Tempering bolts

What did you make them out of?

Hardened fasteners are made from "hardenable" tool steel. If your test bolts have been made from ordinary (mild) steel there is no practical way to harden them any further. Some might suggest you can case harden them by heating in carbonaceous material and quenching. This has to be done numerous times for a case to build up and, at best the bolts will only be more wear resistant, not substantially stronger.
If you make the bolts from tool steel (usu. round drill rod) they can be heat treated in the home shop by following the instructions that come with the steel. This can be done by heating to cherry red (temp. where a magnet is not attracted), quenching, then tempering in a 375 degree oven for around 1 hour. This will yield some amount of hardness between file hard and cutlery hard. You can make acceptable cutting tools such as taps via this method. It is not recommended where you need maximum strength such as in automobile chassis, etc.
Bob Swinney
I've got a little project halfway done. The hydraulic clutch line fittings on my car are damaged (the flare tubing nut hex are all rounded off). Due to its oddball size (12x1mm thread, 6mm tubing), in addition to putting out some feelers for actual parts, I decided to try my hand at turning a couple on the bench lath last night. So far, so good. They appear to fit (hand tight) just fine. But before placing these things into service, what would the best quick and dirty procedure be for hardening/tempering these things before actually torquing them down?
I'm a machine shop newbie with a couple of propane torches and an assortment of motor/light machine/vegetable oil available for quenching. Oh, and a pretty good thermocouple module for my DVM that will go to 700C or so.

In my experience brake and clutch fitting nuts are not hard and so I would see little point in hardening the ones you have made. You could check the old ones but I suspect they're soft. In the UK you can get brass versions of most brake/clutch tube nuts, they're not highly loaded. Maybe the best bet is to get a proper hex spanner intended for them so the nuts can't be rounded.

That's a tough one, Paul. As others have suggested, if you know the specific type of steel you used to make them, you might have a chance. If you don't know, you may be better off leaving them as-is.
If you're careful when wrenching them, the flats shouldn't be a problem. The more likely potential problem is the threads. But even that may not be an issue, because, unless the nipples onto which you're screwing the nuts are fairly hard themselves and the fitting demands a lot of torque to prevent leaks, you ought to be able to get enough torque on them without stripping them.
And consider this: If the nipples are harder and you *do* wind up stripping a nut, the nut is a hell of a lot cheaper to replace than the master and slave actuators for the clutch. I'd just give it a try and be happy if it works; and glad, if you strip the nut, that you didn't strip the other parts.
-- Ed Huntress

[snip Q re hardening] Probably the OD could be increased a few mm also. For example, if the nut is 14mm flat to flat, going up to 17 or 19 mm would make rounding off much less likely.
See "proper hex spanner". The link shows a fuel-line wrench, which is like a box end wrench with a short segment cut out.

If the original fittings were damaged as you say, then they probably were not hardened. Since the subject of this post is "tempering bolts" I assume you made the parts from a bolt. Do you know where the bolt came from, or what it's made of? Do you know about the grade system that's often used in the USA?
If you did harden the fitting after machining it, there's a REAL good chance that something, not good, might happen to the threads during the heating, quenching and cooling process. I am assuming here that you won't be doing the heat treating in a neutral or reducing atmosphere or in a vacuum. Oxidation could easily change the dimensions of the thread for the worse. And, any oxidation left in the threads when you reassemble could cause galling, etc..
Pete Stanaitis ------------------------
Paul Hovnanian P.E. wrote:

Come to think of it, if the nuts were all that hard they'd break long before they rounded off.
Give one of the rounded-off ones a stroke with a file -- if it cuts readily, it ain't hard. If you're really worried, compare the 'cutability' of what you've made with what you're replacing.

They're called Flare Nut Wrenches in the US. If you put some grease or anti-seize on the threads it should be OK.
The steel in Grade 5 bolts isn't difficult to turn with HSS. In my experience Grade 8 bolt shanks can be machined also but I've never cut an internal thread in it.
The flare nut wrench of ultimate desperation: jsw

Any good import parts house will have metric brake line and fittings. JR Dweller in the cellar
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[snip] 1018 cold rolled steel (what I happen to have a lot of lying around).

1018 won't heat treat. Should be fine for your parts, just use a line wrench and you won't round the hex. Put some anti-seize on the threads and rust proof your parts with paint or something.

Sure it will. You case harden it. Heat to 1875 degrees F and then quench in motor oil. You end up with a 60Rc case.
JC

That last bit is a good point. I'm getting pretty good at knocking these things off. So if I bugger up something, these are cheaper than the slave cylinder.

Here=92s one (P/N # M8-6mmbr) similar to what you=92ve made, but in brass: DG

Two things: Might want to put a slight radius on the leading edge of the nut, so you don't risk buggering the threads.
And make sure you do a proper double flare on the tubing with no rips. Brake lines are never single-flared, if the tubing splits a little crack leaks out a lot of fluid at those operating pressures, and you don't have a lot to spare.
-->--

I never heard of case hardening with motor oil, I use KCN when I have to but it scares me. What are the procedures for oil? It doesn't seem that it would supply enough carbon.

As I described Tom. You clean the parts, wrap them in SS foil and then bring them to temp. Once you've done so, you remove them from the furnace, cut open the foil "bags", then toss them in a can of nondetergent motor oil. You need to be quick and there will be smoke. 1018 has just enough carbon in it for this to work but O-1, O-6, or 8620 are much better.
In the olden days, wear plates for molds were made this way when O-6 wasn't avaliable. GraphMo, one of the original modified O-1 trade names and actually ASTM O-6, is basically just high carbon cold rolled steel with graphite added to it.
Getting 1018 done professionally is a better idea if you have to use it and in that case your write the order up as "Carburize and Harden" and specify a case depth or, if the thickness in relatively thin, specify through hardening. Through hardening leaves you with a 60-62 Rc result but it's obviously only useful in compression this way.
Case hardedned bolts are fairly common, or they used to be, because they could be removed when they stripped or rounded. Flame, or torch, hardening works the same way but you aren't heating the entire part, just the surface. You've probably got an entire collection of machines that were advertised with "Flame Hardened" ways when they were built. Induction hardening was the next process improvement and is still used today in many applications.
JC

I think you've been misled about this, John. 1018 won't harden that way. It would need about 40 points of carbon to harden significantly from heating and quenching; 1018 is 18 points.
If you want to *case* harden 1018, you need to supply a carbon-supplying atmosphere at a temp. of over 1400 deg. F or so. That can be a case-hardening compound, or charred bone, etc., or a carburizing atmosphere (CO, most often) in a muffle furnace. Graphite will do it, but it's very slow and often won't raise the carbon potential enough on its own. It's used to make heat-treating boats because it produces an atmosphere that will leave the steel inside the boat roughly of the same carbon content it started with, or just slightly higher.
If you wrap 1018 in foil as you describe, raise it above critical temperature and quench it in oil or water, you will wind up with a *marginally* harder material, because you'll have converted a very small amount of the ferrite to martensite. But, as I say, 18 points of carbon won't harden it much -- maybe 10 points on the Rc scale at the very most.
If you send those parts to a heat-treating shop and specify "carburize and harden," they'll probably do it in a muffle furnace with a carburizing atmosphere. That's not something you can do at home without a pretty elaborate setup, because a CO buildup, at high temperature, will turn your shop into a fuel-air bomb -- if it doesn't poison you to death first.
BTW, you can *color* case harden with a torch, but the carburized layer will be measured in microns of thickness, and will have no effect on mechanical properties of the part. But it will be pretty, if you're good at it.
-- Ed Huntress

All I can tell you is that I've had to make hardened cores for molds out of CRS in emergencies a couple of times Ed. We did it the way I described and if Watkins still makes the thing, you can buy A Hot Spings Spas Grandee with a skimmer weir that has those very core pins in the injection mold that makes them. The weirs are Rovel and if the cores weren't hard they'd have been toast in a couple hundred shots the way the soft steel ones they replaced did. Weatherable ABS is fairly abrasive.
OTOH, you don't stop at ten at night on a Sunday to how hard anything is when your parts have to be on the company President's desk at six AM on Monday morning so who knows exactly what we ended up with. Watkins had forgotten to order a mold, or parts, for what was then their newest addition. I'd have ordered hardened core pins from DME otherwise.
My heat treat tools of choice are a pen, an order book, and a telephone. A good heat treater is a mold makers best friend. Him and the welder I'm not a big fan of using whatever you have laying around but sometimes you go with what you've got.
JC