Thursday, April 14, 2005

The problem of sulphur in steel

I had a commenter in an earlier post ask what was the problem with sulphur and steel. In specific this was in reference to high-sulphur coal being used in forging. I mentioned before that sulphur is added to some steel in carefully controlled amounts to make it easier to machine, but some of these steels, if forged, can actually crumble under the hammer.

I should have mentioned one of the better examples of too much sulphur in steel: the Titanic.

One of the things they found out after testing samples of the hull steel was that it contained a LOT of sulphur; so much that when the steel became cold- say, from water in the North Atlantic- it became brittle. So when the ship scraped against the iceberg, instead of the steel deforming- denting- and maybe some seams springing, the steel itself cracked and split.

I remember reading about a test a few years back. They did a standard test using 'cigarettes', pieces of steel cut to an exact dimensions, one from standard steel used in ship hulls and one from recovered hull plating from the Titanic. They cut a notch of exact size in one side of the cigarette, lock it in a vise, and swing a weighted pendulum against it, then measuring how far it deforms(bends). They chilled them to the same temperature as the water in the North Atlantic at the time the Titanic went down and ran the test. The standard steel bent; the cigarette from the Titanic steel snapped.

So unless very carefully controlled in amount, and for certain uses, sulphur in steel tends to be a bad thing.


Anonymous said...

Is it actually sulpher or is it hydrogen embrittlement caused by the sulpher?

Firehand said...

The actual chemical wherefores I've never gotten ambitious enough to dig up.

One old smith's trick for punching a hole through thick iron was to set the piece level in the fire, and put a lump of sulphur on the spot. By the time the piece was at red heat and kept there a minute, the piece would be much easier to drive the punch through. Whichever it is that causes it, it works.

JD said...

When hot, iron and sulphur combine to form a third substance called iron sulphide. It's not a metal and it's very brittle like the ceramic (or intermetallic?) it is.

The intention when adding to improve machinability is the same, to make the alloy somewhat brittler--when that's no problem for the application at hand--so the chips break earlier and don't turn too long.

Also this iron sulphide melts at far lower temperatures than steel. So when a steel with lots of sulphur is cooled from liquid, the iron sulphide solidifies last, and the resulting structure shows iron grains completely surrounded by a brittle cement, so the alloy doesn't benefit from the properties of iron since the material can break apart without the iron itself breaking apart.

Don't quote me on all this since I'm trying to recall my college, but I think it's basically like this.

Firehand said...

JD, that makes sense. Thanks for the info; and yes, I do need to dig out a book and read back up on it.

JD said...

I know the entry is old, but I was searching Google about the topic and this post actually showed on top. So I decided to post what I (think I) know.

BTW only to complete (I mean correct really) something I said. To improve machinability the idea is not reducing toughness overall. The key is always the microstructure. Not only the amount but the conditions and treatments must be adequate so that the iron sulphide forms as finely distributed particles. That way the effect on a normal part is minimal; but since chips are very thin and long, each of these particles reduce their cross section and so their resistance much further.

Again talking by heart, since my book doesn't include an alphabetical index.