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ABSTRACT
•
This paper is aimed at enumerating the importance of heat treatment and
techniques and determination of hardness of a test piece of component.
INTRODUCTION
• Heat treatment is an operation or combination of operations involving heating and cooling timed and applied to a metal or alloy in the solid state in a way that will produce desired properties. The physical and mechanical properties of metal can be altered so much by heat treatment.
Essentially, a heat treatment cycle consists of three main stages:
• Heating the steel uniformly to some predetermined temperature.
• Holding at an appropriate temperature for the required time.
• Cooling the material at a rate which will produce in it the desired type of structure.
SOME IMPORTANT TERMS
• Annealing: the steady heating of a meta at a certain temperature above the recrystallization phase followed by a gradual cooling process.
• Austenite phase: the phase at which solid steel recrystallizes and has a face centered cubic crystal structure. Austenite steel holds a greater amount of dissolved carbon and exhibits increased formability.
• Bainite: A combination of ferrite and cementite in ferrous metals that is harder than pearlite. Bainite contains needle like grain structures, and it requires an initial rapid cooling followed by gradual cooling.
• Body centered cubic: the crystal structure that contains an atom in the center and one atom in each corner of a cube. Ferrite has a BCC crystal structure.
• Body centered tetragonal: a body centered cubic crystal structure that has been distorted by the presence of extra atoms of carbon. Martensite has a BCT crystal structure.
• Cementite: A compound of iron and carbon that is very hard and brittle. The presence of cementite hardens steel.
ANNEALING
Annealing is one of the most important heat treating operation applied to steel.
It is employed to:
• Obtain a homogeneous structure
• Reduce hardness.
• Remove residual stresses.
• Improve toughness.
• Restore ductility.
• Reduce segregation.
• Alter the mechanical, electrical or magnetic properties of the material.
•
Refine
grain sizes.
Types of heat treatment operations
• Full annealing:
This process consists in heating the steel to the proper temperature and then cooling slowly through the transformation range, preferably in the furnace or in any good heat insulating material.
• Process annealing:
This heat treatment is used in the sheet and wire industries. Cold working often severely strain hardened a metal and to restore its ductility either for service or permit further prolong processing without danger of fracture, process annealing is used.
• Stress relieve annealing:
This process, sometimes called subcritical annealing is useful in removing residual stresses due to heavy machining or other cold working processes. Parts are heated below A1, lower critical point, 550-650 degree and then held for a period of time and cool slowly. The temperature varies with the condition of the component.
• Spheroidization:
This method is employed when high carbon steel is to be prepared for machining or forming. The aim is to produce a structure in which all commentates is in the form of well dispersed spheroid or globules.
• Hardening:
Under slow and moderate cooling rates, the carbon atoms are able to diffuse out of the austenite structure. The iron atoms then move slightly to become BCC. This gamma to alpha transformation takes place by a process of nucleation and growth is time dependent.
Hardening of steel can either be by
• Quench hardening
• Case hardening.
• Quench hardening:
Steel hardening consists of two principal operations, i.e. heating and quenching. Work pieces should be heated to prescribe temp. gradually and uniformly to avoid internal stresses development and excessive slow heating should not be allowed to avoid decarburisation surface of the steel.
• Case hardening:
This involves the packing the low carbon iron within a substance high in carbon, then heating this pack to encourage carbon migration into the surface of the iron. This forms a thin surface layer of high carbon steel, with the carbon content gradually decreasing deeper from the surface.
The resulting product combines much of the toughness of low carbon steel with the hardness and wear resistance of the outer high carbon steel.
• Tempering:
Martensite, although very hard, may also be brittle and hardened steel requires a further heat treatment, known as tempering, before it can be put into service.
When a martensitic structure is heated it becomes possible for the carbon trapped in the supersaturate solid solution to diffuse through the lattice and precipitate from the solution in the form of particles of carbide.
• Austempering:
In this process, the steel is heated to just above the upper critical temperature and then quenched into a molten bath kept at a temperature in the range 250-500 degree.
The steel is kept in the molten bath until the austenite has completely transformed to bainite, after which it is cooled to room temperature at any convenient rate.
The mechanical properties of medium carbon steels in the austempered condition are inferior to those in the fully hardened and tempered condition. Also, austempering is a slow process and the quenching medium must be held at a constant elevated temperature
However, there is no need tempering after the quenching, and austempering is useful in treating components of complicated sections which might distort or crack if directly hardened.
• Martempering:
In this process, the steel is heated to a temperature just about the upper critical point and then quenched into a molten bath kept at a temperature just above the Ms temp. The steel is kept in the molten bath only enough to allow its temp. to become uniform throughout its section.
Hardenability:
The ease with which steel may be quench hardened. The depth of hardening obtained by quenching a given steel bar depends on:
• The composition of the steel.
• The cross-sectional area.
• The quenching medium.
• The quenching technique.
• The influence of cross –sectional area is referred to as the mass effect of heat treatment. The cooling rate across a section becomes slower from the outside to the centre, even with drastic quenching, because of the poor thermal conductivity of steel.
• The quenching mediums are water solution, tap water, fused salts, soluble oil, oil and air.
Quenching techniques:
Hardening of tool steels falls into various categories. Some examples of tool steels requiring different quenching techniques are as follows,
• Water hardening tool steels:
They are covered by AS1239W grades and these are shallow hardening. These steels contain around 1% carbon and may have small additions of vanadium for grain refining and toughness
Depth of hardening is around 3mm when quenched from the normal hardening temperature of 780 degree and will increase to around 6mm by increasing the quenching temperature to 870 degree.
• Applications:
These steels have many uses particularly in wood working tools.
• Tempering:
150-250 degree to achieve the desired hardness.
• Oil hardening tool steel:
An example of oil is AS1239 grade S1A-5 which is hardened from 800-840 degree by quenching into oil.
• Applications:
This steel is normally used for heavier section punches than the W series tool steels and possesses good dimensional stability.
• Applications:
•
This steel is normally used for heavier section punches than the W series
tool steels and possesses good dimensional stability.
• Heat treatment:
Pre heating at 650-700 degree is recommended to allow the tool to equalise at a subcritical temperature prior to rising to the austenitisation temperature. The procedure helps to maintain dimensional stability.
• Tempering:
It is recommended in the range of 170-200 degree which will give hardness in excess of 60HRC. Tempering in the range of 250-350 degree can result in a reduction of impact strength.
• Air hardening tool steel:
Examples of tool steel are grades W and D of AS1239.
• Heat treatment:
They require adequate preheat at 780 degree prior to austenitising and hardening is generally affected by still air cooling. Larger sections may be used to be cooled in an air blast to achieve maximum hardness.
• Tempering:
These steels should be tempered when cooled to a hand warm condition and multiple tempering is sometimes necessary to achieve complete transformation and maximum toughness commensurate with hardness.
• Air hardening hot work steels of H13 type:
These steels may be air hardened in sections up to 60mm. Above this thickness, whilst full hardening will occur, carbide precipitation at grain boundaries will lead to poor tool life and low impact strength.
• Heat treatment:
The preferred procedure is to quench into a fluidised bed furnace or salt bath held just above the Ms point. This allows the cooling rate to miss the critical areas of the S curve where carbide precipitation occurs.
HARDENABILITY TEST
Test for hardenability using the jominy end quench test.
This test is helpful in assessing the hardenability of steel. It involves heating a test piece of about 2.5 cm diameter and 10cm long uniformly to the proper austenitizing temp.; just above the upper critical temperature. It is then removed from the furnace and placed on a fixture where a jet of water impinges on the bottom face of the specimen. The size of the orifice, the distance from the orifice to the bottom of the specimen, and the temperature and circulation of the water are all standardized, so that every specimen quenched in this fixture receives the same rate of cooling.
CONCLUSION
• Components which are required for service are able to meet up with the design specification of the engineer due to proper heat treatment operation.
REFERENCES
Steel
metallurgy for the non-metallurgist By John D. Verhoeven - ASM
International 2007 Page 99-105
The
Medieval Sword in the Modern World By Michael 'Tinker' Pearce - 2007 Page
39
Tool steels
By George Adam Roberts, George Krauss, Richard Kennedy, Richard L. Kennedy -
ASM International 1998 Page 2
Roberts-Austen
By Sir William Chandler Roberts-Austen, Sydney W. Smith - Charles Griffin &
Co. 1914 Page 155-156
Steel castings handbook By Malcolm Blair, Thomas L. Stevens - Steel
Founders' Society of America and ASM International Page 24-9
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