The most extensively used method is conventional hardening for 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:
1. Quenching in two media.
2. Hardening with self-tempering.
3. Stepped quenching or martempering.
5. Isothermal quenching or austempering.
Articles hardened by this method are first quenched in water to a temperature from 300° to 400°C and then quickly transferred to a less intensive quenching medium (for example oil or air) where they are held until they are completely cooled. The purpose of the transfer to the second quenching is to reduce internal stresses associated with the austenite-to-martensite transformation.
It is not advisable to quench first in water and then in oil as this may lead to partial decomposition of the austenite in its zone of the least stability (500° to 600°C) and to the development of high residual stresses due to rapid cooling in martensite transformation range.
Quenching in two media is widely employed in the heat treatment of carbon steel tools (taps, dies, milling cutters etc.) of a shape unfavourable as regards cracking and warping.
Here the article is held in the quenching medium until it is completely cooled but is withdrawn to retain a certain amount of heat in core which accounts for the tempering (self-tempering). The moment when the quenching must be interrupted may be established by experiment. Frequently, more heat is retained in the core than is required for tempering and, when the tempering temperature is reached, the article is reimmersed in the quenching liquid.
This hardening is applied for chisels, sledge hammers, hand hammers, centre punches, and other tools that require a high surface hardness in conjunction with tough core.
After heating the steel to the hardening temperature, it is quenched in a medium having a temperature, from 150° to 300°C. The article is held until it reaches the temperature of medium and then it is cooled further to room temperature in air and sometimes in oil.
The holding time in the quenching bath should be sufficient to enable a uniform temperature to be reached throughout the cross-section but long enough to cause austenitic decomposition. Austenite is transformed into martensite during the subsequent period of cooling to room temperature. This treatment will provide a structure of martensite and retained austenite in the hardened steel.
Martempering has the following advantages over conventional quenching:
(i) Less volume changes occur due to the presence of a large amount of retained austenite and possibility of self-tempering of the martensite.
(ii) Less warping since the transformations occur simultaneously in all parts of the article. (iii) Less danger of quenching cracks appearing in the article.
On the other hand, the extremely low stability of austenite in this range from 500 to 600°C requires a cooling rate of 200 to 500°C per second in this range to obtain super-cooling. At the same time, cooling in hot media is much slower than in water or oil at room temperature.
Therefore, austenite in carbon steel can be cooled through the zone from 600° to 500°C, without decomposition, only in thin articles (upto 5.8 mm in thickness). Such articles are expediently hardened by this method. Alloy steel articles hardened by this method, may be considerably thicker.
It is performed in the same manner principally as martempering but with a longer holding time at hot both temperature (above the martensite point) to ensure a sufficiently complete austenite decomposition (usually with a circular troostite or bainite).
Experience has shown that austempering of many grades of steel provides a substantial increase in structural strength, i.e., strengths of complex specimens. In comparison with conventional hardening followed by tempering at 250° to 400°C, austempering reduces notch sensitivity and sensitivity to eccentric loading and increases the ductility in the notch by 1.5 to 2 times.
It must be noted however, that hardening with quenching in a hot medium is not suitable for all grades of steel and for articles of all sizes. Improper procedure may substantially reduce the mechanical properties.
Molten salts are usually used as a medium in martempering and austempering. The lower the temperature of the salt bath, the higher the cooling rate it provides. Since cooling in molten salt is achieved only by conduction, then cooling capacity is increased to a great extent by agitation.
Steel is oxidized when it is heated in chlorides. The thin film of chlorides, covering the article, protects it against oxidation while it is being transferred to the quenching bath. At the moment of immersion into the molten caustic alkali, the film breaks off (or is dissolved) and bares the metal surface. Contact with caustic alkali, however does not oxidize steel parts to any appreciable extent.
Austempering process is being commercially used for thin steel sections to obtain products free from cracks and with good impact resistance.
The resultant microstructure of a fully hardened steel should consist of martensite. In practice, it is very difficult to have a completely martensitic structure by hardening treatment. Some amount of austenite is generally present in the hardened steel. This austenite existing along with martensite is referred to as retained austenite.
The presence of retained austenite greatly reduces mechanical properties and such steels do not develop maximum hardness even after cooling at rates higher than the critical cooling rate. The amount of retained austenite depends largely on the chemical composition of sted. For plain carbon steels, the amount of retained austenite increases with the rise in carbon contents.
The problem of retained austenite is more complex in alloy steels. Most of the alloying elements increase the content of retained austenite.
In hardened steels containing retained austenite, the strength can be improved by a process known as sub-zero treatment or cold treatment. Retained austenite is converted into martensite by this treatment. This conversion of retained austenite into martensite results in increased hardness, wear resistance and dimensional stability of steel.
a. The process consists of cooling steel to sub-zero temperature which should be lower than Mf temperature of the steel. Mf temperature for most steels lie between -30°C and -70°C. During the process, considerable amount of internal stresses are developed in the steel, and hence tempering is done immediately after the treatment. This treatment also helps to temper mastensite which is formed by decomposition of retained austenite during sub-zero treatment.
b. Sub-zero treatment must be performed first after the hardening treatment. Mechanical refrigeration units, dry ice, and some liquified gases such as liquid nitrogen can be used for cooling steels to sub-zero temperature.
c. This treatment is employed for- High carbon and high alloy steels used for making tools, bearings, measuring gauges and components requiring high impact and fatigue strength coupled with dimensional stability-Case hardened steels.
The special heat treatment given to medium carbon, high carbon and low alloy steel wire rod is called patenting.
The process consist of heating the material well above the austenising temperature to ensure formation of homogeneous austenite. After soaking for sufficient time at this temperature, the steel is quenched in a bath maintained at a constant temperature. For a given steel, the quenching bath temperature is kept in the vicinity of the nose of TTT curve.
This results in transformation of austenite to fine pearlite. Once the transformation is complete, the steel is cooled either in air or by spraying water. Lead bath or salt baths are commonly used for quenching the steel.
The process in mainly used for wires, ropes and springs.
Patenting heat treatment is not employed for low carbon steels since these can be drawn heavily in as annealed condition.