Welding
I tell my students what they learn in my welding classes is the tip of the iceberg when it comes to knowledge of the welding trade. And when I say tip, I mean a very small tip at that. Some people think of the welding trade as some guy standing there all dirty with a stinger in his hand and a cig hanging out of his mouth and that's it.
Ahhhh grasshoppers, those people don't have a clue about the vast amount of knowledge there is when it comes to welding applications, processes, metallurgy, and theory.
I'm going to cover one of those practices here in laymen's terms, and will probably need a couple of more articles to do it. Heat treating steel can be a very important part of welding and is many times not done right, if at all. Most construction steel I worked on in Texas didn't need it unless it was in the Panhandle where it can get really cold.
When I think of heat treating, I think of a job a few years back in Dallas, Texas where Iron Workers were having to heat treat these huge girders at the Penny's corporate headquarters. Unsuspecting "boomers" (travelers from other Iron Worker Locals around the country) would come in and be sent to the job as a heat treater. Heck, it's a job most of us hadn't done and it sounded interesting and pretty easy.
When they got to the job they found the steel they would heat treat was big ol' girders that needed to be heat treated before they were welded. In this case I believe it was to between three and four hundred degrees. It was summer and if you've been to Dallas in summer, you know it is hot! It was well over 100 degrees out with excessive humidity and they had to heat treat the steel with huge torches that put out a LOT of fire.
It was brutal, and most guys worked long enough to get their first paycheck before they were out of there. One guy told me that when they called out for heat treaters at the union hall it looked like someone had thrown a hand grenade in there after word had gotten out about the job.
I can't remember exactly why they needed to heat treat the joints to be welded except that it was a new type of steel they were using, and I would suspect it was because of alloys added to it. Some of the alloys would perform better in the finished weld if they were already three or four hundred degrees before the welding arc, which can be around 6500 degrees F, started to fuse the filler metal to the parent metal.
My definition of heat treating is bringing the heat of the steel or metal to be welded up to a specific temperature to accomplish certain properties.
Heat Treating Metal
Man oh man, is it ever hard to do a brief article on heat treating! I'm usually pretty good at summarizing technical information, and putting it into layman terms. However, with heat-treating there are a lot of complicated definitions and explanations.
Fortunately, for me there are some good resources on the web and I'll list them in my conclusion on pre and post-heat treating.
It is good that more people are starting to realize how important pre and post-heat treating can be depending on the steel used. It is a real shame how many welders out there know nothing about the steel they are welding on. To be a good hand, you HAVE to know about the steel you are working on, by knowing what takes place when it is heated and cooled.
I once stopped a guy who was going to use re-bar for rungs on a ladder for a 200-foot tall tower. What he didn't realize was the re-bar has a high carbon content and when heated and rapidly cooled becomes brittle. Therefore, when some 220-pound Iron Worker with 35 pounds of tools hanging on his belt, stepped down on one of those rungs it could have easily broke. NOT a good scenario 200 feet in the air!
In steel there are microscopic crystals, some bigger than others. The smaller the crystals, and the closer they are to each other, the more strength, and hardness. However, with that comes the downside of the steel being brittle. The larger the crystals, and the farther away from each other, the steel will have less strength, but will be more ductile. (flexible)
Using dashes from my laptop keyboard let's say the small, close ones are from the high strength steel on my pocketknife blade…
- - - - - -- - --
- -- - - - - -
- - - - - - - -
- - - - - - - - -
Using larger dashes, further from each other, a section from a mild steel column or beam on a high-rise office building would be…
___ ___ ___ ___
___ ___ ___ ___
___ ___ ___ ___ ___
___ ___ ___ ___ ___ ___
That's why the old timers changed from cast iron to mild steel when they started building upwards back in the day. Cast iron would crack and shatter when loads shifted from wind or weight of the floors built. With mild steel the building will flex, yet return to its original shape. Another advantage is that mild steel is not changed much when heat treated, thus welding in the field has little effect on the joints.
When welding, there is a HAZ (heat-affected zone.) The steel on each side of the weld is heated by the welding process, and then cooled when the weld cools.
Welding Tip
Cast Iron MUST be pre-heated and post-heated to slow the cooling process or it will crack. When repairing a crack on cast iron it can be very frustrating when the crack spreads just ahead of the welding pool. This can make even the most laid-back welder want to punch a wall!
Drill a hole in front of and at the end of the crack. Pre-heat, weld, then post heat and cool. The hole will keep the crack from spreading, and after properly welding the crack, you can fill in the holes.
Learn more about out Batch Ovens.