KRZYSZTOF ANIOŁEK, JERZY HERIAN, MAREK CIEŚLA
Influence of hot rolling and isothermal annealing on microstructure and mechanical properties of high carbon steel INTRODUCTION Railroads consist, among others, of rail tracks and turnouts. Railway turnouts are complex structures of railroad sections which either fork or join intersecting tracks (Fig. 1, 2). In a turnout, in the place where stretches of rails intersect each other, the so-called frogs are used. The frog allows a rail-vehicle to ride through rail intersections. It is constructed using block sections, type KL60 or KL49, produced via hot rolling. Turnouts are especially exposed to abrasive or fatigue wear and to changes of shape as a result of high dynamic loads of cyclic nature which occur when track vehicles ride through the turnout [1]. The process of a mechanic destruction of the upper layer leads to undesirable changes of the dimensions and shape of the contacting rolling surfaces of a turnout element and a railway wheel. Measurements of the wear of block sections built-in in turnout frogs made in operating conditions have shown irregularities in the wear of the rolling surface profile throughout the actual frog point length and its plastic flattening. The measurement methodology is presented in the authors’ paper [1]. The highest wear occurs in the actual frog point in the place of the biggest load and decreases as the distance from the frog point grows. Such wear is caused by a momentary increase of the dynamic load acting on a small area of the frog point rolling surface, which leads to an intensive wear and plastic flattening of the frog point as a result of the rail vehicle wheel hitting against it. For these reasons, materials used for the production of rail turnout sections must have specific properties and first of all, an increased resistance to abrasive wear and fatigue. High costs of rail turnouts operation and their replacement are the grounds for developing new materials or technologies which will allow producing sections of better durability than those currently in use. Carbon-manganese steels of grade R260 with a pearlitic structure are the basic steels used for the production of rails and “block type” sections. Pearlitic steels are characterised by high strength and hardness, good resistance to abrasive wear [1÷4] and lower ductility and crack resistance. High resistance to abrasive wear of materials with pearlitic structures is ensured by the microstructure consisting of hard cementite lamellae in a soft ferritic matrix. An increased carbon content in the rail steel enhances its strength properties and hardness. At the same time, manganese hardens ferrite and reduces the pearlitic transition point, thereby contributing to the formation of pearlite colonies which are reduced in size. High strength and hardness of steel with a pearlitic structure, with simultaneous sufficient ductility, can be obtained by controlled cooling from the austenitization range to the pearlitic transition point of 620÷480°C, and holding at such temperature PhD Krzysztof Aniołek (
[email protected]) – Institute of Materials Science, University of Silesia, Poland, Prof. Jerzy Herian, Assoc. Prof. Marek Cieśla – Department of Materials Technology, Silesian University of Technology, Katowice, Poland
until total disintegration of austenite. The properties of the steel may be controlled within a wide range by changing the morphology of pearlite [1÷6].
MATERIAL AND RESEARCH METHODS Research material Hot-rolled block sections of KL60 type, made of rail steel grade R260, were chosen for the purpose of the research. The rolling of sections was conducted in a line unit using continuous castings. The temperature at the end of rolling was about 950°C. Cooling was performed in a cooling bed in calm air. The chemical composition and mechanical properties of the steel after hot rolling are presented in Table 1.
Fig. 1. Ordinary railroad switch: 1 – stock rail, 2 – needle, 3 – connecting rails, 4 – frog, 5 – guard rail, L – length, R – radius Rys. 1. Rozjazd zwyczajny: 1 – opornica, 2 – iglice, 3 – szyny łączące, 4 – krzyżownica, 5 – kierownice, L – długość, R – promień
Fig. 2. Scissors crossover : 1 – double frogs, 2 – single frogs Rys. 2. Rozjazd krzyżowy: 1 – krzyżownice podwójne, 2 – krzyżownice pojedyncze Table 1. Chemical composition and properties of R260 steel Tabela 1. Skład chemiczny i właściwości stali R260 Steel grade
R260
Content of chemical elements, % mas. C
Mn
Si
0.71
1.08
0.33
P
S
Cr
Al
V
0.010
0.015
0.040
0.003
0.003
Mechanical properties Rm, MPa
Rp0.,2, MPa
A5 , %
Z, %
HB
935
520
13.9
10.9
268
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Heat treatment
Research of the mechanical properties
The pearlitic structure of R260 steel with a varied pearlitic morphology was obtained in a physical modelling process of isothermal annealing (Fig. 3). Heat treatment was performed in industrial conditions (Vacuum Hardening Furnace Rubig VH 66910-FU). In the modelling of isothermal annealing, cylindrical specimens ø 10 mm were used. The specimens were made of carbon-manganese steel of grade R260. The parameters of the heat treatment applied were defined on the basis of dilatometric examination and were as follows: heating of the cylindrical specimens up to austenitisation temperature of 800°C and soaking in the temperature of 800°C for 1200 s, cooling at a rate of 15°C/s to the isothermal holding temperature of 620, 550 and 480°C, isothermal holding for 480 s and cooling of the specimens to ambient temperature.
Mechanical properties of R260 steel which are significant from the point of view of operational requirements for different conditions of the material were determined in a statistic tensile test. The static tensile test was performed at room temperature on cylindrical specimens with a diameter do = 10 mm, using an MTS-810 strength testing machine. Changes in the specimen’s length were measured on a length equal fivefold diameter of the specimen. An extension meter with a measuring base of 25 mm was used to precisely measure the elongation. The results connected to the basic mechanical properties of hot-rolled and isothermally annealed R260 steel are presented in Figure 5.
Research of the steel microstructure
The pearlitic microstructure of steel R260 after hot rolling is presented in Figure 4a. A pearlitic structure was detected in the steel, with various directions of lamellae arrangement in pearlite colonies. After isothermal annealing, a finer pearlitic structure was detected in steel R260, with colonies smaller (Fig. 4b÷d) than those after hot rolling. The structures showed a smaller interlamellar distance and a smaller thickness of cementite lamellae.
After hot rolling and isothermal annealing the steel microstructure was observed using a scanning electron microscope. The examination was conducted on specimens with a varied pearlite morphology obtained in hot rolling and isothermal annealing processes. In order to detect pearlite colonies, the metallographic specimens, after hot rolling, were etched with 3.5% nital, and after isothermal annealing, with a picric acid solution. During observation, magnifications of 2000, 5000, 10000 and 15000× were used. In each colony, the number of intersections with lamellae was counted for one secant perpendicular to cementite lamella. Various lengths of secants were used, depending on the arrangement of the lamellae in a given pearlite colony. The adopted methodology of measurement allowed determining the real distance between the lamellae lt, µm, by using the dependence:
lt
a n
RESEARCH RESULTS Steel microstructure after hot rolling and isothermal annealing
Quantitative description of the pearlitic structure The measurement results of the interlamellar distance in pearlite colonies are juxtaposed in Table 2. The average interlamellar distance for the as-rolled steel was 0.28 m. A lowering of the isothermal holding temperature caused a reduction of the interlamellar distance. The smallest average interlamellar distance,
(1)
where: a – length of the secant applied, µm, n – number of intersections of the secant with the cementite lamellae.
Fig. 3. Example of a heat treatment diagram for steel R260 at a temperature of 550°C Rys. 3. Schemat obróbki cieplnej stali R260 dla temperatury 550°C
Fig. 4. Steel microstructure after hot rolling a) and isothermal annealing at a temperature of: b) 620°C, c) 550°C, d) 480°C Rys. 4. Mikrostruktura stali po walcowaniu na gorąco a) i wyżarzaniu izotermicznym w temperaturze: b) 620°C, c) 550°C, d) 480°C
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equal 0.07 µm, was obtained for a specimen annealed isothermally at 550°C. A quantitative evaluation of a structure with a varied pearlite morphology and hardness examination have shown that the smaller the interlamellar distance, the higher the hardness of the rail steel (Tab. 2). After heat treatment at 550°C, the interlamellar distance was four times shorter in relation to the asrolled condition. The research findings allow affirming that an advantageous pearlite morphology in the pearlitic steel R260 used for rail turnouts, with an interlamellar distance in the order of 0.07÷0.09 m, which ensures hardness of ca. 310÷320 HB, is facilitated by a hot rolling process combined with isothermal annealing. Such structure can be obtained during austenitisation at a temperature of 800°C, with a cooling rate of 15°C/s to the isothermal holding temperature of ca. 480÷550°C.
of changes in thickness. An increase of strength properties resulting from a reduction of the interlamellar distance caused a change of steel resistance to abrasive wear, which has been proven in authors’ own examinations.
CONCLUSIONS The increase of ride speed, load and pressure as well as the frequency number of transport should be accompanied with the selection of materials which will ensure lower wear of rail sections and will prolong the durability period of switches and railroads. One of the ways leading to the increase of resistance to abrasive wear of high-carbon steel with pearlitic structure is the modification of its morphology. The morphology of pearlite may be regulated to a large degree by means of isothermal annealing parameters. The properties of rail sections material can be regulated by the change of the size of pearlite colonies and the interlamellar distance in cementite. The obtained test results corroborate that the pearlitic microstructure of steel R260 can be effectively modelled through hot rolling combined with isothermal annealing. The microstructure of steel R260 after isothermal annealing in the temperature range of 480÷620°C contained reduced-in-size pearlite colonies, more than two times smaller interlamellar distance in cementite (lt = 0.07÷0.13 µm) than the distance in the steel after hot rolling (lt = 0.28 µm). The shortening of the interlamellar distance as a result of a lowered pearlitic transitionpoint induces an increase in hardness and tensile strength of the steel. A combined process of hot rolling and isothermal annealing ensures obtaining a morphology of pearlite in the pearlitic steel R260 intended for rail turnouts, with an interlamellar distance of 0.07÷0.13 m. The smaller the interlamellar distance can be achieved, the higher the resistance to abrasive wear and the longer the life of rail sections will be.
Properties of the steel The tensile strength of the material, depending on the temperature of isothermal annealing, which is in compliance with the reference data. A lowering of the annealing temperature results in increased tensile strength and improvement of plastic properties (Fig. 5). The increase of tensile strength is caused by a smaller distance between cementite lamellae and their smaller thickness. A comparison of tensile strength for two examined different states of the material has shown that the material after isothermal annealing had tensile strength higher by 120 to 200 MPa compared to the hot-rolled material. This fact is reflected by a similar pattern Table 2. Hardness and interlamellar distance in pearlite colonies Tabela 2. Twardość i odległość międzypłytkowa w koloniach perlitu Interlamellar distance of 1t, µm minimum
maximum
average
Standard deviation
Hardness HB
As-rolled condition
0.18
0.35
0.28
0.05
268
HT 620°C
0.08
0.25
0.13
0.04
288
HT 550°C
0.05
0.09
0.07
0.01
321
HT 480°C
0.05
0.13
0.09
0.02
309
ACKNOWLEDGMENTS The research was performed within the research project No. PBU-70/RM2/2011 financed by National Science Centre Poland.
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