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Lev Isaac Yefimovich, professor, doctor of technical sciences, full professor
National Metallurgical Academy of Ukraine, Ukraine
Shypitsyn Sergey Yakovlevich, заведующий отделом, doctor of technical sciences, full professor
Физико-технологический институт металлов и сплавов, Академии наук Украины
Championship participant: the National Research Analytics Championship - "Ukraine";
the Open European-Asian Research Analytics Championship;
In the present work authors consider the influence of conditions for formation of nitride phases on the cavitation stability of the Cr-Mn-V-N austenitic steels. The influence of isothermal treatment on the mode of austenite hardening had been established by the way of electrochemical phase, X-ray structure and mechanical methods of analysis. The temperature of annealing 700°C determined in this work is new to the subject. It creates the opportunity of forming the biggest quantity of dispersion hardening nitrides. Such temperature ensures optimum conditions for obtaining cavitation resistance for steel 17Х15Г19АФ.
Keywords: nitrides, resistance to cavitation, chromium-manganese-vanadium-nitrogen steels, dispersion hardening of austenite.
One of the main trends in developing the heat power-generating industry is the increase of specific power of energy blocks while increasing correspondingly their efficiency to 48-50%. This predetermines transition to higher steam parameters: pressure up to 35 MPa, temperature up to 650°C [1, 2].
The necessary condition for attaining the set aim is introduction of the new promising scientific and engineering developments, which allow to handle in complex a number of technical problems. These problems arise while developing details of blocking and regulating details, which work under conditions of microimpact action of medium flow, i.e. cavitation.
Analysis of the problem‘s state showed that in the world’s and home practice the main constructional material for manufacturing blocking-regulating units of pipeline fittings is up to now austenitic stainless steel 08X18H10T . But as to the level of strength characteristics and durability under conditions of cavitational action of the medium flow and scratching the contact surface this steel does not meet the present-day demands. It restrains the progress in rising the operating parameters, reliability and service life of equipment. Beside that, the substantial disadvantage of austenitic chromium-nickel steel is the necessity of alloying it with expensive and not readily available nickel. Introduction of manganese and nitrogen presents more economical mode of alloying. The lack of deep scientific and engineering research in this field however constitutes one of the main reasons to explain why the technology of alloying steel with nitrogen, manganese and nitride-forming elements is not demanded for developing corrosion-proof and cavitation resistant steels. The problem of structure formation, distribution of nitrogen between solid solution and nitride phases isolated in the process stages (hot and cold steel deformation) and in the process of microimpact of the surroundings under conditions of cavitation is also one of these reasons. The problem of the influence of chemical composition, schedules of heat treatment, phase steel composition, parameters of micro- and substructure, low degrees of cold plastic deformation and microimpact action on the kinetics of ?→?→? phase transformation and hardening of phases under formation, change of micro- and substructure and the influence of these factors on the destruction resistance of steel during microimpact action remains to be studied.
To solve the existing problem, the alloying system Cr-Mn-V-N had been chosen as the object of studying in the present work. This system creates a possibility of developing high-strength corrosion-resistant grades of steel with nitride hardening.
Smelting of steels was carried out in induction furnace IST 0.16 with acid lining using the method of re-melting technically pure materials. The final deoxidizing and modification of steels was realized with aluminum and silicocalcium.
To attain 3 basic kinds of austenite hardening – solid solution, dispersion and mixed hardening – the quantity and composition of nitride and carbide phases being formed depending on the temperature of isothermal treatment - chemical phase analysis was used.
With the purpose of approaching the steel balance, the time of isothermal soaking had been determined from the work  on the basis of the data about influence of the temperature (from 1200 till 700°C) on the mass speed of isolating VN particles in the Cr-N austenite.
The chemical analysis of the steel under study is presented in the Table 1.
Chemical analysis of the steel grade 17X15Г19АФ
Chemical analysis, %
Phases under study had been isolated by means of electrochemical method of dissolving the metallic samples base . Electrolytic metal dissolving and compounds isolating were carried out on samples ? 20mm and h = 80mm. Electrolysis was carried out in the field of direct dependence of overvoltage value on the current density logarithm. In accord with Tafel’s equation  this field corresponds to electrochemical stage of the process. Extraction of the nitrogencontaining phases had been carried out in the aqueous electrolyte containing 15% NaCl and 2.5% tartrate acid in potentiostatic operating regime created by the potentiostat 10-20PEB. Nitride and carbonitride phases in the shape of anodic precipitate had been isolated from the steel, because their oxidizing potentials in the given electrolyte were bigger than that of metallic base. Implementation of electrolysis with the potential smaller than these of the phases under isolation favored such result. A small collodium bag had been set in the electrolytic cell between sample and cathode for the purpose of collecting inclusions. After finishing the electrolysis the precipitate together with electrolyte had been moved from the collodium small bag into wide polyethylene test-tubes and washed off from the electrolyte by the way of step-by-step operations of centrifuging at 5000rpm and decantation. Isolation of inclusions had been carried out in two series of experiments. In the first series inclusions were prepared for X-ray structure analysis. For this purpose inclusions had been dried up to constant weight in exicator above the concentrated H2SO4. In the second series of experiments isolation of inclusions had been carried out to determine their chemical analysis. Inclusions were dissolved in concentrated H2SO4 with additional wet fusion in the melt of Na2SO4 with H2SO4. Obtained solutions had been transferred to measuring flasks, and the content of elements combined in nitride and carbide phases (Ncomb, Vcomb, Crcomb, Mncomb and Alcomb) had been determined in aliquote parts.
Nitrogen was determined using the photocolorimetric method by the intensivity of complex compound coloring – oxiamidodimercure, formed by the interaction of NH4+ ions with K2[HgJ4] . As a preliminary, nitrogen in the form NH3 had been isolated from the alkaline solution by the method of steam aspiration. Vanadium and chrome had been determined by the oxidation-reducing method by means of volume analysis. Titration had been carried out step-by-step in one and the same solution, at first VO3- ion was reduced using the solution of FeSO4, then, after the oxidation, the sum VO3- + Cr2O72- was reduced in its turn . The content of manganese had been determined by the volume method, by way of reducing MnO4- ions using solution of sodium arsenite . Aluminum had been determined using the photocolorimetric method by coloring intensivity of the complex compound of aluminum with aluminone . The results of analysis are presented in the Table 2.
Chemical analysis of inclusions isolated from the steel 17X15Г19АФ
Conditions of isothermal treatment
Note: after isothermal treatments the samples were cooled in water
As follows from obtained results, after solidification and homogenizing annealing at 1200°C (sample K.1) the steel contains minimum quantity of nitrogen in combined state (Fig. 1,a). Lower temperatures of isothermal soaking within the determined interval lead to increasing Ncomb, i.e., to increasing the formation of nitrogen containing phases. Other alloying elements, with the exception of chromium (at 700°C) show the same trend (Fig. 1,b).
Since isolated phases contain more chromium, the general dependence of the formed phases’ quantity is specified by the curves in the Fig. 1,c.
Figure1. Dependence of the nitrogen content (a), chromium content (curve 1), vanadium content (curve 2), manganese content (curve 3) (b), and the total content of elements combined in nitrides (Ncomb + Crcomb + Vcomb + Mncomb) (c) at the annealing temperature, °C
Determination of the qualitative and semi-quantitative composition of different crystal phases in samples had been carried out on the unit ДРОН-3 (DRON-3) in copper radiation. When identifying structures the authors used Diffraction Data File of American Society for Testing Materials (ASTM File ) and reference data in the tables of File .
The results of the phase analysis of isolated compounds showed that during high-temperature isothermal treatment the formation of nitrides in the steel under study took place to a very insignificant degree. Alumina (? – Al2O3) is present in inclusions isolated from samples subjected to high-temperature treatment (K.1, K.2 and K.3), as well as very little of AlN and VN. The most intensive maxima of alumina had been also determined: d = 3.47?; 2.55?; 2.08?; 1.74? and 1.60?.
The main quantity of dispersion hardening phases are formed at lower temperatures of isothermal treatment. In the first place, these are hexagonal nitrides VN0.35 which gave the most intensive reflections in diffractograms of samples K.5 and K.6 corresponding to d = 2.153?; 1.286?, and tetragonal nitrides VN. It is possible to judge about the presence of VN by values d = 2.091?; 1.235? and 1.116?. Chromium in these samples forms a few phases: hexagonal nitrides Cr2N with d = 2.40?; 2.22?; 2.11?; 1.183? and 1.166?; cubic nitrides CrN with d = 1.463?; 1.249?; 1.197? and 0.846?, as well as cubic carbides Cr23C6 with d = 2.37?; 2.7?; 1.26?; 1.23? and others. Proceeding from the calculation of the element content in precipitates, the data of X-ray structure analysis of precipitates and literary data as to possible presence of non-metallic phases in Cr-Mn-V-N austenitic steels , the authors determined the phase steel composition and the changes depending on the temperature of isothermal treatment (Table 3).
Quantity of non-metallic phases in the homogenized steel (1200°C – 2 hours – water)
depending on the temperature of isothermal treatment
Conditions of isothermal treatment
Mass share of non-metallic phases, %
Note: after isothermal treatments the samples were cooled in water
The obtained results show that nitrogen combines practically only with vanadium. Mass share of vanadium nitrides at the ageing temperature 700°C attains 0.18%, which makes 44% of the theoretically possible quantity of vanadium nitrides in the case of complete combination of the vanadium in steel and nitrogen into nitrides. Experimental conditions of dispersion nitride vanadium hardening of austenitic matrix are attained most completely under ageing at 700°C.
Homogenizing under conditions 1200°C – 2 hours – water ensures practically homogenous state of the matrix, i.e., only its solid phase hardening with C, N, V, Cr.
Under ageing in the interval 900°C - 1000°C the mass share of nitrides VN particles makes only about 50% of their quantity at the ageing temperature 700°C, i.e., it is possible to assume that matrix is in mixed solid solution and dispersion hardening.
An important result of the phase analysis consists in the fact that chromium depletion of the solid austenite solution due to forming chromium nitrides and carbides with mass share not exceeding 40% can’t decrease corrosion resistance of steel.
It must be also noted that only 25% of nitrogen in steel is used for nitrides formation, what means that austenitizing effect of nitrogen is kept.
When using articles of cavitation resistant steels at elevated temperatures, for example, in heat power industry, an important factor of their operating longevity is the heat stability of metal strain hardening.
On the basis of results of testing steel for cavitation resistance, as to mass loss, ?Px102, kg/m2 (Fig. 2,a) and as to hardness increase, ?HB (Fig. 2,b), it was determined in the work that substitution of solid solution hardening of austenite for dispersion hardening raises the degree of strain hardening.
Figure 2: a – mass loss (?P) of steel samples after 20 hours of testing for cavitation resistance; b – influence of the mode of austenite hardening on the degree of strain hardening at upsetting samples by 12-16%. 1 – solid phase hardening (homogenization at 1200°C); 2 – complex hardening (ageing at 900°C); 3 – dispersion hardening (ageing at 700°C).
The results of electrochemical phase analysis and X-ray structure analysis correlate with these data. The temperature of annealing 700°C determined in this work is new to the subject. It creates the opportunity of forming the biggest quantity of dispersion hardening nitrides. Such temperature ensures optimum conditions for obtaining cavitation resistance for steel 17Х15Г19АФ.