Molybdenum plays a critical role in the casting production of high-manganese steel.
1. Molybdenum can effectively inhibit cementite aggregation between 450-600°C, causing molybdenum carbides to be dispersed in austenite in excellent sizes. Through dispersion strengthening, the austenite structure is strengthened, making high manganese The strength and hardness of the steel are increased, and the deformation hardening properties are enhanced, thereby improving the wear resistance.
2. Molybdenum can effectively inhibit the precipitation of grain boundary carbides during the cooling process of high manganese steel. Adding chromium to high manganese steel greatly increases the tendency of grain boundary carbide precipitation. The combined addition of molybdenum and chromium to high manganese steel can bring out the beneficial effects of the two alloying elements at the same time.
3. Molybdenum and nickel have similar effects in inhibiting the precipitation of acicular carbides from austenite. After adding molybdenum to high manganese steel, the acicular carbides become shorter, the number is significantly reduced, and the precipitation temperature increases, which can cause the embrittlement of castings. The temperature increases to about 350°C. Therefore, molybdenum-containing high manganese steel is mainly used for thick-walled castings and is also suitable for parts working at higher temperatures (<350°C).
Many small and medium-sized foundries often ignore the impact of chemical composition on high manganese steel castings when making high manganese steel. Some smelting masters even do not understand the impact of chemical composition on castings. How can we make high-quality high-manganese steel castings in this way? Even qualified products cannot be produced.
Therefore, for small and medium-sized foundries, it is very necessary to strengthen the professional knowledge learning of smelting furnace workers.
Today, we will learn more about the role and mechanism of molybdenum in high manganese steel casting.
When molybdenum condenses in austenitic manganese steel, part of it is dissolved in austenite and part of it is distributed in carbides. Molybdenum can improve the tendency of austenite to develop along dendrites and improve the stability of austenite. Molybdenum can inhibit carbonization precipitation and promote the formation of pearlite. Therefore, the addition of molybdenum has a good effect on improving the crack resistance and water-quenching quality of large-section castings and also has a good effect on preventing embrittlement when welding, cutting, or when the temperature is higher than 275 degrees Celsius. While molybdenum significantly increases the yield strength of steel, the toughness does not decrease or even increase. The content of molybdenum in steel is generally less than 2%.
The effect of molybdenum on the mechanical properties of high manganese steel is manifested in the reduction of brittleness due to the reduction of grain boundary carbides in the cast state. After heat treatment, when the molybdenum content in steel is less than 2%, the yield strength increases, but the plasticity of the tensile strength does not decrease.
After adding molybdenum to high manganese steel, water toughening can be used to make the molybdenum solid dissolve in austenite to play an alloying role. Precipitation strengthening treatment can also be used to precipitate dispersed carbides in austenite to strengthen the steel and improve its wear resistance.
During the cooling phase transformation of high manganese steel, molybdenum inhibits the decomposition of austenite. Molybdenum can also form complex cementite with iron and carbon. Due to the strong binding ability of molybdenum and carbon, special carbides such as molybdenum carbide, molybdenum carbide, etc. can be formed when the molybdenum content is high.
The diffusion rate of molybdenum in steel is much slower than that of carbon, which slows down the dissolution rate of carbides in steel. This rate is prolonged when the molybdenum content exceeds 0.5%. Therefore, the amount of carbides precipitated in high manganese steel containing molybdenum is small after crystallization and solidification, and the carbides that have appeared are difficult to dissolve into the austenite matrix during heat treatment. This effect can delay the precipitation time of needle-like carbides in steel and move its precipitation temperature toward low temperature. When the molybdenum content reaches a certain amount, the carbides in the as-cast structure can be eliminated. Since chromium can precipitate a large number of carbides in the cast structure, and molybdenum has the opposite effect, molybdenum is often added to high manganese steel containing chromium to reduce the brittleness of the cast structure. Precipitated carbides are difficult to dissolve during heating. This effect can be used for precipitation strengthening treatment to make dispersed granular carbides appear in the steel, thereby improving the steel’s ability to resist abrasive wear.
Molybdenum can also change the morphology of carbides. Germanium-like carbides rarely appear in steels containing molybdenum and tend to become unsecured or granular. Since the as-cast carbides are reduced, their distribution morphology also changes, so increasing the carbon content in the steel will not cause excessive carbides to appear. According to research results, the carbon content can be increased to about 1.7%. Increasing the carbon content is beneficial to improving the wear resistance of the material under non-impact abrasive wear conditions.
Molybdenum can also refine the microstructure of steel after water toughening treatment. When the chemical composition, heat treatment method, and process are the same, the grains of high manganese steel containing molybdenum will be finer after heat treatment.
In short, molybdenum is a beneficial element for high manganese. The practice has proved that high manganese steel containing molybdenum has good wear resistance under severe abrasive wear conditions.
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