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The effect of carbon content on thermal properties of cast superinvar alloys subjected to two-stage annealing is studied. It is shown that carbon improves the casting properties of the alloys but raises the temperature coefficient of linear expansion (CTLE). Two-stage high-temperature annealing makes it possible to remove carbon from the solid solution and to transfer it into graphite, which is accompanied by decrease in the CTLE.Key words: superinvar, solid solution, homogenization, graphite, lattice parameter, dendritic segregation, residual stresses. INTRODUCTIONInvar and superinvar alloys belong to a group with special thermal properties [1]. One of the main characteristics of such alloys is the temperature coefficient of linear expansion (CTLE). The alloys have a low and constant CTLE in a specific temperature range known as the invar interval. They are applied primarily in instrument making for fabricating equipment components that should have permanent sizes. By the data available [2] the alloys in question are also used in the oil and gas industry.A typical structure responsible for invar properties of alloys is an iron-nickel ferromagnetic solid solution with an fcc lattice. The invar interval is determined by point M s on the side of low nickel concentrations and by the Curie point on the side of elevated nickel concentrations. A classical invar alloy contains 36% Ni and iron as the remainder. It has been suggested in the middle 1930s (primarily by Japanese researchers [1]) that a part of nickel atoms in an invar should be replaced by cobalt atoms. Such alloys are known as superinvars. The relevant literature contains much information on the thermal properties of deformable invar alloys [1]. Today it has become necessary to produce complex-shape parts from invar and superinvar alloys, which are hard and sometimes even impossible to form by pressure treatment.These are articles for the rocket and space industry, optoelectronics, tracker stations, etc. It is known [3] that alloys with a structure of solid solution have low casting properties and acquire flaws like pores and discontinuities during casting.Processes for casting invar parts with carbon additives are developed today in different countries (Japan, USA, Russia). The action of carbon on the properties of invar and superinvar alloys is ambiguous. A low content of carbon promotes growth in the strength properties of invar and superinvar alloys and widens the invar interval [1]. In the presence 0.5 wt.% and higher concentrations of carbon the casting properties of invars are improved substantially making it possible to obtain quality castings. However, considerable carbon concentrations in invars raise the CTLE, which is not desirable. For this reason, cast articles from carbon invar and superinvar alloys should be annealed for removing carbon from the solid solution and forming graphite. Graphite also raises the CTLE of invar and superinvar alloys. However, the volume fraction of graphite is relatively low and its effect is much lower than that o...

The application of hardening heat treatment process at high temperatures (1100-1170 °C) for high-chromium steels of martensitic-carbide class 95Kh18 and Kh12MFL has been studied. Metallic substrate consisted of high-carbon martensite and residual metastable austenite with some traces of carbide has been obtained. Experiments have shown the resulting structure gains high frictional hardening capacity upon the application of heat. Sufficient amount of cooling martensite can be traced in the analyzed steel after high-temperature quenching (cooling up to the temperature of-70°С). Being combined with residual metastable austenite, it provides the increase of abrasive wear resistance by 25% compared with high temperature annealing. The influence of tempering temperature on hardness and abrasive wear resistance of analyzed steels 95Kh18 and Kh12MFL has also been determined.

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