Welding performance
Metal weldability refers to the adaptability of metal materials to welding processing. It mainly refers to the difficulty of obtaining high-quality welded joints under certain welding process conditions (including welding materials, welding methods, welding process parameters and structural forms, etc.) and whether the joints can operate reliably under specified conditions of use. It includes two aspects: one is the joining performance of the welded joint, that is, the ability to obtain high-quality and defect-free welded joints under certain welding process conditions; the other is the use performance, that is, whether the welded joint or the overall component after welding can meet the various use conditions specified by the technical requirements. There are many factors that affect weldability. For steel materials, there are factors such as the selected materials, the design of the structure and its joints, the process methods and specifications, and the environmental conditions of the joints in service.
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Basic structure of heat-affected zone of welded joint
Welded joints generally include weld metal zone, fusion line and heat-affected zone. The heat-affected zone refers to the area where the structure and properties of the metal on both sides of the weld change due to welding heating. The changes in the structure and properties of the heat-affected zone depend not only on the thermal cycle, but also on the composition and original state of the parent material.
Microstructure distribution and properties of heat-affected zone of hardenable steel
Steel that is not easily quenched refers to steel that is not likely to form martensite under natural cooling conditions after welding, such as ordinary low carbon steel. The heat-affected zone of hardenable steel consists of four parts: fusion zone, overheating zone, normalizing zone and incomplete crystallization zone.
(1) Fusion zone. The fusion zone includes the filler metal melting zone and the semi-melting zone (that is, the heating temperature is between the liquidus and the solidus). The semi-melting zone has poor strength and toughness due to large inhomogeneities in chemical composition and structural properties. , should attract attention.
(2) Overheating zone. The heating temperature is generally around 1100°C, and the grains in this area begin to grow rapidly. After cooling, a coarse overheated structure will be obtained, also called a coarse grain area. This area is prone to embrittlement and cracking.
(3) Normalizing zone (phase change recrystallization zone). The heating temperature is in the temperature range above Ac3 to the temperature range where the grains start to grow rapidly. The grains in this area do not grow significantly. After cooling, uniform and fine pearlite and ferrite are obtained, which is equivalent to the normalized heat treatment structure and has good comprehensive performance.
(4) Incomplete recrystallization zone. The heating temperature is between Ac1 and Ac3. The structure in this area is uneven, the grain size is different, and its mechanical properties are uneven.
The above four zones are the basic structural characteristics of the heat-affected zone of low carbon steel and low alloy steel. However, after some base metals are cold-rolled or cold-worked before welding, the metal will undergo a recrystallization process at a heating temperature close to 500℃-Ac1, causing the work hardening effect to disappear, the strength to decrease, and the plasticity and Increased toughness. However, for steel with aging sensitivity, in the temperature range of Ac1-300℃, if the time is slightly longer, strain aging will easily occur, making this area embrittled. Therefore, this area is also called the age embrittlement area, although its metal structure There is no obvious change, but it is notch sensitive and should be paid attention to when welding.
Microstructure distribution and properties of heat-affected zone of easily quenchable steel
Easy-quenching steel refers to steel types that are easily quenched to form hardened structures such as martensite under air cooling conditions after welding, such as quenched and tempered steel and medium carbon steel.
(1) Complete quenching area. The heating temperature is between the solidus line and A, and due to the growth of grains in this area, coarse martensite is obtained. If the cooling rate is different, a mixed structure of martensite and bainite may also appear. The quenched structure is prone to brittleness and cracks.
(2) Incomplete quenching area. The heating temperature is between Ac1-Ac3, which is equivalent to the incomplete recrystallization zone. Depending on the element content or cooling rate of the base material, mixed structures such as bainite, sorbite, and pearlite may also appear.
(3) Tempering area. If the base metal is steel that has been quenched and tempered before welding, there will also be a tempering and softening zone. For example, when the quenching and tempering temperature of the base metal before welding is t1, during the welding process, when the heating temperature exceeds the tempering temperature t1 (and is less than Ac1), over-tempering softening occurs. If it is lower than t1, its organizational properties remain unchanged.
Welding cracks
Welding cracks can be discovered by naked eyes or flaw detection methods. Classification of welding cracks: For example, according to the location where the crack occurs, it can be divided into weld cracks, fusion zone cracks, root cracks, weld toe cracks, arc crater cracks, etc.; according to the mechanism of crack generation, it can be divided into hot cracks, reheat cracks, and cold cracks. , stress corrosion cracking, etc. Welding cracks are the most serious defects in welded joints and are not allowed to exist in structures or equipment components.
Post time: Nov-04-2024
