Impact of Magnetic-Pulse and Chemical-Thermal Treatment on Alloyed Steels’ Surface Layer

The relevant problem is searching for up-to-date methods to improve tools and machine parts’ performance due to the hardening of surface layers. This article shows that, after the magnetic-pulse treatment of bearing steel Cr15, its surface microhardness was increased by 40–50% compared to baseline. In this case, the depth of the hardened layer was 0.08–0.1 mm. The magnetic-pulse processing of hard alloys reduces the coefficient of microhardness variation from 0.13 to 0.06. A decrease in the coefficient of variation of wear resistance from 0.48 to 0.27 indicates the increased stability of physical and mechanical properties. The nitriding of alloy steels was accelerated 10-fold that of traditional gas upon receipt of the hardened layer depth of 0.3–0.5 mm. As a result, the surface hardness was increased to 12.7 GPa. Boriding in the nano-dispersed powder was accelerated 2–3-fold compared to existing technologies while ensuring surface hardness up to 21–23 GPa with a boride layer thickness of up to 0.073 mm. Experimental data showed that the cutting tool equipped with inserts from WC92Co8 and WC79TiC15 has a resistance relative to the untreated WC92Co8 higher by 183% and WC85TiC6Co9—than 200%. Depending on alloy steel, nitriding allowed us to raise wear resistance by 120–177%, boriding—by 180–340%, and magneto-pulse treatment—by more than 183–200%.

Sour Environmental Severity for HIC Susceptibility

The severity of sour environments has been determined in accordance with the European Federation of Corrosion 16 and NACE MR0175/ISO 15156-2:2015 standards for carbon and low-alloy steels, based on the experimental results of sulfide stress cracking (SSC). However, the severity map obtained from SSC test results cannot be applicable to the hydrogen-induced cracking (HIC) susceptibility. In this study, the hydrogen permeability and crack area ratio (CAR) of HIC under various pH and H2S partial pressures (pH2S) were measured to establish the link between the sour environmental severity and HIC susceptibility using grades X65 to X80 linepipe steels. In addition, the hydrogen concentration at the location of the HIC was calculated by the finite element analysis. The results showed that the sour environmental severity map obtained from hydrogen permeation tests changes with time, because the hydrogen permeability reached maximum values in the early stage and steady-state values in the later stage. Then, the HIC susceptibility did not correspond to the maximum permeability, but to the steady-state hydrogen permeability. In addition, the hydrogen content at the location of the HIC did not correspond to the maximum hydrogen permeability but corresponded to the steady-state hydrogen permeability, because HIC occurred in the center segregation part and the hydrogen atoms required a certain time to diffuse from the metal surface to the mid-thickness. These results suggest that the HIC susceptibility is dominated by the severity map obtained from the steady-state hydrogen permeability.

Features of Machining Rotor Shafts of Diesel Engine Turbochargers with a Multilayer-Coated Tool

Introduction. The paper presents the results of experimental studies of power parameters when hard alloy steels are machined with tools, the cutting units of which have multilayer hard, heat-resistant and wear-resistant coatings. The obtained data will make it possible to optimize machining hard-to-machine materials. Materials and Methods. The aim of the study is to measure the power parameters of turning products and to create experimental formulas of power parameters for different technological modes. For this purpose, a special measuring multicomponent complex was used to estimate the influence of the mode parameters on the change in the cutting force components. Results. The numerically controlled machine tool was retooled by combining it with a three-component dynamometer and tooling. The cutting unit of the tool was coated with a multi-layer hard, heat-resistant and wear-resistant coating. The tool was equipped with instruments connected to a personal computer for measuring and processing experimental data. According to the results of the study, there have been obtained graphical dependences and empirical formulas, which take into account the influence of the mode parameters on the cutting force components when machining the units of alloy steels of high hardness, heat resistance and wear resistance. Discussion and Conclusion.The study allowed us to obtain experimental formulas of cutting force components for different mode parameters when machining parts by the tool equipped with cutting plates. The plates are coated with multilayer hard and wear-resistant coatings of titanium carbonitride, aluminum oxide and nickel nitride. The coating increases significantly the hardness, heat and wears resistance of the tool cutting unit and provides quality machining.

Energy power parameter effect of hot rolling on the formation of the structure and properties of low-alloy steels

The temperature and degree of hot deformation for steel 10HFTBch have been determined. This made it possible to ensure an increase in the mechanical properties of this steel, namely, the ultimate strength up to 540–560 MPa, as well as the relative elongation up to 25–29 %. As a result, it became possible to increase the service life of wheels with increased carrying capacity. This, in turn, will make it possible to increase the load of the transported cargo by motor vehicles several times. The mechanism of the influence of the energy-power parameters of rolling on the formation of the macro- and microstructure of a two-phase steel in the process of hot deformation is disclosed. The applied scheme provided an increase in the homogeneity of the structure of the developed steel, which saved the central part of the rolled section from overheating. It has been established that a decrease in the temperature of the end of deformation leads to a decrease in the size of the recrystallized austenite grain, and, consequently, to a refinement of the ferrite grain. Also an important factor in preventing the growth of ferrite grains in the upper part of the ferritic region is the abolition of cooling of the steel in coils. The recommended mode for multicomponent alloy steel 10HFTBch is as follows: the temperature of the end of rolling is 850 °C, the beginning of accelerated cooling is 750 °C, and the temperature of strip coiling into a coil is 600 °C. The basis for ensuring the increased strength of two-phase steels is the ratio and distribution of structural fractions – ferrite (initial and precipitated from austenite), as well as martensite. When hardened by such traditional "martensite formations" as manganese, the ability to control properties is limited. This is reflected in a narrow range of variation in the strength and ductility of the developed steel. The optimal combination of strength characteristics of plastic properties reduces the metal consumption of the product by 15–25 %.

Effect of Ce Content on Microstructure-Toughness Relationship in the Simulated Coarse-Grained Heat-Affected Zone of High-Strength Low-Alloy Steels

The study aimed to identify a moderate degree of Ce addition to improve the toughness in the simulated coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels, based on the effect of the Ce content on particle characteristics, microstructure and impact toughness. Three steels with 0.012 wt.%, 0.050 wt.% and 0.086 wt.% Ce content were subjected to 100 kJ/cm heat input in their thermal welding cycles. The particles and microstructures in the simulated CGHAZ of each steel were characterized and the impact-absorbance energy levels were measured at −20 °C. The results indicated that Ce2O2S inclusion compounds were gradually modified to CexSy-CeP and CeP with the increasing of the Ce content. A higher fraction of acicular ferrite was formed in the 0.012 wt.%-Ce-treated steel due to the lower mismatch between Ce2O2S and α-Fe. Furthermore, a lower fraction of M-A constituent was obtained in the 0.012 wt.%-Ce-treated steel. As a result, superior toughness and a typical amount of ductile fracture were detected in the simulated CGHAZ of the 0.012 wt.%-Ce-treated steel. Compared with the 0.012 wt.%-Ce-treated steel, a smaller prior austenite grain was observed in the 0.086 wt.%-Ce-treated steel because of the segregation of CeP at the grain boundary. However, the larger size and density of CeP led to poor toughness in the CGHAZ of the 0.086 wt.%-Ce-treated steel.

Hydrogen stress cracking resistance and hydrogen transport properties of ASTM A508 grade 4N

Low alloy steels combine relatively low cost with exceptional mechanical properties, making them commonplace in oil and gas equipment. However, their strength and hardness are restricted for sour environments to prevent different forms of hydrogen embrittlement. Materials used in sour services are regulated by the ISO 15156-2 standard, which imposes a maximum hardness of 250 HV (22 HRC) and allows up to 1.0 wt% Ni additions due to hydrogen embrittlement concerns. Low alloy steels that exceed the ISO 15156-2 limit have to be qualified for service, lowering their commercial appeal. As a result, high-performing, usually high-nickel, low alloy steels used successfully in other industries are rarely considered for sour service. In this work, the hydrogen stress cracking resistance of the high-nickel (3.41 wt%), quenched and tempered, nuclear-grade ASTM A508 Gr.4N low alloy steel was investigated using slow strain rate testing as a function of applied cathodic potential. Results showed that the yield strength and ultimate tensile strength were unaffected by hydrogen, even at a high negative potential of -2.00 V<sub>Ag/AgCl</sub>. Hydrogen embrittlement effects were observed once the material started necking, manifested by a loss in ductility with increasing applied cathodic potentials. Indeed, A508 Gr.4N was less affected by hydrogen at high cathodic potentials than a low-strength (yield strength = 340 MPa) ferritic-pearlitic low alloy steel of similar nickel content. Additionally, hydrogen diffusivity was measured using the hydrogen permeation test. The calculated hydrogen diffusion coefficient of the ASTM A508 Gr.4N was two orders of magnitude smaller when compared to that of ferritic-pearlitic steels. Hydrogen embrittlement and diffusion results were linked to the microstructure features. The microstructure consisted in a bainitic/martensitic matrix with the presence of Cr<sub>23</sub>C<sub>6</sub> carbides as well as Mo and V-rich precipitates, which might have played a role in retarding hydrogen diffusion, kept responsible for the improved HE resistance.

Influence of alloying components on diffusion and bonding processes in powder materials

At present, powder materials are used in practically all branches of industry, from medicine to aerospace technology. This is a wide range of materials ranging from constructional and instrumental materials and ending with special-purpose materials and medical implants. Powder metallurgy methods are most often used where the manufacture of products with desired properties is impossible using traditional methods: casting, stamping, etc. The production of all these materials is based on such basic operations as: obtaining starting materials, molding from these materials blanks of a given shape, size and strength, and sintering, intended for the final formation of the required properties and dimensions. The peculiarity of powder metallurgy technology allows creating a huge variety of developed technological schemes, which puts these technologies to a new level and allows for the rapid development of many industries. Alloying powder steels, in contrast to cast steels, has a number of characteristic features due to the specificity of their production. The structure of powder alloy steels and their properties depend on the methods of obtaining steels and technological features of their production. The following main methods of obtaining powder alloyed steels can be named: preparation of multicomponent mixtures of powders of iron and alloying elements and their subsequent processing; the use of alloyed iron powders, to which, if necessary, add carbon or other alloying elements; impregnation with liquid alloying metals or diffusion saturation of frameworks sintered from iron powders.