Influence of the Modes of Heat Treatment on the Durability of Springs Made of 65G Steel
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INFLUENCE OF THE MODES OF HEAT TREATMENT ON THE DURABILITY OF SPRINGS MADE OF 65G STEEL О. P. Ostash,1, 2 R. V. Chepil’,1 L. І. Markashova,3 V. І. Hrybovs’ka,1 V. V. Kulyk,4 and О. М. Berdnikova3 We study the fatigue life of springs made of manganese steel (0.66% С and 1.02% Mn) after four modes of heat treatment leading to the formation of troostite, martensitic–bainitic, and bainitic–martensitic structures. Springs are studied under the conditions of strain-controlled cyclic loading similar to the operating conditions of products intended for special applications. It is shown that the optimal durability of springs is guaranteed by the mixed structure of lower bainite and martensite obtained after isothermal quenching and special tempering performed by interrupting the process of cooling after quenching in the interval between the points corresponding to the onset and end of the martensitic transformation. The durability of springs is connected with the quantitative parameters of phase composition and dislocation structure and with the micromechanism of fracture of spring steels. Keywords: spring steel, mode of heat treatment, phase composition, dislocation structure, durability.
Elastic elements (springs, carriage springs, rail braces, etc.) are mainly made of 55S2- and 60S2-type steels with a carbon content of 0.55–0.60% and a silicon content of up to 2% [1−3]. As a noticeable shortcoming of these elements, we can mention the elevated susceptibility of their surface layers to decarburization in the stages of formation and heat treatment of the products [4]. This is why 65G steel with a carbon content of 0.64–0.66% and a manganese content of up to 1% proves to be a promising substitute of these materials because it is not only less susceptible to decarburization but also better quenched and, for the same hardness as 60S2А steel, exhibits better characteristics of strength, elasticity, and durability; moreover, it is also lest costly (by 20–25%) [4, 5]. This is especially important for springs operating under the conditions of high dynamic cyclic loads (in particular, in the products for military applications) made of steels of elevated strength, which is achieved by additional alloying with high-cost elements. However, we can mention a disadvantage of 65G steel connected with its susceptibility to brittle fracture under large deformations [5], which can be unacceptable for springs guaranteeing large displacements. This is caused by its low relaxation ability [5], which is obviously connected with the overstressing of local volumes after heat treatment. Springs are usually quenched in oil and subjected to medium-temperature tempering in order to get a troostite structure of steel. At the same time, it is known that, for steels with good quenching ability and, in particular, for 65G steel, the procedure of isothermal quenching (with formation of the bainitic–martensitic or bainitic structures in which thin interlayers of residual austenite enriched with carbon are distributed between the bainitic lathes) seems to be
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