Identification of nitride inclusions in steel using differential thermal analysis and evolved gas analysis

Abstract

A preliminary study has been made of the feasibility of applying differential thermal analysis/evolved gas analysis (DTA/EGA) to the identification and estimation of nitrides in residues extracted from iron base alloys. A set of commercially available nitride powders have also been examined.The results showed that EGA traces are more informative than the DTA observations and, additionally, provide an estimate of the total nitrogen present. Residues from steels often contain elemental carbon and this complicates the interpretation of the DTA and EGA traces. It is generally concluded that the DTA/EGA technique is useful in identifying nitrides but should be ancilliary to other techniques such as X-ray diffraction.In recent years much effort has been given to the identification of non-metallic inclusions in steel since in some cases, these are known to exert important influences on chemical and physical properties. Particular attention has been directed to the quantitative analysis and morphology of carbides, nitrides and sulphides. In general, inclusions may be examined by in situ techniques, using for example the electron probe micro-analyser, or alternatively they may first be extracted from the steel by differential chemical attack and then submitted to a wide range of physical or chemical analytical techniques. The choice of approach depends on the information required. If it is important to determine the character of individual: inclusions then clearly in situ techniques are preferable. However, the examination of many inclusions in this way is prohibitively long and for many purposes, it is better to extract the inclusions in bulk from the steel and subsequently examine them as a powdered sample. This paper is concerned with the identification and determination of nitrides present in such bulk extracted residues.Scholes and White [1 ] stated that with the exception of aluminium nitride [2], no reliable specific techniques exist for the quantitative determination of individual nitride phases, and more recently [3] the hitherto accepted Beeghly method [2] for aluminium nitride determination has been questioned. In the search for new approaches differential thermal decomposition was considered [1] to have potential and this has been developed [4][5][6][7] into a technique involving simulta--neous differential thermal analysis and evolved gas analysis, (DTA/EGA) for the examina~on of carbides, nitrides and sulphides present in residues extracted from steel. White [8] used DTA/EGA to identify the nitrides isolated from two series.