Hydrogen induced cracking (HIC) represents a common challenge, especially in the petroleum and refinery industry where the purity and overall quality of the steel is of paramount importance.
The production process for HIC resistant steels includes critical desulphurization techniques including the injection of magnesium and stirring by bottom argon injection.
The production of clean steel is based on technologies to control and/or remove inclusions in steel. Often, the types and characteristics of non-metallic inclusions in steel have significant influence on the mechanical properties of the steel and also determine the cleanliness of the steel. Precise analysis of these characteristics is therefore needed to optimize the steelmaking process to produce quality steels.
Hydrogen induced cracking (HIC) and sulphide stress cracking (SSC) represent two kinds of a specific hydrogen provoked damage that are frequently met in the petroleum and refinery industry. In the first case (HIC), it is generally recognized that the resistance of steels depends mainly on their microstructural features – non-metallic inclusions and segregation bands.
Elongated manganese sulphides are considered as the most dangerous initiation sites. In the second case (SSC), it is believed that the resistance of steels can be preferentially related to their strength level while microstructure characteristics are less important. The presented paper is devoted to the study of HIC and SSC in carbon-manganese steels, which differ in their heat treatment (microstructure, level of mechanical properties) and their cleanliness.
The MnS inclusion is the most harmful initiator for HIC, therefore, as a basic measure, sulfur content must be reduced for the steels used in acidic environments. Because of the progress in the steel making process, sulfur content can be reduced down to a quite low level; under 0.0008 wt. % (8ppm).
Ca treatment is usually applied in order to prevent formation of elongated MnS and to control the sulfide inclusion into a spherical shape, and if there is an appropriate Ca content for crack resistance property. If the Ca content is relatively high compared with S content, excess Ca can form oxides that can act as an initiator of HIC. Several parameters that represent effective Ca content were introduced, and Ca content needs to be carefully controlled in a narrow range; for example, 3 < [Ca]/[S] < 8, where [Ca] and [S] are contents of Ca and S in weight percent.
Table 1: Chemical composition requirements for high-strength pipe steel, [ppm]
Figure 1 shows the route for HIC resistant steel production. Before the BOF process, the hot metal is desulphurized at the hot metal desulphurization station by the injection of magnesium and CaO. At the BOF, only scrap with low sulphur contents should be charged. The aluminum for deoxidation and the slag forming agents are added during tapping. Main alloying is also processed at that time.
The ladle is then transported to the bubbling station where argon is blown by means of a top lance in order to mix metal and slag for the purpose of desulphurization. Then, the ladle is transferred for vacuum treatment. The steel is stirred by bottom argon injection to assure deep desulphurization and degassing.
Figure 1: Route for producing HIC resistant steel for pipeline