Influence of heat treatment on structure and properties of the alloy KhN73MBTYu (�I 698)

Abstract

We study the influence of heat-treatment temperature atut various aging regimes on the mechanical properties aim toughness of the forging alloy KhN73MBTYu, which is used for the fabrication of gas turbine disks, at room temperature alwl in the range 500-850°C.The alloy KhN73MBTYu is a modification of the alloy KhN77TYuR (Table I), further alloyed with Mo and Nb --elements that improve the thermal stability of the solid solution and the 3" phase and bind the carbon into carbide phases. The increased complication of the chemical composition leads to increase of the mechanical and high-temperature-strength properties of the alloy, but makes it difficult to forge. This means that the low ductility of the alloy and the inadequate power of the existing equipment may not provide in a single heating in the fabrication of large-scale intermediate products a degree of deformation that is uniform in all sections and sufficiently high. One of the techniques for improving the ductility of the highly alloyed nickel alloys and increasing their forgeability is the use of remelting processes in their production.The primary technique used in producing the alloy KhN73MBTYu is open-arc melting (OAM) + vacuum-arc remelting (VAR). A competing process in the production of this alloy is electroslag remelting (ESR), increasing the uniformity and dispersivity of the cast structure, which leads above all to improvement of the ductility, the isotropicity of the mechanical properties, and the fatigue resistance of the metal. In the present work we studied an alloy obtained by the VAR method.The primary normaetallic inclusions in the alloy obtained by VAR are titanium nitrides (volume fraction -0.060%) and oxides (mass fraction -0.0030%). The latter are grains of aluminum-magnesium spinel and corundum. In the electroslag remelted metal the amount, dimensions, and composition of the inclusions depend on the slag that is used in remelting. It has been established [I] that with the use of magnesian slag the inclusion content is minimal (similar to that of the OAM + VAR metal), and the inclusions are fine and enriched with magnesium.Strengthening of the alloy KhN73MBTYu takes place as the result of segregation of the 3" phase --the intermetallic compound Ni 3 (Ti, A1, Nb), containing (at. %): 73.8 Ni, 2.4 Cr, 0.7 Mo, 3.2 Nb, 9.6 Ti, 9.6 AI, 0.I Fe, 0.6 Si. This phase is in equilibrium with the Ni-base solid solution, in which the following elements are dissolved (at. %): 20