Topics CoveredIntroductionOverviewPropertiesComposition Applications
Hot work tool steels are steels developed to tolerate high pressure, heat and abrasion conditions that prevail in manufacturing units. These operations involve shearing, punching or forming of metals at temperatures ranging from 480 to 760°C (900 to 1400°F). Hot work tool steels are also known as group H steels, and are further classified into three subgroups such as chromium (H1-H19), tungsten (H20-H39) and molybdenum (H40-H59) hot work tool steels. They contain 0.35% to 0.45% carbon, and 6% to 25% of chromium, tungsten, molybdenum and vanadium.
H11, H12, and H13 are the most commonly used chromium hot-work steels, but H19 type steel is also used rarely. The H11, H12, and H13 chromium hot-work steels can be air hardened in sections having thickness up to 150 mm. Other group H steels can also be air hardened in section of up to 305 mm. During hardening, the balanced alloy contents and air-hardening characteristic of these steels will cause minimal distortion. Majority of the steels consist of low carbon and alloy contents such that tools produced from these steels can be easily cooled in water without cracking.
Chromium is a carbide former which has high wear resistance and edge retention property. Chromium hot-work steels have been developed with good heat softening resistance due to its medium chromium content and inclusion of carbide-forming elements that include tungsten, molybdenum and vanadium. The low carbon and alloy contents enhance toughness at normal working hardness of about 40 to 55 HRC. Higher molybdenum and tungsten contents increase hot strength and red hardness but reduce toughness of the steel. Vanadium is included in the steels in order to increase resistance to erosive wear at high temperatures. Silicone content is increased in order to improve oxidation resistance at temperature of about 800°C (1475°F). Chromium in these steels also enhances oxidation and corrosion-resistant properties, improves high temperature resistance, and helps achieve high hardenability.
The H10 tool steel has high toughness. The H11 tool steel has high softening resistance, and it also has the ability to withstand temperatures of about 540°C (1000°F). It has high ductility and toughness at room temperature tensile strength of about 1720 to 2070 MPa. It can be hardened at high temperatures due to its secondary hardening characteristic, thus minimizing residual hardening stresses. Chromium hot-work steels have low thermal expansion coefficient, high weldability, corrosion and oxidation resistance, and high thermal conductivity.
The following table shows the composition limits of chromium hot-work steels.
aAll steels except group W contain 0.25 max Cu, 0.03 max P, and 0.03 max S; group W contains 0.20 max Cu, 0.025 max P, and 0.025 max S. Where specified, sulfur may be increased to 0.06 to 0.15% to improve machinability of group A, D, H, M, and T steels.
The applications of chromium hot-work steels are as follows:
- Forging dies, mandrels, hot shears, punching blades, and for extrusion of aluminum and magnesium
- Hot-work and structural applications
- Highly stressed structural parts used in aerospace technology
- Die casting, forging, piercing, heading, extrusion, trimming purposes.