Am I blackening steel correctly?

Further to my previous thread regarding original Myford levelling studs and raising blocks, I de rusted rhen degreased the four levelling screws/bolts, with a view to chemicaly backening them for protection going forward. Here's a photo of before and after:

Although it's acadmeic for these four screws, for future reference can others confirm whether this is a correct method:

Heat with gas torch to a faint dull red then drop into new motor oil.

I'm still unsure of exactly why blackening is recommended for steel components does it both harden (via the quenching) and behave as a good rust inhibitor - does the carbon from the oil effectively etch itself to the steel surface?

I assume for typical modelling threads this won't interfere with thread tolerances?

I tried it a while ago on fabricated stuff. Only had old vegetable oil to hand, no motor oil. Had kinda mixed results.

Somewhere on youtube there is this watchmaker explaining to a small group how to make a centre punch, he gets a fantastic finish but I think I remember him using dirty motor oil.

I do a fair bit of blackening steel. For consistent results use old used diesel oil. It's a better "black" and i've had inconsistent finishes with new motor oil.

I blacken purely for the anti rust side of things.

My 2 Baht's worth...

I use linseed oil, I heat the steel part to around 280 deg. C and apply the oil (and burn the rag afterwards). It seems to work best for smaller parts.

Thor

There are also chemicals for cold blacking which is ideal for heat treated items.

Oil blacking will not harden mild steel components as this type of steel cannot be hardened by quenching as the carbon content is too low also dull red is not a high enough temperature to effect higher carbon steels.

Blacking does seem to give a degree of rust protection but not a great deal in my experience.

Tony

I use old vegetable oil from the deep fat fryer. Works for me.

We used to blacken steel using old engine oil from diesel vehicles the temp that you take your parts to has a great part on how good the blacken turns out

Heat in subdued light. Dull red appearance in bright light is somewhat hotter than in shadow. Probably best done hanging parts from wire in small brazing hearth with cover. I let things cool off a touch to just below red before dunking. On nuts, bolts and similar small parts basically hover over the oil for a few seconds [ count of "one and two .... and six"   ] then dunk. Dunking at red produces significant smoke and vapour. Much reduced if let to cool a touch.

Far as I can see black is just as good. I find old oil blacking good for a year (ish) outside, often more if sheltered, and pretty much indefinite indoors or unheated shed / garage store.

Clive.

Edited By Clive Foster on 02/01/2017 10:13:09

Edited By Clive Foster on 02/01/2017 10:13:28

Edited By Clive Foster on 02/01/2017 10:13:55

When I have seen it done - metal work teacher at school he got it hot and just kept squirting oil on it and carried on apply heat. From memory it was for a farm gate and he used a gas air brazing torch with a big fairly gentle flame for that sort of thing.

John

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Edited By vintagengineer on 02/01/2017 12:45:08

Hi,

Is there any evidence to back that up?

Michael W

I'm also waiting with bated breath for justification from 'vintagengineer' for his original staement

Me too. One of the reasons Japanese cars were pretty well known for not rusting is that they were mainly made there from recycled steel from car bodies gathered from all over the place. Something to stick in empty ships on their way back home at one point.

Education - reprocessed steel will generally be better in all respects than that derived from ore.

Maybe there is some confusion about changes to the steel that is used to make car bodies these days. Pass on that as it's an area I am definitely not up to date on but do know that something has changed. Reprocessing means getting the stuff to the analysis that is required. The fact that some one has already done that to some spec is an advantage.

John

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Used engine oil contains carcinogens. I use rapeseed oil and it works very well.

Neil

Indeed, without proper caution taken, used engine oil can leech into the skin and contribute to cancer, it is supposedly the number 1 reason most car mechanics now wear rubber gloves.

PS. one of my friends thinks that this rule applies to all oils, but this isn't true because not all oils are, atleast, as highly carcinogenic as used engine oil. If it was true, then i'm sure nobody would be rubbing olive oil into their aching joints.

Michael W

Edited By Michael-w on 02/01/2017 15:26:33

For that matter, BMS, or bright mild steel, is made from the same stock as black steel, BMS is cold rolled reprocessed black steel to improve the finish and hold tighter tolerances. I've used lots of BMS and cant say I've had any trouble with it.

Michael W

Ordinary mild steel has always been a bit of a lottery when it comes to exactly whats in it. Usually it is supplied as being within a rather wide range of very limited mechanical properties rather than to a specification. It certainly seems to have got much more variable over the last few years and its quite common to find stuff that doesn't like threading or won't give a decent finish without extra care. Decade or so back you'd generally get quite nice stuff as matter of course.

Higher scrap content from more engineered steels could explain this. Certainly the higher end of common mass produced steel components has become more engineered over the years. Car bodies et al tend to have a fair bit of "high strength" steel in them as compared to the more basic materials optimised mostly for deep drawing and pressing used in days of yore.

I suspect less basic mild steel is used for engineering jobs now too. Almost everything being called out to a steel specification rather than simply taken off the shelf. So simple mild steel tends to be relegated to undemanding work with even less attention paid to detail properties. My big worry is how well the stuff will weld. PIA to machine being obvious when you work it but stress cracking and other issues that may cause failure of apparently sound welds after some service certainly aren't obvious. Last lot of welding I did with ordinary steel and off the shelf box section did not go as easily as I'd hoped. No issues in my application where the weld was basically metal glue to mutually supporting parts but I can think of jobs where the material would have gone back as being clearly out of specification.

Safest to order by number. Remembering that if its supplied by properties rather than by composition what you get inside may be somewhat different to what you expect.

To a degree its always been thus. Bed frames and bed irons were always notorious for being hard to saw through. Allegedly made from re-rolled railway rails. Which may or may not be true but I've worn out my share of hacksaw blades when re-puposing them.

Clive.

I would agree that it's hard to tell whats in it once it's made, but I would reiterate that it isn't in suppliers interest to garner a reputation for poor quality product. As Ajohn said earlier, the steel normally has to be made to a certain analysis and it's hard for them to justify a large deviation from it without it having a knock-on effect.

Michael W