INVESTIGATION OF THE CARBON POTENTIAL IN A LOW-PRESSURE ENVIRONMENT FOR ALLOY STEELS AFTER THE CARBURIZING PROCESS

ABSTRACT

The carbon potential of a low-pressure technological medium (100 Pa) consisting of 100% acetylene has been determined as the limiting saturating ability of such a medium on massive samples of carbon and alloy steels such as 20, 20Cr, 16Cr3NiWVaMoNb, 20Cr2Ni4, 20CrNi4A, 3Cr3Mo3V. It was found that at a temperature of 940 ° C (typical carburizing temperature), the carbon potential is from 1.3 (in the absence of carbon) to 1.4% by weight (with a chromium content of 2% in steel).

АННОТАЦИЯ

Определен углеродный потенциал технологической среды низкого давления (100 Па), состоящей из 100 % ацетилена, как предельная насыщающая способность такой среды на массивных образцах из углеродистой и легированных сталей таких, как 20, 20Х, 16Х3НВФМБ, 20Х2Н4, 20ХН4А, 3Х3МЗФ. Установлено, что при температуре 940 °С (типичная температура цементации) углеродный потенциал равен от 1,3 (в отсутствие углерода) до 1,4 % масс. (при содержании хрома в стали 2 %).

Keywords: carbon potential, cementation, acetylene, chromium, alloy steel, carbon steel.

Ключевые слова: углеродный потенциал, цементация, ацетилен, хром, легированная сталь, углеродистая сталь.

The value of the carbon potential of the medium can simply be determined by saturating a massive sample of carburized steel with a total content of chromium and other carbide-forming elements in it within no more than 2% by weight, since with a higher content of carbide-forming elements in it, alloyed cementite (Fe, Cr)3C begins to actively form on the surface of the part, in which It can dissolve from 70 to 90% of molybdenum, tungsten and vanadium contained in steels [1, 2].

The carbon potential is usually determined in activities, but as shown in [3, 4, 5], when carburizing austenite, it is convenient to use carbon concentrations in mass percentages. This way of representing the potential is justified by the convenience of modeling carburizing processes [6, 7, 8].

In this regard, the purpose of this article was to determine the effective thickness depending on the saturation time, temperature and kinetic coefficient k during vacuum carburizing, as well as to evaluate the effect of carbide-forming alloying elements on the kinetic coefficient.

The studies were carried out on samples from steels 20, 20Cr, 16Cr3NiWVaMoNb, 20Cr2Ni4, 20CrNi4A, 3Cr3Mo3Va. The table shows the chemical composition of the studied steels.

Table 1.

Chemical composition of the studied steels

The objects of the study of the structure of the diffusion layers were microslips prepared according to the traditional method from samples.

During the work after vacuum carburizing in acetylene (C2H2) medium, carried out at the laboratory installation 10.0 VT 4022/24N "Seco/Warwick S. A." (Poland), data on the distribution of carbon over the thickness of the microplate were obtained using the metallographic method. Next, the kinetic curves of carbon are obtained (Fig. 1).

Figure 1. The effect of the vacuum cementation time on the concentration of embedded carbon on the surface at a temperature of 940 °C

In alloy steels, the solubility of carbon in austenite increases with an increase in the amount of chromium. At the same time, the maximum solubility always corresponds to the carbon activity equal to 1 (see Fig. 1).

Based on the processed data, graphs were constructed using computer modeling, which reflect the dependence of the effective thickness of the diffusion layer on the saturation time (Fig. 2)

Figure 2. The effect of temperature on the effective thickness of the diffusion layer after vacuum carburizing of steel 20Cr (a), 3Cr3Mo3V (b) and 16Cr3NiWVaMoNb (c) at an initial cementation time of 2...10 min and temperatures of 880, 910, 940 °C

According to the data obtained, the kinetic coefficients for each steel grade are calculated. Next, the dependence of the kinetic coefficient on the total content of carbide-forming alloying elements for each grade is constructed (Fig. 3).

Figure 3. Influence of the content of carbide-forming alloying elements (Cr, Mo, W) on the kinetic coefficient of various grades of steels: 20, 20Cr, 16Cr3NiWVaMoNb, 20Cr2Ni4, 20CrNi4A, 3Cr3Mo3V at a temperature of 910 °C

With an increase in the concentration of carbide-forming elements in steel, the rate of formation of the diffusion layer slows down (see Fig. 3). Thus, the carbon potential of the atmosphere approaches the value of the limiting solubility of carbon in austenite. Therefore, the carbon potential is 1.0 (in units of activity).

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