01. As-cast waste heat heat treatment of wear-resistant high manganese steel castings
To shorten the heat treatment cycle, the waste heat of casting can be used to toughen high manganese steel. The process is as follows: the casting is removed from the mold at 1100-1180°C. After core removal and sand cleaning, the casting temperature is allowed to cool to 900-1000°C and then put into a furnace heated to 1050-1080°C for 3-5 hours, after water cooling. This treatment process simplifies the heat treatment process but has certain difficulties in production operation.
02. Precipitation strengthening heat treatment of wear-resistant high manganese steel castings
The purpose of precipitation-strengthening heat treatment of wear-resistant high manganese steel is to obtain a certain amount and size of high manganese steel through heat treatment based on adding an appropriate amount of carbide-forming elements (such as molybdenum, tungsten, vanadium, titanium, niobium, and chromium). The dispersed distribution of carbide second-phase particles strengthens the austenite matrix and improves the wear resistance of high manganese steel. However, this heat treatment process is complicated and increases production costs.
03. Solid solution heat treatment of wear-resistant high manganese steel castings
There are a large number of precipitated carbides in the cast structure of water-toughened wear-resistant high manganese steel, so its toughness is low and it is easy to break during use.
The main purpose of solid solution heat treatment is to eliminate carbides within the grains and grain boundaries in the as-cast structure, obtain a single-phase austenite structure, improve the strength and toughness of high manganese steel, and expand its application scope.
To eliminate the carbides in its as-cast structure, the steel must be heated to above 1040°C and held for an appropriate time so that the carbides are completely dissolved in single-phase austenite, and then rapidly cooled to obtain an austenite solid solution structure. This solution heat treatment is also called water toughening treatment.
(1) Temperature of water toughening treatment: The water toughening temperature depends on the composition of high manganese steel, usually 1050~1100℃. High manganese steel with high carbon content or high alloy content should take the upper limit of water toughening temperature, such as ZGMnl3 steel and GXl20Mnl7 steel. However, excessively high water toughening temperature will cause severe decarburization of the casting surface and cause the grains of high manganese steel to grow rapidly, affecting the performance of high manganese steel.
(2) Heating rate: High manganese steel has poorer thermal conductivity than ordinary carbon steel. High manganese steel castings have greater stress and are prone to cracking when heated. Therefore, the heating rate should be determined according to the wall thickness and shape of the casting. Generally, thin-walled simple castings can be heated at a faster rate; thick-walled castings should be heated slowly. To reduce the deformation or cracking of castings during the heating process, a process of preheating at around 650°C is often used in production to reduce the temperature difference between the inside and outside of thick-walled castings, make the temperature in the furnace uniform, and then quickly raise it to the water toughness temperature.
(3) Heating time: The holding time mainly depends on the wall thickness of the casting to ensure that the carbides in the cast structure are completely dissolved and the austenite is homogenized. Usually, the heat preservation time can be calculated based on the casting wall thickness of 25mm for 1 hour of heat preservation.
(4) Cooling: The cooling process greatly impacts the performance indicators and organizational state of castings.
During water toughening treatment, the casting temperature before entering the water should be above 950°C to prevent carbides from re-precipitating. For this reason, the time from casting to water should not exceed 30 seconds; the water temperature should be below 30 degrees. The maximum water temperature after quenching shall not exceed 60 degrees. The mechanical properties of high manganese steel decrease significantly when the water temperature is high. During water toughening treatment, the water volume must be more than 8 times the weight of the casting and hanging rail. If non-circulating water is used, the water volume must be increased regularly. It is best to use clean circulating water or compressed air to stir the pool water. When using a hanging basket for quenching, the cooling of the casting can be accelerated by swinging the hanging basket.
Multi-purpose trolley type for toughening treatment of high manganese molten steel. Heat treatment furnace. Castings are usually quenched by automatic tipping or hanging basket lifting. The former can easily cause deformation of large parts and thin-walled parts with complex shapes, and it is also difficult to take out the castings from the pool after quenching; the latter is easy to take out the castings after quenching, but the hanging basket consumes a lot of money.
04. Heat treatment of wear-resistant medium chromium steel castings
The purpose of heat treatment of wear-resistant medium chromium steel castings is to obtain a martensitic matrix structure with high strength, toughness, and hardness to improve the strength, toughness, and wear resistance of the steel.
Wear-resistant medium chromium steel has high hardenability because it contains more chromium elements. It is usually austenitized at 950 to 1000°C and then quenched in air.
Timely tempering is necessary. To ensure higher hardness and avoid temper brittleness, it is usually tempered at 200 to 300°C.
05. Metallographic structure of high manganese steel after water toughening treatment
After high manganese steel is treated with water toughening, if the carbides are eliminated, it will have a single austenite structure. Such a structure is only possible on thin-walled castings. A small amount of carbide is usually allowed within the austenite grains or on the grain boundaries. The carbides in the structure of high manganese steel are divided into three types according to the reasons for their generation: one is undissolved carbides, which are carbides in the cast structure that cannot be dissolved by water toughening treatment; the other is precipitated carbides, which are Because the cooling rate during water toughening is not high enough, it precipitates during the cooling process; the third type is superheated carbide, which is a eutectic carbide that precipitates due to the heating temperature being too high during water toughening. The first two carbides can be eliminated by heat treatment again, but the eutectic carbides produced by overheating cannot be eliminated by heat treatment again. Castings judged to be unqualified due to excessive eutectic carbides can only be scrapped and are not allowed to be heat treated again.
06. Heat treatment of wear-resistant low alloy steel castings
Wear-resistant low-alloy steel castings are heat treated by water quenching, oil quenching, and air quenching depending on the alloy composition and carbon content. Pearlite wear-resistant steel used in certain working conditions adopts normalizing + tempering heat treatment.
Wear-resistant low-alloy steel castings are usually quenched at 850 to 950°C and tempered at 200 to 300°C to obtain a martensite matrix with high strength, toughness, and hardness to improve the wear resistance of the steel.
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