BritishV8 Forum

It's a good point Curtis, and really, the extra expense and effort only makes sense in an all out racing effort where every ounce of weight reduction counts, or where space is a factor. Then, if you can get at it to tig weld it, you can also follow behind with the rosebud to anneal it. Not much difference at all in terms of space needed and heat generated. Or if space is that tight, the cutting tip will do fine also, just don't hit the lever. It only has to be warmed well. At 400-500 degrees bare steel begins to color. Between that and 1100 it will go through the blue spectrum ending up a very light blue at around 650 degrees. By this point only the deep stresses remain, as the surface stresses have mostly been relieved. From that point until it begins to give off light as a very dim dull red glow, the rest of the stresses are relieved and the metal is fully annealed. It's important to recognize that this is a progressive process. It doesn't just fall of the proverbial cliff at 1100 degrees to the fully annealed state. Plus as already noted any projections or edges will get hot first and therefore get soft first. Inside corners will need the most attention. There is virtually no need to heat soak the metal as the tubing is thin enough that the inside will be at roughly the same temperature as the outside.

But let's take the case of the forward struts on the Jag IRS for the MGB-Roadmaster for an example. Those are made from heavy wall mild steel tube and are probably the weak point in the suspension. To make them a larger diameter would mean making a sharper bend for clearance to the inside of the bend which would make them weaker so it's a compromise set in a dilemma. Perfect application for chrome-moly wouldn't you say? Use the same dimension tube and roughly double the strength without changing the geometry. The entire link could then be heat treated without too much trouble at all.

Jim

In regards to things built in-situ like a roll cage,  if you can't heat treat the entire cage - unless you put the whole car chassis in an oven (don't think that's feasible?) -  Don't you have to hand "heat soak" the entire tubing length not just the weld points.  Are the tubes "pre-heated" and doesn't the high welding heat radiate out along the tube causing other issues?


Trip

You're missing the point Trip. As supplied, the tube is annealed, which means that it is as soft as it ever gets. In order to be hardened it has to be heated, in the case of 4130 probably in excess of 1500 degrees F which would be a bright red and then it is quenched. The only area that ever gets that hot is in the weld zone so that is the only area that is in any danger of being hardened. Therefore that is the only area that has any need of being annealed back to the soft condition.

Annealed 4130 is considerably stronger than mild steel tubing, but is still reasonably ductile. It therefore allows a lighter construction. Only an extreme racing application could ever justify the use of a fully hardened 4130 cage, and that would be a full custom tube chassis. A stock body would never be used due to it's weight.

Jim

Okay, now I understand.  Heated to 1500=Hard=brittle.  Thanks for the clarification, Jim.  I definitely don't have any delusions of turning my home-built project into a full race car but it's fantastic to have the understanding and knowledge, though.  I can see how a custom body ie. in pro-drag car could be used with a fully hardened cage.

Thanks again

Trip

Close Trip. Heated and then Quenched to harden. If just allowed to cool it will not get hard. Welds get hard because they are quenched by the cold steel immediately surrounding them. If preheated before welding they will not quench and will not need to be annealed. If quenched intentionally the steel will then need to be "tempered" or "drawn" in order to reduce brittleness in most cases.

Mild steel or the most common low carbon alloy used for tubing, 1018 steel, has about 60-65,000 psi of tensile strength. It cannot be hardened significantly without the introduction of extra carbon, such as is done in case hardening, by heating the metal and introducing a carbon bearing material to it while it is in a state that will allow it to absorb the carbon. But this is a very valuable and useful property that is used quite commonly to create a hard shell or cutting edge. This steel has a .18% carbon content and no other significant alloying materials, hence the 1018 designation.

By contrast 4130 chromemoly has .30% carbon and as annealed has 115,000 psi of tensile strength, nearly double that of 1018 steel. If hardened and drawn to 400 degrees it has 236,000 psi of tensile strength, nearly four times that of 1018. In that condition as pointed out above it will tear  upon deformation at a less extreme angle than the annealed metal or 1018 also, but it is also quite clear that it will take four times the force to bend it in the first place. Also since it is a medium carbon alloy it is not particularly brittle. Cracks and work hardening are only a concern if stress risers are allowed in the construction, provided it is evenly hardened and tempered.

As such it's easy to see why it is such a widely used alloy in the auto industry, being very common in things like spindles, shafts, suspension links and much more. Any time you see a suspension part and wonder at the small cross section considering the loads it carries, it's a pretty good bet it is heat treated chromemoly, usually either 4130 or 4140. And it may interest you to know, that as a medium carbon alloy steel, in the fully hardened state it can still be machined.

Jim

Where are these numbers coming from? They simply don't match "Engineer to Win", which is still sitting on the edge of my desk. (I've been working on an article about a Formula 5000 racecar this week, and Smith's books are especially full of insight into those designs.)

(from the chart on page 66 of Engineer to Win)
cold-rolled 1018 sheet and tube: ultimate tensile strength = 82,000psi
cold worked and annealed 4130 sheet and tube: ultimate tensile strength = 95,000psi

(from pg 63)

QuoteThe other side of the 4130 coin, often unknown to (or at least unappreciated by) the racer is that it possesses poor deep-heat-treating characteristics and has an inborn dislike of varying cross-sections. These characteristics make 4130 a poor choice for machined or forged parts. It doesn't machine very well at least in the normalized condition - too gummy. Those people who make hubs, steering knuckles and the like from 4130 are kidding themselves - and their customers. It doesn't make very good shafts either, as in drive shaft, or axle, or torsion bar.

4140 is better for deep hardening and machining, but harder to weld... To get to 236,000psi, we're talking about "4340 modified" (i.e. alloys with vanadium) and big budgets.

Curtis, those numbers are from Machinery's Handbook, 20th edition, 1978. (Current edition is 28th and 100 years in print) I think you'd have to agree that they've had ample time to error check their information, and based on practical experience I would have to agree that the roughly double and quadruple numbers are at least in the ball park. I don't know if Carroll Smith has done any actual tensile strength testing to base his numbers on, or if he is using manufacturer's numbers or what, but 82K sounds awfully high for 1018 and only a roughly 15% increase in strength between 1018 and 4130 just doesn't sound realistic.

But, plate, bar, structural and tube often have different characteristics and different sources can vary widely on their testing results so there's no point in arguing the specifics. The point is that an alloy steel (pick your favorite, there are so very many of them) is going to be noticeably stronger than a low carbon steel and a heat treated alloy is going to be noticeably stronger than it's annealed brother. If you want ultimate strength, a hardened electric arc remelt cobalt alloy is hard to beat, but you can always find a way to spend yet more money. If that 235K number seemed high, recall that it was at a 400 degree draw. Hotter draws yield lower tensile numbers and this has a great deal to do with the variations we are talking about. Commercial 4130 HT or 4140 HT are going to be drawn to a higher temperature than that, but if you'll recall, we were discussing a custom heat treat for an entire cage so we can specify the draw we want to use.

For truly accurate results there is no substitute for a reliable supplier and in-house testing. Since most of us don't have that luxury we have to make do with what is commercially available and the results often vary widely. Thankfully once we are accustomed to a given product we can usually spot a different material very quickly, but within the range of noticeable differences there are going to be some pretty significant variations in all commercial supplies, and that's what good quality control is all about. If you don't like the characteristics of what you are getting it's pretty easy to change the specification. If you don't like the way 4130 or 4140 machines, there are free machining varieties available. If you aren't happy with it as a shaft material, pick one you like better. It's not like there was only one choice or something. But just because one guy doesn't like a particular alloy that doesn't make it a bad alloy either. Could just be the way he's using it, or the particular variety of it he's been using, or the draw temperature of the stock he's gotten, or a hundred other things. Just because a change in the material worked for him doesn't mean everyone else who is using it is wrong either, maybe they are all using a leaded alloy for increased machinability. Or maybe they are using stock supplied in heat treated form, which machines very well with carbide tooling and leaves a nice finish.

What really matters is the end result, and the time and money it took to get there. Alloy steels wouldn't be around unless they served that need very well indeed. But they aren't the solution for everything and sometimes black iron pipe is just the ticket. I just wouldn't build a roll cage out of it unless that was a last resort, Mad Max sort of situation. And then I'd build it beefy.

Jim

Wow, this is an AMAZING wealth of knowledge!  I'm going to be reading your posts over many times so I can absorb all the finite details.

Trip

PS. I'm also reading your other posts RE: welding.

Thanks for sharing your insights with me, everyone!