1. To study various kinds of thermal treatment of carbon steels and purpose of these kinds of treatment.
2. To understand the general ideas of hardening thermal treatment of steels (quenching and tempering).
3. To know how to make the correct choice the conditions of steel heating for quenching according to constitutional diagram “iron-carbon”. To study methods and technology of steels quenching, their advantages and drawbacks.
4. To determine experimentally the influence of carbon contents and rate of continuous cooling from austenite area on steels hardening capacity.
5. To investigate experimentally the influence of tempering at various temperatures on structure and properties of quenched steel.
6. To study with the help of metallurgical microscope the steels microstructure after various kinds of thermal treatment.
The theory of thermal treatment of steels is based on four main transformations (figure 1):
1. The transformation of perlite to austenite at heating higher than points A1 or A3.
2. The transformation of austenite to perlite at cooling lower than points A1, A3 (isothermal decomposition of austenite and decomposition of austenite at continuous cooling);
3. Transformation austenite to martensite at quenching of steels;
4. Transformation martensite to ferrite-carbide mixture at heating (tempering of steels).
To full or partial transferring of steels to structural-equilibrium condition various kinds of annealing are applied. At full annealing of hypoeutectoid steels are heated up above point AC3=A3+(30-50°C) with following slow cooling (in furnace).
Steel structure after annealing: perlite + ferrite, perlite or perlite + cementite. Steels strength and hardness are reduced and plasticity is increased as a result of annealing. If steel is heated higher than A3 point and is cooled on the air that steel structure will be not quite equilibrium.
Figure 1. An interval of hardening temperatures for carbon steels.
Such thermal treatment is named as normalization (e - condition, Figure 2, e).
Figure 2. The diagram of decomposition isothermal of austenite with modes of cooling at various methods of quenching: a- continuous quenching; b- interrupted quenching; c- stepped quenching; d- isothermal quenching; e- normalization.
Carbon steels become more strengthly after normalization, than after an annealing. Their plasticity after normalization is lower that maximum possible plasticity. At production of semifinished articles (rods, angle bars, U-beams, sheets, bands etc.) by hot treatment methods after plastic working their cooling is carried out on the air. Structure and properties of such semifinished articles correspond to the normalized condition.
Main hardening types of thermal treatment of carbon steels are - quenching and tempering.
Quenching is steel heating up to temperature of austenite condition, holding at this temperature with consequent cooling with the rate which is more than critical rate. The quenching purpose is to make steel to have non-equilibrium condition. The result of quenching is the transition of austenite to martensite.
Martensite is oversaturated (non-equilibrium) stressed solid solution of carbon in a-iron. The oversaturation level is determined by the contents of carbon in steel. Hardening capacity of steels depends on the contents of carbon in steel. Quench-hardening steels are medium-carbon steels and high-carbon steels (its contain 0,25 % of carbon and more).