Chromoly

[FlatTire]

Anybody have any experience with Chromoly frames? I always hear about cracking on cars built from chromoly tubing. Is this because of they are MIG instead of TIG welded? or is it due to too much flex/fatigue. Does AWS have a weld procedure to do this? I was taught in school that chromoly should always be TIG welded.

[Kromulous]

Most likely due to not stress relieving the steel, or welded areas. Tig is best, small localized heat, but it can still crack.

AWS should have plenty of info on it, i wonder if AMS does as well, i will take a look around if i get some time. Filler wire would be critical i would think, and maybe the gas.

[billetbirdcage]

Anybody have any experience with Chromoly frames? I always hear about cracking on cars built from chromoly tubing. Is this because of they are MIG instead of TIG welded? or is it due to too much flex/fatigue. Does AWS have a weld procedure to do this? I was taught in school that chromoly should always be TIG welded.

There are tons of moly cars out there Mig welded and no problems. Rockets don't have the problem your referring too. In circle track the only moly cars that are TIG welded for the most part are sprint cars then rest are MIG welded.

I could make another 2 page post on this but pretty sure I have before and it's been covered.

[fox1162002]

If you mig weld moly it is best to take a torch and turn the welded area blue this resets the molecules in the moly. Most chassis places have the ovens turned up on the heat before they powder coat them.

[fastford]

There are tons of moly cars out there Mig welded and no problems. Rockets don't have the problem your referring too. In circle track the only moly cars that are TIG welded for the most part are sprint cars then rest are MIG welded.

I could make another 2 page post on this but pretty sure I have before and it's been covered.

this is true, also mig welding with mild steel wire creates a flexible weld that will not crack near as easily, drag car builders discovered this years back.

[billetbirdcage]

If you mig weld moly it is best to take a torch and turn the welded area blue this resets the molecules in the moly. Most chassis places have the ovens turned up on the heat before they powder coat them.

While the first part is true, the temp to anneal moly is around 1200 degree's and a powercoat oven won't get close enough to that to do anything.

It's about spreading the heat thru the rest of the tubing when welding so the weld or more importantly the thin moly right beside the weld to not cool too fast or it will harden and become brittle. Welding slow and allowing the heat to soak up the tube will increase the cool down time and fix the issue. That basic reason TIG is preferred cause it's going to take you a lot longer to weld the joint.

Think of it this way, heat up a 1" x 1" x 1/2" think piece of metal red, it will take a half an hour to cool off by air cooling but heat a 1" x 1" piece of sheet metal .020 thick and it will cool very quickly. Welding with too much heat so you weld real fast will only allow the heat to go up the tube of the joint say 2" and beyond that you can grab the tube so it will cool really fast. However, If you weld with lower heat and therefore have to weld slower the heat will go up 6" of the tube and it will take longer for it to cool thus not heat treating the moly as hard.

Hope that makes sense

Fastford is correct IMO, use regular steel filler.

[fox1162002]

I was just passing on what Mark Richards told me at Rocket when I bought my first Rocket. I weld everyday mig, tig, & stick and was just passing on what I've learned over the years.

[25drtrkr]

There are tons of moly cars out there Mig welded and no problems. Rockets don't have the problem your referring too. In circle track the only moly cars that are TIG welded for the most part are sprint cars then rest are MIG welded.

I could make another 2 page post on this but pretty sure I have before and it's been covered.

I'm pretty sure Grt offeres a tig welded chassis as an option.

[billetbirdcage]

I'm pretty sure Grt offeres a tig welded chassis as an option.

yes, they did (might still do) and IIRC it was a 3500.00 option for moly and TIG welding. I remember talking to one of the welders when they did the first one (thinking it was Clint's car) and they had 40 hours in welding it.

That's the reason I said "MOST" cars are MIG'ed

[billetbirdcage]

I was just passing on what Mark Richards told me at Rocket when I bought my first Rocket. I weld everyday mig, tig, & stick and was just passing on what I've learned over the years.

Only my first paragraph was directed at you, I added the rest just to explain a little more. I wasn't intending on demeaning you in any way, just stating that your typical powdercoat ovens usually only get around 400 degree's which isn't hot enough to have any effect on the moly.

I did agree that heating up the area and letting it cool down is a viable option if you don't have experience with welding moly for a chassis but is sort of unrealistic for production of a chassis as it would eat up a lot of time. They make temp sensitive paint that you can put on the area and it will change colors when you reach the desired temp to anneal the area. You can always do this if your worried that you may not be welding it so it doesn't become brittle.

Peace

[50j]

Tig welding takes a lot longer but is much nicer IMO. You have more control over the amount of heat used as you go around a tube, and can start hotter or turn the heat up for inside joints and back off some as needed in other areas. NHRA certified drag cars can not have Mig welded chrome moly.

[Bubstr]

When working with Molly, the first mistake many make is not knowing the letter N at the end of the properties number is more forgiving to any type of welding. N stands for Normalized. This is a process in the heat treating.

The discussion on how and why to weld high carbon, chrome Molibnium steel has been going on since the 40s. The problem is ruining the heat treat near the weld with heat during the welding process. You end up with hard and softer right where the heat line is. This promotes the cracking so many talk about. The general consensus is to limit the area effected by the heat and still get good penetration with the weld. Up until the mid 60s F1 cars where brased with a torch and held up surprisingly well. Military and experimental Aircraft switched to torch and mild steel and if you build a experimental AC today, you better learn how to use a torch according to the FAA. Stick welding was never a good option, because it was too hard to control the heat. Tig is the most accepted weld, because the heat is so controllable. Not saying Mig can not work if you know about wire size and speed. Heating a welded joint is only one part of normalizing it. How fast it gets cooled is the other. The best possible results come from a good welder, using good equipment, keeping the heat in a area small as possible and still getting good penetration..

If we are realistic, the only advantage CM has over steels that are easy to weld is a small weight savings if you use thinner wall tubing in Molly. Same wall thickness weighs the same, but molly has a little more rigidity.

[let-r-eat]

I've worked with many different makes of metal over the years. Moly is a good option but I'd only TIG. The reason being the filler. The chassis is only as strong as the joints. They are the weak link. Why would you mig them with mild steel? You are defeating the purpose if rigidity is your goal.

Quench and temper is a whole other story.

[MasterSbilt_Racer]

Not to pick on the guys above, but saying moly is more rigid is not really correct. All steel alloys have basically the same spring constant, or modulus of elasticity. The only advantage is simply a higher stress before yield occurs.

This comes in handy in areas such as the cage where strength is important. Otherwise, the rest of the chassis should be designed with rigidity in mind. If you replace mild steel with thinner wall moly, you may maintain or increase strength, but stiffness of the chassis was actually reduced.

[let-r-eat]

Not to pick on the guys above, but saying moly is more rigid is not really correct. All steel alloys have basically the same spring constant, or modulus of elasticity. The only advantage is simply a higher stress before yield occurs.

This comes in handy in areas such as the cage where strength is important. Otherwise, the rest of the chassis should be designed with rigidity in mind. If you replace mild steel with thinner wall moly, you may maintain or increase strength, but stiffness of the chassis was actually reduced.
Last edited by MasterSbilt_Racer; 08-13-2015 at 11:48 PM.


Like the modulus of elasticity of spring steel? That is a perfect example to explain the post I was making. Take a broken spring leaf and weld it together with mild steel. What happens?

I did use the word rigidity in the proper context. A mild steel filler on a moly tube is doing basically the same thing as it would on a leaf spring but on a much smaller scale.

Just clarifying my post.