Lloyd’s Register (LR) is one of the most respected classification societies, and its requirements for AH36 high‑strength Shipbuilding steel plates are particularly rigorous. AH36 is a thermomechanically‑rolled or normalised steel with a minimum yield strength of 355 MPa, commonly used for critical hull structures such as outer shells, longitudinal stiffeners, and deck strakes. To obtain LR class approval, manufacturers must demonstrate that their plates meet strict impact toughness criteria and undergo comprehensive non‑destructive examination. This article summarises the essential impact testing protocols and class approval prerequisites for LR AH36 plates.
Impact testing is the cornerstone of LR’s toughness evaluation. LR mandates Charpy V‑notch (CVN) tests conducted at a specified temperature, which for AH36 is 0°C. The minimum absorbed energy requirement is 27 J average for a set of three specimens, with no single value falling below 20 J. These specimens are taken in both the transverse and longitudinal directions relative to the rolling direction, with the transverse orientation often being the more critical because it is perpendicular to the grain flow. The test pieces must be machined to standard dimensions (10 mm × 10 mm cross‑section) and notched exactly at mid‑thickness. For plates thicker than 50 mm, sub‑size specimens may be permitted, but the energy criteria are proportionally adjusted.
One of the distinctive features of LR requirements is the more stringent ultrasonic testing (UT) regime. LR stipulates that all AH36 plates greater than 15 mm in thickness must undergo 100% ultrasonic inspection to detect internal laminations, porosity, or segregations. This is a notable departure from ASTM A131, which only requires UT for plates exceeding 25 mm. LR’s rationale is to ensure absolute freedom from hidden defects that could serve as crack initiation sites under dynamic wave‑loading conditions. The acceptance criteria for UT are based on the amplitude and extent of echo indications, with any rejectable discontinuity requiring the plate to be downgraded or rejected.
In addition to mechanical testing, class approval demands a comprehensive review of the manufacturer’s production process. The mill must submit historical production records covering at least three representative heats, including chemical composition, rolling parameters, heat‑treatment cycles, and previous test results. LR evaluates the carbon equivalent (CE) and the cracking parameter (Pcm) to assess weldability. If Pcm exceeds 0.25%, LR may require the mill to provide supplementary preheat recommendations and even conduct crack susceptibility tests such as the implant test or y‑groove test.
All approved plates must be accompanied by mill test certificates conforming to EN 10204 Type 3.2, which is a third‑party inspection certificate issued by a recognised independent surveyor. The certificate must list the ladle analysis, tensile properties, impact values, and UT results, along with the plate’s heat number and dimensions. LR also reserves the right to witness random retests at its discretion.
For specialised applications, such as ships operating in polar regions or ice‑breakers, LR can impose impact testing at –40°C instead of 0°C, with the same 27 J energy requirement. This ensures that the steel retains sufficient ductility even in extreme cold. Overall, LR’s rigorous regime ensures that AH36 plates used in LR‑classed vessels offer predictable fracture resistance, reliable weldability, and exceptional structural safety.
Just like you, 70% customers choose long-term cooperation with BBN steel not only for our good product and service quality, good reputation in the international market, but also for our experienced one-stop raw material supply and further steel processing!
