The influence of heating temperature on coercive force and hardness changes in carbon hypoeutectoid steels

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Introduction When a temperature load is applied and subsequent cooling occurs in metals and alloys, two synergistically affecting each other processes take place simultaneously - a change in the structure and properties of the base metal and the appearance of oxide films on the surface. During the oxidation, the following iron oxides are formed: FeO (wustite), Fe3O4 (magnetite) and Fe2O3 (hematite). Oxides are arranged in layers according to the decrease in oxygen content from the outer to the inner layer (Figure 1). Изображение выглядит как текст Автоматически созданное описание Figure 1. Simplified scheme of oxide films on the surface of carbon steel during heating in an oxygen-containing environment Wustite (FeO) has face-centered cubic (FCC) and stable at temperatures above 570 °C. Below this temperature, it decomposes. The largest amount of wustiteis formed in the temperature interval 700-900 °C. Wustite is the softest and loosest layer of the oxide film. Magnetite (Fe3O4) has a cubic crystal lattice, highly abrasiveness and is insoluble in acids. Hematite (Fe2O3) has the highest abrasiveness and practically does not dissolve in acids [1]. With a metal hardness according to Vickers of 140 units, the hardness of FeO is 270-350, Fe3O4 420-500, Fe2O3 1030 units. In mechanical engineering, the study of processes that occur during heating and cooling of steel, as an alloy of iron with carbon and other elements, is typically limited to the study and optimization of the base metal’s structure. The composition, thickness, and properties of oxide films are dealt with either by specialists in hot rolling [2-5], where scale (a high-temperature oxide film on the surface of steel) is defective, or by specialists in processing scale and restoring iron from it[71] [6-8]. Therefore, the purpose of this research is to determine the effect of temperature loading on the hardness and magnetic properties of hypoeutectoid steel, which depend both on the processes occurring in the base metal and on the properties of surface oxide films. 1. Methods and materials The work is experimental. For the experiment, cylindrical samples made of carbon steel 25 (carbon content from 0.25-0.33%) were used. The critical points of the researched steel are Ac1 = 735 ºС, Ac3 = 835 ºС. To study the coercive force (HC, A/m), a verified and certified structuroscope (coercimeter) KIM-2M was used. Figure 2 shows a sample and nozzles for measuring coercive force. For hardness, the Rockwell method B scale (HRB, dimension - arbitrary units) was chosen, implemented in a stationary hardness tester TK-14-250. The samples were heated in a laboratory furnace with a PM-16M-V thermostat. The experiment was carried out as follows: the samples were placed in a furnace heated to a given temperature, held for 15 minutes, and cooled in calm air. The temperature ranged from 200 to 1000 ºС. In each sample, before heating and after cooling, the hardness and coercive force were measured several times along the length of the sample, and then the values were averaged. Before heating, for all samples - the average value of hardness is 89 units of the Rockwell B scale, the average value of the coercive force is 1023 A/m. Figure 2. Sample and nozzles for measuring coercive force Hc, A/m2. Results and discussion On Figure 3, numbers from 1 to 9 show the heating temperatures of the samples under study, plotted on the steel corner of the Fe-Fe3C diagram. Samples 1-6 were heated to temperatures below the critical point Ac1, sample 7 - between Ac1 and Ac3, samples 8 and 9 were completely austenitized during heating. All samples, except the first, were heated above the Curie point of cementite (210 °C). Research the behavior of the coercive force during thermal cycling near the Curie point of cementite (210 ºС) is the subject of works [9-13]. However, the study of the complex influence of processes in the base metal and in the zone of formation of oxide films on engineering characteristics - hardness and magnetic properties during single heating has not been carried out. Figures 4 and 5 are show graphical dependences of the experimental results. Numbers 1-9 correspond to the temperatures of the experiment in Figure 3. SPF + PFF + AGt °CF + CIIIP + CIIAc1 = 735 °CAc3 = 835 °CEAA + CII Figure 3.The steel angle of the Fe-Fe3C diagram, with points (1-9) corresponding to experiment temperatures t °C Figure 4. Dependence of the change in hardness after heating and cooling on the heating temperature: a - average hardness before heating; b - after heating and cooling t °C Figure 5. Dependence of the change in the coercive force after heating and cooling on the heating temperature: a - the average value of the coercive force before heating; b - after heating and cooling Figure 6 shows the temper colors of the samples after heating and cooling. Sample 0 is one of the samples at the same experimental series, not participating in the experiment. Obviously, the visual assessment of temperature by temper colors is subjective, when the temperature threshold of 500 ºС is exceeded (in this work), the visually determined dependence between the temperature and the color of the sample surface after exposure to temperature disappears. In this experiment, both the coercive force and hardness were determined taking into account the properties of the base metal and surface oxide films. The depth of magnetization of the device used and the depth of penetration of the indenter when measuring hardness are significantly greater than the thickness of the total oxide films, therefore, the measurement result is a complex value depending on the properties of the base metal and oxides. Description of the processes occurring in steel during experimental studies is given in Table. Figure 6. Temper colors of samples after heating and cooling Description of the occurring processes in the research samples during heating and cooling Temperature interval Reaction of hardness and coercive force Steel changes 200-300 °С Slight drop from unloaded temperature state Residual stresses decrease in the base metal, a thin single-layer oxide film appears on the surface 300-400 °С Hardness reduction continues. Coercive force decreases slightly Deformed crystal lattice begins to align in the base metal; hematite predominates in the structure of surface oxides (sample 3, Figure 6) 400-500 °С Increased hardness and coercive force There is a similarity of incomplete annealing in the structure of the base metal, but the thickness of a particularly hard layer (hematite) increases on the surface 500-600 °С 600-700 °С Hardness drop. Avalanche-like drop in coercive force due to the appearance of non-ferromagnetic elements in the structure of surface oxide films Incomplete annealing of the base metal occurs, loose wustite predominates in the structure of the surface layers (appeared at a temperature of 570 °С) 700-800 °С Hardness drop. Growth of the coercive force due to the predominance of the wustite ferromagnetic component in the oxide structure, the phenomenon of scale detachment from the base metal The transition during heating to the GSF region of the Fe-Fe3C diagram (Figure 2), an increase in the thickness of oxide films on the surface 800-900 °С Hardness increase. Sharp drop in coercive force Complete austenization of the base metal, hardening takes place in it due to a process of similar normalization, partial removal of oxides from the surface occurs 900-1000 °С Sharp drop in hardness and coercive force There is an overheating of the base metal, uncontrolled growth of austenite grains, self-removal of scale from the surface (Figure 7) Figure 7. High temperature scale on the sample surface Conclusion When applying a temperature load and subsequent cooling in carbon steel, two synergistically influencing processes occur simultaneously - a change in the structure and properties of the base metal and the appearance of oxide films on the surface. Hardness and coercive force (the most structurally sensitive magnetic characteristic of a ferromagnet) are not correlated with each other at all temperature intervals of heating. It is obvious that the visual assessment of temperature by temper colors is subjective, when the temperature threshold of 500 ºС is exceeded (for the research steel in given modes); the visually determined dependence between the temperature and the color of the sample surface after temperature exposure disappears. Therefore, recommendations for determining the temperature of the fire effect on metal structures by the temper colors of steel structures (for example, after a fire) cannot be recognized as sufficiently reliable.

About the authors

Anna V. Kornilova

Moscow State University of Civil Engineering (National Research University)

Email: anna44@yandex.ru
ORCID iD: 0000-0001-5569-9320

Doctor of Technical Sciences, Professor of the Department of Testing of Structures

26 Yaroslavskoye Shosse, Moscow, 129337, Russian Federation

Zaya Kyaw

Moscow State Technological University “STANKIN”

Author for correspondence.
Email: k.kyawzaya@yandex.ru
ORCID iD: 0000-0003-0131-1399

postgraduate student, Department of Composite Materials

1 Vadkovskiy Pereulok, Moscow, 127055, Russian Federation

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