Intergranular Attack on Austenitic Steel
ASTM A262 covers a test for visually inspecting intergranular attack on stainless steel samples which have been immersed in an acid bath. The sample is prepared, and then submergned in various types of acid depending on the specific procedure. The sample is then taken out of the bath, and bent to 180 degrees. The bending of the sample stretches some of the metallurgical grains on the outside of the bent side. These stretched grains are optically inspected and characterized using a high powered microscope.
This summary will cover the fixtures needed to bend the samples.
Samples are rectangular with a length of at least 10x as long as the width. The sample is initially bent to about 150-170 degrees inside the bend fixture. Compression platens are used to finish the job and bend the sample to the full 180 degrees.
Both the bend fixture and the compression platens can be attached to a Universal Testing Machine. A benchtop arbor press can also be used to achieve the 180 degree bend.
The analysis is an optical inspection, so pictures are usually taken with the microscope. Characteristics such as grain size, fracture propagation, and grain distribution can be described using an image.