Steel Hardening by Quenching and Tempering

 Steel Hardening by Quenching and Tempering

Hardening is carried out by quenching steel, which consists of cooling it rapidly from a temperature above the transformation temperature (A?).  The quenching is necessary to suppress the normal breakdown of austenite into ferrite and cementite (pearlite), and to cause a partial decomposition at such a low temperature to produce the new phase called martensite. To achieve this, steel requires a critical cooling velocity, which is greatly reduced by the presence of alloying elements. In such case hardening of steel occurs with mild quenching.

Martensite is a supersaturated metastable phase and has body centered tetragonal lattice (bct) instead of bcc. After steel is quenched, it is usually very hard and strong but brittle. Martensite looks needle like under microscope due to its fine lamellar structure.

Steel is quenched in water or brine for the most rapid cooling, in oil for some alloy steels, and in air for certain higher alloy steels. Water is one of the most efficient quenching media where maximum hardness is required, but it is liable to cause distortion and cracking of the work piece. Where hardness can be sacrificed, whale, cotton seed and mineral oils are used. These tend to oxidize and form sludge with consequent lowering of efficiency. The quenching velocity of oil is much less than water. To minimize distortion, long cylindrical objects should be quenched vertically, flat sections edgeways and thick sections should enter the bath first. To prevent steam bubbles forming soft spots, a water quenching bath should be agitated.

Steel can be hardened by the simple expedient of heating to above the A? transformation temperature, holding long enough to insure the attainment of uniform temperature and solution of carbon in the austenite, and then cooling rapidly (quenching). Complete hardening depends on cooling so rapidly that the austenite, which otherwise decompose through cooling through the A? temperature, is maintained to relatively low temperatures. When this is accomplished, the austenite transforms to martensite on cooling through the Ms – Mf range. Rapid cooling is necessary only to the extent of lowering the temperature of the steel to well below the nose of the S-curve. Once this has been accomplished, slow cooling from then on, either in oil or in air is beneficial in avoiding distortion and cracking. Special treatments, such as time quenching and martempering, are designed to bring about these conditions. As martensite is quite brittle, steel is barely used in as quenched condition, that is, without tempering.

The maximum hardness that can be obtained in completely hardened low alloy and plain carbon steels depends primarily on the carbon content of the steel.

The mass of steel work piece has a large influence on the formation of martensite because of the non uniform rate of abstraction of heat from the steel work piece.  Heat is always abstracted from the surface layers at a faster rate than from the interior. In a given cooling medium the cooling rate of both the surface and interior decreases as the dimensions of the steel work piece increase and the possibility of exceeding the critical cooling rate becomes less. To overcome this, the steel work piece may be quenched in a medium having a very high rate of heat abstraction such as iced brine, but, even so, many steels have a physical restriction on the maximum size amenable to complete hardening regardless of the quenching medium.

The martensite of quenched steel is exceedingly brittle and highly stressed. Consequently cracking and distortion of the object are liable to occur after quenching. Retained austenite is unstable and as it changes dimensions may alter. It is therefore necessary to warm the steel below the critical range in order to relieve stresses and to allow the arrested reaction of cementite precipitation to take place. This is known as tempering.

Tempering during manufacture im­parts shock resistance with only a slight decrease in hard­ness. Tempering is accomplished by heating a quenched part to some point below the transformation temperature, and holding it at this temperature for an hour or more, depending on its size.

The microstructural changes accompanying tempering include loss of acicular martensite pattern and the precipitation of tiny carbide particles. This microstructure is referred to as tempered martensite.

Tempering is the process of reheating hardened martensitic steels to some temperature below the lower critical temperature A1. The rate of cooling is immaterial except for some steels which are susceptible to temper brittleness. As the tempering is increased, the martensite of hardened steel passes through stages of tempered martensite and is gradually changed into a structure consisting of spheroids or cementite in a matrix of ferrite, formerly termed as sorbite. These changes are accompanied by a decreasing hardness and increasing toughness. The tempering temperature depends upon the desired properties and the purpose for which the steel is to be used. If considerable hardness is necessary then the tempering temperature needs to be low. On the contrary if considerable toughness is needed then the tempering temperature is to be high.

Proper tempering of hardened steels requires a certain amount of time. At any selected tempering temperature, the hardness drops rapidly at first, gradually decreasing more slowly as the time prolonged. Short tempering periods are generally undesirable and are to be avoided. Good practice requires at least 30 minutes to two hours at tempering temperature for any hardened steel.

There is necessity of tempering steel after hardening. If fully hardened steel is allowed to cool to room temperature during hardening there is a danger that the steel may crack. Carbon steels and most of the low alloy steels need to be tempered as soon as they cool to a temperature of around 40 -50 deg C. Steels are not to be tempered before they cool to this temperature range since in some steels the Mf temperature is quite low and untransformed austenite may be present. Part or all of the residual austenite transforms to martensite on cooling from the tempering temperature so that the final structure will consist of both tempered and untempered martensite. The brittle untempered martensite, together with the internal stresses caused by its formation, can easily cause failure of the heat treated steel. When there is a possibility of existing of such condition, then a second tempering treatment is given in order to temper the fresh martensite formed on cooling after the initial tempering treatment.

The hardening of steel by quenching and tempering is shown in Fig 1.

Hardening by quenching and tempering

Fig 1 Hardening of steel by quenching and tempering

Role of alloying elements in quenching

Since the work pieces treated are often relatively big and since the alloy­ing elements have the general effect of lowering of the temperature range at which martensite is formed, the ther­mal and transformational stresses set up during quenching tend to be greater in the alloy steel work pieces than those encountered in quenching of the smaller work pieces of plain carbon steels. In general, the greater stresses result in distortion and risk of crack­ing.

Alloying elements, however, have two functions that tend to offset these disadvantages. First and probably most important is the capacity to permit use of lower carbon content for a given application. The decrease in hardenability accompanying the decrease in carbon content may be readily offset by the hardenability effect of the added alloying elements and the lower carbon steel will exhibit a much lower suscep­tibility to quench cracking. This lower susceptibility results from greater plas­ticity of the low carbon martensite and from the generally higher temperature range at which martensite is formed in the lower carbon materials. Quench cracking is seldom encountered in steels containing 0.25 % carbon or less, and the susceptibility to cracking in­creases progressively with increasing carbon content.

The second function of the alloying elements in quenching is to permit slower rates of cooling for a given sec­tion, because of increased hardenabil­ity, thereby generally decreasing the thermal gradient and, in turn, the cooling stress. It should be noted, however, that this is not altogether advanta­geous, since the direction, as well as the magnitude, of the stress existing after the quench is important in relation to cracking.

To prevent cracking, surface stresses after quenching should be either com­pressive or at a relatively low tensile level. In general, the use of a less drastic quench suited to the hardenability of the steel will result in lower distortion and greater freedom from cracking.