Plain carbon steels, alloy steels and cast irons

Low, medium and high carbon steels, effects of alloying elements, stainless and tool steels, AISI and IS designations, the main cast irons, and carbon-equivalent calculations.

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Why it matters

Ferrous alloys make up most of the metal tonnage used in machines, structures and vehicles. A production engineer constantly chooses between a cheap plain carbon steel, an alloy steel that hardens in oil, a stainless steel that will not rust, or a cast iron that damps vibration and casts easily — and must read designations such as C45, 40Cr4 or AISI 304 correctly.

Key ideas

Plain carbon steels contain up to about 1.5 % C (theoretically up to 2.14 %) with small amounts of Mn (0.3–1.0 %), Si (up to about 0.3 %) and impurities S and P (each kept below about 0.05 %).

  • Low carbon (mild) steel, below 0.25–0.3 % C: ferrite + a little pearlite; soft, ductile, very weldable, cannot be usefully hardened by quenching. Sheet, wire, structural sections, rebar, carburising grades.
  • Medium carbon, 0.3–0.6 % C: can be hardened and tempered; shafts, axles, gears, crankshafts, rails.
  • High carbon, 0.6–1.5 % C: high hardness and wear resistance, low ductility and weldability; springs, wire rope (about 0.8 % C), hand tools, cutting tools, dies. Mn deoxidises and ties up sulphur as MnS (avoiding hot shortness, which FeS causes); Si deoxidises; P causes cold shortness (brittleness at low temperature).

Alloy steels add elements deliberately, for hardenability, strength at temperature, toughness or corrosion resistance.

  • Ni: toughness, low-temperature impact strength, austenite stabiliser.
  • Cr: hardenability, wear resistance (hard carbides); corrosion resistance when above about 10.5–12 % (stainless).
  • Mo: hardenability, high-temperature strength, reduces temper embrittlement.
  • V, W: fine grain size, hard stable carbides, hot hardness (tool steels).
  • Mn: hardenability, strength; 12–14 % Mn gives austenitic Hadfield steel that work-hardens under impact (crusher jaws, rail crossings).
  • Si: strength, electrical resistivity (transformer steel), oxidation resistance.
  • Co: the only common element that reduces hardenability; raises hot hardness in high-speed steels.
  • B (a few ppm): very large increase in hardenability. Low-alloy steels (total alloy below about 5 %) such as 40Cr4 or 40NiCr4Mo3 (similar to AISI 4140 and 4340) are quenched and tempered for highly stressed parts. HSLA (micro-alloyed) steels get strength from fine grains and small Nb, V, Ti additions.

Stainless steels (at least 10.5–12 % Cr, forming a self-healing Cr₂O₃ passive film):

  • Austenitic (e.g. 304: 18 % Cr, 8 % Ni; 316 adds Mo): FCC, non-magnetic, very ductile and tough down to cryogenic temperatures, not hardenable by heat treatment (only by cold work).
  • Ferritic (e.g. 430, 17 % Cr): BCC, magnetic, cheaper, not hardenable.
  • Martensitic (e.g. 410, 420, 440C, higher C): hardenable by quenching; cutlery, surgical tools, valves.
  • Duplex: half ferrite, half austenite; strong and resistant to stress-corrosion cracking.
  • Precipitation hardening (e.g. 17-4 PH).

Tool steels include water-hardening, oil-hardening, high-carbon high-chromium die steels (D2), hot-work steels (H13) and high-speed steels (18-4-1: 18 % W, 4 % Cr, 1 % V; M2 with Mo) that keep hardness up to about 600 °C.

Designations. AISI/SAE: 10xx = plain carbon, xx = carbon in hundredths of a percent (1045 = 0.45 % C); 41xx = Cr–Mo; 43xx = Ni–Cr–Mo. Indian IS 1570: C45 or 45C8 = 0.45 % C (the number after C is 10 × mean Mn, so 45C8 has 0.8 % Mn); 40Cr4 = 0.40 % C, about 1 % Cr (the alloy figure is the percentage multiplied by a factor, 4 for Cr).

Cast irons contain 2.14–4.5 % C (usually 2.5–4 %) and 1–3 % Si. Silicon promotes graphite instead of cementite, and the form of carbon decides the properties.

  • Grey iron: graphite flakes in a ferrite or pearlite matrix; fracture is grey. Flakes act as internal notches, so tensile strength and ductility are low, but compressive strength, castability, machinability and vibration damping are excellent. Machine-tool beds, engine blocks, brake drums.
  • White iron: low Si and fast cooling keep carbon as cementite; very hard and brittle, white fracture. Wear parts, and the starting material for malleable iron.
  • Malleable iron: white iron annealed (about 800–900 °C, long times) so cementite decomposes into irregular graphite clusters (temper carbon); appreciable ductility. Small fittings, brackets.
  • Ductile (spheroidal-graphite, nodular) iron: Mg or Ce added to the melt make graphite form spheres; strength and elongation approach those of steel. Crankshafts, gears, pipes, valve bodies.
  • Compacted-graphite iron (CGI): vermicular graphite, between grey and ductile; diesel engine blocks.

Formulas

CE_cast = C + (Si + P) / 3 Carbon equivalent of a cast iron (wt %). CE < 4.3 hypoeutectic, = 4.3 eutectic, > 4.3 hypereutectic. A higher CE favours grey (graphitic) solidification.

CE_IIW = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 Weldability carbon equivalent of a steel (wt %). Roughly: below 0.40 readily weldable; 0.40–0.45 some care; above about 0.45 preheat and controlled heat input needed (thresholds from your welding code).

Worked examples

Example 1 (standard): carbon equivalent of a grey iron. Given: 3.2 % C, 2.1 % Si, 0.15 % P.

  1. CE = C + (Si + P)/3 = 3.2 + (2.1 + 0.15)/3 = 3.2 + 0.75.
  2. CE = 3.95 %, below 4.3, so the iron is hypoeutectic: primary austenite dendrites form first, followed by graphite eutectic.

Example 2 (GATE level): weldability of an alloy steel. Given: C 0.20, Mn 1.40, Cr 0.20, Mo 0.10, Ni 0.30, Cu 0.20 (wt %).

  1. Mn/6 = 0.233; (Cr + Mo + V)/5 = 0.30/5 = 0.060; (Ni + Cu)/15 = 0.50/15 = 0.033.
  2. CE = 0.20 + 0.233 + 0.060 + 0.033 = 0.53.
  3. Since CE > 0.45, the heat-affected zone can form martensite and crack; preheat (typically 100–200 °C, from the code) and use low-hydrogen electrodes.

Example 3 (designation). 40Cr4 → 0.40 % C and Cr = 4/4 = about 1 % Cr, a hardening-and-tempering steel for shafts and gears. AISI 1080 → 0.80 % C plain carbon steel, close to eutectoid, used for springs and wire.

Common mistakes

  • Thinking any chromium makes steel stainless; at least about 10.5–12 % Cr is needed.
  • Assuming all stainless steels are non-magnetic; ferritic and martensitic grades are magnetic.
  • Calling grey iron weak in compression; it is weak and brittle in tension but strong in compression.
  • Confusing malleable iron (made by annealing white iron) with ductile iron (made by Mg treatment of the melt).
  • Reading 45C8 as 8 % carbon; the 8 is ten times the mean manganese content.
  • Expecting austenitic stainless steel to harden by quenching.

For GATE PI

Expect one-mark questions on classification by carbon content, the role of individual alloying elements (Cr, Ni, Mo, Mn, Si, Co, W), types of stainless steel and cast iron and how graphite shape controls properties, high-speed steel composition, and steel designations. Numericals can use carbon equivalent for cast irons or weldability.

Quick check

  1. What element and minimum amount make a steel stainless?
  2. Which cast iron is made by adding magnesium to the melt?
  3. What does AISI 1045 indicate?
  4. Name the alloying element that lowers hardenability.
  5. A cast iron has 3.5 % C and 2.4 % Si (P negligible). Is it hypoeutectic, eutectic or hypereutectic?

Answers: 1. Chromium, about 10.5–12 %; 2. Ductile (spheroidal-graphite) iron; 3. Plain carbon steel with 0.45 % C; 4. Cobalt; 5. CE = 3.5 + 0.8 = 4.3, eutectic.

Interview questions

All Engineering Materials interview questions

Try answering each one aloud before you open it.

  1. 1.What are plain carbon steels, and how are they classified?Concept

    Plain carbon steels are alloys of iron and carbon with small amounts of other elements. They are classified based on their carbon content into low carbon steels (up to 0.3% carbon), medium carbon steels (0.3% to 0.6% carbon), and high carbon steels (0.6% to 1.0% carbon). Each type has different properties and applications, with low carbon steels being more ductile and high carbon steels being harder and stronger.

  2. 2.Explain the difference between alloy steels and plain carbon steels.Concept

    Alloy steels contain additional alloying elements such as chromium, nickel, and molybdenum, which enhance their mechanical properties and corrosion resistance. Plain carbon steels primarily consist of iron and carbon with minimal other elements. Alloy steels can be tailored for specific properties like increased strength, toughness, or resistance to wear, while plain carbon steels are generally more cost-effective and easier to work with.

  3. 3.What are cast irons, and what are their main types?Concept

    Cast irons are a group of iron-carbon alloys with a carbon content greater than 2%. The main types of cast irons include gray iron, ductile iron, white iron, and malleable iron. Gray iron is known for its good machinability and vibration damping, ductile iron for its strength and ductility, white iron for its hardness and wear resistance, and malleable iron for its toughness and ductility.

  4. 4.Why is chromium added to alloy steels?Application

    In low-alloy steels (about 0.5–2 % Cr) chromium raises hardenability and forms hard carbides, improving strength and wear resistance after quenching and tempering, as in 40Cr4 or bearing steel with 1.5 % Cr. Above about 10.5–12 % Cr the steel forms a thin, self-healing Cr₂O₃ passive film and becomes stainless. Chromium also improves oxidation resistance and strength at high temperature, which is why it is in boiler and heat-resisting steels.

  5. 5.What happens if the carbon content in steel is increased beyond 1%?Application

    Above about 1 % C, a slowly cooled steel contains a network of proeutectoid cementite along prior austenite grain boundaries, so hardness and wear resistance keep rising but ductility, toughness and even tensile strength fall. After hardening, more carbides and retained austenite remain, and the steel becomes hard to weld and prone to quench cracking. Such steels (about 1.0–1.4 % C) are used for files, razors, cutting tools and dies, usually after spheroidise annealing for machining and hardening from just above A1.

  6. 6.Explain why ductile iron is preferred over gray iron in certain applications.Application

    Ductile iron is preferred over gray iron in applications requiring higher strength and ductility. Unlike gray iron, which has a flake graphite structure, ductile iron has a nodular graphite structure that provides better mechanical properties. This makes ductile iron suitable for components like automotive parts, where impact resistance and toughness are important.

  7. 7.What is the effect of adding nickel to alloy steels?Application

    Nickel is added to alloy steels to enhance their toughness, strength, and corrosion resistance. It helps in stabilizing the austenitic structure, which improves the steel's ability to withstand impact and low temperatures. Nickel-containing steels are often used in applications like cryogenic tanks and pressure vessels.

  8. 8.How does the presence of silicon affect the properties of cast iron?Application

    Silicon in cast iron acts as a graphitizing agent, promoting the formation of graphite over cementite. This enhances the machinability and thermal conductivity of the cast iron. Silicon also increases the fluidity of the molten iron, making it easier to cast complex shapes. However, excessive silicon can reduce the strength and wear resistance of the cast iron.

  9. 9.Determine the carbon equivalent of a steel with 0.3 % C, 1.5 % Mn and 0.5 % Si using CE = C + Mn/6 + Si/24, and say what it is used for.Numerical

    CE = 0.3 + 1.5/6 + 0.5/24 = 0.3 + 0.25 + 0.021 = 0.57 %. This is the Japanese (WES) form of the weldability carbon equivalent, which includes silicon; the more common IIW formula, C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, ignores Si and would give 0.55 %. Either way the value is above about 0.45, so the heat-affected zone can harden and crack, and the steel needs preheating and low-hydrogen welding.

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