Welding requirements of titanium equipment such as titanium heat exchanger and titanium coil
The welding performance of titanium coil and heat exchanger has many remarkable characteristics, which are determined by the physical and chemical properties of titanium and titanium alloy.
Influence of gas and impurity pollution on welding performance
At room temperature, titanium and its alloys are relatively stable. However, the experimental results show that the liquid droplet and molten pool metal have strong absorption of hydrogen, oxygen and nitrogen in the welding process, and these gases have already interacted with them in the solid state. With the increase of temperature, the ability of titanium and titanium alloy to absorb hydrogen, oxygen and nitrogen also increases obviously. Hydrogen begins to be absorbed at about 250 ℃, oxygen from 400 ℃ and nitrogen from 600 ℃. These gases will directly cause embrittlement of welded joint, which is a very important factor affecting welding quality.
Crack problem of welded joint
When titanium and titanium alloy are welded, the possibility of hot crack in welded joint is very small, because the impurity content of S, P and C in titanium and titanium alloy is very small, the eutectic with low melting point formed by s and P is not easy to appear on the grain boundary. In addition, the effective crystallization temperature range is narrow, and the shrinkage of titanium and titanium alloy during solidification is small, so the weld metal will not produce hot crack. During welding of titanium and titanium alloy, cold cracks can appear in the heat affected zone, which is characterized by cracks occurring in hours or even longer after welding, so it is also called delayed cracks. The results show that the crack is related to the diffusion of hydrogen during welding. During the welding process, hydrogen diffuses from the high temperature deep pool to the lower temperature heat affected zone. With the increase of hydrogen content, the amount of TiH2 precipitated in the zone increases, which increases the brittleness of HAZ. In addition, the volume expansion of hydrides causes large structural stress, and hydrogen atoms diffuse and accumulate to the high stress parts of the zone, resulting in cracks. The main way to prevent the delayed crack is to reduce the source of hydrogen in welded joint.
Porosity in weld
When welding titanium and titanium alloy, blowhole is often encountered. The basic reason for the formation of pores is due to the effect of hydrogen. The formation of porosity in weld metal mainly affects the fatigue strength of joint. The main technical measures to prevent porosity are as follows:
- 1) The protective gas should be pure and the purity should not be lower than 99.99%
- 2) Thoroughly remove the organic matters such as oxide scale and oil stain on the surface of weldment and welding wire.
- 3) Good gas protection is applied to the molten pool, and the flow rate and flow rate of argon are well controlled to prevent turbulence and affect the protection effect.
- 4) The welding parameters should be selected correctly and the residence time of molten pool should be increased to facilitate the escape of bubbles.
Source: China Titanium Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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