International Scientific Journals
of Scientific Technical Union of Mechanical Engineering "Industry 4.0"

The goal of this investigation was to research the nature of the formation of gradient and composite zones near non-metallic inclusions during laser treatment of the steels. The materials for investigation were commercial steels containing different non-metallic inclusions. The specimens of different steels were exposed to laser beaming on the installations GOS-30M. The research methods were applied: petrography, X-ray microscopy (MS-46 Cameca, “Nanolab – 7”) and optical microscopy (Neophot-31) to study steel matrix near non-metallic inclusions and to identify of the inclusions. Nanohardness of the steel matrix near inclusions (“Nano Indenter II”) was analyzed. Peculiarities of saturation of the steel matrix by elements of non-metallic inclusions during different regimes of laser action were investigated. It was shown the role of that process in the formation of local structure of steel matrix near non-metallic inclusions. The features of the formation of gradient and micro composite saturation zones of a steel matrix under conditions of abnormal mass transfer from nonmetallic inclusions during laser processing are discussed. The difference in the rates of abnormal mass transfer of chemical elements of non-metallic inclusions into a steel matrix at the moment of laser melting is shown.

During operation, under the influence of mechanical stress, temperature, and external environment, the initial structure of a railway wheel undergoes changes that lead to its fatigue-corrosion wear.
The wheels with different tread profiles that were removed from operation due to the maximum wear of the rims were studied. The studies were carried out using the metallographic method (Neophot-21) and scanning electron microscope JSM-35.
The causes of cracks and wear particles formation near the tread surface of railway wheels associated with phase and structural changes occurring during operation were analyzed. The possibilities of changing the tread profile due to uneven wear were established. It was shown that structural and phase changes near the tread surface are associated with the development of plastic shifts due to the action of external loads, thermal stresses during braking, and the formation of “white layers” when the metal is heated by the heat of braking and abrupt cooling when the brakes are turned off. The role of non-metallic inclusions and corrosion products of wheel steel in the formation of cracks near tread of railway wheels has been established.

The influence of non-metallic inclusions on the formation of defects in deformed steels and their role in providing crack resistance are considered. It is shown that non-metallic inclusions as stress and strain concentrators are one of the most dangerous sources of defect initiation in steels during pressure treatment. It has been established that the nature of cracks and the features of their growth near nonmetallic inclusions are determined by the type of inclusions, as well as by the scheme of the local stress state, which depends on the loading conditions. An analysis was made of the features of the initiation and development of the cracks near non-metallic inclusions of various types during tensile strain, compressive and bending deformation. It is shown that the most severe way of deformation for the inclusion-matrix system is tensile strain, the softest is compression.

The features of structural changes in the cove zone on the tread of worn out railway wheels were investigated. The influence of laser processing in pulsed and continuous mode on the nature of the structure and properties of wheel steel was shown. The influence of laser treatment on the increase in the hardness and wear resistance of wheel steel has been established. It was shown possibility and necessity of local laser hardening of cove zone of railway wheels tread by coming out of bainite structure. A method for improving the wear resistance of the tread by local laser treatment has been proposed.

It was found that in the process of pulsed laser action, various phase and structural transformations occur in non -metallic inclusions, which take place under nonequilibrium conditions. It is shown that the melting of inclusions under laser action is corresponded with change of their structure and phase composition. Also it is shown that the nature of these transformations depends on the type of nonmetallic inclusion. It was found that nonequilibrium phase transformations contribute to a change in the structure, phase composition, properties and sizes of nonmetallic inclusions, as well as the inclusion-matrix interphase boundaries of steel. It is shown that changes in the structure and properties of non-metallic inclusions affect their behavior and the formation of defects in the laser-strengthened layer of steel
products.

In this research work were analyzed the reasons for fracture of pin machined from 42CrMo44 steel. One pine build in TBU (Tread Brake Unit) of railway wagon breaking system was broken in exploratory condition. Reclamation from purchaser was sent to the producer concerning occurred fracture of pin. Producer sent request to the team of the Faculty of Technology and Metallurgy to perform investigations and to determine the reason of fracture (failure analysis). It was decided to perform the following investigations: visual inspection, determination of chemical composition, hardness measurement, metallographic investigations which include analysis of microstructure and quantitate determination of nonmetallic inclusions. For comparison of results, raw material for pin production i.e. solid bars φ50, and new none used bar from the same steel was investigated too. Performed investigations showed that the main reason for the fracture of pin is the quality of the raw material more concretely segregation of chemical elements and increased concentration of nonmetallic inclusions.

Melting and crystallization of non-metallic inclusions in contact with steel matrix during laser treatment was investigated. It was shown that laser action is the method of local change of inclusion structure in the surface fused layers and also of the properties of non-metallic inclusion surface. Peculiarities of steel matrix saturation with elements of non-metallic inclusions during different regimes of laser action were investigated. It was shown the role of that process in the formation of local structure of steel matrix near non-metallic inclusions. That allows to realize possibility of transformation of the non-metallic inclusions how sources of inside local alloying of steel matrix. It was fixed that in local areas of steel matrix the liquation strengthened zones were formed. They represent different types of composite layers: gradiental zones with cascade and “spot” distribution of elements and nanohardness, dispersal zones with different types of microphases and nanophases, “tunnel” zones, and also zones with combine structure. It was shown the role of non-metallic inclusions in development of micro heterogeneous strengthening of steels under laser treatment.