Austenitic stainless steel is represented by 18%Cr-8%Ni steel. In principle, preheating before welding and heat treatment after welding are unnecessary. Generally, it has good welding performance. However, high-alloy stainless steel with high nickel and molybdenum content is prone to high-temperature cracks during welding. In addition, б phase embrittlement is easy to occur, and ferrite generated under the action of ferrite generating elements causes low-temperature embrittlement, as well as corrosion resistance decline and stress corrosion cracking. After welding, the mechanical properties of welded joints are generally good, but when there is chromium carbide on the grain boundary of heat affected zone, it is easy to form a chromium-poor layer, which is easy to lead to intergranular corrosion during use. In order to avoid problems, low carbon (C≤0.03%) grade or grade with titanium and niobium added should be adopted. In order to prevent welding metal from cracking at high temperature, it is generally considered that it is effective to control δ ferrite in austenite. It is generally recommended to contain more than 5% δ ferrite at room temperature. For steel with corrosion resistance as the main purpose, low-carbon and stable steel grades should be selected and appropriate post-weld heat treatment should be carried out; However, post-weld heat treatment is not suitable for steel whose main purpose is structural strength to prevent deformation and δ phase embrittlement due to carbide precipitation.

The welding crack sensitivity of duplex stainless steel is low. However, the increase of ferrite content in the heat affected zone will increase the intergranular corrosion sensitivity, which may lead to the decline of corrosion resistance and the deterioration of low temperature toughness.

For precipitation hardening stainless steel, there are some problems such as softening of welding heat affected zone.

Selection points of austenitic stainless steel covered electrode;

Stainless steel is mainly used for corrosion resistance, but it is also used as heat-resistant steel and low-temperature steel. Therefore, when welding stainless steel, the performance of covered electrode must be consistent with the use of stainless steel. Stainless steel covered electrode must be selected according to the base material and working conditions (including working temperature and contact medium, etc.). ).

1. Generally speaking, the coating covered electrode can be selected with reference to the material of the base material, and the coating covered electrode with the same or similar composition as the base material can be selected. For example, A 102 corresponds to 0cr19ni9; A 137 corresponds to 1Cr 18Ni9Ti.

2. Because the carbon content has a great influence on the corrosion resistance of stainless steel, stainless steel covered electrode with deposited metal carbon content not higher than the base metal is generally selected. For example, A022 covered electrode must be selected for 3 16L.

3, austenitic stainless steel weld metal should ensure the mechanical properties. This can be verified by welding procedure qualification.

4. For heat-resistant stainless steel (austenitic heat-resistant steel) working at high temperature, the selected covered electrode should mainly meet the hot cracking resistance of weld metal and the high temperature performance of welded joint.

(1) Austenitic-ferritic stainless steel covered electrode is generally used for austenitic heat-resistant steels with Cr/Ni≥ 1, such as 1Cr 18Ni9Ti, and the weld metal should contain 2-5% ferrite. When the ferrite content is too low, the crack resistance of weld metal is poor; If it is too high, it is easy to form σ embrittlement phase and produce cracks during long-term use or high-temperature heat treatment. Such as A002, A 102 and A 137.

In some special applications, when it is possible to weld all austenitic metals, such as A402 and A407 covered electrode can be used.

(2) Cr/Ni

5. For corrosion-resistant stainless steel working in various corrosive media, covered electrode should be selected according to the media and working temperature, and its corrosion resistance should be ensured (corrosion performance test of welded joints).

(1) For corrosive media with working temperature higher than 300℃, covered electrode or ultra-low carbon stainless steel containing stable elements of Ti or Nb should be used. Such as A 137 or A002.

(2) For the medium containing dilute sulfuric acid or hydrochloric acid, stainless steel covered electrode containing molybdenum or molybdenum and copper is often selected, such as A032 and A052.

(3) Stainless steel covered electrode without titanium or niobium can be used for weakly corrosive equipment or only to avoid corrosion pollution.

In order to ensure the stress corrosion resistance of weld metal, super alloy welding material is used, that is, the content of corrosion resistant alloy elements (Cr, Mo, Ni, etc.). ) is higher than the parent metal. For example, 00Cr 18Ni 12Mo2 welding materials (such as A022) are used to weld 00Cr 19Ni 10 weldments.

6. For austenitic stainless steel working at low temperature, the low-temperature impact toughness of welded joints should be ensured at the service temperature, so pure austenitic covered electrode is adopted. Such as A402 and A407.

7. Nickel-based alloy covered electrode can also be selected. For example, using nickel-based welding materials with Mo as high as 9% to weld Mo6 super austenitic stainless steel.

8. Selection of coating type in covered electrode:

(1) Because the weld metal of dual-phase austenitic steel itself contains a certain amount of ferrite, it has good plasticity and toughness. Judging from the crack resistance of weld metal, the covered electrode difference between alkaline coating and titanium-calcium coating is not as significant as that of carbon steel covered electrode. Therefore, in practical application, the coating type code is 17 or 16 covered electrode (such as A 102A, A 102, A 132, etc. ) is widely used in welding process performance.

(2) Only when the structural rigidity is high or the crack resistance of weld metal is poor (such as some martensitic chromium stainless steels and pure austenitic chromium-nickel stainless steels, etc.). ), alkaline coated stainless steel covered electrode (such as A 107, A407, etc.). ) and coating code 15 will be considered.

To sum up, the welding of austenitic stainless steel has its unique characteristics, and the selection of austenitic stainless steel covered electrode is particularly noteworthy. Only in this way can the covered electrode of different welding methods and materials be realized, and the stainless steel covered electrode must be selected according to the base material and working conditions (including working temperature and contact medium, etc.). ). Only in this way can the expected welding quality be achieved.

Defect analysis and preventive measures of austenitic stainless steel

(A) prone to thermal cracking

Austenitic stainless steel is prone to hot cracks in the welding process, including longitudinal and transverse cracks in the weld, crater cracks, root cracks in the backing weld and interlayer cracks in multi-layer welding. Especially for austenitic stainless steel with high nickel content.

1. Reason

(1) Austenitic stainless steel has large liquid-solid line spacing, long crystallization time and strong single-phase austenite crystallization direction, so impurity segregation is serious.

(2) Low thermal conductivity and high linear expansion coefficient will produce large welding internal stress (generally tensile stress in weld and heat affected zone).

(3) Components in austenitic stainless steel, such as carbon, sulfur, phosphorus, nickel, etc. , will form a low melting point crystal in the molten pool. For example, the melting point of Ni3S2 formed by S and Ni is 645℃, while the melting point of Ni- Ni3S2*** crystal is only 625℃.

2. Preventive measures

The weld metal with (1) dual-phase structure should be austenite and ferrite as much as possible, and the ferrite content should be controlled below 3 ~ 5%, which can disturb the direction of austenite columnar crystals and refine the grains. Moreover, ferrite can dissolve more impurities than austenite, thus reducing the segregation of low melting point crystals at austenite grain boundaries.

(2) Welding process measures In the welding process, high-quality alkaline coated covered electrode, small line energy, small current, fast and non-swinging welding, filling the arc pit at the end as much as possible, and argon arc welding as the backing can reduce the welding stress and reduce the arc pit cracking.

(3) Control the chemical composition, strictly limit the content of impurities such as S and P in the weld to reduce the low melting point crystals.

(2) intergranular corrosion

The intergranular corrosion leads to the loss of intergranular bonding force and almost complete disappearance of strength. When stressed, it will fracture along the grain boundary.

1. Reason

According to the theory of chromium deficiency, when the weld and heat affected zone are heated to the sensitization temperature (dangerous temperature zone) of 450 ~ 850℃, the supersaturated carbon diffuses to the austenite grain boundary due to the larger radius of Cr atom, and forms Cr23C6 with the chromium compound at the grain boundary, which is insufficient for corrosion resistance.

2. Preventive measures

(1) Low carbon or ultra-low carbon (W(C)≤0.03%) stainless steel welding material is used to control carbon content. Such as A002.

(2) Adding stabilizers Adding elements with stronger affinity for C than Cr to steel and welding materials, such as Ti and Nb, can combine with C to form stable carbides, thus avoiding chromium deficiency at austenite grain boundaries. Commonly used stainless steel and welding materials all contain Ti and Nb, such as 1Cr 18Ni9Ti, 18Ni 12Mo2Ti steel, E347- 15 covered electrode, H0Cr 19Ni9Ti welding wire, etc.

(3) A certain amount of ferrite forming elements, such as chromium, silicon, aluminum, molybdenum, etc. The welding wire or covered electrode is melted into the weld in a bidirectional structure, so that the weld forms an austenite+ferrite biphase structure. Because the diffusion speed of Cr in ferrite is faster than that in austenite, the diffusion speed of Cr in ferrite to grain boundaries is faster, which reduces the chromium deficiency at austenite grain boundaries. Generally speaking, the content of ferrite in weld metal is controlled at 5% ~ 10%. If there is too much ferrite, the weld will become brittle.

(4) Rapid cooling Because austenitic stainless steel will not harden, we can try to improve the cooling speed of welded joints during welding, such as using copper pads or directly cooling welded joints with water. In the welding process, measures such as low current, high welding speed, short arc and multi-pass welding can be adopted to shorten the residence time of welded joints in dangerous temperature zone to avoid the formation of chromium-poor zone.

(5) After solution treatment or homogenization heat treatment welding, the welded joint is heated to1050 ~1100℃ to re-dissolve the carbide into austenite, and then it is rapidly cooled to form a stable single-phase austenite structure. In addition, the homogenization heat treatment can also be carried out at 850 ~ 900℃ for 2 hours. At this time, Cr in austenite grains diffuses to the grain boundary, and the Cr content at the grain boundary reaches above 12% again, and intergranular corrosion will not occur.

(3) Stress corrosion cracking

Corrosion damage of metals under the combined action of stress and corrosive medium. According to the cases and experimental studies of stress corrosion cracking of stainless steel equipment and components, it can be considered that all existing stainless steels are likely to produce stress corrosion under the combined action of certain static tensile stress and specific electrochemical media at a certain temperature. One of the biggest characteristics of stress corrosion is the selectivity of corrosion medium and material combination. Stress corrosion of austenitic stainless steel is mainly caused by hydrochloric acid and chloride containing chloride ions, as well as media such as sulfuric acid, nitric acid, hydroxide (alkali), seawater, steam, H2S aqueous solution and concentrated NaHCO3+NH3+NaCl aqueous solution.

1. Reason

Stress corrosion cracking (SCC) is a delayed cracking phenomenon of welded joints under tensile stress in a specific corrosive environment. Stress corrosion cracking of austenitic stainless steel welded joints is a serious failure form of welded joints, which shows brittle failure without plastic deformation.

2. Preventive measures

(1) Reasonably formulate the molding process and assembly process, minimize cold working deformation, avoid forced assembly, and prevent all kinds of scars during assembly (all kinds of assembly scars and arc burn marks will become the crack source of SCC, which is easy to cause corrosion pits.

(2) Reasonable selection of welding materials should have a good match with the parent metal, and will not produce any bad structures, such as grain coarsening, hard and brittle martensite, etc.

(3) Adopt proper welding technology to ensure that the weld is well formed without defects such as stress concentration or pitting corrosion, and adopt reasonable welding sequence, such as undercut, to reduce the welding residual stress level. For example, avoid cross weld, change Y-groove into X-groove, reduce groove angle appropriately, adopt short weld bead, adopt small line energy, etc.

(4) Post-weld heat treatment for stress relief, such as complete annealing or post-weld annealing; When heat treatment is difficult, post-weld hammering or shot peening is adopted.

(5) Production management measures: control impurities in the medium, such as O2, N2 and H2O in liquid ammonia medium, H2S in liquefied petroleum gas, O2, Fe3+ and Cr6+ in chloride solution, carry out anti-corrosion treatment, such as coating, lining or cathodic protection, and add corrosion inhibitors.

Brittleness of welded joints

After the austenitic stainless steel weld is heated at high temperature for a period of time, the impact toughness will decrease, which is called embrittlement.

1. Low temperature embrittlement of weld metal (embrittlement at 475℃)

(1) reason

After heating at 350 ~ 500℃, the plasticity and toughness of dual-phase weld structure with more ferrite content (more than 15% ~ 20%) will decrease significantly. Because 475℃ is the fastest embrittlement, it is called 475℃ embrittlement. For austenitic stainless steel welded joints, corrosion resistance or oxidation resistance is not always the most critical performance. When used at low temperature, the plastic toughness of weld metal becomes the key performance. In order to meet the requirements of low temperature toughness, the weld structure usually wants to obtain a single austenite structure and avoid the existence of δ ferrite. The existence of δ ferrite always deteriorates the low temperature toughness, and the more the content, the more serious the embrittlement.

(2) Prevention and control measures

① On the premise of ensuring the crack resistance and corrosion resistance of weld metal, ferrite should be controlled at a relatively low level, about 5%.

② Quenching at 900℃ can eliminate welding embrittlement at 475℃.

2. σ phase brittleness of welded joint

(1) reason

When austenitic stainless steel welded joints are used for a long time in the temperature range of 375 ~ 875℃, a kind of FeCr intermetallic compound called σ phase will be produced. σ phase is hard and brittle (HRC & gt68). The precipitation of σ phase leads to the sharp decline of weld impact toughness, which is called σ phase embrittlement. Generally, σ phase only appears in the weld of biphasic structure; When the service temperature exceeds 800 ~ 850℃, σ phase will also precipitate in single-phase austenitic weld.

(2) Preventive measures

(1) limit the ferrite content in the weld metal (less than15%); Super alloy welding materials, namely high nickel welding materials, are adopted, and the contents of elements such as Cr, Mo, Ti and Nb are strictly controlled.

(2) Adopt small specifications to reduce the residence time of weld metal at high temperature.

(3) carrying out solid solution treatment on the precipitated σ phase when conditions permit, so that the σ phase is dissolved into austenite.

④ Heat the welded joint to 1000 ~ 1050℃, and then cool it quickly. Generally, σ phase does not occur in 1Cr 18Ni9Ti steel.

3. The fusion line is brittle.

(1) reason

When austenitic stainless steel is used for a long time at high temperature, a few grains outside the fusion line will be brittle fracture.

(2) Prevention and control measures

Adding Mo to steel can improve the brittle fracture resistance of steel at high temperature.

Through the above analysis, the above welding defects can only be avoided if the above welding process measures or welding materials are reasonably selected. Austenitic stainless steel has excellent weldability, and almost all welding methods can be used to weld austenitic stainless steel. Among all kinds of welding methods, covered electrode arc welding has the advantages of adapting to various positions and different plate thicknesses, and is widely used.

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