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The different types of manganese sulphide (MnS) inclusions

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The original classification of MnS inclusions was proposed by Simms and Dahle[1]. They divided the MnS inclusions into types I, II and III. This classification applies to the inclusion morphology in cast steels. Subsequent classifications have not changed the terminology, but the classifications are applied to wrought steels as well as cast steels.

In cast steels, Type I MnS is globular and randomly distributed throughout the structure. It is often formed as a duplex structure with silicates. This is typical in semi-killed and silicon-killed steels, where the oxygen content is high. Steels with this type of MnS inclusions generally have high ductility.

Type II MnS often has a dendritic structure in cast steels, distributed as chain-like formations or thin precipitates on the grain boundaries. This type of MnS is found in steel de-oxidised without an excess of aluminium. These inclusions are associated with extremely low ductility in steel.

Type III MnS inclusions in cast steel are more isolated than type II, still in the grain boundaries. The shape of this type of MnS is irregular, and distributed randomly at the grain boundaries. This type of MnS is found in steels de-oxidised with an excess of aluminium and is associated with a recovery of the steel ductility from the values associated with Type II MnS.

It is not the de-oxidising elements themselves that control the MnS morphology, but the oxygen content. Zirconium, titanium, calcium and vanadium are also used as deoxidisers. However, the effect of these elements on the MnS morphology is less straightforward to quantify than the effect of aluminium.

Simms and Dahle made some 'rule of thumb' observations about the type of MnS likely to be present in steel:

  1. Type I. These exist when there is practically no aluminium content, usually in silicon-killed steels.
  2. Type II. These appear with the first traces of aluminium, above 0.005 wt%.
  3. Type III. These initially appear alongside type II at levels of 0.01 to 0.03 wt% total aluminium.
  4. Type III. This is practically assured as the only type to occur with total aluminium of ≈0.04 wt%.

The type of MnS inclusions in wrought steels depends on the type of sulphide formed in the original cast steel.

Type I MnS inclusions are much harder than the other types. During rolling, type I MnS deforms to a 'lozenge' shape. Any silicates usually deform more, ending up at the tips of the lozenge. These two features can be seen in Fig.1a.

Fig.1. Different inclusion types Fig.1a) Type I manganese sulphide particle with (darker) silicate
Fig.1. Different inclusion types Fig.1a) Type I manganese sulphide particle with (darker) silicate
Fig.1b) Type II manganese sulphides
Fig.1b) Type II manganese sulphides
Fig.1c) Strings of broken silicates
Fig.1c) Strings of broken silicates

Types II and III MnS inclusions become much more elongated than type I upon rolling. It is much more difficult to distinguish between these types in wrought steels than in cast steels. Type II are characteristically in clusters, rather than isolated inclusions.

The final shape of the inclusions in wrought steel is particularly important with reference to hydrogen induced cracking in sour service and lamellar tearing.

The elongated MnS inclusions in wrought steel (Type II, III) act as initiation sites for hydrogen induced cracking in sour (H2S containing) environments[2] .However, type I MnS inclusions have been reported to trap hydrogen[3], inhibiting hydrogen diffusion and thus inhibiting hydrogen induced cracking both at the initiation and propagation stage.

Elongated (Types II and III) inclusions also act as initiation sites for lamellar tearing [4-6]. Farrar[6]found that lamellar tearing could also be initiated by type I MnS, but the volume fraction of Type I MnS required for initiation was very high, and such a high level of only type I inclusions is unlikely to be found in structural steels. In such (high oxygen) steels, susceptibility to lamellar tearing is generally controlled by oxides.

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References

  1. Sims C.E, and Dahle F.B: AFS Trans, 1938, Vol 46, 65.
  2. Taira T, Kobasi Y, Inagaki H and Watanabe T: 'Sulphide corrosion cracking of linepipe for sour gas service', Wet H2S cracking of Carbon Steels and Weldments Publ. NACE International, 1996, p359-378.
  3. Kikuta Y, Araki T and Hirose A: 'Effect of non-metallic inclusions on hydrogen assisted cracking', Transactions of the Japan welding society, 19 (1), 1988, p60-65.
  4. Farrar JCM and Dolby RE: 'Lamellar Tearing in Welded steel Fabrications' Welding Institute Publication, 1972.
  5. Dolby, RE; Hart, PHM; Bailey, N; Farrar, JCM 'Material Aspects Controlling Weld Defects In Offshore Structures' Pre-prints, 1973 Offshore Technology Conference, Houston, Texas, 30th April-2nd May 1973. Vol.2, Paper No. OTC 1908, p823-834.
  6. Farrar JCM: 'Inclusions and susceptibility to lamellar tearing of welded structural steels', Welding Journal, 58 (3), 1979, p321s-331s.

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