Any idea what metal I may have? (55HRC)

I purchased this block of round stock off eBay. Was told it is 1045. Realizing I need to slice in half and couldn’t fit in my band saw I figured I’d mill four flats. Wow, have to tell you that was something else it was extremely difficult to machine. I machine 1045 gear stock all the time and never experienced anything like this. I noticed the excessive shaking and pounding right away. I proceeded, swapped cutters out for carbide, it helped, and then decided it was a bit rough on my tooling and mill I called in the white flag and pulled out the Rockwell test kit. 65hrc? Nope it scratched, 60hrc? Nope it lightly scratched, 55HRC? Yep the file slid across the surface. Uhmm that’s not 1045 unless someone hardened it but we’re not talking surface hardening. There isn’t any carburizing that’s been done to this. It’s very raw looking yet and I tested where I cut a good 1/4” off the rounded side. This has a 55HRC internal hardness. I think I may have a different material on hand. Any educated guesses? Unless it was through hardened but the seller made no mention nor does it look like it’s gone through the process. I harden 1045 steel at least once a week so I’m familiar with the process and you typically have those tell tale signs. Only thing remotely suggesting hardening is the scaling on the outside but it looks more like a rust treatment to me.

Here is the label that was displayed in the seller listing. Looks like ASTM a434 bc. I’m no expert but I thought 1045 was ASTM A108 ASTM A29 UNS G41400. I think he sent me 4140 which would make sense it’s a bear to machine and I wouldn’t have hoses it for this project. Am I missing something?

It could be tool steel maybe a stainless.

Didn't see the label before posting

Why not just drop Timkensteel.com an email and ask them

Here is a photo after quickly hitting the surface with a face mill. It is struggling to clean up. It’s definitely nothing I’m going to probably machine here unless I can find out what it is and possibly anneal it to make it machineable. I went through a $40 box of carbides already and that’s without pulling out the drill bits, end mills, taps, etc. I had planned to make a tool turret out of it. I’m sure it would be great material for it. It almost has that 4140 look to the finish. I’d have to chuck it in the lathe to know better and I can usually identify 4140 and 17-4 when I’m turning it but I just haven’t machined it much; especially something of this size. I’m only out $25 on the round block I’m sure I can put it up and someone will want it that has a larger mill that can handle cutting through it. I can already hear my gear box rattling and haven’t done a belt conversion yet not wanting to kill it before I get that far.
Edit: Temp after machining it is 100 plus degrees.

I suppose I could try annealing it in the oven by 4140 specs and see what happens. Might get lucky and it works.

Have you tested it with a magnet? That may reveal content of a stainless variety. I have seen stainless turn hard like carbide if the cutting tool slips and generates local heat. Slow cutter speed and lots of force will complete the cut but any excess heat and you have a hardness problem. You may need to save this purchase for some other purpose like a small anvil and start again.

Did it initially cut OK, then start to act up? If so it could be air hardening tool steel. I've run into that before with 1/4 inch plate I was drilling holes in, they initially had nice curls, then destroyed the bit. It was mystery metal from the salvage yard.

Further research, this looks like it is 4340 steel, and in the hardened state is between Rc 56 to 60.

Yeah it’s magnetic and it’s not stainless. It turned out that it is indeed 1045 however it is hardened to 55hrc. The seller didn’t realize it got that hard but it was the end piece and apparently the cut with a hot knife because he said the bandsaw would get chewed up by trying to cut the end pieces. I suggested he let people know when he is selling it was a bear to machine.
I did find my good 1204 carbide inserts I save for these kind of tasks and it cut much better but in making an octagon it probably took a good 4 hours. I’m not quite finished it now has to go to the lathe and have a dust/Swarf collar machined into it as well a center hole and then I will attach by center hole to rotary and finish cut the ends down to 59/60mm per side dependent where the center aligns, etc... this was a first for me I hadt run into a surprise like this before. Typically when I buy material hardened I know what it is and and it’s never 55HRC. I probably could’ve annealed it but I decided to go for it. Originally the material was purchased to make curvic couplers and because it’s so hard I’m not destroying expensive tapered mills on it nor band saw blades slicing into two pcs so it’s going to be the face of the 8 tool turret instead of the stainless 17-4 I have; which by all means is far easier to machine than this. In fact, I like high speed machining 17-4 you just have to make sure to use coolant in through holes and such not to work harden it. Essentially that’s what I got was a work hardened by hot knife, round end bar chunk. It reminded me a lot of 4140 moly in appearance and machining. I would liken them to be very similar and I’m choosy about how I use 4140 so
I don’t have to struggle with too much time spent machining. Wouldn’t be an issue if I was paid by the hour regardless of spent time but this is a house project for a retrofit and on my spare time which is plenty limited these days.

DJP said:

Have you tested it with a magnet? That may reveal content of a stainless variety. I have seen stainless turn hard like carbide if the cutting tool slips and generates local heat. Slow cutter speed and lots of force will complete the cut but any excess heat and you have a hardness problem. You may need to save this purchase for some other purpose like a small anvil and start again.

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It would make sense to temper the material to HRC 28 to 32 which is the hardness of stress proof. At this hardness 4140 machines extremely well with good surface finish. I would expect similar results with 1045.

We make gun parts from 4140 Prehard HRC52+-. Rigid holding, good quality carbide cutters. Then we don't have to deal with warping post heat treat to get the needed tensile. Hope you don't have any warpage as you machine. Sounds like your metal was hardened accidentally and not necessarily soaked and stress releaved. Spell check has pretty limited manufacturing vocabulary and is really annoying.

The brittle-to-ductile transition temperature for fully hardened (non-tempered) chrome-moly steels (4130, 4140, 4340) is about 200 degrees F, making the metal very brittle at room temperature. This is why gun barrels (and B7 & B16 steels) are tempered between 1100 and 1200 degrees F - because it shifts the brittle to ductile transition temperature below freezing (permitting safe firing during the winter seasons). The US Army had a run of howitzer failures in the 70s - due to 700 degree temper on 4340 - causing brittle-fracture failures when the guns were fired at temperatures less than 50 degrees F. Look up the heat-treatment tables for the alloy - pay particular attention to the Charpy Impact Energy at various tempers.

I honestly can't see any reason to have fully hardened 4140/4340. If you need 50 plus HRC with serious toughness, S7 or D2 steel is the way to go.