cast iron, cast steel, and otherwise

[TimB]

Maybe, instead of trying to reason this through, I should just come right out and ask what the difference is, between cast iron--with a higher carbon % than steel, and cast steel. Does the cast steel have yet higher carbon content than cast iron? Why is cast iron called "iron" even though it has a higher % carbon content than steel.

[matt87]

Does the cast steel have yet higher carbon content than cast iron?

'Cast steel' can refer either to crucible refined or produced steel (e.g. Huntsman or Wootz process) or steel that has been cast into shape. The latter is a much more modern process and can be used with virtually any steel. The former is generally quite a plain steel with carbon content from around 0.6% to 2.2%.

'Cast iron' almost exclusively refers to a material, i.e. an iron-carbon alloy with carbon content somewhere between 2% and 6%.

Why is cast iron called "iron" even though it has a higher % carbon content than steel.

When cast iron was first encountered by the English-speaking world (for simplicity's sake, let's say the 15th century and ignore issues such as excessively reducing bloomery furnaces) no-one knew about iron-carbon alloys; this was only discovered in the 19th century. 'Cast iron' doesn't share many useful properties with steel (edge holding, springiness etc.) so its applications were largely to replace bloomery iron (and stone). I suppose this is probably where the name comes from. Agreed there isn't much logic to it knowing what we do, but think how confusing and pointless changing it would be; just look at so-called damascus steel as an example :D

[Charlotte]

The simple answer is this: Cast steel may be one of many formulations that contain a may contain some of a wide variety of alloying componets but which usually contain carbon at substantially less than 1%.

Cast Iron is called cast iron because it was recognized as a seperate material from wrought iron eary on, had a lower melting point, and was fluid enough to be cast.

Cast Iron may take many different formulations but contains Carbon in excess of 1% and is characterized by Iron carbide precipitation. The control of the size of the precipitants is the feature that distinguishes the different forms and mechanical properties.

Cast Iron is a paradox: It is a matrix of, often nearly pure, iron that contain Iron carbide particles. Thus it drills easily, and machines freely, and properly heat treated, dent readily, but it will wear well and dull tools like nothing else.

In between Cast Steel and Cast Iron are hundreds to alloys that share properties of one or the other depending on the exact formulation and heat treatment.

Wootz steel Is concidered a "Super Carbon" steel with very high levels of carbon characterized by iron carbide dentrites which exhibit spectacular patterns when properly forged and etched.

Wrought Iron is (was) a nearly pure Iron with that was never truely melted but consolidated from a bloom by hammering and which often exhibits silicia inculsions.

So no simple answer, except that at the forge cast iron falls apart rather than forges when it gets hot.

[Nakedanvil - Grant Sarver]

Very good explainations, guy! I wood only add that most texts define cast iron as having over 2.2% carbon. I've had "steels" that were around 1.5% C. Darn hard to work too. It had been rolled for the manufacture of reamers before high-speed had taken over completely for that purpose. It actually took and held a better edge and reamers are usually run slow anyway.

[Quenchcrack]

The only thing that was not mentioned previously is that the cast iron has most of it's carbon in flake form. It does not really play as big a roll in contributing to hardness as it's high per centage would suggest. The flakes are really graphite and make a fractured surface appear sooty and grey. Hence, it was called grey iron. It does not ring when struck like steel does because the graphite flakes attenuate the sound.

[ThomasPowers]

Except that should really be 2% in Charlotte's post.

[Charlotte]

Except that should really be 2% in Charlotte's post.

Thomas there is difference between an offical definition and a functional definition.
I don't pay a lot of attention to "official" definitions. As a practical matter when steels get high carbon contents they begin to look like cast iron when handled differently.

There is a whole category of materials that are called "semi-steel"

From Wiki=Semi-steel casting is a lower cost method to produce a casting that is not quite as strong as a steel casting but less expensive to manufacture. It was used more commonly as a marketing term.

The carbon and silicon percentages are reduced to the amount approximately consistent with those in steel. This is done using pig iron or gray iron casting scrap and reducing the amount of carbon through the additon of relatively pure steel or wrought iron scrap in a well heated cupola. The percentage of carbon is typically between foundry cast iron and wrought iron.

Which really helps no one except that many items that we think are cast iron may really be puddled steel with uncertain carbon content and other alloys.

In East Tennessee I knew of a foundry that was producing "Cast Iron" from scrap and pig iron for various construction items like sewer grates, lids,etc.
not any sign of chemical analysis. Might have been cast Iron greater than 2 % or steel less than 2 percent. (back around 1970 before ISO 2001)

Essentially there is a vast grey area that gets murky and really depends on the item's history. In response to original question some where around there is close enough.

[ThomasPowers]

I disagree, lots of steels I work with are over 1% and not just the new high alloy steels; old black diamond files were 1.2%. They work like steel.

I will agree that some of the high alloy steels will cottage cheese on you like trying to forge cast iron (H13, D2, etc) but they are not a cast iron as they don't have carbon in them as graphite but rather as complex carbides. (May we leave white cast iron out of the discussion?) It's the graphite that makes a "cast iron" in my opinion!

Semi steels, nodular cast irons, etc still have graphite it is just in roundish forms rather than lenticular ones.

Puddled steel is a method of going from cast iron to wrought iron only stopping somewhere in the middle and items made by that methodology will be forged to shape and so should show forging rather than casting tattletales.

Cast iron has a lower melting temp so if your foundry is not controlling the carbon content it's wasting fuel and putting a greater strain on the refractories as well.

The line is a bit murky these days with some high alloy steels up there at the 2% C line; but as I previously mentioned no graphite!

[Charlotte]

I was looking at the question from the view point of someone who is new to smithing and was looking at a pile of scrap steel and wondering what what to pass by and what to pick up and what to expect in the forge and why.

Incidently, what you do with the metal after you puddle it depends on what you want to do with it.

[nitewatchman]

There is a simple answer. In steels the carbon is in solution and in cast irons it is not and resides in the grain boundaries as free carbon which is very weak and brittle.

Think of sweetened tea where the sugar is in solution. The sweetening can be increased until it the tea is saturated and the liquid can hold no more and then the crystals settle out.

In some steels the carbon can be very high. Manganese Steels can have a carbon content of 1.8%. The carbon is taken up in the solution at this range when the temperature is above 1900F and completely disolved. If however the steel is allowed to cool slowly the carbon comes out and forms a brittle material full of free carbon. For this material the manganese content would be about 18% (a 10:1 ratio) and the manganese acts like an Austenite stablizer. This allows the material to be water quenched from 2000F, retain the dissolved carbon, stay as Austenite and be dead soft after quenching. The cooling here occurs faster than the carbon can come out of solution.

[territorialmillworks]

I appreciate that there is a wealth of good info out there. However,I've concluded that I'm now more ignorant (confused) about cast iron vs cast steel than I was before reading the posts. Getting dizzy....got to lay down....lol

[Charlotte]

Nitewatchman's simplified answer is a good one! The first step to new knowledge is to recognize confusion.

[TimB]

Yeah, thanks guys. Night, your responce was the string that the solution precipitated out and crystalized on, thanks... , and yes, I was asking from the standpoint of looking at a scrap heap wondering what to try out next, and you all gave me more than I asked for, but that's only because I'm too ignorant to know to ask for it. It was not more than I wanted to know. Thank you

[philip in china]

Where do the very hard cast irons come into the equation? Some cast iron is so hard you need carbide tools to work it! Also what is meant by cold casting? Some old gun emplacement armour was cold cast iron (I think) although it could have been steel.

[TimB]

...uuhh, I stumped myself again. ( it's getting easier these days...) Are you all saying that cast steel is just steel that's been cast instead of forged? such that if you took a "cast steel " sumpthin and put it in the forge, it would heat and beat out like a similar chunk of steel?

Night, your responce was the string that the solution precipitated out and crystalized on, thanks... ,

evidently, the crystal needs some polishing :) Edited June 15, 2009 by TimB

[matt87]

...uuhh, I stumped myself again. ( it's getting easier these days...) Are you all saying that cast steel is just steel that's been cast instead of forged? such that if you took a "cast steel " sumpthin and put it in the forge, it would heat and beat out like a similar chunk of steel?




evidently, the crystal needs some polishing

'Cast steel' can mean either a steel object directly cast into shape (e.g. a Vaughn/Brooks anvil) or 'crucible' steel that has been cast into an ingot and then forged to shape. This was the typical method for the Huntsman process, where steel was refined by melting in a crucible but the available technology precluded casting directly to shape (I understand this is to do with grain structure and gas absorption).

From the point of view of the scrapyard scrounger, I would say that the majority of old tools marked 'cast steel' would be the latter, i.e. forged from a cast ingot. Assume it to be a plain steel with around 1% carbon unless otherwise tested.

[Charlotte]

White cast iron with lower silicon, that is cooled quickly. has more dissolved carbon and so is extremely hard.
Gray cast iron with higher silicon, cooled slowly, has more free carbon which precipitates as graphite flakes and thus is softer.
The amount of silicon dissolved in cast iron controls to a great extent the presence of free carbon. More silicon=More Free Carbon(graphite) all other things being equal. The hardness of Cast Iron depends in part on the carbon content, other alloying ingredients, and in the rate at which it cools after being cast. Followed by susequent heat treatments.

Phillip, there is a process called chill casting in which steel is poured against a steel shell of the same or different alloy. This was one of the big items in manufacturing hot and cold rolled steel as chill cast rolls were used in the rolling mills. One of my steady dates' in the 60's father was the forman of a US steel crew that did that casting. The purpose is to have a hard, wear resistant surface, and tough resiliant and flexible core that would resist permanate deformation.
I don't know if they still do it but railroad car wheels used to be cast steel chill cast against a cold rim.

.

[nitewatchman]

Railroad wheels are indeed still cast with the tread against a chill in some cases. We also cast manganese steel railroad frogs in our foundry with chills against the running surface.

In both of these cases the chill freezes the metal very quickly at the contact zone resulting in very fine grain and promotes directional solidification. This tends to give better mechanical properties in the zone subjected to highest stress.

[TimB]

'Cast steel' can mean either a steel object directly cast into shape (e.g. a Vaughn/Brooks anvil) or 'crucible' steel that has been cast into an ingot and then forged to shape. This was the typical method for the Huntsman process, where steel was refined by melting in a crucible but the available technology precluded casting directly to shape (I understand this is to do with grain structure and gas absorption).

From the point of view of the scrapyard scrounger, I would say that the majority of old tools marked 'cast steel' would be the latter, i.e. forged from a cast ingot. Assume it to be a plain steel with around 1% carbon unless otherwise tested.

So steel, depending on the carbon content, (in combination with the catalyst used to keep the C in suspention) must be forged at a higher temperature than, say, mild steel, in direct proportion to the level of it's carbon content, in increasing temperatures, in order to avoid a situation where the C may precipitate out during forgeing?

And then cast iron has a C content that precipitates out of suspention at the temperature where it begins to freeze? Edited June 16, 2009 by TimB

[matt87]

So steel, depending on the carbon content, (in combination with the catalyst used to keep the C in suspention) must be forged at a higher temperature than, say, mild steel, in direct proportion to the level of it's carbon content, in increasing temperatures, in order to avoid a situation where the C may precipitate out during forgeing?

And then cast iron has a C content that precipitates out of suspention at the temperature where it begins to freeze?

I'm not so sure on the carbon precipitation but as a rule the working temperature window narrows in proportion to the carbon content; the higher the carbon the lower the burning/sparkler threshold, the harder it is to move under the hammer (hot hardness) and the more likely it is to be hot-short. Hot-shortness can be likened to forging cottage cheese; it crumbles and falls apart into a nasty mess. To see what this is, try forging a piece of old file steel at a yellow heat, or cast iron at just about any heat. The stiffness under the hammer unfortunately makes you want to work it at a higher heat for obvious reasons, however not only do you risk burning and hot-shortness but decarburisation occurs faster at higher temperatures. This is not often much of a problem to a general blacksmith (as opposed to, say, a bladesmith). Other alloying elements (Mn, Ni, Cr etc.) confuse the issue somewhat.

[TimB]

Hmmm...decarburization, is that what's going on with the "sparkler effect"?


And that would explain why the splitting maul head I'm trying to forge into an anvil horn is arguing with me so much

[ThomasPowers]

Well in the extreme form, when it's sparkler-ing it's pretty much getting trashed for high carbon work and gets cut off and discarded.

Decarb happens all the time even at low temps---if scale is forming it also can be decarbing depending on the exact set up.

Since low carbon steel is SOFTER under the hammer than medium to high carbon steel I don't understand your comment as to forging the splitting maul head. It should be harder to forge because it does have a higher carbon content than mild steel not that it's decarburizing. Or were you referring to the more limited forging ranges for steels with more carbon in them?

One beginning knifemaker's common mistakes is to take longer to forge a blade and so have a decarb layer that when they heat treat it and then try it with a file makes the blade look like it's soft when really the core may be quite hard and if they re-quench with a more aggressive quenchant may crack on them. All they had to do is to remove the decarb layer and continue making the knife.

[TimB]

... Or were you referring to the more limited forging ranges for steels with more carbon in them?

Yep, I was. Sorry it wasn't clearer.

[MattBower]

Well, the nice things about something the size of a splitting maul are that: (1) it'll hold the heat for quite a while; and, (2) it shouldn't be all that high in carbon, probably. (Maybe 40-50 point range?) But if you're forging something that size you'll be much happier if you find yourself a striker and arm him with a sledge. Or make friends with someone who has a power hammer.

[TimB]

;)

I put a hammer handle on an 8# sledge head I had laying around, so I been having fun with that set up. I love the way my arms feel after swinging that around a while. :D