MODELOWANIE INŻYNIERSKIE 2016 nr 58, ISSN 1896-771X

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MICROSTRUCTURE AND PROPERTIES EVALUATION OF RAIL-STEEL AFTER ISOTHERMIC HEAT TREATMENT

Karol Dopierała1a, Janusz Ćwiek1b, Krzysztof Labisz1c

1Instytut Materiałów Inżynierskich i Biomedycznych, Politechnika Śląska akarol.dopierala@polsl.pl, bjanusz.cwiek@polsl.pl, ckrzysztof.labisz@polsl.pl

Summary The material used for the production of a rail steel is composed of an appropriate combination of alloying ele-ments and requires a particular heat treatment. There is a wide range of grades of steel, differing in terms of hardness and wear resistance; appropriate selection is done for operating conditions of the track, the durability can be achieved by providing obtain the desired microstructure, lubrication and sanding surface. This study on the changes of the standard heat treatment applied to either bainitic steel in order to reduce the energy needed for production purposes.

Keywords: rail steel, bainite, heat treatment, microstructure

OCENA MIKROSTRUKTURY I WŁASNOŚCI STALI SZYNOWEJ PO IZOTERMICZNEJ OBRÓBCE CIEPLNEJ

Streszczenie Materiał wykorzystywany do produkcji stali szynowej składa się z odpowiedniej kombinacji dodatków stopowych oraz wymaga zastosowania określonej obróbki cieplnej. Istnieje szeroki zakres gatunków tych stali różniących się pod względem twardości i odporności na zużycie ścierne; odpowiedni dobór odbywa się pod kątem warunków eks-ploatacyjnych torów, odpowiednią trwałość można osiągnąć, zapewniając otrzymanie wymaganej mikrostruktury, odpowiednie smarowanie oraz wyszlifowanie powierzchni. Niniejsze badania dotyczą zmiany standardowej obrób-ce cieplnej, stosowanej dla klasycznej stali bainitycznej w celu zmniejszenia energii koniecznej dla celów produk-cyjnych.

Słowa kluczowe: stal szynowa, bainit, obróbka cieplna, mikrostruktura

1. INTRODUCTION

The first steel rails used anywhere in the world were laid in Derby station on the Midland Railway in 1857 (Fig. 1). The metallurgical structure of those rails was essen-tially the same as that of the rail steel still used today - a pearlitic structure based on a carbon/manganese composition [1,2].

Rails not only wear, they also break. Their inherent toughness is poor as a result of the presence of the brittle carbide phase. Fracture can occur from relatively minor stress-concentrating features inside the rail, or on the surface, as a result of manufacture or subsequent handling damage. At least one European railway net-

work suffers almost 4000 rail fractures every year. These are rarely dangerous, as modern track signalling systems and routine inspections will find most. They do however have a high replacement cost and can be very disruptive to the network [1,3,4].

Up until the 1970s, rails for passenger and freight trains were regarded as relatively simple undemanding prod-ucts and the specifications had changed very little for decades. However, investments in railway systems, the advent of high-speed passenger trains and the require-ment for longer life track imposed a demand for rails of high quality, greater strength and tighter geometric

KAROL DOPIERAŁA, JANUSZ ĆWIEK,

tolerances. Therefore there have been major innovations in the past 20 years in terms of the method of manufature, degree of inspection and range of products

Fig. 1. Evolution of Sections [Key to metals A.G.]

As we mentioned above, the rails are subject to heavy contact cyclic loading that accompanies increased car size and loading, to 100 and 125 ton capacity, increased train size, and increased train speeds used to transport bulk products over the last several decades

These increasing demands require manufacturing and metallurgical approaches that offset wear and other types of failure that limit rail life. Depending upon the properties required, the rails are either cooled normally in air or subjected to enhanced cooling for the develoment of high strength. On cooling to room temperature, the rails are passed through a roller-straightener mchine which subjects the section to a number of severe bending reversals and emerge with a very high degree of straightness. Finally, the rails pass through a series of ultrasonic, eddy current and laser inspection stations which monitor non-metallic inclusions, external defects and the flatness of the running surface [1,2,5]

Table 1. Composition (in %wt) of typical Rail steels [Key to metals A.G.]

2. INVESTIGATIONS PROCEDURE METHOD

For investigations the rail steel grade with the chemical composition presented in Table 2. The test of the chemical composition was made on the optical spectrometer LECO GDS850A, three measurments were made and then calculated their average. Table 3 gives the chemical composition of the steel

KAROL DOPIERAŁA, JANUSZ ĆWIEK, KRZYSZTOF LABISZ

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ances. Therefore there have been major innovations in the past 20 years in terms of the method of manufac-ture, degree of inspection and range of products [1,2,5].

Evolution of Sections [Key to metals A.G.]

subject to heavy contact cyclic loading that accompanies increased car size and loading, to 100 and 125 ton capacity, increased train size, and increased train speeds used to transport bulk products over the last several decades [1,5].

demands require manufacturing and metallurgical approaches that offset wear and other types of failure that limit rail life. Depending upon the properties required, the rails are either cooled normally in air or subjected to enhanced cooling for the develop-ment of high strength. On cooling to room temperature,

straightener ma-chine which subjects the section to a number of severe bending reversals and emerge with a very high degree of

s through a series of ultrasonic, eddy current and laser inspection stations

metallic inclusions, external defects [1,2,5].

Table 1. Composition (in %wt) of typical Rail steels

METHOD

R350 was used, with the chemical composition presented in Table 2. The test of the chemical composition was made on the optical spectrometer LECO GDS850A, three measure-

de and then calculated their average. Table 3 gives the chemical composition of the steel

tested according to PN-EN13674has been subjected to an isothermal heat treatment consisting of austenitizing at 850 °C for 30 minutes and then accelerated cooling with compressed air to teperature 350 °C in 14 seconds . Steel was then annealed this temperature for 25 minutes, after which time a further cooling took place with furnace to 200 °C then rest cooling have place in air (Fig.2). Heat treparameters being selected in accordance with the graph CTP (Fig.3). Austenitizing have place in furnace Czylok FCF 2,5P, isothermic hardening in Czylok FCF 4/160M/PG

Table 2. Chemical composition of the investigaterail steel

Chemical composition, wt.%

Grade C Si Mn P

R350HT

0,67 0,34 0,80 0,02

R350B 0,71 0,36 0,82 0,01

Table 3. Chemical composition of the steel R350HT in PN-EN 13674 – 1

Chemical composition, wt.%

Grade C Si Mn P

min 0,7 0,13 0,65 -

max 0,82 0,6 1,25 0,025

Fig. 2. Isotermic heat treatment of steel R350B

Fig. 3. CTP diagram of the R350HT steel

KRZYSZTOF LABISZ

EN13674-1: 2006. Steel R350B has been subjected to an isothermal heat treatment consisting of austenitizing at 850 °C for 30 minutes and

ccelerated cooling with compressed air to tem-perature 350 °C in 14 seconds . Steel was then annealed this temperature for 25 minutes, after which time a further cooling took place with furnace to 200 °C then rest cooling have place in air (Fig.2). Heat treatment parameters being selected in accordance with the graph CTP (Fig.3). Austenitizing have place in furnace Czylok

ermic hardening in Czylok FCF

Chemical composition of the investigated

composition, wt.%

S Cr Al

0,02 0,02 0,03 0,004

0,01 0,01 0,04 0,005

position of the steel R350HT

Chemical composition, wt.%

S Cr Al

0,008 - -

0,025 0,03 0,15 0,004

Isotermic heat treatment of steel R350B

CTP diagram of the R350HT steel [5]

MICROSTRUCTURE AND PROPERTIES EVALUATION OF RAIL-STEEL AFTER (...)

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The microstructure of the steel was examined in the light metallography microscope Zeiss and scanning microscope Zeiss Supra 25 on four samples, two from steel R350HT and two of steel R350B, in both cases it was a longitudinal section and a transverse section. Samples were taken from the head rail. In addition, on samples with a cross section of micro Vicekers examina-tion was performed with a load of 9800 mN to hardness tester Wilson Wolpet microVickers 401MVD, hardness was automatically converted to HB. Metallographic samples been prepared in accordance with the informa-tion provided in Table 4.

For investigations the following test were carried out:

- Metallographic investigations were performed also using scanning electron microscope ZEISS Supra 35 with a magnification up to 500 times. For microstructure evaluation the Back Scattered Electrons (BSE) as well

as the Secondary Electron (SE) detection method was used, with the accelerating voltage of 20 KV.

- Structure investigation was performed using the light microscope Leica MEF4A supplied by Zeiss in a magni-fication range of 50 - 500x. The micrographs of the microstructures were made by means of the KS 300 program using digital camera.

- The hardness was measured with Brinell hardness tester with a load chosen for the HB scale, with a load of 9800 Kgf.

- Tribometer module with a Al2O3 counterball radius of 6 mm with a line speed 15,00 cm/s. The normal load was applied with a value of 10 N and acquisition rate: 5,0 [hz]. The environment conditions were: temperature: 25 °C and humidity: 0.00 %.

Table 4. Performance parameters of samples

Paper / polishing wheel

Pressure [N]

Rotation speed wheel [rpm]

Rotation

Speed samples

[rpm]

Direction Time [s]

Cooling / Suspen-sion

Gri

ndin

g

220 25 200 150

Cou

nter

110

Water 600 25 200 150 120

1200 25 200 150 240

Pol

ishin

g

MDMol

Struers 10 150 150

Con

curr

entl

y 600 Buehler MetaDi Dia-mondSuspension; 3Sm

MDNap

Struers 15 150 150 180

Buehler MetaDi Dia-mondSuspension; 1Sm

3. INVESTIGATIONS RESULTS

On the basis of analysis carried on the light micro-scope structure investigations, it was found, that the microstructure of the investigated steel is characterized by relative large irregular grains (Fig. 5), with the size in the range up to 200 µm while the R350HT steel have a microstructure consisting of smaller grains in the size up to 50 µm (Fig. 4). The same relationship is valid for the cross-section and longitudinal section (Figs. 6 and

7). Perlite grain growth is caused by a long time austen-itic stainless R350B compared to steel R350HT wherein the cooling process begins immediately after the comple-tion of hot rolling (rolling end temperature - 850 0C). On metallographic sample steel R350B there are areas of ferritic structure, it allows us to think further increase the cooling rate will create the structure of the upper bainite. The use of the proposed heat treatment resulted in getting rid of the structure of low-melting sulphide formed at the time of rolling [5,6,7].

KAROL DOPIERAŁA, JANUSZ ĆWIEK, KRZYSZTOF LABISZ

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Fig.4. Microstructure of the R350HT steel, cross section, SEM

Fig.5. Microstructure of the R350B steel, cross section, SEM

Fig.6. Microstructure of the R350HT steel, longitudinal section, SEM

Fig.7. Microstructure of the R350B steel, longitudinal section, SEM

Based on the observations of the perlite structure in a scanning electron microscope it was determined the interlamellar spacing in the perlite plates. It was found that the interlamellar spacing in the R350 HT steel is larger – equal 0,36 Sm, where as In the R350 B steel lower – equal 0,26 Sm (Fig. 8 and 9). The absence of the vanadium in the steel caused a lowering of the tempera-ture Bs leading to select a too low cooling rate and the existence of high temperature annealing [5,6].

Fig. 8. Microstructure of the R350B steel, SEM

Fig. 9. Perlitic microstructure of the R350HT steel, SEM

MICROSTRUCTURE AND PROPERTIES

However, the cooling temperature was low enough to reduce the interlamellar distance of perlite.resistance investigations the wear profile was obtained (Figs. 10 and 11). For the R350B steel the maximal wear depth was measured as 7.5 µm, whereas for the

Fig. 10

Fig. 11 The steel R350B was an increase in impact strength after heat treatment (Fig 12). Breakthroughs for all four samples exhibited the characteristics of brittle fracture during the macroscopic observation. The confirmation of the observation of macroscopic studies are scannelectron microscope (Fig. 13 and 14), we see there also small areas suggest a ductile breakthrough steel R350B which may explain the increasing impact of the sample. In some places you can see the effects of torn material for steel R350HT accept it the appearance of broken tiles perlite and steel R350B empty craters

Fig.12. Fracture surface of the R359HT

MICROSTRUCTURE AND PROPERTIES EVALUATION OF RAIL-STEEL AFTER

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However, the cooling temperature was low enough to reduce the interlamellar distance of perlite. For the wear

r profile was obtained ). For the R350B steel the maximal

wear depth was measured as 7.5 µm, whereas for the

R350B steel it was determined as 5 µm. The lower wear resistance for the R350B steel is of practical interest because of the steadily wear occurred during usage, what lowers the costs of maintaining the rails, without the necessity to machine it during service [

Fig. 10. Wear profile of the R350B steel

Fig. 11. Wear profile of the R350HT steel

The steel R350B was an increase in impact strength Breakthroughs for all four

samples exhibited the characteristics of brittle fracture The confirmation of

the observation of macroscopic studies are scanning ), we see there also

breakthrough steel R350B which may explain the increasing impact of the sample.

can see the effects of torn material for steel R350HT accept it the appearance of broken tiles perlite and steel R350B empty craters [9].

Fracture surface of the R359HT steel

Fig.13. Fracture surface of the R359HT

Fig. 14. Fracture surface of the R359B

STEEL AFTER (...)

R350B steel it was determined as 5 µm. The lower wear resistance for the R350B steel is of practical interest because of the steadily wear occurred during usage, what

ers the costs of maintaining the rails, without the ty to machine it during service [8,9].

Fracture surface of the R359HT steel

Fracture surface of the R359B steel

KAROL DOPIERAŁA, JANUSZ ĆWIEK,

4. CONCLUSIONS

The presented tests results carried out using light microscopy allowed the determination of the micrstructures obtained after the production process of rail steels. The observations made using the scanning electron microscope revealed the microstructurethe performed heat treatment, characterized by a regular fine grained perlitic structure.

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Ten artykuł dostępny jest na licencji Creative Commons Uznanie autorstwa 3.0 Polska. Pewne prawa zastrzeżone na rzecz autorów. Treść licencji jest dostępna na stronie http://creativecommons.org/licenses/by/3.0/pl/

KAROL DOPIERAŁA, JANUSZ ĆWIEK, KRZYSZTOF LABISZ

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The presented tests results carried out using light microscopy allowed the determination of the micro-structures obtained after the production process of rail steels. The observations made using the scanning

microstructure after the performed heat treatment, characterized by a

The wear resistance decreases generally, but also the risk of contact damages decreases what can lead to minimise of the service cost of rails in form of lover wearing process. The hardness does not change due to the appliance of the proposed heat treatment parters change as well as lower vanadium content in the chemical composition of the alloy

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Ten artykuł dostępny jest na licencji Creative Commons Uznanie autorstwa 3.0 Polska. Pewne prawa zastrzeżone na rzecz autorów.

dostępna na stronie http://creativecommons.org/licenses/by/3.0/pl/

KRZYSZTOF LABISZ

ear resistance decreases generally, but also the risk of contact damages decreases what can lead to minimise of the service cost of rails in form of lover wearing process. The hardness does not change due to the appliance of the proposed heat treatment parame-ters change as well as lower vanadium content in the chemical composition of the alloy [2,5,10].

2004. anie sił podłużnych w szynach toru bezstykowego.

ion of stress distribution in railway rail. Berlin

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dispersive precipitations in the structure of high Journal of Iron and Steel Research International”

J.: Simulation of dynamic interaction between train and railway turnout. “Vehi-

rail contact model for railway dynamics. “Vehicle System

quality railway transportation, Siemens Transportation

Ten artykuł dostępny jest na licencji Creative Commons Uznanie autorstwa 3.0 Polska.

dostępna na stronie http://creativecommons.org/licenses/by/3.0/pl/