HEATING OF TVCH HEAD OF RAILS FROM PROTECTIVE STEEL

Heating tvch rail head of eutectoid steel. Usually, railway rails are supplied in hot-rolled condition or after volume hardening (quenching in oil). During operation, only the head (mainly the surface) wears out the rails. In this regard, surface hardening is possible for hardening rails, including the progressive and well-proven method of hardening and high-frequency heating. This article is dedicated to this article.

UkrNIIMET has developed a technology for the combined heat treatment of rails from proeutectoid steel [1-3], according to which surface hardening and heating of HDTV of railheads previously subjected to cyclic spheroidizing annealing to obtain a granular pearlite structure is the final technological operation. At the same time, according to [4-5], heating of the high-frequency rail head should be chosen taking into account the shape, dispersion and distribution in the matrix of the carbide phase formed during the preliminary heat treatment.

Optimum heating of high-frequency current of the rail head made of steel containing 0.88% С; 0.87% Mn; 0.33% Si; 0.011% Ti; 0.02% V; In the final heat treatment, 0,0012% Се was chosen according to the presence of a homogeneous macrostructure in the quenched layer, a fine-grained and dispersed microstructure, the thickness of the hardened layer with increased hardness and mechanical properties. Critical points of steel were determined during heating and cooling, physical characteristics (ρ, Нс), metallographic, electron-microscopic and X-ray studies and mechanical tests were performed. The results of the research were used in the development of modes, heating the HDTV head of full-profile rail samples with their subsequent use in the production of batches of rails from eutectoid steel under the conditions of the Azovstal metallurgical combine.

The critical points of the proeutectoid steel were determined on a DKV-5AM dilatometer, as well as by differential-thermal and magnetometric methods on samples with a diameter of 2.5 and a length of 40 mm with slow and accelerated heating to 930 ° C and cooling at a rate of 3 ° C / min (Table. one).

Table 1

 Source structure Ас Ас Асm
°С
Plate Perlite
720/720
760/775
820/830
Granular Perlite
725/725
770/795
835/850
Granular pearliteNote. In the numerator, critical points are given, determined at νн = 0.8 ° С / s, in the denominator – at νн = 10 ° С / s.

It was established that regardless of the morphology of the initial carbide phase and the rate of heating used, the P → A eutectoid rail steel transformation begins at 720–725 ° C and ends at different temperatures depending on the initial structure. In particular, the Ac1k point of steel with the initial structure of granular perlite with slow heating is 10, and with accelerated it is 20 ° C higher than steel with the structure of lamellar perlite. With accelerated heating, the temperature of dissolution of excess cementite (Acm point) in steel with lamellar and granular perlite is 10 and 15 ° C higher than with slow.

The kinetics of phase and structural transformations of austenite with slow and accelerated heating was investigated in the temperature range 750–950 ° C on samples with a cross section of 4 × 4 and a length of 50 mm. Accelerated heating of the samples was carried out in an electric furnace, overheated to a temperature of 1000-1200 ° C, and cooling – in water.

The metallographic analysis of the samples was carried out on a Neophot-2 microscope at × 500 magnification. Investigated the structure formed in the steel during quenching from different temperatures with the definition of points and lengths of martensite needles according to GOST 8233-56 and austenite grain size according to GOST 5639-82. Electron-microscopic examination was performed on a Tesla electron microscope using the method of two-step plastic-platinum-carbon replicas on thin sections, etched in a 2% solution of nitric acid in alcohol. The microstructure of the rail proeutectoid steel with the initial structure of the plate perlite hardened from 750, 850 and 900 ° С is shown in Fig. 1, and – in. It can be seen that during hardening from 750 ° C, the γ-solid solution does not completely homogenize in the steel and the lamellar perlite grade 2 remains with a plate lamella distance of approximately 0.25 μm, while the cementite plates partially dissolved (Fig. 1, a) . During quenching from 850 and 900 ° C, a full martensitic transformation occurs with the formation of medium and large-needle martensite of a score of 4-5 and 6-7 with a needle length ≥8 and ≥12 μm, respectively (Fig. 1, b, c). The grain size of austenite steel with the original structure of the plate perlite, subjected to quenching from 850 and 900 ° C, corresponds to No. 8-9. With increasing temperature, the grain size of austenite increases (Table 2).

The microstructure of the rail carbon proeutectoid steel after quenching from accelerated heating to different temperatures

Img.1. The microstructure of the carbon rail eeutectoid steel with the original plate (a – c) and granular (d – e) perlite after quenching from accelerated heating νн≅10 ° C / s to various temperatures (× 6000):
a, g – 750 ° C; b, d – 850 ° C; b, e – 900 ° C.

In steel with the initial structure of granular perlite, only after quenching from 800 ° C and above, partial dissolution of the carbide globules and incomplete martensitic transformation are observed (Img. 1, d). Hardening of steel from 850 ° С leads to the formation of fine-needle martensite with a score of 3-4 with the presence of insignificant residues of the globules of carbides of the former granular perlite (Img. 1, e). A fine-needle martensite grade 4 with a needle length of 6 μm is formed after quenching from 900 ° С (Fig. 1, e). With a further increase in the heating temperature to 950 and 1000 ° C, there is no significant change in the characteristics of martensite (a score of 5 and 6 martensite is formed with a needle length of 8 and 10 μm, respectively). The grain size of austenite after quenching from 850 and 950 ° С is No. 10 and 9, respectively (Table 2).

table 2

tзак, °C Бм Type of martensite Austenite Grain Number Бм Type of martensite Austenite Grain Number
Original structure – lamellar perlite Исходная структура — зернистый перлит
750
800 2-3 Small needle 8-9 1 Medium needle 9
850 5 Medium needle 8-9 3-4 Small needle 10
900 6-7 Medium and large-needle 8 4 Small needle 9-10
950 8-9 Large needle 7-8 5 Medium needle 9-10
1000 9-10 Large needle 6-7 6 Medium needle 8-9
Legend: BM is a martensite score.

The process of dissolving the carbide phase under accelerated heating of steel was investigated by the electrical resistivity ρ and demagnetizing current Ip (proportional to the coercive force Hc) of samples quenched from different temperatures. The electrical resistivity of the samples was measured at the U303 bridge DC unit, and the demagnetizing current was measured at the KIFM-1 coercimeter [7]. The curves of dependences of the studied characteristics on the temperature of accelerated heating for quenching have a similar character, however, the curves of ρ and Ip of steel with the initial structure of granular pearlite are shifted to the right and are higher than those with a plate-like pearlite structure (Fig. 2, curves 1 and 2).

The maximum demagnetizing current Ip of steel with the initial structure of lamellar pearlite corresponds to quenching from 820 ° С, and for steel with granular pearlite structure to quenching from 850 ° С, after which the current stabilizes. The maximum specific electrical resistance q of steel with the initial structure of plate-like perlite is achieved after quenching from 850–900 ° C, and steel with granular perlite – after quenching from 850–930 ° C (Fig. 2). The change in ρ is apparently due to the processes of austenite formation and its homogenization.

.

Dependence of electrical resistivity and demagnetizing current of rail carbon proeutectoid steel on the temperature of accelerated heating for quenching

Image2. Dependence of specific electrical resistance ρ and demagnetizing current Ip of rail e-eutectoid carbon steel on the temperature of accelerated (νн ≅10 ° С / s) heating for quenching:
1 – initial structure – lamellar perlite (after rolling); 2 – dispersed granular perlite (after cyclic spheroidizing annealing).

Thus, the results show that with accelerated heating (νн≅10 ° C / s) for quenching in the temperature range used, the rail eutectoid steel with the initial structure of granular perlite saturates austenite with carbon and alloying elements (in this case, manganese) flows more completely and higher temperature than steel with lamellar perlite. This is due to the peculiarities of the transformation of granular perlite into austenite [6], as well as the presence in the initial structure of the lamellar perlite of excess cementite in the form of coarse precipitates. In granular pearlite, excess cementite after cyclic spheroidizing annealing is not in the form of a solid mesh, but in a globular form.

The parameters of a fine crystal structure (dislocation density ρd and stresses of type II Δa / a) were determined by X-ray diffraction on a DRON-3 and DRON-UM-1 diffractometer in cobalt Kα radiation according to the method [8, 9].

It was established that after final heat treatment using accelerated heating of steel with the initial structure of dispersed granular perlite, the substructural parameters ρd and Δa / a are higher than those with a plate-like perlite structure and are: ρd = 10.2 · 1010 and 7.1 · 1010 cm-2; Δa / a = 12.8 · 10-4 and 8.6 · 10-4, respectively.

Taking into account the results of the research, experiments were carried out to test the mode, heating the high-frequency current at a rate of νн≅10 ° С / s of the head of full-profile rail samples (~ 500 mm) of steel of similar chemical composition. Such technological parameters of quenching as current frequency, cooling method and self-tempering temperature corresponded to the parameters of heat treatment of rails from industrial steel. The heating of the rail sample head with the initial structures of the lamellar and granular pearlite was carried out in the temperature range of 750-1000 ° C.

Studies have shown that the thickness of the hardened layer varies from 7-8 to 14-16 mm (along the axis of the head) after quenching from 750-800 ° C and 850-950 ° C, respectively. The hardness of the rail sample head with the initial structure of granular perlite is distributed evenly over the cross section with the presence of a zone (at a depth of up to 8 mm) of increased hardness of 41.5-38.5 HRCE with a troostite structure in which the plate-to-plate distance is 0.2 μm. In a layer ~ 6 mm thick, located at a distance of h ≅ 8-10 mm from the surface, the hardness gradually decreases. The structure of this layer consists of sorbitol with an inter-plate distance of 0.6-0.8 μm, rolling at a greater depth into the initial structure of granular pearlite. For rail samples without preliminary heat treatment (with the initial structure of lamellar pearlite), the area of ​​increased hardness is located at a shallower depth, and the structure of the hardened layer with similar hardness consists of troisto-sorbitol with a slightly longer plate-to-plate distance of 0.6 μm.

During the final heat treatment of rail samples with the initial structure of granular perlite, the maximum hardness values ​​(61-64 HRC3) and temporary tensile strength (σв = 1430-1440 N / mm2) with a high level of ductility and toughness (Table 3) temperature range of 880-950 ° C. Samples with the initial structure of the perlite plate (in the rolling state) after
The final heat treatment with HDTV heating have lower mechanical properties and hardness. At the same time, the maximum hardness of 58-61 HRCE and temporary resistance to rupture σв = 1385-1400 N / mm2 of the samples is achieved after quenching, heating with HDTV in the temperature range 850-900 ° C. A subsequent increase in the heating temperature for quenching leads to a significant decrease in mechanical properties, especially ductility and toughness (Table 3).

Table 3

tзак, °C σв σ0,2 δ5 ψ a1, Дж/см2
Н/мм2 %
750 1085 755 9,2 16,8 21,5
1020 660 13,8 28,0 34,0
800 1260 905 10,4 26 28
1235 890 14,1 30 37,5
850 1385 1180 11,0 28 31,0
1390 1160 14,8 36 39,5
880 1410 1100 10,2 25,5 32,5
1430 1110 14,4 38,5 36,4
900 1400 1200 8,2 26,5 30,5
1435 1190 16,0 41,0 40,6
950 1360 1190 5,9 23,2 24
1440 1170 12,0 32,8 34,6
1000 1320 1160 5,7 13,7 21,2
1390 1155 9,9 27,4 27,5
По ТУ-14-2-499-82 ≥1275 ≥8 ≥8 ≥25 ≥19,6
Note. In the numerator, the properties of the samples with the initial structure of the plate-like perlite are given, in the denominator – the granular perlite; requirements for specifications are given for the original structure of granular perlite.

It has been established that the shape and dispersion of the carbide phase affect the kinetics of the P → A transformation and the final indicators of the structure, substructure and mechanical properties of steel. At the same time, in steel with the initial structure of granular perlite, the P → A transformation proceeds somewhat more slowly and at higher temperatures, and the saturation of the γ-solid solution with carbon and alloying elements is more complete than in steel with the initial plate-like perlite. This ultimately contributes to the improvement of the mechanical properties of steel after quenching from accelerated heating [6]. The optimum temperature of quenching from accelerated heating (νн≅10 ° С / s) of rail proeutectoid steel with the initial structure of lamellar perlite corresponds to 850-900 ° С, and with granular perlite – 880-950 ° С. After quenching from accelerated heating to the optimum temperature range of steel with dispersed granular perlite, obtained after cyclic spheroidizing annealing, fine-grained (horn size No. 9-10) fine-grained structure of hardening troostite with 0.2 mm micron spacing is formed in the quenched layer high dislocation density ρd = 10.2 · 1010 cm2. The formation of such a structure provides a high complex of properties in a quenched layer: σв = 1430..1440 N / mm2, δ5 = 12.0..14.4%; ψ = 32.8 ..38.5%; A1 = 34.6. 36.4 J / cm2. A significantly smaller complex of properties after quenching from accelerated heating is achieved in steel with the initial structure of lamellar perlite (hot rolled state), where the structure of the hardened layer consists of troisto-sorbitol with interplate distance of 0.6 μm, dislocation density ρd = 7.1 · 1010 cm- 2 and a larger grain size of austenite (No. 8-9). In this case, σв = 1385..1400 N / mm2; δ5 = 8.2 ..11.0%; ψ = 26.5 ..28.0%; A1 = 30.5 ..31.0 J / cm2.

Thus, by changing the morphology of the initial structure of the rail proeutectoid steel and applying the optimum temperature of accelerated heating, it is possible to provide in the hardened layer a high complex of properties with the presence of fine-grained, highly dispersed structure and substructure. Such treatment contributes to a significant increase in contact fatigue endurance and wear resistance during operation in particularly difficult conditions and at low temperatures.

According to the developed regimes of accelerated heating of the rail head under the conditions of the Azovstal metallurgical combine, batches of rails were manufactured from carbon eutectoid steel with a carbon content of 0.88; 0.92 and 0.95%, previously subjected to cyclic spheroidizing annealing on dispersed granular perlite. Rails are characterized by a high complex of properties; σв = 1450..1370 N / mm2; σ0.2 = 1050..945 N / mm2; δ5 = 12.5..9.6%; ψ = 37..34%; A1 = 41..32 J / cm2.
Lots of rails weighing 350 tons undergo operational tests under the most difficult conditions of the Far North.

Bibliography:

1. Combined heat treatment of rails made of carbon eeutectoid steel / DK Nesterov, N. F. Levchenko, V. Ye. Sapozhkov, etc. // Ferrous metallurgy. 1988. № 9. S. 44-47.
2. Steel for rails of increased strength / DK Neste

D.K. Nesterov, N.F. Levchenko, V.E. Samozhkov, V.A. Dubrov
ISSN 0026-0819. “Metallurgy and heat treatment of metals”, № 11. 1991
This article was taken from this resource.