Heat treatment of plain carbon steel

Heat treatment

Material is heat treated to

• relieve stresses

• increase softness, ductility, and toughness

• produce a specific microstructure

Types of annealing (depends on composition)

• Process annealing

• Stress relief annealing

• Full anneal

• Spheroidising anneal

• Process annealing

• Process annealing

  • Used for steels below 0.4% to negate affect of CW, for low C steels.

  • Used during extensive CW to allow continuation of CW without breaking. Recovery and recrystallisation are allowed to occur.

  • Temp. 630 to 700

  • Component is allowed to cool in furnace

  • a fine grain structure is desired

Stress relief annealing

A steel heated or cooled quickly or unevenly can form residual stresses ex: welding, machining, quench hardening. The strength of the workpiece is reduced and distortion can occur.

Internal stresses may be removed by stress relief annealing heat treatment. Part is heated between 550 and 650 degrees. Relatively low such that affects from CW or other heat treatments are not affected. Slowly cooled ex: in switched off furnace or air cooled.

Full annealing

Solidification of steels occurs at temps. much higher than heat treatment process temps. Production of large components ex: casting or forging can result in very large austenite grain sizes. On air cooling, ferrite forms along the austenite grain boundaries and also within the grains to produce a mesh like structure (Windmanstatten). The latter impacts negatively the toughness of steel.

Steel is heated to above A3 (hypoeutectoid) or A1 (hypereutectoid) and allowed to fully form austenite/austenite-cementite grain structure.

Material transformed into the equilibrium microstructure as it is allowed to cool very slow (furnace cooling for alloy steel). Microstructure is coarse pearlite (in addition to any proeutectoid phase). Microstructure with uniform small grains result. Material will get increased ductility, relieved internal stresses, lower hardness and refined grain size and suitable for undergoing other heat treatment processes.

Spheroidising (soft) anneal

Most effective for steels above 0.4%C (significant amount of pearlite). Steel is heated, soaked and cooled slowly to produce spheroidal pearlite/ globular forms of carbides. Generally heat below lower critical temps. but may also involve cycling above and below lower critical temps (+-50) or above lower critical temps. (steel alloys) followed by slow cooling. Reduced hardness; maximises ductility and machinability. For low C %(less than 0.4%)this is not needed as the steels are already quite soft.

This improves the machinability and formability of the steel ex: sheets. Ductility of these steels is higher as dislocation movement is less hindered by spheroidal cementite compared to lamellar pearlite. Spheriodite is easier to machine via continuous cutting operations.

Normalising

Steels which have been plastically deformed consist of grains of pearlite (and a proeutectoid phase) which are irregular in shape and relatively large but vary substantially in size.

During normalising, the steel is heated to above A3 or Acm and allowed to turn into fine austenite. Steel is then cooled freely in air (or a protective gas). Cooling in air, faster.

For hypoeutectoid steels, the fine grains of austenite will then transform into fine ferrite and pearlite on cooling, producing a uniform and desirable distribution of fine grained structure which is tougher than coarse grained.

For hypereutectoid steels, the material is heated above the Acm line to transform to austenite and then cooled to transform into fine pearlite and cementite contributing to a higher strength. Sometimes used to stress relief between rough and fine finishing of large castings and forgings, provides dimensional stability.