The Shapes and Compositions of Nonmetallic Inclusions in Mild Steel Weld Metals (Report 4)

Abstract

In the previous reports, the author separated the oxide inclusions using alcoholic iodine method and observed, analysed or determined these crystal forms. In this report, sulphide inclusions in mild steel weld metals are separated and determined these crystal forms using x-ray diffractometer. This method of separation is alcoholic iodine method but is different from the previous one in a few point. A piece of 10-15g of specimen is rotated slowly dipping in alcoholic iodine solution under pure H₂ atmosphere, stop the rotation when the half of specimen is desolved, filter the solution and wash the residue. It is proved analytically that about 80% of sulphides are separated as residue after these process, but all of sulphides are desolved and oxides are only remained as residue if the method in the previous report is used.
From the results of diffraction, three forms of sulphides are detected; FeS, MnS and FeS-MnS solid solution. FeS-MnS solid solution has the same crystal system (Cubic) with pure MnS but slightly smaller lattice parameters. Ionic radius of Fe is smaller than Mn and FeS-MnS solid solution can be exist according to the phase diagram, so the crystal form of this solid solution seemed to be same as of FeS-MnS.
The specimens used in this experiments have several contents of Mn and S. Only MnS can be detected in the weld metals containing more than 1% Mn, only FeS in the ones containing fewer than 0.1% of Mn and MnS, FeS-MnS or FeS-MnS and FeS can be detected in the ones containing 0.3-0.9% Mn. After these experiments the relation between the crystal forms of sulphide inclusions and ratio [Mn]/[S] in weld metals is examined. If [Mn]/[S]>5, the sulphides are only MnS, 5>[Mn]/[S]>4 FeS-MnS, 4>[Mn]/[S]>1 FeS-MnS and FeS, 1>[Mn]/[S] only FeS.