Explosive treatment of metals
Explosive strengthening (depth hardening) of metals
Explosive strengthening by means of a shock wave generated by the detonation of a high explosive has been used for some mechanical engineering products for more than half a century. It is one of the few methods of using the construction effects of an explosive and obtaining materials with superior properties, mainly with greater strength and hardness, namely to a considerable depth beneath the surface. This method has spread the most in strengthening castings made of Hadfield steel. It is manganese-alloyed austenitic steel capable of great strengthening, and it is used for components exposed to the action of powerful impacts and wear. One of the assets of explosive strengthening is the possibility of strengthening components of rather complex shape. This method thus competes with the conventional way of strengthening during which volume changes and wear occur already during strengthening. The classic example of this technology is explosive strengthening of railway frogs, crushing elements or mining technology components. Yet, the strengthening of components for railway infrastructure, whether entire railway crossings, inserts or guard rails or other components, clearly dominates in this area.
Mechanism of strengthening
The detonation of a high explosive in close contact with metal generates a shock wave with high pressure (around 20 GPa) of very short duration. The passing shock wave and reflected waves will generate a greater or smaller number of defects in the crystal lattice of the metal (vacancies, dislocations, twinning). The strength and hardness of the metal being processed by the passing shock wave is increased in this way without significant deformations. Moreover, the material is able, despite considerable strengthening and increase in hardness, to preserve a relatively high toughness, even higher than it is conventionally manifested by cold-formed steel with the same degree of strengthening. An increase in the hardness of manganese (Hadfield) steel can be measured on average to a depth of 25 mm. A hardness of 350 HB and more can usually be reached on the surface of the metal compared to the initial hardness, which reaches values around 180 to 200 HB following homogenisation. Hardness continuously decreases in depth to the initial level of hardness of the material.
The procedure for strengthening railway frogs is carried out in accordance with the requirements of ČSN EN 15689 or according to the customer’s requirements and related foreign standards.
The procedure for strengthening railway frogs is carried out in accordance with the requirements of ČSN EN 15689 or according to the customer’s requirements and related foreign standards.
