STUDY OF CHANGE IN WEAR RESISTANCE OF STAINLESS STEEL SURFACE OBTAINED BY LASER PULSE ALLOYING IN THE COATING LAYER

The paper describes an experimental study of laser pulsed modification of the surface of stainless chromium-nickel steel of the austenitic class 12Cr18Ni10T with a graphite coating previously applied to it. The microhardness of the treated surface and its dependence on the modes of laser radiation were determined. The abrasion method was used to study the change in the wear resistance of the modified layer and the possibility of its increase. For a quantitative assessment of the surface resistance to wear, a gravimetric analysis of samples subjected to ball abrasion was carried out, the results of which are fully consistent with the calculation of the dimensionless wear resistance index. It was found that the value of the microhardness is largely influenced by the voltage of the pulses. Thus, the highest microhardness indicators were observed during processing by laser pulses in the voltage ranges of 250-280 V and 340-440 V and take values of 5-6 GPa and 4-5 GPa, respectively. A comprehensive study of abrasion resistance, including the calculation of a dimensionless wear resistance index and an analysis of the weight loss of steel samples as a result of the test, showed that, to the greatest extent, the surface hardening effect is characteristic of samples subjected to treatment with laser pulses focused into a spot 1 mm in diameter at a pulse voltage of range 300-400 V.

COMPARATIVE ANALYSIS OF THE INFLUENCE OF THE COMPOSITION OF THE ALLOYING COVERING ON THE CHANGE IN THE STRUCTURE AND MICROHARDNESS OF STAINLESS STEEL 12KH18N10T

The article is devoted to the experimental analysis of the influence of the process of laser pulsed alloying in a layer of alloying mixture on the qualitative and physical and mechanical characteristics of the modified surface of stainless chromium-nickel steel of the austenitic class 12Cr18Ni10T. It was found that the use of graphite paste as an alloying compound leads to a significant increase in microhardness, a change in micromorphology, and the formation of a heat hardening zone in the near-surface layer of steel. The smoothing of the boundaries of structural changes is revealed and the effect of surface hardening is determined when a finely dispersed powder of titanium dioxide (anatase) is added to the alloying coating. According to the data obtained, a comparative analysis of the dependence of the microhardness of the modified surface on the voltage of the pump lamp and the diameter of the laser pulse spot is carried out. The maximum microhardness value, equal to 9,56 GPa, was achieved as a result of laser processing of a series of samples, where graphite paste was applied as a preliminary surface preparation. Rational technological modes of laser modification of the surface of 12Cr18Ni10T steel that have previously undergone abrasive blasting, modes of laser pulsed alloying in a layer of graphite coating and laser pulsed alloying of steel in a layer of coating consisting of graphite paste and anatase powder in a ratio of 4:1, respectively, are recommended.

Assessment of Electrochemical Compatibility of Structural Materials of some Dental Products

Orthopedic treatment of tooth anomalies in children and adolescents is provided the long-term use of various metal constructions and devices in the oral cavity – braces, retreaders, locks or rings with struts, wire arches, and so on. They are usually made of corrosion-resistant metals and alloys, most often they are made of stainless chromium-nickel austenitic steels of Х18Н9Т type (import analogue – steel 304), martensitic 08Х17 (import analogue – steel 430), nickel-titanium or nickel-molybdenum alloys. The main disadvantage of all metal products is their manifestation of electrochemical properties and participation in electrochemical processes which can flow into the oral cavity and provoke galvanoses, especially for their joint use. In the "in vitro" conditions, according to a specially developed method, investigations of electrode potentials of directly 4 types of very small (2–3 mm) orthodontic products, in recommended for such products environment were carried out: 3 % solution of sodium chloride (pH=6,8), 2 % solution of citric acid (pH=0.5) and 2 % solution of baking soda (pH=8.65). It is found that the considered elements of orthodynamic systems have similar values of electrode potentials in neutral and weakly-alkaline environments and, accordingly, in the absence of other metal inclusions in the oral cavity, can be used jointly without the risk of galvanosis. The most heterogeneous construction is an individual ring with a strut, in which the difference in the values of the potentials between the individual parts in the acidic medium is more than 120 mV, which is a prerequisite for increasing the likelihood of galvanosis. For simultaneous use of other elements, in particular standard doping brackets, the value of EMF can increase up to 160 mV.

Determination of Nickel in Welding Fume by Gravimetric Method Using Dimethylglyoxime

Welding fume emitted while welding stainless chromium-nickel steels contains mainly nickel oxides (NiO, NiO2 and Ni2O3). Welding in the inert gas shielding as well as plasma and laser cutting of parent materials of a high content of nickel ranks among the welding processes characterised by the highest concentrations of nickel oxides in welding fume. The predicted content of this element in the mass % range of 0.7 to 6% has been adopted as a criterion for the selection of the analytical method of the determining nickel content in the welding fume. Determination of nickel is based on dissolution of the sample in a mixture of acids, separation of nickel in the form of solid dimethylglyoxime nickel (II) and gravimetric determination. The adopted method of nickel determination in the welding fume proved to be an effective and inexpensive solution.