The difference between iron and steel is carbon. Steel is iron with a small, controlled amount of carbon, roughly 2 percent or less, while cast iron carries much more carbon and wrought iron carries almost none, and that one number changes how each metal bends, breaks, rusts and gets made into parts.
In everyday talk people say “iron” for anything heavy and gray, and old buildings still have “iron” beams that are really steel. This guide sorts out pure iron, wrought iron, cast iron and steel, compares their strength and corrosion behavior, and explains why modern structural shapes are all steel.
Iron Is the Element, Steel Is the Alloy
Iron (Fe) is a chemical element. The US Geological Survey puts the relationship simply on its USGS iron and steel statistics page (read 25 October 2026): steel, by definition, is a combination of iron with a small amount of carbon, and the metallic iron it is made from has to be extracted from iron ore first.
Pure iron is soft and not much use for structure. What people actually buy and build with are iron alloys, and the amount of carbon in them sorts them into families. Our article on what steel is made of goes deeper into the other elements, such as manganese, that show up in steel.
The Three Irons and Steel, Compared
The clearest public description of the iron family comes from the National Park Service, which deals with all of them in historic buildings. Its NPS Preservation Brief 27 on architectural cast iron (read 25 October 2026) gives these definitions.
Cast iron
Cast iron is an alloy with a high carbon content, at least 1.7 percent and usually 3.0 to 3.7 percent, plus varying amounts of silicon, sulfur, manganese and phosphorus. Molten, it pours easily into molds, which is why old storefronts, columns and decorative railings were cast. It is too hard and brittle to be hammered, rolled or pressed. NPS explains that free graphite flakes in gray cast iron cause that brittleness, and that brittleness is the key difference between cast iron and mild steel.
Wrought iron
Wrought iron is almost pure iron, with less than 1 percent carbon (usually 0.02 to 0.03 percent) and 1 to 4 percent slag mixed through it physically rather than alloyed. That slag gives it a fibrous, layered grain. NPS describes it as soft, malleable, tough, fatigue resistant and easy to forge and bend. NPS also notes that mild steel is now used to make new hand-worked metalwork and to repair old wrought iron elements.
Steel
NPS describes mild steel as an iron alloy with not more than 2 percent carbon, strong but easily worked. The Specialty Steel Industry of North America’s SSINA designer handbook (read 25 October 2026) gives the same limit for carbon steel, at least 95 percent iron with up to 2 percent carbon, and notes that higher carbon makes the steel stronger. Structural grades sit at the low end of that range so they stay ductile and weldable. Our guide to mild steel covers that low-carbon end in detail.
| Property | Wrought iron | Cast iron | Steel (mild / structural) |
|---|---|---|---|
| Carbon content | Under 1%, usually 0.02-0.03% | At least 1.7%, usually 3.0-3.7% | Up to about 2%; structural grades at the low end |
| How it is shaped | Forged, bent, drawn | Poured into molds | Rolled, forged, welded, cut |
| Behavior under load | Tough, ductile, fatigue resistant | Strong in compression, weak in tension, brittle | Strong in tension and compression, ductile |
| Corrosion (NPS comparison) | More resistant than mild steel | More resistant than wrought iron or steel | Least resistant of the three when bare |
| Joining | Riveted or forge welded | Bolted or screwed | Welded or bolted |
| Used for new structural framing? | No; mild steel is used instead | No | Yes, modern beams and columns |
Which Is Stronger, Iron or Steel?
For anything that bends or gets pulled, steel. NPS notes that cast iron withstands great compression loads, which is why it made good columns, but it is relatively weak in tension and fails under tensile loading with little warning. A beam carries both compression on one face and tension on the other, so a brittle material is a poor choice.
Steel is ductile. It stretches and yields visibly before it breaks, which gives warning and lets designers rely on a defined yield point. Modern grades are specified by that number: A36 steel for plate and angles, and ASTM A992 steel for wide flange beams.
Which Rusts Faster?
Bare mild steel generally rusts fastest. NPS states that cast iron’s high carbon makes it more resistant to corrosion than either wrought iron or steel, and that mild steel is less resistant than both. All three still rust when left wet and unprotected. NPS also notes that rust forms quickly on cast iron once relative humidity is above about 65 percent, and sooner where salt or acid pollution is present.
Cast iron has its own failure mode called graphitization, where the iron corrodes out of the graphite network and leaves a part that looks intact but is structurally weak. NPS suggests testing suspect areas by scraping with a knife to see if the metal crumbles. Steel avoids that, but needs paint, galvanizing or a stainless alloy to stay rust free. Stainless is still steel, just with at least 10.5 percent chromium added.
Iron vs Steel in Buildings
Older buildings can contain cast iron columns, wrought iron beams or early steel, and they are not interchangeable. If you are renovating and find an old column or beam, do not assume its strength from its size. Cast iron columns can be brittle, and early iron beams do not share the properties of modern steel shapes. A licensed structural engineer has to identify the material and check its capacity before anything is added, cut or removed.
New work uses steel. Basement posts are usually steel pipe or lally columns, and beams are steel wide flange or I-shapes. Our steel beam types guide shows the common shapes and what each is used for.
How to Tell Iron from Steel
- Look for casting marks. NPS points to mold lines, flashing, casting flaws and air holes on cast iron, and very uniform repeated pieces.
- Check how it is joined. NPS notes cast iron is usually bolted or screwed, while wrought iron is riveted or forge welded. Welds suggest steel.
- Look at the shape. Rolled sections with constant flanges and a stamped mill marking are steel. Ornate, thick, sculptural pieces are often cast.
- Check broken edges. NPS describes a gray, grainy fracture surface on gray cast iron.
- When it matters, test it. A lab or engineer can confirm the material from a small sample.
Frequently Asked Questions
Is steel just iron?
Mostly. Carbon steel is at least 95 percent iron, with up to about 2 percent carbon and small amounts of other elements. That small amount of carbon makes it much stronger than pure iron.
Is cast iron stronger than steel?
Only in a narrow sense. Cast iron handles compression well but is brittle and weak in tension, so it can crack suddenly. Steel is stronger overall and bends before it breaks, which is why it is used for beams.
Are wrought iron railings really iron?
Often not. NPS notes that mild steel is now used for new hand-worked metalwork, so many new “wrought iron” railings are steel. Genuine wrought iron is mainly found on older buildings.
Is an I-beam made of iron or steel?
Modern I-beams and wide flange beams are steel. “Iron” in names like “angle iron” is a holdover from the days before steel took over.
Can you weld cast iron to steel?
It is possible with suitable filler metal and careful heat control, but cast iron is brittle and prone to cracking. For anything structural, get a qualified welder and a structural engineer involved.
The Short Version
Steel is iron with a little carbon, roughly 2 percent or less. Cast iron has more carbon, pours into molds, handles compression and is brittle. Wrought iron has almost none, is tough, and has been replaced by mild steel in new work. Modern beams and columns are steel because it is strong, ductile and weldable, and any old iron member you find should be identified and checked by a licensed structural engineer.
Reference information only, not engineering advice. Disclaimer.