1. Technical and economic analysis and comparison of new and old steel connection methods
The connection methods of steel bars in cast-in-place reinforced concrete are mainly: lashing method, tower welding method for manual arc welding, tie strip welding method, electroslag welding in the 1960s and gas pressure welding developed in recent years.
These connection methods have their own characteristics, but compared with the technical and economic indicators, the narrow gap welding technology of steel bars is superior to the original various steel connection methods in various indicators such as technology and economy. Take the horizontal steel bar connection method as an example: although the lap joint method is simple in construction, does not require energy and mechanical equipment, and is not affected by the climatic environment, but wastes a lot of steel materials, and the stress performance is not good; the manual arc welding Although the welding method and the tie strip welding method are superior to the lashing and lap joint in technical performance and production cost, the construction is troublesome and the technical requirements are high. The workers need to master the welding techniques of various angles such as flat, vertical, horizontal and vertical. The power consumption is large, the work efficiency is low, and a certain amount of steel bars are also wasted; the electroslag welding can only be used for the welding of vertical steel bars so far, and the welding power supply voltage has strict requirements, otherwise the welding quality is affected or the arc cannot be started. welding. Although gas pressure welding saves steel and reduces costs, its welding process requirements are very strict. Under the condition of poor hydropower construction conditions, this technology is difficult for workers to accept and limits its scope of use, and both methods need to be more expensive. Special equipment and fixtures; sleeve connection method, first, the construction cost is high, and second, it is difficult to find the spot supply in the domestic market, which brings inconvenience to the construction preparation work.
The narrow gap welding technology of steel bars has the following advantages to overcome the above shortcomings.
(1) The joint of the narrow gap welding technology of the steel bar is the centered force butt joint, which not only improves the stress condition of the steel bar, but also ensures the design position of the steel bar, which provides convenient conditions for concrete watering.
The use of narrow gap welded steel joints, reliable quality, short-trained welders, in accordance with the regulations, the mechanical properties of the joints can meet the design and hydraulic specifications, the pass rate can reach 100%.
(2) Using narrow gap welding technology, no need to add equipment, ordinary ordinary AC and DC welding machines can be used, with a certain number of molds (each mold costs 300 yuan) can be welded. For example, for a horizontal steel bar with a welding diameter of 28 mm or more, the cost of the narrow gap welded joint is 1/11 of the cost of the welded joint head, 1/5 of the cost of the lap joint, and the work efficiency can be increased by about 5 times.
(3) The narrow gap welding technology of steel bars has a wide range of application. Horizontal, vertical and oblique welding can be carried out on I II III steel bars with diameters of 18mm to 40mm. Since the basic technical action and welding method of the narrow gap welding of the steel bars are the basic actions and the transportation method of the manual arc welding technology, the welder training work is much easier, and the initial work of the manual arc welding work permit has been held. For the intermediate welder, it is actually just a process of adapting to the new method. Generally, after 7 to 15 days of special training, the horizontal, vertical and oblique steel joints can be welded.
2. Application of narrow gap welding technology of steel bars in Daxia Hydropower Project
2.1 Project Overview
Daxia Hydropower Station is located on the main stream of the Yellow River at the junction of Baiyin City and Yuzhong County of Gansu Province. The total installed capacity of the power station is 300,000 kw, which is an important construction project in Gansu during the â€œEighth Five-Year Planâ€ period. The power station is designed as a low-head riverbed power station. The hub building is a reinforced concrete structure with a total concrete content of more than 600,000 square meters. The designed steel bar is about 15,000 tons, and the various types of steel joints are about 600,000.
2.2 Technical training
At the beginning of 1994, we cooperated with the Materials Welding Department of Southwest Jiaotong University, citing the patents of narrow gap welding technology and welding molds, and held the first training courses at the site of the construction site of the Daxia Power Station. The 13 junior, middle and senior welders were selected for the period. Intensive training for 15d, all qualified after rigorous examinations, and among the first batch of students, the four best candidates were selected as the on-site quality inspectors for steel narrow gap welding.
In May 1994, more than 20 experts, professors, engineers and technicians of the Daxia Power Station project construction unit, design unit, Southwest Jiaotong University, supervision engineer, construction and other units, the principle, process flow, mechanical performance indicators of the technology, and The test data and process records were equally seriously reviewed and evaluated, and the technology was approved for use in the Daxia Hydropower Project.
At present, the horizontal narrow gap welding technology has been fully adopted in the Daxia power station project. Vertical narrow gap welding is used in some engineering parts. Unqualified joints have not been found in more than 30 batches of samples, see attached table.
Partial sampling test results of the site of the Daxia site
Engineering site reinforcement grade steel diameter / mm yield strength / MPa resistance strength / Mpa
Chamber section 7 II 36383 526
Unit 4 3 II 32 356 520
Left bottom hole 3-9 II 36 381500
Lock chamber section 10 blocks II 32 366 522
Hole body section 12 pieces II 36 371 501
Unit 4 II-1 Block II 36 361 501
GB: 1499-91 II 8ï½ž25 28ï½ž40 355 355510 490
Note: (1) The test strength is only required to be above GB1499-91;
(2) The yield and tensile strength in the table are the average of a set of test pieces at that location.
2.3 Field narrow gap welding of horizontal steel bars
With the narrow gap welding technology of steel bars, the increased length of steel bars should be removed during the processing of steel bars. When the steel bar is installed, it can be constructed according to the conventional process. The joints of the two steel bars are placed in a special mold, a certain gap is left in the middle, the steel bars are laid at the side, the mold is installed, and a few upper steel bars are temporarily vacated at the joint portion. After the joint is welded, the part of the steel bar is tied.
In this way, a narrow gap weld is made to the horizontal bars in all positions. If you have the conditions, you can also weld the steel joints before installing them, and you can get twice the result with half the effort.
2.4 Field narrow gap welding of vertical steel bars
For on-site installation and welding of upright steel bars, the stability problem of the upper section of the steel bars must first be solved. The traditional method is to first bind and fix the steel bars, or to weld the upper steel bars and the lower steel bars together, and then tie the welding and insert them.
Due to the narrow gap welding technology process, a certain gap must be left in the middle of the steel joint, which requires a slight change to the traditional steel installation method to adapt to the new process requirements. The specific method is: firstly, in the working part where the steel bar is to be installed, spot welding an upright steel bar every 2 to 3 m by conventional methods, and tying 2 to 3 horizontal steel bars at an appropriate height to form a skeleton, and then installing a narrow gap welding joint The steel bar is installed with a small wooden block prepared in advance in the middle of the joint of the steel bar (the thickness of the wooden block is the gap width required for the narrow gap weld). The upper part and the horizontal steel bar are firmly tied. Next, the welding mold is installed, and the wooden block is removed during welding to be welded. After all the welding is completed, the remaining unreinforced horizontal steel bars are installed. Construction by this method does not increase the workload, and has no significant effect on the ergonomics. Since the problem of stability of the upper section of the narrow gap welding is solved, the narrow gap welding of the vertical steel bars is used, and the comprehensive work efficiency is improved, and the diameter of the steel bars is larger. The overall benefits of construction are higher.
2.5 Construction work
The narrow gap welding technology of steel bars has the advantages of high speed and high efficiency. It can improve the work efficiency by the construction method. Because the process requires a joint to be welded, it takes several minutes to remove the mold to improve the joint quality and protect the mold. Each welder is equipped with 3 to 5 molds.
When installing the mold, it is often necessary to adjust the gap of the steel joint. A welder's two auxiliary workers are better labor combinations, so that the joint gap can be adjusted, the mold, the welding and the mold can be interspersed with each other, and the flow operation can maximize the production. effectiveness.
2.6 Quality inspection and remedial measures
The quality inspection is carried out in two levels. The first inspection is self-inspection and the first inspection is final inspection. The contents of the self-inspection include: preparation before welding; baking of welding rod; inspection of the welding certificate of the welding personnel, execution of the welding procedure; visual inspection of the welded joint one by one. The self-inspector must be a narrow gap welder with excellent training skills and strong sense of responsibility.
The final inspection is carried out by the professional quality inspection department. The method is to visually inspect the welding heads on the site one by one. The samples are randomly selected at the site, and the mechanical properties are tested by the laboratory. Three specimens are generally taken for tensile testing. Three test pieces can also be taken for cold bending test as needed.
If it is difficult to extract the test piece from the installed steel mesh, the simulated test piece can be made, and the position of the test piece and the welding procedure specification must be consistent with the on-site construction conditions. The inspection work is carried out in accordance with the "Hydraulic Concrete Construction Code (SDJ207-82).
In order to ensure the quality of the project, it is considered that there are substandard joints in the inspection, such as: undercut, not welded, not welded, etc., should be repaired or reinforced by tie strip welding.
3. Analysis of technical economy and social benefits
3.1 Technical Benefit Analysis
The narrow gap welding technology of steel bars solves the problem of eccentric stress of steel bars caused by the connection technology of steel bars. In the construction of reinforced beams, slabs and columns, the design position of the steel bars is often not guaranteed due to the splicing and lap joints of the steel joints. The ties of the steel joints also increase the local density of the steel bars, which brings the concrete pouring. difficult. The narrow gap welding technology not only ensures the design position of the steel bars, but also provides good conditions for the concrete pouring.
The narrow gap welded joint is forcibly formed in the copper mold, the welding quality is suitable to ensure, and the operation difficulty is also reduced. During the welding process, the welding heat absorbed by the copper mold prevents the joint from overheating and produces brittle structure. After the joint is welded, the joint is tempered to prevent the formation of cold and brittle structure and improve the welding quality of the joint.
3.2 Calculation of economic benefits
According to the market price of various materials in 1994, the calculation includes: steel bar 3,500 yuan / t, electrode rod 7 yuan / kg, electricity fee 0.26 yuan / kW? h, labor costs 15 yuan / day, welding a 36mm diameter tie strip The actual cost of the welded joint and the narrow gap welded joint are as follows:
(1) Band welding head cost
Consumable steel (tie): 5.75kg, price 20.16 yuan consumption electrode: 1kg, price 7 yuan consumption of electricity: 9.3kW?h, price 2.4 yuan labor costs: 0.1 working days, price 0.94 yuan total: 30.39 yuan
(2) Narrow gap welding head cost
Consumable steel: 0
Consumption of welding rod: 0.26kg, price 1.84 yuan consumption of electricity: 2.33kW?h, price 0.61 yuan labor costs: 7mim, price 0.22 yuan total: 2.67 yuan
From the rough calculation of the cost combination, the cost of the narrow gap welded joint is only 1/11 of the cost of the strip welding.
For the joints of smaller diameter steel bars, if the narrow gap welding technology is used, the saving value is also considerable: after calculation, welding a 25mm diameter lap joint welding head, the total cost is 5.33 yuan, and the narrow The gap welding technology costs only 1.08 yuan, even if it is economical to consider narrow gap welding to assist workers.
3.3 Social Benefit Assessment
The appearance of the narrow gap welding technology of steel bars has greatly improved the work efficiency, reduced the engineering cost, improved and improved the engineering quality, and reduced the labor intensity of workers. Since the welding process is carried out in the mold, the damage of the welding arc to other workers in the working part and the occurrence of "electro-optic eye" are greatly reduced, and the attendance rate is improved.
In summary, the narrow gap welding technology of steel bars is a new technology that enables enterprises to obtain economic benefits, promote engineering construction, and has good social benefits, has broad application prospects, and has promotion value.
Graphite is an elemental element of elemental carbon. Each carbon atom is surrounded by three other carbon atoms (arranged in a honeycomb-like hexagon) to covalently bond to form a covalent molecule.
Since each carbon atom emits an electron, the electrons are free to move, so the graphite belongs to the conductor. Graphite is one of the softest minerals, and its use includes the manufacture of pencils and lubricants. Carbon is a non-metallic element that is located in the second periodic IVA family of the periodic table.
The specific use of graphite
1, for refractory materials, including refractory bricks, crucible, continuous casting powder, casting core, mold detergent and high temperature materials.
Magnesium carbon brick refractory material is the mid-60s, developed by the United States; in the world of magnesium carbon brick has been widely used in steel, and has become a traditional use of graphite. Aluminum carbon brick aluminum carbon refractories are mainly used for continuous casting, flat billet from the pipeline of the protective cover, underwater nozzle and oil well blasting tube.
Made of graphite and refractory crucible and its related products, such as crucible, curved neck bottles, plugs and nozzles, etc., with high fire resistance, low thermal expansion, melting metal process, by metal infiltration and erosion But also stable, good thermal shock stability at high temperatures and excellent thermal conductivity, so the artifacts and its related products are widely used in direct melting metal process.
Graphite and other impurity materials can be used as carbonizers in the steelmaking industry. Carburizing uses a wide range of carbonaceous materials, including artificial graphite, petroleum coke, metallurgical coke and Natural Graphite. In the world of steel-making carbon-based graphite is still one of the main uses of earth-like graphite.
3, for the conductive material
Graphite in the electrical industry is widely used as electrodes, brushes, carbon rods, carbon tubes, mercury rectifier cathode, graphite gaskets, telephone parts, TV picture tube coating and so on. Among them, Graphite Electrode is the most widely used, in the smelting of various alloy steel, ferroalloy, the extensive use of graphite electrodes. The graphite used in the electrical industry is very demanding on grain size and grade.
4, for wear and lubrication materials
Graphite is often used as a lubricant in the machinery industry. Lubricants are often not used in high-speed, high-temperature, high-pressure conditions, and graphite wear-resistant materials can be 200 â„ƒ a 2000 â„ƒ temperature and at a high sliding speed (loom / s) without lubricating oil work. Many of the equipment that transport corrosive media, widely used graphite materials made of piston rings, seals and bearings, they run, do not need to add lubricants, graphite milk is also a lot of metal processing (wire drawing, pull tube) when the good lubricant.
5, for corrosion-resistant materials
Graphite has good chemical stability. After special processing of graphite, with corrosion resistance, good thermal conductivity, low permeability and other characteristics, and widely used in the production of heat exchangers, reaction tank, condensate, combustion tower, absorption tower, cooler, heater, filter Pumps and other equipment. These equipment for the petrochemical, hydrometallurgical, acid and alkali production, synthetic fiber, paper and other industrial sectors, can save a lot of metal materials.
6, for the atomic energy industry and the defense industry
Graphite has a good neutron deceleration performance, the earliest as a slowdown agent for atomic reactors, "uranium-graphite" reactor is currently used more of an atomic reactor. As the power of the atomic energy reactor in the slowdown material should have a high melting point, stable, corrosion-resistant properties, graphite can meet the above requirements.
7, for anti-scaling anti-rust materials.
Graphite can prevent boiler fouling. Tests have shown that adding a certain amount of Graphite Powder to the water prevents the surface of the boiler from scaling. In addition, graphite coated with metal chimneys, roofs, bridges, pipes can be anti-corrosion and rust.
8, graphite new use
1) Flexible graphite products. Flexible graphite, also known as Expanded Graphite, is the 1970s developed a new graphite products. 1971 US research success of flexible graphite sealing material, to solve the problem of atomic valve leakage, followed by Germany, Japan, France also began to develop production. In addition to this product has the characteristics of natural graphite, but also has a special flexibility and flexibility. Therefore, it is an ideal sealing material, widely used in petrochemical, atomic and other industrial fields, the international market demand growth year after year.
2) the production of semi-metallic friction materials. Since the 1970s, clutches and automatic liners have been widely used semi-metallic friction materials. Semi-metallic friction material is the graphite and metal powder, steel fiber, clay powder made of synthetic resin. These automatic lining can be used for high-speed equipment such as aircraft, trucks and off-road vehicles, and clutch pads.
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