View Full Version : Salient qualities of cast iron

02-27-2002, 03:39 PM
What makes cast iron so useful? I don't believe it's any stronger than say mild steel, and it's not cheaper, so why are so many things casted with iron? Why not cast things with steel? I remember reading something about fine grain structures, but most mild steel doesn't even have any grain structures that you can see with naked eyes.


02-27-2002, 04:57 PM
It is great for dampening....look at a good vise...like Kurt...the base of the vise is made of Ductile Iron....I make hydraulic pump housings out of ductile Iron...It is very strong stuff...You also have good lubrication properties from cast iron..You can cast alot of different shapes and sizes that you can't do with other materials...


02-27-2002, 05:31 PM
Hi Brent,

I don't understandwhat you mean by dampening. Do you mean that cast iron is able to absorb lot of energy without deforming or cracking? If so, why is that cast iron isn't used for holding tools (e.g. boring bar)? My metal handbook show that steel has higher tensile strength than cast iron so what do you mean by "very strong stuff". Do you mean stronger than non-ferrous metal which are often used for casting? I'm not trying to challenge your information since I'm as green as they get when it comes to machining. Just trying to learn and understand.

Does low carbon steel simply not cast well, since this seems to me like a better material all around.


02-27-2002, 05:58 PM
go here...

www.me.mit.edu/2.01/taxonomy/characteristics/iron (http://www.me.mit.edu/2.01/taxonomy/characteristics/iron)

click on Iron


02-27-2002, 06:01 PM
that website doesn't work....sorry...

try this one...

www.me.mit.edu/2.01/taxonomy/html/materials.html (http://www.me.mit.edu/2.01/taxonomy/html/materials.html)

02-27-2002, 06:02 PM
sorry...I don't know why it won't work.....


02-27-2002, 07:14 PM
I remember reading somewhere that cast iron is very stable. ie it won't change shape over time. Not the best structural material however.

02-27-2002, 07:34 PM
it absorbs vibration very well which is an advantage when building machinery

02-27-2002, 07:45 PM
It's also good for tool bases, or stuff that's going to take knocks. Mild steel will dent and ding pretty easily, while the cast iron will weather this without showing much wear and tear. It also seams to hold lube longer, perhaps the grain structure is open and will hold oil. This is speculation on my part, I dunno. It does take a slighty different approach for machining, I found out, when replacing a bandsaw wheel I made from an exercise machine flywheel.

02-28-2002, 01:34 AM
Cast iron has a natural ability to put up with dings, vibration, and wear better than steels. It is the best material for machines and things like lathe backplates or faceplates. Steel "rings" like a bell, iron does not.

If you really want to know why go to the library and have them get you US Steels "The making, shaping, and treating of steel" (the title may be slighly off - I cannot get to my copy to confirm its title - too much "stuff" in the way/lazy). This book will tell you everything you could want to know about Iron and its alloys and the processes involved with them. 1400 pages of Joy-Joy!


02-28-2002, 10:03 AM
I claim no special expertise here, and can't cite any specific references, but I've formed the following opinions from my various readings: in addition to its excellent lubricating and damping qualities C.I., tho not as strong as steel, is extremely strong in compression, thus not as prone to denting and dinging (already mentioned above). I've also formed the impression that it lends itself better to casting (more fluid, etc.) and it's is easier to control the metallurgical quality when melting/casting iron. Cost? Tho small quantities obtained by you and I may be as expensive as other metals, I suspect that when cast on a production basis it turns out to be much cheaper. As with most other industrial decisions I would bet that ultimatly cost is the determining factor.

Part of it can probably be traced to the early evolution of the steel industry: steel required additional processing steps which added to the cost. So if the item could be satisfactorily made from the molten iron, why take the additional steel making steps. And we continue doing things the same way, until more profitable alternatives arise.

One question that comes to my mind is the issue of sand molds. Can steel be as readily cast in sand as can iron? Maybe someone more knowledgeable can address that.

This is a fascinating issue and I hope others with more expertise help out here. Can't recall his name at the moment but there's a guy down around Jacksonville FL that's mentioned in HSM who's into casting iron. He has ads for his book and video, and Steve Acker (gunsmith contributor) referenced him in a recent article (he'd cast a faceplate for Steve). I'm sure he could shed additional light on this question.

02-28-2002, 11:27 AM
Far as price of cast iron is concerned, my local metal supplier charges more for cast iron per pound thatn regular mild steel, but quite possibly for production applications it's more economical.

What's funny is that I have books and books on metal and it's properties, but they talk about the individual physical properties, and fail to explain where they are typically used and why. It would be nice if someone put together a big chart that gave information about where they are, how they are used, how much they cost (relatively), how easily they are worked, etc, instead of lots of numbers which leaves you to figure out what they are good for. My experience has been that metal supplier don't generally know what they're selling. I find this board very useful for this very reason. Appreciate everyone's input.


02-28-2002, 10:22 PM
I learned in my material and design classes that it’s basically this simple.
Iron is to concrete like steel is to wood.
Iron has very strong compressive forces like concrete. Basically it is used for machine bases, tool bases, and frames. Irons compressive force is much higher than its tensile force. Iron will crack and break when introduced to high tensile loads! (Think of the tombstone the mill head is attached too!)
Need tensile loads? Use steel! Steel will take tensile loads much greater than iron, but does not have nearly the same compressive strength. (Think of wires in a suspension bridge).

This is where it gets a little confusing.
Iron is fairly easy to machine, will eliminate harmonics better (vibrations), and is hard to “dent”. Most iron will be found in “cast” stock and will be found to be fairly light and less dense for its relative size. Also can be used for bearing material, and is the most easily processed ores from the “steel family”. It looks trick too!

Steel machines less easily, is very susceptible to harmonics (vibrations), and in its softer forms is easily “dented”. Steel usually comes in “cold roll” stock which is comparatively more dense, stronger, and heavier than typical gray iron. Steel should cost more, because of the extra ingredients in its composition. (Unless there is so much more than iron that it’s just cheaper because of logistics) Steel is iron blended with other metals to achieve different properties. Thus heat-treating steel like 4140, A2, and O6 happens because of the other materials in the steel make up. If you really what to know more I can dig up my old research paper I wrote on heat treating steels from school and email it too you! I also worked in a foundry that poured everything but iron for about 10 years. Machining castings is more fun than stock in my opinion. At least the brass & bronze chips instead of sprays. But that’s a whole other topic! Thanks for the soap box!

Also everybody before is right about what the said about iron verses steel.


02-28-2002, 10:51 PM
I am not a metallurgist, all I know is from what I've broken over the years or picked up from machinists that I've pestered.
The alloy of iron or steel is only one difference. The way that the metal is formed into shape also affects its strength. Taking the same alloy and hot rolling it will make it have different characteristics than casting or cold rolling. Cast iron has a more random grain. I think that hot rolled has grain going the long way. I don't know what cold rolling grain is like but it is harder than hot rolled. Forging is even stronger. please correct me if I'm wrong. Thanks--Mike.

03-01-2002, 02:56 AM
Iron has a very random grain structure and poor distribution of its alloying elements - part of the reason it does not "ring" when struck.

Steel rings because it has a more uniform grain structure. Cold forming or hot forging helps align the grain structure and more evenly distribute alloying elements thus increasing the strength of the material.

It is important to realize that metal alloys are not Chemical compounds - that is the different metals do not bond together to form a chemical. Samurai Sword Steel is unique - having been folded, hammer welded, folded, & rewelded over and over again until the process produces a steel that is almost a "homogenous" compound. The steel is nearly a uniform compound with its alloying elements evenly distributed. Hence its outstanding properties.

It is a error to assume that cast iron is "cheaper" than steel - this simply is not so. Some patented casting alloys cost more to produce than steel because alloying Elements are adjusted to give the best physical properties just as with steel alloys. Cast Iron can be one of the most difficult materials to machine.

Or, so I have been told...

[This message has been edited by Thrud (edited 03-01-2002).]

metal mite
03-01-2002, 01:21 PM
Cast iron bar is a rip-off.
I think the steel houses are just taking advantage of the situation.
Many foundries have gone overseas now, thank you EPA.
Real castings are still cheap compared to the hogging you got to do on many jobs.
Ever cut ductile iron? That smell brings back memories of my youth.
Metal Mite

Besides cast steel is a pain to machine
Hard spots, and those nasty chips will tear you up.

[This message has been edited by metal mite (edited 03-01-2002).]

03-01-2002, 01:25 PM
I use ductile Iron quite often...I hate the stuff, coolant goes rancid quick....it machines nice, but if you like bad smells and black hands then this is the stuff for you.....


03-03-2002, 06:02 PM
The biggest componenet that give Cast Iron all of these properties, is the carbon content, and how it is casted and chilled.
When the cast iron cools down a lot of the carbon in solution comes out and forms graphite. This can come out in the form of layers, or spheres and each type will impart different characteristics to the iron.
When it is in the layed for it will have good damping characteristic and aborb the vibrations with out transmitting to much of it because the layers of graphite act as rubber mats.
The self lubricty of cast iron is also because of the graphite formation, this acts as an internal lubricant and because of the random order of the layers keeps the chip size small.
The high carbon content in solution lowers the melting point and increases the fluidity of the molten iron making it an easy material to work with.
An interesting piece of trivia that I remember reading about says that in 1 cubic foot of iron, there can be up to fifty pounds of carbon in solution.
Sorry for the long winded posting, but I am on night shift and have nothing to do for the moment. Hope this was helpful.

Herb W
03-03-2002, 08:44 PM
I came across a website a while back that has lots of info on ductile iron. I was not aware that it (d.iron) could be made to have so many different characteristics, depending on how it is made, alloying elements etc.
No connection to site.

www.ductile.org/dimg/Default.htm (http://www.ductile.org/dimg/Default.htm)

[This message has been edited by Herb W (edited 03-04-2002).]

Rich Carlstedt
03-05-2002, 11:22 PM
A Little info for you guys
Wrought Iron ,is pure iron . It is soft and makes great artistic stuff, but is very hard to find today.
When carbon is added to Iron,(.10%, Thats one tenth or a percent) it becomes steel.
As you increase the carbon content, steel gets stronger and harder.
So .18% makes 1018 steel, your normal bar stock.
.45% makes 1045, a good steel for hardening and making tools.
.8% makes the steel so hard, it is called Spring steel ie.1080

When the carbon gets to greater than 1.4%, it becomes Cast Iron
Bessimer developed the modern steel mill when he put a oxygen lance in a vat of cast iron and the oxygen burned off the carbon as carbon monoxide and carbon dioxide leaving steel with a low carbon content.

Now Cast Iron can have carbon upto 3%, but that is wicked stuff. It also has many forms that I cannot cover here.
There is Ductal Iron,Mallable Iron,Perlitic Iron, Mehanite Iron,White Iron etc. Each has a good/bad point.Iron comes in "grades". THIS is the tensile strength(pull apart)of the iron.
So a 20 grade has 20,000 PSI strenth.
A 40 has 40,000 etc.
Normal steel is 60,000 FYI.

Now here is the interesting stuff about cast iron:
It is the only material that can be used as a bearing or slide against itself ! It doesn't gall . The carbon lubes the joint and prevents the parts from ahering to each other. Thats one reason it is hard to weld !.
Steel cannot run against steel and bronze against bronze..no way, but Cast Iron does.

Cast Iron absorbes vibration 10 times better than steel, so it is self-dampening. The old saying is" you will never see a Cast Iron tuning fork ! "
This makes it great for machines to keep resonating down.

Cast Iron can be strong...are you ready ???
Mehanite 120 is used for diesel engine crankshafts....4140 maybe stronger (130,000 PSI versus 120,000) but Irons vibration dampening ability means that it suppresses high vibration loads where steel will FAIL.
Thats right, they use iron and the cranks hold up, but steel will fail..
If this is hard to understand, think of waves in a swiming pool. if they hit hard concrete sides , the spray shoots up into the air, but if the sides are rubber like and move with the pressure , the "spike" is subdued, and then the sides push back after the major force has past. (Conservation of energy law !)

Cast Iron has superior flow abilty when melted and poured.
It is stable.

AND lastly,It has greater corrosion resistence then steel or wrought iron.
Thats why you doen't see steel blocks in cars.( although Crosley tried it back in 48 )

Love it !