Construction technology of low temperature welding of steel structure of futures building

The low temperature welding technology in steel structure engineering has always been the subject of common concern in the academic and engineering circles. Especially in the rapid development and large number of applications of the construction steel structure industry in recent years, the construction of steel structure in a low temperature environment has been mentioned in a quite high position, which has attracted high attention from all aspects. Considerable human and material resources have been invested to study and solve the direct impact of construction technology in low temperature environment, especially on welding quality. Because of the poor welding quality control caused by the decline of welding quality and even cause unsafe risks, and even the resulting safety accidents occur constantly, and the welding quality defects caused by the environment is a very important aspect. Therefore, the research and experiment in this field are constantly strengthened. Taking Futures Building steel structure engineering as an example, the welding technology of steel structure under low temperature environment is briefly analyzed.
The main structure of the Futures building is 242.8m high super high-rise building steel structure project, the total construction area of the project is 131,500 square meters, and the comprehensive impact of the land climate and tropical Marine climate is more serious. From September 2008 to June 2009 for the main structure construction phase of Dalian futures, steel structure installation through the entire low temperature winter period.
A large number of welding steel structure failure accidents show that low temperature is the main cause of brittle fracture, especially if there are defects (notch effect) in the structure, the brittle fracture effect is more serious. When the temperature is below the critical transition temperature of the material, under the action of much less than σs, the σs of the steel increases and approaches σb, and a completely non-yielding fracture occurs. This is the fundamental reason for low-temperature welding tests. The test results bring us a lot of enlightenment, but also our winter construction provides a theoretical basis and operational guidance. Many low temperature tests are quite successful, which provide us with reliable theoretical and technical support in the process of engineering construction, and provide more reliable test data for the preparation of welding process evaluation plan.
Through laboratory research and analysis, the direct characterization of the harm of low-temperature environment to weld metal is the occurrence of cracks and brittle fracture under working conditions, and the brittle fracture mechanism is changed by the change of the rate of temperature drop, which is mainly summarized as follows: 1. Under the condition of low temperature welding, the cooling rate of the weld increases compared with the normal temperature, and the direct consequence is that the t8/5 decreases, and the grain size becomes coarse, so the sensitivity of cold crack also increases correspondingly. 2. Under the premise of large structural constraint, the cooling speed is too fast, which is easy to cause the segregation of the weld metal. Under the action of strong tensile stress field, the crystallization crack occurs at the segregation of the weld, that is, the center part of the weld, which is a form of hot crack.
3. The delay effect of cold crack increases. In the cooling process of weld metal, the solubility of free hydrogen decreases, the cooling speed becomes faster, and the time of hydrogen leakage becomes shorter. Therefore, the proportion of residual metal increases, which increases the effect of cold crack. The delay effect is proportional to the amount of hydrogen remaining in the metal. 4. When the working temperature of the component is lower than the brittle transition temperature of the material, the static load strength of the structure is greatly reduced under the combined action of tensile stress and welding residual stress, which is highly likely to cause brittle fracture under the action of external forces far below the σs point of the material.
Due to the special geographical environment and the rapid development of the building steel structure industry in recent years, more and more steel structure construction, especially steel structure welding, must go through a relatively long low-temperature welding process in winter. Through a large number of tests and analysis, research, people gradually summed up a set of low-temperature welding quality control essentials. First of all, in order to eliminate the impact of welding stress on the construction quality, in the welding process of high-rise steel structures, it is usually followed from the center to the periphery, the use of structural symmetry, node symmetry welding sequence. In order to reduce the damage caused by welding stress, both ends of the same component shall not be welded at the same time, and shall not be welded from the outer ring of the structure to the center, and the weld shrinkage amount shall be reserved during installation.
Secondly, when welding construction in a low temperature environment, not only must there be strict welding protection measures, but also must use a large range of pre-welding heating method to eliminate the strong temperature difference between the base metal and the weld zone when welding without pre-welding heating, to avoid causing cracks between the base metal and the welded joint. Strict heating before welding, continuous, stable and high interlayer temperature during welding process, the whole process of narrow-pass welding, the regular use of left and right alternating welding pass can effectively control the generation of welding cracks.
In the whole winter construction process of Dalian Futures steel structure project, we organized the construction in strict accordance with the welding process evaluation report and low-temperature welding technical regulations prepared by the welding research Institute of the company through a large number of low-temperature welding tests and construction experience. The weld length of the project is 5723.45 meters. In the welding quality self-inspection and third-party testing of the whole project, the pass rate of a self-inspection inspection reaches more than 98%, and the repair pass rate is 100%. The project was finally awarded the gold medal of steel structure.
Through this project, our low-temperature welding construction technology is mainly summarized into the following aspects:
According to the characteristics of the engineering structure, the welding sequence is arranged reasonably to reduce the welding residual stress. In the welding process, the steel itself should achieve positive temperature (that is, to use a variety of different preheating methods to improve the small ambient temperature around the weld), select the appropriate preheating method; Take emergency heat preservation and slow cooling measures after welding to reduce welding cracks caused by sudden drop in temperature.