Knowing the heat-affected zone can improve the welding quality, and the welding seam quality standards are explained in detail
Welding heat affected zone
The welded joint is a welding consisting of three parts: the weld, the fusion zone and the heat-affected zone.
Welding heat-affected zone: referred to as HAZ (Heat Affect Zone), under the action of welding heat cycle, the area where the solid base metal on both sides of the weld undergoes significant changes in structure and performance is called the welding heat-affected zone.
Microstructure distribution of hardened steel
Features: Martensite is not easy to form under welding air cooling conditions. Such as low carbon steel, 16Mn, 15MnV and 15MnTi, etc.
According to heating temperature and organizational characteristics, it can be divided into four areas: overheating zone, normalizing zone, partial normalizing zone and recrystallization zone. as the picture shows.
Overheating zone (coarse crystal zone)
The temperature is between the solidus to 1100°C, and the width is about 1 to 3 mm. During welding, the austenite grains in this area grow up severely. After cooling, a superheated structure with coarse grains is obtained, and the plasticity and toughness are obviously reduced.
Phase change recrystallization zone
Phase change recrystallization area (normalizing area or fine-grained area)
The temperature is between 1100℃～Ac3, and the width is about 1.2～4.0mm. Air cooling after welding makes the metal in this area equivalent to normalizing treatment, so its structure is uniform and fine ferrite and pearlite, and its mechanical properties are better than the base material.
Incomplete recrystallization zone
Incomplete recrystallization zone (also called partial normalizing zone)
The heating temperature is between Ac3 and Ac1. When welding, only part of the structure is transformed into austenite; after cooling, fine ferrite and pearlite are obtained, and the rest is still the original structure, so the grain size is uneven and the mechanical properties are also poor.
If the base material undergoes cold working deformation before welding, the temperature is between Ac1～450℃, and there is a recrystallization zone. The mechanical properties of the metal in this area have not changed much, but the plasticity has increased. If there is no cold plastic deformation before welding, there is no recrystallization zone in the heat-affected zone.
Weld quality standards and the first, second and third level classification of welds
1. Guaranteed items
1. Welding materials should meet the design requirements and relevant standards, and the quality certificate and baking records should be checked.
2. The welder must pass the examination and check the qualification certificate and assessment date of the welder's corresponding welding conditions.
3. Welds of class I and II must be inspected by flaw detection, and should meet the design requirements, construction and acceptance specifications, and check the weld flaw detection report.
4. The surface of the welded seam should not have cracks, weld bead, burn through, arc crater and other defects. Class Ⅱ welds must not have defects such as surface pores, slag inclusions, arc craters, cracks, arc scratches, etc., and class Ⅰ welds must not have defects such as undercuts and insufficient welds.
2. Basic items
1. Appearance of weld seam: uniform weld seam appearance, smooth transition between weld bead and weld bead, weld bead and base metal, and clean welding slag and spatter.
2. Surface pores: Class Ⅰ and Ⅱ welds are not allowed; for class Ⅲ welds, the allowable diameter of each 50mm length of the weld is ≤0.4t; and there are 2 pores ≤3mm; the spacing of pores is ≤6 times the diameter.
3. Undercut: Class I welds are not allowed.
Class Ⅱ weld: depth of undercut ≤0.05t, and ≤0.5mm, continuous length ≤100mm, and total length of undercut on both sides ≤10% of weld length.
Grade III weld: undercut depth ≤0.lt, and ≤lmm.
Note: t is the thinner plate thickness at the joint.
4. The allowable deviation items are shown in Table 5-1.
3. Finished product protection
1. Don't hit the joint after welding, and don't water the steel that has just been welded. Slow cooling measures should be taken at low temperatures.
2. It is not allowed to ignite the arc on the base metal outside the weld at will.
3. Welding can only be performed after the various components are calibrated, and the shim and fixtures should not be moved at will to prevent the size deviation of the components. The welds of concealed parts must go through the concealed acceptance procedures before proceeding to the next concealed process.
4. It is not allowed to clean the slag immediately after low-temperature welding, and it should be done after the welding seam cools down.
Fourth, the quality issues that should be paid attention to
1. The size exceeds the allowable deviation: For the deviation of the weld length, width, thickness, center line offset, bending, etc., the relative position and size of the welding part should be strictly controll