Hardening:
This is the process of in which the carbon containing or alloys steels is heated to the hardening temperature and then cooled rapidly to room temperature by quenching in a suitable quenching medium such as water, oil or a salt bath. The high hardness developed by the process is due to the phase transformation accompanying the rapid cooling. Rapid cooling results the transformation of austenite into non-equilibrium products at low temperatures. The product of low temperature transformation of austenite is Martensite, which is a hard micro constituent of steel.
Hardening temperature depends on chemical composition. For plain carbon steels it depends on the carbon content only. Hypo eutectoid steels are hated to about 30-50° C above the UCT, where as hyper eutectoid steels are heated about 30-50° C above the LCT. In hypo eutectoid steels ferrite and pearlite transforms to austenite at hardening temperature and this austenite transforms to martensite on rapid quenching from the hardening temperature. The presence of martensite accounts for the high hardness of the quenched steel. For hyper eutectoid steel the preferred hardening temperature is between LCT and UCT. There are two advantages for the hardening temperatures in this range. First one is the formation of cementite in the structure. Since cementite is harder than martensite, the formation of two-phase structure will result in a higher hardness and wear resistance than that obtained by martensite alone. Second one is that at this particular temperature range fine martensite is attained in the final structure.
Successful hardening requires two condition to be met.
1. Formation of homogenous Austenite
2. Rapid cooling of steel in a suitable medium.
Hardening temperature:
Medium carbon steel : 820- 870° C
High carbon steel : 790- 830° C
Alloy steel : 1000-1300° C
Soaking time is generally determined as one hour for every 25mm thickness.
The main purpose of hardening is to develop high hardness. This enables steel to cut other material. Wear resistance also is improved by this process. That is the sole purpose of this process for components like gears, shafts and bearings. Tensile strength and yield strength are improved considerably when structural steels are hardened. Yield strength is more important since that determines the safe limit of maximum permissible stresses. Since yield strength is the limit of stress to which elasticity is maintained, an increase in it will result in the increase of stress levels, which a material can withstand with out loss of elasticity. This is a very important property of steels.
Internal stresses are developed in hardened steel due to the rapid cooling from the heat treatment temperature. Hence the hardened parts are rarely used as-hardened condition since in hardened condition steels are brittle. Hardening is there fore followed by another process known as tempering. This reduces internal stresses and brittleness and makes the hardened steel relatively stable. Hardening followed by tempering results in improved wear resistance and optimum combination of strength and ductility and enhanced elastic characteristics.
The properties of hardened steel developed by hardening depend on various factors.
ü Chemical composition,
ü Size and shape of steel part,
ü Hardening cycle (heating and cooling rates),
ü Homogeneity and grain size of austenite,
ü Quenching media,
ü Surface condition of the steel part.
Hardening methods:
Method of hardening mainly depends on quenching procedures. Various methods of quenching are:
a) Conventional or direct quenching:
This is the simplest form of quenching and is extensively used hardening method. The process is cooling the steel part from the hardening temperature in the quenching medium and allowed to cool till the temperature of the quenching bath. In addition to the development of internal stresses the hardened steel parts develop a tendency towards distortion and cracking due to drastic cooling rates involved. Cooling rates cane be controlled by selecting a less severe quenching medium, oil in place of water for example. But only smaller sections can be quenched using a quenchant having lesser quenching power. Hence this method is adopted only for simpler shaped and smaller sized parts.
b) Quenching in stages in sequence in different media:
This method consists of quenching the steel part from hardening temperature in a quenching bath maintained at a predetermined temperature. The medium used is water in general. The part is then transferred quickly to a milder quenching medium where it is cooled to the room temperature. Oil and air are preferred for the second stage of quenching. Since the cooling rates are not so severe, the amount of internal stresses developed are considerably less than that developed in direct quenching.
c) Spray quenching:
It is a specific hardening method in which steel part is rapidly cooled from hardening temperature by spraying the quenchant continuously. The rate of heat extraction from the steel is much higher compared to direct quenching process. It is because a continuous stream of quenchant is always in contact with steel surface. So the possibility of formation of a vapor blanket is reduced. Water is the most commonly used quenching media in this method.
d) Hardening with self-tempering:
Tempering in order to achieve an optimum combination of hardness, strength and toughness always follows hardening. In general hardened steel possess uniform mechanical properties through out the section. But for certain applications this condition is not desirable. For example, parts, which are to be subjected to impact loads, must have a soft and tough core and a hardened and toughened case. Such condition can be achieved by a process known as hardening with self-tempering.
In this process the steel part is quenched from hardening temperature and after some time it is withdrawn from the quenching bath. Hence the steel is not allowed to cool down completely. So a considerable amount of heat is retained in the core portion. Now the steel part is cooled in a milder quenching media such as air or oil. The first quenching results in formation of Martensite. Cooling during second quenching result in homogenization of temperature. The core is now cooled at a much slower rate and hence the transformation is from austenite to pearlite. The Martensite already formed will be tempered by itself with out any further tempering treatment. Thus the process results in the formation of a soft and tough core with a hardened and toughened case.
The process is usually applied for chisels, hammers, shafts, gears etc. the close control of time in the first quench is the only problem associated with this operation. It is difficult to know the amount of heat content left after the first quench. An estimation of the temperature can be done by observing the temper color of the surface.
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