Heat treatment of a metal or alloy is a technological procedure, including controlled heating and cooling operations, conducted for the purpose of changing the alloy micro-structure and resulting in achieving required properties.

Normalising

  • For this process, the metal is placed in the furnace and heated to just above its ‘Upper Critical Temperature’.
  • When the new grain structure is formed it is then removed from the furnace and allowed to cool in air as it cools new grains will be formed.
  • These grains, although similar to the original ones, will in fact be smaller and more evenly spaced.
  • Normalising is used to relieve stresses and to restore the grain structure to normal.

This is particularly useful after heavy machining where grains may have become stressed or after the prolonged heating of a forging process has allowed the grains to grow large.

Quenching

  • It is a heat treatment when metal at a high temperature is rapidly cooled by immersion in water or oil.
  • Quenching makes steel harder and more brittle, with small grains structure.

Annealing (Softening)

  • Annealing is a heat treatment procedure involving heating the alloy andholding it at a certain temperature (annealing temperature), followed bycontrolled cooling.
  • Annealing results in relief of internal stresses, softening, chemical homogenising and transformation of the grain structure into more stable state.
  • The annealing process is carried out in the same way as normalising, except that the component is cooled very slowly. This is usually done by leaving the component to cool down in the furnace for up to 48 hours.
  • Annealing leaves the metal in its softest possible state and is usually carried out to increase ductility prior to cold working or machining.
  • Annealing is carried out in different stages which are classified as:
    • Stress relief (recovery
      • A relatively low temperature process of reducing internal mechanical stresses, caused by cold-work, casting or welding.
      • During this process atoms move to more stable positions in the crystal lattice. Vacancies and interstitial defects are eliminated and some dislocations are annihilated.
      • Recovery heat treatment is used mainly for preventing stress-corrosion cracking and decreasing distortions, caused by internal stresses.
    • Recrystallization 
      • It can be easily said to be alteration of the grain structure of the metal.
      • If the alloy reaches a particular temperature (recrystallization or annealing temperature) new grains start to grow from the nuclei formed in the cold worked metal. The new grains absorb imperfections and distortions caused by cold deformation. The grains are equi-axed and independent to the old grain structure.
      • As a result of recrystallization mechanical properties (strength, ductility) of the alloy return to the pre-cold-work level.
      • The annealing temperature and new grains size are dependent on the degree of cold-work which has been conducted. The more is the cold-work degree, the lower is the annealing temperature and the fine recrystallisation grain structure.
      • Low degrees of cold-work (less than 5%) may cause formation of large grains. Usually the annealing temperature of metals is between one-third to one-half of the freezing point measured in Kelvin (absolute) temperature scale.
    • Grain growth (over-annealing, secondary recrystallization)
      • Growth of the new grains at the expense of their neighbours, occurring at temperature, above the recrystallization temperature.

This process results in coarsening grain structure and is undesirable.

04-Heat-treatment (1)

Hardening

  • Hardening also requires the steel to be heated to its upper critical temperature (plus 50°C) and then quenched.
  • The quenching is to hold the grains in their solid solution state calledAustenite; cooling at such a rate (called the critical cooling rate) is to prevent the grains forming into ferrite and pearlite.
  • Hardening is a process of increasing the metal hardness, strength,toughness, fatigue resistance.
  • The rate of cooling affects the hardness of the metal, in that the faster the cooling rate, the greater the hardness.
  • The cooling liquid can therefore be selected to suit the hardness required. If a steel is quenched too rapidly it may crack, this is especially true with thin walled components.
    • Strain hardening (work hardening)
      • Strengthening by coldwork (cold plastic deformation),
      • It causes increase of concentration of dislocations, which mutually entangle one another, making further dislocation motion difficult and therefore resisting the deformation or increasing the metal strength.
    • Grain size strengthening (hardening)
      • Strengthening by grain refining.
      • Grain boundaries serve as barriers to dislocations, raising the stress required to cause plastic deformation.
    • Solid solution hardening
      • Strengthening by dissolving an alloying element.
      • Atoms of solute element distort the crystal lattice, resisting the dislocations motion. Interstitial elements are more effective in solid solution hardening, than substitution elements.
    • Dispersion strengthening 
      • Strengthening by addition of second phase into metal matrix.
      • The second phase boundaries resist the dislocations motions, increasing the material strength.
      • The strengthening effect may be significant if fine hard particles are added to a soft ductile matrix (composite materials).
    • Hardening as a result of Spinodal decomposition
      • Spinodal structure is characterized by strains on the coherent boundaries between the spinodal phases causing hardening of the alloy.
    • Precipitation hardening (age hardening)
      • Strengthening by precipitation of fine particles of a second phase from a supersaturated solid solution.
      • The second phase boundaries resist the dislocations motions, increasing the material strength. The age hardening mechanism in Al-Cu alloys may be illustrated by the phase diagram of Al-Cu system.
      • When an alloy Al-3%Cu is heated up to the temperature TM, all CuAl2 particles are dissolved and the alloy exists in form of single phase solid solution (α-phase). This operation is called solution treatment.
      • Slow cooling of the alloy will cause formation of relatively coarse particles of CuAl2 intermetallic phase, starting from the temperature TN.
      • However if the the cooling rate is high (quenching), solid solution will retain even at room temperature TF. Solid solution in this non-equilibrium state is called supersaturated solid.
      • Obtaining of supersaturated solid solution is possible when cooling is considerably faster, than diffusion processes. As the diffusion coefficient is strongly dependent on the temperature, the precipitation of CuAl2from supersaturated solution is much faster at elevated temperatures (lower than TN).This process is called artificial aging. It takes usually a time from several hours to one day. When the aging is conducted at the room temperature, it is called natural aging.

04-Heat-treatment (2)

Tempering

  • As there are very few applications for very hard and brittle steel, the hardness and brittleness needs to be reduced. The process for reducing hardness and brittleness is called tempering.
  • Tempering consists of reheating the previously hardened steel.
  • During this heating, small flakes of carbon begin to appear in the needle like structure. (See below) This has the effect of reducing the hardness and brittleness.
  • The temperature to which the steel is reheated depends on the hardness required by the application of the component. The higher the tempering temperature, the less hard will be the resulting steel.

04-Heat-treatment (3)

If the steel is polished before tempering, the range of oxide colours that the steel goes through during heating can be used as a guide to its temperature.

Stress Relieving

  • When a metal is heated, expansion oc­curs which is more or less proportional to the temperature rise. Upon cooling a metal, the reverse reaction takes place. That is, a contraction is observed.
  • When a steel bar or plate is heated at one point more than at another, as in welding or during forging, internal stresses are set up.
  • During heating, expansion of the heated area cannot take place unhindered, and it tends to deform. On cooling, contraction is prevented from taking place by the unyield­ing cold metal surrounding the heated area.
  • The forces attempting to contract the metal are not relieved, and when the metal is cold again, the forces remain as internal stresses. Stresses also result from volume changes which accompany metal transformations and precipitation.
  • The term stress has wide usage in the metallurgical field. It is defıned simply as bad or force divided by the cross-sectional area of the part to which the bad or force is applied.
  • Internal, or residual stresses, are bad because they may cause warping of steel parts when they are machined.

To relieve these stresses, steel is heated to around 1100 0F (595 0C) assuring that the entire part is heated uniformly, then cooled slowly back to room temperature. This procedure is called stress relief annealing, or merely stress relieving

Allotropic Forms of Steel

  • The temperature 723oC is known as Curie temperature, below it steel shows magnetic properties and above it steel becomes non-magnetic.
  • In the diagram, the carbon percentage is plotted on X-axis and temperature plotted on Y-axis.
  • The melting point of iron is about 1539oC. The melting temperature of iron varies with increasing carbon percentage.
  • The iron has carbon upto 2% known as steel. The cast-iron is a form of having 2 to 4.5% carbon. The iron has carbon upto 6.67% known as pig iron.

04-Heat-treatment (4)

Eutectcid Point

Eutectoid reaction in this diagram occurs when temperature 1oC and carbon is 0.83%. At this point a solid form iron (γ) changes into two solid forms α–iron and cementite (Fe3C).

04-Heat-treatment (5)

  • A eutectoid mixture of ferrite (α–iron) and cementite is known as pearlite. This is a microconstituent.
  • The fraction of ferrite in eutectoid steel is 88%.

Eutectic Point

  • In this diagram, the location of this point is at 1175oC temperature and 4.3% at the carbon. At this point eutectic reaction occurs.
  • In this reaction, a liquid phase changes two solid phases γ-iron (austenite) and cementite (FeC).
  • Peritectic reaction occurs at 1495oC and at this point carbon composition is 0.18%. This temperature is known as peritectic temperature.

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