Black Steel

[Runner]

Hi all,

I obtain my steel from a local supplier and invariably it has a black or dark coating which I have to remove by abrasive means. What is this type of steel and is it inferior for use in model steam locomotives?

Thanks in advance

Brian

PS I googled 'Black Steel' and the returns where invariably NOT about metal.

 

[Tin Falcon]

Sir I believe what you are referring to is hot rolled steel the common type being A-36 commonly used for steel structures and formed in shapes such as flat bar angle c and I beams.

the dark gray coating is oxidation formed when the hot steel hits the air.

I see no problem using it for locomotive frames or event the structure or frame for other engines steam or IC. It is not the most machinable steel so do not expect a super fine finish. but it is weldable .

in contrast 12L14 cold roll is ultra machinable has a light oil finish but not weldable.
Tin

 

[GWRdriver]

Hello Brian,
That's hot rolled mild steel (HRS) and it has its place. It's usually a good mild steel, but whether it's inferior depends entirely upon what you intend to use it for on a steam loco. I rarely use a hot rolled shape (rounds, bars, channels) on my loco but I do use quite a bit of HRS angle. The blue-black surface is an oxide coating imparted during the the rolling and cooling process.

Hot rolled sheet or plate on the other hand, sometimes called "black-iron" in the fabrications shops, is a hot-rolled mild steel. It can be had black, with the oxide left on, or pickled, where the sheet surface is acid etched to remove the oxide. Some suppliers have only black, and some will have both black and pickled.

Personally, HRS is the only structural plate material I use on my projects, but then I build British locos which use far more plate in their frames than do American locos. In general it's one of my favorite materials for a number of reasons. It resists rust, it's tough but cuts and machines relatively easily, has minimal stress and deformation, absorbs the bumps and grinds of ongoing construction without marring, primes and paints well, it's relatively inexpensive, and just about any good sheet metal shop is going to have stocks. The only drawback I've come across is if I want to solder some together the oxide must be filed off anywhere I want the solder to flow. Most of these attributes apply to HRS shapes also.

Hot rolled shapes and plate aren't for everything but are very good for specific uses.

 

[Entropy455]

Ditto.

It sounds like you are describing the foundry oxide layer. Hot-rolled steel is formed while it’s in the semi-plastic state (aka hot). This greatly extends the life of the forming dies. During the manufacturing process, the hot steel is not protected from the atmosphere, and the outer layer reacts with oxygen and nitrogen to form a black oxide layer. Again, the colder the “hot” steel is during the forming process, the greater the wear and tear on the dies will be.

If the temperature is kept low during the drawing and forming process, the oxide layer will be thin and lightly adhering – where you can remove it relatively easily (good for the customer, bad for the foundry). If the temperature is kept high during the drawing and forming process, the oxide layer will be heavy and tightly adhered, where you'll need a grinder to remove it. (bad for the customer, good for the foundry).

Cold-rolled steel is formed at or near room temperature. The cold-rolling process is extremely hard on the dies, which is a large reason why cold-rolled steel is more expensive. Cold-rolling will increase the yield strength of the steel by approximately 15% (typical), however it does not increase the ultimate tensile strength. Cold-rolled steel has a pretty surface finish, and is normally completely void of any foundry oxide layer.

 

[Runner]

Thanks Tin, Harry and Entropy455 for your replies.
It appears that the oxide coating should be left on as it accepts painting without problems. However where it is machined and not painted such as coupling and connecting rods, crossheads etc, I find that it rusts fairly easily that's why I thought that it was an inferior steel. Rusts removal is a frequent occurance, is there any protection that can be applied without detracting from the look of bare metal?

I live in the driest state in the driest country in the world, so humidity should not be a major concern.

Brian

 

[Tin Falcon]

Rusts removal is a frequent occurance, is there any protection that can be applied without detracting from the look of bare metal?

An old fashioned method is to heat the steel to a black heat. Just hot enough to oxidize the metal then while still hot coat with paste wax.

another alternative to try is is spray conversion coating it turns metal black and is intended as a primer coat. so will likely need wax or some other top coat.

Just because a metal is prone to rust does not mean it is inferior . corrosion resistance is but one physical quality of a metal . weldabilty, machinabilty strength , ductility ect are all things to consider when when selecting material.
Tin

 

[///]

.... weldabilty, machinabilty strength , ductility ect are all things to consider when when selecting material.
Tin

Availability.
It is so damned hard to find metals in Western Australia.
I use mystery metals from various sources (scrapped printers, scrap yards etc.)

 

[Niceonetidy]

I use black steel here in the UK, it is good and stable, without distorsion, and will case harden very good too if you need hardness,

Cheers

Colin

 

[OrangeAlpine]

I have found that wire wheels are the best way to remove mill scale. Also, HRS has one property that makes it superior to CRS. CRS has internal stresses from the cold rolling and can warp when a surface is removed. HRS does not.

Bill

 

[ZipSnipe]

Ditto.

Cold-rolled steel is formed at or near room temperature. The cold-rolling process is extremely hard on the dies, which is a large reason why cold-rolled steel is more expensive.

Here in Florida cold rolled is the cheaper steel, I get it for .96 per pound, hot rolled is 2.89 per pound. Over at ACME in Sanford

 

[abby]

Are we are talking steel bar ? By cold rolled I presume we actually mean cold drawn ?
Cold drawn bar starts life as hot rolled , it has to be pickled to remove scale , de-coiled and drawn down to size , usually to within 2 thou.
The process is expensive , the dies are solid lumps of tungsten carbide which costs a fortune.
Please explain how you can buy cold drawn steel for 1/3 the price of hot rolled.

 

[OrangeAlpine]

We are talking about steel that has gone through the hot rolling process, pickled to remove the mill scale, then cold rolled.

In my experience, cold rolled is more expensive than hot rolled.

Bill

 

[sivtek]

I have found the best way to remove the scale before machining is to submerge in a solution of vineger and commen salt. leve for a day or two.

Sivtek

 

[dajt]

Availability.
It is so damned hard to find metals in Western Australia.

I thought the ground was full of the stuff!

 

[Swifty]

I thought the ground was full of the stuff!

Yes it is, but we have to dig it up first, send it off to China where they make steel out of it and then we buy it back at an inflated price.

Paul.

 

[Busydad]

The only problem i have ever encountered was with machining the diameter of Cold Rolled Steel is it will bend if you reduce the dia too much. You break through the outer "tough" layer and it will bend. I use HRS for frames and such , things that are welded together.