Effect of the Depth of Decarburized Layer in SKL15 Tension Clamp on Fatigue Strength

The surface of a quenched and tempered spring steel may have a decarburized layer from which the carbon component has been reduced. The fatigue strength of the decarburized layer is low compared to the base metal, which can easily develop fatigue cracks. Recently, fatigue failure was reported in the tension clamp (SKL 15) of the DFF-300 rail fastening system during use on one urban transit route in South Korea. As a result of measuring the depth of the decarburized layer of the SKL 15 tension clamp where the fatigue failure occurred, a decarburized layer thinner than the manufacturer’s maximum allowable decarburized layer was found in one of the eight tension clamps. To check the depth of the decarburized layer where the fatigue crack may have initiated, the decarburized layer was assumed to be the initial crack, and fatigue crack initiation was assessed based on the linear elastic fracture mechanics. The manufacturer’s maximum allowable decarburized layer depth of 0.2 mm may result in fatigue cracks.

STRUCTURE TRANSFORMATION IN THE EXPLOSION WELDED BIMETAL STEEL 20 + STEEL 50Cr15Мo2V AFTER THERMAL TREATMENT

The results of studies of structural changes in a two-layer explosion-welded composite material are presented: carbon steel 20 + alloyed stainless steel 50Cr15Мo2V after normalization at temperatures of 800 - 1100 С and holding time 1 hour. It was found that at 800 ° C there is a “reverse” diffusion of carbon - from its lower to higher concentrations and the formation of a decarburized layer in steel 20, and with increasing temperature, carbon diffuses back into steel 20 together with chromium, changing the structure of the heat-affected zone: with an increase in the amount of perlite , the formation of the structure of Widmannstätt and carbide colonies in steel 20 and the formation of martensite in steel 50Cr15Мo2V. The features of structural changes in the alloys that were formed after explosion welding are considered.

Resource-saving preparation of the 38KHGNM steel surface for heading of automobile components

To produce high-quality fasteners for motor group components of automobiles it is necessary to follow the increased requirements to calibrated rolled stock in terms of surface defects. Therefore, the goal of this paper is to study the reasons, types and depth of the surface defects on the calibrated rolled stock from steel 38KHGNM Ø 12.0 mm on the basis of the metallographic analysis. Before cold upsetting, the hot-rolled products are subjected to metal flow and removal of unacceptable surface defects by means of expensive turning operation during which the screw cuts and cracks might appear. It has been defined, that the hot-rolled stock from steel, grade 38KHGNM, diameter 12.0 мм has nonuniform mechanical properties, grooves, laps and partial decarburization on the surface. The heat treatment of the rolled stock with a decarburized layer on the surface contributes to its further decarburization. Poor alignment of calibrated stock during its turning at the turning machine does not enable to completely remove the decarburized layer with minimum skinning of rolled stock. It has been shown that the use of rolled stock from steel 38KHGNM with surface defects and unreasonably high decarburized layer on the surface increases its rejection by 8% and raises the consumption of rolled stock for manufacturing of important fasteners for the motor group of automobiles.

Surface Analysis, Microstructural Characterization and Local Corrosion Processes in Decarburized SAE 9254 Spring Steel

The aim of the present work was to study the surface chemistry, microstructure, and local corrosion processes at the decarburized layer of the SAE 9254 automotive spring steel. The samples were austenitized at 850°C and 900°C, and oil quenched. The microstructure was investigated using confocal laser scanning microscopy and scanning electron microscopy. The surface chemistry was analyzed by x-ray photoelectron spectroscopy. Electrochemical impedance spectroscopy and potentiodynamic polarization were used to assess the global corrosion behavior of the decarburized samples. Scanning electrochemical microscopy was used to evaluate the influence of decarburization on the local corrosion activity. Microstructural characterization and x-ray photoelectron spectroscopy analysis indicate a dependence of the local electrochemical processes with the steel microconstituents and Si oxides in the decarburized layer.

Study on Microstructure and Properties of a New Warm-Stamped Niobium-Alloyed Steel

The warm stamping technology is a promising technology to meet the needs of car weight reduction and energy conservation. In order to compare with the mechanical properties of the traditional hot-stamped boron-alloyed steel 22MnB5, a new warm-stamped niobium-alloyed steel 22Mn3SiNb was designed and tested. The optimal heating parameters for warm forming process were explored through mechanical tests, and the process of their microstructure evolution was investigated by scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD), etc. The experimental results indicate that the optimal heating parameters for the niobium-alloyed steel 22Mn3SiNb are a heating temperature of 800 °C and a soaking time of 5 min. Compared to the hot-stamped boron-alloyed steel 22MnB5 under their respective optimal heating parameters, the properties and microstructure characteristics of 22Mn3SiNb are greatly improved, and nearly no decarburized layer is found on the surface of the niobium-alloyed steel 22Mn3SiNb. In addition, the addition of Nb produces the effects of grain refinement and precipitation strengthening due to the introduction of plenty of nano-precipitated particles and dislocations. In the end, it can be predicted that the new warm-stamped niobium-alloyed steel will replace the conventional hot-stamped boron-alloyed steel.

Directions of development of production of small- size billets for long products

The aim of the work is to study the technological features of the transfer of metallurgical enterprises to the production of small-size and wire metal products from continuously cast billets. The main parameters of the rolling technology are considered: the size of the billet, the choice of the location of the continuous casting machine, the transfer scheme of continuously cast billets to small- size and wire mills. It is shown that when using a continuously cast square billet with a size of 130x130 mm and 150x150 mm, direct combination of a continuous casting machine with a rolling mill is almost impossible due to the difference in the speed of continuous casting and the roughing group of the mill. The performed calculations show that the scheme of cutting the billets and combining with the intermediate furnace is technologically feasible, however, further heating of the billets leads to significant energy losses. It has been established that an effective option for combining the continuous casting machine with a rolling mill is to use a complex of equipment with a furnace-thermostat for a billets 120 m long. This flowchart reduces the energy consumption for heating the billets by at least 53%. The proposed technology of combining the continuous casting machine with a rolling mill will provide metal savings by reducing the thickness of the scale to 0.4-0.73 mm (an average of 1.1% of the mass of the billet), improve its quality by reducing the depth of the decarburized layer from 1.1 mm to 0.3 mm. The technology provides for the transportation of liquid steel in the ladle from the steel-smelting shop, casting on the continuous casting machine, identification of defects in the billets in a hot condition, supply of the billets with a temperature of 8500С to high-temperature furnaces for heating and subsequent rolling.