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Recreating an interesting hardened part.


Jul 18, 2023
I trying to choose a steel to re-create an old car suspension part. The original (a trunnion cap) will not file, indicating that it is hard or hardened. When tested it showed Rc4, which should definitely file. Looking at spark test charts my best guess is that it is mild steel. The mating part, a trunnion (threaded pivot) Rockwell tested at 19Rc and is a case-hardened steel casting. Neither part showed wear. I need to match but not exceed that hardness so I don’t have to remake these anytime soon. The original also self-tapped with a weird broad profile thread into the stamped steel control arm. The originals stripped the arms in disassembly.

My best guess is that the original was medium carbon steel with flame/induction hardening thin enough that Rockwell tester didn't "see" it. We also don't have the shallow penetration set up as far as anyone at the shop knows.

I am remaking the part with a home-brew Class 5 interference thread (I need to match the -11 pitch of the trunnion at 1-1/8 OD to maintain 1/8” wall thickness) so that it doesn’t eat itself the next time I have to work on it. The handbook recommends Grade 5 (ballpark of QT 1045 correct?) for Class 5 (interference) external threaded parts.

My best choice by mechanical property and application was 1144 stress or fatigue -proof but they are harder than the trunnion untreated (25 and 30 Rc respectively). These cannot be heat treated further, as they are previously stress-relieved. Next best was 1045 and 4140/ 41L40. These are listed as 90 and 95 Rb respectively as supplied which seems too soft. They also are said to harden into the 50-60 Rc range. I have heard both require annealing to machine well, particularly the ChroMoly although the leaded stuff should be ok. I also hoped to avoid asking to use the heat treat oven as playing with old cars in not my real job. Then, again all they can say is no.

The part is roughly 1-1/8 long with a 1-1/8 hex and 1/8 walls at the internal and external threaded areas. Parts will be turned, bored, ext. threaded and int. threaded on a manual engine lathe with carbide tools.

The trunnion pivots inside the part on the threads, they are greased. The cap is the wear item as I cannot replace the original cast trunnion without even more difficulty.

What steel will harden to 19Rc by flame and have strength and toughness of xx40 steels? Could I anneal a harder steel without loosing strength or toughness? How much harder or softer than 19Rc could I get away with?

Thanks to anyone who finishes this book.


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I'd have to learn more first, but my first course of action would be to get a piece of higher quality steel, something in the 1000 series or better, than machine it out of that. Then....I wouldn't worry at all about hardening it. That's because I suspect it has no need to be hardened.
19 Rc is very soft steel. O1 tool steel is in that general range as supplied in its annealed state. 1018 will be around 10.

4140 prehard will be around 28 to 30 and is cheap, machines sweetly, and is widely available.

I agree that there seems to be little reason for needing to heat treat your part. Are you planning on running this “old” car 100000 more miles?

Those thread which are 'weird and broad' sound a lot like the threads on a Mopar upper ball joint of the 60's/70's. I have cut 'nuts' for them...I measured the TPI (I think it was 5 or 6) and, as it turned out, the profile matched my boring bar insert pretty well. The thread depth, I made to match. I cut some on the looser side, and some on the tighter side, until I settled on a depth that took some torque to assemble. I figured that's a good thing as it help retain the fit. So far...no issues.

I have heard that those threads are used as they essentially self-tapping...the female part would have no threads, just a bore, and the male part would be cranked in until it bottomed out. Personally, I think that would be successful if you had a factory-assembly type tool that did the job, but you won't achieve the same results in a shop. That's why I cut the threads on the nut.
When a suspension arm ball joint fails ,interesting things happen......pucker factor maxes out.

Makes some awfully worrisome noises too! The weird hardness reading could have been a number of things, from a nitride layer to a layer of plating, etc. You could always just use some off-the-shelf 4140 Q&T. That should be plenty good enough.
You could take the original and have it tested by a lab. If you have any scrapyard contacts, many large ones have very expensive "guns" that can reliably tell what is in front of it.
Thanks everyone! What a difference a day makes.

I did a little more homework myself and found different hardness charts I could jump between. As many said, off the shelf 4140 or even the 1045 should work. They are in the same range as the cap untreated, and the trunnion is hardly hard at all. 4 Rc works out to around 85 Rb and 160 BHN (low/mild steel area). The trunnion's 19 Rc works out to be just over 100 Rb. If I match the hardness of the new caps there shouldn't be a problem. The original caps and trunnion showed no wear so they must have got it right back in the sixties!

These parts are not ball joints but failure would be just as disturbing (think rapid unscheduled disassembly). Thanks for your concern. They come from a 1963 Rambler American. The trunnion design didn't change at all from the pre-war Nashes up to this year. I have heard of the Mopar ball joints but didn't think to look that way for ways to fix my Rambler. I'd be surprised if I put 100k miles on this car but it'd be cool to see if the parts last. My next project is gonna be one I can buy parts for I think.