Different kinds of Heat Treatments Given to Carbon Steels

Kinds of Heat Treatments Given to Carbon Steels

Carbon steels are subjected to the following basic heat-treatment processes:

1. Annealing,
2. Normalising,
3. Hardening,
4. Tempering,
5. Surface hardening,
6. Case hardening, and 
7. Ageing. 

Each of them has a number of varieties. We shall now describe these processes in the next posts.

Annealing

          This is a kind of heat treatment after which a metal or alloy acquires a structure close to the equilibrium one. A material is exposed to an elevated temperature for an extended time period and then slowly cooled. Normally, annealing is carried out to:

1. To improve machinability,
2. To relative internal stresses,
3. To increase softness, ductility and toughness, 
4. To produce a specific micro-structure, 
5. To remove gases,
6. To refine grain size due to phase recrystallisation,
7. To modify electrical and magnetic properties.

          The approximate temperatures to which the steel-sample should be heated will depends upon its carbon content. The recommended temperatures are shown in the following table:

          There are a variety of annealing heat treatments are possible. These are characterized by the changes that are induced, which many times are microstructural and are responsible for the alteration of the mechanical properties.

An annealing process consists of three stages: 

      (i) heating to the desired temperature, 

      (ii) holding or 'soaking' at that temperature, and 

      (iii) cooling, usually to room temperature. 

             In these annealing procedures, time is an important parameter. There exist temperature gradients between the outside and interior portions of the piece during heating and cooling. The magnitudes of temperature gradients depend on the size and geometry of the piece. Soaking time may be given at the rate of 3-4 minutes for every one mm thickness of the cross-section of material. If the rate of temperature, change is too great, temperature gradients and internal stresses may be induced that may lead to warping or even cracking. Moreover, the actual annealing time must be long enough to allow for any necessary transformation reactions. An annealing temperature is also an important consideration. Since diffusional processes are normally involved and therefore annealing may be accelerated by increasing temperature. The various types of annealing operations are: 

1. Full annealing,
2. Isothermal annealing,
3. Process (or) sub critical annealing,
4. Spheroidise annealing,
5. Diffusion annealing, and 
6. Recrystallisation annealing.

Full Annealing

          If it is desired to reline the grain structure and produce a lamellar pearlite, a full annealing cycle should be used. This consists of heating the steel to a temperature above the transformation range, holding for one or two hours, and then cooling at a predetermined rate to obtain the desired microstructure. Grain refinement is accomplished in this instance by the re-crystallisation of the steel in passing through the critical range both in heating and in cooling. The microstructure obtained in cooling any steel from above the critical temperature range is dependent both upon the temperature range in that ranges. Thus, it is obvious that the rate at which any steel is cooled determines the final microstructure, since the degree of transformation will depend on the amount of time allowed for it to occur. Therefore, the slower the rate of cooling and the higher the temperature at which complete transformation occurs during full annealing, the coarse the pearlite will be with correspondingly lower hardness.

          Such treatment is performed usually on steel of 0.3 to 0.6 % carbon content which is to be machined.

Isothermal annealing

          It is a type of full annealing in which the steel first is cooled to the temperature at which it is desired to have transformation occurs, at a rate sufficiently rapid to prevent any structural change above that temperature. The steel then is held at the selected temperature for the time necessary to complete such transformation. Thus it possible, with this process to obtain a more uniform micro-structure that could be expected by continuous cooling. However, since it is necessary to drop the temperature rapidly to prevent any transformation above the desired temperature, there are definite limitations as to the mass that can be co-treated. It is applicable, therefore, only to small sections and would be suitable for large bars or large load in batch type furnace since in would be impossible to cool them at a rate sufficiently rapid to prevent some transformation.

          Isothermal annealing process not only improves machinability in general, but also results in a better finish by machining. However, it has the following important limitations:

          It is suitable only for small components. Heavy components cannot be subjected to this treatment because it is not possible to cool them rapidly and uniformly to the holding temperature at which transformation occurs. 

Process or Sub critical annealing

          Another type annealing called Process or Sub critical annealing consists of heating the steel to a temperature first under lower critical point and holding at this temperature for the proper time (2-4 hrs) followed by air cooling. This type of annealing results in softening the steel due to particle coagulation of the carbide to form the spheroids or small globules of carbide. It is not suitable when a close control of hardness or structure is desired, because the prior structure of steel determined to a marked degree the extent of spheroidisation which will occur.

          This process is quite satisfactory for rendering bars more suitable for cold sawing or shearing and is used to great extent for these purposes. Since the temperature to which the bars are heated is somewhat lower than in full annealing there is less scaling and warping can be controlled.   

Spheroidisation annealing

          It is a type of annealing which causes practically all carbides in the steel to agglomerate in the form of small globules or spheroids. There may be wide range of hardness with such a structure for any grade of steel since the size of the globules has a direct relation to hardness. Spheroidising may be accomplished by heating to a temperature just below the lower critical and holding for sufficient period of time. A more desirable and commonly used method for spheroidising is to heat to temperature just above the critical and cool very slowly (6°C per hour) through the critical range or to heat to a temperature within the critical range but not above the upper critical and cool slowly.

          This treatment is used for practically all steels containing over 0.6 percent carbon that are to be machined or cold formed.  

Diffusion annealing

          This process, also known as homogenizing annealing, is employed to remove any structural non-uniformity, like dendrites, columnar grains and chemical inhomogeneities are generally observed in the case of ingots, heavy plain carbon steels castings. These defects promote brittleness, and reduce ductility and toughness of steel.

Process:

• Steel is sufficiently above the upper critical temperature (say 1000-2000°C), and held at this temperature for prolonged time, usually 10-20 hours, followed by slow cooling.
• Segregated zones are eliminated, and a chemically homogeneous steel is obtained by this treatment as a result of diffusion.
• Heating to such a high temperature results in considerable coarsening of austenitic grains & heavy scale formation. The coarse austenite thus obtained further transforms to coarse pearlite on cooling, which is not a desirable structure as mechanical properties are impaired.
• The main aim of homogenising annealing is to make the composition uniform, i.e., to remove chemical heterogeneity.
• The impact energy and ductility of the steel increases as the homogenizing temperature increases and the hardness yield strength and tensile strength decrease with an increase in the homogenizing temperature.
• Homogenizing annealing has a few shortcomings as well. It results in:
• Grain coarsening of austenite, there by impairing the properties.
• Thick scales on the surface of steels.
• It is an expensive process.

Recrystallisation Annealing

• Practically all steels, which have been heavily cold worked, are subjected to this treatment.
• The process consists of heating steel above the recrystallisation temperature, holding at this temperature and cooling thereafter. It results in decrease in hardness or strength and increase in ductility. The process is used both as an intermediate operation and as a final treatment. The treatment is very important and is frequently employed in industries manufacturing steel wires, sheets and strips.
• As little scaling and decarburisation occurs in recrystallisation annealing, it is preferred over full annealing.
• High carbon steels and alloy steels require higher recrystallisation temperatures. 

Normalising

          Normalizing is the term applied to the process of heating the steel approximately 4 above the critical temperature followed by cooling below this range in still air. This is one of the simplest treatment.

          Due to air cooling which is slightly fast as compared to furnace cooling employed in full annealing, normalised components show slightly different structure and properties than annealed components. 

• The steel produced by this treatment is harder and stronger but less ductile than annealed steel having the same composition.
• This treatment is frequently applied to castings, forgings, etc., to refine grain structure and to relieve stresses set up in previous operations.
• It is commonly applied after cold working, overheating, or any other operation resulting in non-uniform heating or cooling.

The objects of normalising are:

1. To eliminate  coarse gain structure obtained during forging, rolling, and stamping.
2. To reduce internal stresses.
3. To improve machinability of low carbon steels.
4. To increase strength of medium carbon steel.
5. To improve structure of welds.
6. To achieve desired results in mechanical and electrical properties.  

Hardening

          Hardening is a process in which steel is heated to a temperature above the critical point, held at this temperature and quenched (rapidly cooled) in water, oil or molten salt baths.

          As earlier mentioned that if a piece of steel is heated above its upper critical temperature and plunged into water to cool it an extremely hard, needle-shaped structure known as martensite is formed. In other words, sudden quenching of steel greatly increases its hardness. However, in order to harden, the carbon content in the work piece should be at least 0.25%. Therefore, dead mild steel cannot be hardened in this way. Mild steel will also harden slightly for specimens containing over 0.25% carbon. Higher the carbon percentage, higher will be resulting hardness. Hardened pieces become brittle and their extreme brittleness becomes a great disadvantage. They tend to fail in-service. Therefore hardening process is invariably followed by a tempering process.

          The hardening process is based on a very important metallurgical reactions of decomposition of eutectoid. This reaction is dependent upon the following factors:

1. Adequate carbon content to produce hardening.
2. Austenite decomposition to produce pearlite, bianite and martensite structure.
3. Heating rate and time.
4. Quenching medium.
5. Quenching rate.
6. Size of the part.
7. Surface conditions.

Hardening methods

          The most extensively used method is conventional hardening or quenching in a single medium. The disadvantage of this method, however, is that the cooling rate in the martensitic transformation range will be very high. It will differ only slightly from the rate on the upper zone of super-cooled austenite of low stability and, therefore, cracks, distortion and other defects may occur in this method. Other hardening methods, which shall be briefly described, are generally employed to avoid these defects and to obtain the required properties.

          The various hardening methods are:

• Quenching in two media.
• Hardening with self tempering.
• Stepped quenching or martempering.
• Isothermal quenching or austempering.

Mass Effect

          Mass effect is the variation in hardness across of the components having higher thickness through heat treatment.

          Hardness of plain carbon steel depends upon its carbon content and the rate of cooling from the hardening temperature. A part having less thickness will cool more quickly than a part having higher thickness if both are cooled in the same quenching bath. In a thicker component, outer layer will cool faster than the core and heat will get trapped at the centre. This leads to a variation in hardness across a section of the component. The result can be another layer of martensite and inner core of article, this variation in hardness is referred to as mass effect. 

          Owining to "mass effect" plain carbon steels having large sections are said to have a poor hardenability, as they cannot be fully hardened throughout.

Hardenability

          Hardenability is defined as the ability of steel to develop its maximum hardness when subjected to the normal hardening heating and quenching cycle. High hardenability means that the steel hardens more nearly throughout rather than just at the surface. In order words, good hardenability is indicated by a greater depth of hardening below the surface.

          Often a confusion arises about the meaning of hardening power and hardenability. It may be remembered that hardening power refers to the maximum haardness obtainable with steel of given composition while hardenability refers to the depth of hardening in a given thickness of steel. Only the carbon content affects the maximum hardness that can be realised with any steel. High carbon content makes more carbon and iron carbide available to distort the martensitic structure and consequently allows higher hardness. The hardenability of a steel becomes greater as the percentage of carbon increases and is further improved by the addition of such alloying elements as manganese, nickel, chromium, molybdenum and vanadium, and by increase in the austenite grain size.

          The factors which affect the depth of hardening of steel components are given below:

1. Hardenability of the steel from which the component is made.
2. Severity of quench used.
3. Size and shape of the piece.
4. Surface condition and the austenite grain size. 

Jominy hardenability test or Jominy end quench hardenability test

          One of the important properties of metals is hardenability. Hardenability is the capacity of an alloy to be hardened by heat treatment. Hardenability is not same as hardness. 

      There are a number of methods available for measuring the hardening response of a steel to heat treatment. One of the most useful and widely accepted and the only discussed here is the Jominy test or Jominy end quench hardenability test. Here all the factors which effect the depth of hardening of steel, except composition are fixed or held constant. Any variations in the depth of hardening, therefore, reflect, a variation in hardenability. The data obtained from this test are relatively easy to interpret, appropriate plotting the hardenability of different steels may be readily compared. Further more Jominy test results are useful in prediciting the hardness at any location within a quenched steel part without a specific test, it is done on the basis of the steel.

Fig. 1

Fig .2

          In this test we use the standard specimen consist of a cylinder rod either 100 mm and 25 mm in dia , alternatively 102 mm X(by) 25.4 mm is obtained. In making test the specimen is first heated to a suitable austenising temperature and held there long enough to obtain a uniform austenite structure. It is then rapidly  placed in a jig, where it is held vertically and stream of water allowed to strike the end of specimen as shown in Fig.(1). The cooling conditions within the Jominy bar during quenching change very little across the diameter but very rapidly at the end and progressively less rapidly at points towards the opposite end. After being thoroughly  cooled, shallow flats are ground on the bar along the flats at 1/16" intervals for the first inch followed by reading at 1/18" intervals for the next 3/2" . A plot is made of the hardness reading against the distance from the quenched end of the bar. such plot is shown Fig.(2).  

Tempering

          Martensitic structure formed by direct quenching of high-carbon steel are hard and strong, but unfortunately are also brittle. They cannot be plastically deformed and have very little toughness, and although strong they are unable to resist impact loads and are extremely sensitive to stress concentrations. Some of the hardness and strength must be sacrificed to obtain suitable ductility and toughness. This is done by tempering the martensitic steel.

          Tempering means giving up a certain amount of hardness but shedding a great deal of brittleness acquired in the process of hardening. It is a trade off between hardness and brittleness, so that hardened component may give useful service without failure. Tempering involves heating the carbon steel part to a temperature varying from  150°-600°C (depending upon how much trade off is required) and cooling the component in an oil or salt bath or even in air.

Thus tempering process is carried out to:

• Increase toughness,
• Decrease hardness,
• Stabilise structure,
• Relieve stress, and
• Change volume.

Method Of Tempering

          Tempering of steel may be carried out in liquid bath such as oil, salt or lead, the bath being heated to the correct temperature and steel immersed in the bath for the determined length of time after which component (steel) is removed and allowed to cool to room temperature. Air tempering furnaces are very successfully employed for tempering. These furnaces are fully automatic and the hot air (heated by gas or electric means) is circulated around the parts to be tempered.

Temper Colors

          Tempering can be judged by the temper color which appear on the bright red surface, and experienced eyes are generally guided by those colors while heating steel materials for tempering. The following are the colors formed on steel in the process of tempering:

English Color Text	Fahrenheit	Color	Celsius	SAE 1040	SAE 1050	SAE 1070	SAE 1095
Clear - As fully hardened	100°		38°	54	59	64	67
Pale yellow 2	420°		216°	51	55	59	63
Very pale yellow 1	430°		221°
Light yellow 1, 2, Straw 3	440°		227°
Pale straw-yellow 1, Straw 3	450°		232°	50	54
Straw-yellow 1, 2	460°		238°				62
Deep straw-yellow 1, 3	470°		243°
Dark yellow 1, Light orange 3	480°		249°			58
Yellow-brown 1, Orange 3	490°		254°	49	53		61
Brown-yellow 1, 3, Bronze 2	500°		260°
Spotted red-brown 1, Dark Brown 2	510°		266°
Brown with purple spots 1, 2	520°		271°		52	57
Light purple 1, 2, Purple 3	530°		277°	48			60
Full purple 1, Purple 2	540°		282°
Dark purple 1	550°		288°
Full blue 1, 3	560°		293°	47	51	56	59
Dark blue 1, Blue 2	570°		299°
Dark blue 2	590°		310°				58
Pale blue 3				46	50	55
Light blue 2	610°		321°
Greenish blue 2, Grey 3	630°		332°	45		54	57
Light blue 1	640°		338°
Steel grey 2	650°		343°	44	47	53	56
Steel grey	700°		371°	42	46	51	54
	750°		400°	40	44	50	52
	800°		427°	38	42	47	49
	900°		482°	35	37	43	44
Lowest visible red	1000°		538°	30	32	36	39
English Colour Text	Fahrenheit	Colour	Celsius	SAE 1040	SAE 1050	SAE 1070	SAE 1095

Effects of tempering temperature on the mechanical properties of steel:

Tempering process decomposes the martensite into a ferrite-cementite mixture and thus the properties of steel are strongly affected.

1. At low tempering temperatures (upto 200 to 250)
• bending and true tensile strength are increased, and
• hardness changes to a small extent.
2. If the temperature is further increased the following result:
• hardness, true tensile strength , proportional limit and yield point are reduced.
• reduction of area and relative elongation are increased.
3. Tempering at 250 to 400 reduces the impact strength of steel. Therefore, the temperature range 250 to 400 should be avoided in assigning tempering temperature.
4. The properties after structural improvement i.e., hardening followed by high tempering are always higher than those of annealed steel. This is due to the difference in structure of the ferrite-cementitic mixture.