O’zbekistоn respublikasi оliy va o’rta maxsus ta`lim vazirligi

Lecture №20
Methods of chemical treatment and coating of rubbing surfaces of parts.
20.1 . Thermal hardening of friction surfaces . There are three main types of heat treatment: annealing, hardening and improvement.
Annealing . Diffusion annealing aims to equalize the inhomogeneities of the chemical composition of the alloy, especially segregation, by means of long-term heating. Heating is carried out, as a rule, to high temperatures (from 1100 to 1300C) in the region of the γ-solid solution. By means of diffusion annealing, the differences in the content of alloying elements due to intracrystalline segregation are primarily achieved.
Annealing with phase recrystallization (normalization) aims to create a fine-grained structure different from the original one by means of a double γ-α transformation. In shaped casting, the coarse-grained cast structure, which causes low mechanical properties, is eliminated. Differences in structure due to differences in deformation conditions and uneven cooling are eliminated in rolled and forged blanks. The structure after recrystallization has the finer the grain, the faster the heating and cooling are carried out, the closer the heating temperature is to the AC 3 point, and the shorter the exposure at this temperature.
Annealing to relieve stresses arising during welding and cold deformation is carried out in the temperature range of 450–650C, but not higher than the tempering temperature during the previous improvement. After annealing , it is advisable to carry out slow cooling to minimize new stresses.
Hardening . Quenching is the process of cooling the workpiece from the temperature region of the solid solution at such a rate that the transformation in the pearlite and intermediate regions is suppressed and martensite is formed. Steel goes into the state of the greatest possible hardness. There are: normal hardening used in the processing of medium and high carbon steels, and hardening after chemical-thermal treatment (carburization of high-temperature cyanidation) used for steels with a relatively low carbon content (see section "Cementation").
During hardening, significant macroscopic, microscopic and submicroscopic stresses (of the 1st, 2nd and 3rd kind) arise, due to the uneven plastic deformations during cooling or heating of the workpiece (strain stresses); volume changes that occur during the transformation (phase stresses); distortion of the atomic lattice during martensitic (or similar) transformations (structural stresses). Stresses of various kinds are in a complex relationship, which ultimately determines the resistance to fatigue and wear. Hardening cracks and warping of workpieces can be a consequence of the stressed state.
In some cases, it is advisable to strengthen only the surface layers. Then only this surface layer is heated to the hardening temperature, and the core remains unhardened. The most common methods of surface hardening are gas-flame and especially electric induction (HFC). By adjusting the speed of the inductor, the supplied thermal energy and the cooling rate, they change the depth and quality of the hardened layer. Since energy is consumed only for heating thin layers of the workpiece, surface hardening is more economical than bulk hardening.
The method of plasma hardening meets modern requirements. Out-of-drain installation for surface hardening of large-module gear wheels with a capacity of 75 running m/h (wheel diameter up to 1600 mm, module 9 ... 22). Processing is carried out with the help of two plasmatrons (power source of the VPR-602 type), which move along the wheel tooth (heating rate 500 ... 600C / s). Low-temperature plasma is used as a coolant. Cooling is carried out by water, which is fed through a sprayer moving after the plasma torches. Protection by inert gas excludes oxidation and decarburization of a surface.
To give the hardening structures greater plasticity, to relieve residual stresses, the workpieces are subjected to tempering operations. Tempering consists in reheating the steel to a temperature between room temperature and AC 1. During tempering, structural changes take place, which, due to lack of time, could not occur at the same temperature during the cooling process during the hardening operation. Sometimes multiple vacations are required.