Defect Analysis for Rail Thermit Welding

1. Loose

From sectional cutting inside inspection, the surface is not smooth, it occurs in the central triangle area of the rail waist and the rail bottom.

Analysis why it will happened?

-Preheating time is too long, with overheating phenomenon because of high temperature in part area, especially in rail waist.

- Offset installation of preheating gun or sand mold.

-Large welding gap.

How to prevent?

-Appropriate preheating time, do not burn the rail end.

-Good control flame length between 12~15mm.

-The preheating gun is installed in the center of the rail gap.

-The rail gap must between 25~28mm.

2. Porosity. The surface is smooth and round shape.

Analysis why it will happened?

-Incomplete cleaning of rails

-Thermit portion is wet

-Sand mould is web

-Too much luting sand, or too wet.

- Low Preheat temperature.

How to prevent?

 -cleaning of both ends of the rail to be welded thoroughly.

 -Strictly prevent the welding material from getting wet.

 -The sealing paste should not be too wet or used too much.

 -Rail preheating temperature needs to reach 700~900℃.

3. Slagging. The slagging can be formed when the slag entering the welding gaps.

Analysis why it will happened?

-The cleaning of the rail surface is incompletely, there are containing metal oxide and iron oxide slag.

-Weld gap is too large, or "running out of sand mould"

-The calming time is not enough, when reaction is not completed before pouring and the slag is not completely separated from the molten steel.

How to prevent?

- Cleaning of both ends of the rail to be welded thoroughly.

- The rail gap must between 25~28mm.

- Carefully check the crucible not damage before use, put the crucible in the centre of sand mould.

4. Hot break. The surface often has an oxidized blue color.

Analysis why it will happened?

-The welding gap moves position during the welding process, large tensile stress existed.

- Premature unpacking makes the weld head acting force too fast.

- Premature shearing process makes the welding head acting force too fast.

- External hammering is received when solidification is not completed during the cooling process

- The rail shrinks before completing solidification during the cooling process.

How to prevent?

-In the whole welding process DO NOT stepping, knocking, prying to be welded rail.

-Unpacking sand mould and shearing according to process requirements.

-Strictly prohibited to remove the rail equipment when weld head temperature higher than 600 ℃.

-Big shrink according to the temperature, rail tensioner can be used to maintain the size of the rail gap.

5. Unwelded. The two rail sections to be welded have unmelted areas.

Analysis why it will happened?

-The cleaning of the rail surface is incompletely, there are containing oxide scab.

-Preheating temperature is too low, aluminum hot steel is not enough to melt the rail head.

-Rail gap is too small, the capacity of aluminum hot steel liquid within the weld is small and fails to fully melt the rail head.

How to prevent?

- Cleaning of both ends of the rail to be welded thoroughly.

-The preheating temperature shall up to 700~900℃.

-Appropriate welding gap between 25~28mm.