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

Lecture No. 19
Chemical-thermal treatment of working surfaces of parts.
Thermomechanical treatment (TMT)
Thermomechanical treatment provides a significant increase in the mechanical properties of steel parts as a result of exposure to plastic deformation at austenite temperatures, followed by hardening and low-temperature tempering. At the same time, strength increases and a sufficiently high ductility is maintained. For practice, it is also important that hardening during TMT can be combined with the shape change of workpieces.
There are several options for volumetric or only superficial DM. At low-temperature TMT, deformation occurs in the region of supercooled austenite. Low-temperature TMT consists of deformation, quenching and low-temperature tempering, as a result of which increased brittleness is eliminated. Most often, this method is used to harden parts made of alloyed steels, which have increased stability of supercooled austenite. Since in this case the degree of deformation reaches (50...90)%, and recrystallization does not occur, there is a greater hardening than with high-temperature thermomechanical processing, however, plasticity is somewhat lower.
The most promising is high-temperature thermomechanical treatment (HTMT). It is performed by heating the metal of the workpiece to the temperature of the existence of austenite, after which it is plastically deformed and immediately hardened so that the deformed austenite does not recrystallize. After that, low-temperature tempering of HTMT is performed, which consists in solid solution homogenization at a temperature of 1200C and plastic deformation by (25...30)% after preliminary cooling to (1100...1000)C. This allows you to significantly increase the strength of parts even at temperatures up to 900C.
The strengthening effect of HTMT is explained by the improvement of the structure and physical and mechanical characteristics of the metal, the formation of a dislocation texture. The degree of hardening depends both on the method and mode of the HTMT itself, and on its rational combination with PPD or chemical-thermal hardening operations. So, during ordinary hardening, tensile stresses arise on the surface of the samples, the magnitude of which depends on the chemical composition of the steel, for example, for samples from articles 60C2A, 40X and 45, they amount to 500, respectively; 80 and 100 MPa. Bulk hardening with HFC heating during water cooling creates an even higher level of axial residual tensile stresses on the surface of the samples (700 ... 750 MPa).
Carrying out HTMT significantly changes the nature of residual stresses. In this case, both the magnitude and sign of the residual stresses depend on the degree of compression prior to quenching. The nature of the dependences is basically the same for all the studied steels. HTMT in almost all cases leads to a change in the sign of residual stresses on the surface. This applies to both axial and tangential residual stresses.