Size stability and strength of multiphase metastable materials

Abstract

The changes in the clearances (interferences) caused by the differences in the temperature coefficients of linear expansion (TCLE) of joined materials are considered. In operation and long-term storage of precision parts made of high-strength steels these changes can exceed the admissible ranges due to the instability of the phase composition even when the parameters of the heat-treatment process correspond to the existing technological recommendations. A study of the nature of the size instability of high-strength corrosion-resistam steel 25Khl7N2B-Sh allowed the authors to recommend regimes for its final heat treatment that provide maximum size stability at the specified level of mechanical properties.In recent years the requirements on the precision of the dimensions of precision machines and devices have become as strict as the requirements on plane-parallel final measures of length. It is natural that designers of precision equipment have a fight to expect that the dimensions of parts manufactured with a high accuracy will be maintained during the whole of the storage time and the operating period. However, in actual practice the dimensions of parts can change even in the absence of corrosion, creep, and wear, causing changes in the clearances and interferences in joints.The causes of the size changes under the effect of stresses a < a0. 2 in metals include inelasticity of various origins, microcreep, relaxation of residual stresses, and phase instability [1,2]. Metallic materials used in a high-strength state are commonly characterized by phase and structural instability that is a potential source of instability of size. The situation is aggravated substantially if the part functions or is stored under conditions of thermal cycling, because elevation or reduction of the temperature t i relative to the normal value t20 can promote phase transformations and changes in the phase composition and, ultimately, in the dimensions and properties of the part.In the general case the clearances (interferences) M between joined parts, which are fixed at normal temperature, become a function of the deviation of the temperature from the normal value (At = t i --120 ), the thermal coefficient of linear expansion ct of the materials of the parts, and the rated size 10 of the joint under conditions of thermal cycling, i.e., A/=f(At, ct i, 10 ). It is commonly accepted that the changes in the clearances (interferences) are mainly caused by the difTula State University, Tula, Russia. ferences in the temperatures of the joined parts or a substantial difference in the TCLE of the materials of which they are made (for example, steel and bronze [3]). However, a simulation of the changes in the clearances for various fits [4] shows that the design clearance (interference) in precision joints can be disturbed even for the same temperature of parts produced from the same material but having different structures or phase compositions.To provide the design reliability of the parts the specification envisages an admissible change of ...