Pressure Vessel 20°C Impact Energy Test: Standards, Methods, and Acceptance Criteria

The 20°C impact energy test, typically performed as a Charpy V-notch pendulum impact test, is a core requirement for verifying the notch toughness of pressure vessel steels, forgings, and welded joints before fabrication and service. Conducted in accordance with standards such as GB/T 229, ISO 148-1, and ASTM A370, the test measures the absorbed energy (KV2) of standardized specimens to confirm resistance to brittle fracture at the design temperature. This article explains the technical basis, sample requirements, testing methods, acceptance criteria, and typical applications of the pressure vessel 20°C impact energy test.

Technical Overview

The pressure vessel 20°C impact energy test is a destructive mechanical test used to evaluate the notch toughness of metallic materials intended for pressure-bearing equipment. A machined specimen with a V-shaped notch is struck by a calibrated pendulum hammer, and the energy absorbed during fracture—reported as KV2 in joules—is recorded. Higher absorbed energy indicates greater resistance to brittle, sudden fracture, which is a critical safety property for pressure vessels operating under internal pressure, thermal stress, and cyclic loading.

For most carbon and low-alloy steel grades used in pressure vessel construction, 20°C serves as the reference test temperature because it corresponds to ambient design conditions for standard service. The test is mandated by pressure vessel codes and material standards such as GB 150, TSG 21, GB/T 713, and NB/T 47008, and it is widely performed by accredited third-party laboratories for material qualification, incoming inspection, and weld procedure qualification. Materials intended for low-temperature or cryogenic service are additionally tested at their design minimum temperature rather than relying solely on the 20°C result.

Testing Samples

Pressure Vessel Steel Plates

Carbon steel and low-alloy steel plates are the most frequently tested product form. Typical grades include Q245R, Q345R, Q370R, 15CrMoR, 14Cr1MoR, and 13MnNiVR for ambient and elevated-temperature service, as well as low-temperature grades such as 16MnDR, 15MnNiNbDR, 09MnNiDR, and 08Ni3DR. Impact specimens are usually taken transverse to the rolling direction at quarter-thickness locations, and the notch orientation is defined by the applicable material standard.

Forgings and Bar Stock

Forged components such as flanges, tube sheets, nozzles, and gasket blind plates are sampled in accordance with NB/T 47008 or equivalent forging standards. Common grades include 20, 16Mn, 20MnMo, 35CrMo, and low-temperature forging steels. Specimens are oriented tangentially or longitudinally depending on the forging geometry and code requirement.

Pipes and Tubes

Seamless and welded steel pipes used for shells, nozzles, and heat exchanger tubing may require impact testing when wall thickness, material grade, or design temperature triggers the requirement. Specimens are taken longitudinally or transversely as the applicable pipe standard specifies.

Welded Joints

For welding procedure qualification and product weld test plates, impact specimens are extracted from three distinct zones: the weld metal, the heat-affected zone (HAZ), and the fusion line. The notch is positioned precisely in the zone of interest, and each zone is evaluated against its own acceptance requirement. This sampling strategy reveals whether the welding process, consumables, or post-weld heat treatment have degraded toughness.

Testing Items

• Charpy V-notch absorbed energy (KV2) at 20°C, reported for each of three specimens together with the arithmetic mean.
• Verification of individual specimen values against the minimum single-value requirement, typically not less than 70 percent of the specified average value.
• Zone-by-zone evaluation of welded joints, covering weld metal, HAZ, and fusion line toughness.
• Lateral expansion measurement and shear (fibrous) fracture appearance percentage when specified by the contract or standard.
• Instrumented force–displacement records for crack initiation and propagation energy when advanced characterization is required.

Testing Methods

The test is performed according to GB/T 229, ISO 148-1, EN ISO 148-1, ASTM A370 combined with ASTM E23, or JIS Z 2242, depending on the jurisdiction and customer specification. Testing machines are verified and calibrated in accordance with GB/T 3808 or ISO 148-2. The principal elements of the method are as follows:

• Specimen preparation: standard specimens measure 10 mm × 10 mm × 55 mm with a 45° V-notch, 2 mm notch depth, and 0.25 mm root radius. Where thickness does not permit full-size specimens, subsize specimens of 7.5 mm or 5 mm width are used, and acceptance values are adjusted in proportion as the governing standard directs.
• Equipment: a pendulum impact machine of 300 J or 450 J capacity is used, and the absorbed energy should fall within the calibrated working range of the machine.
• Temperature control: the specimen temperature is maintained at 20°C ± 2°C, verified with a calibrated thermometer or thermocouple, and the specimen is fractured within the time limit specified by the standard after removal from the conditioning environment.
• Test execution: the specimen is centered on the anvil supports with the notch aligned against the striking edge, the pendulum is released in a single uninterrupted blow, and the absorbed energy is read from the dial or data acquisition system.
• Acceptance and retest: acceptance criteria are set by the product or construction standard, not by the test method standard. As an example, common carbon-manganese boiler and pressure vessel plates such as Q245R and Q345R typically require an average KV2 of at least 41 J at 20°C, with no single specimen below 70 percent of the specified value. Where results are marginal or non-conforming, the specific retest provisions of the governing standard, which generally involve testing additional specimens and re-evaluating combined or individual minima, must be followed.

Applications

• Incoming material inspection to confirm that delivered plates, forgings, and pipes meet the impact requirements of their certified material test reports.
• Welding procedure qualification, verifying that the weld metal and heat-affected zone achieve adequate toughness with the proposed process, consumables, and heat treatment.
• Product weld test plates for manufactured vessels, satisfying the mandatory requirements of pressure vessel codes before delivery.
• Validation of post-weld heat treatment cycles, since improper tempering or stress relief can significantly reduce impact energy.
• Failure analysis and fitness-for-service assessments, where measured 20°C toughness is compared against minimum expectations to judge structural integrity.
• Research and development of new steel grades, welding consumables, and heat treatment parameters.

FAQ

Why is the test performed at 20°C?

Twenty degrees Celsius is the standard reference temperature at which many ambient-service pressure vessel steels are qualified by their material standards. It represents normal atmospheric design conditions and allows consistent comparison of toughness between heats, products, and suppliers.

What does KV2 mean?

KV2 denotes the absorbed energy of a V-notched specimen fractured with a striker having a 2 mm nose radius. It is expressed in joules and is the standard reporting symbol in GB/T 229, ISO 148, and ASTM specifications.

How many specimens are tested?

A set of three specimens is normally tested for each location, orientation, and zone. The reported result is the average of the three values, and each individual value is also checked against the single-specimen minimum, commonly 70 percent of the specified average.

Can subsize specimens be used for thick plates?

Subsize specimens are used only when the product thickness cannot yield a full-size 10 mm × 10 mm specimen. The required energy values for reduced-size specimens are scaled down in accordance with the governing standard, and results from different specimen sizes are not directly interchangeable.

Does passing at 20°C cover low-temperature service?

No. Vessels designed for low-temperature or cryogenic service must be impact tested at the design minimum temperature specified by the code and design documents. The 20°C test verifies baseline toughness for ambient service but does not substitute for low-temperature qualification.

What should be done when results are below the specified value?

The applicable material or construction standard defines the retest rules, which usually permit testing additional specimens from the same lot or location under defined conditions. If the retest still fails to satisfy the requirements, the material or welded joint is rejected or must be re-heat-treated and re-verified.