Non-metallic inclusions are an ever present issue which adversely affects mechanical properties and therefore overall material performance.
Interestingly Sulphur has dual effects meaning that it can improve machinability potential of a material but also have a deleterious effects other key service properties such as forgeability, ductility, toughness, weldability and corrosion resistance.
Mechanical properties of steels are highly damaged by nonmetallic inclusions, because they are discontinuities in the metallic mass and therefore reduce the active section, are local power concentrators, reduce the mobility of dislocations and cause a notable indenture’s effect. Nonmetallic inclusions have the ability to be crack primers and reduce the strength, plasticity, toughness, resistance to fatigue, corrosion, wear and weldability.
Properties are negatively influenced by the intercrystalline inclusions and also by the coarse intercrystalline inclusions which are in higher concentrations. Plastic inclusions keep a better grip on the request matrix. The hard inclusion, especially the rough oxide inclusions favors local concentrations of stress and the occurrence of cracks.
Crack propagation speed is influenced by the nature of the inclusion: the fragility, which can be broken by the stress field, forming secondary cracks and accelerating crack propagation. Hard inclusions that remain supportive on the matrix, decrease the velocity of the crack propagation.
Non-metallic inclusions usually have significantly different thermal and mechanical properties from those for the metal matrix. This leads to stresses, cracks, creep, microstructure instability and many other detrimental effects during thermomechanical processing and the service loading of steels. The removal of inclusions to improve the cleanliness of the steels has been a continuous effort in both academia and industry.
There are a number of clean steel fabrication techniques applied in large scale productions, e.g. electromagnetic stirring, bubbling and filtration. However, these conventional methods are not efficient in eliminating particles whose sizes are smaller than 20 μm, and accompanied with significant energy consumption.
Generally, sulphur improves machinability in steel. However, the presence of sulphur also produces deleterious effects on some service properties such as forgeability, ductility, toughness, weldability and corrosion resistance. It is known that solid solubility of sulphur in iron (Fe-S system) at temperatures below 769°C is very low.
Since the solubility of sulphur in iron and steel is very low (less than 0.01 % at room temperature), it is usually present as a sulphide. The sulphide inc!usions formed during solidification of steel are predominantly manganese sulphide. MnS as a common inclusion adversely influences the mechanical properties, physical properties and corrosion resistance of the steels.
MnS inclusions have high melting temperature (1610°C) and these are a form of primary idiomorphic crystals.
MnS inclusions are found in most steel and their beneficial effects in improving machinability and retarding grain growth in steels are well known. Since the morphology of these sulphide inclusions has significant effects on the various properties of steel, numerous studies focusing on morphology and distribution of the MnS inclusions have been conducted over the years.
According to work of Sims and Dhale, the morphology of MnS can be broadly classified into three types:
Type 1: randomly dispersed globular sulphides
Type 2: grain boundary sulphides and
Type 3: angular sulphides
Figure 1: Distribution of the MnS inclusions at different temperatures (Etching: KLEMM Na2S2O3-K2SO4 x200