Heat treatment of high manganese steel castings

Update:21-11-2018
Summary:

The International Standard specifies austenitic high ma […]

The International Standard specifies austenitic high manganese steels for wear –resistant service. The Grades covered by the ISO 13521 have GX120MnMo7-1(C: 1.05% to 1.35%, Mn: 6% to 8%, Mo: 0.9% to 1.2%), GX100Mn13, GX120Mn13, GX110MnMo13-1, GX120MnCr13-2, GX120MnNi13-3, Mn: 11.0% to 14.0%), GX120Mn18, GX120MnCr18-2 ( C: 1.05% to 1.35%, Mn: 16.0% to 19.0%) and GX90MnMo14(C: 0.7% to 1.00%, Mn: 13% to 15%, Mo: 1.0% to 1.8%). The heat treatment of high manganese steel is to heat the high manganese steel castings to the temperature of carbide solid solution, and keep it for a certain time, and then rapidly cool in water to form a single austenite structure, which greatly improves the strength and toughness. Unlike ordinary carbon steel, high manganese steel does not become hard after quenching in water, but becomes soft. Therefore, the heat treatment of high manganese steel is called water toughening treatment. In the heat treatment process, the carbide is dissolved in the austenite in a solid solution state, so it is also called solid solution strengthening treatment. The parameters of solid solution treatment of high manganese steel mainly include furnace temperature, heating rate, holding temperature, holding time and placement position.

    Before the high manganese steel castings are placed in the furnace, the sand and gating system on the surface of the castings should be cleaned. The sticky sand has heat insulation effect on the heating or cooling of the casting, so that the cooling of the casting and the cooling after entering the water are not uniform, and the severe sanding will reduce the cooling speed of the casting after entering the water, causing the grain boundary carbide to re-precipitate. High-manganese steel has low thermal conductivity and a large thermal expansion coefficient. Although the casting does not undergo phase transformation stress during low-temperature heating, heating to 300 °C or higher causes a phenomenon of brittle carbides in the crystal and grain boundaries, and pearlite transformation sometimes occurs. For example, the structure of the high manganese steel frog is complicated, and the wall thickness of the same casting is very different, and the casting itself has casting stresses ranging from small to large. There is a large temperature difference in different parts during the heating or cooling process of the heat treatment, and thermal stress is generated. Thus, the thermal stress and the casting stress are superimposed, which causes cracks in the frog. Therefore, it is necessary to control the furnace temperature and heating rate of the high manganese steel frog.

    The solution treatment temperature and holding time are determined based on sufficient dissolution of carbides, appropriate grain size of austenite, uniform chemical composition in steel, optimum mechanical properties, and prevention of overheated structures. Under industrial conditions, quenching of castings is conducted at 1050-1150 °C and followed by cooling in cold running water in order to provide complete dissolution of the carbides and accelerate the diffusion processes in austenitization. When the quenching temperature is raised to 1200 °C, the austenite grains in the casting grow and superheating-induced carbides appear.

    For high manganese steel frogs without other alloying elements, water toughness treatment temperature is about 1000-1100 °C. The cementite-type carbide dissolution process is the diffusion of carbon from the carbides into the austenite. The iron atoms of the original cementite phase self-diffuse and form a face-centered cubic austenite. The carbon atoms and other atoms in the (Fe, Mn) 3C type carbide have weaker force, the diffusion process is easy to proceed, and the dissolution rate is faster. When heated to 1000 °C, (Fe, Mn) 3C can be completely decomposed. In order to accelerate the decomposition, dissolution and diffusion, and promote the homogenization of the components, a solid solution temperature of 1050 to 1100 °C is sufficient. When the temperature exceeds 1050 °C, the austenite grains start to grow. When the temperature reaches 1120 °C, the austenite grains grow significantly. When the temperature is greater than 1150 ° C, the crystal grains are coarse and excessive heat is generated.

    For high manganese steel containing carbide forming elements such as chromium, molybdenum, vanadium, titanium, etc., there are special carbides in the structure, which is difficult to dissolve, and the solid solution temperature should be increased by 30 to 50 °C. The holding time can be such that the carbide is sufficiently dissolved and the components are substantially uniform. Excessive holding time does not contribute to mechanical properties.

    For Grade GX90MnM014, steel casings can be supplied without heat treatment if the casting thickness is less than 45 mm and the carbon content is less than 0.8% according to the ISO 13521:2015(E) standard.