| The introduced of alloy steel |
| Because alloy structural steel is the permeability steel, after appropriate metal heat treatment, the microstructure is homogeneous sorbite, bainite or fine pearlite, thus it has higher tensile strength and yield ratio(around 0.85), high toughness and fatigue strength, and low transition temperature of ductile to brittle , can be used in the machine parts of larger section size. |
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| The role of alloying elements in steel |
| 1, Increase the hardenability of steel.Hardenability means when the steel is quenching, how long is the depth from surface into martensite, it is the main parameters of comprehensive performance .In addition to the Co, almost all alloy elements such as Mn, Mo, Cr, Ni, Si and C, N, B can improve the hardenability of steel, the Mn, Mo, Cr, and B are the strongest, and the second is Ni, Si and Cu.And strong carbide forming elements such as V, Ti, Nb, it is only soluble in austenite can increase the hardenability of steel. |
| 2,Influence steel tempering process.Due to the alloy elements on tempering can hinder the diffusion of atoms in steel, so compared to carbon steel, under the same temperature, it can delay the decomposition of martensite and carbide gathered grew up, so as to improve the stability of temperingl, namely improve tempering softening resistance, V, W, Ti, Cr, Mo, Si are more significant effect, the effect of Al, Mn, Ni is not obvious.Contain higher content of carbide forming elements such as V, W, Mo, during tempering under 500 ~ 600 ºC, precipitate fine dispersion of special carbide particles such as V4C3, Mo2C and W2C, replace part of the relatively bulky alloying cementite,increase the strength of steel, Mo is prevented or reduced for temper brittleness steel. |
| 3,Influence reinforcement and toughening.Ni strengthening ferrite in solid solution strengthening way;And carbide forming elements such as Mo, V, Nb, both in dispersion hardening ways and solid solution strengthening way to improve the yield strength of steel;The most significant carbon reinforcement.In addition, add the alloy elements, usually refine austenite grain size, increase in grain boundary of the reinforcement.The factors affecting the toughness of the steel is more complex, Ni improve the toughness of the steel;Mn for austenite grain coarsening, sensitive to temper brittleness;Reduce the content of P, S, increase steel purity, an important role to improve the toughness of the steel. |
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| The classification of alloy structural steel |
| Quenched and tempered alloy constructional steeland surface hardening steel |
| 1, Quenched and tempered steel such as the carbon content of steel is generally about 0.25% ~ 0.25%, for a given cross section size of the structure, the tempering treatment, quenching and tempering, if along the cross section of hardening, the performance is good, if quenching, have ferrite in microstructure, the toughness is reduced.With temper brittleness tendency of steel such as manganese steel, no steel, nickel chromium steel, etc., should be rapid cooling after temper.This kind of steel quenching critical diameter, along with the increase of grain size and content of alloy elements, for example, 40 cr and 35 simn steel is about 30 ~ 40 mm, while 40 crnimo and 30 crni2mov steel is about 60 to 100 mm, often used in the manufacture of a shaft, connecting rod structure under high load. |
| 2, Surface hardening steel used to manufacture the surface is hard wear-resisting and heart of flexible components, such as gear, shaft, etc.High toughness of heart to make the parts, carbon content in steel should be low, generally in 0.12 ~ 0.25%, and the right amount of alloy elements, in order to ensure appropriate hardenability.Nitriding steel need to join to form nitrides alloy elements (such as Al, Cr, Mo etc.).Carburizing and or carbon oxygen permeability of steel, the 850 ~ 950 ºC after carburizing and carbonitriding, quenching and low temperature tempering to use (about 200 ºC) condition.Nitrided steel after nitriding treatment (480 ~ 580 ºC), used directly, no longer by quenching and tempering treatment. |
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| Alloy structural steel production process |
| According to the quality requirements of steel and steel, alloy structural steel smelting, can use oxygen top-blown converter and open hearth furnace, electric arc furnace;Or add electroslag remelting, vacuum degassing.Ingot casting or die casting can be used.Ingot should slow cooling or hot forging, rolling.Steel ingot heating, should strive to temperature uniform and there is enough heat preservation time, to improve the segregation defects and avoid uneven deformation of forging, rolling,After forging, rolled steel, small size, especially the carburizing steel carbon by about 0.2%, at 600 ºC or more should be fast cooling, in order to avoid aggravating banded structure;Cross section of large forgings, measures should be taken to eliminate internal stress and white dots.Quenched and tempered steel shall be quenched martensite structure, as far as possible and then to tempered sorbite organizations;Carburizing steel in the process of carburizing, carbonitriding layer concentration gradient shoulds not be too big, lest the carbonitriding layer continuous carbide network appear on the grain boundary;Nitrided steel required by heat treatment to get the required performance, then through the final finishing nitriding can proceed.After nitriding treatment in addition to the fragile and thin layer "white" of grinding to remove, no longer processing. |
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