Complex metallophysical studies of structural steels after two-component diffusion hardening based on boron

The studies of the structure, phase composition and microhardness of 45; 40X13 and (25...30) CMT steels during hardening in mixtures containing boron, boron and silicon, as well as boron, silicon and aluminum, are presented. Boration was carried out at a temperature of 890°C for 3 hours. The results of the study show that as a result of boration, the layer thickness is greater than in the case of borosiliconizing and boro-alumino-siliconizing, however, the needles of the boride phases are sharper. By obtaining diffusive layers that differ in structure with different phase ratios FeB, Fe2 B, Fe3 Si, it is possible to significantly influence the resistance to brittle failure of the surface layers of parts that are operated under real conditions in friction pairs under periodic or constant shock effects. So, if the wear process proceeds without ever manifesting dynamic effects, then it is possible to recommend the use of the boration process – both for small-sized parts with CTR in powder mixtures using unpressurized containers, and large-sized parts - in coatings that are applied only to the wearing surfaces of the parts. If the wear occurs under conditions of a relatively low level of periodically manifested shock effects, it is possible to use the borosiliconizing process at (800...900)°C. English version of the article is available at URL: https://panor.ru/articles/complex-metal-physical-studies-of-structural-steels-after-twocomponent-boron-based-diffusion-hardening/69778.html

Effect of In-Situ Synthesized Boride Phases on the Impact Behavior of Iron-Based Composites Reinforced by B4C Particles

The objective of this study is to investigate the impact behavior of iron-based composites reinforced with boron carbide (B4C) particles and in-situ synthesized iron borides (Fe2B/FeB). The composite specimens (Fe/B4C) were fabricated by hot-pressing under a pressure of 250 MPa at 500 °C, and sintered at a temperature of 1000 °C. The effects of the reinforcement ratio on the formation of in-situ borides and impact behavior were investigated by means of different volume fractions of B4C inside the iron matrix: 0% (un-reinforced), 5%, 10%, 20%, and 30%. Drop-weight impact tests were performed by an instrumented Charpy impactor on reinforced and un-reinforced test specimens. The results of the impact tests were supported with microstructural and fractographical analysis. As a result of in-situ reactions between the Fe matrix and B4C particles, Fe2B phases were formed in the iron matrix. The iron borides, formed in the iron matrix during sintering, heavily affected the hardness and the morphology of the fractured surface. Due to the high amount of B4C (over 10%), porosity played a major role in decreasing the contact forces and fracture energy. The results showed that the in-situ synthesized iron boride phases affect the impact properties of the Fe/B4C composites.

Structure and characteristics of boron-containing steels for fasteners

According to TU 14-1-4486-88 and TU 14-1-5490-2004, in accordance with international standards DIN EN 10263-4, OJSC «BSW – Management Company of the Holding «BMC» produces hot-rolled products of the following grades of steel: 20Г2Р, 30Г1Р, 20MnB4, 30MnB4 and 32CrB4 microalloyed with boron. They are used for the manufacture of fasteners instead of previously used steels 35X, 38X and 40X. The use of boron-containing steels reduces the surface defects of hardware; to increase the stability of the mechanical properties of bolts, screws, studs, strength class 8.8 and 10.9 by GOST 1759.4-87; to achieve a high level of toughness at negative temperatures. It is shown that steel 20Г2Р provides stable mechanical properties of products up to M27 (in contrast to steel grades 35X, which are designed exclusively for bolts of class M16 8.8). The boron-containing steels in the delivery state with the structure of granular perlite have high technological plasticity – in the cold, the bolt head is formed and the thread is rolled without tearing in the thread hollows, as is the case with chromium steels of type 40X. It was found that their ductility increases due to the low content of carbon and chromium, as well as the formation of dispersed carbonitride-boride phases of a globular form.

Study of structure and properties of steel 38KhN3МFА after low-temperature liquid borating

The effect of low-temperature liquid borating on the structure and hardness of steel 38KhN3МFА is studied. It is found that in the borating process at temperature 600...660 °C and duration 8...32 hours boride coating with thickness of 6...19 μm with surface hardness of 1900...2000 HV is formed on the steel surface. The optimal borating regimes are determined, in which hardened layer with solid core is formed. The presence of two boride phases FeB and Fe2B in the boride layer is established by metallographic, X-ray and electron microscopic analyzes.

Synthesis of Al-B system nanostructures by interaction of disperse aluminium with boron and diborane in arc discharge plasma

Experimental studies of aluminium boride synthesis as a result of interaction of disperse aluminum with diborane B2H6 and disperse boron in a flow of thermal plasma of different composition generated in electric arc plasma torch have been carried out. Experimental work on the synthesis of aluminium boride nanoparticles from elements (a mixture of disperse aluminum and boron) has shown the possibility of obtaining in thermal plasma arc discharge of such phases of the boride as AlB12 and AlB31. The specific surface of the powders obtained is from 3 to 27 m2/g. According to X-ray phase analysis, the powders obtained, except for aluminum boride phases, also contain boron, aluminum, aluminum nitride and boric acid phases. The greatest yield of aluminum boride phases is provided by using the nitrogen plasma with hydrogen and enthalpy 4.5 kWt∙h/m3 in the reactor with increased high-temperature zone. The use of gaseous diborane made it possible to eliminate restrictions on the evaporation of boron particles but did not provide an opportunity to obtain aluminum borides in the plasma-chemical process. It was concluded that it is necessary to perform quenching of high-temperature gas flow containing boron and aluminum vapor to form aluminum borides from the gas phase in plasma-chemical synthesis. Such an approach should ensure that the temperature is reduced to the values at which aluminum borides are stable and that the formation of aluminum boride nanoparticles will occur as a result of condensation from the gas phase under these conditions.

Materials for Gas-Thermal Spraying, Obtained by Diffusion Alloying from Powders Based on Austenitic Steels

The article presents experimental studies of composite powder materials for plasma spraying, obtained by the method of diffusion doping of powder materials based on austenitic steels. It is indicated that the main factors forming the diffusion layer on a powder material are a composition of the required saturating medium, treatment temperature and duration of chemical and thermal exposure. Creation of single-phase diffusion layers is possible only in the case of a minimum level of temperature-time characteristics during heat treatment. This is also facilitated by the use of media with a low concentration of boron and introduction of additives inhibiting saturation process (such as carbon, aluminum, silicon) into a saturating mixture of powder. Structure and composition of powders have been thoroughly investigated with the help of X-ray microanalysis that has made it possible to study location of elements contributing to powder alloying and micro-durametric characteristics. A component of high-boride phase is increasing due to higher degree of powder material alloying. Significant changes in phase composition, as well as the chemical one, are noticeable in diffusion processing of the following alloyed powder materials: РR-Х18N9, РR-Х18N10, РR-Х18N15. Free carbon being displaced by boride into a transition zone creates dispersed complex carbide compounds with chromium. This is confirmed by distribution nature of carbide-forming components in a powder particle. All carbide-forming elements have characteristic concentration peaks-bursts in contrast to non-carbideforming silicon. Silicon is practically not present in the considered FeB phase and it is found only in a very small amount in the studied Fe2B phase; it is pushed aside by high-boride phases to a sublayer. The change in microhardness of the FeB and Fe2B phases under study is associated with dissolution of corresponding alloying elements in them and distortions of a crystal lattice in borides. A similar phenomenon is also characteristic for saturation while using boron or while making chemical and thermal treatment of alloyed steels, it has been noted in a number of studies. The increase in microhardness of a particle nucleus during its boriding is caused by displacement of carbon and alloying elements by growing front of boride phases. A core zone moves with an increased microhardness to a particle core while increasing temperature mode and time of boronization and up to realization of the effect with counter diffusion.

On the Bonding Strength of Fe-Based Self-Fluxing Alloy Coating Deposited by Different Methods on the Steel Substrate

In the present paper, the bonding strength of Fe-based self-fluxing alloy coating deposited by plasma spraying, gluing and laser remelting and alloying on the steel substrate have been investigated. When flame melting, a globular structure is formed. Against the background of the solid solution carbide-boride phases are clearly distinguishable, between which the Fe–Fe2B and Fe–FeB eutectic colonies are situated. Laser remelting leads to the formation of metastable structures, reinforced with dendrites, consisting of alloyed Fe-α and Fe-γ. At the low laser beam speeds the coating is melted completely with the formation of a cast structure with the dendrites. When the laser beam speed is increased, the dendritic structure gets fragmented. Structures of coatings alloyed with B4C and remelted by the laser beam vary with the increase of the spot speed. The bonding strength of coating without subsequent remelting decreases by 4–5 times in comparison with remelted. The bonding strength of the reinforced glue coating has adhesive and adhesive-cohesive character. When the load increases in the coating, microcracks develop, which gradually spread to the center of the bonding surface. For plasma coatings after laser remelting without additional alloying, the maximum bonding strength is observed with the minimum laser beam speed. With increasing the laser beam speed it decreases almost 1.5 times. In glue coatings reinforced with B4C particulates by laser remelting, the bonding strength is lower by 1.2–1.4 times in comparison with plasma coating.

Fatigue and Cyclic Behavior of 304L Stainless Steel in the Presence of Boriding Surface Treatment

In this work, we will study the cyclic behavior (tension-compression under strain control) and the fatigue of 304L austenitic SS after application of a boriding thermochemical treatment. 304L specimens was borided in a solid medium at 900 °C for 4 hours. The phase analysis of boride layers formed at the surface was performed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The obtained results show that the layers formed on 304L steel contained the following boride phases FeB, Fe2B, CrB, Cr2B, NiB and Ni2B. The fatigue tests show that the boriding treatment improves the life a factor which may be greater than four. The borided specimens show a negative average stress during cycling which may explain the increase of fatigue life. Analysis of the fracture surface by SEM show that crack initiation takes place in the substrate rather than in the boron-treated area.