Hao Kang，Xian-Ming Zhao，Di Wu

(State Key Lab of Rolling and Automation，Northeastern University，Shenyang 110819，China)

1 Introduction

With the development of railway transportation industry， traction weight， driving speed， transport density and total volume passed on rail have increased dramatically，which make the burden on heavy rail increase accordingly，and rail damage is also increased，especially the wear on the surface of rail head.So it is very important to increase the strength of heavy rail，which can improve wear resistance and extend the service life.There are two methods to increase strength ofheavy rail:oneisthe heat treatment;the other is the alloying.Researches show that heat treatment is better than the alloying because of low cost and high efficiency，and heat treatment can extend service life of heavy rail at least one time than before［1－4］.

Heat treatment has experienced a process from QT(Quench Tempering)to S-Q(Slack Quenching)［5］;in United States and Russia，Q-T process has been applied(oil and water as coolant)，but S-Q process(compressed air as coolant)has been mainly applied in Japan and China.Through the use of heat treated heavy rail on high-speed and heavy-haul railway，it can be proved that the S-Q process is better than Q-T process because of low requirement for surface condition of rail head and uniformity of hardened layer.The S-Q process is used in this paper，that is:the whole heavy rail samples are heated in resistance furnace，then，cooled by spraying compressed air，as a result，fine pearlite can be obtained.At present，most results about the temperature field on the surface of heavy rail before and after heat treatment are obtained by the simulations，but there is a large gap compared with heavy rail production.In this paper，it is researched the temperature field on the surface of U75V 60 kg/m heavy rail and thickness of hardened layer under different S-Q processes， which provides the experimental basis for industrial development of on-line heat treatment processes of heavy rail［6－8］.

2 Experiment Schemes

Samples cut from U75V 60 kg/m 100 m heavy rail are 250 mm long，whose chemical composition are shown in Table 1.

Table 1 Chemical composition of U75V steel

Firstly，the samples are heated to 900℃ in resistance furnace，after 50 min，the samples are austenitized completely，simulating the temperature at the outlet of universal rolling.Under this condition，grain size is 6 grade in the center of rail head.Secondly，samples are dragged out of furnace and cooled in open air for 25 s，simulating the temperature drop of heavy rail on rollers near the cooling bed.Then，the samples are put into air spraying channel to be cooled on the top and both sides of rail head by spraying compressed air. During this period， the samples are moved back and forth slowly in order to ensure the cooling on the surface uniform.Finally，the samples are rapidly dragged out of air spraying channel and cooled on rollers in open air，simulating cooling of heavy rail on the cooling bed.

Experiment equipment as shown in Fig.1，air spraying channel with three air boxes is 500 mm long，and there are three rows air nozzles on top box and two rows air nozzles on side boxes，and all this nozzles are arranged alternately［9］，which can ensure air spraying uniformly.The distance between spraying nozzles and surface of rail head(thereafter referred to“air spraying distance”)is 15 mm.Major technical parameters and air pressure are shown in Table 2，and observing macrostructure of hardened layer，testing Rockwell hardness on cross-section of hardened layer according to“Technical specifications for the heat treated rails”(TB/T2635-2004)［10］.

Fig.1 Heat treatment equipment of heavy rail

Table 2 Change of air pressure during U75V 60 kg/m heavy rail heat treating

3 Results and Analysis

3.1 Cooling Rate on the Surface of Rail Head

The surface temperature before and after heat treatment arerecorded by FLIR Systemsthermal imager，and emission rate ε is 0.95，and the surface temperature before and after heat treatment are shown in Fig.2 when air pressure are 0.16 MPa and 0.26 MPa，respectively.

The surface temperature after heat treatment are constantly recorded by thermal imager，and time interval is 1min.In order to avoid the effect on results，100 mm in middle of samples is chose，and average temperature on the surface of four positions is computed as shown in Fig.3(a).It can be known from the image constantly taken by thermal imager after heat treatment that there is a self-temper on the rail head，and the tempering temperature reaches the largest when the samples are cooled for 3 min in open air.At that time，the ambient temperature is 26 ℃，and the tempering temperature at four positions under different air pressure are shown in Fig.3(b).

From the resuts in Fig.3，It is indicated that during the tempering after heat treatment，the tempering temperature of rail head is lower with the increase of air pressure，the tempering temperature of rail head is more than 570℃when air pressures are 0.16，0.20 and 0.23 MPa respectively，which are higher than finishing temperature of pearlite transformation at the cooling rate of 3℃/s according to CCT curve of U75V steel［11－12］.when air pressures are 0.26，0.30 and 0.33 MPa respectively，the largest tempering temperature is 529℃lower than finishing temperature of pearlite transformation at the cooling rate of 3℃/s.Under this condition，pearlite transformation is finished totally，so air pressure for U75V heavy rail heat treating should be more than 0.26 MPa，which can make sure that pearlite transformation is completely finished during 80 s air spraying.

Fig.2 Surface temperature of heavy rail before and after heat treatment

Fig.3 Tempering temperature at different positions of rail head

The cooling rate at four positions(as shown in Fig.3(a))of rail head is calculated according to the tempering temperature，and the results are shown in Table 3，which indicate that the largest cooling rate occurs at position 2 during heat treatment，and the smallest cooling rate occurs at position 4，and it is not sensitive to the change of air pressure because of not facing air spraying nozzles at position 4.

3.2 Photos of Hardened Layer

The 20 mm testing parts are cut from the middle of samples，and etched in 10%Nitric acid and alcohol solution after grinding and polishing.The photos of hardened layer are shown in Fig.4 when air pressures are 0.16 MPa and 0.26 MPa separately.

From the results in Fig.4，it is indicated that there is a hardened layer in rail head but it is not clear to be seen，the hardened layer is thicker than that obtained in off-line heat treatment heating only on the rail head，and hardened layer transits uniformly，which can effectively avoid the hardness collapse and softening zone［13－15］.

Table 3 Cooling rate at different positions

Fig.4 Photos of hardened layer under different air pressure

3.3 Rockwell Hardness on Cross-Section

Rockwellhardness on cross-section is tested according to“Technical specifications for the heat treated rails”，testing positions as shown in Fig.5(a)，and the distance between two testing points is 3 mm，because of symmetry of rail head，only list Rockwell hardness at the position A，B and D under different air pressure，as shown in Figs.5(b)，5(c)and 5(d).

From the results in Fig.4，it is indicated that the thickness and hardness on cross-section of hardened layer increase with the increase of air pressure.When air pressure is more than 0.26 MPa，the thickness of hardened layer are more than 24 mm at the A，B and C，and more than 15 mm at the D and E，and the hardness on cross-section of hardened layer are more than 37.4 at the A1，B1and C1，and more than 36 at the D1and E1，which completely meet the need that“the thickness of hardened layer is more than 15 mm at the A，B and C，and more than 10 mm at D and E，and the hardness on cross-section of hardened layer are more than 37 at the A1，B1and C1，and more than 35 at the D1and E1”stated in“Technical specifications for the heat treated rails”.So in order to reduce air consumption and control the cost，the proper air pressure of U75V 60 kg/m heavy rail heat treatment should be 0.26 MPa.

Fig.5 Rockwell hardness under different air pressure

In fact，the heat treatment of heavy rail is a procedure of accelerated cooling during the pearlite transformation，which can form the“cap-shaped”hardened layer on the rail head，microstructure is fine pearlite.Because pearlite transformation is the typical diffusion phase transformation，according to Zener-Hillert model，the growth rate of pearlite is free adjust and undercooling is fixed［16］. The growth rate of pearlitic colony［17］v as shown in Eq.(1):

where k is thermodynamic coefficient;Dbis interfacial

In Eq.(2)，it is indicated that S0decreases with the increase of ΔT，but ΔT increases with the increase of cooling rates，so average pearlite interlamellar spacing of hardened layer is decreased from about 300 nm before heat treatment(cooling rate 0.49℃/s in the open air)to about 100－120 nm after heat treatment(coolingratemorethan 3.21 ℃/s).Hardness increases with the decrease of S0，so hardness at the position A1is increased from 28-31 HRC(before heat treatment)to 37-40 HRC(after heat treatment，air pressure more than 0.26 MPa).diffusion coefficients of solute atoms;ΔT is undercooling.

In Eq.(1)，it is indicated that v increases with ΔT，but ΔT increases with cooling rate，and it can be seen from Tab.3 that cooling rate increases with the increase of air pressure，so hardened layer thicken increases with the increase of air pressure.

The relationship［18］between average pearlite interlamellar spacing(S0)and undercooling(ΔT)is shown as Eq.(2):

4 Conclusions

1)With the increase of air pressure，the thickness and hardness of hardened layer increases.When air pressure is more than 0.26 MPa，the thickness is more than 24 mm at the center and top fillets of rail head，and 15 mm at the blow fillets of rail head，which can fully meet needs that hardened layer is more than 15 mm at center and top fillets，and 12 mm at blow fillets of rail head stated in“Technical specifications for the heat treated rails”.So in order to reduce air consumption and control the cost，the proper air pressure of U75V 60 kg/m heavy rail heat treating should be 0.26 MPa.

2)During tempering after heat treatment，the tempering temperature of rail head is more than 570℃when air pressures are 0.16，0.20 and 0.23 MPa respectively，which are higher than finishing temperature of pearlite transformation at the cooling rate of 3 ℃ /s according to CCT curve of U75V steel.When air pressures are 0.26，0.30 and 0.33 MPa respectively，the largesttempering temperature is 529℃ lower than finishing temperature of pearlite transformation.So air pressure of U75V heavy rail heat treating should be 0.26 MPa，which can make sure that pearlite transformation has completely finished during 80 s air spraying

3)With the increase of air pressure，the cooling rate increases.The average cooling rate on the surface of rail head is more than 3.21℃/s when air pressure is more than 0.26 MPa，and the largest cooling rate occurs at top fillets of rail head.

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