Electrolytic de-rusting and Hydrogen embrittlement

[Noddy]

I was taking a look at the rust killing post in this link:
www.frets.com/FretsPages/Machining/index.html

The use of a wet towel hadn't occurred to me before for treating large surfaces!

The author doesn't say anything about the risk of hydrogen embrittlement if the process is used on critical items. (apparently baking at 200 to 400c for 24 or 48 hours drives the hydrogen out).

That got me thinking; brittle generally means slightly better machining, has anyone any thoughts on this, as it seems possible (in theory at least ) to de rust some stringy, chewy scrap, and perhaps get some better machining out of it before baking the hydrogen out again

Keith

[GWRdriver]

I'm not sure I could agree with that . . . I'm presently machining C.I. cylinders which are somewhat more brittle than they should be, something amiss with the pour it appears, and I can't get a decent surface finish anywhere no matter what I do. I will be forced to sleeve the bore s and use valve face inserts. :-(
'driver

[Noddy]

Hmmmm,

You've got a point there,

I suppose Glass isn't exactly known for it's easy machining properties either, although it and some stones do grind well

I really don't know, hence floating the question.

Perhaps I'm completely wrong and somesort of lubricant such as graphite or sulphide is needed on grain boundaries to allow easy shearing at the tip of the tool, rather than brittleness and micro cracks


Keith

[abby]

Hydrogen enbrittlement is mainly encountered when pickling sheet metal components after heat treatment and prior to further press forming , the hydrogen released from the acid pickle enters the surface of the metal causing possible cracks when bending, I don't think this is a problem with lumps for machining or castings but like you say low temperature heat treatment is the cure.
There is a process for renovating files using sulphuric acid and electrolysis - I believe this was covered in ME some years ago - and I don't think enbrittlement was a problem - or advantage.
From my experience with cast iron it is usually, but not always , machined dry and its machinability is totally down to the foundry. If you do use coolant notice how quick your lovely clean oil turns to black sludge.
The best solution Harry is to let me cast your cylinders in gun-metal LOL!

[GWRdriver]

Abby,
Next go-around!
'driver

[steam4ian]

G'day all.

Hydrogen embrittlement is a problem with steel rails (as in rail tracks/roads). At a steel rolling mill at which I worked we had hot boxes into which the just rolled and cut rails were loaded and allowed to cool. This delayed cooling allowed the hydrogen to be liberated.
I think hydrogen entered the steel during the rolling because the rolls were cooled with water sprays.

The point, H2 embritttlement affects larger steel items.

For all that some steel is "stringy" forged bolts from the hardware store are typical. I think this is part of the forging.

Regards,
Ian

[Nigel Bennett]

Another area where hydrogen emrittlement is a problem is in zinc-plating high-tensile steels - eg hexagon socket ("Allen") screws and the like. Occasionally we do these at work, and the trick seems to be to plate them and then get them in the oven PDQ. What this does to the strength of the steel by tempering it is a moot point...

[Noddy]

I'd better clarify a couple of points that with hindsight weren't clear in the first post.

Atomic H will be liberated during the electrolytic de rusting (as it would during electro-plating) and some will inevitably disolve in the steel / iron, potentially making it more brittle.

Hydrogen embritlement shows up in all sorts of odd places, for example in steel which is in contact with stagnant saline water (the hydrogen sulphide puts H into the steel), there was a report of a railway / metro rail cracking in a tunnel a couple of years ago, due to a 2" deep pool of stagnant saline water!

fortunately it was found during an inspection, not a derailment.

Crude oil and sour natural gas pipes also need to be of embrittlement resistant steels.

anyway!

I wonder how the embrittlement affects machineability, are we possibly missing a trick here?

In my comments about "Lubricants" I was thinking of internal constituents of the metal, such as lead concentrated around grain boundaries, ditto sulphur, or discreet particles of lubricant such as sulphide stringers or graphite, rather than anything like cutting oils or suds.

Keith

[Lee]

Hydrogen embritlement can also appear in forged steel crankshafts after a worn crankpin has been reclaimed by the "flash chrome" process once popular here in OZ. I was told that the hydrogen atoms stick together and will gather in any small flaw in the forging creating a huge stress. The cranks snapped like carrots some lasted years some only hours.

Lee

[steam4ian]

G'day Noddy

Liquid lubricants should not be used on cast iron, it makes an awful mess and is itself abrasive. I was taught that an air blast can be used, certainly would get the dust away ;D.

Near the start of this topic cast iron was mentioned. I think the problem has more to do with the quality of the casting and the crystal geometry. The problem could also be high levels of silicon or sulphur in the iron; but I am no metallurgist. Intuitively it may be worth heating the castings to a high temperature (cherry red) and holding them at this temperature for a while before slow cooling, this could let the grain structure sort itself out.

Regards,
Ian