One of the most common steels used to make fixtures, dies,
and tooling are tool steels. Tool steels are both strong, tough, and highly
resistant to wear due to their high hardness. Tool steels come in many
varieties with some suited to cutting tools such as high-speed steel or others
more suited to shock loads such as S1 tool steel. However, there are other tool
steels such as A2 which are popular for their balanced properties, medium hardness,
and ability to be machined in the annealed condition and subsequently heat
treated, air cooled, and tempered for final properties.
A2 is an air-hardening tool steel which is commonly used to
make durable tooling for many manufacturing and testing applications. A2 tool
steel is hardened by austenitizing and quenching in oil or air (from
temperatures around 1700-1800 degrees Fahrenheit). Once hard, the tool steel is
tempered at 400-1000 degrees Fahrenheit for 30 minutes to increase ductility
and prevent brittle fracture.
The below photomicrographs are of a sample of an A2 tool
steel test fixture that failed in a salt water stress experiment at
magnifications of 200X, 500X, and 1000X. The sample was removed from the
fixture, mounted, ground, and polished to a submicron finish. Then the sample
was etched using 4% nital to reveal the microstructure. The microstructure
consists of massive carbides and spheroidal carbide particles in a matrix of
tempered martensite.
4% Nital:
96 mL Ethanol, 4 mL Nitric Acid
