Where Hydrogen Comes From and Why It Matters
Hydrogen embrittlement in welded hull and offshore structures is not a single material property; it is the result of three factors arriving together. A susceptible microstructure, residual tensile stress and a supply of diffusible hydrogen at the weld are enough to produce delayed cracking, often hours after the joint has cooled and sometimes after the ship has left the yard. High-strength grades are more exposed because their yield strength leaves less capacity to absorb local strain before a crack initiates.
The hydrogen itself almost always enters through the consumable or the joint preparation: damp flux, un-dried basic electrodes, moisture on the plate surface, rust, mill scale, cutting fluid or condensation in a cold shop. Because the source is process-related, the countermeasures are process-related too, and rule sets address them through chemistry limits, welding procedure requirements and inspection duties rather than through a single specification number.
Chemistry and Cleanliness Controls in EH36 and FH40
Chemistry sets the baseline resistance of the steel. CCS rules for higher-strength hull structural grades limit carbon and sulphur, and the microalloying route keeps carbon low while thermomechanical rolling provides the strength. Typical controls include:
Carbon held at or below about 0.18 per cent for the E and F grades, with tight limits on sulphur and phosphorus to reduce centreline segregation and inclusion content.
Carbon equivalent capped around 0.40 per cent using the IIW formula, with a cracking parameter typically held in the 0.20 to 0.24 band for thermomechanically rolled plate.
Calcium treatment and low-sulphur practice for improved through-thickness behaviour on restrained joints such as hatch coamings and block-to-block connections.
Optional nickel additions of roughly 0.2 to 0.5 per cent on plates intended for colder service, where extra low-temperature toughness is requested.
Chemistry alone cannot carry the requirement, however. A plate delivered with a compliant analysis will still crack if it is welded wet, and that is why the welding procedure and the consumable control regime carry equal weight in approval.
Low-Temperature Toughness: What the Rules Require
Toughness requirements are expressed as a minimum absorbed energy at a defined test temperature, measured on a Charpy specimen in the required orientation. The grade letter fixes the temperature, and the strength level fixes the energy floor. The figures below reflect the values normally specified for hull structural steels under the unified requirements that class societies apply.
| Grade | Minimum yield strength | Impact test temperature | Typical minimum energy |
|---|---|---|---|
| CCS D, normal strength | 235 MPa | minus 20 °C | 27 J |
| CCS E, normal strength | 235 MPa | minus 40 °C | 27 J |
| CCS DH36 | 355 MPa | minus 20 °C | 34 J |
| CCS EH36 | 355 MPa | minus 40 °C | 34 J |
| CCS FH36 and FH40 | 355 MPa and 390 MPa | minus 60 °C | 41 J |
Three details decide whether a plate genuinely meets the intent of the table. The first is orientation: transverse specimens are more demanding than longitudinal ones and are required for many applications. The second is consistency, meaning every heat must be tested rather than a single representative sample. The third is the heat-affected zone, whose toughness after welding has to be demonstrated through procedure qualification, not assumed from the parent plate result.
Welding Procedure Controls and Post-Weld Heat Treatment
Hydrogen control at the arc follows a well-tested sequence.
Consumables: basic-coated electrodes and flux-cored wires with a low diffusible hydrogen classification, dried and stored at the temperature recommended by the manufacturer, with electrodes taken from the holding oven in quantities that can be consumed within the working period.
Joint preparation: grinding or wire brushing to remove rust, scale, moisture and cutting residue immediately before welding, and keeping the joint dry in humid or cold conditions.
Thermal control: preheat of about 75 to 125 °C on thin sections and higher values on thick, restrained joints; interpass temperature limited to avoid grain coarsening; and where the geometry permits, post-heating or a hydrogen release treatment to let hydrogen diffuse out before cooling continues.
Post-weld heat treatment: specified where thickness, service temperature or design code require it, with a controlled heating and cooling rate and a soak time based on section thickness.
On FH40 and thick EH36 sections, the combination of preheat, interpass control and, where applicable, post-weld heat treatment is what keeps the heat-affected zone both crack-free and tough. Reducing preheat to save time is the most common way to turn a compliant procedure into a repair job.
Testing and Verification
Verification covers the material, the procedure and the finished weld. On the material side, tensile testing, impact testing and chemical analysis are reported per heat, and through-thickness or ultrasonic examination can be specified for restrained joints. On the welding side, procedure qualification records document the consumable, preheat, heat input and thermal history, and the procedure is backed by mechanical testing of the welded joint, including hardness where hydrogen service or sour conditions are involved.
Two specialised tests are worth knowing when hydrogen is the main concern. Diffusible hydrogen content in the deposited weld metal is determined by the standardised mercury or gas chromatography method, with limits typically set at or below 5 ml per 100 g of deposited metal. Slow strain rate testing in a hydrogen-containing environment, carried out under the relevant part of the stress corrosion testing standard, is used to rank the susceptibility of candidate steels and weld metals where hydrogen service or liquid hydrogen storage is planned.
Cryogenic and Alternative-Fuel Applications
Requirements tighten again as soon as the service temperature drops. Liquefied natural gas cargo containment at around minus 163 °C uses a nine per cent nickel steel specified under a dedicated pressure vessel plate standard, or its nickel alloy steel equivalent in the European low-temperature standard system, together with austenitic stainless steel and aluminium alloys depending on the containment concept. The approval path covers the plate, the welding procedure, the consumables and the toughness testing as a single package.
For hydrogen-fuelled vessels the industry is still building experience. Austenitic high-manganese steels are under development for cryogenic tanks because their austenitic structure resists hydrogen-assisted cracking better than ferritic grades, and slow strain rate testing in hydrogen is used to compare candidates. The practical lesson for a shipyard today is that the welding controls described above - cleanliness, low diffusible hydrogen, controlled heat input and preheat - are the same controls that make new fuel-related designs manufacturable.
Frequently Asked Questions
Q: What causes hydrogen embrittlement in welded EH36?
Hydrogen introduced by damp consumables, moisture, rust or scale, combined with a susceptible heat-affected zone and high restraint stress, produces delayed cracking. All three must be present, so controlling the welding process is as important as the steel chemistry.
Q: What is a safe diffusible hydrogen limit for hull welding?
The limit is set by the procedure and the project specification, with values at or below 5 ml per 100 g of deposited metal commonly required for high-strength, highly restrained joints.
Q: How do I verify low-temperature toughness?
By Charpy impact testing at the grade's specified temperature and orientation for every heat, supported by heat-affected zone toughness results from weld procedure qualification.
Q: What preheat does CCS EH36 need?
About 75 to 125 °C for typical sections, increasing with thickness, restraint and the carbon equivalent of the actual heat. The figure is confirmed by the qualified welding procedure.
Q: When is post-weld heat treatment required?
When the design code, the thickness or the service conditions demand it. Thick high-strength joints and components subject to hydrogen or sour service are the usual candidates.
Q: Which steel is used for LNG containment instead of EH36?
Nine per cent nickel steel to the relevant pressure vessel plate specification, or its nickel alloy steel equivalent in the European system, with austenitic stainless steel also used depending on the containment design.
