HEAT TREATMENT

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Heat Treatments

Heat treatments are used to change the physical & mechanical properties of the metal without changing its shape. They are important processes in metal manufacturing which increase required characteristic of the metal, while allowing for further processing to take place.

Various heat treatment processes involve carefully controlled heating & cooling of the metal. Steel, for example, is commonly heat treated for use in a variety of the commercial applications.

Common objectives of the heat treatment are to:

  • Increase strength
  • Increase hardness
  • Improve toughness
  • Improve machining
  • Improve formability
  • Increase ductility
  • Improve elasticity

Why do you need to Heat treat the material?

While fabricating the materials by rolling, extrusion or by forming there are internal stresses developed in the parts. So the final stage of any fabrication process is heat treatment .

Heat treatment process is used to:

  • Refine grain structure
  • Relieve of internal stresses
  • Hardness &  toughness

Various heat treatment process are…

  • Annealing
  • Normalizing
  • Tempering
  • Hardening      
  • (a) Quenching
  • (b) Carburising
  • (c) Nitriding
  • (d) Induction hardening
  • (e) Flame hardening

Annealing

The cooling stage has different effects based on the metal & process. When steel is cooled quickly it hardens, whereas the rapid cooling stage of the solution annealing will soften aluminum.

While there are many types of the heat treatment, two important types are annealing & tempering.

The cooling stage has different effects based on the metal & process. When steel is cooled quickly it hardens, whereas the rapid cooling stage of the solution annealing will soften aluminum.

While there are many types of the heat treatment, two important types are annealing & tempering.

Annealing involves heating steel to a specified temperature & then cooling at a very slow & controlled rate.

Annealing is commonly used to:

  • Soften a metal for cold working
  • Improve machinability
  • Enhance electrical conductivity

Annealing also restores ductility. During the cold working, the metal can become hardened to the extent that any more force will result in cracking.

Annealing is used for the steel, but, other metals including copper, aluminum & brass can be subject to the process called solution annealed.

Large ovens are used for the annealing steel process. The inside of the oven must be large enough to permit air to circulate around the metal. For large pieces, gas fired conveyor furnaces are used while car-bottom furnaces are more practical for the smaller pieces of the metal.

During the annealing process, the metal is heated to a fixed temperature where recrystallization can occur. At this stage, any defects caused by deformation of the metal are rectified. The metal is held at that temperature for a fixed period, then cooled down to the room temperature.

The cooling process should be done very slowly to produce a refined microstructure, thus maximizing softness. This is often done by immersing the hot steel in the sand, ashes or other substances with low heat conductivity, or by switching off the oven & allowing the steel to cool with the furnace.

Heat the part above Re-crystalization temp of the material and cool it down in the furnace very slowly.

  • Austenite to pearlite

There are three stages of annealing

  • Recovery
  • Re-crystalization
  • Grain growth

Can be done on steel, copper, aluminium and brass

Performed to have: softness, ductility, machinability and good electric conductivity.

Normalizing

Normalizing is an annealing process for steel where it is heated 150-200°F higher than in annealing & held at the critical temperature long enough for the transformation to happen. Steel treated in this way must be air cooled. The heat treating in the normalization causes smaller austenitic grains, while air cooling produces more refined ferritic grains. This process improves machinability, ductility, & strength of the steel. Standardization is also useful to remove columnar grains & dendritic segregation that can occur during the casting of a part.

Ferrous metals are normalized to relieve the internal stresses caused by machining, forging, or welding. Normalized steel are harder and stronger than annealed steels.
• Steel is considerably tougher in the normalized condition than in any other condition. Parts which will be subjected to impact and parts that require maximum toughness and resistance to external stresses are usually normalized.
• Normalizing prior to hardening is beneficial in obtaining the desired hardness, provided the hardening operation is performed correctly.
• Low carbon steels do not normally require normalizing, but no harmful effects result if these steels are normalized.
• Normalizing is achieved by heating the metal to a set temperature (higher than either the hardening or annealing temperatures), soaking the metal till it is uniformly heated, and cooling it in still air.

Tempering

Tempering is a method of the heat treating used to increase the resilience of the iron-based alloys like steel. Iron-based metals are very hard, but they are often too brittle to be useful for most purposes. Tempering can be used to change the hardness, ductility, & strength of the metal, which normally makes it easier to machine. The metal will be heated to a temperature below the critical point as lower temperatures decreases brittleness while maintaining hardness.  For increased plasticity with less hardness & strength, higher temperatures are needed.

The Tempering Process

Tempering is done by elevating the steel to a set point below its lower critical temperature, generally following a hardening operation. Once this temperature is reached, it is held there for a some amount of the time. The exact temperature & time depend on several factors such as the type of steel & desired mechanical properties.

To get the steel to its critical temperature, some type of heating device must be used. Common devices include gas furnaces, electrical resistance furnaces, or the induction furnaces. Often, this heating is done in a vacuum or with an inert gas to protect the steel from the oxidation. Once the furnace achieves the desired temperature, a dwell time occurs. Following the dwell time, the furnace is shut off and the steel is allowed to cool at predetermined rate.

• Steel is usually harder and too brittle for practical use after being hardened.
• Severe internal stresses are set up during the fast cooling of the metal.
• Steel is tempered after being hardened to relieve the internal stresses and decrease its brittleness.
• Tempering consists of heating the metal to a specified temperature and then leaving the metal to cool. The rate of cooling generally has no effect on the metal structure during tempering. Therefore, the metal is generally permitted to cool in still air.
• Temperatures used for tempering are generally much lower than the hardening temperatures.
• At higher tempering temperature the metal becomes softer. High-speed steel is a metal that becomes harder instead of softer after it is tempered

Why Is Steel Tempered?

Tempering steel after a hardening process allows for a middle ground of hardness and strength. This is achieved by allowing the carbon diffusion to occur within a steel microstructure. When steel is hardened, it can become excessively brittle and hard. However, when not hardened, the steel may not have the strength or abrasion resistance needed for its intended application. Tempering also improves the machinability and formability of a hardened steel, and can reduce the risk of the steel cracking or failing due to internal stresses.

When Is Tempering Used?

  • Tempering is most commonly used following a quenching operation. Heating a carbon steel  and rapidly quenching it can leave it too hard and brittle. Tempering it can restore some of its ductility.
  • Tempering can reduce the hardness and relieve the stress of a welded component. Welds can create a localized zone that has been hardened due to the heat of the welding process. This can leave undesirable mechanical properties and residual stress that can promote hydrogen cracking. Tempering helps prevent this.
  • Work hardened materials often require tempering. Materials can become work hardened through processes such as punching, bending, forming, drilling, or rolling. Work hardened materials have a high amount of residual stresses that can be alleviated through a tempering process.

What’s the difference between annealing and tempering?

The difference between annealing & tempering comes down to how it is treated. Annealing involves heating steel to a specified temperature & then cooling at a very slow & controlled rate, whereas tempering involves heating the metal to a precise temperature below the critical point, & is often done in air, vacuum or in the inert atmospheres.

Hardening

In heat treating to harden the metal, the metal is heated to a temperature where the elements in the metal become a solution. Before doing this, defects in the crystal lattice structure of the metal are the primary source of plasticity. Heat treating addresses those deficiencies by bringing the metal into a reliable solution with fine particles to strengthen the metal. Once the metal is heated to the right temperature to produce a solid solution, it is quickly quenched to trap the particles in the solution.
In precipitation hardening, impurity particles are added to metal alloy to increase strength further.

Quenching: Hardening involves heating of steel, keeping it at an appropriate temperature until all pearlite is transformed into austenite, and then quenching it rapidly in water or oil. The temperature at which austentizing rapidly takes place depends upon the carbon content in the steel used. The heating time should be increased ensuring that the core will also be fully transformed into austenite. The micro structure of a hardened steel part is ferrite, martensite, or cementite

Martensite: it is the hardest known form of iron or steel

Cooling from austenite we get pearlite, binite and martensite.

Carburizing: Adding carbon to the surface of iron based alloys by heating the metal below its melting point (normally 1,560 and 1,740°F) and putting it in contact with carbon rich solids, liquids, or gasses. This increases the metal’s strength and makes the surface hard and more abrasion resistant, but also decreases it toughness. Carburizing is generally done on finished parts.

Pack carburizing: solids containing (50% charcoal, 20% bi carbonate,  5% calcium carbonate and 5-12% sodium bi carbonate)

450-500 degree C for 6-8 hrs.

Liquid carburizing: dipping in sodium cyanide for 930-950°C for 3-4 hrs

Gas carburizing: passed gases like CH4, propane, Butane for 930-950°C for 3-4 hrs.

Nitriding: Gas nitriding is a surface hardening process, where nitrogen is added to the surface of steel parts using separate ammonia as the source.

• Gas nitriding forms a very hard case in a component at relatively low temperature, without the need for quenching.
• Nitriding is done at temperatures below the transformation temperature of alloy steels, with proper manufacturing techniques, there is little or no distortion as a result of the process.
• Parts to be nitrided are heat treated to the proper strength level, and finally machined. The parts are then exposed to active nitrogen at a carefully controlled temperature, typically in the range of 925°F to 985°F.
• This temperature is usually below the final tempering temperature of the steel so that nitriding does not affect the base metal mechanical properties. As a result, a very high strength product with extremely good wear resistance can be produced, with slightly or no dimensional change.

Flame hardening:

FLAME HARDENING is a heat treatment process in which a thin surface shell of a steel part is heated rapidly to a temperature above the critical point of the steel. After the grain structure of the shell has become austenitic (austenitized), the part is quickly quenched, transforming the austenite to martensite while leaving the core of the part in its original state

Flame hardening employs direct impingement of a high-temperature flame or high-velocity combustion product gases.

Depths of hardening from about 0.8 to 6.4 mm

Induction hardening:

Induction hardening is a heat treating process used to increase the hardness or wear resistance of a material. Induction hardening heats a material using a form of induction heating to a set temperature. Once the temperature is reached, it is rapidly cooled by a quenching media. This rapid cooling forms a material micro structure that is hard and strong.

  • Induction hardening has several benefits over other hardening methods.
  •  It is easier to control because it is an electrical process rather than a combustion process.
  •  It begins heating the surface of the material, rather than the core. This makes induction hardening an excellent option for case hardening. Since precise controls can be used in induction hardening, a uniform case hardened surface can be created. The depth of the hardened material can also be readily controlled.