Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

High temperature wedges
Standard angle beam assemblies are designed for use at normal
environmental temperatures only. For situations where metal must be
inspeced at elevated temperature, special high temperature wedges are
available. Some of these wedges will tolerate brief contact with surfaces as
hot as 480° C or 900° F. However, it is important to note that high
temperature wedges require special attention with regard to the sound path
they generate. With any high temperature wedge, sound velocity in the wedge
material will decrease as it heats up, and thus the refracted angle in metals
will increase as the wedge heats up. If this is of concern in a given test,
refracted angle should be verified at actual operating temperature. As a
practical matter, thermal variations during testing will often make precise
determination of the actual refracted angle difficult.
Surfaces as hot as 480°C / 900°F