Implementing multi-layer and multi pass welding thermal cycle adjustment

1 Principle

1.1 Welding thermal cycle adjustment

When implementing multi-layer and multi pass welding, due to the unrestricted cross-section of the weld seam, it is possible to adjust the heat input and welding speed within a large range, thus requiring significant adjustments to the welding thermal cycle.

For non brittle steel, the high temperature residence time in the overheated zone can be reduced to prevent grain coarsening and reduce the Weinstein structure. In addition, after multi-layer and multi pass welding, the weld bead reheats the previous weld bead and the heat affected zone. In the reheating zone above AC3, a normalizing like structural transformation occurs, forming fine equiaxed grains and refining the grains. The columnar crystals in the weld seam disappear within this temperature range, and the plasticity and toughness are improved.

For brittle fire steel, in the tempering temperature heating zone, the hardening structure is softened, and the plasticity and toughness are improved. At the same time, by adjusting the heat input and cooling time of multi-layer and multi pass welding, the welding sequence can be changed to achieve control of structural deformation and stress.

1.2 Adjustment of weld crystallization

Welding seam crystallization usually starts from unmelted grains on the fusion line, grows in the opposite direction to heat dissipation, and forms columnar crystals. Therefore, near the centerline of the welding seam, it often crystallizes later. Due to the segregation of the chemical composition of the welding seam, the metal with high solidification temperature crystallizes first, and the metal with low solidification temperature crystallizes later. There are eutectic impurities with low melting points at the crystallization point, causing macroscopic segregation, resulting in hot cracks, inclusions, and pores. Multi layer and multi pass welding, due to dividing a single pass into multiple passes, causes segregation and dispersion of impurity elements in the weld during crystallization, which will not concentrate on the centerline of the weld, thus avoiding the occurrence of thermal cracks on the centerline of the weld.

1.3 Improve production efficiency and control integration ratio

In the welding of heterogeneous steel, multi-layer and multi pass welding adjusts the weld seam width

For the side of the base material with poor weldability, low welding energy should be used and the transition layer should not be swung; On the side with good weldability, using a larger wire energy can not only control the fusion ratio but also improve production efficiency.

2 Classification and Application

In actual production, multi-layer and multi pass welding can be divided into:

(1) Long section, multi-layer, and multi pass welding. The so-called long section, multi-layer, and multi pass welding refers to each weld seam being longer and above 1rn in length. Therefore, before welding the next weld seam, the previous weld seam has been cooled to a lower temperature, and the welding sequence can be adjusted according to the structural heating situation to effectively control welding deformation.

(2) Short section multi-layer multi pass welding. The so-called short section multi-layer multi pass welding refers to segmented and layer by layer welding, with each section having a shorter weld bead length of only 50-500 mm. When using short section multi-layer multi pass welding, a narrow thermal cycle curve can be obtained, which shortens the residence time above the austenite transformation temperature AC3 and avoids austenite grain coarsening. At the same time, due to the use of short section multi-layer welding, when the weld seam cools to the martensitic transformation temperature, the second layer is immediately welded. This way, the first layer of weld seam and the heat affected zone metal are affected by the heat of the second layer of weld seam welding, causing the temperature to rise and slowing down the cooling rate, effectively avoiding the occurrence of hard structures.

Short section, multi pass, and multi-layer welding can solve the contradiction between high temperature residence time and difficulty in reducing cooling speed simultaneously. This thermal cycle regulation is very suitable for steel grades that are prone to grain growth and overheating after welding. The application of short section multi-layer welding is mainly to reasonably determine the length of each weld seam. If the weld bead is too long, the previous weld seam has cooled to below the martensitic transformation temperature and produced martensitic structure, which may cause cracks; If the weld bead is too short, it will be due to the weld seam and heat affected zone staying at high temperature for too long, causing grain coarsening.

The characteristics of multi-layer and multi pass welding

Multi layer and multi pass welding can improve the quality of weld metal, especially plasticity, because the back layer (pass) weld has a heat treatment effect on the front layer (pass) weld, which is equivalent to a normalization treatment on the front layer (pass) weld, thus improving the secondary structure. For the last weld seam, an annealing weld seam can be applied on top of it. Some factories, when the bending test of the welded joint fails, take measures to change the original welding process parameters, changing single-layer welds to multi-layer welds, and using small currents for rapid welding, which has a certain effect on improving the qualification rate (plasticity index) of the bending test.

It should be pointed out that multi-layer and multi pass welding has a good effect on improving the quality of manual arc welding. During submerged arc welding, the heat treatment effect is poor due to the fact that the thickness of each layer of weld can reach 6-10mm, but the heat effect of the next layer of weld only reaches 3-10mm

4 Operation precautions

(1) The weld surface should have a smooth transition to prevent stress concentration;

(2) Each layer of welding seam should avoid creating angles, and except for the first layer of welding seam, the welding seam should be hammered to eliminate stress. Each weld seam should be cleaned of slag to prevent slag inclusion;

(3) Do not use beveled corner welds. The first layer should use a higher current to ensure root penetration, while the cover layer should use a lower current to ensure beautiful forming;

(4) During multi pass welding, the welding gun does not need to swing. Swinging during multi pass welding can easily result in interlayer non fusion defects, and the angle of the welding gun is also important for multi-layer and multi pass welding. Incorrect welding gun angle can also easily cause interlayer non fusion defects.

6 Conclusion

Practice has proven that using multi-layer and multi pass welding and selecting reasonable welding process measures can not only control the welding deformation and stress of the structure, but also ensure the microstructure and performance of the weld, effectively improving product quality.