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Basic principles of heat treatment
Steels are iron-carbon alloys whose characteristics can be influenced by changing the chemical composition (C-content and by adding alloying elements) as well as through heat treatment. This means that there is a large number of constructional steels which fullfil all requirements. In order to understand the various heat treatments, it is necessary to be familiar with the processes which take place in constructional steel during heating and cooling.
The constitutional diagram iron-carbon (Figure 1) forms the intial basis for heat treatment. It shows the microstructural constituents and amounts present in a condtion of equilibrium. It can be seen from the constitutional diagram that austenite and, in hypereutectoid steels, austenite and cementite are present at temperatures above the GSK line. Very slow cooling leads to conditions of equilibrium at room temperature and causes the austenite to convert into other types of microstructure.
Figure 1 (To enlarge, please click on the picture in question )
Constitutional diagram iron-carbon
Steels with less than 0.8 % carbon content segregate ferrite out of the austenite during cooling and the remaining austenite disintegrates at under 723°C into perlite (figure 2 - ferrite and perlite). With a carbon content of 0.8 %, perlite (Figure 3) only forms as a mixture of ferrite and cementite. In steels with over 0.8 % carbon content, perlite and cementite (Figure 4) form, whereby the secondary cementite is segregated out at the grain boundaries.
By adding alloying elements, the transformation temperatures and lines of equilibrium can be changed and the formation of carbides influenced.
The constitutional diagram does not give us any information about the microstructure of steels which cool rapidly from the austenite area. Austenite is converted at higher cooling speeds, not according to equilibrium conditions and so other types of microstructure - sorbite, troostite, bainite, martensite - are formed which cannot be displayed in the FeC diagram. The types of transformation are described in time-temperature transformation diagram; these form an additional basis for heat treatment.
Figure 2
Ferrite and perlit
(hypoeutectoid steel) |
Figure 3
Perlite (eutectoid steel) |
Figure 4
Perlite and cementite
(hypereutechtoid steel) |
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In the time-temperature transformation diagram, the ordinate gives the temperature and the absciss the time on a logarithmic scale. The beginning and end of austenite transformation into the individual microstructural constituents - ferrite, perlite, bainite and martensite - are shown as curves. Depending on whether the temperature was maintained at constant level in the calculation used in the diagram or whether it was continually changed during transformation, a differentiaton is made between an isothermal time-temperature transformation diagram and the time-temperature transformation diagram for continual cooling.
The isothermal time-temperature transformation diagram (Figure 5) shows the time which is required at each temperature from the beginning of austenite transformation until its completion and which temperature range has the greatest transformation tendency. Furthermore, the cooling speed which has to be reached to achieve complete formation of martensite has to be estimated. For this, a level below the line Ms must be acchieved when cooling down from the austenitisation temperature without there having been any transformation in the microstructure.
The formation of martensite is spontaneous and is not influenced by the temperature holding time. The proportion of martensite is increased by cooling down further to room temperature (characteristic line for 50 and 90% proportion of martensite). The hardness of the microstructural constituents formed is assigned in each case to the "transformation " curve and is identified in circles with information regarding the HRC or HV values. The time-temperature transformation diagram gives the transformation of the following microstructural constituents: below Ms martensite, to the right of the "end of transformation" curve at higher temperatures perlite, at lower temperatures bainite. The proportion of transformation microstructure is identified by numbers given as percentages.
Figure 5 (To enlarge, please click on the picture in question )
Isothermal time-temperature diagram
The time-temperature diagram for continuous cooling (Figure 6) describes all the transformation processes which run when various cooling speeds are used. Therefore a number of cooling curves are shown based on the austenitisation temperature. The cooling speeds are marked on the 500° isotherm in °C/min. and, in the case of rapid cooling, the cooling parameters are noted. The cooling parameters give the cooling time of between 800 and 500°C by multiplying by 100; a parameter of 0,1 says that cooling from 800 to 500°C takes place within 10 secs.
For each cooling curve, the individual transformation stages of the microstructural consituents are noted as percentages. The hardness values of the microstructure present at room temperature are shown on the absciss axis as HV.
Each time-temperature diagram is set for a melt of a quality which is named and can therefore only be utillzed taking a certain scattering into account which comes about as a result of the different chemical compositions. The austenitisation temperature and the grain size also have an influence on how the transformation processes take place.
Figure 6 (To enlarge, please click on the picture in question )
Time-temperature diagram with continuous cooling
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