An Introduction to Heat Treatment Processing Types

An Introduction to Heat Treatment  Processing Types

Introduction
Engineering components often pass through a number of forming and finishing processes during the production cycle. These may be sub-divided into primary and secondary processes. The primary
processes include moulding, casting, forging, drawing, rolling and extrusion. They are performed on the raw material to give it form and shape. Sometimes, as is the case with polymers and die-cast
metals, the finished product can be made in one primary forming process. More often, a component requires secondary forming and finishing by machining, heat treatment and surface protection.
The properties of a material affect the choice of production process and conversely, a production process may affect the properties of a material. In the case of metals, it is the primary forming
processes and subsequent heat treatment which have the greatest effect on the final properties of a component

Types of Heat Treatment 

1.Annealing 

2. Normalizing 

3. Quenching or Hardening
4. Surface Hardening 

5. Tempering 
6. Ausforming

7. Case Hardening

1- Annealing

The purpose of annealing is to restore the ductility and malleability of work hardened material after cold working. The material is heated in a furnace to what is known as the recrystallisation

temperature. At this temperature, new crystals or grains start to form and grow in the regions where the old grains are most distorted. The process continues until the new grains have completely

replaced the old deformed structure. When recrystallisation is complete the material is cooled. If the annealing process is carried on for too long a time, the new grains will grow by feeding off each other to give a very course grain structure. This can make the material too soft and weak (Figure 1).

FIG 1

Carbon steels, copper brass and aluminium can all have their grain structure reformed by annealing. Figure1 shows the range of temperatures at which annealing is carried out for plain carbon steels. After annealing for a sufficient length of time, steel components are cooled slowly in the dying furnace. Pure aluminium is annealed between 500 ⁰C and 550 ⁰C, cold worked brass between

600 ⁰C and 650 ⁰C and copper between 650 ⁰C and 750 ⁰C. Unlike steels, these materials can be quench-cooled after annealing.
The most favorable microstructure for machinability in the low- or medium-carbon steels is coarse
pearlite. The customary heat treatment to develop this microstructure is a full annealing, illustrated
in Fig. 2

FIG 2

.

Anealing is reheating steel followed by slow cooling.   It is completed
a) to remove internal stress or to soften or
b) to refine the crystalline structure (This involves heating to above the upper critical temperature ).
The steel is heated about 25^oC above the upper critical temperature, held for a set time and then cooled slowly in the furnace.   This process is used to remove internal stresses built up as a result of cold working and fabrication processes.   Following annealing the dislocations are rearranged in to a lower energy configuration, new strain free grains are formed and grain growth is encouraged.
Isothermal Annealing
Annealing to coarse pearlite can be carried out isothermally by cooling to the proper temperature for
transformation to coarse pearlite and holding until transformation is complete. This method, called
isothermal annealing, is illustrated in Fig. 3. It may save considerable time over the full-annealing

FIG 3

process described previously, since neither the time from the austenitizing temperature to the transformation temperature, nor from the transformation temperature to room temperature, is critical; these may be shortened as desired. If extreme softness of the coarsest pearlite is not necessary, the transformation may be carried out at the nose of the IT curve, where the transformation is completed
rapidly and the operation further expedited: the pearlite in this case is much finer and harder.
Isothermal annealing can be conveniently adapted to continuous annealing, usually in specially designed furnaces, when it is commonly referred to as cycle annealing.

Spheroidization Annealing
Coarse pearlite microstructures are too hard for optimum machinability in the higher carbon steels.
Such steels are customarily annealed to develop spheroidized microstructures by tempering the asrolled, slowly cooled, or normalized materials just below the lower critical temperature range. Such
an operation is known as subcritical annealing. Full Spheroidization may require long holding times
at the subcritical temperature and the method may be slow, but it is simple and may be more convenient than annealing above the critical temperature.

The annealing procedures described above to produce pearlite can, with some modifications, give
spheroidized microstructures. If free carbide remains after austenitizing, transformation in the temperature range where coarse pearlite ordinarily would form proceeds to spheroidized rather than
pearlite microstructures. Thus, heat treatment to form spheroidized microstructures can be carried out
like heat treatment for pearlite, except for the lower austenitizing temperatures. Spheroidization annealing may thus involve a slow cooling similar to the full-annealing treatment used for pearlite, or
it may be a treatment similar to isothermal annealing. An austenitizing temperature not more than
55 oC above the lower critical temperature is customarily used for this supercritical annealing.

FIG 4

FIG 5

Process Annealing
Process annealing is the term used for subciritical annealing of cold-worked materials. It customarily
involves heating at a temperature high enough to cause recrystallization of the cold-worked material
and to soften the steel. The most important example of process annealing is the box annealing of
cold-rolled low-carbon sheet steel. The sheets are enclosed in a large box that can be sealed to permit
the use of a controlled atmosphere to prevent oxidation. Annealing is usually carried out between
590 and 700 C. The operation usually takes ～24 hr, after which the charge is cooled slowly within
the box; the entire process takes ～40 hr.

FIG 6

2- Normalizing

Normalizing is a process which is mainly carried out to refine the grain structure and relieve internal stress concentrations in components which have been hot formed to shape. Components formed by hot forging and pressing are very often normalized prior to machining. The process for steel components is similar to annealing except for carbon contents above 0.83%. Here the normalizing temperature is higher than that used for annealing as shown in Figure 7. Normalizing also differs from annealing in the rate at which components are cooled after recrystallisation.

The usual practice is to allow steel components to cool more quickly in still air.

FIG 7

Normalizing is used to

             -To refine the grain structure and to create a more homogeneous austenite when a steel is to                   be reheated for quench hardening or full annealing
             - To encourage reduced grain segregation in castings and forgings and provide a more                            uniform structure
             - To provide moderate hardening

3- Quench hardening

Medium and high carbon steels which have a carbon content of above 0.3% can be hardened by heating them to within the same temperature band as for annealing and then quenching them in

water or oil. Structural changes take place at these high temperatures.

The iron atoms rearrange themselves from a body-centred to a face-centred cubic structure, which is known as austenite, and all of the carbon atoms are taken into solution.

In this condition the steel becomes very malleable and ductile which is why hot working is done at these high temperatures. It must be remembered that the change of structure takes place with

the metal still in the solid state. When the steel is quenched, it does not have time to revert to its original grain structure. The result is a new grain formation called martensite, which consists of hard

needle like crystals. Quenching in water gives the fastest rate of cooling and the maximum hardness. The violence of the cooling can however cause cracking, particularly with high carbon steels. Oil quenching is slower and less violent. The steel is slightly less hard but cracking is less likely to occur. High carbon steels should always be oil quenched. Mild steel with a carbon content below 0.3% does not respond to quench hardening. It can, however, be case hardened on its surface, as will be described.

FIG 8

Quenching Medium
There are a number of fluids used for quenching steels listed below in order of quenching severity

• Brine
• Water
• Oil
• Special liquids
• Air

Note: Agitation of medium increases its quenching severity

Soft distilled water is the preferred medium when using water for quenching carbon steels.  The water should have no impurities such as oil, grease or acids as they could result in uneven hardening if they stick to the surface of the steel being hardened an provide local thermal insulation.   Hard water is unsatisfactory because it may release scale as the temperature is raised.   Soap is sometimes added to adjust quenching rates.  Cold brine or water is used to provide the most severe quench with the consequent maximum hardness.  Extreme care is require in the selection of sections shapes hardened as the process result in severe thermal shock with consequent cracking and distortion.

Oil bath quenching is used where extreme hardness is not required and where freedom from quenching shock is needed.  Oils used are mainly mineral oils with the viscosity selected to suit the type of steel to be quenched.   Oil cooling systems are required when significant quenching capacity is required to prevent the oil from breaking down and to maintain the quenching conditions. Air cooling is used for mild hardening process when a tough hard pearlitic structure is required.

4- Surface Hardening 
Surface hardening, treatment of steel by heat or mechanical means to increase the hardness of the outer surface while the core remains relatively soft. • Surface-hardened steel is also valued for its low and superior flexibility in manufacturing. • The oldest surface-hardening method is carburizing, in which steel is placed at a high temperature for several hours in a carbonaceous environment. The carbon diffuses into the surface of the steel, rendering it harder.
Carburizing
In carburizing, low-carbon steel acquires a high-carbon surface layer by heating in contact with
carbonaceous materials. On quenching after carburizing, the high-carbon skin hardens, whereas the
low-carbon core remains comparatively soft. The result is a highly wear-resistant exterior over a very
tough interior. This material is particularly suitable for gears, camshafts, etc. Carburizing is most
commonly carried out by packing the steel in boxes with carbonaceous solids, sealing to exclude the
atmosphere, and heating to about 925⁰C for a period of time depending on the depth desired; this
method is called pack carburizing. Alternatively, the steel may be heated in contact with carburizing
gases in which case the process is called gas carburizing; or, least commonly, in liquid baths of
carburizing salts, in which case it is known as liquid carburizing. on three primary carburizing processes: Pack carburizing, Gas carburizing and Liquid carburizing.

FIG 9

Nitriding
The nitrogen case-hardening process, termed nitriding, consists of subjecting machined and (preferably) heat-treated parts to the action of a nitrogenous medium, commonly ammonia gas, under conditions
The process consists of maintaining the steel component at a carefullly controlled temperature of 490 ^oC to 530 ^oC under the action of nascent of active nitrogen produced on the surface of the component by the decomposition of gaseous ammonia
Whereby surface hardness is imparted without requiring any further treatment. Wear resistance,
retention of hardness at high temperatures, and resistance to certain types of corrosion are also
imparted by nitriding.

FIG 10

Flame Hardening
This process involves direct an oxy acetylene flame on the surface of the steel being hardened and heating the surface above the upper critical temperature before quenching the steel in a spray of water. This is also known as the shorter process.figure 11

This is a surface hardening process resulting in a hard surface layer of about 2 mm to 6 mm deep. The main difference between this process and other surface hardening processes is that the composition of the steel being hardened is not changed. The steel must itself have sufficient hardenability . This limits this process to steels having carbon contents of above 0,35%. Steels with carbon contents of 0,4%-0,7% are most suitable for this process. Steels with higher content and high alloy steels may not be suitable as they a liable to cracking.  This process produces similar result to the conventional hardening process but with less hardness penetration

FIG 11

Induction Hardening:
Induction hardening provides a similar surface treatment regime to flame hardening .   The steel component is located inside a water cooled copper coil which has (AC) alternating current through it.  This causes the outer surface of the component to heat up.  Depending on the AC frequency and current, the rate of heating as well as the depth of heating can be controlled. This process is well suited for surface heat treatment.  Figure 10 

Hardenability
The hardenability of a steel is broadly defined as the property which determines the depth and distribution of hardness induced by quenching. Hardenability is a characteristic determined by the following factors

• Chemical composition
• Austenite grain size
• Structure of alloy before quenching

The hardenability is the depth and evenness of hardness of a steel upon quenching from austenite.

Thickness Considerations
The properties of heat treated steel are significantly affected by the thickness of the section.   Hardening consist of heating the steel through and just above its critical range to obtain the condition of solid solution and quenching with sufficient rapidity to retain this condition.   If a steel has a large thickness it is practically impossible to obtain an even temperature throughout and the middle of the section is always at a lower temperature compared to the outside surfaces.    On quenching the heat is absorbed rapidly from the outside and it is impossible even with the most drastic quench processes to remove heat from the core region sufficient to obtain the desire structure.   For thin sections it may be possible to obtain the desire structure throughout the section with a comparative mild quenching process.

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5- Tempering

Quench hardened components are generally too hard and brittle for direct use. The tempering process removes some of the hardness and toughens the steel.

TABLE 1

Tempering is achieved by re-heating it to temperatures between 200 ⁰C and 600 ⁰C and quenching again in water or oil. The temperature to which steel components are re-heated depends on their final use as shown in Table 1. 

Large batches of components are hardened and tempered in special temperature-controlled furnaces. Small single items may be hardened in the workshop by heating in a gas flame to a bright cherry red color and quenching. The surface is then polished and they are then gently re-heated until oxide color films start to spread over the surface. When the color film that corresponds to the required tempering temperature starts to appear, the components are quenched.
Martempering
A modified quenching procedure known as martempering minimizes the high stresses created by the
transformation to martensite during the rapid cooling characteristic of ordinary quenching ( Fig. 12). In practice, it is ordinarily carried out by quenching in a molten-salt bath just above the Ms temperature. Transformation to martensite does not begin until the piece reaches the temperature of
the salt bath and is removed to cool relatively slowly in air. Since the temperature gradient characteristic of conventional quenching is absent, the stresses produced by the transformation are much lower and a greater freedom from distortion and cracking is obtained. After martempering, the piece may be tempered to the desired strength.

FIG 12

Austempering
lower bainite is generally as strong as and somewhat more ductile than tempered martensite. Austempering, which is an isothermal heat treatment that results in lower bainite, offers an alternative heat treatment for obtaining optimum strength and ductility.
In austempering the article is quenched to the desired temperature in the lower bainite region,
usually in molten salt, and kept at this temperature until transformation is complete ( Fig. 13).
Usually, it is held twice as long as the period indicated by the IT diagram. The article may be
quenched or air cooled to room temperature after transformation is complete, and may be tempered
to lower hardness if desired.

FIG 13

FIG 14

Tempering Colors with their Corresponding Temperatures – The various colours obtained after tempering with their corresponding temperatures are: 

FIG 15

6- Ausforming 

 Ausforming also known as Low and High temperature thermomechanical treatments is a method used to increase the hardness and stubbornness of an alloy by simultaneously tempering, rapid cooling, deforming and quenching to change its shape and refine the microstructure.

FIG 16

7- Case hardening
Mild steel does not respond to quench hardening because of its low carbon content. Case hardening increases the surface hardness of the material whilst leaving the core in its soft and tough
condition. The first part of the process is known as carburising where the components are ‘soaked’ for a period of time at high temperature in a carbon bearing material. The traditional method is to pack them in cast iron boxes with a carbon rich powder.
This may be purchased under a variety of trade names or made up from a mixture of charcoal and bone meal. The carbon slowly soaks into the steel to give an outer case with a high carbon content. The depth of the case depends on the time of soaking. The second part of the process is to re-heat the components to refine the grain size in the core and then to quench harden and
temper the outer case. Case hardened component have a hard and wear resistant outer case and a tough impact resistant core. This is an ideal combination of properties for many engineering components.
Case hardening is not confined to mild steels. Medium and high carbon steels in the normalised condition can be case hardened by rapidly heating and quenching the outer surface.
Induction hardening is such a process, where an induction coil induces a high frequency electric current into the component as it passes through the coil. This has a rapid heating effect after which
the component passes through a water or oil jet to quench the surface.

FIG 17