An impact test certificate records the results of Charpy V-notch impact tests performed on specimens cut from a material or weld. It quantifies the material's resistance to brittle fracture at a specified low temperature — a critical property for pressure vessels, pipelines, and structural components operating below ambient temperature.
Quick Answer
Quick Answer
A Charpy impact test certificate reports the absorbed energy (in joules or ft-lbs) measured when a notched specimen is struck by a pendulum hammer at a defined temperature. It is used to verify that a material meets the minimum toughness requirement of the applicable specification or code, particularly for low-temperature or cryogenic service applications.
Why Impact Toughness Matters
Metals can exhibit a ductile-to-brittle transition (DBTT) as temperature decreases. Above the transition temperature, failure is ductile (absorbs significant energy, deforms plastically). Below it, failure is brittle (low absorbed energy, little deformation, catastrophic).
For carbon and low-alloy steels, this transition occurs in the range of -50°C to +50°C depending on composition and microstructure. Austenitic stainless steels and most nickel alloys do not exhibit a DBTT and remain ductile at cryogenic temperatures.
Pressure vessels operating in cold climates, cryogenic service, and LNG applications require material with a guaranteed minimum toughness at the Minimum Design Metal Temperature (MDMT) to prevent catastrophic brittle fracture.
The Charpy V-Notch Test Method
Specimen geometry: 10 mm × 10 mm × 55 mm bar with a 2 mm deep, 45° V-notch at mid-length. Sub-size specimens (7.5 mm × 10 mm or 5 mm × 10 mm) are used when material thickness is insufficient for full-size specimens.
Test procedure (ASTM A370, EN ISO 148-1):
- Specimen is cooled (or heated) to the specified test temperature in a liquid bath and transferred to the anvil within 5 seconds of removal
- A 300 J (or larger) pendulum hammer strikes the specimen on the face opposite the notch
- The energy absorbed by the specimen as it fractures is measured from the pendulum's residual swing
- The fracture surface is examined — percentage shear area and lateral expansion may also be reported
Units: Joules (J) per EN standards; foot-pounds (ft-lb) per ASTM. Conversion: 1 ft-lb = 1.356 J.
Test Temperature Selection
Test temperature is selected based on the MDMT of the equipment:
| Standard | Test temperature rule |
|---|---|
| ASME VIII-1 | Per UCS-66/UCS-66M impact test temperature curves, or MDMT marked on nameplate |
| ASME B31.3 | Same as VIII-1 for piping material curves |
| EN 13480 | Lowest operating temperature (LOT) or design temperature, whichever is lower |
| API 5L PSL2 | Specified in the pipe grade designation (e.g., X65 QO at -20°C) |
| EN 10028-3 | Grade-specific test temperature (e.g., P355NL1 tested at -40°C) |
For welding procedure qualification (ASME IX QW-406.31), the required Charpy test temperature is specified by the referencing code (e.g., ASME VIII-1 or B31.3) when supplementary essential variable impact testing is invoked.
Minimum Energy Requirements
Acceptance criteria vary by code and application. Common benchmarks:
| Standard / application | Minimum average energy | Minimum single value |
|---|---|---|
| ASME VIII Div. 1 (UG-84) | 27 J (20 ft-lb) for full-size | 19 J (14 ft-lb) |
| EN 10028-4 (cryogenic plate) | 27–47 J depending on grade | 70% of average minimum |
| API 5L PSL2 (body) | 40 J (29 ft-lb) for X65 and above | 27 J (20 ft-lb) |
| NORSOK MDS (Y30) | 45 J average at -46°C | 30 J |
| DNV-ST-F101 subsea pipeline | 100 J minimum depending on grade | — |
For sub-size specimens, minimum values are proportionally reduced. ASME VIII-1 UG-84(b)(4) specifies the correction factors.
What an Impact Test Certificate Must Contain
- Material identification — heat/lot number, specification, grade, product form, dimensions
- Test laboratory — name, ISO 17025 accreditation number
- Applicable test standard — ASTM A370 or EN ISO 148-1
- Specimen dimensions — full-size or sub-size (state reduction factor if applicable)
- Notch orientation — transverse (T-L or T-S) or longitudinal (L-T) relative to rolling direction; orientation significantly affects results
- Test temperature — specified and actual (bath temperature with calibration reference)
- Individual specimen results — energy absorbed for each specimen (typically 3 specimens per set)
- Average absorbed energy — calculated from the three results
- Fracture appearance — % shear area (if required)
- Lateral expansion (if required by the specification)
- Pass / Fail against stated acceptance criteria
- Technician signature and laboratory authorization
Weld Impact Testing (PQR)
When impact testing is required for a welding procedure qualification, specimens are cut from three locations:
- Weld metal (WM): Specimens centered in the weld deposit
- Fusion line (FL): Notch centered on the fusion line between weld and base metal
- HAZ (heat-affected zone): Notch at 2 mm from the fusion line into the HAZ
All three sets must meet the minimum energy requirements. HAZ results are frequently the most challenging to pass because the microstructure in the coarse-grained HAZ tends to be less tough than either the base metal or weld deposit.
What is the difference between Charpy and Izod impact tests?
Both measure absorbed energy in a notched specimen struck by a pendulum. The Charpy specimen is supported at both ends and struck in the middle; the Izod specimen is cantilevered (fixed at one end) and struck near the notch. Charpy is the standard for metals in industrial and pressure equipment codes. Izod is used mainly in plastics testing.
Does austenitic stainless steel require Charpy impact testing?
Generally no. Austenitic stainless steels (300-series), nickel-based alloys, and most non-ferrous alloys do not exhibit a ductile-to-brittle transition and maintain good toughness at cryogenic temperatures. ASME VIII-1 exempts these materials from impact testing in most applications. However, weld metal in austenitic welds may be impact tested when the referencing code specifically requires it.
What causes a heat of steel to fail Charpy impact testing?
Common causes: high sulfur and phosphorus content (sulfide inclusions reduce toughness), coarse grain size from improper heat treatment, high carbon or low manganese, presence of bainite or martensite microstructures from rapid cooling, and improper specimen orientation (using longitudinal specimens where transverse is required).
Can impact test results be retested if one specimen fails?
Yes, under a specific re-test protocol. ASTM A370 Supplementary Requirement S5 and EN ISO 148-1 allow re-testing if no more than one specimen falls below the minimum individual value and the average still meets the minimum. A re-test of three additional specimens may be performed; all three must meet the minimum average, and no single specimen may fall below two-thirds of the minimum average.
What is the MDMT and how does it relate to impact testing?
The MDMT (Minimum Design Metal Temperature) is the lowest temperature at which a pressure vessel is designed to operate in full pressurized service. ASME VIII-1 requires that the material and its welds be impact tested at or below the MDMT unless the combination of material grade, thickness, and temperature falls within the exemption curves of UCS-66. The MDMT is permanently marked on the nameplate of the completed vessel.
Ready to automate your certificate workflow?
Try TestCert free