STEEL in contact with impure hydrocarbons containing H2S and water will corrode, producing hydrogen at its surface. H2S inhibits the natural hydrogen formation and escape, but instead promotes its absorption into the steel.
Those hydrogen atoms can become trapped causing embrittlement and ultimately cracking in the normal working conditions of the equipments.
H2S is actually responsible for numerous types of problems in steels, such as sulphide stress cracking (SSC), hydrogen induced cracking (HIC) and stress oriented hydrogen induced cracking (Sohic). Wet H2S cracking is thus a major issue in petroleum refining and upstream oil and gas since the 1950s and the criticality increases as fields richer in H2S get exploited.
Many equipments are fabricated in potentially sensitive C-Mn steels that have been therefore developed into materials with improved properties, says Dr Patrick Toussaint, marketing manager for pressure vessel steel plates with Industeel, a subsidiary of ArcelorMittal specialised in advanced technology carbon steel, alloy and stainless steel plates.
Better H2S resistance of the steel is provided by a combination of improved chemistry, cleanliness, homogeneity and microstructure, he says.
Industeel developed its family of C-Mn CarElso grades in order to satisfy the requirements for plates from 6 to 200 mm thick and beyond. A sample-averaged CLR of maximum 5 per cent is commonly achieved whereas excellent individual specimen CLR values down to also 5 per cent can be guaranteed for the thinner and 7 per cent for the thicker ranges, thanks to the high level of homogeneity of the steel. More than 15,000 tonnes of SSC and HIC resistant Industeel CarElso are delivered yearly.
Steelmaking includes vacuum ladle refining after electric arc furnace melting of selected scraps, to enable the making of very clean high quality C-Mn steel with:
• Soft-reduction continuous casting or ingot casting under argon protection with optimised mould shape, to control solidification conditions and limit occurrence of segregation;
• Very low phosphorus (P≤ 0.008 per cent with an average level of 0.006 per cent usually obtained); to avoid crack-propagating intergranular segregation effects;
• Very low oxygen (O≤0.0015 per cent), to minimise total inclusions count;
• Very low sulphur (S≤0.001 per cent), whereby Ca treatment is unneeded and even detrimental to HIC resistance; and
• Very well controlled chemical composition with optimised alloying to guarantee low while sufficient Ceq to achieve required mechanical characteristics.
The typical Ceq range is 0.38 to 0.45 depending on thickness and delivery conditions, as thicker plates need higher Ceq whereas a ‘quenched and tempered’, or ‘normalised accelerated cooling and tempered’ route allows for lower Ceq than a ‘normalised’ route.
At the final stage of fabrication, proper PWHT of the entire equipment is highly recommendable.
