Analysis of welding technology of titanium pipe
Table of Contents
- (1) The melting point of titanium rod is 1668 °C, with low density and low thermal conductivity. During welding, it is easy to produce coarse grains in the weld area, form overheated structure and reduce the plasticity; The welding cooling is fast and easy to produce unstable brittle titanium martensite, which also reduces the plasticity of the welded joint. In the welding construction, the linear energy and cooling rate should be strictly controlled.
- (2) Titanium has small elastic modulus and large welding deformation; Cold deformation has large resilience and difficult forming. Take measures to prevent deformation.
- (3) Titanium has stable chemical properties and strong corrosion resistance at room temperature. When it reaches the molten state, titanium can react violently with the surrounding gaseous elements hydrogen, oxygen, nitrogen and carbon to form compounds, which seriously affects the mechanical properties and corrosion resistance of welded joints. Tungsten argon arc welding must be adopted and fully argon protected.
- (4) The existence of iron will establish spontaneous batteries in the iron rich phase zone and the heat affected phase zone, resulting in galvanic corrosion, especially in the weld and heat affected zone. At the same time, iron pollution will accelerate hydrogen absorption and form hydrogen embrittlement cracks. Therefore, iron pollution should be avoided in welding.
- (5) Titanium cannot be welded with other metals, and it is easy to form brittle intermetallic compounds, resulting in weld embrittlement.
Main technical measures
Pollution prevention measures
- (1) Prevent impurity pollution, especially when plasma cutting pipe and groove processing are used, the surface pollution layer shall be removed in time.
- (2) To prevent iron pollution, try to avoid using iron, carbon steel and low alloy steel tools to process titanium pipes, and it is strictly prohibited to use carbon steel wire brush. When mechanical cutting and beveling of pipes are adopted, the iron contaminated layer shall be removed with a grinder. Paint shall be used for marking on titanium pipe.
- When mechanically cutting pipes and beveling, the temperature of the cut surface shall not exceed 300 °C, and water spray can be used for cooling.
- (3) Storage protection measures. The titanium pipes to be processed and processed shall be covered with plastic cloth to prevent the pollution of dust, oil and sundries. Titanium pipes shall be stored by category and shall not be mixed with carbon steel materials.
Take strict cleaning measures
- (1) Clean the surface dirt. All oxide scale, scale, paint film, metal debris, dust, etc. on the groove end face of titanium pipe welded joint, the inner and outer surfaces of pipe wall and the surface of base metal within 40mm from the end face of pipe shall be cleaned with stainless steel wire brush and cotton yarn. Surface burrs and other defects shall be polished with a fine file.
- (2) Degreasing treatment. After the surface dirt is cleaned, alkali wash and degrease with na3h aqueous solution to remove the grease and hydrocarbons on the surface. The NaOH concentration is 20% ~ 30%, the temperature is between 50 ~ 60 °C, and the cleaning time is 5 ~ 10MH. After alkali washing, wipe the alkali washed surface with absorbent cotton.
- (3) Pickling treatment. After degreasing treatment, in order to remove the oxide film on the surface, the surface needs to be pickled. The pickling solution is prepared from hydrochloric acid, nitric acid and sodium fluoride. The formula proportion is: 270m1 hydrochloric acid (concentration 1.19), 55m1 nitric acid (concentration 1-4) and 40g sodium fluoride. After mixing the three, add warm water (40 °C) to 1000ml and pickle until the surface is silvery white. Too long pickling time is easy to cause hydrogen absorption of titanium. The surface after pickling shall be washed immediately with clean water with a temperature of 60 °C to remove the residual acid on the surface, and then wiped dry with a clean white cloth.
- (4) Wash with acetone. Before welding, the weld area surface of titanium pipe shall be cleaned with acetone for the last time, so as to completely remove all kinds of impurities left on the surface.
- (5) The welding wire shall be cleaned according to the same procedure above.
Inert gas protection measures
The purity of argon shall not be less than 99.99%, and the moisture content shall not be greater than 50mgm.
The double-sided protection method is adopted, the protective cover is set outside the pipe, and the sealing diaphragm is used inside the pipe; For fixed welded junction, water-soluble paper shall be used for sealing in the pipe.
- (1) When titanium pipe is welded, the area above 400 °C is 80mmx60mm. The area of the protective cover depends on this. The width, length and shape of the protective cover depend on the pipe diameter. The protective cover is composed of shell, air inlet pipe and porous sieve plate and is made of austenitic stainless steel. Argon flows into the protection area through the intake pipe and porous sieve plate to protect the welding quality from the outside of the pipe.
- (2) For the titanium pipe with small diameter, the whole pipe ventilation protection shall be adopted, and the two ends of the pipe shall be blocked with rubber plugs. One end shall be ventilated, and the other end shall be opened with a small hole to ensure a slight positive pressure of argon in the pipe during welding. Since argon is heavier than air, the inlet hole should be located below the outlet.
Source: China Titanium Pipe Manufacturer: www.titaniuminfogroup.com