For steel products with section thicknesses of about 10mm or less that are required to be impact tested, it is necessary to use Charpy V-notch test specimens that are smaller than the standard 10mm x 10mm test specimens (sub-size specimens).
For sub-size specimens it is normal practice for standards to specify proportionally lower impact energy values than for full-size specimens due to their smaller cross-sectional areas.
In addition, some American standards specify that particular sub-size impact specimens are tested at a temperature below that specified for full-size specimens. The general rule is that a test temperature reduction is required when a sub-size test specimens has a width across the notch that is less than 80% of the base material nominal thickness.
The requirement to apply a test temperature reduction for sub-size specimens is not applicable if lateral expansion is specified as the acceptance criterion rather than impact energy.
Details of requirements for test temperature reductions to be applied for sub-size impact test specimens for certain material grades can be found in the following codes:
| AWS D1.1 (Structural Welding Code) |
Table 4.15 |
| ASTM A333 (Steel Pipe for Low-Temperature Service) |
Table 6 |
| ASME VIII Division 1 (Construction of Pressure Vessels) |
Table UG-84.2 |
| ASME B31.3 (Process Piping) |
Table 323.3.4 |
A further consideration has to be taken in to account when a standard specifies that the impact test temperature must be reduced for a base material thicknesses below 10mm but also requires a test temperature reduction when particular sub-size test specimens are used. The rule that is applied is that the actual test temperature to be used is the difference between the two specified temperature reductions.
An example of how the test temperature is calculated under these circumstances is shown below for ASTM A333 Grade 6 thin walled pipe:
| Pipe size: |
6 inch, schedule 40 pipe
(168.3mm outside diameter x 7.11mm wall thickness) |
Standard test temperature
(for Grade 6) |
-45°C |
| Charpy V-notch width: |
5mm
(max. size that can be taken) |
Table 6 of ASTM A333 specifies a test temperature reduction of 4°C below the specified test temperature for Grade 6 because the material thickness is nominally 7mm.
The impact test specimen width is only 5mm (71% of the material thickness) and Table 6 specifies a temperature reduction of 11°C.
The actual reduction in test temperature below the standard test temperature for this situation is the difference between these two temperatures, namely:
| Temperature Difference |
= 11°C - 4°C
= 7°C |
| Actual Test Temperature |
= -45 - 7°C
= -52°C |