Hard Surface on Black Mild Steel?

A recent post mentioned the mill-scale on hot-rolled mild-steel being hard or abrasive, reminding me of some unexpected problems with such steel.

The mill-scale (oxide) is very thin, however, can there be an appreciably thicker layer of hard steel just below it?

In the first instance, two of us were hole-sawing large bolt-holes through a piece of standard channel section. The saw cut most of the way through but really struggled with the last part of the cut, perhaps as much as a sixteenth of an inch.

In the second I was tapping the holes in the baseplate of my Harrison lathe cabinet - a welded steel fabrication - to take hefty great levelling-screws. It was a brute of a task anyway, but the last part, when the tap was breaking though the 5/8 inch plate, was the worst.

In both cases it was the exit side of the work that objected. The channel was a decidedly pre-loved piece that had lost any mill-scale long ago.

So could there be a hard layer in the steel itself, under the oxide? If so, would I expect to find it on both sides or was the similarity above, co-incidental?

Hi Nigel, quite possible that you hit upon some kind of inclusion like small amounts of slag. Back in the 70/80's there used to be a lot of steel plate, even that produced in the UK that would have this and when Oxy-Acetylene cutting you would suddenly hit a bit and get a horrible blowback hole where the cutting steam would go anywhere except through the offending inclusion, other times you may see a patch of the surface lifting, even as much as 2mm or so thick. Didn't always end there, as if the part/s that had these inclusions had to be welded, you'd get blow holes in the weld and had to go digging with an angle grinder to tidy up the bad bit of weld and sometimes it would take two or three go's at it to get the rubbish out.

Regards Nick.

According to engineering folklore, when a lot of railway track was taken up after the Beeching cuts, it was hot rolled into angles and channels.. This stuff is manganese iron, (actually an alloy steel). It is very work-hardening and I can remember drilling some angle for a little job and seeing the drill glow red. It could be that it was this steel that you were trying to saw, rather than mild steel. I believe the 'trick' to cutting this material is to make sure the tool is cutting and not rubbing, or to grind through the steel.

Roy Moss

Either the tool, the material or the operator. Or a combination of all three.

Perhaps as Roy says it's not mild-steel!

Inclusions are unlikely in modern metal, but maybe slag or mill scale contaminated the channel surface when it was rolled. Both are possible.

Operator error? I've a bad habit of hacksawing metal thin section first because it saves effort, and I use whatever old blade is in the frame. I also tend to race whilst leaning hard on the blade to get a tedious job over quickly, and might be sloppy about work holding too. It's very bad - I should be choosing a blade with suitable teeth per inch, arranging the cut to maximise the number of teeth in contact, letting the tool do the work, and holding the job firmly.

But maybe Nigel's examples are down to what happens when a blade or drill breaks through. Both of his different examples involve trouble at the end.

Well known twist drills are likely to grab hard as they emerge from sheet metal, especially brass. Other materials blow-out rather than leave a clean hole when drilled or sawn. The cure is to provide a sacrificial backing sheet, or restart from the other side. Not sure what the cause of this bad behaviour is, but I think cutters only work properly when the work is rigid because metal tears when it's not supported. And metal certainly isn't supported just before a blade breaks out. Tearing metal is likely to cause work-hardening too. Other possibilities are the cut slot closing up and gripping the blade as it slices through the last bit, and teeth being at their bluntest at the end of cut.

So I suggest mechanical causes. If so, turning the work over and cutting from the clean side should work better. Be interesting to know if Nigel's tricky channel shows any hint of having a hard face when tested with a sharp file. I suspect not but wouldn't bet the farm on it!

Dave

Thank you gentleman for the various ideas.

I can't speak for the origins of the channel section, which could have had inclusions. We noticed this problem on at least two holes, but I think we did manage it eventually by turning the piece over and coming in from the other side.

I think Harrison would have bought good-quality steel for making lathe cabinets.

The cabinet base is a single, massive steel plate about 5/8-inch thick. It's occurred to me that as the holes I were trying to tap are close to flame-cut edges and welds, I wonder if steel has small, partially-carburised areas, a sort of case-hardening.

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I managed it but as the holes were a bit under 1.125 BSW tapping-size I had to "pilot tap" them with a metric tap , having carefully drawn the threads to an enlarged scale to assess the number of pitches possible before the mis-match became excessive. By the time I finished I wished I'd simply used a crow-bar and shims!

Nigel, when we made LARGE jigs and fixtures for aircraft wings etc, we used quite small studding to keep joint faces aligned, then poured a hard setting plastic resin between the faces. Ie the studding took minimal load and the plastic resin made the incompressible "shim".

For a big lathe,  it could probably stand on 4 x 1/2' levelling studs then fill the void with cement ?

Edited By Speedy Builder5 on 27/06/2020 08:27:35

I did consider that, and it would make sense, though I think it would need more and heavier supports than just those small studs. Though I have used this method for other machines, I decided against it here for two reasons.

First, I was not certain I would keep the lathe in one place as I developed the workshop.

Secondly the floor below the cabinet is completely hidden by the base-plate so I could not guarantee grouting the entire cavity properly, especially since the back of the box is only a few inches from the workshop wall.

As it happens the lathe will probably stay in that location. For a start I have mounted the motor on a sub-frame fitted to a frame supporting the overhead hoist rails, so moving the machine is not just a simple matter of crow-bars and rollers. The workshop floor is reasonably smooth and level concrete but not to machine-tool accuracy, and the cabinet overlaps a rough shuttering-scar a few inches wide, along the walls.

The Harrison manual does refer to levelling screws but my lathe stand had only plain holes, so probably pre-dates that edition of the handbook.

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I have used at work an equivalent to your aircraft-engineering technique. Two specially-profiled aluminium wedge pairs had to hold a rectangular assembly to the narrow inner ends of an elliptical fibre-glass shell in such a way the wedges could be pressed in to a calculated pre-stress. The rear faces of the outer wedges were therefore machined to a close-fitting elliptical arc, but bedded into the shell itself by a similar resin compound allowed to cure under a minimal load before the full insertion. On completion the projecting wedge ends were trimmed back flush and everything stayed permanently in place - and they proved extremely difficult to dismantle safely anyway.

The biggest headache was of the wedges' bearing faces galling, so they had to be coated liberally with anti-seize grease, and usually successfully.

I bet Harrisons did not 'make' their cabinets. it is likely that this was a job farmed out to a local firm with bending kit. Perhaps the same firm that made cabinets for lots of firms. And so - likely to use whatever was cheapest.

Cheers, Tim

If the problem is a regular one, then it might be worth finding out of the tungsten carbide brazed grit hole cutters come in the size you use. At least they could cut through the skin, giving the HSS ones a chance.

Hard spots in steel can be caused by old carbide tips thrown into the swarf tray of machines the the scrap is reused into new steel the offending tip does not melt and mix with the new steel and you hit it when machining the new billet.

I've never been so unlucky to encounter carbide in steel, but I did have to machine about an inch off the diameter of a so called continuously cast billet that I made into a backplate. There would have been tears if I didn't have carbide to machine it. The hard bit was about 3/4" x 1/4" by 3/16" deep on the outside edge. It was like an interrupted cut, and fortunately was gone before I reached the required diameter.

Tim -

Good point. I must admit I'd not thought of that.

There is a lot of steel in it . The main box is quite an elaborate assembly of thick sheet, the base a single 5/8-inch plate, the top also a single piece a bit thinner, and planed all over.

The motor was originally on a massive box welded to the back of the cabinet, heavy enough to held a bus engine let alone a half-horsepower motor. I cut that off, as it would have put the nice new and physically smaller motor down close to the floor and forced the entire machine to take up more workshop space than justified. I have already cut the plinth up for its one-eighth and half-inch thick plate materials!

I also had to cut a rather untidy hole in the back of the cabinet to gain access to the machine's mounting bolts and nuts. How Harrisons, who apparently matched lathe to cabinet, assembled it in the first place, I was unable to work out!

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Old Mart & Colin -

Luckily this was a very unusual event and finding it twice in fairly quick succession in different places a co-incidence, but still one that made me wonder.

I know cheap grades of steel can be a cuss to work: my club once used any number of old steel bed-frames and some of that "angle-iron" was awful to saw and drill.

Usually I rather enjoy using hot-rolled steel even if it's very tatty, as long as there is enough of it to allow to me to cut away the corroded surfaces to reveal "new" metal.

Edited By Hopper on 28/06/2020 07:29:52

Hi, that shouldn't make any difference in mild steel, I've had to do that many times during my day jobs and haven't come across that situation in particular.

Regards Nick.

I've come across it before.

Edited By Hopper on 28/06/2020 09:40:44