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Vacuum Hardening Strengthening Process for Low Carbon Steel

Some parts in the textile machine were originally made of malleable cast iron, which had low mechanical properties and often broke during work. 20 steel vacuum hardening was used to replace the original malleable cast iron material. The actual operation proved that the service life was increased by more than 10 times, and the shape and precision of the parts were also high. The machine runs smoothly and the product quality has also improved. The experiment also proves that the vacuum hardening strengthening effect of low carbon steel is not only affected by the chemical composition of the steel, heating temperature, holding time and cooling medium, but also has a great relationship with the original group state and surface quality of the steel. Therefore, in order to obtain stable low-carbon martensite, these factors must be considered comprehensively in the vacuum hardening strengthening process. For this reason, experiments and researches were carried out on the vacuum hardening strengthening process of low carbon steel.

1 Experimental method for vacuum hardening strengthening of low carbon steel

The steel used in the experiment is 20 steel, and its chemical composition (mass fraction, %) is: 0.17~0.24C, 0.27Si, 0.50Mn, 0.40P, <0.40S. The parts are made of cold-rolled or hot-rolled steel plates, which are formed by punching. Vacuum hardening furnace is used for vacuum hardening heating, the heating temperature is 880-980°C, the holding time is calculated as 1 min/mm, the cooling medium is w(NaCI)=10% solution (temperature control ≤40°C), and it is used without tempering after treatment. And do hardness test, metallographic structure observation and physical tensile test.

2 Analysis of experimental results of vacuum hardening of low carbon steel

2.1 Comparison of mechanical properties

The manufacturing size and assembly precision of textile parts are high, and they are subjected to multiple impact tensile pulsating stresses under low load (maximum tensile load 350kg) during operation. It was originally made of malleable cast iron, which has poor craftsmanship, low performance, and low dimensional accuracy, and often breaks during operation. For this reason, malleable cast iron was replaced by 20 steel which was strengthened by fire, and physical simulation performance tests of the two materials were carried out. It can be seen that the elongation of 20 steel is similar to that of malleable cast iron under the maximum load after vacuum hardening and strengthening (920℃x4min, 10%NaCl aqueous solution). But the tensile strength is higher than that of malleable cast iron, and the fracture rate of malleable cast iron parts is more than 60% under the load of less than 4000 N. The use proves that the parts strengthened by vacuum hardening of low carbon steel are more than 10 times higher than the malleable cast iron parts, and the shape and precision of the parts are also high, the machine runs smoothly, and the product quality is improved.

2.2 Factors affecting the vacuum hardening strengthening process

2.2.1 Chemical composition

Carbon content is the main factor affecting the properties of low carbon martensite. With the increase of carbon content in steel, the strength and hardness of martensite after vacuum hardening increase, while the ductility and toughness decrease accordingly. Generally, the range of carbon content is required to be narrower, but the carbon content of 20 steel is 0.17%~0.24%. Even if it is the same 20 steel, but the origin and furnace number are different, the carbon content is also different, often resulting in the same hardening strengthening process and surface The hardness fluctuates in a large range. Due to the wide range of carbon content in 20 steel, it is difficult to ensure the stability of the hardness of parts after vacuum hardening. In addition, the inhomogeneity of chemical composition also directly affects the stability of the obtained performance.

When the composition is uneven and vacuum hardening, the supersaturation of solid solution carbon in the area with high carbon concentration is large, and low-carbon martensite with high hardness can be obtained, while the low carbon concentration around the low-carbon martensite group forms a strengthened Pearlite organization (or troostite). Thereby exhibiting uneven hardness. Therefore, the influence of various factors should be considered comprehensively when formulating the vacuum hardening heating process of low carbon steel. According to these factors, the temperature range of vacuum hardening heating for low carbon steel is summarized.

2.2.2 Austenitizing temperature

The original microstructure of the material is the same, but the difference in the vacuum hardening heating temperature will lead to the difference in the martensite microstructure, which will affect the mechanical properties. We choose A3 hot-rolled steel plate with wc=0.12%～0.18% for metallographic observation and hardness and tensile (plate sample) test at different vacuum hardening temperatures.

Generally, the vacuum hardening heating temperature of hypoeutectoid steel is selected according to Ac3+(30～50)℃. Considering the improvement of vacuum hardening strengthening effect, appropriately increasing the hardening heating temperature (Ac,+100℃) is conducive to the uniform composition of austenite and The martensitic strengthening effect is favorable. Especially the strengthening effect of A3 steel is more remarkable.

From the above results, it can be seen that with the increase of heating temperature, the obtained microstructure state is different, and the microstructure at 880~920 °C is mainly strengthened pearlite + lath martensite (the amount of martensite increases with the increase of temperature), and the strength Also improve. When the heating temperature is 950~980℃, lath martensite is obtained, and the martensite lath grows, but the performance does not decrease significantly.

2.2.3 Original organization

It is found in the production that the material and vacuum heat treatment process are the same, but the original structure state is different, and the effect of vacuum hardening and strengthening of A3 steel is also different. Due to the different original microstructures, the hardnesses of cold-rolled steel and hot-rolled steel after vacuum hardening are 38-44 HRC and 35-40 HRC, respectively.

Due to cold-rolling deformation, the grains of cold-rolled steel are elongated along the rolling direction, and the structure becomes thinner, resulting in different thicknesses of martensite after vacuum hardening, resulting in differences in hardness. The reason is that grain refinement, grain boundary distortion and dislocation density increase during the cold rolling deformation process, and increase significantly with the increase of deformation degree. In the state of a large number of dislocations, when heating, on the one hand, the nucleation rate of austenite is increased, the initial grain is refined, and the driving force for grain growth is also increased, and the refined austenite grain can be obtained. . On the other hand, the deformation texture formed in the cold deformation is inherited to the α phase through the phase transformation of rapid heating and hardening (α→y→α), and the stronger the deformation texture, the stronger the inherited texture after rapid heating and hardening, increasing The stronger the effect of the fire, the more obvious it is. Therefore, if cold-rolled and hot-rolled steel parts with the same carbon content are used to obtain the same vacuum hardening strengthening effect, the lower limit temperature of vacuum hardening heating for cold-rolled steel parts is not only beneficial to grain refinement. It is also beneficial to reduce the fire stress of parts.

2.2.4 Effect of steel surface quality

Due to the different manufacturing processes, the surface quality of cold-rolled steel is better than that of hot-rolled steel. The quality of the surface has a great influence on the quality of vacuum hardening, especially for the parts that are directly formed and quenched without machining. Due to repeated heating and rolling of hot-rolled steel sheets, defects such as surface decarburization and oxidation will make the hardenability of the steel worse, and the hardness will decrease after hardening. If the parts with severe scale are pickled or sandblasted before vacuum hardening, the surface hardness can be improved.

3 Conclusion of vacuum hardening strengthening process for low carbon steel

(1) The vacuum hardening strengthening effect of low carbon steel is not only affected by the chemical composition of the steel (mainly carbon content) and heating temperature, but also has a great relationship with the original microstructure and surface quality of the steel.

(2) Using low-carbon steel vacuum hardening instead of malleable cast iron to produce textile machine parts with good comprehensive performance. Through the comparison of actual production and use, the life of the parts is increased by more than 10 times, and the machine runs smoothly, which has obvious social and economic benefits.

(3) Low-carbon martensite has good comprehensive mechanical properties, and has been widely used in machinery manufacturing industries such as petroleum, automobiles, and machine tools, and has broad prospects in the textile industry. We further study to expand its range of application in textile machinery.