Layers of thermobimetallic materials (TBM) are soldered together over the entire contact plane. Components with a larger TCLE value are called the active layer; those with a smaller one — the passive. On heating, due to different elongation of the layers, thermobimetals bend, allowing them to be used as sensing elements of measuring instruments and protection components in circuit breakers, fuses, and relays.
To achieve the greatest bending effect in the working temperature range, pairs of materials with the maximum difference in temperature coefficients are used. Because of this TBM bends along a circular arc, causing high internal stress; to relieve it, thermobimetals are annealed in a reducing atmosphere during production.
Precision steels for making TBM are chosen according to the purpose and requirements for the elements manufactured. Iron-nickel alloys with various alloying additions are predominantly used.
Alloys based on iron (Fe), nickel (Ni), and chromium (Cr) are widely used as the active layer, including steel grades 19NKh, 20NG, 24NKh, 27M, 28NKhTYu, and 75GND.
The main steel grade used for the passive layer of thermobimetals is 36N (invar), but above the Curie point it loses its properties. Therefore for elements whose temperature range exceeds 200°C, alloys with nickel content above 40–42% are chosen, including steel grades 42N, 45NKh, 45NKhTYu, 46N, 50N, and 52NTYu.
The main property of thermobimetals is temperature sensitivity — the ability to bend along a circular arc when working temperature rises. This parameter depends directly on the TCLE difference of the alloys in the thermobimetallic material. Thermal sensitivity is characterized by the change in material curvature when temperature rises or falls by 1 K. Also important for TBM are maximum heating temperature, electrical resistivity, and strength and elastic properties.
According to physical and mechanical characteristics, thermobimetals are divided into six groups:
All types of thermobimetallic materials are characterized by a linear dependence of deformation on temperature and by absence of mechanical hysteresis: after temperature exposure ceases the metal returns to its initial state.
TBM are produced as cold-rolled strips and bands 10 to 250 mm wide with thickness from 0.1 to 2.5 mm. Allowable deviations range from 0.015 mm (for bands 0.1–0.2 mm thick) to 0.12 mm (at thickness 2.0–2.5 mm) and are regulated by GOST 503-71. Length of bands and cut pieces is allowed from 200 to 1,300 mm; for coiled materials this parameter is not regulated.
