Types of steels and cast iron

Alloy steels contain elements such as Cr, Ni, Mo, V, Mn, Si added to plain carbon steel to improve strength, hardenability, toughness, corrosion or wear resistance.

(a) HSLA steels

• Composition: low carbon (0.05-0.25% C), up to 2% Mn, small amounts (below 0.1%) of Nb, V, Ti, plus Cu, Cr, Ni.
• Strengthening: fine grain size, precipitation of carbonitrides, and solid-solution strengthening.
• Properties: yield strength 275-550 MPa (higher than mild steel), good weldability and toughness, better atmospheric corrosion resistance (weathering steels).
• Uses: bridges, pressure vessels, pipelines, vehicle frames, and building structures, where lighter sections reduce weight.

(b) Stainless steels

Contain at least 10.5% Cr; a thin, self-healing film of Cr$_2$O$_3$ protects against corrosion.

Other types: duplex and precipitation-hardening stainless steels.

(c) Tool steels

• Used to make cutting, forming and shearing tools; contain 0.6-1.5% C with W, Mo, Cr, V, Co that form hard carbides.
• Properties: high hardness, wear resistance, hot hardness (red hardness), toughness.
• Types: high-speed steel (18-4-1: 18% W, 4% Cr, 1% V), cold-work (die) steels, hot-work steels, shock-resisting steels.
• Uses: drills, taps, lathe cutting tools, punches, and dies.

Cast iron is an iron-carbon alloy with 2.14-6.67% C (usually 2.5-4%) and 1-3% Si. Carbon may be present as free graphite or as cementite, depending on composition and cooling rate.

Production notes

• Grey iron: slow cooling with high C and Si, which promote graphite.
• White iron: rapid cooling (chilling) and low Si keep carbon as cementite.
• Malleable iron: white iron annealed at about 900 °C for 2 days, which decomposes cementite into temper carbon.
• Ductile iron: Mg (or Ce) added to the melt before casting turns graphite flakes into spheres, which removes the sharp stress-raising edges.

Machine-tool beds

Grey cast iron absorbs vibration (graphite flakes damp vibrations), is cheap to cast in complex shapes, machines easily, wears well on slide ways and has high compressive strength.

Weldability of steels

Weldability is the ease with which a steel can be welded to give a sound joint of adequate strength and toughness. Poor weldability arises from the hard, brittle martensite that forms in the heat-affected zone (HAZ) on rapid cooling.

Factors affecting weldability:

• Carbon content: below 0.25% C is readily weldable; above 0.45% needs preheating and post-weld heat treatment.
• Alloying elements: Mn, Cr, Mo, Ni raise hardenability, increasing the risk of cracking. This is expressed by the carbon equivalent:

$$CE = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15}$$

Steels with CE below about 0.4 are generally weldable without preheat.

• Thickness and cooling rate: thick sections cool faster.
• Hydrogen, sulphur and phosphorus: promote cracking and hot shortness.
• Remedies: preheating, low-hydrogen electrodes, controlled heat input, post-weld heat treatment.

Embrittlement

• Temper embrittlement: alloy steels (containing Ni, Cr, Mn) held or slowly cooled between about 375 and 575 °C lose toughness, because impurities (P, Sb, Sn, As) segregate to prior-austenite grain boundaries, causing intergranular fracture. Remedies: add Mo (about 0.5%), keep impurities low, quench rapidly through the range.
• Hydrogen embrittlement: atomic hydrogen absorbed during pickling, plating or welding diffuses into the steel and reduces ductility and causes delayed cracking at stress well below yield. High-strength steels are most susceptible. Remedy: baking (about 200 °C), low-hydrogen processes.