Abstract : Through the comparison of new and old ladle structure, stress analysis and microstructure and gold equivalent analysis, it is concluded that the main cause of ladle failure is crack, and then the characteristics and types of cracks are studied, and thermal fatigue cracks are analyzed. The mechanism and its influencing factors.
Key words : ladle, welded structure, crack mechanism, analysis
0 Preface
At present, Baosteel currently uses a total of 78 steel and iron bags, including 63 steel ladle and 15 ladle. The first batch of ladle and iron bag were imported from Japan. Through the analysis of these serviced steel ladle, it is found that the life of the imported ladle is longer than that of the domestic one, but both are less than 15 years. The main drawback of both is cracking.
1 Introduction of new and old ladle welding structure
The Japanese ladle and the domestic imitation ladle that were used extensively in the Baosteel Phase I and II projects are T-shaped fillet flat bottom structures, that is, the "T" type welded joints are included in the wall and the bottom of the package. As shown in Figure 1.1. The main structural feature is that the bottom of the plate is connected to the ladle cylinder by a T-joint. There is a clear defect in the design of this old-fashioned structure. During the working of the ladle, the inner wall of the joint between the ladle cylinder and the bottom of the plate will generate a very large additional tensile stress, which is mainly due to the extremely rigid flat bottom and the less rigid. The deformation of the barrel at the joint is not coordinated. Therefore, the crack starts from the inner wall of the ladle, and the old ladle is often found after the crack spreads deep.
Figure 1.1 Old-fashioned ladle structure and its cracking parts
The biggest structural improvement of the new ladle is to change the flat bottom plate to a flat bottom with a circular flange, and the T-shaped fillet welded structure is changed to the butt welded structure. The new ladle structure is shown in Figure 1.2.
Figure 1.2 Partial schematic view of the new ladle structure
1.1 New ladle appearance crack
A steelmaking 5# ladle is a new ladle. It was put into use in January 2000, and the cumulative operation was about 3,500 times. In the beginning of February 2004, on the side of the lathe counterweight, intermittent shallow surface cracks were found in the butt welds of the cylinder and the flat-bottom arc transition section. The crack in the heat-affected zone is about 30 mm long, and the total length of the crack in the heat-affected zone under the weld is about 2800 mm. The schematic diagram of the cracking part is shown in Figure 1.3.
Figure 1.3 Photograph of cracked lamination metallography (crack morphology after magnification)
2 ladle crack type discrimination
The cracking position of the new ladle is obviously different from that of the old ladle, but the crack directions of both are developed along the longitudinal direction of the girth weld.
The cracking of the old ladle is caused by the unreasonable structural design, which causes large strain and low-cycle fatigue cracking failure at the inner surface of the ladle and the T-weld of the bottom of the ladle - strain fatigue. There are two main reasons for fatigue cracking. One is that there is a large additional stress in the steel ladle, and the other is that the work of the ladle is intermittent. The combination of the two causes large periodic stress fluctuations in the ladle, which greatly reduces the material. Anti-fatigue performance. Figure 2.1 shows the calculation results of the ladle cylinder stress of the old ladle above the T-weld joint.
Figure 2.1 Stress on the outer surface of the T-weld of the old ladle
The new ladle has been improved by local structure, which greatly reduces the cyclic stress amplitude of the inner surface of the ladle, but the axial stress on the outer surface of the joint between the cylinder and the bottom seal may change from a negative value to a positive value.
Whether it is an old-fashioned ladle or a new ladle, since the load is cyclic, the stresses in the ladle are also cyclically fluctuating. The stress from the empty steel is approximately zero to the maximum stress after filling the molten steel, and returns to the empty ladle stress. It is approximately zero, so the new ladle also has fatigue life problem. For a specific material and working temperature, as long as the stress fluctuation amplitude reaches a certain value in the ladle, it may cause fatigue crack initiation and expand into macro crack until the ladle cracking failure.
It can be seen from Fig. 2.2 and 2.3 that the surface crack of the new ladle is multi-source crack, which is obviously different from the surface crack pattern of the old ladle. Therefore, the outer surface crack of the new ladle is not mechanical fatigue or at least not pure mechanical fatigue. The result of a combination of fatigue cracking or mechanical fatigue and thermal fatigue.
Figure 2.2 Multi-source crack morphology on the outer surface of 5# ladle
Figure 2.3 Magnified surface of the outer surface crack of 5# ladle
3 Finite element stress analysis of the bottom joint of ladle
In order to find out the mechanical causes of cracks in the ladle, the finite element method is used to analyze the numerical stress of the bottom structure of the ladle. According to the actual situation of ladle, the ANSYS software of international general finite element analysis is used to analyze the stress of each part of the ladle, and the detailed stress calculation of the weld part at the bottom of the ladle is carried out from the viewpoint of analytical design. The finite element analysis process is omitted. The conclusion is basically consistent with the above analysis.
4 5# ladle analysis of external cracks
4.1 Further identification of thermal fatigue cracks
Figure 4.1 shows the microstructure of the outer surface crack of the 5# ladle. It can be seen that the outer surface crack has typical thermal fatigue (corrosion thermal fatigue) crack characteristics. And the Metallographic Atlas
Figure 4.1 High-magnification microstructure of 5# ladle crack
Comparing the thermal fatigue cracks, it is found that the cracks are transgranular and the ends are sharp, which is almost consistent with the metallographic morphology of the corrosion thermal fatigue crack. The external crack of the ladle is compared with the metallographic morphology of the typical thermal fatigue crack. It can be seen that the external crack is a thermal fatigue crack, and the thermal fatigue crack characteristics of the crack microstructure are obvious.
4.2 Preliminary analysis of the causes of thermal fatigue cracking of 5# ladle
4.2.1 Advantages of the new ladle
The finite element analysis of the stress of the new ladle shows that the maximum axial stress at the girth weld on the inner surface of the ladle is reduced to 48.76 MPa, which is only the maximum stress of the old ladle, regardless of the influence of temperature. /10, considering the force and maximum stress, the new ladle structure is more reasonable than the old ladle structure, which can effectively avoid the low cycle fatigue failure of the material due to excessive local mechanical stress, eliminating the upper part of the T-joint of the old ladle inner wall. Hidden danger of fatigue cracking.
The T-shaped fillet welded structure is another structural defect of the old ladle. Since the thickness of the flat bottom reaches 80 mm, it is difficult to ensure the welding quality, and the fatigue strength of the welded joint is also low. The new ladle changes the T-shaped fillet weld to the butt weld, which greatly improves the fatigue strength of the welded joint.
4.2.2 Insufficient new steel ladle
(1) The new ladle adopts the flat bottom structure with arc-folded transition. It is reasonable, but because the flat bottom thickness is 80mm, the hemming processing is extremely difficult. The manufacturer uses the segmented heating semi-mechanical bending method to ensure the processing quality.
(2) In order to butt weld with the ladle cylinder with a thickness of only 32mm, the new ladle folds from 80mm to 32mm at the weld seam are treated by double-sided thinning, according to the requirements of GB159-98 "Steel Pressure Vessel", thinning The length should be greater than 3×(80-32)/2=72mm, the double-sided thinning length of the 5# ladle is close to the minimum thinning length, the local stress concentration is large, and the weld is close to the thinned portion. Seam fatigue is extremely unfavorable.
(3) The new ladle also has a very unfavorable structural design, that is, the outer surface weld of the ladle is susceptible to the baking of high-temperature molten steel. The high-temperature radiation of the molten steel causes the wall of the outer cylinder to rise sharply on the side of the ladle, accelerating the material. High temperature aging, especially caused by thermal fatigue damage in the weld fusion zone and heat affected zone, cracking and shortening the service life of the ladle. There are several hanging slags on the outer surface of the 5# ladle. It also shows that there is a possibility of being affected by high temperature.
4.2.3 Reasons for thermal fatigue cracking of 5# ladle
The microstructure of the crack surface of the 5# ladle is shown in Figure 4.2. The microstructure of the 20g steel ladle has changed significantly. The pearlite area in the metallographic analysis has been difficult to find, and the material has undergone extremely serious pearlite spheroidization. It shows that the outer surface of the ladle ring weld often reaches a higher temperature, which greatly exceeds the 360 ​​°C generally considered by the old ladle. This conclusion can also be confirmed from the hardness test results of the crack portion of the ladle, and the measured hardness value is significantly lower than the normal hardness of 20 g steel. It indicates that the pearlite spheroidization of this part is more serious. Studies have shown that 20g steel pearlite spheroidization will significantly reduce the strength and hardness of the material. The spheroidization level can be referred to Figure 4.3 and Table 4.1.
Figure 4.2 Microstructure of cracks on the outer surface of 5# ladle
Table 4.1 No. 20 steel pearlite spheroidization reference level
Item name Spheroidization level Organizational characteristics
After the fifth stage of the globalized pearlite form has disappeared, the spheroidized carbides are distributed on the grain boundary and the ferrite matrix, and the dispersion is large. The spheroidized sixth-order grain boundary and the carbide on the ferrite matrix have gradually become gradually. Growing up, large dispersion
Globalization (Level 5) Severe spheroidization (Level 6)
Figure 4.3 20 steel pearlite spheroidization reference level (680 ×)
According to the analysis of the influencing factors of thermal fatigue, the precipitation of carbides, especially at the grain boundaries, will reduce the thermal fatigue strength of the material. Therefore, the outer surface of the 5# ladle will be seriously pearlite spheroidized, which will cause thermal fatigue. Life expectancy is greatly reduced.
4.2.4 Causes of weld cracking before parent metal
There are two major characteristics of the cracked part of the 5# ladle. The hoop feature is that the crack has a weight-bearing part on the side of the molten steel. It has been mentioned that this is probably related to the damage of the ladle material caused by the hot steel water radiation.
The height direction is characterized by cracks in the weld fusion zone and heat affected zone, and the weld is mostly in the lower part of the weld, which is closely related to the deterioration of the material structure and performance degradation caused by the welding process, and the heat affected zone on the lower side of the weld At the transition of the wall thickness of the ladle cylinder and the flat bottom flange, there is a large stress concentration or edge additional stress in both the mechanical stress and the thermal stress.
5 Conclusion
The crack of the new steel ladle cylinder is generated by the thermal fatigue crack at the outer surface of the girth weld. It is completely different from the cracking position of the old ladle surface and has a large concealment. It is difficult to find nondestructive testing. This time, the on-site laminating metallographic technique is adopted. found.
The main causes of thermal fatigue cracking are:
(1) The temperature difference between the inner and outer surfaces of the cylinder is large, and the tensile thermal stress generated at the outer wall of the cylinder has cyclic fluctuation characteristics;
(2) The outer part of the cracked weld is welded with a weight equal to the thickness of the cylinder. The lower part of the weld is a flat bottom with a hem. The thickness of the hem is changed from the wall thickness of the cylinder by 2 times to the cylinder. Thickness makes the difference in cooling speed near the weld seam of the cylinder large, and the distribution in the vertical direction near the weld is uneven, and additional tensile thermal stress is generated in the weld.
(3) When hot steel water is poured, it has high-temperature heat radiation effect on the ladle cylinder, which leads to deterioration of the material of the ladle body without de-weight barrier and performance degradation, which greatly reduces the mechanical fatigue strength of the affected joint of the cylinder. The thermal fatigue strength promotes the initiation and propagation of thermal fatigue cracks in the ladle.
(4) Due to the coarse grain of the weld fusion zone and the heat-affected zone, the performance is embrittled, and the ring girth of the cylinder is at the junction of the cylinder and the transition section of the flat-bottomed flange with different thicknesses, so the small thermal fatigue cracks are found. The fusion zone and the heat affected zone under the weld.
E27 Led Bulb,Led Bulb Lamp,Led Lamp Light,Dimmable Led Light Bulbs
Guangdong Smart Street Lighting Co., Ltd , https://www.gdfldlight.com