In this article we will discuss about:- 1. Definition of Heat Treatment 2. Iron-Carbon Equilibrium Diagram 3. Theory of Heat Treatment of Steel 4. Martensitic Transformation (In Steel) 5. Critical Cooling Rate 6. Retained Austenite 7. Austenite Grain Size and Grain Size Control 8. Heat Treatment of Tool Steels 9. Alloy Steels and their Heat Treatment and Other Details.
Contents:
- Definition of Heat Treatment
- Iron-Carbon Equilibrium Diagram
- Theory of Heat Treatment of Steel
- Martensitic Transformation (In Steel)
- Critical Cooling Rate
- Retained Austenite
- Austenite Grain Size and Grain Size Control
- Heat Treatment of Tool Steels
- Alloy Steels and their Heat Treatment
- Defects due to Heat Treatment of Steels
- Surface Finish after Heat Treatment
1. Definition of Heat Treatment:
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Heat treatment is defined as an operation or combination of operations, involving heating and cooling of a metal or alloy in its solid-state with the object of changing the characteristics of the material.
Heat treatment is generally employed for following purposes:
1. To improve machinability.
2. To change or refine grain size.
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3. To relieve the stresses of the metal induced during cold or hot working.
4. To improve mechanical properties, e.g., tensile strength, hardness, ductility, shock resistance, resistance to corrosion etc.
5. To improve magnetic and electric properties.
6. To increase resistance to wear, heat and corrosion.
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7. To produce a hard surface on a ductile interior.
2. Iron-Carbon Equilibrium Diagram:
Fig. 5.10 shows the iron-carbon equilibrium diagram. Any iron-carbon alloy in the molten state may be considered to be solution of Fe3C in iron. When this solution cools and solidifies, it depends upon the amount of carbon present whether the alloy will solidify as a solid solution or form a eutectic (In this system the two constituent metals are soluble in all proportions in liquid state but completely insoluble in the solid state).
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If the carbon does not exceed 2.0% the alloy will solidify as a solid solution. A solid solution in γ-iron is known as austenite, and this name applies to all such solutions regardless of the amount of carbon, which may range from almost nil upto the saturation point 2.0%. The saturated solution is sometime’ known as “2.0%” austenite. If the alloy contains more than 2.0% of carbon it solidifies with the formation of eutectic, which freezes at 1130°C and contains 4.3% of carbon.
In other words everything to the left of the line AB in the diagram (Fig. 5.10) freezes as a solid solution and everything to the right of it freezes selectively. The limit of 2.0% of carbon is recognised as the division between steel and cast iron, an iron-carbon alloy containing less than this amount being defined as steel and any that contains more being defined as cast iron.
Note:
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Hypoeutectoid steels contain upto 0.80 percent carbon whereas hypereutectoid steels contain higher percentage (more than 0.80%).
3. Theory of Heat Treatment of Steel:
The theory of heat treatment is based on the fact that a change takes place in the internal structure of steel at specific temperatures. Steel in its simplest form is composed of pure iron to which a small percentage of carbon has been added. At normal temperatures the steel consists of the pure iron, known as ferrite combined with iron carbide, which is called cementite.
The hard brittle iron carbide does not become fully combined with the iron however, until a temperature of approximately 800°C is reached. If the steel is heated progressively it will be found that at this point the temperature of metal does not increase, although heat is still being added.
This is owing to the fact that the increased heat is for the time being, used up in bringing about a structural and chemical change in the metal. The carbon is in fact, going into what is termed as solid solution with the iron, although the steel is well below the melting point. This is termed the decalescence point or upper critical point by metallurgist.
When the change has taken place the temperature of steel begins to rise again. If the steel is removed from heat at this stage and allowed to cool, a point will again be found at which the cooling appears to be arrested, and the metal may even appear to rise slightly in temperature.
This is termed the recalescence point or lower critical point and indicates the metal is changing back from a solution to a mixture of iron and iron carbide. Once the change has taken place, the metal continues to cool at a normal rate. The critical points are separated by a temperature difference of from 30 to 105°C, according to the composition of the steel the recalescence point being lower than decalascence point.
The iron-carbon equilibrium diagram (Fig. 5.11) shows the manner in which the carbon content affects the structure of steel during heat treatment. The terms on the diagram may be simply understood, pure iron ferrite and iron carbide, termed cementite has already been referred to.
The ferrite and cementite combine in layers and the resultant structure is termed ‘pearlite’. If the heating process is continued beyond the critical temperature, the pearlite dissolves to form “austenite’. When the steel contains 0.89% of carbon, a true pearlite structure is obtained, with less carbon some free iron remains uncombined, whereas an excess of carbon over 0.89% results in uncombined cementite.
If a piece of steel is heated above its upper critical point until it becomes cherry red, i.e., its austenitic condition, which is equivalent to about 900°C, and is then plunged into cold water to cool it, the sudden cooling traps the carbon in its solid solution state and an extremely hard, needle-shaped structure known as martensite is formed.
4. Martensitic Transformation (In Steel):
Martensite is the supersaturated solution of carbon in α-iron. The unit cell of martensite looks like a cube stretched into a rectangular parallelepiped. The iron atoms are immobile during the martensite transformation and so remain dissolved interstitially in the a-iron lattice of martensite.
Martensite is hard and possesses low ductility Martensite formation from austenite is accompanied by an increase in specific volume (about 3%). This is the main reason why large stresses are set up in hardening that distort the article being hardened so the cracks may appear.
The main characteristics of the martensitic transformation are:
(i) High rates of nucleation and crystal growth of low temperatures.
(ii) Limited crystal growth; crystal grows rapidly to a certain limited size after which growth ceases.
(iii) Rapid attenuation of the transformation upon stopping the cooling process.
The martensite transformation occurs in a wide temperature range. It begins at a temperature corresponding to a point Ms (Martensite start). When the cooling process passes through point Ms, austenite begins to transform into martensite.
The lower the temperature, the more martensite will be formed. The amount of martensite formed, according to the temperature, may be represented by the so called martensite curve (Fig. 5.14). At a definite temperature for each steel, further transformation of an austenite into martensite ceases.
This temperature is usually denoted as Mf (Martensite finish). The positions of points Ms and Mf do not depend upon cooling rate and are determined by the chemical composition of the austenite. More carbon in a steel lowers points Ms and Mf.
A characteristic feature of martensite transformation is that it is practically never completed. That is why a hardened steel contains retained austenite, as a rule, in addition to austenite. The higher the carbon content, the more austenite will be retained (Techniques for the measurement of retained austenite in quenched steels involve quantitative X-ray diffraction measurements and are capable of measuring retained austenite in amounts as small as 0.3 percent). The cooling rate below the martensite point also affects the amount of retained austenite. The lower this cooling rate, the more austenite will be retained in the hardened steel.
The presence of retained austenite in hardened steel is undesirable since it has a detrimental effect on its mechanical properties in majority of the cases. To reduce the retained austenite refrigeration treatment (also called sub-zero treatment and cold treatment) is applied.
In order to have more complete transformation of austenite into martensite, cooling must be continued to the Mf temperature.
The fully martensite steels have much less practical usefulness than the pearlite, bainitic, or ferrite types, or the cold worked and age hardenable alloys. Steels with martensite microstructures retain magnetization well and are ideal for permanent magnets.
Heat treatment magnets involve only a quenching operation. For structural and machine applications, hardened martensitic steels are tempered to develop ductility and toughness. The amount of tempering is a compromise between service requirements for ductility and the greater strength of martensite.
5. Critical Cooling Rate:
The minimum cooling rate at which all of the austenite is super-cooled to point Ms and is transformed into martensite is called the critical cooling rate. The magnitude of the critical cooling rate depends on the stability of the austenite. The higher its stability, the less the critical cooling rate will be.
The critical cooling rate may vary in wide range in accordance with the carbon content of a steel and size of austenite grains. Eutectoid steel has the lowest critical cooling rate. Most alloying elements increase the stability of austenite and this lowers the critical cooling rate.
6. Retained Austenite:
The degree of completeness of the transformation of austenite to martensite is not determined by cooling rate or holding time but by the temperature to which the steel is cooled. At any given temperature, quasi-equilibrium is reached when a definite proportion of martensite has formed.
Since the coefficient of thermal expansion of martensite is considerably greater than that of austenite, further cooling decreases the stress and “equilibrium” is upset. Additional austenite transforms to martensite until a new “equilibrium” is reached. With each decrease in temperature there is an incremental transformation of austenite to martensite until Mf is reached, i.e., the transformation is complete.
Austenite, unless extensively cold-worked, is relatively soft, ductile, and tough but has only moderately high strength. We might expect retained austenite in a hardened steel to reduce strength and hardness and increase plasticity and toughness. The expected effect on strength and hardness is found to some extent.
It is possible, with 30-40% retained austenite, to perform light cold-working operations on the quenched alloy, but smaller amounts of retained austenite contribute nothing to plasticity or toughness. In fact, smaller proportions often appear to make hardened steel more brittle than if it were 100% martensite.
Retained austenite transforms very slowly at room temperature even without mechanical deformation. This probably occurs by transformation to exceedingly fine bainite with a small increase in total volume. This change is unimportant for many applications but can give trouble in many precisely machined parts and is disastrous in precision gauges or test blocks.
Retained austenite is undesirable in many hardened steels, particularly tool steels, since retained austenite prevents attainment of the best possible combination of strength, hardness, toughness and dimensional stability.
Retained austenite can be eliminated by the following two practical methods:
1. Tempering.
2. Cold treatment (which simply requires cooling the steel below Mf).
7. Austenite Grain Size and Grain Size Control:
Concept:
Grain size normally refers to austenite grain size. Austenite grain size is not altered much by rate of cooling to room temperature.
Grain size is very important factor in relation to strength, usefulness and other physical properties of steel and it is also very important in developing fundamental theories of metallic behaviour.
Importance of Grain Size:
Fine Grains:
(i) Increase impact toughness;
(ii) Improve machining finishes;
(iii) Mitigate quenching cracks, distortion in quenching and surface decarburisation.
Coarse Grains:
(i) Improve rough machinability;
(ii) Raise hardenability, tensile strength as normalised and creep strength.
The austenite grain size is important in determining the hardening response of the steel.
Grain Size Measurement:
The grain size specified by the American Society for Testing and Materials (ASTM) is as under:
n = 2N-1 … (5.1)
where n = Number of grains per inch square at a magnification of 100 x.
N = ASTM grain size number.
I. Small grain size number I to 5 indicates the coarse grain size steel. ASTM number one is the coarsest grain size which represents about one grain per inch square at 100 x magnification. ASTM number 3 represents 4 grains per inch square at 100 x and so on.
II. Grain size numbers 6 and above indicate fine grained steel. ASTM grain size numbers with corresponding average number of grains and grain diameter are given in Table 5.1.
The following factors govern the grain size:
(i) Nature and amount of deoxidizers.
(ii) Composition of steel.
(iii) Metallic and non-metallic inclusions.
(iv) Mechanical working processes like rolling, forging etc.
(v) Heat treatment processes, working temperature and environment.
(vi) Grain size, time of limit of heating and cooling and tendency to grain growth. (The size of austenite grain increases with increase in temperature and time of heating. By minimising austenitizing time and temperature, small size of austenite grains can be obtained. Once a particular size is achieved during heating, it cannot be decreased by rapid cooling.)
Grain size of steel can be controlled by the addition of alloying elements, e.g., aluminium, boron, vanadium, titanium during manufacturing process.
Excess grain coarsening at high temperatures during heat treatment can be avoided by addition of titanium, boron, vanadium, molybdnum, tungsten. These are known as ‘grain growth inhibitors’. They prevent grain coarsening for forming stable carbides at high temperature.
ASTM has recommended the following methods for measuring grain size:
1. Comparison method.
2. Intercept (or Heyn) method.
3. Planimetric (or Jeffries) method.
1. Comparison Method:
I. This method is applicable for equiaxed grains. Hence the grain size of steel is reported by ASTM grain size number.
II. In this method the grain size is measured by comparison, under a microscope with a magnification of x 100 (after etching), with standard grain size ASTM charts. Grain sizes of steel are usually graded into eight classifications.
Steels with grain size numbers from 1 to 5 inclusive are coarse grained while numbers 6, 7, and 8 are fine-grained (Steels having ASTM grain size number greater than 8 are called ultra-grained steels). By trial and error a best match is determined and the grain size of the steel is noted from the index number of the machining chart.
Sometimes, mixed grain sizes are observed in steels. These are expressed by giving the estimated area percentages occupied by each of the ranges of sizes.
2. Intercept Method:
I. This method is particularly used when the grains are not equiaxed.
II. In this method a photo-micrograph is used. On any straight line, the length of line drawn in mm divided by the average number of grains intercepted by it gives the grain diameter (or size).
3. Planimetric Method:
I. This method is used for measuring the grain size of equiaxed grains, and in case of dispute the results of this method are preferred over the comparison method.
II. In planimetric method a photo-micrograph is taken and a circle or a rectangle of known area is drawn on it. Then number of grains within this area is counted and from this the number of grains per square mm is calculated.
8. Heat Treatment of Tool Steels:
The heat treatment of the various high-speed steels varies considerably and the manufacturer’s directions should generally be sought in each case.
There are, however, two important properties of high speed steel which are common. Firstly, the temperature at which the steel must be hardened, so as to give the maximum cutting efficiency, is higher than in case of carbon tool steel.
Thus, in case of plain carbon steels or steels containing small percentage of tungsten and chromium, the quenching temperatures are normal, i.e., usually from 700° to 800°C, according to the particular composition employed. In the case of high-speed steels it is necessary to heat these to a much higher temperature, namely a little short of fusion (from 1150° to 1350°C) to obtain the full advantages of these steels.
The second characteristic feature of high speed steel is its marked resistance to tempering. Unlike carbon steel which tempers at definite temperatures, with its hardness decreasing gradually as the tempering temperatures increase, from the initial reduction in hardness value at about 250°C; high speed steels do not lose their initial hardness even tempered, in some cases, at a temperature as high as 610°C. When the carbon content is high, the steel after tempering is actually harder than it is in the quenched condition.
It follows from the above fact that these high-speed steels can be used for tools that heat up by friction when machining to relatively high temperatures and that the tool will retain its temper indefinitely provided it is not heated above 580° to 610°C, the actual value depending upon condition.
As supplied, high speed steels are generally annealed and can be filed or machined without difficulty. In the case of small pieces for tool holders, however, the manufacturers generally supply the steel in the hardened state.
Although it is possible to render high-speed steel extremely hard by the process of quenching in oil or water from, say, 100°C, and thus to give it a high degree of hardness, the material would not possess the necessary resistance to tempering.
The correct methods of hardening high-speed steel are to some extent governed by the composition but it is a general rule that high speed steel should not be quenched in water; it should be cooled in oil or by means of an air blast.
The usual procedure with high speed steel is to preheat the part in a muffle (gas or electric-heated for preference) to a bright red heat (800°C to 870°C), after a certain short period of “soaking” to transfer to another final heating furnace. In this the temperature is raised quickly to about 1200°C to 1300°C, and the part quenched in oil or an air blast.
The time of “soaking” plays an important part in the process of hardening, for, if too long the steel will become over-hardened and brittle. If it has not been soaked long enough, it will be too soft from the Brinell hardness point of view.
When hardening cutting tools of high speed steel the nose only is heated rapidly to 1120°C to 1300°C after the preheating period at the lower temperature.
It is important to warm steel parts before they are placed in the preheating furnace, and then to heat them slowly until the proper temperature is reached. The reason for this is that high speed steel is denser and, therefore, requires a longer period of heating; if a cold bar of this steel is placed in a hot furnace, the outside would expand more quickly than the interior, causing severe internal stresses resulting in the condition known as a “clink”.
Although the bar to the eye appears as before, and exhibits no flow, yet when in use the tool may break, the fractured surface showing “the cup and egg” appearance associated with incorrect heating procedure.
After hardening the tool steel should be tempered at the temperature (580°C to 610°C) recommended for the particular composition.
9. Alloy Steels and their Heat Treatment:
1. Carbon-Manganese Spring Steels:
C = 0.45% to 0.65%
Si = 0.1 to 0.35%
Mn = 0.5 to 1.0%.
These steels are quenched and tempered to give a Brinell hardness of about 350. They are widely used for laminated springs for railway and general purposes.
2. Hyper-Eutectoid Spring Steels:
C = 0.9 to 1.2%
Si = 0.30% (max.)
Mn = 0.45% to 0.70%.
These steels are oil quenched and tempered at a low temperature. They are used for volute and helical springs.
3. High Carbon Tool Steel:
C = 0.90 to 1.40%.
These steels are heat treated according to the experience of the tool smith. As a rule they are water-quenched and tempered. They are used for general tool making, e.g., for shears, drills, chisels, taps, punches, pick axes, planning tools, razors etc.
4. Bearing Steels:
C = 0.90% to 1.20%
Si = 0.10 to 0.35%
Mn = 0.30 to 0.75%
Cr = 1.0 to 1.6%.
After machining in the annealed condition this steel is hardened in oil and tempered below 200°C to give a Brinell hardness of 800 to 880. It is used for the manufacture of ball bearings and ball races.
5. Silico-Manganese Spring Steels:
C = 0.33% to 0.6%
Si =1.5 to 0.2%
Mn = 0.6 to 1.0%.
These steels are hardened and tempered to give a Brinell hardness of about 450, and are used for the manufacture of railway and road springs generally.
6. Heat Resisting Steels:
Steels which must be resistant to creep at high temperatures must contain molybdenum. Silicon and chromium impart resistance to oxidation and scaling.
Steels which are satisfactory upto about 700°C operating temperature are:
C = 0.15% max.,
Si = 0.5 to 2.0%,
Mn = 0.5% max.,
Ni = none,
Cr = 1.0 to 6%,
Mo = 0.50%.
These are used in the as rolled or as forged condition, particularly for the valves of internal combustion engines. For higher temperatures upto 1000°C, steels containing upto 22% nickel and 26% chromium are used. These are stainless steel tyres, but their mechanical properties are not so good at high temperatures. If 20% molybdenum is added they have good resistance to creep upto 650°C.
7. Non-Magnetic Steels:
Some steels contain so much alloy that the austenite is stabilized at ordinary temperatures. Such steels are non-magnetic. The commonest non-magnetic steels containing 16 to 25% Cr and 8 to 22% Ni, and 13% Mn are wear resisting steels. If the latter is quenched from 1000°C, the austenite is retained. Cold work converts this to martensite. In service therefore, as wear resisting part, the outside becomes converted to intensely hard martensite while the inside remains as soft but very much tougher austenite.
10. Defects due to Heat Treatment of Steels:
1. Overheating.
2. Burning.
3. Decarburisation.
4. Excessive hardness of hot worked annealed steel.
5. Black fracture.
6. Quenching cracks.
7. Deformation and volume change after hardening.
8. Warping.
9. Insufficient hardness after quenching.
10. Soft spots.
11. Excessive hardness after tempering.
12. Insufficient hardness after tempering.
13. Erosion.
14. Corrosion.
11. Surface Finish after Heat Treatment:
Due to heat treatment certain defects like overheating, burning, decarburisation, quenching cracks, deformation, scaling, soft spots etc., may occur.
In order to remove these defects following finishing operations are carried out:
1. Acid pickling.
2. Sand blasting.
3. Degreasing and alkaline detergent cleaning.
4. Straightening.
1. Acid Pickling:
If the treatment is not performed in a controlled gas atmosphere oxide or scale layer is formed on the surface of the heat treated component.
Descaling can be performed by acid pickling in the following two way:
Direct Dissolution of a Component in an Acid Bath:
FeO + H2O= FeSO4 + H2O
Mechanical Circulation of Acid Solution:
Fe + H2SO4 = FeSO4 + H2O
a. 8 to 12% H2SO4 or 20% HCl is taken for the above purpose.
b. The acid bath is used at 50 to 70°C.
c. The components removed from the pickling bath are thoroughly cleaned with steam and then neutralised in a week alkaline bath for 5 to 10 minutes. Finally they are rinsed by warm water.
2. Sand Blasting:
a. The surface to be cleaned is exposed to a jet of compressed air carrying sand composed of dry, sharp, and quartz grain of 1 to 12 mm size. This process gives fine surface and is useful to large areas to be cleaned. However, operators need to take proper precautions since a good amount of dust is generated in this process.
b. In another method surface cleaning is done by centrifugal action of abrasive material like iron shots (shot blast) instead of sand blowing with pneumatic pressure. This method also produces cold casehardening of the surface, increasing the fatigue resistance of the articles.
3. Decreasing and Alkaline Detergent Cleaning:
a. Salts are deposited on the surfaces of the components when treated in salt baths. These are removed by soaking the component in boiling water.
b. Tools tempered and quenched in oil bath have a greasy material deposited on the surface. These are cleaned by 10 percent (by volume) soda ash solution at 80 to 90°C or 3 percent solution of caustic soda.
c. Both salts and oil deposits can be cleaned by immersing the components in a tank containing boiling data solution and then washing in water. The operation is known as vat or sak electing.
4. Straightening:
During heat treatment certain tools and machine parts get distorted or warped. In order to bring them to original shape, straightening is carried out.
Straightening of rolled stock is done by straighteners or hydraulic straightening process.