Tool steels are commonly used to make tools, moulds and dies for cutting, forming or shaping a mate
rial into a part for component because of their high strength and wear resistance. There are various grades
of tool steels, such as cold work tool steels, hot work tool steels, and high speed tool steels. Hot work tool
steels are commonly used at temperatures higher than 200°C. They resist softening up to 550°C temper
ature [1–2].
AISI H13 steel alloy is a Cr–Mo–V based hot work tool steel which is characterized by high resistance
to thermal shock and thermal fatigue, good high temperature strength, exellent toughness, ductility,
machinability, air hardenability, resistance to abrasion and high hardness. Typical applications of AISI
H13 hot work tool steel are hot punches and dies for blanking, bending, swaging and forging, hot extrusion dies
for aluminium, cores, ejector pins, inserts and nozzles for aluminium, tin and lead die casting. Hardness and
microstructure of the tool steels for specific applications should be in a defined range. Hardness of the hot work
tools steels for die casting should be 52–54 HRC, steel for extrusion should be 40–48 HRC, steel for hot
pres should be 44–52 HRC, and steel for wear resistant parts should be 50–52 HRC [1–2].
Steel is an attractive material due to possibility of making changes in its mechanical properties by
changing the microstructure. Steel properties can change from charge to charge or after heat treatment.
In addition, properties of steels can change during service. Therefore, it is necessary to develop a test sys
tem which implements fast nondestructive way for determination of mechanical properties by character
izing the microstructure.
Ultrasonic measurements can be used to characterise the microstructure and to estimate grain size in
materials [3–12]. It is advantageous to use a nondestructive method for evaluate microstructure and mea
surement of grain size of a material since the optical method is time consuming and requires cutting spec
imens from component. Additionally optical method allows only surface or limited volume measurements
to be made and gives the results only at these selected locations.
1 Th