causes of aluminium heat welding defects
Analysis of The Causes of Aluminium Heat Welding Defects In Steel Rails and Preventive Measures
Aluminium thermal welding of rails is a commonly used railway rail joining technique, the basic principle of which is to use the high temperatures generated by the thermal reaction of aluminium to melt the ends of the rails, thus enabling them to be welded. Due to the difference in welding materials and the lack of standardised welding operations, the probability of injury and breakage of aluminium heat welded joints is much higher than that of the parent material of the rails, and it has become one of the weak links of the line.
1. Shrinkage and loosening
Loose inner surface is not smooth, generally easy to occur in the centre of the weld rail waist and rail bottom central triangle, as shown in Figure 1.
(Figure 1 Transverse fracture due to loosening)
Causes
The main cause of such defects is high local temperatures and varying cooling times.In the aluminium heat welding process: preheating time is too long or preheating pressure adjustment is not correct; local high temperature, overheating phenomenon, especially the rail waist and rail bottom part of the local overheating phenomenon; preheating gun or sand mold installed bias, position is not centered, the formation of one side of the phenomenon of overheating; rail seam is too large.
Preventive measures
(1) Preheating time is appropriate and must not burn the track.
(2) Preheater flame length is moderate.
(3) The preheater is placed in the centre of the rail joint.
(4) The rail seam is moderate.
2. Porosity
Porosity is the steel in the reaction process of the release of gas, solidification is not timely discharged from the mouth, stay in the weld sub-surface and internal round holes. Pneumatic holes are the most common defects in aluminium heat welding, generally in the bottom of the rail and the bottom of the rail on both sides of the sloping surface is easy to produce, its surface is smooth, generally round (Figure 2).
(Figure 2 Morphology of porosity inside the weld)
Formation causes
(1) The end face of the rail to be welded is not thoroughly cleaned, with water or oil stains.
(2) Flux is damp.
(3) Dampness of sand mould.
(4) Sealing mud is too much and too wet.
(5) Preheating temperature is too low.
(6) The operation time from the end of preheating to pouring is too long, so that the temperature after preheating is lowered more.
Preventive measures
(1) Thoroughly clean both end faces of the rails to be welded.
(2) Strictly prevent welding materials from moisture
(3) The sealing mud shall not be too wet and it is not easy to apply too much.
(4) The preheating temperature of the rails should reach 800-900℃.
(5) Pour the flux into the crucible in advance, and prepare high temperature matches, after preheating. The sub-grade is placed in the crucible and ignited for pouring.
Secondary preheating: shunt plug to prevent.
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3. Slag
Slag is a low-level inclusions defect on the surface or inside the aluminium heat weld. In the casting process, such as steel and water part of the slag did not float away in time, and in the welding bar is solidified inside or attached to the surface of the welding bar, then the formation of slag-type inclusions, usually referred to as slag (Figure 3).
(Figure 3)
Formation reasons
(1) When the clamp pot for aluminium heat reaction, slag from the clamp pot out into the cavity, adhering to the surface of the rail, when the steel into the cavity can not be part of the slag flushed away floating and the formation of slag.
(2) Sedation time is not enough, the reaction is not completed on the pouring (for example, Konyi hit the nail too early or automatically off the plug, etc.), the slag is not sufficiently separated from the liquid steel, so that the slag is not completely uplifted, oxide and inclusions into the weld;
(3) Rail gap is too large, not enough steel or rail bottom casting ‘running iron’ and so on, so that the slag into the cavity.
Preventive measures
(1) Thoroughly clean the two end faces of the rail to be welded.
(2) Rail seam moderate.
(3) Check carefully whether the crucible is broken before use, and place the crucible so that the pouring mouth is placed in the centre above the sand mould.
4. Sand inclusion
Sand entrapment is also the surface of the aluminium heat welding weld or the internal presence of low-frequency inclusions defects.In the pouring condensation process, the local snail, sand moulds and other refractory materials are burned, or fall off the debris, floating sand, etc. into the molten steel is solidified, the formation of sand (Figure 4).
(Figure 4)
Causes of formation
(1) The floating sand is not completely removed after sand grinding.
(2) There are impurities into the cavity.
(3) The sand type is not fastened at one time, and the sand type is squeezed and falls into the cavity during the movement.
(4) Excessive preheating, burned sand.
Preventive measures
(1) Thoroughly clean the floating sand in the cavity after grinding.
(2) During the operation, it is strictly prohibited to drop foreign objects into the cavity.
(3) If there is any misalignment after the sand mould is closed, the sand mould must be reopened to clean the inside of the mould cavity.
5. Thermal cracks
Aluminium hot welding weld casting tissue in the solidification of the high temperature state before the formation of tensile stress cracks that is thermal cracking.Thermal cracks are usually located in the weld size of the weld bar in the middle, along the transverse direction of development, the fracture has a bluish oxidation colour or temperature colour, as shown in (Figure 5).
(Figure 5)
Formation causes
(1) There is movement of the weld during the welding process, which is subject to tension or the presence of large tensile stresses.
(2) Premature unboxing, so that the weld head is stressed in advance.
(3) Pushing the tumour in advance, so that the weld head is stressed in advance.
(4) Welding head in the cooling process, not fully solidified by external hammering.
(5) Welding head in the cooling process, not fully solidified when the rail shrinkage.
Preventive measures
(1) Strictly prohibit stepping on, hammering and prying the rail to be welded during the whole welding process.
(2) Carry out the dismantling and tumour pushing operation according to the time required by the process.
(3) It is strictly prohibited to dismantle the rail pairing equipment too early.
(4) When the rail shrinks greatly with the temperature, take measures to keep the rail gap size.
6. Not welded together
Rail aluminium heat welding is the aluminium heat flux in the clamp pot ignition, reaction formed after the high temperature steel water poured into the cavity composed of sand mold and to be welded rail, melting to be welded rail end face, steel water after cooling and solidification and to be welded rail into a whole. If the preheating temperature of the end face of the rail is not enough or the temperature of the steel water is low, the end face of the rail to be welded is not completely melted, then it can not achieve the melting combination, resulting in unwelded defects.
(Figure 6)
Formation causes
(1) The end face of the rail to be welded is not thoroughly cleaned, and there are oxidised skin and other foreign substances.
(2) The preheating temperature is too low, and the aluminium-heated steel is not enough to melt the rail end.
(3) The rail seam is too small. The heat capacity of the aluminium-heated steel liquid in the weld seam is small, and it fails to melt the rail end sufficiently.
Preventive measures
(1) Thoroughly clean both end faces of the rail to be welded.
(2) The preheating temperature of the rail needs to be reached, and the preheating temperature is evenly distributed.
(3) The rail seam is moderate.
7. Bite edge
Bite edge is due to improper selection of welding parameters or incorrect operation process, along the toe of the base material parts of the groove or depression (see Figure 7).Bite edge is often caused by stress concentration, so that the rail in the dynamic load of the train, it is possible to produce cracks in the bite edge, resulting in early fatigue fracture of the rail in use.
Figure 7
Formation causes
(1) The rail gap is too big.
(2) Welding of old and new rails.
(3) Excessive preheating of rail head.
The causes of aluminium heat welding defects in rails may involve a number of factors such as materials, process parameters, operating techniques, environmental factors and equipment.Each of these factors may individually or collectively lead to different types of defects.In order to make a comprehensive analysis, it is necessary to systematically check each link, from material preparation to the welding process to post-welding treatment, to identify possible problem points one by one.
