With the continuous development of China's petroleum enterprises, the establishment of many large oil storage, gas storage, etc., due to the special requirements of the transport medium. Need to use a large number of stainless steel pipes with strong corrosion resistance. In order to ensure that the oil and gas pipeline can last, safe operation, the need for oil and gas pipeline welding process, parameter selection and analysis of its welding defects, the following is a simple shallow discussion.
Stainless steel according to the normalized state of the steel tissue state, divided into martensitic stainless steel, ferritic stainless steel, austenitic stainless steel and austenitic a ferritic stainless steel. The following analysis from the austenitic stainless steel pipe performance.
(A) the performance of stainless steel (welding-related physical properties)
- 1. The thermal conductivity of stainless steel is lower than that of carbon steel, especially the thermal conductivity of austenitic stainless steel, about 1/3 of carbon steel.
- 2. High resistivity of stainless steel, especially austenitic stainless steel resistivity, about 5 times that of carbon steel.
- 3. Austenitic stainless steel coefficient of linear expansion is about 50% larger than carbon steel, martensitic stainless steel and ferritic stainless steel coefficient of linear expansion is roughly equal to that of carbon steel.
- 4. The density of austenitic stainless steel is greater than carbon steel, martensitic stainless steel and ferritic stainless steel density is slightly less than that of carbon steel.
- 5. Austenitic stainless steel is not magnetic, martensitic stainless steel and ferritic stainless steel is magnetic.
(B) austenitic stainless steel tube mechanical properties (1Cr18Ni9Ti tube for example)
Stainless steel mechanical properties
(C) austenitic stainless steel pipe welding properties
Austenitic stainless steel plasticity and toughness is very good, with good weldability, welding generally do not need to take special welding process measures.
(D) austenitic stainless steel pipe welding process performance
- 1. When welding begins, do not casually arc on the weld to avoid damage to the surface of the weld, affecting corrosion resistance.
- 2. Due to the high resistance of austenitic stainless steel, the resistance heat generated during welding is also large, so the same diameter of the wire, the welding current value should be about 20% smaller than that of mild steel wire.
- 3. The welding process, the use of small current, fast welding speed. A weld should not be too wide, preferably no more than three times the diameter of the wire.
- 4. Multi-layer multi-channel welding, each weld after a thorough removal of oxidation slag, and control the inter-channel temperature, wait until the previous weld cooled to below 60 ℃ before welding the next.
From the mechanical properties of stainless steel, weldability, welding process and chemical properties compared to the above, only austenitic stainless steel it to chromium, nickel as the main alloying elements. It has a better corrosion resistance; lower strength, while plasticity, toughness is excellent; good welding performance. Therefore, austenitic stainless steel pipe is widely used in oil and gas pipelines.
Austenitic stainless steel pipe has excellent weldability. Generally used fusion welding methods can weld austenitic stainless steel. But from the economic, practical and technical performance considerations, it is best to use tungsten argon arc welding. If electroslag welding it is characterized by long residence time at high temperature, slow welding speed, slow cooling speed, high line energy, so that the joint resistance to intergranular corrosion, and in the fusion line near the susceptible to serious knife corrosion, so rarely applied. C02 gas shielded welding with oxidation, alloying elements burned serious, has not been used to weld austenitic stainless steel. In the actual welding, usually choose tungsten arc welding process, because argon is an inert gas, not chemical reaction with the metal, alloying elements will not be oxidation burn, and argon does not dissolve in the metal. The welding process is basically a simple process of metal melting and crystallization, therefore, the protection effect is good, can obtain a more pure and high-quality weld, especially the quality of the weld inspection X-ray film pass rate is high; arc by the argon gas flow cooling and compression, the heat of the arc is concentrated, and the temperature of the argon arc is very high, so the heat-affected zone is very narrow. Welding deformation and stress is small, suitable for small burn loss of alloying elements, in addition, due to the small heat input, the welding of austenitic stainless steel sensitive to overheating; can all metal materials can be argon arc welding, especially suitable for welding welding austenitic stainless steel pipe priming welding, welding process, good weld formation, weld surface clean and slag-free.
Causes and preventive measures of common defects in welding process
Here to austenitic stainless steel horizontal fixed tube as an example, combined with their own welding teaching and practical operation process common defects in the causes and preventive measures for a shallow analysis:.
Porosity is one of the common welding defects in the welding of austenitic stainless steel pipe. It exists in the weld bottoming layer internal pores or cover layer surface pores, the defect form for a single pore, dense pores, continuous pores, etc.. The harm of porosity is mainly manifested in reducing the bearing pressure of the pipeline, which can lead to increased local corrosion perforation factors.
(B) not welded through
Not welded through is the phenomenon of welding wire and base material is not completely melted through. Here easy to produce stress concentrations, under the action of external forces may lead to cracking. The main causes of formation are as follows.
- 1. Assembly gap is small, the use of small welding current, wire speed is too fast, so that the base metal can not be completely melted, resulting in the root not welded through.
- 2. The tilt angle of the welding gun is too small, incorrect filling method is easy to produce the defect of not welded through.
- 3. There is a wrong edge when assembling the pipe mouth. Irregularity in the orifice roundness leads to a large amount of misalignment in the local position, and the root is not welded through easily in the lower side of the tube during the bottoming layer welding process.
- 4. In the tube in the inner mouth when filled with argon gas, the use of argon gas flow rate is large, easy to produce argon turbulence blowing the root of the ferrofluid movement, the root appears not welded through.
(C) serious oxidation
In the welding process, the welding area and welding pool protection is not appropriate, it will make the weld is oxidized, causing intergranular corrosion and stress corrosion of stainless steel pipe in the weld area during production and operation, thus reducing the overall corrosion resistance of stainless steel pipe.
(D) not fused
Austenitic stainless steel pipe welding between the weld material and the pipe or between the weld layer, not completely fused phenomenon called not fused. Produced by the following reasons.
- 1. In the pipe mouth under the welding, welding torch and welding wire did not reach into the bevel root heating or filling wire and produce defects such as unfused.
- 2. Improper selection of welding parameters fails to control the interlayer temperature produces defects such as unfused.
- 3. Bottom welding channel when the pipe argon gas flow rate selection is large, resulting in turbulent flow blowing ferrofluid movement, resulting in the root of the unfused.
The following measures can be taken to prevent welding unfused.
- 1. In the pipe mouth under the welding, the welding gun and welding wire into the root of the bevel and make full heating, using the internal filling method, adjust the angle of the welding gun for 800-900.
- 2. During the welding process, the welding gun is best not to swing laterally, using a small current, fast welding speed. A weld should not be too wide, preferably not more than 3-5 times the diameter of the wire; multi-layer multi-channel welding, each weld after a thorough removal of slag, and control the inter-channel temperature, wait until the previous weld cooled to 60 ℃ or less before welding the next
- 3. Bottoming weld channel when the pipe argon gas flow rate is adjusted to 4-6L / min, so as not to produce turbulence blowing ferrofluid movement, so that the root appears unfused.
The use of appropriate post-welding treatment
Austenitic stainless steel pipe welding should be pickled, surface passivation treatment, remove the weld and heat-affected zone surface such as indentations, dents, rough spots, stains, to play a role in corrosion resistance.
The welded austenitic stainless steel pipe as a whole in the pickling tank soak 24-45min, take out and rinse with clean water; specifically with nitric acid (density 1.42) 20%, hydrofluoric acid 5%, the rest is water. Soaking temperature is carried out at room temperature.
Passivation is carried out after pickling, with the passivation solution will be wiped over the surface of the steel pipe, stay 1h, then rinse with cold water, carefully scrubbed with a cloth, and finally rinse clean with hot water and blow dry it. After passivation treatment of stainless steel, the appearance of silver-white, with high corrosion resistance. Specifically with nitric acid 5mL, potassium dichromate 1g, water 95ml, at room temperature.
Austenitic stainless steel pipe has the special characteristics of the strict adoption of austenitic stainless steel pipe welding process in the application of welding, taking into account the welding material, pipe, pipe diameter, pipe thickness, welding environment and other factors, a comprehensive summary of the causes of welding defects in the welding process, in order to take effective preventive measures, it is possible to avoid the generation of welding defects, improve the quality of the weld inside and outside, so as to ensure the quality of engineering construction and production and operational safety.