Heat treatment is defined as an operation or combination of operations, involving heating and cooling of a metal or alloy in the solid state to obtain desirable properties.
Heat treatment consists of three phases.
- Heating of the metal
- Soaking of the heat into metal
- Cooling of the metal
Heating temperature of metal for the purpose of heat treatment depends upon its grade, grain size, type and shape of a metal or alloy. Generally, the metal is never heated much beyond its upper critical temperature. Plastic deformation takes place because of heating. This deformation depends upon the chemical composition of the metal or alloy and the temperature at which it is heated.
Metal or alloy to be given heat treatment, is held at the specified temperature for a specified period so that there is a uniformity of temperature throughout the mass. The period of soaking depends upon the size and shape of the component.
Main changes in the properties of metal or alloy take place in the cooling process. Change in properties or transformation depends mostly on the rate at which the cooling takes place. The different media used for cooling are,
- Caustic Soda solution
- Brine solution (NaCl)
Various types of heat treatment processes are:
Annealing is one of the most important heat treatment operation applied to steel. It is the process of heating the steel in a furnace to a point not exceeding 50° above its upper critical point and maintaining the steel at that temperature for a considerable time (30-60 minutes) to convert the whole steel to austenite. Steel is allowed to cool down slowly through a medium of hot sand, hot ashes or hot lime dust. The rate of cooling is to be maintained at 150-200ºC per hour.
Purpose. The various purposes of annealing are:
- To soften the metal. i.e. While working on metals in cold condition, it becomes hard. For further work on the metal without any cracks, it should be soften by annealing process.
- To improve machinability.
- To refine grain size, structure and to improve mechanical properties.
- To relieve internal stresses which were developed during working over the metal.
- To modify physical properties.
- To increase ductility of metal.
To prepare the steel for cold working.
Types of Annealing
Low Temperature Annealing (Spherodising)
This process consists of heating the steel to a temperature little below the lower critical point and cooling is done to carbon steels at a very slow rate (25 to 30ºC per hour) before cold working. This process reduces hardness to the minimum and brings the steel to elastic limit and yield point. Full annealing after the cold working will restore its original property.
Severely cold worked steels, which are quite hardened and have a very high yield point, are heated to 300ºC (blue colour) in an open furnace and cooled down slowly. It helps to work further on the sheet without crack.
In this process the job is kept in a closed annealing pot or box, heated to a sub-critical temperature and cooled down slowly together with box. It is used mainly for sheet, strip, or wire.
In this process, the iron base alloys are heated to 400ºC and cooled down slowly. After this the job appears in a black colour, which is free from oxide.
When hollowing on a sheet or working on a particular part or area of large job, the area tends to become hard due to work hardening. It is impossible to work further. For further working, the part or area of a job is to be softened. It is done by heating the job by the oxy-acetylene flame to light red colour (800ºC) and cooled down slowly.
In this process the articles are covered with sand (pack), heated to a light red colour (800ºC) and cooled down slowly together with pack. It is done on various shock resisting tool steel like chuck key, power tools, etc.
This is a process of heating the article to above its upper critical point, slowly cooling it down to black heat (approx. 400ºC) and then finally quenching in water. This is carried out to speed up the annealing process when there is lack of time.
Isothermal annealing reduces the total time required for an annealing operation. In this process, steel is heated to austenite state and then cooling it down to a temperature of about 650°C at a relatively faster rate. Then it is held at constant temperature i.e. isothermally for some time and then cooling it down to the room temperature at a rapid rate.
Salt Bath Method
It is generally recognised that, the satisfactory method of obtaining uniform high temperature throughout the considerable volume of metals is by immersing in a bath of liquids. The liquid employed would naturally depend largely upon the temperature that has to be obtained and also upon the nature of metal to be heat treated. For light alloys, the most convenient medium is a mixture of salt, prepared by mixing in equal proportion of Sodium Nitrate and Potassium Nitrate.
Size of Baths
The size of the bath varies according to the size of job on which heat treatment is to be carried out. Thus larger baths are capable of taking big sheets and smaller baths are used for comparatively smaller components, such as rivets, nuts and bolts, etc.
This is used for heat treatment of Dural rivets and small parts. The Salt bath consists of a metal cabinet containing a cast iron bath and a pair of kerosene heaters, which can be used alternatively to enable the heat to be supplied continuously. A bath is centrally mounted on the cabinet and the access is through the sliding door in the upper part of the cabinet. A pyrometer is provided to indicate the temperature of the bath. When it is heated, fumes from the bath go out into the atmosphere through the pipe at the rear side of the cabinet. When the burner is in position, the fuel container rests outside the heater compartment and is shielded from the burner by the sliding door. This door has slots cut in its lower edge to clear the supply pipe, which connects the burner and the container.
Methylated spirit is used for preheating the burner. A full container will provide 2 to 3 hours of burning.
For recording the temperature each bath is provided with a pyrometer.
In this operation, the nitrate must be heated to the required temperature and the salt when melted should not be more than 3/4th of the capacity of the bath. Nitrates can be used again and again but must be cleaned once in a month, so as to remove any residues left while heating. The heat must be applied very gradually.
In hardening process, the steel is first heated to a point exceeding 50ºC above the upper critical point for hypo-eutectoid steels and 30-50ºC above for hyper-eutectoid steel. Then the steel is soaked at this temperature for a considerable time to ensure that all the pearlite and cementite have changed into austenite. After that the steel is cooled rapidly to keep the austenite to remain as such at room temperature. This process consists of two operations – heating and quenching. If these two operations are properly carried out, then the required structure is obtained.
Heating may be carried out in a furnace, fired by oil, gas or coal, in which the job is in direct contact with the flame. It can be heated in a muffle furnace where the job is held in a compartment and is not in direct contact with the flame or electric current. Also, it can be heated in a bath type furnace where it is immersed in a molten salt or lead bath.
The job should be heated gradually and uniformly. Sudden or uneven heating causes internal stresses, while a slow rate of heating causes grain growth.
The structure of steel is affected by the rate of cooling in general and more particularly in the temperature range of 650-550ºC where austenite decomposes more rapidly into pearlite, cementite or ferrite. Between 300-200ºC martensite is formed. This in turn, causes an increase in the volume of the metal thereby developing high internal stresses and strains.
Effects of Hardening
The effects of hardening are:
- Maximum hardness.
- Smallest grain size.
- Minimum ductility.
- Maximum tenacity.
Types of Hardening
Low carbon steels which have been cold rolled or hammered, become hard to a certain extent, thereby increasing yield point and ultimate strength with reduction of ductility and toughness.
This is a surface hardening process done by the oxy-acetylene flame. In this process heat is applied to the skin of the job and then before the heat penetrates to the core, it is suddenly cooled. This method is normally used on pinions, gear surface, crown wheels, cams and camshafts.
This is a surface hardening process, in which the heating medium is the high frequency current. No sooner is the surface heated, the supply of the current is shut-off and a high-pressure jet of water sprayed on the job.
The hardening temperature of Ni-Cr steel of 900-1000º C and that of high-speed steel is 1100-1300ºC.
Martensite is stable only up to 200ºC. If a piece of steel, which has been hardened, is subsequently heated to a temperature above 200ºC, the decomposition of martensite will start taking place. This decomposition is in the order of troostite first and then sorbite.
Martensite decomposes into troostite, which is a finely dispersed mixture of cementite and ferrite, in the temperature ranges of 200-300ºC. Tempering at temperature between 500-600ºC will lead to the formation of the globular structure of sorbite.
The object of tempering is to remove excessive brittleness and induce toughness.
Tempering Colors with their Corresponding Temperatures
The various colours obtained after tempering with their corresponding temperatures are:
Different Methods of Tempering
The different methods of tempering are:
1. Austie Tempering
Steel jobs of smaller diameter not exceeding 1/4” and containing 0.9% carbon are heated to above the upper critical point and quenched in a salt or lead bismuth bath of 260-340C. When the job reaches the temperature of bath, it is removed and quenched in water. This method does not promote the formation of martensite. Thus there are no stress/strain effects, but strength, ductility and hardness are induced.
2. Mar Tempering
In this process the steel is heated above its upper critical point and quenched in a bath (260˚C). It is held in the bath for a definite time and then cooled down to room temperature in still air. The transformation of martensite takes place under conditions of slow rate of cooling and therefore, internal stresses are reduced to a greater extent.
Salts for Various Temperatures
The various salts of combination of salts for different operating temperature are as given below.
|Barium Chloride||100% by weight||1050-1325C|
|Barium chloride &||67% by weight||1050-1325C|
|Potassium chloride||33% by weight||750-1100C|
|Sodium chloride &||50% by weight||650-800C|
|Potassium chloride||50% by weight||650-800C|
|Potassium nitrate&||50% by weight||650-800C|
|Sodium nitrate||50% by weight||650-800C|
If a piece of clean polished steel is heated, it will be seen that a series of colours appear on the surface as the temperature rises. By heating the hardened job until a particular colour appears, a definite amount of brittleness is removed. Always quench the job when the required colour appears. The colour should be observed on a dark background or in the shade, because colours vary with the intensity of light. There are two methods of hardening and tempering by colours:
- Single heating method
- Double heating method
Single Heating Method
This method is used when the body of the tool is to be left soft and tough, while the working edge is to be hardened and tempered e.g. cold chisels, screw drivers, punches, scribers and drifts etc.
Process. Heat approximately half the tool from the working edge upwards to a cherry red colour. Now dip half the heated portion into water, moving the tool up and down to prevent the formation of water line. When the cooled part becomes black, remove the tool and quickly polish the tip. The heat from the upper part will flow down to the lower part. When a dark purple colour (290C) appears on the polished tip, quench the whole tool in water.
Double Heating Method
This method is employed where the whole body is to be hardened and tempered and the shank alone left soft and tough. In this method the body (tool) is heated up to its upper critical point and quenched drastically for hardening. In the second heating, it is heated up to tempering temperature and again quenched. This method is followed for drills, reamers, scrapers, hacksaw blades etc.
An alloy bath consists of lead and tin in varying proportions which, when melted, will have temperatures varying between 180-320C depending upon the percentage of each of the constituents. Hardened articles are immersed in this molten bath, which is maintained at the tempering temperature required, till they reach the temperature of the bath. They are then quickly cooled in water.
Oil Bath Method
In this method, oil, having a high flash point (where oil starts burning into flame), is heated to the required temperature. The article to be tempered is immersed in it until the article attains the temperature of the oil. The job is then cooled in water.
In this method the job is heated to its upper critical point and then quenched in hardening oil. It is then removed from the oil and held in a clean fire until the oil flashes. It is then cooled in water. This method is normally used for coil springs.
Hot Sand Method
Large hardened articles are covered with sand and heated to tempering temperature. It is then removed and quenched as usual. The correct temperature is ascertained by means of a thermometer.
Salts with low fusion points, such as potassium and sodium are mixed and melted and the hardened articles are immersed in it. After they attain the temperature of the bath, the articles are removed and quenched in water.
The process consists of heating of steel to a point 40 to 50°C above its upper critical temperature. Hold at that temperature for a short duration and subsequently cooling in still air at room temperature. This is also known as air quenching. It produces microstructures consisting of ferrite and pearlite for hypo -eutectoid steels and pearlite and cementite for hypereutectoid steels.
Normalizing is done for the following purposes:
- To eliminate coarse grain structure which is produced during forging, rolling, etc.
- To improve machinability.
- To reduce internal stresses.
- To improve certain mechanical properties.
Effects Of Normalizing
- Normalizing raises the yield point, ultimate tensile strength and impact strength of steel.
- Normalized steels are harder and stronger but less ductile than annealed steels with the same composition.
- Reduces the grain size caused by over heating or by slow cooling.
- Produces uniform granular structure.
- Improves the machineability of the steel.
- It prevents the cracking of High Carbon Steel, High Speed Steel and High Tensile Steel, when these steels are hardened.
This is a heat treatment process carried out in order to break down large, coarse grains formed by the overheating of steels. The process involves heating the job to above the upper critical point and then quenching it. This is repeated three or four times, the maximum temperature being lowered by 50C each time, e.g. first heating to 900C, second heating to 850C and the fourth heating to 750C. This process is done in addition to normalizing. Where the jobs are such shapes as to give rise to severe strains on quenching, they are air-cooled.