Pressure Vessel Steel Inclusion Chart Comparison Testing: Micrographic Rating of Non-Metallic Inclus

Pressure vessel steel inclusion chart comparison testing is a metallographic evaluation method that rates non-metallic inclusions, including sulfides, aluminas, silicates, and globular oxides, against standardized rating diagrams in accordance with ASTM E45, ISO 4967, and GB/T 10561. This test is essential for verifying steel cleanliness, toughness, and fatigue resistance in boiler and pressure vessel applications. This article explains the principle, sample types, testing items, rating procedure, acceptance criteria, and practical applications of the chart comparison method for pressure vessel steels such as Q245R, Q345R, SA-516, and SA-387 grades.

Technical Overview

Non-metallic inclusions are inevitable by-products of steelmaking, deoxidation, and secondary metallurgy. In pressure vessel steels, elongated sulfides and hard oxide stringers act as stress concentrators that degrade transverse ductility, impact toughness, fatigue life, and resistance to hydrogen-induced cracking. Because pressure vessels operate under sustained internal pressure, often at elevated or sub-zero temperatures, the inclusion content of the steel must be quantified and controlled against limits defined by the governing product specification.

Inclusion chart comparison testing, also known as the micrographic comparison method or JK rating method, is the internationally accepted technique for this purpose. A polished, unetched metallographic specimen is examined under a metallurgical microscope at 100× magnification, and the observed inclusion morphology is compared directly with standard rating diagrams. The principal standards are ASTM E45, ISO 4967, GB/T 10561, EN 10247, and JIS G 0555. Inclusions are classified into four morphological categories plus a single-particle category:

• Type A: sulfide inclusions, typically manganese sulfide, which are highly deformable and elongated along the rolling direction with rounded ends and a high length-to-width ratio.
• Type B: alumina inclusions, appearing as stringers or rows of small, hard, angular particles aligned in the deformation direction with a broken or discontinuous distribution.
• Type C: silicate inclusions, elongated particles with sharp or tapered ends that are brittle and prone to fragmentation during hot working.
• Type D: globular oxide inclusions, discrete rounded particles randomly distributed and essentially non-deformable, generally smaller than 13 µm in diameter.
• Type DS: single globular oxides, isolated round particles with a diameter of 13 µm or greater.

Each category is rated on a severity scale divided into a thin series (typically 0.5 to 3.0) and a heavy series (typically 1.5 to 5.0), where severity is governed by inclusion length for Types A, B, and C, by particle count for Type D, and by particle diameter for Type DS. The final result expresses the worst field observed for each type and series, providing a concise cleanliness index for acceptance decisions.

Testing Samples

Carbon and Low-Alloy Pressure Vessel Steel Plates

Typical materials include Q245R, Q345R, Q370R, and 16MnDR plates supplied to GB 713, SA-516 Gr. 60/65/70 plates supplied to ASME SA-20, and P265GH/P355GH plates supplied to EN 10028-2. These grades are widely used for shells, heads, and knuckles of medium- and low-pressure vessels.

Alloy Steel Plates for High-Temperature Service

Chromium-molybdenum grades such as 15CrMoR, 14Cr1MoR, and 12Cr2Mo1R to GB 713, SA-387 Gr. 11/12/22 to ASME, and 13CrMo4-5 or 11CrMo9-10 to EN 10028-2 are tested for hydroprocessing reactors, reformers, and other hot-service equipment, where inclusion control is critical for creep and hydrogen service performance.

Forgings, Pipes, and Tubular Products

Pressure-containing forgings, seamless tubes, and pipe materials, including 16Mn and SA-335 P-series grades, can be evaluated using the same chart comparison approach on appropriately oriented sections.

Weld Metal and Heat-Affected Zone Specimens

Although chart comparison is standardized for base metal, the method is frequently applied to weld metal and heat-affected zone cross sections during welding procedure qualification and failure investigations to characterize oxide and slag-type inclusions.

Specimen Orientation and Dimensions

For plates, specimens are normally taken at the quarter-width position from a full-thickness section. The polished surface is prepared on the longitudinal plane, parallel to the principal rolling direction, because Types A, B, and C ratings are defined relative to the deformation direction. A polished observation area of approximately 300 mm², for example 20 mm by 15 mm, is recommended by ISO 4967 and GB/T 10561 to ensure representative sampling.

Testing Items

The chart comparison experiment delivers a complete inclusion rating package covering the following items:

• Worst-field severity rating for Type A (sulfides), thin and heavy series.
• Worst-field severity rating for Type B (aluminas), thin and heavy series.
• Worst-field severity rating for Type C (silicates), thin and heavy series.
• Worst-field severity rating for Type D (globular oxides), thin and heavy series.
• Worst-field severity rating for Type DS (single globular oxides).
• Auxiliary indices where required, such as cumulative ratings, average severity per field, or cleanliness indices computed over the entire observed area.
• Comparison of results with acceptance limits specified by the product standard or purchase order.

Testing Methods

Specimen Preparation

Specimens are sectioned with adequate coolant to avoid thermal alteration of inclusion morphology, mounted, and ground through successive silicon carbide abrasive grades. Final polishing with diamond or oxide suspensions produces a scratch-free, flat, unetched mirror surface. Preparation quality is critical: inclusions must not be pulled out, pitted, smeared, or corroded, because any preparation artifact will bias the severity rating. The specimen is examined in the polished, unetched condition so that inclusion contrast and geometry are preserved.

Microscopic Examination

Examination is performed on an upright metallurgical microscope in bright-field illumination at a calibrated total magnification of 100×. The field of view corresponds to the standard diagram geometry, nominally 0.8 to 1.0 mm across on the specimen surface. Modern systems may use a digital camera with the standard diagrams displayed beside the live image for side-by-side comparison, which improves repeatability compared with wall-chart comparison.

Chart Comparison and Rating Procedure

The operator traverses the entire polished area systematically, field by field. For every field, inclusions present are assigned to Types A, B, C, D, or DS based on morphology, distribution, and alignment relative to the rolling direction. Each field is mentally matched against the standard rating diagrams for both the thin and heavy series, and the severity number of the most closely matching diagram is assigned. Where a field falls between two diagrams, the nearest severity step is recorded, with half-step interpolation permitted where the standard allows. The worst field for each type and series over the entire specimen is retained as the final result.

Evaluation, Acceptance Criteria, and Reporting

Results are reported in the standard notation, for example A2.0, B1.5t, C1.0h, D1.0, and DS1.5, together with specimen identification, sampling location, orientation, observed area, magnification, and the edition of the rating standard used. Typical acceptance levels for pressure vessel plates under specifications such as GB 713 and common supplementary requirements to ASME SA-20 limit Type A, B, C, and D inclusions to 2.0 in the thin series and 1.5 in the heavy series, with DS ratings also controlled, commonly to 2.0 or below; the governing product specification or purchase order always prevails. Because chart comparison involves operator judgment, accredited third-party laboratories control subjectivity through trained raters, calibrated microscopes, reference specimens, and periodic proficiency comparisons, typically achieving reproducibility within half a severity step.

Applications

Inclusion chart comparison testing supports quality control and engineering decisions across the entire pressure vessel supply chain:

• Mill certification and release testing of boiler and pressure vessel plates, forgings, and tubes against specification limits.
• Incoming material verification by fabricators before forming and welding operations.
• Acceptance documentation for vessels built to codes and standards such as GB/T 150, ASME Boiler and Pressure Vessel Code Section VIII, and EN 13445.
• Steelmaking process improvement, including evaluation of calcium treatment, ladle refining, vacuum degassing, and electroslag or vacuum arc remelting on steel cleanliness.
• Failure analysis of fatigue cracks, lamellar tearing, and hydrogen-induced cracking initiated at elongated inclusions, often alongside HIC testing per NACE TM0284.
• Welding consumable qualification and research into inclusion-controlled microstructures such as acicular ferrite weld metals.
• Arbitration and re-test resolution between purchasers and producers when cleanliness disputes arise.

FAQ

What is the difference between the chart comparison method and automatic image analysis?

Chart comparison relies on visual matching of microscope fields against standard diagrams and yields severity numbers directly comparable with specification limits. Automatic image analysis, standardized in ASTM E1245 and EN 10247, measures inclusion length, width, and area electronically to produce statistical distributions. Chart comparison remains the referee method for most pressure vessel plate specifications, while image analysis is often used for research and process monitoring.

Why is the examination performed at 100× on an unetched specimen?

The standard diagrams were constructed for a calibrated field at 100× magnification, so any other magnification invalidates the comparison. Etching would reveal microstructural constituents that obscure inclusion boundaries and alter apparent morphology, so the specimen must be rated in the polished, unetched condition.

How large an area must be examined?

The entire polished surface is surveyed field by field, and ISO 4967 and GB/T 10561 recommend a total observed area of approximately 300 mm² so that sparse heavy inclusions are not missed and worst-field ratings are statistically meaningful.

What do the letters and numbers in a report such as A2.0, B1.5t, D1.0h mean?

The letter identifies the inclusion type, the number is the severity rating, and the suffix t or h indicates the thin or heavy series. A2.0 therefore means sulfides of severity 2.0, while D1.0h denotes heavy-series globular oxides of severity 1.0.

How do ASTM E45, ISO 4967, and GB/T 10561 compare?

All three standards use the same principle of comparing 100× microscope fields with rating diagrams for Types A, B, C, and D, and all now include the DS category. GB/T 10561 is technically aligned with ISO 4967, while ASTM E45 defines several practices, of which Method A, worst-field rating, corresponds most closely to the ISO/GB approach. Rating values are broadly transferable, but the exact reporting format and acceptance limits follow the standard cited in the product specification.

Can the test be performed on a failed component?

Yes. A polished section taken away from the fracture can be rated to determine whether excessive or abnormal inclusions contributed to the failure, and the results can be correlated with fracture surface features and HIC or sulfide stress cracking susceptibility assessments.