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MindMap Gallery Chapter 2 Section 1 Cutting and Welding
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Chapter 2 Section 1 Cutting and Welding
Summary of the content of Chapter 2, Section 1, Cutting and Welding of the first-level cost engineer examination training textbook "Construction Engineering Technology and Measurement" (Installation Engineering 2023 Edition).
Edited at 2024-03-15 01:33:40
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Chapter 2 Section 1 Cutting and Welding
- bacteria
This is a mind map about bacteria, and its main contents include: overview, morphology, types, structure, reproduction, distribution, application, and expansion. The summary is comprehensive and meticulous, suitable as review materials.
- Plant asexual reproduction
This is a mind map about plant asexual reproduction, and its main contents include: concept, spore reproduction, vegetative reproduction, tissue culture, and buds. The summary is comprehensive and meticulous, suitable as review materials.
- Reproductive development of animals
This is a mind map about the reproductive development of animals, and its main contents include: insects, frogs, birds, sexual reproduction, and asexual reproduction. The summary is comprehensive and meticulous, suitable as review materials.
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- Outline
Section 1 Cutting and Welding
cutting
(1) Mechanical cutting: shearing machine, hacksaw machine, steel bar shearing machine, grinding wheel cutting machine, electric pipe cutting and threading machine, etc.
(2) Flame cutting
1. Oxygen-acetylene flame cutting (gas cutting): Cast iron, stainless steel, copper and aluminum are not suitable for gas cutting. Suitable for gas cutting include: pure iron, low carbon steel, medium carbon steel, low alloy steel and titanium.
2. Oxygen-propane flame cutting: high ignition temperature, narrow explosion range, high safety; low cost, easy to liquefy and fill, little environmental pollution; no obvious burning collapse, no slag hanging on the lower edge, and good cutting surface roughness. Disadvantages: low flame temperature, long cutting time, high oxygen consumption; the total cutting cost is much lower than that of oxygen-acetylene flame cutting.
3. Oxygen-hydrogen flame cutting: low cost, high safety, environmentally friendly and pollution-free.
4. Oxygen flux cutting: mainly used for cutting thick stainless steel profiles and casting risers. It is a fast and economical way to cut difficult materials.
(3) Arc cutting
1. Plasma arc cutting: It can cut most metal and non-metal materials, and has better cutting effect on non-ferrous metals. Advantages: fast cutting speed, smooth cut surface, small thermal deformation, and almost no heat-affected zone.
2. Carbon arc gas cutting: can process grooves on metal; can be used to process grooves (especially U-shaped grooves); not suitable for cutting stainless steel with corrosion resistance requirements; the equipment is simple, the operation is convenient, flexible and safe; clean When welding defects and cleaning welding roots, the shape and depth of defects can be clearly observed, resulting in high production efficiency.
(4) Laser cutting: small incision width (about 0.1mm), high precision, fast speed, small heat-affected zone, and the workpiece is almost deformed; it can cut a variety of materials (metal, non-metal and composite materials, etc.). Disadvantages: It can only cut medium and small thickness plates and pipes; the equipment cost is high and the one-time investment is large.
(5) Water jet cutting: cold cutting, no thermal effect; one-time processing and cutting is possible; excellent incision quality, smooth surface, no need for secondary processing; safe, environmentally friendly, and efficient. Widely used in ceramics, stone, glass, metal, composite materials, chemical and other industries.
welding
Classification and characteristics of welding
1. Fusion welding (fusion welding)
(6) Electron beam welding—omitted
(1) Gas welding
No power supply required; production efficiency is low; heat-affected zone is large and welding deformation is large; welding quality is poor and automation is difficult to achieve. Mainly used for welding thin steel plates (0.5~3mm thick), copper and copper alloys and repair welding of cast iron. When welding non-ferrous metals, cast iron and stainless steel, welding powder (flux) can be used to eliminate refractory oxide films and impurities.
(2) Arc welding
Manual arc welding (hand arc welding): simple equipment, flexible operation, low investment, wide application range; low production efficiency, poor working conditions, and unstable welding quality.
Submerged arc welding is divided into two methods: automatic submerged arc welding and semi-automatic submerged arc welding. It is mainly used for the production of pressure vessels, pipe sections, box beams, columns and other important steel structures. Suitable for long welds of medium-thick plate structures and girth welds of large-diameter cylinders. Advantages: high degree of mechanization, high production efficiency; high thermal efficiency, large penetration, smaller workpiece groove; high welding speed, welding speed of 8~10mm steel plate can reach 50~80cm/min; good welding quality, middle weld It is not easy to produce defects such as pores and cracks; in windy environments, the protective effect of submerged arc welding is better than other welding methods. Disadvantages: only suitable for horizontal welding seam welding; cannot weld aluminum, titanium and other highly oxidizing metals and their alloys; cannot directly observe the arc and groove position, and is prone to welding deviation; only suitable for long welding seam welding, and cannot weld space Welding seams with limited position; not suitable for thin plates and low current welding.
(3) Gas shielded arc welding (seven electric welding)
Features of tungsten inert gas welding (TIG welding): tungsten electrode does not melt, stable welding, easy to mechanize, good protection effect, high weld quality; suitable for connecting thin plate metals and bottom welding, and can be used for connecting almost all metals, especially Suitable for chemically reactive metals. Disadvantages: low production efficiency, high cost, poor wind resistance, not suitable for field operations; only suitable for welding of thin plates (below 6mm) and ultra-thin plates.
Features of gas metal arc welding (MIG welding): suitable for welding non-ferrous metals, stainless steel, heat-resistant steel, carbon steel, alloy steel, etc.; fast welding speed, high deposition efficiency, high productivity; DC reverse connection possible, cathode atomization can Effectively removes oxide films and improves welding quality; the cost is lower than TIG welding.
Features of CO2 gas shielded welding: high production efficiency, 1 to 4 times that of manual arc welding; low cost, 40% to 50% of that of submerged arc welding and arc welding; high welding quality, and good crack resistance of the weld seam; visible It has good performance, simple operation and can be welded in all positions. Thin sheets can be welded. Disadvantages: poor surface formation, not suitable for welding of non-ferrous metals and stainless steel, poor wind resistance; complex welding equipment and difficult to use AC power for welding.
(4) Plasma arc welding
The power density of non-melting arc welding is more than 100 times higher than that of free arc; the energy is concentrated, the temperature is high, the welding speed is fast, and the productivity is high; the penetration ability is strong, and the keyhole effect can be obtained on most metals; the weld is dense and beautiful; it can Welding ultra-thin plate structure (metal foil welding below 1mm). Disadvantages: complex equipment, large gas consumption, and high usage costs; suitable for indoor welding.
(5) Electroslag welding
Features: The efficiency is 2 to 5 times higher than that of submerged arc welding, and the groove preparation is simple. It is mainly used for large workpieces with a thickness of 30mm or more, and is suitable for heavy machinery manufacturing. Large area repair welding and surfacing welding can be carried out. Disadvantages: Welds and heat-affected zones are prone to form coarse structures and require normalizing treatment after welding; most welding is performed in a vertical position, and flat welding is not possible.
(7) Laser welding: suitable for welding workpieces with strong heat sensitivity; can weld metal and non-metal materials; the energy conversion rate is low, and it is not suitable for materials with low laser beam wavelength absorption. It is mostly used for welding precision devices in the electronic and instrument industries with high dimensional accuracy. It can also weld silicon steel sheets and galvanized steel sheets.
2. Pressure welding (pressure welding) (1) Resistance welding: The three basic types are spot welding, seam welding and butt welding. (2) Electroslag pressure welding: used for connecting steel bars in cast-in-situ reinforced concrete structures and used in the construction of column and wall steel bars in high-rise buildings.
3. Brazing: According to the melting point of the solder, it can be divided into: hard soldering and soft soldering. Features: The stress and deformation caused are small; multi-slit parts can be connected at one time; it is easy to realize the connection of dissimilar metals, metals and non-metals.
Selection of commonly used welding materials and welding equipment
Welding equipment - briefly
Welding rod selection principles
According to mechanical properties: For non-alloy and low-alloy steel, choose welding rods with deposited metal tensile strength equal to or slightly higher than the base metal; for those with large structural rigidity, high joint stress, and easy cracking of the weld, choose welding rods that are one level lower than the strength of the base metal. welding rod. According to chemical composition: If the content of carbon, sulfur, phosphorus, etc. in the base material is relatively high (the weld is easy to crack), choose a low-hydrogen welding rod with good crack resistance.
According to usage performance: low-hydrogen welding rods with higher plasticity and toughness should be used to withstand dynamic loads and impact loads. According to working conditions: Use stainless steel welding rods or other corrosion-resistant welding rods if they are in contact with corrosive media.
According to the welding structure and stress conditions: (ultra) low hydrogen welding rods and high toughness welding rods are selected for thick and large weldments with complex structural shapes and high rigidity. According to the welding conditions: choose acidic electrodes if the force is not large and the welding part is difficult to clean. Where possible, use acidic welding rods.
Welding parameter selection
Welding rod diameter: Generally, choose a welding rod with a larger diameter.
Welding current: For alloy steel electrodes with more alloy elements, the welding current should be reduced accordingly. (High current can easily cause the drug skin to fall off prematurely)
Arc voltage: determined by the arc length; too long arc can easily cause defects such as pores, and the arc length is required to be less than or equal to the diameter of the electrode, that is, short arc welding; acidic electrode - long arc welding.
Power supply type and polarity: DC power supply has stable arc, small spatter and good welding quality. However, AC welding machines are generally preferred because of their simple structure, low cost, and easier maintenance. DC positive connection: the workpiece is connected to the anode, and the welding rod is connected to the cathode. The workpiece is heated a lot and is suitable for welding thick and large workpieces. Acidic welding rods usually use direct current connection. DC reverse connection: The workpiece is less heated and is suitable for welding thin and small workpieces such as thin plates; alkaline electrodes use DC reverse connection.
(3) Welded joints, grooves and combinations
1. The function of welded joints is to connect and transmit force; they are divided into 5 basic types: butt joints, T-shaped (cross) joints, lap joints, corner joints and end joints.
2. Bevel classification of fusion welding joints. Basic types: I-shaped, V-shaped, single-sided V-shaped, U-shaped and J-shaped grooves, etc. Combined type: It is composed of two or more basic types of grooves, such as Y-shaped, VY-shaped, U-shaped with blunt edge, double Y-shaped groove, etc. Special type: slightly.
3. Beveling, assembling and welding of pipes
(1) There are three main types of pipe bevels: I shape: suitable for pipe mouth welding with wall thickness less than 3.5mm. The groove does not need to be processed and can be directly butt welded. V-shaped: suitable for medium and low pressure steel pipe welding, the groove angle is 60°~70°, and the root of the groove has a blunt edge about 2mm thick. U-shaped: suitable for welding high-pressure steel pipes with a wall thickness of 20~60mm. The root of the groove has a blunt edge about 2mm thick.
(2) Bevel processing method: Low-pressure carbon steel pipes with nominal diameter ≤50mm - beveling with portable grinding wheels; Low-pressure carbon steel pipes with nominal diameter ≤50mm - oxygen-acetylene cutting and beveling, grinding with portable grinder. Medium pressure carbon steel pipe, medium and low pressure stainless steel pipe, low alloy steel pipe, high pressure steel pipe - beveling machine or lathe processing. Non-ferrous metal pipes - hand-beveled.
3. Welding process quality inspection (before welding, during welding inspection and post-welding appearance inspection - omitted)
Post welding inspection - non-destructive testing. (1) Surface and near-surface defect inspection: penetrant inspection and magnetic particle inspection; internal defect inspection: radiographic inspection and ultrasonic inspection. (2) Strength test of welded joints of pressure pipes and vessels. (3) Tightness inspection (leak test): air tightness test, ammonia leak test, kerosene leak test, vacuum box test.
Welding heat treatment
Heating methods for post-weld heat treatment—omitted
Commonly used heat treatment methods
Heat treatment before welding—omitted
2. Post-weld heat treatment
annealing Cool slowly after keeping warm
Complete annealing: refines the structure, reduces hardness, improves processing performance, and removes internal stress. Suitable for casting, welding and rolling parts of medium carbon steel and medium carbon alloy steel.
Incomplete annealing: reduces hardness, improves cutting performance, and removes internal stress. Commonly used for annealing tool steel workpieces.
Stress relief annealing: heating to a temperature above the critical point Ac3, keeping warm in the furnace and then cooling slowly; the purpose is to remove residual stress.
Normalizing air cooling
The steel parts are heated to an appropriate temperature above the critical point, maintained for a certain period of time and then cooled in the air to obtain a pearlite matrix structure. Purpose: to eliminate stress, refine the structure, and improve cutting performance. It is a preheating treatment before quenching, or the final heat treatment of some components. The cooling speed is fast and the degree of supercooling is large; the strength, hardness and toughness of normalized workpieces are higher than those of annealed ones, and the production cycle is short and the energy consumption is low, so normalizing treatment should be given priority.
Quenching rapid cooling
After the steel piece is austenitized, it is rapidly cooled at an appropriate cooling rate, causing the workpiece to undergo an unstable structural transformation of martensite in the cross section. The purpose is to improve the strength, hardness and wear resistance of steel parts; it is mostly used in various tools, molds, bearings, parts, etc.
Tempering Air cooling after insulation
Low temperature tempering: stabilizes the structure, improves hardness and wear resistance, and reduces internal stress and brittleness. Used for the tempering treatment of various high carbon steel cutting tools, molds and rolling bearings.
Medium temperature tempering: improves elasticity, toughness and corresponding hardness, generally suitable for medium hardness parts, springs, etc.
High-temperature tempering, that is, quenching and tempering treatment, can obtain higher mechanical properties, such as high strength, elastic limit and higher toughness, which significantly exceeds the normalizing treatment. Mainly used for important structural parts.
3. Selection of heat treatment method
(1) Generally, single high-temperature tempering or normalizing plus high-temperature tempering treatment is used; (2) The gas welding joint is treated with normalizing and high-temperature tempering; (3) Single medium temperature tempering is only suitable for assembly welds of large ordinary low carbon steel containers assembled on construction sites.
5. Non-destructive testing
1. Radiographic flaw detection
X-ray flaw detection: High sensitivity for displaying defects, short irradiation time, and fast speed. The disadvantages are that the equipment is complex and heavy, troublesome to operate, and costly to use.
γ-ray flaw detection: stronger penetrating power, short exposure time, low sensitivity, its flaw detection equipment is light and flexible, and low cost; it is mostly used at construction sites.
2. Ultrasonic flaw detection
Features: Compared with X-ray, flaw detection has high sensitivity, short cycle, low cost, high efficiency and harmless to human body. The disadvantage is that the surface of the test piece is required to be smooth, and the expression of defects is not intuitive and requires professional judgment. Suitable for workpieces with larger thickness.
3. Eddy current flaw detection
Disadvantages: only suitable for conductor surface and near-surface defect detection, not suitable for test pieces with complex shapes; advantages: fast detection speed, no direct contact between the probe and the test piece, no coupling agent required; can measure multiple parameters at the same time, such as the inside and outside of the pipe diameter, wall thickness and eccentricity, etc.
4. Magnetic particle inspection
Advantages: Suitable for surface and near-surface defect detection of magnetic materials; simple flaw detection equipment and easy operation; fast inspection speed and high sensitivity; not limited by the size and shape of the specimen; the minimum detection width is about 1μm; wide and shallow defects are difficult to detect .
5. Penetrant inspection
Advantages: It is not limited by the shape, size, chemical composition, internal structure and defect orientation of the test piece to be inspected, and can simultaneously inspect all defects on the opening and surface; it can be inspected in batches, is fast, easy to operate, has high sensitivity, and the defect display is intuitive and unobtrusive. Complex and expensive equipment is required, and the inspection cost is low.
Disadvantages: It cannot display the depth, internal shape and size of defects, and can only detect surface defects.
It is widely used for surface defect inspection of ferrous and non-ferrous metal parts, ceramics, glass, plastics, etc.; it is not suitable for materials with loose structure and porous structure.