Why is cast steel not often used ?

A friend asked me teh other day why cast iron is used nearly always for cast ferrous parts instead of steel and all I could think of was that steel would be more expensive and perhaps has properties that lend itself more to rolling than casting. Anyone else have a more comprehensive answer ?

Btw, nice forum.

A friend of mine is a metallurgist. He keeps his references over his desk. One is a book "Cast Iron" about 1 3/4 thick and another "Properties of Cast Steel and its Alloys" is even thicker - all in fine print and not many pictures. The question of cast iron Vs cast steel is a big one if approached from an engineering perspective connected with a product to be mass produced.

Major castings for machine tools are cast iron for reasons of stability, self lubricity, and economy in machining. Cast steel in the same application suffers from small scale metallurgical instability (sophisticated heat treatment is required to ensure dimensional stability in most cast steels), tendancy to gall when used in machine slides, and, while it machines very well, cast steel is also malleable and has lower compression strength and far greater tendancy to bruise and scratch when used as machine tables.

Cast iron of the optimim alloy for machine tool castings is superior to the best competitive steel.

Some will tell you that cast iron is stiffer than cast steel. That is not true. The modulous of elasticity of cast iron is nearly identical to cast steel. Others will assert that cast iron damps vibration better. That is true to a certain extent.

Aren't a lot of items today that are called "cast iron" actually cast from iron that is highly contaminated with scrap steel and are actually semi-steel castings? Especially a lot of Chinese and Korean stuff out there? Thought I heard that somewhere, like with engine blocks and heads, the hard inserts etc just get melted down with the iron.

Barry, count your blessings that your iron and steel are "contaminated" with scrap. Otherwise your metal products would cost over double, four times in the case of aluminum. The airplanes you fly in are likely 60% scrap and recycled metals by weight.

Part of steel making and iron making is efficient utilization of scrap which is metal far cleaner and purer than ore taken from the ground. If contaminants like cobalt chromium inserts, copper, or other matals are present in the scrap, the metallurgist will know and adjust his melt accordingly. Some very small proportion of copper and aliminum and other metals are important for controlling grain structure, response to heat-treatment, and other attributes of the melt.

There's a good many BS artists who love to scoff about "contaminated metal" and who spout other nonsense to make themselves sound important to the cedulous. Many Taiwan and Chinese made machines are built with inferior materials but the fact that scrap is a major constituent is a tiny factor compared with poor metallurgy and slack foundry practice.

Forrest I agree 100 percent with your comments on scrap contents in casting (with some caveots based on events in story below). Woodworking machinery sales folk seem particurally prone to spouting BS regarding the merits of their machine castings. I was looking at an RBI scroll saw at a local festival/market event a few months ago where the salesman proclaimed their machine superior to others based on the "virgin" aluminum used in it's casting and extrusions. I tried to explain how this was total BS, that even on the offhand chance that it was virgin material that it wouldn't be superior in the least, but he would hear none of it. I asked what he did in his previous life...a policeman in NYC

Speaking of using scrap in iron casting, apparently the foundry has to be somewhat particular of what the scap actually is. When I first designed my dovetail jig (later called Omnijig) back in 1985, I made my own crude wood pattern for the base and set about to find a foundry to cast it. I was living in Raleigh, NC where the only foundry was one that cast manhole covers. Yep, that's *all* they did..manhole covers ! The place had dirt floors and antique Bullard vertical lathes for "finish" turning the manhole covers. Anyway, the new owner who just bought the place from the previous codger was rarin' to expand to vast new casting horizons and agreed to produce a few samples using my pattern. He admitted to throwin' in auto springs and practically anything this resembled ferrous material but I didn't think much of it until later.

The results looked fine, but was a bit dismayed when my 6 inch Sandvik carbide insert mill wouldn't even think about actually finishing the surface of that casting...the sparks were not a good sign. Since they threw anything and everything into the melting pot the results were so irratic hardness wise that I had to find another foundry.

Found a great one in Greensboro, NC that was already doing castings for some of the woodworking machinery manufacturers in the area (but not Neuman Whitney who has their own foundry as I recall). Only catch was a proper foundry could not use my improper pattern, so I had to finagle a deal (traded a woodworking machine cuz I couldn't afford to pay him) with the nearby patternmaker to make a new pattern. But the end results were fantastic...the new castings milled with ease, excellent surface finish and had very few blo-holes and imperfections.

I eventually changed over to aluminum castings, but that's another story.

[This message has been edited by D. Thomas (edited 12-31-2000).]

Thanks guys, I forgot about teh self lubricating and ease of machining advantages of cast iron, good info.