I am researching knifemaking/steel types and have come upon a question i havent seen answered sufficiently. Why isnt steel in knifes, structural beams etc cast? I am aware of the drawbacks of cast iron, but why is only cast iron, cast? What prevents you from taking a suitable steel(not cast iron) melting it, casting it into shape and then using or heat treating it(in the case of a blade). I assume all steel starts out molten, but why is the casting technique used only with cast iron?
Your premise is wrong although for most thin items rolled steel would be an easier starting point.
There are cast steels. For example, the crankshaft in nearly every automobile engine produced today is probably made from cast steel. There are significant cost advantages to casting over forging or rolling. However, forging and/or rolling the steel improves its internal structure, which improves the strength.
When just about any metal solidifies, it contracts. This creates small voids in the internal structure of the metal which are literally small cracks. If the loads are light or well controlled, these cracks may not be detrimental. In addition, when steel (which is, by definition, an alloy, or mixture) solidifies, the pure metal tends to solidify at first, rejecting the alloy to the remaining liquid so that the last to solidify has a higher alloy content, creating chemical segregation in the casting. Again, in some applications, this is OK, but the more homogeneous the material is, the more uniform the properties will be. When the cast ingot or billet is rolled and/or forged, these small internal cracks will weld themselves shut and the chemical segregation that occurs is also broken-up, improving the overall strength.
Cast iron pours like water, cast steel is hotter but flows more like treacle.
Crucible steels were the first tool steels made this way.
if it’s cast, it’s probably cast iron. steel crankshafts are typically forged or machined from billet.
http://www.hotrod.com/how-to/engine/116-0308-crankshafts-how-to/
A joke in Joyce’s Ulysses:
“What opera is like a railway line? … The Rose of Castille. See the wheeze? Rows of cast steel.”
I don’t get it
Chrysler used cast steel crankshafts is all but their high performance V8s (340’s, 4bbl 383s, HP440’s, and 426’s) since the late 1960’s (about 1968 and on). All 360 cranks were cast steel from the factory. I assume Chevy and Ford did the same. I am pretty sure that, with the exception of the Diesel and V10 truck engines, and maybe some turbocharged V6’s, cast steel is used in the vast majority of production engines.
A lot of the cast steel crankshafts are incorrectly referred as Cast Iron crankshafts, but they are in fact cast steel. The aftermarket folks cloud the issue by ignoring the difference between cast iron and cast steel, since their forged products are obviously better. But, these are aftermarket crankshafts sold to rebuilders who are trying to greatly increase the power over stock.
Cast iron has to be cast because it’s too brittle to forge. Casting steel is possible but a lot of other forming methods are faster, cheaper, or produce higher quality.
As I understand it, when steel is made in the highest quality forms, it gets cast into standard forms such as bars and plates. The steel mill has a recipe that has been tuned empirically and they know just how to get a product that is very high strength and consistent, so long as they put it into the standard form. Then all the end manufacturers just machine out the piece they need.
Casting an object with a complex shape, like mentioned above, like casting a gear with the teeth part of the gear mold, causes problems with the grain structure of the metal and loss of strength. So instead you cast a rectangular plate and cut the gear out of the plate with machine tools. Ditto swords - you would cut one out of a plate instead of trying to cast the blade itself.
Some day we may have a better way than this, but this is what we got.
And, of course, all the bits that get machined or cut off can be tossed back into the furnace.
cite.
So the high carbon content makes cast iron somehow more suitable for casting?I assume so, because virtually everything that is cast into shape is cast iron not cast steel. Also wouldnt normalizing and heat treating cast steel improve the grain structure?
Depends.
The machining happens at the customer’s plant, not the steel mill. What they do with the scrap and where it ends up is an interesting question.
Aerospace metals are 100% straight from ore with zero recycled bits. For some big or intricate parts they end up machining away 90% of the virgin metal to make the finished part.
They can sell the cuttings as scrap which can be reworked into lesser alloys. But usually the cuttings sell for about 5% of new. Unless they’re titanium cuttings. Those sell for more like 0.5% of new since there’s almost no market for titanium outside aerospace and medical devices. Both of which demand pure virgin material.
I don’t think this is the way it’s done. Structural shapes and plate are formed after the molten steel has been poured into ingots and blooms. These are allowed to cool over the course of hours to a workable temperature and then rolled to the appropriate shape or thickness in a rolling mill. Rolling acts as a continuous forging process, homogenizing and lengthening the crystalline structure of the steel as well as squeezing out imperfections in the cast. I expect there are more expert posters coming along but this is what I know from having been a machinist/welder level consumer of steel products.
And since it’s more enjoyable to correct oneself: regarding my previous post it looks like I need to read up on continuous casting.
There are plenty of cast steel components.
Look no further than to the chemical/oil/gas industry. Most pump casings are cast. Also, in various grades; carbon, stainless, “high performance” and high nickel alloy steels.
Despite years of experience and best intentions, castings are still susceptible to porosity. This becomes a strong disadvantage when a high polish or stringent quality specs are required. Polishing removes surface material, which reveals porosity, which must be filled in, and repolished, and so on. Some oil/gas/chemical manufacturers that deal with high temperatures/pressures or lethal service require X-ray inspections of all castings to verify no/minimal porosity.
I’d imagine that this would pose a disadvantage for something like a knife, which requires a porosity free fabrication to maintain its sharpness and sanitation.
There are direct ore-to-steel processes now, but traditionally iron ore was made into cast iron in blast furnaces first, then refined into steel in converters as a second step. So cast iron needed that much less work. Everything’s about how much extra work and extra steps are needed to produce the final product. If you’d have to heat-treat a cast steel object, it’s no savings on just milling it out of stock steel.
An older term for cast steel was “semi-steel” This denoted that it was a bit better than cast iron, but not quite as good as rolled or forged steel.
Investment casting of stainless steel is a pretty popular process. Major portions of many guns are made this way, though not the barrel, or other parts highly loaded in tension.
What is there about stainless steel that makes it non-magnetic? (I hate stainless - steel refrigerators, that you can’t stick magnets to.) :mad: