HIC-resistant steels are used when carbon steel is exposed to hydrogen sulphide (H2S) in the acidic or humid H2S environment of electrochemical corrosion reactions.
HIC (Hydrogen Induced Cracking) is prevalent in wet H2S environments, which is often known as sour service.
Therefore HIC resistant carbon steels are offered on the market so that these steels can be used primarily in the oil and gas industry for the use of steel pressure vessels so that they are protected against hydrogen-induced cracks and can be operated safely. The hydrogen induced cracking damage increases as the level of hydrogen sulphide increases.
Conditions for exemption from HIC test for carbon steel and low alloy steel
In order to guarantee the required HIC resistance, materials for sour service are following the most stringent production process route and are passing following production steps such as
- desulfurization of pig iron to achieve very low levels of sulfur
- hot rolling
- vacuum degassing
- tress relieving after the obligatory normalising heat treatment is mandatory (N+T)
Note: The additional heat treatment after normalising can also be carried out by the customer after welding (PWHT).
The specifications NACE MR0175 / ISO 15156–2 contain requirements for HIC resistant carbon steels: Ni ≤ 1 %, S ≤ 0.010 % (seamless product), S ≤ 0.003 % (flat rolled product), P≤0.010% and a hardness of HRCmax. = 22 (237 HB).
The yield strength of the material is less than 355 MPa and the tensile strength is less than 630 MPa.
The limited nickel, phosphorus and sulphur content of the HIC-resistant steels guarantees good weldability.
Testing to prove HIC resistance (HIC-Test)
A standard HIC-test is to be carried out in a certified laboratory and in accordance with NACE TM0284 “Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking”. Depending on the plate thickness the tested sample consists of three or more specimen, which are exposed to the testing solution A for 96 hours. Two different test solutions in accordance with NACE TM0284 are available for this purpose: Solution A (5 % sodium chloride with 0.5 % acetic acid), with a pH of 3, and Solution B (artificial seawater), with pH 5. If not specified differently, one HIC-test will be carried out per heat. Testing shall be performed at ambient temperature [25 °C ± 3 °C]
After immersion, the specimens are cut into three pieces to perform metallographic crack evaluation. The crack dimensions are put into relation to the specimen dimensions and are expressed by CLR (crack length ratio), CTR (crack thickness ratio) and CSR (crack sensitivity ratio) values.
The acceptance criteria CLR, CTR and CSR can either be determined per each single cross section, as arithmetical mean value of one specimen or as arithmetical mean value per sample (all specimen).
The ordering customer specifies the acceptance criteria to be applied.
Acceptance criteria (HIC test) acc. to NACE MR175/ISO 15156-2 Table B.3
| Solution | CLR [%] (Crack Length Ratio) | CTR [%] Crack Thickness Ratio) | CSR [%] (Crack Sensitivity Ratio) |
|---|---|---|---|
| A | ≤ 15 | ≤ 5,0 | ≤ 2,0 |
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Standards and specifications for steel materials in hydrogen systems
