Effect of heating rate and process atmosphere on the thermodynamics and kinetics of the sintering of pre‐alloyed water‐atomized powder metallurgy steels

Abstract

AbstractDuring the sintering of powder metallurgy steels the full removal of the iron oxide layer is required in order to develop strong inter‐particle necks. Although this iron oxide layer has low thermodynamic stability, its removal from the powder compact is a very complex process that is determined by a number of parameters such as temperature profile, sintering atmosphere, compact properties, powder properties, and additives. This paper summarizes these sintering parameters in correlation with the powder properties through the use of thermogravimetry analysis. In this work, hydrogen additions were identified as the most effective agent for the removal of the surface iron oxide during the early stages of sintering (at temperature range between ~300 and 600°C). The process depends on the heating rate and a rather low activation energy of 48 kJ/mol was determined for this reaction. Carbothermal reduction plays the largest role in the oxide reduction at high temperatures where two main reactions can be distinguished. The first was the reduction of the surface oxide residue and particulates, which occurred at temperatures between 950 and 1150°C. This reaction is characterized by an activation energy of 253 kJ/mol. Second was believed to be associated with the reduction of the internal oxides, occurred at temperatures above 1150°C. This reaction is characterized by rather high activation energy of 422 kJ/mol.