The hottest small diameter homogeneous cast iron e

2022-08-09
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Small diameter homogeneous cast iron electrode

this paper studies the welding processability of small diameter homogeneous cast iron electrode cast 268 electrode under different welding current and preheating temperature. The experimental results show that cast 268 electrode can weld cast iron under appropriate conditions. When the welding current is 95A and the preheating temperature is 270 ℃, the welded joint has good mechanical properties and machinability. After machining, the color is consistent with the base metal, and there is no crack

key words: cast iron welding small diameter homogeneous cast iron welding rod welding process

preface

cast iron is a kind of cast metal material with low production cost and many good properties. The annual production of various cast iron in China is now about 80million tons, and the castings with various casting defects account for about 10% - 15% of the annual production of cast iron, that is, the generally said scrap rate is 10% - 15%. If these castings are scrapped, the loss is as high as more than 1billion yuan per year based on the cast iron price in 1997. If it can be repaired by welding, it will be conducive to the completion of production tasks and save a lot of money

after China's entry into WTO, especially since the beginning of the 21st century, China has become a large country in the production of iron castings. According to incomplete statistics, 1/3-1/4 of iron castings in the world are produced in China, and most of them are exported. If the exported iron castings are welded, the color after machining is required to be consistent with the base metal, otherwise they are not allowed to be exported. Under such technical conditions, nickel based electrodes with expensive lattice (300 ~ 400/kg) are powerless. These export castings have many small defects, and there is nothing we can do to solve the small defect welding problem of export castings with the past large-diameter homogeneous cast iron electrode. The development of small diameter homogeneous cast iron electrode is one of the ways to solve the welding problem of small defects in export castings. After six years of research, the research was successful φ 2.5 homogeneous cast iron electrode cast 268, which has excellent welding process performance, good crack resistance, and good machining performance with the cooperation of reasonable process. It solves the requirement that the exported castings should have the same color with the base metal after welding repair machining

1 test method and conditions

1.1 test materials and conditions

the base metal used for the test is HT200, and the specification of the test plate is 200mm × 100mm × 1. Finally, install the jaw clamping plate for 5mm, and weld on the test plate; Self made cast iron electrode Z268 with diameter of φ 2.5mm; The welding equipment is jbx-60 arc welding machine and temperature measuring instrument; The welding current is 95A, 100A, 105a, 110A and 120a respectively, The preheating temperature of the test plate is 230 ℃, 240 ℃, 250 ℃, 260 ℃, 270 ℃, and the room temperature is 20 ℃. Table 1 chemical composition of gray cast iron (%)

Table 2 chemical composition of deposited metal of welding rod (%)

1.2 test method

first dry the cast 268 welding rod at 250 ℃ and keep it warm for 1.5 hours, and then select the above five different welding currents to start welding five welds at equal distance on the polished test plate. The welding speed is 3.07mm/s, the weld length is 80 mm, and each weld is welded with two layers, Weld the next weld after the previous weld is cooled to room temperature. Take metallographic samples at the starting point of each weld, observe the microstructure of the metallographic samples made of five welds under the metallographic microscope, make hardness and make hardness curve, and then a suitable welding current can be obtained. If the result is not ideal, preheat the test plate to the above five different temperatures, then take the metallographic sample of the weld and observe its structure under the metallographic microscope. A suitable preheating temperature can be obtained by hardness and hardness curve

2 test results

2.1 electrode processability test

2.1.1 influence of welding current on electrode processability

influence of welding current on electrode processability is as follows: Table 3 results of first layer weld processability experiment

Table 4 results of second layer weld processability experiment

from the above test results, it can be seen that when the temperature of welding test plate is 20 ℃ and the welding current is 95A, the welding processability is the best, At this time, there is no crack in the weld, and the slag has good coverage and no splash When the welding current is 120a, the effect of welding current is the worst, whether it is weld formation or weld surface, and there are cracks

2.1.2 effect of preheating temperature of the test plate on the processability of the electrode

the welding current is 95A. Changing the preheating temperature of the test plate, the processability of the electrode is as follows: Table 4 Effect of preheating temperature on the processability of the electrode

from the above test results, when the welding current is adjusted to 95A, the welding effect is the best when the temperature of the test plate is preheated to 270 ℃. At this time, the weld formation is good, and the weld surface is smooth, Under the microscope, there is no crack and white mouth structure

2.2 analysis of properties and microstructure of welded joints

2.2.1 effect of welding current on properties and microstructure of welded joints

with the increase of current value, a series of changes have taken place in the properties and microstructure of welded joints. The influence of welding current on the performance of welded joints can be seen from the following five groups of hardness curves in different areas

it can be seen from the four groups of curves in Figure 1, figure 2, figure 3, figure 4 that the higher the welding current is, the higher the hardness of the weld and fusion zone is

when the welding current is 95A, the hardness value of the upper layer of the weld is 33.7hv, and its structure is troostite, a small amount of cementite, spheroidal graphite and ferrite. In the middle of the weld, its structure is graphite ball, troostite and ferrite, but with the increase of cementite, its hardness value also increases to 38.5hv. The hardness value at the bottom of the weld is 37.6hv, and its structure is flake graphite and tempered troostite. The hardness decreases slightly, while in the fusion zone, the cementite structure decreases, pearlite increases, and the hardness value is 30.9hv

when the welding current is 100A, the hardness of the upper layer of the weld is 54.6hv, and its structure is pearlite, divergent graphite and a small amount of cementite. The hardness value in the middle of the weld is 59.6hv, and its structure is tempered troostite, spheroidal graphite and carbide, and its directionality is not strong. The hardness at the bottom of the weld is 51.2hv, and the structure is a small amount of pearlite, graphite ball, ferrite and cementite. At the fusion zone, the hardness becomes 35.9hv, because there is a small amount of white mouth structure, but it is discontinuous

when the welding current is 105a, the hardness of the upper layer of the weld is 59.8hv, and its structure is pearlite, long rod carbide, with strong directivity, graphite balls and a small part of white mouth structure The middle part of the weld is pearlite, spherical graphite and carbides tending to be long blocks, but there is no white mouth structure, so the hardness decreases to 51.5hv. The hardness at the bottom of the weld is 45.6hv, and its structure is troostite, spheroidal graphite and a small amount of carbide, so the hardness is reduced. When it comes to fusion, it needs to be specially pointed out that in the area of GB 4806.1 (2) 016 national standard for food safety general safety requirements for food contact materials and products, its structure has changed into troostite, graphite and cementite, but there is no white mouth structure, and its hardness is 33hv

when the welding current is 110A, the hardness of the upper layer of the weld is 62.2hv, and its structure is a small amount of pearlite, spheroidal graphite and a large amount of carbide, so the hardness is higher. The proportion of Troostite in the middle of the weld is increased, and a small amount of graphite in the shape of graphite and dots appears. In addition, there are some carbonized materials with low hardness and no obvious directivity, so the hardness decreases, and the hardness value is 58.9hv. The hardness at the bottom of the weld is 37.6hv, and the microstructure is pearlite, a small amount of carbides and some dotted graphite. The microstructure in the fusion zone is pearlite and graphite pellets, and the hardness value is 32.4hv

finally, when the welding current increases to 120a, the hardness of the upper layer of the weld is 99.3hv, and its structure is more graphite balls and pearlite, as well as a large number of eutectic carbides, so the hardness is larger. The structure in the middle of the weld is pearlite, graphite and white mouth structure, and the hardness decreases to 85.7hv at this time. At the bottom of the weld, the hardness is 84.4hv, because the structure is hypoeutectic white mouth structure, pearlite and some fine eutectic ledeburite, and the hardness is 33.6hv

for the above five current values, not any current can make the welding performance better, nor is it the greater the welding current at the same temperature, the better. The results of welding test show that the more carbide structure it contains, the higher its hardness is; The more graphite and pearlite structures it contains, the lower its hardness

the hardness value is the smallest when the welding current value is 95A. And there is no crack in the weld at this time. However, when the welding current value is 95A, the hardness value of the weld and fusion zone is more than 30 HV, and the machining performance is not ideal. In order to further reduce the hardness of the joint, the base metal is preheated

2.2.2 effect of preheating temperature on the properties and microstructure of welded joints

when the preheating temperature of the test plate is 230 ℃, 240 ℃, 250 ℃, 260 ℃ and 270 ℃, the hardness curve is shown in the following figure:

from Figure 5, Figure 6, Figure 7, figure 8, four groups of curves can be seen, with the increase of the preheating temperature of the base metal, the hardness of the weld and fusion zone shows a downward trend. When the base metal is preheated to 270 ℃, the hardness of the weld and fusion zone is the lowest, That is, it has the best machinability. Through microscopic observation, its structure is ferrite, pearlite and spheroidal graphite, and there is no crack and white mouth structure

the experimental results show that under the conditions of this test, the welding effect is the best when the welding current is 95A and the preheating temperature is 270 ℃. Not only can the interaction between starch macromolecules be very strong to prevent the occurrence of cracks, white spots and hardened structures, but also the weld surface is relatively smooth and the molding is relatively good

conclusion

(1) the diameter developed by ourselves is φ The influence of different welding current on the processability of 2.5mm homogeneous cast iron electrode Z268 is also different. In this test, the larger the welding current is, the more unstable the arc is, the worse the slag coverage is. The inside of the slag shell is not smooth and it is not easy to deslagging. The most obvious feature is that there are cracks in the welding process, and the weld surface is not smooth and not well formed. Under the conditions of this test, the appropriate current is 95A

(2) under the conditions of this test, with the increase of preheating temperature of base metal, the hardness of weld and fusion zone shows a downward trend. When the base metal is preheated to 270 ℃, the welded joint has good machinability

(3) adopt self-developed diameter of φ The 2.5mm homogeneous cast iron electrode Z268 adopts 95A welding current and the preheating temperature is 270 ℃. When welding gray cast iron, the electrode not only has excellent welding process performance, the welded joint has good crack resistance, machining performance, the color after machining is consistent with the base metal, but also the price is low. It solves the problem that the color of cast iron export parts is consistent with the base metal after welding and machining

references

1 Zhou Zhenfeng, et al., welding metallurgy and weldability of metals, machine press, 1988.11

2 Ou etc., development of high crack resistance electron for coldwelding of gram iron casting with thin walls collection of partners, 1984wi of CMES

3, edited by Li Fengyun, "the work of mechanical engineering materials also considers material forming and application", Beijing: Higher Education Press, 2004

4 edited by welding society of China Mechanical Engineering Society, welding metallographic atlas, Beijing: Mechanical Industry Press, 1987

5 (end)

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