Cast iron vs ductile iron vs steel

I heard a discussion at a hardware store about the types of metal that tools are made of, and the impact it has on cost and on flexibility. I thought it was worthwhile to share and elaborate on some of what I heard. What’s the difference between cast iron vs ductile iron vs steel? Generally, it’s strength, hardness, and flexibility. And those factors generally come down to the carbon mixture.

Ductile iron is a form of cast iron, but it has carbon mixed into the alloy to make it more flexible, less brittle, and give it a higher strength overall. Steel is also a mixture of carbon and iron, but it has less carbon than ductile iron, the precise mix depends on the properties you want.

Cast Iron

Humans have been casting objects from iron for centuries. It involves making a mold out of another material, heating up the iron until it is liquid, and pouring it into the mold and letting it cool.

The result is a metal object that is harder than other metals like bronze, yet abundant enough in nature to be relatively inexpensive. But the hardness comes at a cost. Iron has low tensile strength, which is the up-and-down strength like you saw in old super glue commercials that supported the full weight of an adult male.

The other downside to cast iron is that cast iron, like granite, is brittle. If you drop it on another hard surface, it can break. I’ve personally had two different incidents with old toy trains made of cast iron falling onto concrete. The train that fell two feet survived. The one that fell four feet broke into several pieces.

The problems with cast iron led to numerous other developments. Sometimes making metal parts or metal objects out of pot metal or diecasting is more appropriate. Though softer than iron, both types of metal are less brittle, and tend to be cheaper.

But when you need the hardness of iron and its good casting properties without the brittleness, there is an answer.

Ductile Iron

It didn’t take long for humanity to discover that by adding carbon, they can make iron more flexible and less brittle. The flexibility keeps it from breaking. Ideally it will spring back into its form rather than bending permanently, but either is better than breaking.

Carbon, if you remember your high school science, is another very common element. Graphite is made of carbon, and so is diamond. For that matter, you are also mostly carbon. It’s not the carbon that makes carbon-bearing alloys expensive so much as it’s getting the mixture right.

Ductile iron contains at least 3 percent carbon. The carbon improves flexibility, giving it a tendency to bend in response to force rather than snap. You can still cast with it, so ductile iron is really a form of cast iron. But it’s better in many regards than plain cast iron, especially for making tools.

The other thing that distinguishes ductile from steel is its good casting properties. It’s also easier to machine, and cheaper than steel as well.

Why Cast Iron Is Better

The main advantage of cast iron is cost. It is cheaper than ductile iron or steel. And that isn’t always a bad thing.

Years ago, an engineer explained cost to me like this. Let’s say you have a budget for a car. I can get you a Rolls-Royce. It’s a great car, and you’ll love it. But if you don’t have budget for a Rolls-Royce, it doesn’t matter. You don’t care how great the car is if you can’t afford the other things you need.

Is a Rolls-Royce a better car than a Toyota? Only if you can afford the Rolls-Royce.

So if a cast iron vise is all you can afford and you need a vise and can’t find a used one, it’s better than doing without.

And for some things, like cooking, cast iron has advantages. Foodies agree that while a cast iron skillet needs care that cookware made of more modern materials doesn’t need, the results from cast iron cookware made it worth it.

Why Ductile Iron Is Better

Ductile iron has the positive attributes of cast iron, but is less temperamental. It’s harder to break. So in the event of an accident, where something made of ductile iron gets dropped or bumped, or has something dropped on it, the ductile iron is much less likely to snap.

A cheap vise, like the cheapest vises you can get at Harbor Freight, and even those sold as light duty vises at the big-box stores, are made of cast iron. And what you typically hear about the cheap Harbor Freight vise is that it was great until it broke, and that it broke unexpectedly. A vise made of ductile iron costs more, but it will last the rest of your life. And whoever gets it from you after that will find it will last the rest of their life too.

And the price difference isn’t necessarily as high as you may think. Harbor Freight sells a 6-inch cast iron vise for $80. Home Depot sells a 6-inch ductile iron vise for $100.

A lot of what we call cast iron pipe today is actually ductile iron pipe. It’s common for pipes to encounter shocks when being transported and handled on all the stops between the manufacturing facility and the job site. And shocks are absolutely guaranteed to happen during installation. Cast iron pipes are cheaper but once they’re installed, they look about the same.

The carbon also improves the pipe’s corrosion resistance, which is important, considering a pipe’s job is carrying water.

Steel

Like ductile iron, steel is iron mixed with carbon. So what’s the difference? What distinguishes ductile iron vs steel? Steel contains less carbon. Steel can contain up to 2 percent carbon, but it is usually less than 0.4%. The carbon content changes the properties of the steel. The trade off is hardness vs malleability.

Low-carbon steel has less than .3% carbon. Medium has .3-.6 percent. High carbon has .6-1.5%, typically. High carbon steel is often used for tools. More carbon makes it stronger and harder but less ductile and malleable.

This means all steel tends to be more ductile than ductile iron. Steel also contains trace amounts of other metals such as nickel and chromium. They may sometimes also include silicon and/or sulphur. Adjusting the mixture in the alloy changes the properties slightly. It’s also common to plate sheet steel, used to form complex shapes, with tin to improve its corrosion resistance.

But steel isn’t always better than ductile iron. Ductile iron has better abrasion resistance, partly because of its higher carbon content in the form of graphite flakes. The graphite makes it self lubricating. Ductile iron also has better corrosion resistance than steel.

But steel has better impact resistance, which makes it better for making things like a hammer.