Effect of Hot Mounting Samples on PIP Results

Mounting of samples is commonly used as part of the surface preparation of metals, particularly for samples which are non-uniform in shape PIP tests (unlike uniaxial tests) can be carried out on mounted samples, and so on samples which may be unusual in shape or taken directly from a part.

Hot mounting in Bakelite is the most used method for mounting metallic samples. This process involves surrounding the metallic sample with Bakelite powder, which then undergoes a regime of heating to ~200°C whilst under pressure, causing the Bakelite to set hard around the sample.

For many metals, being subjected to a heat treatment of 200°C for around 5 minutes will have minimal effect on its material properties. However, some lower melting point metals, such as aluminium alloys, may be more sensitive to the heat treatment imposed by hot mounting. It is possible for these metals to be microstructurally changed during the hot mounting, altering its mechanical properties. This could cause a discrepancy between the PIP-derived results and the results from uniaxial testing, since the material tested during the uniaxial testing is not necessarily the same as the material tested using PIP.

The effect on material properties from the heat treatment of metal samples during hot mounting may be particularly pronounced for alloys which are strengthened by precipitation hardening. Precipitates within an alloy are fine regions within the microstructure that are usually a different phase from the bulk matrix. These strengthen the alloys by inhibiting the movement of dislocations through the matrix, increasing the stresses required for plastic flow. Heating the sample can result in growth of these precipitates. The effect of this precipitate growth on the mechanical properties of these alloys depends on the thermal history of the alloys. If they are in an underaged condition, the hardness may increase, whereas if they are in an overaged condition, the hardness may decrease.