The following points highlight the seven main processes used for heat treatment of steel. The processes are:- 1. Annealing 2. Normalizing 3. Hardening 4. Tempering 5. Case Hardening 6. Induction Hardening 7. Flame Hardening.
Heat treatment is a combination of heating and cooling operation and applied to a metal or alloy in the solid state in a way that will produce desired properties. All basic heat treating processes for steel involve the transformation or decomposition of austenite.
The nature and appearance of these transformation products determine the physical and mechanical properties of any given steel. The first step in heat treatment of steel is to heat the material to some temperature in or above the critical range in order to form austenite.
In most cases, the rate of heating to the desired temperature is less important than other factors in heat-treating cycle. Highly stressed materials produced by cold work should be heated more slowly than stress free materials to avoid distortion.
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The difference in temperature rise within thick and thin section of articles of variable cross-section should be considered and whenever possible, provision should be made for slowing the heating of the thinner section to minimize thermal stress and dispersion.
Some of the processes of heat treatment are described below:
Process # 1. Annealing:
Annealing is a heat treatment process generally used to:
(i) Relieve internal stresses
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(ii) Increase, softness, ductility and toughness
(iii) Produce a specific microstructure.
It consists of three stages:
(i) Heating to a desired temperature
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(ii) Holding or ‘Soaking’ at that temperature
(iii) Cooling usually to room temperature.
It is employed for low and medium carbon steels that will be experiencing plastic deformation during forming operation.
It refines the structure.
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It is carried out a 15° C to 40° C above upper critical temperature for hypo-eutectoid steels and above lower critical temperature for hyper-eutectoid steels.
Holding time is generally 3-4 min. per mm thickness of the largest sections.
After heating furnace is turned off and both alloy and furnace cool together to room temperature at the same rate.
The microstructure product of full annealing is “coarse pearlite” in addition to any pro-eutectoid phase (α or Fe3C).
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Grains are small and uniform.
Process # 2. Normalizing:
Steels that have plastically deformed for e.g.: a rolling operation, consists of grains of pearlite and a pro-eutectoid phase, which are irregularly shaped and relatively large and vary in size.
Normalizing is used to refine these grains (i.e. to decreases average grain size) and produce more uniform and desirable size distribution.
The microstructure formed is fine grained pearlite with a pro-eutectoid phase.
It is accomplished by heating alloy (55° C to 85° C) above the upper critical temperature After sufficient time has been given to alloy to completely transform to austenite (Austenizing), the treatment is terminated by cooling in air. It raises hardness and strength of alloy, but decreases ductility of steels.
Normalizing is done to increase the strength of medium carbon steel.
Improve machinability of low carbon steel.
Improve structure of weld.
Mechanical properties is better than annealing and process is of short duration because of cooling in air.
The selection process of annealing, normalizing and spheroidizing normalizing and spheroidizing depend on carbon content.
Steel produced in Normalizing is less ductile than annealed steel having the same composition.
It helps in achieving desired result in mechanical and electrical properties.
Process # 3. Hardening:
“Hardenability” is the ability of alloy to get hardened by the formation of martensite microstructure after suitable heat treatment.
Hardening of steels involves three process:
(i) Heating the object to a temperature above the critical point.
(ii) Holding the object at this temperature for a definite period.
(iii) Quenching in a suitable medium.
Hypo-eutectoid steels are heated 30 – 50° C above the Upper critical temperature while hyper-eutectoid steels are heated above Lower Critical Temperature (L.C.T).
Upon heat transformation to austenite takes place (Austenizing). Steels are than cooled at a rate faster than critical cooling rate that enables austenite to transform directly to martensite. Hardening must be followed by tempering to improve mechanical properties of steel.
Hardening Depends Upon:
1. Adequate Carbon Content:
In order to produces structures like martensite at least 0.5% C is required.
Carbon increases hardness upto 1%.
2. Heating Rate and Time:
Heating rate depends upon the composition of steel, its structure, residual stresses, form & size of the part to be hardened.
If heating rate is too high there will be temp-gradient between surface & core of the part.
Best way is too heat the part & hold it for a sufficient time so that entire volume attain the same temperature. Heating time of tool & high steels is 50% more than carbon steels.
3. Quenching Medium:
It has to provide cooling rates greater than Critical Cooling rate to avoid decomposition of austenite to pearlite or bainite.
Most widely used quenching media are water, brine, oil air, molten salt etc.
Water Quenching:
Water & aqueous solution are most widely used as quenching media for hardening carbon & low alloy steels.
But water quenching tends to form a vapour blanket which reduces the cooling rate and there is a sharp reduction in cooling capacity at higher temperature.
It also leads to high stresses leading to distortion and even quenching cracks.
Adding Nacl, H2SO4 soda to water substantially increases its cooling capacity and provides more uniform cooling. Maximum-cooling rate is obtain when 10-15% salt is added.
Mineral oil Quenching:
Suitable for quenching alloy steels in which austenite is highly stable and critical cooling rate is therefore low.
Advantages:
i. Due to its high boiling point the cooling rate in the martensite range is comparatively low. This prevents quenching defects.
ii. Compared to water, oil cools steel more uniformly over the entire temperature range which reduces quenching stresses.
Disadvantages:
i. Low cooling rates as compared to water.
ii. High inflammability of oil.
iii. It has a tendency to thicken in the course of time, which reduces its quenching capacity.
Quenching Rate:
It depends upon quenching medium. The higher the quenching rate the more is the temperature gradient between the core and the furnace.
Size and Shape:
Long articles of cylindrical & other cross section should be quenched with their axis perpendicular to both surface.
Long articles, should be immersed about an edge and the direction of movement during cooling should coincide with direction of immersion.
Heavy article should be held stationary and the liquid should be agitated.
Martempering (or Stepped Quenching):
Martempering is performed in two steps, first the steel quenched in water to a temperature 300-400° C and then quickly transferred to a less temperature medium like oil where it are held until it is completely cooled.
As this is done in step so it is called as stepped quenching, after heating steel to hardening temperature. It is quenched in a medium to a temperature slightly above Ms (240° C). The Holding time must be enough so that part reached the temp of cooling medium.
The holding time must be sufficient to bring entire volume to a uniform temp, but should riot be long enough to cause decomposition by austenite to Bainite. This treatment give a micro structure of martensite & retained austenite.
Advantages:
1. Less volume change due to presence of retained Austenite
2. Less distortion since transformation occurs simultaneously in the entire volume.
3. Less chance of quenching cracks.
Disadvantages:
It can be applied to sections of 5-8 mm thickness only.
Austempering:
Longer holding times as compared to martempering in hot bath. Austenite transforms to Bainite. Here molten salts & alkalis are used as quenching media. Temperature is maintain between 150°C-450°C.
Here, sample has quenched below the nose of TTT diagram but above the martensite start line then it hold for longer period.
Advantages:
1. No martensite formation thus no brittleness.
2. No quenching cracks, distortion.
3. Improved ductility.
4. Impact strength & toughness are increased.
Disadvantages:
It can be applied to small sections only as big sections cannot be cooled rapidly enough to avoid pearlite formation.
Process # 4. Tempering:
It is done- (i) To relive residual stress (ii) To improve ductility (iii) Toughness is increased.
Tempering is carried out in the temperature Range of 150°C-650°C.
It is heating of steel below lower critical temperature (hardened steel), holding it for some time and then slowly cooling.
It is the final operation in the heat treatment.
It is done to improve the mechanical properties of martensite which become very brittle during quenching.
Thus hardening should be followed by tempering.
High temperature tempering causes reduction in hardness and increase in toughness and by slow cooling residual stress generated during hardening will be decreased.
i. High Temperature Tempering (500-650°C):
Resulting structure is sorbate, residual stress completely vanished.
ii. Medium Temperature Tempering (350-500°C):
Resulting in Troostite formation. This process increases endurance limit. This process is used for making “spring steels and die steels”. After tempering work is cooled to water, leads to enhanced endurance strength.
iii. Low Temperature Tempering (150°C-350°C):
Holding time is 1-3 hrs. This apart from above properties of tempering, provides additional “wear resistance”. Used for measuring & cutting tools bending.
Process # 5. Case Hardening:
Here hard surfaces are produced with relatively soft core. Hard surface has good wear resistance while soft core has good toughness. Hard surface is called case.
Type of Case Hardening:
i. Carburizing:
It is applied to low carbon steels having upto 0.18%C (Temp range is between (870- 950°C) and result in formation of Austenite phase, which has maximum solubility of carbon.
Methods are as follow:
a. Pack Carburizing:
Piece is surrounded by a carburizing mixture and packed in a steel box. It is heated to around (870-950°C).
b. Carburizing Mixture:
50% charcoal + 20% BaCO3 + 5% CaCO3 + 5-12% Na2CO3.
i. BaCO3 acts as an energizers and increases action of carbon on low carbon steel.
ii. Heating time is 6-8 hours and carbon diffuses into layer of part.
Fe + 2CO → Fe3C + CO2
iii. Fe3C is austenite which dissolves carbon.
Disadvantages:
High labour cost, time consuming, difficult to quench directly from carburizing temperature.
c. Gas Carburizing:
Here work part is treated in an atmosphere of gases containing carbon and hydrocarbon gases such as CH4, butane etc.
Part is heated to temperature of 950°C for 3-12 hrs (holding time).
2CO → CO2 + C (atomic)
CH4 → 2H2 + C (atomic)
This atomic carbon diffuses into austenite and produced hard surfaces.
Advantages:
High quality, quick process, low labour cost, process is clear, production cost is low, more flexibility of operation
d. Liquid Carburizing:
Workpiece is heated to 950°C and dipped in a molten salt bath containing 20% NaCN, which provides Carbon & Nitrogen. At this temperature Carbon and small amount of Nitrogen diffuses into surface. The only disadvantage is that cost of carburizing salt is very high.
In carburizing operations carbon can be penetrated upto the depth of 0.3mm with hardness obtained around (55-65 HRC).
Applications:
Gears, cams, shafts, bearings, piston pins, sprockets.
2. Nitriding:
Here nitrogen content of the surface is increased. It is done by heating steel in the atmosphere of NH3 gas. Piece to be nitrided are placed in an air tight container, workpiecs is heated to a temperature in range (500-600°C), at which NH3 dissociates into-
2NH3 → 2N + 3H2
This atomic nitrogen diffuses to the surface making case hardened. This process make the hardest case on steels. Maximum thickness of case is around (0 – 0.5mm). After nitriding the work in cooled in the furnace is the spring of ammonia. Holding time is 30-40 hrs
Advantages (Hardest Case Obtained):
i. High wear resistance endurance limit and resistance to corrosion.
ii. No heat treatment is required after nitriding.
Applications:
Gears, shaft, valves, boring bars, fuel injection pump parts. Generally alloy steels, HSS, stainless steels are heat treated.
Disadvantages:
(i) Case produced is brittle
(ii) Costly compared to carburizing
3. Cyaniding:
Here work part is immersed in molten salt bath containing sodium cyanide (NaCN), which is heated to 820-860°C. This is usually followed by water quenching.
The cyanide both consists of:
20-30% NaCN + 25-50% NaCI + 25-50% Na2CO3
Time required is 30-90 minutes depending upon depth of case required, which is in the range 0.15-0.5mm.
The following reaction takes place:
The atomic C & N diffuse into steel surface. The work can be directly quenched as soon as it is taken from molten bath. Cases obtained by cyaniding are more wear resistance and have high endurance compared to carburizing.
Applications:
Small shafts, worms, gears, nuts, springs etc.
4. Carbonitriding (Gas Cyaniding):
A mixture of NH3 and a hydro carbon gas is used. The work is heated to 850°C for 2-10hrs. This is followed by quenching and low temperature tempering. Carbon diffuses at a rate higher than nitrogen. Troosite is the product of tempering here. It is better process compared to carburizing and cyaniding. In this process carbide and nitride lower the stability of austenite leads to formation of troosite which reduce the endurance limit ductility and toughness of steel. This is used for steel having percentage of carbon in the range of medium carbon steel.
(i) This process involves lower temperature
(ii) Higher corrosion and wear resistances.
(iii) Clean operation
(iv) Toxic salts are not required
(v) Less time consume
(vi) Work can be machined
(vii) Nitrogen and Carbon contents can be controlled
Process # 6. Induction Hardening:
Workpieces heated in an induction furnace surrounded by copper coils which are water cooled. High frequency AC current is passed through Cu coils and thus alternate magnetic field set up which induce eddy currents on the surfaces. Heat is generated on the surface by eddy currents.
The surface of work piece is heated in austenitic range then quenched immediately to form martensite. The structure of core remains unchanged because it is not affected by heat. The composition should contain 0.4-0.5% C or sufficient alloying element as Cr, Ni or Mo approximately 90% of the heat generated in the work layer of thickness X.
Where, X = 5000 √ρ/µf
Where, f = frequency, µ = magnetic permeability, ρ = electrical resistivity.
Heating rate is 300°/sec for hypo-eutectoid steel and subsequently heating time is very small (2-50 sec.) quenching temperature depend upon rate of heating if the rate is 500°C/sec, it is 1000°C & if it is 250°C then 900°C.
Induction hardening is done in three ways:
(a) Whole workpieces is heated at one go and quenched. This process is used for hardening small shafts and tools.
(b) Sections of work are heated and quenched consequently. Used for hardening crank shaft journals, gear teeths and cams of camshaft.
(c) Work is made to travel w.r.t stationary inductor and spray quenching is done. Used for hardening long shafts and axles.
Advantages:
Process is very quick and highly productive:
i. Scale is not formed.
ii. Process can be automated and depth of hardness can be controlled easily.
iii. Distortion is reduced.
iv. Both internal and external surfaces can be hardened.
v. Maximum production method.
Disadvantages:
Special fixtures are required for holding workpieces.
Application:
Used in hardening surfaces of camshaft, crank shaft, gears, automobile components, splines, spindles, brake drum etc.
Process # 7. Flame Hardening:
Process consists of heating the surface of high carbon steels by a high temperature gas flame at 2400-3500°C, followed by immediate cooling is air or water. Heat is supplied by oxy-acetylene torch. The heat is supplied so quickly to the surface that the core remains unaffected. The thickness of hardened layer is 2-4mm and its structure is martensite.
Advantages:
i. Practically no distortion of workpiece because only small sections of workpiece is heated.
ii. Due to high heating rate workpiece surface remains clean.
iii. Easy Automation.
iv. It is more efficient for large works as compared to induction hardening.
v. Very economical for large works.
Disadvantages:
i. This sections may get distorted extensively.
ii. High heat may causes cracks.