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Repair welding technology and operation technology of gray c
Pubdate date:2019-10-17Views:0
Based on the analysis of the weldability of gray cast iron, this paper gives a detailed introduction to the repair welding process and the specific operation techniques and methods. It has great reference value for the welder to improve the welding technology at the production site.

I. Introduction

Gray cast iron is a kind of cast iron. The carbon of gray cast iron is distributed in the form of flake graphite in the cast iron base. The section is dark gray, so it is called gray cast iron. Since the flake graphite cuts the matrix structure of the cast iron, the gray cast iron has low tensile strength and lacks plasticity. Gray cast iron has good castability and cutting performance, and because graphite in gray cast iron exists in the form of flakes, it has good wear resistance, shock resistance and machinability and has high compressive strength, so it is industrially It is widely used.

Gray cast iron is currently used in the form of castings. Due to the characteristics of the casting process, castings often have various degrees of defects. There are also many castings damaged by various reasons in the production site. The welding of cast iron is actually repair welding of castings with defects or damage. Therefore, casting repair welding has great economic significance.

1. The welding performance of gray cast iron is poor, and the following problems are easy to occur during welding.

1.1 white joint tissue after welding

When repairing gray cast iron, a layer of white mouth tissue is often formed in the fusion zone. The reason for the white-mouth structure is that the near-seam area of ​​the base metal is heated by high temperature during welding. When the heating temperature is above 860 °C, the initial part of the graphite in the original gray cast iron is also melted in the iron. The higher the temperature is. The more graphite is melted in the iron. When cooling, it is generally considered that under rapid cooling conditions of 30-100 ° C / s, the carbon melted in the iron is not as precipitated as graphite, but appears as cementite, so-called white mouth. Also. The lack of graphitized elemental carbon, silicon, etc. in the weld pool is also a major cause of white mouth. Generally, in a narrow high temperature fusion zone, white mouth tissue is easily generated after welding. The white mouth tissue is hard and brittle, making the weld difficult to machine after welding and even causing cracking. The main measure to prevent white-mouth formation is to properly adjust the chemical composition and cooling rate of the filler metal. Improve the chemical composition of the weld technology, increase the content of graphitization elements, and prevent the weld metal from producing white mouth under certain conditions. For example, the carbon of cast iron welding wire for gas welding is higher than that of the base metal (C3.0%-3.8%, Si3.6%-4.8%), especially when cold-welded grey cast iron, the silicon content in the welding wire can be as high as 4.5. % Slow cooling after welding and prolonging the time when the fusion zone is in red hot state, the graphite is charged and analyzed, which is the main process to avoid white mouth in the fusion zone. The specific measures taken are pre-weld preheating and post-weld insulation. The cooling rate is slower due to gas welding. therefore. It is extremely powerful to prevent white mouth.

1.2 cracks appear on the street

Cracks are a problem in welding gray cast iron. Cracks on the welded joints of gray cast iron may occur in the weld metal or on the base metal or base metal. The cracks in the base metal generally appear in the near seam area, which may be longitudinal, lateral or oblique. Due to the extremely poor plasticity of the gray cast iron, almost no plastic deformation can occur, and the strength is low. Therefore, under the joint action of the welding stress and the stress of the casting itself (tissue stress), when the local stress is greater than the strength limit, cracks are generated. In severe cases, the weld metal and the base metal are separated, that is, the weld is detached from the base metal, so-called peeling. If the weld strength is high and the base metal strength is low, or the joint is white, the shrinkage rate of white cast iron (1.6%-2.%) is larger than that of gray cast iron (0.9%-1.8%). And the plasticity is also poor, so peeling occurs. Cracks in the weld metal are generally common in transverse cracks, sometimes in longitudinal and oblique cracks, and there is no common blue color at high temperature oxidation at the fracture of the weld. Crack formation often results in a crisp metal cracking sound. Usually the crack occurs after the dark red color of the hot weld metal disappears, ie below 600 °C, until the weld and the overall temperature of the weldment are uniform. The temperature at which the crack is most likely to occur is below 400 ° C. Usually, such a crack occurring under the combined action of thermal stress and tissue stress is called a thermal stress crack.

(1) Pre-weld preheating and post-weld slow cooling measures:

Pre-heating and partial warm-up of the weldment before welding can not only reduce the white-mouth tendency of the weld, but also reduce the welding stress and prevent the weldment from cracking.

(2) Measures to reduce welding stress by arc cold welding:

Use plastic materials with good plasticity, such as nickel, copper, nickel copper, high vanadium steel, etc. as filler metal, so that the weld metal can relax the stress by plastic deformation and prevent cracks; use fine straight precision welding rod, small current, intermittent The welding and dispersive welding method can reduce the temperature difference between the weld and the base metal and reduce the welding stress; the hammer weld can eliminate the stress and prevent the crack.

When the weld is cooled, it can compensate for the free shrinkage of the bottom, thereby avoiding excessive force and causing cracks.

(3) Using heat welding and controlling the temperature.

When the temperature is higher than 600 °C, some internal stress is eliminated due to a certain plastic deformation. Generally, no thermal stress crack will occur when welding at 600 °C or higher.

Second, the gray cast iron has poor welding performance, and the following problems are likely to occur during welding.

1. Producing white mouth tissue after welding

When repairing gray cast iron, a layer of white mouth tissue is often formed in the fusion zone. The reason for the white-mouth structure is that the near-seam area of ​​the base metal is heated by high temperature during welding. When the heating temperature is above 860 °C, the initial part of the graphite in the original gray cast iron is also melted in the iron. The higher the temperature is. The more graphite is melted in the iron. When cooling, it is generally considered that under rapid cooling conditions of 30-100 ° C / s, the carbon melted in the iron is not as precipitated as graphite, but appears as cementite, so-called white mouth. Also. The lack of graphitized elemental carbon, silicon, etc. in the weld pool is also a major cause of white mouth. Generally, in a narrow high temperature fusion zone, white mouth tissue is easily generated after welding. The white mouth tissue is hard and brittle, making the weld difficult to machine after welding and even causing cracking. The main measure to prevent white-mouth formation is to properly adjust the chemical composition and cooling rate of the filler metal. Improve the chemical composition of the weld technology, increase the content of graphitization elements, and prevent the weld metal from producing white mouth under certain conditions. For example, the carbon of cast iron welding wire for gas welding is higher than that of the base metal (C3.0%-3.8%, Si3.6%-4.8%), especially when cold-welded grey cast iron, the silicon content in the welding wire can be as high as 4.5. % Slow cooling after welding and prolonging the time when the fusion zone is in red hot state, the graphite is charged and analyzed, which is the main process to avoid white mouth in the fusion zone. The specific measures taken are pre-weld preheating and post-weld insulation. The cooling rate is slower due to gas welding. therefore. It is extremely powerful to prevent white mouth.

2. Cracks appear on the welding street

Cracks are a problem in welding gray cast iron. Cracks on the welded joints of gray cast iron may occur in the weld metal or on the base metal or base metal. The cracks in the base metal generally appear in the near seam area, which may be longitudinal, lateral or oblique. Due to the extremely poor plasticity of the gray cast iron, almost no plastic deformation can occur, and the strength is low. Therefore, under the joint action of the welding stress and the stress of the casting itself (tissue stress), when the local stress is greater than the strength limit, cracks are generated. In severe cases, the weld metal and the base metal are separated, that is, the weld is detached from the base metal, so-called peeling. If the weld strength is high and the base metal strength is low, or the joint is white, the shrinkage rate of white cast iron (1.6%-2.%) is larger than that of gray cast iron (0.9%-1.8%). And the plasticity is also poor, so peeling occurs. Cracks in the weld metal are generally common in transverse cracks, sometimes in longitudinal and oblique cracks, and there is no common blue color at high temperature oxidation at the fracture of the weld. Crack formation often results in a crisp metal cracking sound. Usually the crack occurs after the dark red color of the hot weld metal disappears, ie below 600 °C, until the weld and the overall temperature of the weldment are uniform. The temperature at which the crack is most likely to occur is below 400 ° C. Usually, such a crack occurring under the combined action of thermal stress and tissue stress is called a thermal stress crack.

2.1 Pre-heating and post-weld slow cooling measures:

Pre-heating and partial warm-up of the weldment before welding can not only reduce the white-mouth tendency of the weld, but also reduce the welding stress and prevent the weldment from cracking.

2.2 Measures to reduce welding stress by arc cold welding:

Use plastic materials with good plasticity, such as nickel, copper, nickel copper, high vanadium steel, etc. as filler metal, so that the weld metal can relax the stress by plastic deformation and prevent cracks; use fine straight precision welding rod, small current, intermittent The welding and dispersive welding method can reduce the temperature difference between the weld and the base metal and reduce the welding stress; the hammer weld can eliminate the stress and prevent the crack.

When the weld is cooled, it can compensate for the free shrinkage of the bottom, thereby avoiding excessive force and causing cracks.

2.3 Using hot welding and controlling temperature

When the temperature is higher than 600 ° C, part of the internal stress is eliminated due to a certain plastic deformation, and thermal stress cracking is generally not generated when welding at 600 ° C or higher.

Third, gray cast iron repair welding method

The welding method of gray cast iron mainly adopts electrode arc welding, gas welding and brazing. According to whether the weldment is preheated before welding, the electrode arc welding can be divided into cold welding, semi-heat welding (preheating temperature below 400 °C) and heat welding (preheating temperature 600-700 °C).

4. Repair welding process of gray cast iron

4.1.1 Cold welding method. The arc welding cold welding method means that the weldment is not preheated before welding, and the heating process does not assist the heating, so it can accelerate the welding repair productivity, reduce the cost, improve the working conditions, and reduce the weldment due to preheating. The deformation caused by uneven heat and the oxidation of the processed surface of the weldment. At present, the cold welding method is being promoted and developed rapidly. However, the cold welding method has a large cooling rate due to the weld and the heat affected zone after welding. The formation of white mouth tissue. In addition, due to the uneven heating of the weldment, it often forms a great internal stress, which will cause cracks. At the time of cold welding, the following points should be noted:

1 Oil should be thoroughly cleaned before welding. Cracks should be made at both ends of the crack. The shape of the groove should be guaranteed to facilitate welding and reduce the amount of weldment.

2 Use welding rods other than steel core or cast iron core. Small diameter welding rods should use small welding current as much as possible to reduce the internal stress and the width of the heat affected zone.

3 Short-weld welding method is used. Generally, it is welded 10-40mm each time, and then welded after it is fully cooled.

4 using segmented reverse welding. This can reduce the tensile stress and is good for crack prevention.

5 After each welding a short weld bead, use a round hammer to hammer outward along the weld.

Cold-welded welding rods are divided into two categories according to the machinability of welds after welding: one type of castings that do not require machining after welding, such as steel cored iron welding rods (EZCQ), which are only suitable for small thin-walled castings with little rigidity. The defect weld repair; another type of casting used for machining after welding, such as pure nickel electrode (EZNi-1) ferronickel electrode (EZNiFe-1) nickel-copper cast iron electrode (ENiCu-1).

4.1.2 Hot welding method

The heat welding method is a welding method in which the weldment is heated to 600-700 ° C in whole or in part before welding, and maintained at a certain temperature during the welding process, and slowly cooled in the furnace after welding. When using the heat welding method, the weldment is cooled slowly and the temperature distribution is uniform, which is beneficial to eliminate the white mouth structure, reduce stress and prevent cracks. However, the heat welding method has high cost, complicated process, long production cycle, and poor labor conditions during welding, so it should be used as little as possible.

4.2 Repair welding process of gas welding and ash-filled cast iron:

The gas welding flame temperature is much lower than the arc temperature, so the heating and cooling of the weldment is relatively slow, which is advantageous for preventing the white mouth structure and crack generated by the gray cast iron during welding. Therefore, the quality of castings repaired by gas welding is generally better, because gas welding is a common method for repairing cast iron. However, compared with electrode arc welding, the welder has high labor intensity, large deformation of the weldment, and it is difficult to weld through large welds. However, due to the good quality of gas-welded castings and easy cutting, many small and medium-sized gray-mouth castings in many factories are still more commonly used for gas welding.

4.2.1 Preparation before welding

1 After the weldment is removed, check the defects. The defects on the weldment can be observed at least, and can also be found with a magnifying glass 10-20 times.

2 After the crack is found, the hole with diameter φ4-6mm is drilled at both ends of the crack to prevent crack propagation. When welding gray cast iron, cast iron wire, wire 401A or wire 401B can be used. Gas welding flux is selected when welding 201, cast iron The melting point of the gas welding flux is 650 ° C. It can change the silica (melting point of 1350 ° C) generated during the gas welding of cast iron into a fusible salt. When welding the gray cast iron with gas welding flux for cast iron, it should be larger. The welding torch is used to improve the efficiency of the welding head flame, which is beneficial to the defects such as the air hole slag. The nozzle hole diameter can be determined according to the wall thickness of the welding joint.

4.2.2 Operating Technology

In the gas welding process, a neutral flame or a weak carbonized flame must be used; at the end of the welding, the carbonized flame can be used to slowly cool the weld, which can reduce the burning of carbon and silicon, eliminate the excessively thick oxide film, and prevent the white mouth from being cold. Phenomenon; when the bottom of the defect or the opening is eliminated, an oxidizing flame can be used. When welding, the welding wire is added after the basic metal is melted to prevent poor fusion; when there are small pores and white bright spots in the molten pool, it can be Adding a small amount of gas welding flux to the molten pool helps to eliminate flat slag, but the gas welding flux should not be added too much, otherwise it will easily produce slag and pores; properly increase the power of the flame and increase the temperature of the molten iron in the molten pool, which is beneficial to gases and impurities. Floating, thus reducing the pores, slag; operation should pay attention to the flame always cover the molten pool; when adding the wire, often use the wire to gently stir the molten pool, to promote gas, slag floating; welding repair will be completed The weld seam is slightly higher than the surface of the weldment, and the surface layer with more impurities is scraped off with the wire. Since the surface layer contains more impurities and has higher hardness after cooling, scraping off the surface layer can improve the cutting performance of the weld.

When the repair welding is more rigid or the repaired weld stone is larger, and the material is poor, the loose structure of the casting, hot welding can be used. After welding, the casting is covered with asbestos cloth or char, so that the weld is slow. Cool to prevent cracks and white-mouth tissue.

V. Conclusion

Gray cast iron adopts reasonable welding repair process, which can obtain better machinability and compactness of the weldment. Therefore, the weld repair of gray cast iron is as long as the welding repair process is proper, and the white mouth structure and crack are generated.
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