Common problems in welding of elbows of air-conditioning copper pipe

2022-09-08 15:59

(1) Poor forming ability of refractory welds

The thermal conductivity of copper elbows is more than 7 times greater than that of iron at 20°C, and more than 11 times greater at 1000°C. During welding, the heat is quickly transmitted to the heating zone, which expands the heating range. The greater the thickness of the weldment, the more serious the heat dissipation. It is difficult for the welding zone to reach the melting temperature, so the base material and the filler metal are difficult to fuse. At the melting temperature of copper, its surface tension is 1/3 smaller than that of iron, and its fluidity is l~l.5 times larger than that of steel. Therefore, the surface forming ability is poor.

(2) Large welding stress and deformation

The expansion coefficient of copper is 15% larger than that of iron, and the shrinkage rate is more than 1 time larger than that of iron. Because of the strong thermal conductivity of the air-conditioning copper pipe elbow, the amount of deformation is large when it is cooled and solidified. When welding a rigid weldment or welding deformation is hindered, a large welding stress will be generated, which becomes the mechanical cause of welding cracks.

(3) Easy to produce hot cracks

Red copper elbows may produce hot cracks on the weld and heat-affected zone. The main reason is that copper is easily oxidized in the liquid state to form cuprous oxide. It dissolves in liquid copper but not solid copper. It forms a slight melting point with copper during the condensation process. Cu2O+Cu eutectic (melting point 1064℃) lower than copper. If there are impurities such as bismuth (Bi) and lead (Pb) in copper, low-melting eutectic Cu-Bi (melting point 270°C) and Cu+Pb (melting point 326°C) will also be generated during the crystallization process of the molten pool. The material is distributed between the dendrites or grain boundaries of the weld metal. When the weld is at a high temperature, the low-melting eutectic in the heat-affected zone re-melts, and under the action of welding stress, thermal cracks will occur on the weld or the heat-affected zone.

(4) Easy to produce stomata

When red copper elbows and copper alloys are welded, the pores produced by the weld are much more serious than when welding steel. This is related to the metallurgical and physical properties of copper and copper alloys. From the aspect of metallurgical properties, there are soluble gases and gases produced by oxidation-reduction reactions in copper during welding. The solubility of hydrogen in copper is related to temperature and increases or decreases with the rise and fall of temperature. When copper is in the liquid-solid transition, there is a sudden change, as shown in Figure 7-8-1. It shows that a large amount of diffusible hydrogen is precipitated during the condensation process; Cu2O in the molten pool is precipitated because it is insoluble in copper during solidification, and then reacts with hydrogen or CO to generate water vapor or CO2 gas, which escapes due to insoluble in copper.

In terms of physical properties, the thermal conductivity of copper is more than 7 times greater than that of iron, and the crystallization rate of weld metal is very high. Under these conditions, the diffusion and escape of hydrogen and the rise of water and carbon dioxide are extremely difficult, and it is often too late to escape. Floating up creates pores.

(5) Joint performance

Since the elbow of the air-conditioning red copper pipe and copper alloy generally do not undergo phase transformation, the grains of the weld and heat-affected zone are easy to grow; various brittle low-melting eutectic crystals appear at the grain boundary, which results in significant joint plasticity and toughness Decrease, and the corrosion resistance of copper alloys depends on the addition of gold-containing elements such as aluminum, zinc, manganese, nickel, etc. These gold-containing elements evaporate and burn during the welding process, all of which reduce the corrosion resistance of the joint to varying degrees .

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