Manganese Sulphide Inclusions, Lamellar Tearing, and HIC Susceptibility in Carbon Steel Pipe Fittings

Formation and Morphology of MnS Inclusions

Sulphur is present in conventional carbon steels at concentrations of 0.010–0.030% (the ASTM A234 WPB maximum is 0.058%). During solidification, sulphur combines with manganese to form manganese sulphide (MnS) particles that precipitate in the interdendritic spaces of the solidifying steel. These MnS particles are relatively soft and ductile at hot-working temperatures (900–1200°C), so during hot rolling, forging, and extrusion (the forming operations used to make buttweld fittings), they deform and elongate in the direction of metal flow. The result is flat, elongated MnS stringers aligned parallel to the surface of the fitting — in an elbow, these stringers follow the curved geometry of the fitting wall; in a tee or reducer, they follow the extrusion direction. The morphology of these inclusions — elongated and parallel to the surface — is what makes them harmful.

Lamellar Tearing: Through-Thickness Stress

Lamellar tearing occurs when weld shrinkage during solidification and cooling applies a tensile stress in the through-thickness direction (perpendicular to the rolling plane) of the steel. This stress acts perpendicular to the plane of the MnS stringers — which have very low through-thickness ductility because the MnS/steel interface has essentially no ductile tearing resistance in that orientation. The crack propagates stepwise: through the MnS inclusions (decohesion along the stringer-matrix interface) and then up through the steel between adjacent stringers, creating the characteristic stepped fracture surface that gives lamellar tearing its name. Lamellar tearing is a risk in: heavy weld-on fittings where a thick weld bead is applied to the fitting body; boss or nozzle welds where through-thickness contraction is significant; and stub-in connections on large-diameter tees. The material specification parameter that controls lamellar tearing risk is through-thickness ductility, quantified by the through-thickness reduction of area (TTRA) from tensile specimens cut perpendicular to the plate surface. EN 10164 specifies three quality classes: Z15 (TTRA ≥ 15%), Z25 (TTRA ≥ 25%), and Z35 (TTRA ≥ 35%). For highly restrained joints, Z35 steel should be specified.

HIC in Wet H₂S Service

In wet hydrogen sulphide service (H₂S dissolved in water, aqueous phase pH below approximately 6), atomic hydrogen generated by the cathodic reaction at the steel surface (H₂S + Fe → FeS + 2H) diffuses into the steel and accumulates at MnS inclusion interfaces. The hydrogen pressure builds up until the stress at the inclusion tip exceeds the steel's fracture toughness, and a crack initiates parallel to the steel surface along the stringer. These cracks — hydrogen-induced cracks — propagate along adjacent stringers and eventually link up through short transverse cracks to form the characteristic step-cracking pattern seen in HIC. Critically, HIC occurs at ambient temperature with no applied stress — it is driven entirely by hydrogen pressure from the environment, making it distinct from SSC (sulphide stress cracking), which requires applied or residual stress and typically affects hard zones. Standard carbon steel WPB at sulphur levels of 0.010–0.030% is susceptible to HIC in wet H₂S service. HIC-resistant steel (to NACE TM0284) requires sulphur below 0.003% and typically calcium treatment to modify inclusion morphology.

Calcium Treatment: Changing Inclusion Shape

Calcium treatment of the steel melt — adding calcium silicide or calcium wire to the ladle — reacts with MnS to form calcium aluminate and calcium sulphide inclusions. Calcium sulphide (CaS) inclusions are more spherical and less deformable than MnS, so they do not elongate during hot working. The result is a dispersion of small, spherical inclusions rather than elongated stringers. Spherical inclusions have a much lower stress concentration factor at their poles than elongated stringers, and the through-thickness ductility is significantly improved. Calcium-treated steels are specified in HIC-resistant pipe fitting procurement by requiring NACE TM0284 testing with acceptance criteria of: Crack Length Ratio (CLR) ≤ 15%; Crack Thickness Ratio (CTR) ≤ 5%; and Crack Sensitivity Ratio (CSR) ≤ 2%. These criteria must be demonstrated on test coupons from the actual heat used for the fittings — NACE TM0284 results from a different heat cannot be used as a substitute.

Sulphur Content and the Certificate

For standard WPB fittings, the EN 10204 3.1 certificate should report sulphur content of the heat. Values above 0.020% should be reviewed for any application involving: lap-joint configurations with significant through-thickness weld stress; wet H₂S or sour service; or thick-wall heavy fittings where lamellar tearing risk is elevated. For sour service, the purchase order should specify: maximum sulphur 0.003%; calcium treatment; and NACE TM0284 test with results on the MTC. Without these explicit requirements in the PO, a supplier will typically supply to the standard ASTM A234 sulphur maximum (0.058%), which may be inadequate for the application.