Explosive Hardening of Metals

the change of the mechanical properties of metals caused by a shock wave, which results in deformation.

Explosive hardening of metals has been known as an independent process since the beginning of the 1950’s. The shock wave arises in the metal as a result of the detonation of a contact explosive charge. Explosive hardening of metals also occurs as a side effect of stamping and explosive welding. A shock wave of 10-50 giganewtons per sq m (GN/m²), or 100,000-500,000 kilograms-force per sq cm (kgf/cm²), causes high-velocity deformation at a high stress level, which leads to intensive development of plastic displacement in microvolumes. In this case the density of defects and, consequently, the hardening are found to be considerably greater than in the case of deformation under normal conditions (that is, at moderate strain rates). The quality of hardening depends on the pressure at the shock-wave front and on the properties of the metal. Explosive hardening of metals in-creases the hardness and the strength, whereas the plasticity and impact strength decrease. For example, in high-manganese G13L steel, shock waves of 20 GN/m² (200,000 kgf/cm²) increase the Brinell hardness from 200-220 to 300-350 and the tensile strength from 6.0 to 10.0 MN/m² and decrease the impact strength from 1,700 to 950 kilojoules per sq m and the elongation per unit length at break from 15 to 7 percent.

The main characteristics of the explosive hardening of metals are the small residual change of the dimensions of the hardened workpiece (up to 2-5 percent, depending on the processing) and the great depth at which changes of the metal properties are achieved (up to 50-100 mm, depending on the height of the charge or the impact plate thickness). Explosive metal hardening is used for increasing the wear resistance of the cores of railroad frogs, the teeth of power-shovel buckets, rock-crusher jaws, and bearing bushings. The service life of parts hardened by explosion is increased by a factor of 1.5 to 2. Explosive deformation may be a preliminary operation for the subsequent change of the metal’s structure by annealing.