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Self-temperature Process for Railway Fish Plate Heat Waste Quenching

Current Status of Research And Use of Railway Fish Plates

Railway fish plate (Also named railway joint) is an important accessory for railway construction. Railway fish plates are used for the connection between two steel rails. They play a role in fixing steel rails during train operation, maintaining the continuity and integrity of the rail track. With the increases of speed and load of the railway and the prevalence of high-speed railway lines, the railway fish plate is facing with increasing higher performance demanding. 
 
A pair (Two pieces) of railway fish plates is connected by six bolts. They function by the close contact with the slopes of the upper and lower shoulders to transmit and bear the load when the train passes. They need to afford complex stress effects such as fretting wear, tension, bending and shearing. 

Study Based on Related Experiments of Railway Fish Plates

Chemical composition of experimental steel

Element C Si Mn P S Cr Mo
Content 0.25 0.23 1.28 0.008 0.009 0.19 0.1
Element

 

V Ni Cu Ti      
Content

 

0.07 0.02 0.03 0.02      

Next, we will make the researches on two different sample experiments. First, we must be familiar with the sectional drawing of fish plate.
section drawing of railway fish plate
Section Drawing of Railway Fish Plate
  • 1# Sample Process 
A fish plate is heated to 900°C, and kept like this for 30mins. Then, we will put it into the water at 80°C by the method of big web up for quenching for 25seconds.
  • 2# Sample Process 
A fish plate is heated to 900°C, and kept like this for 30mins. Then, we will put it into the water at 80°C by the method of small web up for quenching for 25seconds.

Experimental Methods of Railway Fish Plates

Our experimental methods include: 
  1. Determination of CCT curve
  2. Microstructure experiment
  3. Temperature field measurement
  4. Tensile test
  5. Transmission electron microscopy

Determination of Continuous Cooling Transition Curve of Experimental Steel

Heat Temperature

The sample will be heated to 900°C at 20°C/s, and kept for 5mins for fully austenization. And then, it will be cooled down to room temperature at 1, 2, 5, 8, 10, 14, 16, 18, 25°C/s cooling rate. 

CCT curve

1-5 °C/s: ferrite
5-16 °C/s: bainite
25 ° C/s: martensite

Metallographic analysis of different cooling rates

(a) Cooling rate 1°C/s: Pro-eutectoid ferrite, bainite and a small amount of pearlite
(b) Cooling rate 2°C/s: small granular ferrite, bainite and ferrite
(c) Cooling rate 5°C s: acicular bainite
(d, e, f, g) Cooling rate at 8, 10, 14, 16°C/s: bainite

Cooling rate microstructure analysis at 1°C/s

The main transformation product is ferrite. The base cementite strip is covered in the upper part, and made into pearlite. Besides, there are also local bainite strips. These correspond to the CCT curve.

Cooling rate microstructure analysis at 14°C/s

When the cooling rate is at 14°C/s, it is mainly bainite, and there are granular bainites. There are also twins in which the bainite is more clearly seen. Comparing the tissue of 1°C/s with 14°C/s, the latter structure is more delicate and corresponds to the CCT curve. However, as the cooling rate increases, the grain size becomes fine. 

Hardness analysis

When the cooling rate is 1, 2, 5, 8, 10, 14, 16, 18, 25°C/s, the linearity of hardness increases. The bainite content increases, while the ferrite and pearlite contents decrease or even disappear. And as the cooling rate increases, the degree of super-cooling increases, and the nucleation point of the bainite increases, so the structure becomes relatively small and appears tiny granular.

Determination of temperature field and study on self-tempering of quenching fish plate

Drawing of Quenching curve of fish plate

curve of railway fish plate
The sample will be tested for being kept in the quenching medium in the temperature field for 25 seconds. The quenching temperature field of the fish plate is uniform.

Residual heat self-tempering temperature field of fish plate

Residual heat self-tempering temperature field of railway fish plate
The drawing above shows unevenness in temperature decrease. There is unevenness and unpredictability of the quenching temperature field.

The temperature rise curve of the web of fish plate (25seconds)

The temperature rise curve of the web of railway fish plate
The surface reddening temperature as the web is facing upward is significantly about 100°C lower than that as the web is facing down. The temperature changes are slow when the big web is facing up.

Microstructure Analysis of Railway Fish Plates

The 1# sample is mostly bainite in its microstructure, which is in the form of block and slat, and the crystal grains are fine.
The 2# sample has bainite in its microstructure, which is in the form of massive and lath, and the crystal grains are coarse, the performance is unstable, and the fluctuation is large.
From the CCT curve and the mechanical properties, it can be seen that the 1# sample is better than the 2# sample.
Because it is inconsistent during the quenching process, the cooling rate during the temperature decrease is relatively stable and uniform as the big web is facing up, while the small web is inconsistent in the cooling rate. As a result, its microstructure and mechanical properties are lower than the big web facing up.

Mechanical Properties of Railway Fish Plates

The test of mechanical properties includes:1 tension sample, 1 cold bending sample, 6 impacting samples and 5 pieces of hardness samples. 
The result shows that different cooling rates and different water entering methods cause different structures and performances. Similarly, If the quenching medium has different residence time, it will cause different structures and performances. If the degree of water quenching in the heat treatment process is different, that will cause that the hardness and impact in the 1# sample are higher than 2# sample.

Conclusion of Railway Fish Plate Heat Waste Quenching

The CCT curves of the fish plate and the fiber structures at different cooling rates can be proved that like the following results. 
1. In phase transition, the experimental steel is ferrite transformation at below 15°C/s is, bainite transformation at 5°C/s to 25° C/s and martensite transformation at above 25°C/s.
2. As temperature increase, cooling and residual heat of the quenching and the tempering temperature field are shown, the temperature curve of the big web facing up is more stable and uniform than that of the web facing down in the water entering method. 
3. For the self-tempering process for quenching residue heat of fish plate, two different ways into the water can be drawn that because the depth of the big web upwards into the water is higher than that of the big web downwards, so the water temperature decrease rate is slower and the structure stability strength is higher. The microscopic structure is uniform and the mechanical properties are superior.

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