Chapter 7 · 2 hours
Material Properties
Practice questions
Practice questions and answers
2 exam-style questions on this chapter, written for this site from the official syllabus. We haven’t found past IOE papers for this subject yet; if you have some, share them in the community.
- Practice · 8 marks
Explain annealing, normalizing and tempering of steel. State the purpose, heating temperature and cooling method for each, and differentiate between annealing and normalizing.
Answer
Heat treatment is controlled heating and cooling of a metal in the solid state to change its properties such as hardness, strength, ductility and grain size.
Annealing
- Heat the steel to 30 to 50 °C above the upper critical temperature (A3 for hypoeutectoid; above A1 for hypereutectoid), soak, then cool very slowly in the furnace.
- Purpose: soften the steel, improve machinability and ductility, relieve internal stress, refine grain, and make it ready for further working.
- Structure: coarse pearlite (and ferrite), which is soft.
- Types: full annealing, process (sub-critical) annealing for cold-worked low carbon steel, spheroidising for high carbon steel, stress-relief annealing.
Normalizing
- Heat to 40 to 50 °C above the upper critical temperature, hold briefly, then cool in still air.
- Purpose: refine grain, remove stress left by forging, rolling or casting, give uniform structure and better strength than annealed steel.
- Structure: fine pearlite.
Tempering
- Done after hardening (quenching), which makes steel hard but brittle with residual stress. The steel is reheated below A1 (150 to 650 °C), held and cooled in air.
- Purpose: reduce brittleness and internal stress, improve toughness, with some loss of hardness.
- Higher temper temperature gives lower hardness and greater toughness (150 to 250 °C for cutting tools, 350 to 450 °C for springs, 500 to 650 °C for shafts and gears). Colour changes (straw to blue) indicate temperature on a polished surface.
Annealing versus normalizing
| Annealing | Normalizing |
|---|---|
| Cooling slowly in the furnace | Cooling in still air |
| Softest condition | Harder and stronger |
| Coarse pearlite | Fine pearlite, small grains |
| Better machinability and ductility | Better strength; moderate machinability |
| Longer, more costly cycle | Shorter, cheaper |
- Practice · 3+3 marks
List the properties required of a cutting tool material. Compare high speed steel, cemented carbide and ceramic tools, and state the carbon content and uses of low, medium and high carbon steels.
Answer
Required properties of a cutting tool material
- Hot hardness: retain hardness at the high temperature generated at the tool tip.
- High wear resistance and hardness (much harder than the work).
- Toughness to withstand shock and interrupted cuts.
- Low coefficient of friction with the chip and good thermal conductivity.
- Reasonable cost and ease of grinding.
Comparison
| Property | High speed steel | Cemented carbide | Ceramic (Al2O3) |
|---|---|---|---|
| Composition | 18% W, 4% Cr, 1% V (18-4-1), or Mo type | Tungsten carbide grains with cobalt binder | Aluminium oxide, sintered |
| Hot hardness limit | About 600 °C | About 900 °C | Over 1200 °C |
| Cutting speed | 20 to 40 m/min | 100 to 200 m/min | 300 to 600 m/min |
| Toughness | High | Medium | Low (brittle) |
| Form | Solid tools, ground | Brazed tips or inserts | Inserts |
| Uses | Drills, taps, form tools, milling cutters | Turning steel, cast iron, mass production | High speed finishing of cast iron and hard steel |
Carbon steels
| Type | Carbon % | Properties and uses |
|---|---|---|
| Low (mild) | 0.05 to 0.25 | Soft, ductile, weldable; sheets, wires, nails, bolts, structural sections |
| Medium | 0.25 to 0.60 | Strong, can be heat treated; shafts, axles, gears, connecting rods |
| High | 0.60 to 1.5 | Hard, wear resistant, less ductile; files, chisels, springs, hand tools, dies |
Hot rolled steel is rolled above recrystallisation temperature (scaly surface, loose tolerance); cold rolled steel is finished at room temperature (smooth, accurate, stronger).
Written from the official syllabus. Questions and answers are written for this site; check them against your class notes.
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