Determination of Plastic Inclusions in Pressure Vessel Steel: Standards, Methods, and Applications

Plastic inclusion determination is a key metallographic evaluation for pressure vessel steel, quantifying deformable non-metallic inclusions such as sulfides and silicates that reduce transverse ductility, impact toughness, fatigue resistance, and hydrogen-induced cracking performance. This article explains the governing standards (GB/T 10561, ISO 4967, ASTM E45), sample preparation, rating procedures, typical acceptance criteria, and industrial applications of plastic inclusion testing for pressure vessel steels.

Technical Overview

Plastic inclusions are deformable non-metallic inclusions in steel—primarily manganese sulfides (Type A) and silicates (Type C)—that elongate into stringers during hot rolling, forging, or other hot-working operations. In pressure vessel steels, these elongated inclusions act as internal stress concentrators and preferred sites for crack initiation and hydrogen accumulation, degrading transverse ductility, impact toughness, fatigue strength, and resistance to hydrogen-induced cracking (HIC) and stress-oriented hydrogen-induced cracking (SOHIC).

Because pressure vessels operate under sustained internal pressure, elevated or sub-zero temperatures, and frequently in corrosive media, inclusion cleanliness is a mandatory quality characteristic in material standards such as GB/T 713, EN 10028, and the ASME Boiler and Pressure Vessel Code (Section II). Plastic inclusion determination is performed by quantitative metallography on a polished, unetched longitudinal section using the standard-diagram comparison method of GB/T 10561 (technically aligned with ISO 4967) or the microscopic methods of ASTM E45. Inclusions are classified into categories A (sulfides), B (alumina), C (silicates), and D (globular oxides), of which Types A and C are the plastic, deformable categories. Each observed field is rated for severity from 0.5 to 3.0 in thin and heavy series, and the worst-field or statistically averaged result is compared against the specification limits agreed in the purchase order.

Testing Samples

Plates for Pressure Vessels

Specimens are normally taken from boiler and pressure vessel plates—such as carbon and low-alloy grades (e.g., Q245R, Q345R, Q370R), Cr-Mo hot-service plates (e.g., 15CrMoR, 12Cr2Mo1R), low-temperature nickel-alloyed plates, and their ASTM equivalents (e.g., SA-516, SA-387). The polished test surface is a longitudinal section parallel to the principal rolling direction, commonly located at the quarter-thickness (1/4 T) position, with the exact sampling position agreed between the manufacturer and purchaser or specified by the applicable product standard.

Forgings and Seamless Tubes

For pressure vessel forgings (including large reactor-grade components) and seamless tubes and pipes, specimens are prepared on a longitudinal section parallel to the major deformation axis so that elongated plastic inclusions are fully revealed in their most detrimental orientation. Transverse sections may be additionally prepared when cross-sectional distribution or through-thickness anisotropy is of concern.

Sample Preparation Requirements

• Careful sectioning with abundant cooling to avoid overheating, burning, or plastic smearing of inclusions.
• Mounting followed by grinding through successive abrasive grades and polishing with diamond or oxide suspensions to a scratch- and pull-out-free mirror finish.
• No etching—the inclusion rating must be performed on the unetched polished surface.
• A minimum examined area of approximately 200 mm² per GB/T 10561 / ISO 4967, achieved by scanning multiple adjacent fields of view.
• Verification of field calibration (0.8 mm circular field of view at 100× magnification) using a certified stage micrometer before rating.

Testing Items

The scope of plastic inclusion determination typically covers the following measurable items, each of which is reported against the applicable acceptance limits:

• Type A (sulfide) inclusion rating in the thin series, severity scale 0.5 to 3.0.
• Type A (sulfide) inclusion rating in the heavy series, severity scale 0.5 to 3.0.
• Type C (silicate) inclusion rating in the thin series, severity scale 0.5 to 3.0.
• Type C (silicate) inclusion rating in the heavy series, severity scale 0.5 to 3.0.
• Worst-field rating and conformance determination per ASTM E45 Method A.
• Measured inclusion length, width, and projected area per ASTM E45 Method B or automated image analysis.
• Average severity ratings and calculated cleanliness indices over the entire examined area.
• Inclusion morphology and chemical identification of ambiguous inclusions using SEM-EDS.
• Estimation of the maximum expected inclusion size using statistics of extreme values (ASTM E2283).

Testing Methods

Standard Diagram Comparison Method (GB/T 10561 / ISO 4967)

The specimen is examined at 100× magnification on a calibrated metallograph with a circular field of view of 0.8 mm diameter. Each field is compared with the standard rating diagrams for inclusion categories A, B, C, and D in both thin and heavy series, and the matching severity number (0.5 to 3.0) is recorded. The test report documents the worst field for each category and series, or the frequency distribution of ratings across all examined fields, in accordance with the reporting option selected by the purchaser. This method is the reference procedure in Chinese pressure vessel material practice and is directly compatible with ISO 4967 ratings.

ASTM E45 Microscopic Methods

ASTM E45 Method A assigns a severity rating to each field by pictorial comparison and reports the worst field for each inclusion type—suited to acceptance testing against a maximum allowable severity. Method B records the actual length and thickness of inclusions exceeding defined minimum dimensions and computes inclusion-length sums and severity indices—suited to statistical process control. Methods C and D provide inclusion-count and low-magnification screening approaches for specific product forms. Thin and heavy series limits (for example, worst-field limits commonly in the range of 1.0 to 2.5 depending on application class) are specified in the purchase order or referenced material specification.

Automated Image Analysis (ASTM E1245 / EN 10247)

Automated scanning systems with dedicated software segment inclusions on the polished surface using gray-level thresholds, classify them by shape factor into elongated (plastic), broken-chain (brittle), and globular families, and output length, width, aspect ratio, area fraction, and severity statistics. Image analysis improves repeatability and reproducibility compared with manual chart comparison and is particularly valuable for large-area cleanliness surveys of critical vessel plates and forgings.

Supplementary Characterization

Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) confirms whether an elongated inclusion is manganese sulfide, a complex silicate, or an exogenous slag-type contamination, which directs corrective action in steelmaking (calcium treatment, sulfide shape control, ladle refining). Extreme value analysis per ASTM E2283 extrapolates the largest inclusion likely to exist in a much larger steel volume from measured field maxima, supporting fatigue- and HIC-critical designs. Ultrasonic and macro-etch examinations may supplement micro-ratings for detecting coarse or clustered inclusions.

Applications

• Release testing and certification of carbon-steel and low-alloy plates for boilers, drums, reactors, heat exchangers, and spherical storage tanks.
• Qualification of HIC-resistant steels for wet hydrogen sulfide (sour) service, where elongated sulfide stringers are the dominant crack-initiation sites.
• Quality control of Cr-Mo hot-service plates and forgings for hydroprocessing equipment in refinery and petrochemical plants.
• Cleanliness verification of large forgings for nuclear and heavy-wall pressure boundary components.
• Evaluation of low-temperature vessel steels, where inclusion stringers reduce through-thickness and transverse toughness.
• Failure analysis and remaining-life assessment of in-service vessels exhibiting lamellar tearing, stepwise cracking, or fatigue initiation at inclusions.
• Independent verification testing by accredited third-party laboratories for project specification compliance and import/export material certification.
• Steel mill process optimization, including sulfide shape control by calcium treatment, argon stirring, vacuum degassing, and tundish metallurgy.

FAQ

What are plastic inclusions in pressure vessel steel?

Plastic inclusions are non-metallic inclusions—chiefly manganese sulfides (Type A) and silicates (Type C)—that soften and deform at hot-working temperatures. They elongate into stringers aligned with the rolling or forging direction, producing pronounced anisotropy in mechanical properties.

Why are plastic inclusions harmful in pressure vessels?

Elongated inclusion stringers act as stress raisers and hydrogen traps. They lower transverse tensile ductility and impact toughness, reduce fatigue life, promote lamellar tearing in welded T-joints and nozzle attachments, and provide initiation sites for hydrogen-induced cracking in sour service—failure modes directly relevant to pressure integrity.

Which standards govern plastic inclusion determination?

The principal standards are GB/T 10561 (micrographic method using standard diagrams, aligned with ISO 4967), ASTM E45 (inclusion content of steel by microscopic methods), JIS G 0555 (microcleanliness counting method), and EN 10247 together with ASTM E1245 for automated image analysis. Acceptance limits are set by product standards such as GB/T 713 and EN 10028 or by the purchaser's specification.

How is the sample taken and prepared?

A specimen is cut with the polished surface on a longitudinal section parallel to the deformation direction, typically at the quarter-thickness position for plates. It is ground and polished to a mirror finish and examined unetched at 100× magnification over an area of at least about 200 mm² using a 0.8 mm calibrated field of view.

What do thin and heavy series ratings mean?

The thin series applies to narrower, finer inclusions while the heavy series applies to thicker, more coarse inclusions; each is rated from 0.5 to 3.0 against the standard diagrams. Heavy-series inclusions are generally more damaging, and most specifications assign them stricter limits than the corresponding thin series.

What are typical acceptance criteria?

Exact limits depend on the steel grade, section thickness, and service class, and must be stated in the purchase order. For common pressure vessel plates, worst-field limits of about 1.5 to 2.5 for Type A and Type C inclusions are frequently specified, with tighter limits applied to HIC-resistant and nuclear-grade materials.

Can the test distinguish sulfides from silicates?

Yes. Under the chart method, Type A sulfides are highly elongated with high deformability and uniform gray contrast, whereas Type C silicates are elongated but more variable in outline and optical behavior. When morphology alone is ambiguous, SEM-EDS analysis provides definitive chemical identification.

How often should plastic inclusion testing be performed?

Testing frequency follows the applicable product standard or purchase agreement—commonly per heat-treatment lot or per plate for critical applications. Retesting protocols after process changes (e.g., calcium treatment adjustment) and periodic verification by accredited third-party laboratories are standard practice for safety-critical vessel programs.