API 2H Grade 50
Intermediate-strength carbon-manganese offshore structural steel plate · API Specification 2H, 9th Edition, July 2006, reaffirmed September 2020 · ASTM A6/A6M, general requirements for rolled structural steel plate · ASTM A370, mechanical testing of steel products
API 2H Grade 50 is a normalized, 50 ksi-class carbon-manganese plate developed for critical welded offshore structural locations exposed to impact loading, cyclic plastic strain and through-thickness stress. Its defining features are transverse tensile and low-temperature Charpy requirements, controlled sulfur and carbon equivalent, and optional supplementary requirements for ultrasonic examination, Z-direction ductility, lower-temperature toughness and strain-aged performance. The base designation alone does not guarantee tested through-thickness properties or suitability for arctic service.
- Plate up to 4 in. [101.6 mm]Specified thickness range · Actual mill availability may be narrower.
- 50 ksi [345 MPa] through 2.5 in.; 47 ksi [324 MPa] over 2.5 in.Minimum transverse yield strength · Requirements apply to transverse tensile specimens.
- 70–90 ksi [483–620 MPa]Tensile strength · Specified across the API 2H Grade 50 thickness range.
- 30 ft·lbf [41 J] minimum average at −40 °F [−40 °C]Base Charpy requirement · Full-size transverse specimens; API 2H also permits specified sub-size specimen/temperature combinations.
- 0.43 through 2 in.; 0.45 over 2 in.Maximum carbon equivalent · Calculated from heat analysis using the IIW-type equation specified by API 2H.
- S4: 30% minimum reduction of areaOptional Z-direction qualification · Applies to plate 3/4 in. [19 mm] and thicker when S4 is ordered.
Overview
- Designation system
- American Petroleum Institute offshore structural plate specification
- Product forms
- Rolled steel plate up to 4 in. [101.6 mm] thick, Plate subsequently cold formed and welded into offshore structural components or structural pipe
- Condition
- Normalized by the plate manufacturer (standard condition), As-rolled/green for subsequent normalization by the fabricator under an established procedure, Quenched and tempered for Grade 50 plate over 2.5 in. [63.5 mm], only with purchaser approval
- Density
- 7850 kg/m³ (Representative density for carbon-manganese steel; API 2H does not specify density.)
What the designation covers
API Specification 2H covers Grade 42 and Grade 50 intermediate-strength plates for selected critical portions of welded offshore structures. Grade 50 is commonly associated with tubular joints, stiffened-plate intersections and other details where impact resistance, plastic fatigue resistance and resistance to lamellar tearing are important. The specification applies to plate—not automatically to finished tubulars, weld metal, forgings, castings or other product forms.
API's 2025 publications catalog identifies the governing document as API Specification 2H, 9th Edition, July 2006, effective February 1, 2007 and reaffirmed in September 2020. ASTM A6/A6M supplies the general delivery requirements unless API 2H states otherwise; in a conflict, API 2H controls.
The designation is not a blanket statement of offshore suitability. Structural category, design temperature, fatigue demand, weld detail, forming strain, thickness direction stress, inspection level and the applicable fabrication code must still be established by the project specification.
Manufacturing route, heat treatment and marking
| Condition | API marking suffix | Practical significance |
|---|---|---|
| Normalized by the plate manufacturer | N | The normal and expected delivery condition; a Grade 50 plate is marked API 2H-50N. |
| As-rolled/green for normalization by the fabricator | G | Final compliance depends on normalization using procedures established by the material manufacturer and qualification of the heat-treated plate. |
| Quenched and tempered | QT | Permitted for Grade 50 plate over 2.5 in. [63.5 mm] only with purchaser approval. API 2H specifies austenitizing at 1500–1700 °F [815–925 °C] and tempering at 1100–1300 °F [595–705 °C]. |
Delivery condition is part of the material identity and must agree with the purchase order, plate marking and material certificate.
The standard requires visual examination of all plate surfaces. Base marking is API 2H-50 with the applicable heat-treatment suffix. Plates accepted under S4 receive a Z marking, and low-sulfur S5 material receives an LS marking. If certain otherwise restricted elements are intentionally added with purchaser approval, the marking uses 2HC rather than 2H. The API Monogram may be applied only by an authorized licensee; compliance with the material specification and authorization to use the Monogram are related but distinct purchasing considerations.
Chemical composition and weldability controls
| Element | Requirement, wt.% | Comment |
|---|---|---|
| Carbon | 0.18 max | Product-analysis maximum is 0.22%. |
| Manganese | 1.15–1.60 | Primary strength and hardenability contributor. |
| Phosphorus | 0.030 max | Heat analysis. |
| Sulfur | 0.010 max | S5 lowers the heat-analysis maximum to 0.006%. |
| Silicon | 0.05–0.40 | Product-analysis range is 0.05–0.45%. |
| Columbium/niobium | 0.01–0.04 | Specified microalloying addition for Grade 50. |
| Titanium | 0.020 max | Any minimum value must be agreed and stated on the order. |
| Total aluminum | 0.02–0.06 | Fine-grain and nitrogen-control practice. |
| Nitrogen | 0.012 max | Shall not be intentionally added; S7 reduces the maximum to 0.009%. |
| Vanadium | Not intentionally added without purchaser approval | If approved, heat- and product-analysis limits must be agreed and stated on the order. |
| Boron, zirconium, cerium and other rare-earth metals | Not intentionally added without purchaser approval | Intentional additions trigger additional marking requirements. |
These are specification requirements, not a typical producer analysis. All intentionally added elements and every element used in the carbon-equivalent calculation must be reported.
The chemistry and CE limits support weldability but do not define a welding procedure or eliminate the need for preheat. Required preheat, interpass control, heat input, consumable classification, hydrogen control and HAZ toughness qualification depend on plate thickness, actual certificate chemistry, restraint, joint geometry, ambient conditions and the governing welding code. Welding procedure qualification should use the ordered delivery condition and should address the base-metal toughness that must be retained after welding.
Transverse tensile requirements
| Nominal thickness | Minimum yield strength | Tensile strength | Minimum elongation |
|---|---|---|---|
| Through 2.5 in. [63.5 mm] | 50 ksi [345 MPa] | 70–90 ksi [483–620 MPa] | 21% in 2 in. [50 mm] or 16% in 8 in. [200 mm] |
| Over 2.5 in. to 4 in. [63.5–101.6 mm] | 47 ksi [324 MPa] | 70–90 ksi [483–620 MPa] | 21% in 2 in. [50 mm] or 16% in 8 in. [200 mm] |
API 2H specifies transverse test specimens. The reduced yield requirement for thicker plate must be recognized in design, purchasing and substitution reviews.
Default testing represents material by heat and relevant thickness extremes rather than necessarily testing every individual plate. Supplementary Requirement S3 can be ordered when individual-plate mechanical testing and/or product analysis of each mother plate is required. This distinction is important for highly critical components and for projects requiring plate-specific traceability.
Impact toughness and temperature limitations
| Option | Specimen size | Minimum average | Minimum single value | Test temperature |
|---|---|---|---|---|
| A | 10 × 10 mm | 30 ft·lbf [41 J] | 25 ft·lbf [34 J] | −40 °F [−40 °C] |
| B | 7.5 × 10 mm | 30 ft·lbf [41 J] | 25 ft·lbf [34 J] | −40 °F [−40 °C] |
| C | 5 × 10 mm | 30 ft·lbf [41 J] | 25 ft·lbf [34 J] | −40 °F [−40 °C] |
| D | 7.5 × 10 mm | 23 ft·lbf [31 J] | 19 ft·lbf [26 J] | −50 °F [−46 °C] |
| E | 5 × 10 mm | 15 ft·lbf [20 J] | 13 ft·lbf [18 J] | −80 °F [−62 °C] |
Three transverse specimens are tested. The producer may use any listed size/energy/temperature combination; therefore a certificate showing a sub-size option is not necessarily evidence of nonconformance.
API 2H describes its base notch-toughness provisions as suitable below water or above water in temperate climates with a minimum service temperature of 14 °F [−10 °C]. This is a scope statement, not permission to equate a −40 °C Charpy test directly with a −40 °C allowable service temperature. Colder service requires explicit consideration of component thickness, stress state, flaw tolerance, weld and HAZ toughness, forming history and the applicable structural design rules.
| Requirement | What it adds | When it matters |
|---|---|---|
| S2 | Lower-temperature or higher-energy toughness requirements. The defined Grade 50 full-size Charpy option is 35 ft·lbf [48 J] average and 30 ft·lbf [41 J] single at −76 °F [−60 °C]; an associated drop-weight route is also provided. | Colder environments or project-specific fracture-control criteria. |
| S7 | Nitrogen limited to 0.009% maximum. | Improving resistance to toughness degradation after strain aging. |
| S8 | Charpy testing after at least 5% tensile strain and aging for one hour at 480 °F [250 °C]. | Heavily cold-formed components or fabrication outside established experience. |
| S12 | ASTM E208 drop-weight testing using two P-3 specimens at −30 °F [−35 °C], both meeting no-break criteria. | When nil-ductility transition behavior is specified in addition to or in place of the normal route. |
The standard advises considering S7 and/or S8 when strain exceeds 5%, or when nitrogen content in wt.% multiplied by percent strain exceeds 0.040.
Through-thickness performance and lamellar-tearing control
API 2H was developed for details where through-thickness stress can be significant, but the basic API 2H-50 designation does not guarantee a measured minimum Z-direction ductility. Plate cleanliness, joint configuration, weld-metal shrinkage, restraint and through-thickness loading must be considered together. A low-sulfur certificate alone must not be treated as proof of Z-direction tensile performance.
| Requirement | Specified control | Important limitation |
|---|---|---|
| Base API 2H | Sulfur limited to 0.010% on heat analysis | No minimum through-thickness reduction of area is guaranteed. |
| S1 ultrasonic examination | Each plate examined by the pulse-echo method to ASTM A578/A578M, Acceptance Level A, with API 2H reporting and rejection provisions | Detects specified internal discontinuities; it does not directly measure Z-direction ductility. |
| S4 through-thickness testing | Two Z-direction tensile tests from each plate as rolled for plate 3/4 in. [19 mm] and thicker; each specimen normally requires at least 30% reduction of area | Must be ordered. The accepted plate receives a Z marking. |
| S5 low-sulfur steel | Sulfur limited to 0.006% maximum on heat analysis | Does not require Z-direction tensile testing and is expressly not a substitute for S4. |
| S4 plus S5 plus S1 | Direct ductility testing, enhanced sulfur control and ultrasonic examination | Often the more defensible combination for highly restrained, thickness-loaded joints, subject to project engineering requirements. |
Under the S4 retest rule, one initial result between 25% and 30% may be retested using two adjacent specimens, both of which must achieve at least 30%.
Fabrication implications
API 2H Grade 50 is intended to be cold formed and welded, but forming and welding can alter the properties established on flat mill plate. Cold forming introduces plastic strain, reduces local toughness and can create strain-aging sensitivity, particularly where attachment welds subsequently heat the strained region. Forming procedures should control strain, bend radius, surface damage and orientation; S7 or S8 should be considered when the standard's strain criteria are exceeded.
Weld procedures should address hydrogen-assisted cracking and HAZ toughness rather than relying only on the nominal 50 ksi strength. Low-hydrogen consumables and handling, dry joint surfaces, suitable preheat/interpass control and qualified heat input are normal engineering considerations. Consumable selection should satisfy the required weld-metal strength and toughness without unnecessary overmatching that increases restraint and residual stress.
If post-weld heat treatment is planned, its effect on tensile and toughness properties must be addressed. Supplementary Requirement S9 provides for simulated post-weld heat treatment of test material and should be considered where the finished fabrication will receive a thermal cycle capable of changing plate properties. The grade has no inherent corrosion-resistance designation; offshore corrosion protection remains a separate coating, allowance and cathodic-protection design matter.
Ordering and acceptance checklist
| Item | What to specify or verify |
|---|---|
| Specification identity | API Specification 2H, Grade 50, required edition and any project amendments. |
| Dimensions and tolerances | Thickness, width, length, permissible under-tolerance, flatness and whether S14 reduced thickness over-tolerance is needed. |
| Delivery condition | N, G or purchaser-approved QT, with the corresponding marking and test condition. |
| Retreatment | Whether renormalizing or quench-and-temper retreatment is permitted following a failed test. |
| Chemical additions | Approval and agreed limits for titanium minima, vanadium, zirconium or other restricted intentional additions. |
| Toughness basis | Base Charpy requirement or S2 project temperature/energy requirements; identify any required S12 drop-weight testing. |
| Through-thickness controls | Whether S1 UT, S4 Z-direction testing, S5 low sulfur, or a combination is required. |
| Forming and thermal history | Expected cold strain and whether S7, S8 or S9 testing is required. |
| Test frequency and traceability | Default heat/thickness testing or S3 individual-plate testing; required certificate type and plate-to-certificate traceability. |
| Inspection and certification | Purchaser or third-party inspection, API Monogram requirement if applicable, test reports, product analysis and UT records. |
The supplementary requirements apply only when invoked by the purchaser. They should not be assumed from the words “API 2H Grade 50.”
Before accepting a substitution or multicertified plate, compare the complete requirements: governing product specification, edition, thickness range, heat treatment, transverse mechanical properties, impact specimen orientation and option, test frequency, carbon equivalent, supplementary requirements, dimensional tolerances, marking and certification. Similar strength does not establish interchangeability.
Relationship to other API offshore plate specifications
| Specification | Processing basis | Relationship to API 2H Grade 50 |
|---|---|---|
| API 2H Grade 42 | Normalized carbon-manganese plate | Lower-strength grade within the same specification; not the same design grade. |
| API 2W Grade 50 | Thermo-mechanically controlled processed offshore plate | Companion 50 ksi-class offshore plate with different chemistry, processing and qualification requirements; not automatically interchangeable. |
| API 2Y Grade 50 | Quenched-and-tempered offshore plate | Companion high-strength offshore plate specification with a different metallurgical route and thickness provisions; not automatically interchangeable. |
| API RP 2Z | Preproduction qualification practice for offshore structural plate | May be used to qualify a producer's specific composition and processing route, including weld HAZ toughness, but it does not replace production material requirements. |
Substitution among API 2H, 2W and 2Y requires engineering and contractual approval even where nominal yield strength is the same.
Sources
- API Specification 2H, Ninth Edition — Specification for Carbon Manganese Steel Plate for Offshore StructuresAmerican Petroleum Institute; licensed reproduction
- API 2025 Exploration and Production Publications CatalogAmerican Petroleum Institute
- API Standards Purchase and Publications CatalogAmerican Petroleum Institute
- API Monogram and APIQR Program DocumentsAmerican Petroleum Institute
- ASTM A6/A6M — General Requirements for Rolled Structural Steel Bars, Plates, Shapes, and Sheet PilingASTM International
- API 2H Grade 50 Technical Product InformationSSAB
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