Effect of Combined Laser Thermal and Shock Wave Effects on the Mechanical and Tribological Properties of Steels

Herein, we present the results of an experimental study on the mechanical properties of Fe-C alloys with different carbon contents (0.2, 0.45, and 0.8%) in a wide range of deformation rates (10-3-103 s-1) and abrasive wear resistance, which underwent combined laser thermal (laser surface hardening-LSH) and laser shock wave (Laser Shock Peening-LSP) processing. The combined treatment modes included a different sequence of exposure to laser thermal and laser-induced shock pulses on the material. The amplitude and duration of laser-induced shock waves were measured using a laser Michelson interferometer. The mechanical properties of steel samples were studied under conditions of uniaxial tension under static loads on a standard universal testing machine, the LR5KPlus, and under dynamic loading, tests were carried out on a specialized experimental complex according to the H. Kolsky method using a split Hopkinson rod. The abrasive wear resistance of hardened surfaces was studied using the Brinell-Haworth method. Studies have shown that the use of a combination of LSH and LSP treatments leads to an increase in both the mechanical properties of steels and abrasive wear resistance compared to traditional laser hardening. It has been established that in the combinations considered, the most effective is laser treatment, in which LSP treatment is applied twice: before and after LSH. Thus, after processing steels using this mode, an increase in the depth of the hardened layer was recorded-by 1.53 times for steel 20, by 1.41 times for steel 45, and by 1.29 times for steel U8-as well as a maximum increase in microhardness values by 22% for steel 20, by 27% for steel 45, and by 13% for U8 steel. The use of this mode made it possible to obtain the maximum strength properties of the studied materials under static and dynamic loading, which is associated with an increase in the volume fraction of the strengthened metal and high microhardness values of the strengthened layer of traditional LSH. The dependences of abrasive wear of the studied steels after various combinations of LSP and LSH impacts were established. It is shown that the greatest wear resistance of the studied steels is observed in the case when the LSH pulse is located between two LSP pulses. In this case, abrasive wear resistance increases by 1.5-2 times compared to traditional LSH.

Comparative analysis of dynamic strength and impact toughness of pipe steels

To study the dynamic strength of pipe steels, a cycle of dynamic tests of three grades of pipeline steels (X80, X90, and X100) was performed, which included three types of experiments based on the Kolsky method: uniaxial tension of smooth specimens, as well as tension of specimens with a V-shaped annular notch and Charpy impact test according to the three-point bending pattern of beam-type specimens with a V-notch. In the last two cases, the fracture force and impact toughness, as well as the nominal fracture stress in the specimen, were determined. To assess the effect of “cold brittleness” and determine the temperature of the ductile-brittle transition, the specified test cycle was carried out in the temperature range from +20°C to –100°C. As shown by the test results, the studied steels have a fairly high ductility: the elongation at break is 25-30%. Therefore, the destruction of specimens directly in the first load cycle is not always possible. The use of an original system for registering additional loading cycles makes it possible to determine in which loading cycle the specimen ruptured. The limiting characteristics of plasticity (ψ and δ) of all tested steel grades are weakly dependent on both the strain rate and temperature (within the range of their variation). Whereas the impact toughness of steels significantly (2-3 times) decreases with a decrease in temperatures from room temperature to –100°C.

Dynamic properties of stainless steel under direct tension loading using a simple gas gun

The combined experimental and theoretical approach was applied to the study of high-speed deformation and fracture of the 1810 stainless steel. The material tests were performed using a split Hopkinson pressure bar to determine dynamic stress-strain curves, strain rate histories, plastic properties and fracture in the strain rate range of 102 ÷ 104 s-1. A scheme has been realized for obtaining a direct tensile load in the SHPB, using a tubular striker and a gas gun of a simple design. The parameters of the Johnson-Cook material model were identified using the experimental results obtained. Using a series of verification experiments under various types of stress-strain state, the degree of reliability of the identified mathematical model of the behavior of the material studied was determined.

Creation of database for strength calculation of constructions

The problems of creation of an automated system for obtaining, processing and storing experimental information on the physical and mechanical properties of structural materials, building materials and various soil media used to provide information support for automated computing systems for analysis of strength and design of structures are discussed. The developed database is focused on studying the effects of the behavior of structural materials under highly parametric influences of force, temperature and other physical fields, as well as on equipping and experimental substantiation of mathematical models describing these effects. In the proposed variant, the database is implemented as a client-server application executed on the Microsoft.Net Framework using a powerful database management system MS SQL Server. The content of the created bank is made up of sections of primary information, material properties and material models. In order to automate the processing of data at all stages of information conversion, a special “Desktop” subsystem is included in the bank, which allows aggregating data obtained from several different experiments, correcting the selected data, processing and converting it. Processing and transformation of data in the subsystem “Desktop” can be carried out either in manual mode or on the basis of special algorithms using appropriate mathematical methods.