Layers of thermobimetallic materials (TBM) are soldered together along the entire contact plane. Components with a higher TCLR value are called an active layer, while components with a lower value are called a passive layer. When heated due to different lengthening of the layers, thermal bimetals bend, which makes it possible to use them as sensitive elements of measuring instruments and protection components in automatic switches, fuses and relays.
To achieve the greatest bending effect in the operating temperature range, material pairs with the maximum difference in temperature coefficients are used. Due to this, TBM bends along the arc of a circle, causing high internal stress, to relieve which, during production, thermobimetals are annealed in a reducing atmosphere.
Precision steels for the manufacture of TMB are selected in accordance with the purpose and requirements for the manufactured elements. Iron-nickel alloys are mainly used with the addition of various alloying materials.
Alloys based on iron (Fe), nickel (Ni) and chromium (Cr) are widely used as an active layer, including 19NH, 20NG, 24NH, 27M, 28NHTU and 75GND steels.
The main steel grade used for the passive layer of thermal bimetals is 36N (invar), but at temperatures above the Curie point, it loses its properties. Therefore, for elements whose temperature range exceeds 200°C, alloys with a nickel content above 40-42% are chosen, including 42N, 45NH, 45NHTU, 46N, 50N and 52NTYU steels.
The main property of thermal bimetals is temperature sensitivity — the ability to bend along an arc of a circle when the operating temperature rises. This parameter directly depends on the difference in TKLR alloys that make up the thermobimetallic material. Thermal sensitivity is characterized by a change in the curvature of the material when the temperature rises or falls by 1°C. The maximum heating temperature, electrical resistivity, strength and elastic properties are also important for TBM.
In accordance with their physical and mechanical characteristics, thermal metals are divided into six groups:
All types of thermobimetallic materials are characterized by a linear dependence of deformation on temperature, as well as by the absence of mechanical hysteresis: after the thermal effect ceases, the metal returns to its original state.
TBM is produced in the form of cold-rolled belts and strips with a width of 10 to 250 mm with a thickness of 0.1 to 2.5 mm. Permissible deviations range from 0.015 mm (for strips 0.1-0.2 mm thick) to 0.12 mm (with a thickness of 2.0-2.5 mm) and are standardized by GOST 503-71. The length of strips and scraps is allowed from 200 to 1,300 mm; this parameter is not regulated for roll materials.
