In the manufacturing process of wind power tower, welding is a very important process. The quality of welding directly affects the production quality of the tower. Therefore, it is necessary to understand the causes of weld defects and various prevention measures.
1. Air hole and slag inclusion
Porosity: Porosity refers to the cavity formed when gas in the molten pool does not escape before metal solidification and remains in the weld. Its gas may be absorbed by the molten pool from the outside, or it may be generated by reaction in the welding metallurgy process.
(1) The main reasons for air holes: there are rust, oil stain, etc. on the surface of base metal or filler metal, and the amount of air holes will increase if the welding rod and flux are not dried, because the rust, oil stain, and the moisture in the coating and flux of the welding rod decompose into gas at high temperature, increasing the content of gas in the high-temperature metal. The welding line energy is too small, and the cooling speed of the molten pool is large, which is not conducive to the escape of gas. Insufficient deoxidation of weld metal will also increase oxygen porosity.
(2) Harm of blowholes: blowholes reduce the effective sectional area of the weld and loosen the weld, thus reducing the strength and plasticity of the joint and causing leakage. Porosity is also a factor that causes stress concentration. Hydrogen porosity may also contribute to cold cracking.
Prevention measures:
a. Remove the oil stain, rust, water and sundries from the welding wire, working groove and its adjacent surfaces.
b. Alkaline welding rods and fluxes shall be used and thoroughly dried.
c. DC reverse connection and short arc welding shall be adopted.
D.Preheat before welding to slow down the cooling speed.
E. Welding shall be carried out with relatively strong specifications.
Crackle
Measures to prevent crystal cracks:
a. Reduce the content of harmful elements such as sulfur and phosphorus, and weld with materials with low carbon content.
b. Certain alloy elements are added to reduce columnar crystals and segregation. For example, aluminum and iron can refine grains.
c. The weld with shallow penetration shall be used to improve the heat dissipation condition so that the low melting point material floats on the weld surface and does not exist in the weld.
d. Welding specifications shall be reasonably selected, and preheating and afterheating shall be adopted to reduce the cooling rate.
e. Adopt reasonable assembly sequence to reduce welding stress.
Measures to prevent reheat cracks:
a. Pay attention to the strengthening effect of metallurgical elements and their influence on reheat cracks.
b. Reasonably preheat or use afterheat to control the cooling rate.
c. Reduce residual stress to avoid stress concentration.
d. During tempering, avoid the sensitive temperature zone of reheat cracks or shorten the residence time in this temperature zone.
Measures to prevent cold cracks:
a. Low hydrogen type alkaline welding rod shall be used, dried strictly, stored at 100-150 ℃, and used when taking.
b. The preheating temperature shall be increased, the post heating measures shall be taken, and the interpass temperature shall not be less than the preheating temperature. The reasonable welding specification shall be selected to avoid brittle and hard structures in the weld.
c. Select reasonable welding sequence to reduce welding deformation and welding stress.
d. Conduct hydrogen elimination heat treatment in time after welding
Post time: Nov-08-2022