Powder Metallurgy Technology
Giving particular emphasis to those processes which are of industrial significance, this useful text provides detailed coverage of metal powder production, compaction, sintering, hot consolidation, and much more.
TABLE OF CONTENTS Powder Metallurgy Technology
6.8: Sintering Atmosphere
6.8 Sintering Atmosphere
Nearly all metals of technical importance react with the gas of their surrounding atmosphere even at room temperature, but more so when treated at higher temperatures. The most important reason for using special sintering atmospheres is to provide protection against oxidation and re-oxidation of the sintered metal powders. There are many other ways in which a sintering atmosphere can influence the basic sintering process. By reducing the oxides the atmosphere may create highly mobile metal atoms. Gas atoms of the sintering atmosphere can enter the sintering compact via interconnected pores. They may later get trapped in closed pores, thus hindering their shrinkage. Gas atoms of the sintering atmosphere may also diffuse into the metal. At times these atoms might also alloy with the metal. In the following sections the details of different sintering atmospheres are given.
6.8.1 Hydrogen
Reducing atmospheres are, by far, most commonly used for sintering metal parts. Pure hydrogen is an excellent reducing gas, but it is not economical except in case of high valued products. Hydrogen is very flammable, having an extremely high rate of flame propagation. Because of its high flame propagation, hydrogen burns with a short, hot flame immediately upon contact with air. The flame is an almost colourless blue. Hydrogen is the lightest element; its specific gravity being only 0.069 as compared to 1.0 for air. It is easily displaced by air, and rushes out the top of the furnace door openings rapidly when free to do so. The gas...
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