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Influence of chemical elements on properties of steel and iron (5)

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 Effect of tungsten on properties of steel

Element 29: W (tungsten)


In addition to the formation of carbide in steel, tungsten is partially dissolved into iron to form a solid solution. Its effect is similar to that of molybdenum, and the general effect is not as significant as that of molybdenum according to the mass fraction. The main samples of tungsten in steel are increased tempering stability, red hardness, thermal strength, and wear resistance due to carbide formation. So it is mainly used for tool steel, such as high speed steel, hot forging die steel, etc.


Tungsten forms refractory carbide in high quality spring steel, which can alleviate the process of carbide accumulation and maintain high temperature strength when tempering at higher temperature. Tungsten can also reduce the overheating sensitivity of steel, increase hardenability and increase hardness. 65SiMnWA spring steel hot rolling after air cooling has a very high hardness, 50mm2 section of spring steel can be quenched in oil, can be used to withstand large load, heat resistance (no more than 350), the impact of the important spring. 30W4Cr2VA high-strength heat-resistant spring steel, with large hardenability, quenching at 1050 ~ 1100, tempering at 550 ~ 650, tensile strength reaches 1470 ~ 1666Pa. It is mainly used for making springs used at high temperature (not more than 500).


Due to the addition of tungsten, the wear resistance and cutting ability of steel can be significantly improved, so tungsten is the main element of alloy tool steel.


Effect of lead on properties of steel

Element 30: Pb (lead)


Lead can improve machinability. Lead-based free-cutting steels have good mechanical properties and heat treatment properties. Due to the pollution of the environment and the harmful effects in the smelting process of scrap recycling, lead is gradually replaced.


Lead and iron are difficult to form solid solution or compound, and tend to agglomerate in the grain boundary in spherical form.


Effect of bismuth on steel properties

Element 31: Bi (bismuth)


Adding 0.1~0.4 bismuth to free cutting steel can improve the cutting performance of steel. When bismuth is evenly dispersed in the steel, the particulate bismuth melts in contact with the cutting tool, acting as a lubricant and breaking the cutting to avoid overheating, thereby increasing the cutting speed. Recently a lot of bismuth has been added to stainless steel to improve the cutting performance of stainless steel.


Bi exists in three forms in free-cutting steel: it exists alone in the steel matrix, is surrounded by sulfide, and is between the steel matrix and the sulfide. In s-bi free-cutting steel ingot, the deformation rate of MnS inclusions decreases with the increase of Bi content. Bi metal in steel can inhibit sulfide deformation during ingot forging.


Adding 0.002-0.005% bismuth to cast iron can improve the casting performance of malleable cast iron, increase the tendency of white mouth and shorten annealing time, and improve the extension performance of parts. Adding 0.005% bismuth to ductile iron can improve its seismic and tensile resistance. It is difficult to add bismuth to iron and steel because at 1500 the bismuth has become so volatile that it is difficult to permeate bismuth evenly into steel and iron. At present, bi-mn composite plate with melting point of 1050 is used as additive to replace bismuth, but the utilization rate of bismuth is still only about 20%.


Nippon steel, posco iron, kawasaki iron and other enterprises have proposed that adding Bi can significantly increase the B8 value of oriented silicon steel. According to statistics, Nippon iron, JFE and Bi production of high magnetic induction oriented silicon steel invention total has more than 100, add Bi, magnetic induction reached 1.90t above, the highest reached 1.99t.


Other elements: Re rare earth


Influence on steel properties:


Rare earth elements, commonly refers to the periodic table of atomic number from 57 to 71 lanthanides (lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium) plus 21 scandium and yttrium 39, a total of 17 elements. They are close in nature and not easily separated. Unseparated rare earth is called mixed rare earth, which is relatively cheap. Rare earth can be deoxidized, desulfurized, and microalloyed in steel, which can also change the deformation capacity of rare earth inclusions. Especially, it has denatured effect on brittle Al2O3 to some extent, which can improve the fatigue performance of most kinds of steel.


Rare earth elements, like Ca, Ti, Zr, Mg and Be, are the most effective deformation agents for sulfide compounds. Adding proper amount of RE to steel can make oxide and sulfide inclusions become small and dispersed spherical inclusions, thus eliminating the harmfulness of MnS and other inclusions. In production practice, sulfur exists in the form of FeS and MnS in steel. When Mn in steel is high, the MnS formation tendency is high. Although its high melting point can avoid the generation of hot brittleness, MnS can extend into ribbons along the processing direction when processing deformation, and the plasticity, toughness and fatigue strength of steel are significantly reduced. Therefore, it is necessary to add RE into the steel for deformation treatment.


Rare earth elements can also improve the oxidation resistance and corrosion resistance of steel. The effect of oxidation resistance exceeds that of silicon, aluminum, titanium and other elements. It can improve the fluidity of steel, reduce nonmetallic inclusions, and make steel compact and pure.


The functions of rare earth in steel mainly include purification, modification and alloying. With the gradual control of oxygen and sulfur content, the traditional purification of molten steel and metamorphism gradually weakened, and replaced by more perfect cleaning technology and alloying.


Rare earth elements in ferrochrome aluminum alloy increase the oxygen resistance of the alloy, maintain the fine grain of steel at high temperature, and improve the high temperature strength, so that the life of electrothermal alloy is significantly increased.

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