Manganese sulfide in A529 steel

Manganese sulfide in A529 steel

Figure 1: Sulfide inclusions found in A529 billet sample: manganese sulfide – additional morphologies. SEM, scale bars: 1, 2 µm.

Inclusion name: Manganese sulfide
Record No.: 805
Inclusion formula: MnS
Inclusion type (Macro/Micro/Nano): Micro
Inclusion type (Exogenous/Indigenous): Indigenous
Inclusion classification: Sulfide
Inclusion composition in weight %: No data
Sample: A529 billet sample, high strength low alloy (HSLA) steel
Steel composition in weight %: 0.18% C, 0.80% Mn, 0.01% P, 0.03% S, 0.18% Si, 0.33% Cu, 0.10% Ni, 0.05% Cr.
Note: The cleanliness of high strength low alloy (HSLA) steel was assessed qualitatively and quantitatively. The determination of inclusion type and inclusion morphology were carried out using Selective Potentiostatic Etching by Electrolytic Dissolution (SPEED) method allowing in-situ examination of inclusion morphology by analytical techniques such as SEM/EDS.
Sulphur is soluble in iron at steelmaking temperature, therefore no formation of common metal sulfides are expected throughout the molten steel operations. As the liquid steel solidifies, solubility of sulphur decreases to about 0.05 wt% S at 1365°C and further cooling to 988°C decreases the solubility limit to 0.012 wt% S. During steel solidification, insoluble sulphur segregates and forms metal sulfides with iron and manganese. Stability of the two sulfides are comparable at steelmaking temperature, but MnS is the more stable phase at room temperature.
Although smaller in particle diameter, sulfide inclusions were found in greater quantity than oxide inclusions. Sulfide precipitates were frequently observed in all specimens as a result of decrease in solubility upon cooling, which may not truly reflect the condition at liquid steel temperature. Therefore only sulfides found in billet samples are discussed in this section.
Good sulfide inclusion retention on sample’s surface was achieved by SPEED etching technique. The variations of sulfide morphology, spherical and cubic, are shown in inclusions No. 803, 804 and 805; where the images were taken from SPEED etched samples. It is otherwise difficult to present the details of surface feature on inclusions, shown in Fig. 1. Referring to the work by Liu et al., the dark fine precipitates observed on the surface of MnS inclusions were believed to be copper sulfides hence the trace copper signal shown in the EDS spectra.
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