1.  Carbon Steel

 Carbon steel is iron + carbon (no major alloying elements). It is classified based on carbon content.

1.1  Low-Carbon Steel (Mild Steel)

Carbon content: 0.05–0.25% C

Examples: ASTM A36, SAE 1020, 1018

Mechanical Properties 

·         Tensile strength: 370–550 MPa

·         Yield strength: 200–350 MPa

·         Hardness: 120–180 HB

·         Excellent ductility and toughness

·         Very good weldability and machinability

Chemical Properties 

·         C: 0.05–0.25%

·         Mn: 0.4–1.0%

·         Si: 0.05–0.3%

·         Very low alloy content

·         Corrosion resistance: low (requires coating/painting)

1.2  Medium-Carbon Steel  

Carbon content: 0.25–0.60% C

Examples: 1045, 1050

Mechanical Properties

·         Tensile strength: 600–800 MPa

·         Yield strength: 400–600 MPa

·         Hardness: 170–250 HB

·         Good strength but reduced ductility

Chemical Properties

·         C: 0.25–0.60%

·         Mn: 0.6–1.5%

·         Suitable for heat treatment (quenching & tempering)

1.3  High-Carbon Steel Carbon content: 0.60–1.25% C

Examples: 1095, tool blanks

 

Mechanical Properties

·         Tensile strength: 800–1200 MPa (after heat treatment)

·         Hardness up to 65 HRC (after quenching)

·         Very high wear resistance

·         Low ductility and toughness

Chemical Properties

·         C: 0.6–1.25%

·         Mn: 0.3–0.9%

·         High hardenability

·         Used for springs, blades, high-strength wires

 

2.  Alloy Steel

Steels with intentional alloying elements: Cr, Ni, Mo, V, W, B, Mn.

2.1  Low Alloy Steel (2–5% alloying elements)

Examples: 4140, 4340, 40Cr, 35CrMo Mechanical Properties

·         Tensile strength: 700–1100 MPa

·         Yield strength: 500–900 MPa

·         Excellent fatigue strength

·         High impact toughness

·         Good hardenability

Chemical Properties

·         Cr (0.5–1.5%) → hardenability & strength

·         Mo (0.1–0.3%) → creep resistance

·         Ni (1–3%) → toughness

·         Mn (0.6–1.2%)

2.2  High Alloy Steel (>5% alloy elements)

Typically stainless steels, tool steels, heat-resistant steels.

3.  Stainless Steel

 Contains ≥10.5% chromium, forming a protective Cr₂O₃ layer.

3.1  Austenitic Stainless Steel (Non-magnetic)

Examples: 304, 316, 310

Composition: Cr 18–20%, Ni 8–12%

Mechanical Properties

·         Tensile strength: 500–750 MPa

·         Yield strength: 200–300 MPa

·         Very high ductility

·         Excellent corrosion resistance

Chemical Properties

·         304: Cr 18–20%, Ni 8–10.5%, C ≤ 0.08%

·         316: Cr 16–18%, Ni 10–14%, Mo 2–3% (improves marine resistance)

3.2  Ferritic Stainless Steel (Magnetic)

Examples: 430, 446

Composition: Cr 12–18%, Low C

Mechanical Properties

·         Tensile strength: 400–600 MPa

·         Lower ductility than austenitic grades

·         Good formability

Chemical Properties

·         Cr: 12–18%

·         C: ≤ 0.12%

·         Good oxidation resistance but poor welding at high thickness

3.3  Martensitic Stainless Steel

Examples: 410, 420, 440A/B/C Hardenable by heat treatment.

Mechanical Propertie

·         Tensile strength: 700–1700 MPa

·         High hardness up to 58–65 HRC

·         Good wear resistance

Chemical Properties

·         Cr: 11–17%

·         C: up to 1.2%

Used for: knives, turbines, tools.

 

4.  Tool Steels

 High hardness, wear resistance, and heat resistance.

Types of Tool Steels

 

Type	Example	Main Alloy
Water-hardening (W series)	W1, W2	High C
Cold-work (O, A, D series)	D2	High Cr
Hot-work (H series)	H13	Cr-Mo
High-speed steel (HSS)	M2, T1	W, Mo, V

Mechanical Properties

·         Hardness: 58–70 HRC

·         Very high wear resistance

·         High hot hardness (for HSS)

Chemical Properties

Typical elements:

·         C: 0.7–1.5%

·         Cr: 4–12%

·         V: 1–4%

·         Mo/W: 5–20%

 

5.  Cast Steel

 Similar to cast iron but with lower carbon (0.2–0.6%).

Mechanical Properties

·         Tensile strength: 400–800 MPa

·         High toughness compared to cast iron

·         Good fatigue resistance

Chemical Properties

·         C: 0.2–0.6%

·         Si: 0.2–0.8%

·         Mn: 0.3–1.0%

·         May include Ni, Cr for special grades

6.  Structural Steels

Used in buildings, bridges, cranes.

Examples: S235, S275, S355 (European), ASTM A992 (USA)

Mechanical Properties

·         Yield strength:

o S235: 235 MPa

o S355: 355 MPa

·         Tensile strength: 350–550 MPa

·         Good weldability and ductility

Chemical Properties

·         Low C: 0.1–0.2%

·         Mn: 0.5–1.5%

·         Small Si, P, S

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