Auto axles aren't made from 4130 or 4140 or 4340 or any other desirable alloy worth salvaging. Some are just plain medium carbon, usually 1055, but most are 1541. 1541 is a manganese strengthened medium carbon steel that's cheap, but still has good toughness properties after induction hardening. However, the high manganese content content makes 1541 difficult to machine and near impossible to weld with reliability.
The poor machinability isn't a problem in the manufacture of axles as there's comparatively little machining involved relative to the size of the part. The only close tolerance turned dimension is the diameter of the area to be splined. The splines are roll formed, and the diameter prior to rolling has to be held close to avoid a partially formed spline, or wrecking the tooling in the case of an oversized diameter. The flange and pilot diameters aren't close tolerance as machining goes and can be plowed off with little regard to surface finish. Same for the roughing of the bearing seat diameter as it's either ground or hard turned after heat treat.
But, if you take a piece of this material and try to make any sort of close tolerance part with good finishes and a nice overall appearance, this **** will fight you all the way. Face and turn a part with a cnmg insert and then come back with a snmg set at 45° to chamfer the corners and it'll cut the chamfers and roll up a couple tiny razor sharp burrs in the process. With a CNC lathe, you can program the finish facing, turning, and chamfering as continuous motions and get around that sort of problem, but you still have to contend with the fact that it'll kill tool edges at about twice the rate of more common high strength alloys such as pre heat treated 4140 or ETD 150, neither of which requires post machining heat treatment to develop it's strength like 1541 does.
I ran some sample parts from this stuff a few years ago for a customer who'd hired some new genius who claimed they could switch from ETD 150 to 1541 and save a bunch of money. I'd previously run a few batches of these, totaling around a couple thousand parts, so I had a well optimized program already. I was running these on a 21,000# Okuma LC series turning center which is among the most rigid lathes ever built, yet running these at my previously established feeds and speeds, the parts came out looking like I might've made them on a HF lathe using tools ground with a side grinder. By the time I diddle-dicked around with the program to the point where I was making a sorta decent looking part, I'd added about $3 per part in additional cycle time.
The 1541 parts would require heat treatment after machining, and one diameter was +0/-.0004 for a bearing seat. I hardened and tempered a few pieces as I was suspicious of that diameter holding size, and sure enough it went out of tolerance on most of the samples. Only solution to that would be to leave a machining allowance on that diameter and hard turn it after heat treatment. Between handling the parts again and the actual turning, that would add another $2 per part. On top of that would be the cost of outside heat treatment plus transportation to and from, none of which was present with the pre treated ETD 150 stock. All things considered, the "savings" amounted to an increased cost of about 25% on a finished part.
Even if it's free, and even if you're only machining as a hobby, the aggravation of dealing with some monkey metal intended for high volume production parts just ain't worthwhile unless continuous frustration is your idea of fun.
