Selection of Heat Treatment Parameters for a Cast Allvac 718Plus(®) Alloy

Abstract

The first selection of best set of parameters for heat treating a cast Allvac 718Plus alloy is presented in this paper. The high interest from several aeroengine manufacturers in this new alloy with improved temperature capabilities over widely used In718 has driven the development of the alloy also in the direction of making available a cast form of it. A first step in this direction was to use some available material (billet originally intended to be used for forging other parts) as remelting stock for producing a couple of cast rings and use them as test material for assessment of different sets of heat treatment parameters. In particular, the application of homogenization before HIP and the solution temperature after HIP are discussed. Use of hardness measurements and optical microscopy were made to help in the assessment of the effect of the homogenization cycle, and also the effect of the different solution temperatures. Other techniques such as Scanning Electron Microscope and X-Ray Diffraction were tried for identification and quantification of hardening phases. Introduction Alloy Allvac 718Plus has been marketed by its inventor, Dr W.D. Cao, and producer company, Allvac, and several articles have been published or fostered from the company to show the advantages over the widely used In718 [1, 2, 3]. The new alloy claims to offer better mechanical properties at those temperatures where In718 has already exhausted its applicability, in the 650oC range. Allvac 718Plus shows improved metallurgical stability at even higher temperatures, up to 700oC. In this upper range, no degradation of mechanical properties has been noticed for the new alloy [4]. The alloy was originally developed to be used in its wrought form, but requests from aeroengine companies have driven Allvac to perform activities in order to tune-up the chemical composition of the alloy in its cast form and also trim the heat treatment of this application. This paper shows one of the first efforts to select the heat treatment to be applied to this kind of material. ITP and CEIT were partners in the European VITAL R&D program, together with Volvo Aero Corporation, where several activities were performed to evaluate the new alloy. One of these was the production of castings in different geometries, such as separately cast test bars, stair shaped plates, hollow rings and a demonstration casting to finally assess castability [5]. The present study consisted of heat treatment trials and mechanical property assessments of cast rings. This paper covers the heat treatment studies.