Engineering materials and heat treatment
Engineering materials and heat treatment tests knowledge of ferrous and non-ferrous metals, tool steels, the iron-carbon diagram and the purpose of annealing, normalising, hardening, tempering and surface treatment.
Core concepts
- Plain carbon steels are classified by carbon content: low-carbon is ductile and weldable; medium-carbon responds to hardening; high-carbon, about 0.6 to 1.0% C, suits cutting tools, springs and dies.
- Alloying elements such as Cr, Mo, V and W improve hardenability, wear resistance or hot hardness. Tool steels include water-hardening, oil-hardening, cold-work, hot-work and high-speed steels.
- Austenitising, quenching and tempering are a sequence. Quenching produces martensite, which is hard and brittle; tempering relieves stress and adjusts toughness.
- Annealing softens and relieves stress; normalising refines grain structure; case hardening gives a hard skin with a tough core.
- Hardness is usually checked by Brinell, Vickers or Rockwell methods; each has a different indenter and scale.
Worked example
A cold chisel is to be made from 0.8% carbon steel. Forge to shape, anneal or normalise for machining, then austenitise at about 780 to 800 degrees C, quench in water, and temper immediately at about 200 to 250 degrees C. The quench gives a hard cutting edge; the low-temperature temper reduces brittleness while keeping wear resistance. The cutting edge may be left harder and the struck end tempered more.
Traps
Do not temper before hardening; do not quench high-alloy tool steel in water unless the data says so; overheating causes coarse grain and cracking; and polishing away decarburised skin before hardness testing prevents a false low reading.