Determining the thermal diffusivity coefficient of parts made from powdered metals

Abstract

where Ais the mathematical expectation of the reliability index; A~, Au are the lower and upper limits of the unilateral confidence interval.The values of R and Q were taken in accordance with the tabular data in [I]. The results~of calculation for N are given in Table 2. As is seen, for compensators Nos. 3 and 4, 6 max > 6 = 0.2, i.e., the confidence level will be less than the given y = 0.9 for observing the condition 6max ~ 0. At this time, a large number of the components used in engneering equipment are manufactured using the technology of powder metallurgy wherein the powdered metals are atomized with water (PMAW) [i]. One of the most important stages in the processing of components using PMAW' is heating of the billets to temperatures higher than i000~ before pressing.Heating of the billets is usually carried out in muffle or induction electric furnaces.However, when this is done, one does not obtain a uniform heating of the billet over its entire volume.The temperatures which arise in the billet can lead to cracking or complete failure of the part.The application of a pseudo-liquified, dispersed layer may help to avoid this problem and simultaneously increase the heating rate [2]. High values of the thermal diffusivity coefficient between the pseudo-liquified layer and the body which is immersed in it, coupled with an intense mixing of the dispersed material, will eliminate the possibility of localized overheating.The choice of an optimum heating regime by one or another means, and also calculations of the length of this regime required in the design of the equipment to be made, cannot be done without knowing the thermal diffusivity coefficient of the article undergoing heat treatment.In this study a method is presented for experimentally determining the thermal diffusivity coefficient of billets which have been pressed from iron powder with a zinc stearate additive and a porosity of i0-12%. This method is Nased on solutions of the differential equation describing heat conductivity in a body of classical form [3].Under linear conditions, for a body in the shape of an infinite platewhere T(T) and T O are the mean and initial temperature of the part, respectively; Tc is the temperature of the media; Bi = ~R/k is theBio criteria; R is half the plate thickness; k and are the coefficients of thermal conductivity and emissivity, respectively; Fo = aT/R 2 is the Fourier criteria; a is the coefficient of thermal diffusivity; r is the time; and ~n is the root of the characteristic equation cot ~n = Dn/Bi.As was shown in [4], by using the first component of the series in Eq. (i), a linear approximation can be used in calculating the heating rate of a body. The error which arises when the remaining components of the series are ignored can be determined in the following way: