Joules to Foot-Pounds for Charpy Impact Certificates

One Impact Requirement, Two Unit Systems on the Certificate

A Charpy V-notch test is a brutally simple experiment: a pendulum of known energy swings through a notched bar and the machine records how much of that energy the specimen swallowed before it broke. What arrives on the mill test report is that absorbed energy — in joules if the certificate follows EN 10204 and ISO 148, in foot-pounds if it follows ASTM A370. Fabricators who buy plate in Europe and qualify it to an American code end up holding both, and the review is a comparison of two numbers that describe one pendulum swing.

Conversion factor: 1 J = 0.737 562 149 ft·lbf. The 27 J minimum that appears on so many European certificates is therefore 27 × 0.737 562 = 19.91 ft·lbf, and a 40 J requirement comes out at 29.50 ft·lbf.

What the Absorbed Energy Actually Records

The Temperature Is Part of the Number

An energy value with no test temperature beside it is meaningless. The same steel that absorbs 150 J at room temperature may take 15 at minus forty. Whenever a converted figure is written down, the temperature has to travel with it.

A Transition, Not a Constant

Ferritic steels do not lose toughness gradually — they fall off a cliff over a narrow band of temperature, the ductile-to-brittle transition. Codes set their test temperature to keep service conditions on the upper shelf of that curve, well clear of the drop.

Lateral Expansion Reads the Same Break

Beside the energy column most reports carry lateral expansion in mils or millimetres and a percentage shear area. They describe the same fracture from different angles, and low-temperature codes often accept on expansion rather than on energy alone.

Izod Swings at a Different Bar

Izod clamps the specimen vertically as a cantilever and strikes above the notch; Charpy supports it horizontally and strikes behind the notch. Both report absorbed energy in the same units, which is exactly what makes them so easy to confuse on a data sheet.

Checking a Mill Certificate Line Against an ASTM Requirement

Incoming inspection usually comes down to one question — does this heat meet the toughness clause the purchase order invoked — asked against a report written in the wrong unit.

Take the three individual values, not just the average

A Charpy result is a set of three specimens. Enter each one rather than the mean, because acceptance normally carries a floor on the single lowest value as well as on the average, and it is the low specimen that fails a heat.

Put the joule figure in the left-hand box

The foot-pound equivalent forms as the digits land. Certificates printed in continental Europe often write 27,5 with a comma; that is accepted as typed, and so is a value with spaces separating the thousands.

Swap when the requirement is the imperial one

Sometimes the specification is the American document and the certificate is the metric one. The swap arrows turn the pair round so a 20 ft·lbf clause can be read as the 27.12 J it demands — and compared directly with the 27 J the mill reported.

Copy the plain value into the review record

The copy control above each field, and Ctrl+C from inside it, hand over the digits alone — ready for a receiving-inspection sheet, a nonconformance report or the traceability spreadsheet, with no unit text to delete.

Converting never lowers a requirement: a heat reported at 27 J is 19.91 ft·lbf, which does not meet a clause written as 20 ft·lbf minimum. The gap is under half a per cent, but an acceptance criterion is a threshold and not an approximation — if the two documents disagree, the governing specification decides, not the arithmetic.

Charpy V-Notch Requirements by Grade and Test Temperature

Each row shows a widely used product specification, the impact requirement in the unit its own standard writes it in, and the same energy converted for the other document. All values are for full-size 10 × 10 mm specimens, averaged over a set of three.

Grade / product	As written	Converted	Test temperature	Orientation
EN 10025 S355JR	27 J	19.91 ft·lbf	+20 °C	Longitudinal
EN 10025 S355J0	27 J	19.91 ft·lbf	0 °C	Longitudinal
EN 10025 S355J2	27 J	19.91 ft·lbf	−20 °C	Longitudinal
EN 10025 S355K2	40 J	29.50 ft·lbf	−20 °C	Longitudinal
API 5L X65 PSL2 line pipe	40 J	29.50 ft·lbf	0 °C	Transverse
ASTM A516 Gr. 70 vessel plate	20 ft·lbf	27.12 J	−29 °C	Transverse
ASTM A350 LF2 forged fittings	20 ft·lbf	27.12 J	−46 °C	Longitudinal
ASTM A333 Gr. 6 low-temp pipe	13 ft·lbf	17.63 J	−45 °C	Transverse
ASTM A320 L7 bolting	20 ft·lbf	27.12 J	−101 °C	Longitudinal

Read down the converted column and the pattern behind the two traditions becomes obvious: 27 J and 20 ft·lbf are the same intention rounded in two directions, as are 40 J and 30 ft·lbf. Note also that transverse specimens carry the same numerical requirement as longitudinal ones while being the harder direction to pass — rolling stretches inclusions along the plate, so across-grain toughness is systematically lower.

What This Page Does During a Certificate Review

Certificate and Clause Read Back Both Ways

A metric report and an imperial acceptance limit can be compared from whichever end you happen to be holding, so nothing has to be transcribed into a calculator before the two documents will line up.

Each of the Three Specimens in Turn

Overtype the value and the answer follows immediately, so a set of three results plus the average takes four keystroke sequences rather than four separate calculations, and the low specimen is easy to spot against the floor.

A Plain Number for the Nonconformance Report

What lands on the clipboard is the bare figure with no unit attached, which is what a quality database field, a receiving-inspection form or an email to the mill actually wants pasted into it.

Inch-Pounds for Miniature-Specimen Work

Search either dropdown and the inch-pound is there alongside the rest of the energy list, which is what miniaturised and instrumented impact work tends to report when the absorbed energies drop to single figures.

Questions From the Certificate Review Desk

Why are 27 J and 20 ft·lbf treated as one requirement when they are not equal?

Because both are round numbers chosen independently to express the same engineering judgement. Twenty foot-pounds is 27.116 J and twenty-seven joules is 19.914 ft·lbf, so the two thresholds differ by about 0.43 % — far below the scatter of a real Charpy set, where three specimens from one plate routinely spread over ten joules or more. Standards writers therefore treat them as equivalent in intent while keeping them distinct in law. For a purchase order it matters which document governs: a heat at exactly 27 J satisfies an EN clause and misses an ASTM one written as 20 ft·lbf minimum, however academic the difference looks.

The specimens were sub-size — can I compare those joules with a full-size limit?

Not directly. When material is too thin for a 10 × 10 mm bar, testing moves to a 10 × 7.5 mm or 10 × 5 mm specimen, and less material breaking means fewer joules absorbed for identical toughness. Codes handle this by scaling the acceptance figure down in proportion to the ligament remaining under the notch — three quarters of the area for the 7.5 mm bar, half for the 5 mm one — so a 27 J full-size requirement becomes roughly 20 J, or 14.94 ft·lbf, on the 7.5 mm geometry. Check the exact factor in the governing code before applying it; the tables are not identical between them, and converting the units does nothing to correct for the geometry.

The report gives three results and an average — which one has to pass?

Both, under two separate criteria. The average of the set must meet the stated minimum, and no single specimen may fall below a lower floor — commonly around 70 % of it, so a 27 J average requirement typically carries an individual minimum near 20 J. The two-part rule exists because impact energy in the transition region is genuinely scattered: one specimen may catch an inclusion or a slightly coarser grain and read far below its neighbours. Most standards also allow a retest on a fresh set when one specimen alone falls short, provided the failure is not gross.

Does a converted value tell me anything about the transition temperature?

Only in combination with the temperature it was measured at. A single point, in either unit, is one coordinate on a curve that climbs steeply from a brittle lower shelf to a ductile upper one. If a certificate reads 47 J at −20 °C — 34.67 ft·lbf — that says the steel is comfortably above the drop at that temperature, but it does not locate the drop. Establishing a transition temperature means testing a series of temperatures and fitting the curve, then reading off the point where the energy reaches a defined value or the fracture reaches 50 % shear.

Can an Izod figure be read as a Charpy one once the units match?

No. Converting joules to foot-pounds only changes how the energy is written; it cannot change the test that produced it. The specimen is clamped rather than simply supported, the notch faces the striker rather than away from it, and the stress state at the notch root differs as a result. On the same steel at the same temperature the two methods give different numbers, and there is no general factor between them — any correlation is empirical and specific to one material and geometry. If a code calls for Charpy V-notch data, an Izod certificate does not satisfy it in any unit.

Joules to Foot-pounds