Physical and chemical mechanisms of lubricant removal during stage I of the sintering process

by Gateaud, Arnaud .

The present study focuses on the physical and chemical mechanisms of lubricant removal during the first step of the sintering process during powder metallurgy (P/M) processing of ferrous systems. Previous works on the kinetics of delubrication made it possible to develop an empirical model which accounts for the typical weight loss profile observed upon heating of green compacts. It has been established that the rate at which the parts are heated dictates the overall process kinetics, and fitting curve methods yield two parameters which contain the corresponding information: (i) TMAX is the temperature of 50% lubricant removal, and (ii) b is representative of the slope of the curve during weight loss stage. Phase I of this study aims at determining the dependencies of these two parameters with respect to a series of physical variables: green density of the compacts; presence of an alloying element potentially catalytic for the reaction of lubricant pyrolysis; and procedure of compaction and geometry of the compacts. Also, it is suggested that the two parameters obtained from the fitting curve methods can be related to the main two mechanisms of delubrication: evaporation of the lubricant and conversion of the lubricant molecules into smaller hydrocarbons, assuming that these two mechanisms are the kinetically limiting mechanisms. Furthermore, recent studies of the delubrication process have been opening the way to the potential development of gas sensors, which could eventually allow the direct monitoring of the emissions of gaseous species. Several key features have been reported in the literature, including a peak emission of hydrocarbons at the delubrication temperature, as well as strong emissions of CO and CO2 at temperatures above 700°C. The scope of Phase II of this project was thus to verify that these features were retained under various processing conditions, so that the development of a sensor suitable for various sintering environments is viable. Variations in the emission profiles of gaseous species were observed as the processing conditions were changed, and when possible, potential justifications for these changes have been proposed. ii