Iron vs steel vs cast iron: carbon content and what it changes

What is the difference between iron, steel and cast iron?

The difference is carbon content. By general engineering knowledge, not in our data, iron holds below about 0.02 wt% carbon, steel up to about 2 wt% and cast iron more. In our data the 20 steel profiles print carbon limits up to 1.60 wt% (AISI D2 cold-work tool steel), gray cast iron prints 2.5 to 4.0 wt% (per Wikipedia), and no profile prints a carbon limit between 1.60 and 2.5 wt%.

Drag the line across the carbon axis below. Each bar is one profile at the carbon limits its record prints; the readout shows the yield strength, tensile strength, elongation, hardness and density of the profiles nearest the line, each from one stated condition. The three shaded territories use the general engineering convention, which our data does not define.

Show beside each bar Tensile strengthYield strengthElongationHardnessDensity

Iron Steel Cast iron

1. Stainless steel 304L max 0.030 wt% C min 485 MPa
2. Carbon steel 1215 max 0.090 wt% C 540 MPa
3. Carbon steel 1010 0.08 to 0.13 wt% C 350 MPa
4. Stainless steel 430 max 0.12 wt% C min 448 MPa
5. Carbon steel 1018 0.14 to 0.20 wt% C 430 MPa
6. ASTM A36 structural carbon steel 0.26 wt% C 400 to 552 MPa
7. Alloy steel AISI 4130 0.28 to 0.33 wt% C 670 MPa
8. P20 plastic mould steel 0.28 to 0.40 wt% C 965 to 1030 MPa
9. 1.2316 stainless plastic mould steel 0.33 to 0.45 wt% C 1100 MPa
10. AISI H13 hot-work tool steel 0.39 wt% C 1420 MPa
11. 1.2738 plastic mould steel 0.35 to 0.45 wt% C 993 MPa
12. 300M ultra-high-strength steel 0.38 to 0.43 wt% C min 1862 MPa
13. Alloy steel AISI 4140 0.38 to 0.43 wt% C 1020 MPa
14. Alloy steel AISI 4340 0.38 to 0.43 wt% C 850 to 1000 MPa
15. Carbon steel 1045 0.43 to 0.50 wt% C 570 to 700 MPa
16. Carbon steel 1070 0.65 to 0.75 wt% C 640 to 700 MPa
17. AISI A2 air-hardening cold-work tool steel 0.95 to 1.05 wt% C not printed
18. AISI 52100 bearing steel 0.980 to 1.10 wt% C not printed
19. Silver steel 0.95 to 1.25 wt% C 700 to 850 MPa
20. AISI D2 cold-work tool steel 1.40 to 1.60 wt% C not printed
21. Gray cast iron 2.5 to 4.0 wt% C 200 to 350 MPa
22. Ductile iron 3.55 to 3.65 wt% C 448 MPa

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Carbon content, wt%. The axis is stretched below 1.0 wt% so the low-carbon steels do not overlap; the break is marked at 1.0. Zone limits of about 0.02 and about 2 wt% are general engineering knowledge, not in our data.

Carbon values are the limits or nominal figures printed in each profile's composition line, not measured heats. Stainless steel is limited to two grades here (Stainless steel 304L and Stainless steel 430) to keep the axis readable; the other stainless grades have their own pages. No profile in our data prints a carbon limit between 1.60 wt% (AISI D2 cold-work tool steel) and 2.5 wt% (Gray cast iron), so where steel ends and cast iron begins is not decided by our data.

What are the physical properties of iron?

The Iron (pure and ingot iron) profile prints a density of 7.874 g/cm3 (per periodictable.com), a melting point of 1,538 °C (per periodictable.com), an elastic modulus of 211 GPa (per WebElements) and a thermal conductivity of 79 W/(m K) (per periodictable.com) and 80 W/(m K) (per WebElements).

The profile named iron oxide in our database describes a ceramic compound, not metallic iron, so it is not a substitute. The left column is the iron profile, with every record it holds; where two sources print different values both are shown. The right column is the lowest-carbon steel with data, as a stand-in for comparison only, with its condition.

How does more carbon change strength, hardness and ductility?

In our records, within one condition, more carbon raises strength and lowers elongation. Hot rolled carbon steel 1010 (0.08 to 0.13 wt% C) prints 350 MPa tensile strength and 30 % elongation; carbon steel 1018 (0.14 to 0.20) prints 430 MPa and 27 % (23 °C, per MfgRobots). Heat treatment matters more: alloy steel 4340 (0.38 to 0.43 wt% C) prints 850 to 1,000 MPa in condition T and min 1,550 MPa in condition Z (per voestalpine).

Hardness points the same way across three datasheets, though not under one condition. Carbon steel 1010 (0.08 to 0.13 wt% C) prints 95 to 105 HB (per Total Materia, form not stated), carbon steel 1045 (0.43 to 0.50 wt% C) prints 170 to 210 HB when hot rolled under 250 mm (per voestalpine), and carbon steel 1070 (0.65 to 0.75 wt% C) prints 192 to 212 HB (per Gravity Cast, condition not specified in the table). Hot rolled 1045 also prints 570 to 700 MPa tensile strength, above the 430 MPa of hot rolled 1018 from a different source, so treat these as pointers, not a controlled measurement.

The pattern breaks as soon as the condition changes. Alloy steel AISI 4140 and alloy steel AISI 4340 print the same 0.38 to 0.43 wt% carbon, yet 4340 ranges from 850 to 1,000 MPa in condition T to min 1,550 MPa in condition Z (per voestalpine), and 4140 prints 1,020 MPa when normalised at 870 °C (per Elgin). Carbon sets how strong a steel can become; the heat treatment decides how far it gets.

A dash means that value is not printed for that row's condition; other conditions of the same profile may print it, and the profile page lists them. “min” and “max” are limits, not typical values. Rows use different conditions on purpose: values are never averaged across conditions. Chosen condition per profile: the one that prints most of yield, tensile, elongation and hardness, preferring a plain hot rolled, annealed or normalised state when equal.

Why is gray cast iron brittle while ductile iron is not?

Gray cast iron holds its 2.5 to 4.0 wt% carbon mostly as flake graphite, and the flakes act as notches, so EN-GJL-250 prints only 0.3 to 0.8 % elongation at 20 °C (per MfgRobots). Ductile iron holds 3.55 to 3.65 wt% carbon as substantially spheroidal graphite and prints 18 % elongation in grade 60-40-18 (per Goldens), falling to 2 % in grade 120-90-02.

The gray cast iron profile says graphite may take 6 to 10 percent of the volume and that the flake tips act as notches under tensile stress or shock. The ductile iron profile describes the graphite as substantially spheroidal and notes that elongation falls as the grade rises, from 18 % in grade 60-40-18 to 2 % in grade 120-90-02 (per Goldens). Where the carbon goes, and in what shape, is what separates these two irons in our data; the carbon percentage alone does not, because the two profiles print overlapping carbon limits.

What are the types of cast iron, and what is gray cast iron used for?

Our database holds two cast iron types: gray cast iron (flake graphite iron, ASTM A48, EN-GJL-250) and ductile iron (nodular or SG iron, ASTM A536, EN-GJS). Gray cast iron is used for machine bases, frames, pump housings and engine blocks; ductile iron for high-strength gears and rollers and pressure-containing valve and pump bodies. Malleable iron and white cast iron: not found in any source we hold.

Where sources disagree both values stay: the ASTM A48 class 20 tensile strength is 152 MPa in Wikipedia's table (printed 22 ksi) and 138 MPa at Align (printed 20,000 psi); neither source says whether its figure is a minimum or typical. Malleable iron and white cast iron: not found in any source we hold.

The gray cast iron profile lists machine bases, frames, pump housings and engine blocks as the fit for the grade, and the ductile iron profile lists high-strength gears and rollers and pressure-containing valve and pump bodies. The ductile iron profile also notes that elongation falls as the grade rises. Both statements are from the profiles; the grade tables above are the numbers behind them.

How do density, stiffness and heat flow change from steel to cast iron?

Gray cast iron is less dense and far less stiff than steel in our records. EN-GJL-250 prints a density of 7.2 g/cm3 and an elastic modulus of 103 to 118 GPa at 20 °C (per MfgRobots), against 7.9 g/cm3 and 210 GPa for hot rolled carbon steel 1018 at 23 °C (per MfgRobots). Ductile iron grade EN-GJS-400-15 prints 169 GPa (per Total Materia, temperature and treatment not stated).

Heat flow is closer: gray cast iron EN-GJL-250 prints 50 W/(m K) at 20 °C (per MfgRobots), and cold drawn carbon steel 1018 prints 51.9 W/(m K) (per Elgin, a value the datasheet says is estimated from similar materials). Carbon steel changes little in density and modulus between grades, so the table is mainly a way to see where cast iron breaks from the rest.

A dash means no record of that property in the profile. Thermal expansion is a mean coefficient over the temperature range shown; thermal conductivity and expansion are not limited to room temperature, so each cell carries its own temperature. Records in °F that could not be converted are left out of this table and kept in the CSV.

What melting point and compressive strength data do we hold?

Both are thin. Melting point is printed for 4 of 22 profiles on this page and for no cast iron. Compressive strength is printed only for gray cast iron: 752.5 to 1,053.5 MPa for EN-GJL-250 (per Total Materia). No steel profile prints a compressive strength.

The other 18 profiles, including both cast irons, print no melting point in any source we hold. The melting range shown for carbon steel 1018 is labelled by its source as typical for low carbon steel. The ASTM A48 class values are as printed, with no statement of minimum or typical.

Which data is missing from this comparison?

Not found in any source we hold: yield strength, tensile strength and elongation for iron itself; a profile for malleable iron or white cast iron; a compressive strength for any steel; a melting point for any cast iron; and room-temperature yield or tensile strength for AISI A2 air-hardening cold-work tool steel, AISI 52100 bearing steel and AISI D2 cold-work tool steel. Those gaps limit every comparison on this page.

• Iron itself. The iron profile prints no yield strength, tensile strength or elongation record; each of its thermal and elastic values comes from one or two reference sites.
• Only two cast irons. Malleable iron and white cast iron are not covered, and the gray iron class tables do not say whether a figure is a minimum or a typical value.
• Carbon is a printed limit. The axis uses composition limits or nominal values from each profile, not measured heats; a maximum-only limit is drawn as a faded bar from zero.
• Conditions differ between rows. Each row shows the one condition that prints most of the four mechanical values. Other conditions are on the profile pages and in the CSV, where every record is listed with its condition and test temperature.
• Elevated-temperature records are excluded from the mechanical and density columns (for example gray cast iron tensile strength at 300 and 600 °C) and kept in the CSV with the reason.
• Hardness is patchy. Where a profile prints hardness only for an annealed or delivery condition, the scrubber says so and lists any hardness from another condition under that condition's own label, never as part of the same row.
• Stainless steel is limited to two grades on the axis; the stainless collection holds the rest.

Questions people ask about iron, steel and cast iron