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

Views:1     Author:Site Editor     Publish Time: 2019-06-03      Origin:Site

Element 15: V (vanadium)

Vanadium has a strong affinity with carbon, ammonia and oxygen, forming corresponding stable compounds. Vanadium is found mainly in the form of carbides in steel. Its main function is to refine the structure and grain of steel and reduce the strength and toughness of steel. When dissolved into solid solution at high temperature, increasing hardenability; Conversely, the presence of carbides, for example, reduces hardenability. Vanadium increases tempering stability of quenched steel and produces secondary hardening effect. Vanadium content in steel, except high-speed tool steel, is generally no more than 0.5%.

Vanadium in ordinary low carbon alloy steel can refine grain, improve the strength and yield ratio and low temperature properties after normalizing, and improve the welding performance of steel.

Vanadium is often used together with manganese, chromium, molybdenum, tungsten and other elements in structural steel due to its lower hardenability under general heat treatment conditions. Vanadium in tempered and tempered steel is mainly to improve the strength and yield ratio of steel, refine grain, pick up overheating sensitivity. In the carburized steel, because of the grain refinement, the steel can be quenched directly after carburizing, without the need of secondary quenching.

Vanadium in spring steel and bearing steel can improve the strength and yield ratio, especially increase the proportional limit and elastic limit, reduce the sensitivity of decarburization during heat treatment, and thus improve the surface quality. Bearing steel containing vanadium with five chromium has high carbonization dispersion and good service performance.

Vanadium refines grain size in tool steels, reduces overheating sensitivity, increases tempering stability and wear resistance, thus prolonging tool life.

Element 16: Cr (chromium)

Chromium can increase the hardenability and secondary hardening of steel, can improve the hardness and wear resistance of carbon steel without making the steel brittle. When the content exceeds 12%, the steel has good high temperature oxidation resistance and oxidation corrosion resistance, and also increases the thermal strength of the steel. Chromium is the main alloying element in acid - resistant stainless steel and heat - resistant steel.

Chromium can improve the strength and hardness of carbon steel in rolling state and reduce the elongation and section shrinkage. When chromium content exceeds 15%, strength and hardness decrease and elongation and area shrinkage increase correspondingly. High surface processing quality can be easily obtained by grinding chrome-containing steel parts.

The main role of chromium in the quenching and tempering structure is to improve hardenability, so that steel after quenching and tempering has better comprehensive mechanical properties, in the carburizing steel can also form a carbide containing chromium, so as to improve the wear resistance of the material surface.

Spring steel containing chromium does not readily decarbonize during heat treatment. Chromium can improve the wear resistance, hardness and red hardness of tool steel, and has good tempering stability. In electrothermal alloys, chromium can improve the oxidation resistance, resistance and strength of alloys.

Element 17: Mn

Mn can improve the strength of steel: because Mn is relatively cheap and can dissolve indefinitely with Fe, while improving the strength of steel, the impact on plasticity is relatively small. Therefore, manganese is widely used as a strengthening element in steel. Basically, all carbon steels contain Mn. Our common pressed soft steels, double phase steels (DP steels), phase transition induced plastic steels (TR steels), martensitic steels (MS steels), all contain manganese. Generally, Mn content in mild steel does not exceed 0.5%. Mn content in high strength steel increases with the increase of strength level, such as martensitic steel, manganese content can be up to 3%.

Mn improves the hardenability of steel and improves the thermal processing performance of steel: the typical examples are 40Mn and no.40 steel.

Mn can eliminate the influence of S (sulfur) : Mn can form MnS with high melting point with S in iron and steel smelting, thus weakening and eliminating the adverse influence of S.

However, Mn content is also a double-edged sword. The higher the Mn content, the better. The increase of manganese content will reduce the plasticity and weldability of steel.

Element 18: Co (cobalt)

Cobalt is commonly used in special steels and alloys. High-speed steels containing cobalt have high temperature hardness, and can obtain ultra-high hardness and good comprehensive mechanical properties by adding molybdenum into martensitic aging steels at the same time. Cobalt is also an important alloy element in hot steel and magnetic materials.

Cobalt reduces the hardenability of steel. Therefore, adding carbon steel alone will reduce the comprehensive mechanical properties after quenching and tempering. Cobalt can strengthen ferrite and add it into carbon steel. When annealed or normalized, it can improve the hardness, yield point and tensile strength of steel, and have an adverse effect on elongation and section shrinkage. Impact toughness also decreases with the increase of cobalt content. Cobalt is used in heat resistant steel and alloys because of its oxidation resistance. Cobalt - based alloy gas turbine shows its unique role.

Element 19: Ni (nickel)

The beneficial effects of nickel are: high strength, high toughness and good hardenability, high resistance, high corrosion resistance.

On the one hand, it strongly increases the strength of steel, and on the other hand, it always keeps the toughness of iron to a very high level. Its brittleness temperature is extremely low. When nickel < 0.3%, the brittle temperature is below ‐100℃, and when Ni increases, about 4-5%, the brittle temperature can be reduced to ‐180℃. Therefore, the strength and plasticity of quenched structural steel can be improved simultaneously. Cr free steel with Ni=3.5% can be cavity quenched, Cr steel with Ni=8% can also be converted to M body at very low cooling rate.

The lattice constant of Ni is similar to gamma iron, so it can be a continuous solid solution. This is conducive to improving the hardening of steel, Ni can reduce the critical point and increase the stability of austenite, so the quenching temperature can be reduced, good hardenability. Generally large section of the thick and heavy parts with Ni steel. When it is combined with Cr, W, or Cr, Mo, its hardenability increases. Nickel-molybdenum steel also has a very high fatigue limit. Ni steel has good thermal fatigue, working in hot and cold repeatedly. Sigma, alpha k height)

Ni is used in stainless steel to make the steel have uniform a-body structure to improve corrosion resistance. Ni steels are generally not prone to overheating, so they prevent grain growth at high temperatures and maintain fine grain structure.

Element 20: Cu (copper)

The prominent role of copper in steel is to improve the atmospheric corrosion resistance of ordinary low-alloy steel, especially when used together with phosphorus, the addition of copper can also improve the strength and yield ratio of steel, without adverse impact on the welding performance. Rail steel (u-cu) containing 0.20% ~ 0.50% copper has a corrosion resistance life of 2-5 times that of ordinary carbon rail, except for wear resistance.

When the copper content exceeds 0.75%, it can be strengthened by aging after solution treatment and aging. When the content is low, the effect is similar to nickel, but weak. When the content is high, it is unfavorable to hot deformation processing, and leads to copper embrittlement in hot deformation processing. In austenitic stainless steel, 2% ~ 3% copper can resist the corrosion of sulfuric acid, phosphoric acid and hydrochloric acid and the stability of stress corrosion.

Element 21: Ga (gallium)

Gallium is an element in the closed gamma region in steel. Trace gallium dissolved easily in ferrite and formed subrogation solid solution. It is not a carbide-forming element, nor does it form oxides, nitrides, or sulfides. In gamma +a two-phase region, trace gallium is easy to diffuse from austenite to ferrite, and it has high concentration in ferrite. The effect of trace amount of gallium on mechanical properties of steel is mainly solid solution strengthening. Gallium has little effect on corrosion resistance of steel.

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