Water toughening treatment is to improve the structure of some austenitic steels to improve toughness. The steel casting parts are heated to a high temperature to dissolve the excess phase, and then water-cooled heat-treatment process.
Water toughening treatment is actually a solid solution treatment, commonly used in high manganese steel casting parts, because the as-cast structure of high manganese steel is composed of austenite, carbides and a small amount of phase transformation product pearlite. Carbides that precipitate along the austenite grain boundaries reduce the toughness of the steel. In order to eliminate carbides, the steel is heated to the temperature of the austenite region (1050-1100℃, depending on the fineness or coarseness of the carbides in the steel), and kept for a period of time (1h per 25mm wall thickness) to make the as-cast structure. The carbides in the alloy are basically solid-dissolved into austenite, and then quenched in water to obtain a single austenite structure.
Most of the steel casting parts can be accepted and delivered after the water toughening heat treatment in the foundry. Because the raw materials of the steel casting parts contain a large amount of carbides, the toughness is very low and it is very easy to break.
Therefore, steel casting parts need to be subjected to water toughening heat treatment, and the main purpose of heat treatment is to remove carbides. When water toughening steel casting parts is carried out, the following two points are mainly required.
1. The temperature of water toughness treatment. The temperature of the water toughening treatment depends on the composition of the high manganese steel, and is usually 1050 to 1100°C. The water toughness temperature of high manganese steel with high C content or high alloy content is taken as the upper limit. However, an excessively high water toughness temperature will cause severe decarburization on the surface of steel casting parts, and the grains will grow rapidly, affecting their performance.
2. The speed of heating and heating. The thermal conductivity of high-manganese steel is worse than that of ordinary carbon steel. If the stress is too large (the heating rate is too fast) during the heating process, the steel casting parts is easy to crack, and the carbonization of the surface of the steel casting products leads to a large difference in the C content and mechanical properties of the inner and outer layers. , then the increased C layer is prone to micro-cracks.
In general, thin-walled and simple parts can be heated at a faster rate, and thick-walled and surface-enhanced parts should be heated slowly. In production, it is often used to keep the temperature at about 650 ℃ for 1 to 3 hours in advance, so that the temperature inside and outside the furnace and steel casting parts tends to be uniform, and then quickly heat up to the temperature of the water toughening process.