Heat Treatment of Steel

Heat Treatment of Steel

• Heat treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape.
• Steels are particularly suitable for heat treatment, since they respond well to heat treatment and the commercial use of steels exceeds that of any other material.

Reasons for Heat Treatment

• Softening: to reduce strength or hardness, remove residual stresses, improve toughness, restore ductility, refine grain size, or change the electromagnetic properties of the steel.
• Hardening: to increase the strength and wear properties.
• Material Modification: to modify properties of materials in addition to hardening and softening, such as stress relieving, cryogenic treatment, or spring aging.

Heat Treatment Processes

• Annealing: heating the material above the critical temperature, holding long enough for transformation to occur, and slow cooling.
• Normalizing: heating a steel above the critical temperature, holding for a period of time long enough for transformation to occur, and air cooling.
• Quenching: heating the material to a suitable temperature and then quenching in water or oil to harden to full hardness.
• Tempering: reheating the steel at a relatively low temperature to develop the required combination of hardness, strength, and toughness or to relieve the brittleness of fully hardened steels.

Critical Temperatures

• Decalescence point: the temperature at which steel continues to absorb heat without appreciably rising in temperature.
• Critical point: the temperature at which the steel actually increases in temperature, although its surroundings may be colder.

Hardening

• The use of this treatment will result in an improvement of the mechanical properties, as well as an increase in the level of hardness, producing a tougher, more durable item.
• Alloys may be air cooled, or cooled by quenching in oil, water, or another liquid, depending upon the amount of alloying elements in the material.

Austempering and Martempering

• Austempering: a technique used to form pure bainite, a transitional microstructure found between pearlite and martensite.
• Martempering: similar to austempering, but the goal is to create martensite rather than bainite.

Embrittlement

• Occurs during tempering when, through a specific temperature range, the steel experiences an increase in hardness and a reduction in ductility.
• Two types: tempered martensite embrittlement (TME) or one-step embrittlement, and temper embrittlement (TE) or two-step embrittlement.

Recrystallization

• A process by which deformed grains are replaced by a new set of defects-free grains that nucleate and grow until the original grains have been entirely consumed.
• Usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility.

Time-Temperature Transformation (TTT) and Continuous Cooling Transformation (CCT) Diagrams

• TTT diagrams: measure the rate of transformation at a constant temperature.
• CCT diagrams: measure the extent of transformation as a function of time for a continuously decreasing temperature.
• CCT diagrams are more practical than TTT diagrams as most of the processes employ continuous cooling rather than isothermal transformation.

Hardenability

• The ability of the material to be hardened by forming martensite.
• Measured by the Jominy end-quench test.
• Hardenability is then given as the dependence of hardness on distance from the quenched end.### Cooling and Thermal Gradients
• Fast cooling of a metal piece can cause warping and crack formation due to large thermal gradients.
• The shape and size of the piece, along with its heat capacity and heat conductivity, influence the cooling rate for different parts of the metal.

Heat Capacity and Heat Conductivity

• Heat capacity is the energy content of a heated mass that needs to be removed for cooling.
• Heat conductivity measures how fast energy is transported to the colder regions of the piece.

Precipitation Hardening

• Precipitation hardening, also known as age hardening, enhances hardening by forming extremely small precipitates that hinder dislocation motion.
• Precipitates form when the solubility limit is exceeded.

Steps to Achieve Precipitation Hardening

• Solution heat treatment: dissolve all solute atoms to form a single-phase solution.
• Rapid cooling across the solvus line to exceed the solubility limit, resulting in a supersaturated solid solution.
• Precipitation heat treatment: heat the supersaturated solution to an intermediate temperature to induce precipitation and keep it there for some time (aging).

Overaging

• If the precipitation hardening process is continued for a very long time, the hardness eventually decreases, a phenomenon known as overaging.