What is black steel?

Its as it comes from the rolling process not pickled in acid to get ri

of the ash left from being hot rolled , it usually is cheaper

-- olmo

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Black steel is as it comes from the mill and not descaled. Has a hard skin of scale.

Bright Mild Steel is the same but descaled.

Peter

-- Peter A Forbes Prepair Ltd, Luton, UK snipped-for-privacy@easynet.co.uk

Isn't there another key difference - at least according to the text books? You can machine the black stuff without it distorting as it's supposed to have no internal stresses. BTW I've successfully removed the scale with "electolytic derusting". Lots of refs on Google to method, I used washing soda solution as electolyte

Because black bar is hot rolled it is annealed as it cools, in it's finished shape. Bright bar is cold rolled (achieves better tolerances and finish) but cold working any material does lock in residual stresses, which can be released by heating or machining.

Regards

Kevin

Black refers to the way it is finished after rolling down to size from a 5" billet. The black bar has been cooled in air from rolling temperature - in modern mills in a cooling rack and in the old days on the mill floor. This allows the outside to oxidise - which gives the blue-black oxide form (this turns to the red oxide form if you let it). The description for this simple cooling is 'normalised'. If for metallurgical reasons you want the material dead soft you could anneal it by decreasing the cooling rate further eg by cooling in a controlled furnace.

The softness of the bar depends on the heat treatment effected by the cooling process and the chemical constituents of the steel. So called mild steel has very little carbon and significantly more manganese. It is relatively unaffected by heat teatment so the normalised material is not far different from the annealed . It became the material of choice for boilers and structural steelwork only after the French discovered how to produce low carbon ferro-manganese in about

1880. Prior to this, bulk manufactured steel had a middling high carbon content, which restricted its structural uses, although it made good railway tracks.

Bright bar has had the black oxide removed, usually by pickling in dilute hydrochloric acid; although they used to do peeled bar at one time. The bar can then be cold worked to a precise size by drawing it through a die (wire drawing), reeling or cold rolling. This cold works the material and induces residual stresses, particularly near the surface of the material. If you then machine the material assymetrically, the stresses on the side which still has the material will pull relative to the neutral axis and bend the workpiece.

So : mild steel is low carbon bright bar has been cold worked and may be susceptible to residual stresses black bar is usually slightly softer and less ssusceptible to residual stresses black bar should be cheaper but is not always available at model engineer stckists

HTH Bill H Derby x-no-archive: yes

There doesn't seem to be a formal 'definition' of mild steel, or not one that I've seen. Some stockholders include EN8 in the mild steel category, & that can be hardened by heat treatment alone. In my experience it can be difficult to get a decent finish when machining black bar, especially the smaller diameters - am I alone in this?

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

Does that mean it's better than bms for things like model IC engine crankshafts where the skin stresses can warp the part as you cut it out ? Can you get leaded or otherwise free-turning black ms that'll give a good machined finish ?

Absolutely right Tim, technically "mild steel" does not exist as such

-it is simply a generic term for low carbon steel. EN8 is a slightly tricky one, as EN8A is a mild steel, but EN8D is a stressproof tough steel ideal for crankshafts (in model engines -for the previous poster). If we're being technically correct, of course, EN8 doesn't exist either as that british Standard was replaced in 1969. From memory the new specification for "mild steel" is 070M20 (if I could be bothered to look it up I could probably work out what all the numbers meant).

This all takes me back to my HNC in Fab & Weld at peoples college in Nottingham -do they still have an engineering department?

Regards

Kevin

EN8 is not a mild steel as it contains 0.4% carbon,which means it can be hardened without any additions.Mild steel,which can be hot rolled,cold drawn,bright or black can not be hardened on its own.

070M20 is the ISO equivalent for EN1A which is available all diameters in bright and from 50mm dia up in black.O8OA15 is EN3B which is considered to be true mild steel as it is the cheapest and has no controlled ingredients.EN1A is also mild steel but has a specification. As a matter of interest I had a job calling for EN32B last week,what kind of steel do you guys reckon that was? I was most surprised when I looked it up. regards,Mark.

How embarrasing, mixing up EN1A & EN8A (perhaps showing my age that I knew the "metric" equivilent, but not the "EN"). Funny how the EN numbering has stuck, although it's over 20 years ago that the standard was officially replaced.

Regards

Kev>EN8 is not a mild steel as it contains 0.4% carbon,which means it can

I would describe EN8 and EN9 as medium carbon steels.

I always liked the old EN system of materials but as steelmakers developed new recipes it became plagued by all the extra suffix letters.

Having killed off EN (ENgineering steels) specifications when I was an apprentice and replaced them with BS970 steeels such as 070M20 they've been superceded by EN numbers where EN stands for EuroNorm. These seem to be based on the old German designations. The americans still seem to have their own system AISI, ASTM & SAE. I suppose it's similar to where ISO covers both metric and UNC/UNF screw threads as endorsed world standards.

If you're referring to Nottingham Trent Uni (Poly as was), they are closing their mechanical and electrical engineering departments this year and concentrating on softer subjects. One of my colleagues is taking his degree finals this year and has suffered due to the remaining skeleton staff of engineering lecturers having to do their best to cover all the syllabus. Bill H Derby x-no-archive: yes

Well, EN31 is good old ball bearing steel (1%C and 1 - 1.5% Cr) while EN40 and EN41 are nitriding steels, the former 3 Cr and the latter 1 Cr and a dash of Al, ISTR. So by interpolation......

but OTOH since when did logic have anything to do with steel classifications. So go on, surprise us.

EN stood for Emergency Number,introduced during the war. EN32B is a case hardening version of EN3B/O8OA15,in other words mechanically crap. For a very concise breakdown of all the steel specs/equivalents Macreadys Orange book is hard to beat.If you spend a lot with them you might scrounge it free,otherwise it was =A315,no vat. regards.Mark.

Well you learn something new every day! Thanks for that.

Enjoy the sunshine!

Bill H Derby x-no-archive: yes

It's worth the money IMO, and the previous books come up in secondhand bookshops as well, so keep your eyes peeled.

Peter

It's also sod to silver solder to. Even after diligently cleaning the surface with emery cloth, there is still a residue of scale incrusted in the surface. The only way (if it's not machined) is to remove a little bit using a file, grinder, etc.

mike a =E9crit :