Molybdenum exists in the steel solid solution and carbide, have solid solution strengthening effect, also can improve the hardenability of steel.
Adding a trace amount ofboron powder (0.001% to 0.005 %) can significantly improve steel’s hardenability, improve the technological properties of steel and mechanical properties of the steel. This is due to trace amounts of boron is mainly distributed in the austenite grain boundaries, resulting in reduced crystal defects in the grain boundaries, make the rate of austenite decomposition slows down, increase the hardenability. The effect of trace boron to improve the hardenability is equivalent to 2.6% Ni, 0.3% Cr and 0.2 % Mo. So that to improve the hardenability, boron can replace scarce or expensive alloying elements.
Molybdenum and boron are have significantly delayed effect on pearlite transformation, and less affected to bainite, therefore, in a fairly large range of cooling rates can get all bainite organization. Molybdenum or boron were the basic added elements of bainite micro alloy non-quenched and tempered steel. In order to reduce the cost of steel production, recent years have developed some boron-containing but without molybdenum bainitic steels.
The trend of molybdenum formed carbides in the steel is similar to chromium, that is can be solid dissolved in ferrite or austenite, also can be formed of carbide. When the content is higher, will successively form of Mo23C6, Mo2C, Mo6C and other types of carbides. Molybdenum in the steel is both play the role of solid solution strengthening and precipitation strengthening, so that can significantly improve the strength and hardness of the steel.
According to metal powder supplier, boron resource is rich and the price is cheap. Added boron to steel can significantly save expensive nickel, chromium, molybdenum and other alloying elements, have considerable economic benefits. The main advantage of boron steel is cheap, while ensure the steel has the required hardenability and mechanical properties, steel’s heat and cold processing performance is better. The main disadvantage is that the hardenability’s fluctuations not larger than the boron- containing steel. The main role of boron in steel is to increase the steel’s hardenability, thereby saving more rare and expensive metals, such as nickel, chromium, molybdenum, etc. It can instead of 1.6% nickel, 0.3% chromium, 0.2% molybdenum, boron instead of molybdenum should be noted that, because the molybdenum can be prevented or reduced temper brittleness, but boron is slightly have a tendency of promoting temper brittleness, so it can not completely with boron instead of molybdenum.
Molybdenum allows C curve remove to right side, thereby significantly improving the hardenability of steel, molybdenum can significantly improve steel’s recrystallization degree (every 1% molybdenum enables the steel’s recrystallization temperature increased to 115°C), strongly increase creep resistance of iron ferritic. In tempering, molybdenum can be effectively suppressed carbides and aggregation, so that make the steel maintaining a high strength and hardness at a higher temperature, can inhibit two types of temper brittleness, improve the uniformity of the distribution of carbides, improve the toughness within the entire scope of the tempering temperature. Mo can reduce the thermal sensitivity of steel, refined steel grain. Molybdenum is an indispensable element in the high temperature heat-resistant steel alloy.
Usually in alloy structural steel, about 0.3% to 0.40% of Mo can be effectively inhibited temper brittleness. Mo can significantly improve the hardenability in the quenched and tempered steel, refined austenite grain, improve tempering resistance and inhibit the role of second temper brittleness, in the quenched and tempered steel, the add amount of Mo is generally less than 0.6%.
Mechanism of action of boron to improve the hardenability has been proposed variety of claims, wherein the more consistent understanding is: due to boron delayed the ferrite nucleation process (but does not affect the thermodynamic properties of austenitic or ferritic matrix, that boron can reduce the ferrite nucleation rate, but does not affect the grow rate, and the rate of forming pearlite and martensite) to improve the hardenability of the steel mold.
Source:http://www.mhcmp.com
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