Charpy Impact Testing: KV Values, Transition Temperature and J0/J2/K2

The test

A pendulum hammer strikes a small notched specimen and the energy absorbed in breaking it is recorded in joules. The standard specimen is 10 × 10 × 55 mm with a 2 mm deep V-notch; the result is written KV (K for the test type, V for the V-notch geometry). The governing methods are EN ISO 148-1 in Europe and ASTM E23 in the US. Alongside the energy, laboratories may report lateral expansion and the percentage of ductile (shear) fracture on the broken faces.

Why temperature changes everything

Ferritic steels do not absorb a constant amount of energy. As temperature falls, the absorbed energy drops — sometimes steeply — from a high upper shelf (tough, ductile fracture) to a low lower shelf (flat, brittle fracture). The range in between is the ductile–brittle transition. A structure is safe in the cold when its service temperature stays above the transition range of its steel, which is exactly why standards specify a test temperature and a minimum energy at that temperature rather than a room-temperature value.

Austenitic stainless steels, with their face-centred cubic structure, show no sharp transition — one reason they dominate cryogenic service.

Reading J0, J2 and K2

In EN 10025 structural steels, the designation suffix encodes the impact requirement:

Suffix	Minimum energy	Test temperature
JR	27 J	+20 °C (room temperature)
J0	27 J	0 °C
J2	27 J	−20 °C
K2	40 J	−20 °C

The letter gives the energy level (J = 27 J, K = 40 J) and the number gives the temperature (R = room, 0 = 0 °C, 2 = −20 °C). So S355J2 and S355K2 are tested at the same −20 °C, but K2 must absorb 40 J instead of 27 J. Other product standards extend the ladder to colder duty — normalized grades in EN 10025 and pressure-vessel grades in EN 10028 are commonly tested at −40 °C or −50 °C.

Details that decide pass or fail

• Orientation. Product standards state whether impact specimens are longitudinal or transverse to the rolling direction; transverse values are typically lower, and a certificate quoting the easier direction does not prove the required one.
• Set of three. Acceptance is normally judged on the average of three specimens; individual values are usually allowed to fall to about 70% of the specified minimum. Defined retest rules apply when a set fails.
• Sub-size specimens. Where the product is thinner than 10 mm, specimens of 7.5, 5 or 2.5 mm width are used. The required minimum average energy is then reduced in proportion to the specimen cross-section, as set out in the product standard — sub-size results are not directly interchangeable with full-size values.
• Notch quality and machine calibration matter: a blunt notch or a worn striker inflates results, which is why accredited labs verify their machines with certified reference specimens.