Abnormal structural defects of alloy rails can be easily dealt with by medium frequency induction heating power supply

Rails are important components of railway transportation. As we all know, rails must have high strength and wear resistance, as well as high fatigue resistance. The carbon mass fraction of rail steel is generally 0.55%-0.75% high carbon steel, while alloying elements such as manganese, silicon, molybdenum, etc. are added to improve its performance. The rail metallographic structure requires a full pearlite structure, and abnormal structures such as martensite and bainite are not allowed. Abnormal structures will significantly reduce the toughness and fatigue properties of the steel and cause microcracks, which can easily lead to the risk of rail explosion.

The rails that suffered abnormal fractures were inspected and analyzed. Metallographic observation found that several abnormally bright bands with a width of about 10 microns were found intermittently distributed in the tissue. The tissue in the white bright area of the abnormally bright band was not a pearlescent tissue; there were needle-like or bamboo leaf-like tissue characteristics inside, with typical high Morphological characteristics of carbon martensite. Martensite needles are found in two forms: black and white. Black needles are slightly tempered martensite, and small precipitates can be seen under an electron microscope. This is because during the continuous cooling of the rail, the martensite formed first produces slight tempering and precipitates carbides due to the residual heat. During the continuous cooling, the martensite formed later does not appear due to the low temperature, so this tempering precipitation phenomenon does not occur. After corrosion, the difference in morphology of black and white martensite appears. The microhardness test confirmed that the abnormal structure is martensite, and the microhardness of the abnormal area is about 2 times that of the normal area.

Analysis shows that the martensite formed in the rail is high-carbon lamellar martensite. Among various structures (pearlite, bainite, martensite), martensite has the highest hardness and strength and the lowest plasticity. , its performance characteristics are hard and brittle, with high crack sensitivity. Martensite is a supersaturated interstitial solid solution in α-iron, and its microscopic lattice distortion causes large residual stress. This residual stress is the source of microcracks in the workpiece. The existence of microcracks is the main reason for the initiation and expansion of cracks in the martensitic structure, leading to rail bursts. It is also an important factor causing rail transverse fatigue fracture, horizontal fracture at the rail waist, and longitudinal crack damage at the rail head.

In order to explore the characteristics and causes of the abnormal martensite structure of the fractured failed rail, further physical and chemical examination and analysis were conducted on the samples at the cracked parts of the workpiece. The inspection found that there was severe component segregation at the rail waist fracture part, and the elements with higher content delayed the pearlite phase transformation process, which was not conducive to the formation of pearlite structure. In particular, the segregation of Mo content was serious, which seriously affected the pearlite transformation. It is precisely because The composition of the rail waist is segregated, and Mo, V, Cr, Mn and other alloying elements are enriched there, which delays and inhibits the pearlite transformation, preventing the pearlite phase transformation from occurring there, so a martensite-like structure is generated during cooling. 

In order to eliminate martensite structure, we must first eliminate segregation defects that occur in steel smelting. This requires strict smelting production processes and strict control of steel quality inspection standards to prevent steel with serious segregation or inclusion defects from flowing into the product processing process.

On the other hand, for rail parts containing martensite structure, medium frequency heating power is used for tempering heat treatment to eliminate residual stress, reduce and adjust the hardness of the workpiece, and improve the toughness of the workpiece to meet the technical requirements for excellent comprehensive mechanical properties.

Performance tests have shown that the toughness and plasticity of the rails tempered by medium-frequency induction heating power are significantly improved, and the strength is maintained at a very high level, which meets the technical requirements of the rails.