Re: 4130 Debate

Every so often this comes up on the bicycle framebuilders list

which has about

1300 members. I'm one of only two of us who has ever seen a problem with brass penetrating the 4130. In both cases the tubes had been heated far beyond normal for fillet brazing, AKA bronze welding. (In my case the joint design was so stupid I don't care to describe it here.)

I think the reason to discourage this on airframes is that airframe weldors use darker lenses than we do, and are used to applying more heat on their joints. More likely for them to overheat the tubing. Another reason I heard is that the joints would be heavier, and could cause trouble due to vibration.

An engineer for True Temper tubing told me that they found that post welding (or brazing) heating of joints to 600F significantoy increased fatigue life. According to the text books it shouldn't be hot enough to make a difference. But their testing machines put the joints through hundreds of thousands of cycles, and showed a clear advantage. The theory is that putting the entire joint through an expansion/contraction cycle evens out residual stresses.

However, few bike builders do this, and failure of custom built frames is extremely rare.

BTW for us bike types, thick wall tubing is 0.035".

The figures I posted came from MatWeb. Did you find something different there, for cold-rolled 1020? As-drawn works out very closely to as-rolled.

If you're looking at some other condition, only normalized is a reasonable one to consider. And then your yield strength goes to pot, with plain-carbon steels.

Ed Huntress

When I surveyed some industry brazing experts a couple of years ago, including the author of _The Brazing Book_, their collective opinion was that Finch may have had some bad experiences due to overheating, and then jumped to the conclusion that it was a problem inherent with steel grade rather than one of technique. The book's author had been involved in testing and development for brazed joints on aircraft during WWII (he's in his '80s now), including 4130, and said he never saw anything like Finch described.

They all pointed out what you're saying, however, that overheating is bad news in brazing, particularly with thin-walled tube or thin sheet. It leads first to excessive intergranular penetration, and then to the formation of intermetallic compounds. Both are weak. But that applies to brazing most grades of steel, not particularly to 4130.

Ed Huntress

By "fifty n-joule hammer blows" I meant fifty blows of n joules apiece, not 50 nanojoules. Sorry that wasn't more clear.

I was just making a rhetorical point that the failure mode of a material that receives fifty small impacts does not necessarily look like the failure mode of a material that receives one big impact, even if the total kinetic energy applied is the same in both cases.

MGD

Perhaps the reason that this problem is most often associated with

4130 is that it is often used because you can use it in thinner sections, which are more suseptable to overheating.

If you are building something where tubing is loaded in torsion or bending and you've been using 1" x 0.065" 1020, you can get 121% of the stiffness, at 74% of the weight by switching to 1.25" x 0.028". A tube of this dimension would dent too easily in 1020, but at least for bikes is usually plenty tough enough in 4130. However, if you've been used to brazing or welding 0.065" wall, it's very easy to overcook

0.028".

Mark Stonich

Sounds good in theory, but the newer, large diameter frames have a higher failure rate than the traditional 1" top tube and 1.125" seat and down tubes. Eddy Merckx only broke two frames in his entire career. I know 145lb, 18 year old roadies who've broken two frames.

The older frames were supple. The new oversized tubes are thinner and lighter, but stiffer in bending and torsion. They don't flex much so all the impact is concentrated at the joints.