Welding technology and application of 2205 duplex stainless steel
2205 duplex stainless steel has high yield strength, not only has strong chloride stress corrosion resistance, but also has good comprehensive mechanical properties. The market application prospect is broad. The welding process is complex, and it is very important to ensure the phase proportion balance of the weld and HAZ.
2205 duplex stainless steel in our company to undertake a large number of materials used in the project, its weldability analysis, process qualification test, to master the welding technology of the material, has practical significance for the promotion of the project.
Welding analysis of duplex stainless steel
Weldability analysis of duplex stainless steel
- 1) Hot cracks. The sensitivity of hot cracking is much less than that of austenitic stainless steel. There are few impurities which are easy to form eutectic with low melting point, so it is difficult to produce low melting point liquid film;
- 2) Embrittlement in HAZ. The main problem of duplex stainless steel welding is heat affected zone. Under the effect of welding thermal cycle, the HAZ is in fast cooling non-equilibrium state, and more ferrite is always retained after cooling, which increases the corrosion tendency and sensitivity of hydrogen induced cracking (embrittlement);
- 3) Ferrite embrittlement occurs at 475 ℃. Duplex stainless steel contains about 50% ferrite and brittleness at 475 ℃, but it is not as sensitive as ferritic stainless steel.
(1) Phase ratio requirement: the mechanical properties and corrosion resistance of duplex stainless steel welded joint depend on whether the welding joint can maintain proper phase proportion. When the content of ferrite and austenite is close to 50%, the properties of similar base metal are better. If the ratio is changed, the corrosion resistance and mechanical properties will decrease; too low (< 25%) or too high (> 75%) ferrite content is not conducive to duplex stainless steel welding.
(2) The influencing factors of phase ratio are as follows
- 1) The effect of alloying elements. According to research, nitrogen content in base metal is very important. The ability of nitrogen is much greater than that of nickel. At high temperature, the ability of nitrogen to stabilize austenite is greater than that of nickel; nitrogen addition can delay the precipitation of intermetallic phases, and improve the strength and corrosion resistance of weld metal. 2) The influence of thermal cycle. The biggest characteristic of duplex stainless steel welding is the influence of welding thermal cycle on the microstructure of joint. Therefore, multi-layer and multi pass welding is beneficial, and the subsequent weld bead has heat treatment effect on the front weld bead, which can significantly improve the microstructure and properties of the whole welded joint.
- 3) The influence of process parameters. The welding parameters, i.e. welding wire energy, play a key role in the balance of dual phase structure. The dual phase stainless steel is 100% ferrite at high temperature. If the linear energy is too small or too large, the cooling rate of HAZ will be affected.
- 4) The effect of shielding gas: adding nitrogen during TIG welding can prevent the loss of nitrogen on the weld surface due to diffusion, and contribute to the balance of ferrite and austenite.
Two problems must be paid attention to in the welding process of duplex stainless steel:
- 1) Maintain proper phase ratio to prevent unstable austenite phase content in weld metal and HAZ;
- 2) Ferrite phase is unstable in the high temperature range, and harmful substances will be precipitated, which should be prevented.
To solve the above problems and welding parameters. Due to lack of welding experience and process parameters of duplex stainless steel, welding test must be carried out.
Put forward and determine the welding method, welding material and process plan
1. Selection of welding method: according to the site construction and tooling equipment, SMAW and GTAW welding methods are selected.
2. Put forward and determine the welding material scheme: the welding material should be the dual phase steel welding material with similar chemical composition and higher nickel content than the base metal, so as to ensure that the austenite in the weld is dominant. Er2209 welding wire and E2209 welding rod are selected by comparison.
3. Put forward and determine the welding process plan:
- Scheme one: GTAW: argon protection on the front side and argon protection on the back side of the weld
- Scheme two: GTAW: argon protection on the front side and nitrogen protection on the back side of the weld
- Scheme 3: SMAW: full penetration butt welding.
According to the above three schemes, the feasibility of the process can be verified.
Selection of welding parameters
In order to ensure the balance of two-phase structure, the welding line energy is too large or too small, which is generally controlled in the range of 0.5-2.5kj/cm. Preheating is not conducted before welding, and the interlayer temperature is controlled below 150 ℃. In view of this, the welding process parameters of the two welding methods adopted in the welding procedure qualification are shown in the table below:
|Number of layers||Welding method||Welding material||Specifications||Polarity||Current a||Voltage V||Speed mm/s||Baking temperature ℃||Holding time h|
- 1. The welding procedure qualification is carried out according to Nb/T47014-2011. Scheme 2 and scheme 3 are adopted, the average hardness test value is hbw275, the average tensile strength of tensile test is 880mpa, and the plastic fracture is in the heat affected zone; the side bending sample is qualified without crack, and the side bending sample in scheme 1 is unqualified.
- 2. Phase ratio evaluation: the microstructure morphology observed by metallographic method in scheme 2 and 3 is that there are light gray lath and massive ferrite on the white austenite matrix. After assessment, the ferrite content of weld is about 55%; the heat affected zone is 55%; after quality inspection, all meet the design requirements, and the welding double comparison is effectively controlled.
- 3. Welding inspection: in the construction of the project, GTAW: argon protection on the front side of the weld and nitrogen protection at the back of the weld were adopted. A total of 303 pieces of rt-ray inspection were completed for 2205 duplex stainless steel welding, among which 303 pieces were qualified once, and the first pass rate was 100%. Practice proved that the process effect was good.
The results show that the welding process of shielded metal arc welding and argon arc welding is suitable for the welding parameters. The comprehensive mechanical properties and duplex structure content per unit area of 2205 duplex stainless steel can meet the standard requirements. The mechanical energy of welded joint is improved by filling pure nitrogen in argon arc welding, which indicates that nitrogen promotes the transformation of ferrite to austenite in welding process.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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