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Austenitic stainless steel welding characteristics and electrode selection

2016-01-10 15:51:11 Hot:

Abstract: austenitic stainless steel is one of the petrochemical production in the most widely used metal materials, this paper briefly describes some of the problems of stainless steel welding process prone to address these issues proposed appropriate preventive measures, and based on these proposed measures the selection principles of electrode.

Keywords: austenitic stainless steel welding characteristics of measures to prevent the electrode selection

304 stainless steel plate stainless steel refers to the main elements of Cr is higher than 12% increase, to make steel in a passive state, but also has the characteristics of stainless steel. Stainless steel according to the microstructure into ferritic, martensitic, austenitic, austenitic + ferritic and precipitation hardening stainless steel. Austenitic stainless steel is usually at room temperature, pure austenite organization, there are some small amount of austenite + ferrite.
First, the austenitic stainless steel welding characteristics

Stainless steel is the petrochemical production in one of the most widely used austenitic metal material, good welding performance, but in the welding process is also prone to many problems, mainly for the following categories:

1, intergranular corrosion

304 austenitic stainless steel weldment likely to occur in the welded joint intergranular corrosion, according to chromium depletion theory, the reason is when welding the weld and heat affected zone heated to a temperature range of 450 ~ 850 ℃ to stay for some time joints, In the grain boundary precipitation of high chromium carbide (Cr23C6), causing the grains to reduce the amount of chromium-containing surface layer, forming chromium depleted zone, the role of the corrosive medium, the grain surface chromium depleted zone is formed by corrosion intergranular corrosion. Then the etched surface of welded joints no significant changes, it will fracture along grain boundaries under stress, almost completely lost strength.

In order to prevent and reduce intergranular corrosion welded joints, preventive measures in general are: (1) low or ultra-low carbon welding consumables, such as A002, etc., or the use of titanium, niobium and other stabilizing element electrode, such as the A137, A132 and the like; (2) by a wire or rod to weld a certain amount of ferrite forming elements, so that the weld metal into an austenite + ferrite two-phase structure (ferrite general control 4 -12%); (3) to reduce overheated weld pool, use a smaller welding current and welding speed faster, speed up the cooling rate; and (4) of the resistance to intergranular corrosion can be demanding weldments after welding stable annealing process.

2, welding hot cracking

The main reason is that the weld heat cracks in the dendrites strong directional conducive S, P and other elements of the low melting point eutectic formation and aggregation products. In addition, the thermal conductivity of such steel is small (about 1/3 of low carbon steel), linear expansion coefficient (50% larger than the low-carbon steel), welding stress is so large, exacerbated by heat cracks. Which prevents approach is:
    (1) selection of a low carbon content welding material, welding material containing an appropriate amount of Mo, Si, etc. ferrite forming elements, the formation of the weld duplex austenite and ferrite, reducing segregation; (2) Try to use quality alkaline drug skin electrode, in order to limit the weld metal S, P, C and other content.

3, stress corrosion cracking

Stress corrosion cracking is welded joints under certain corrosive environment by the delayed cracking phenomenon tensile stress generated. Stress corrosion cracking of austenitic stainless steel welded joints are welded joints more serious failure mode, showed no brittle failure of plastic deformation.
    Stress corrosion cracking prevention measures: (1) take the appropriate welding process to ensure good weld, do not produce any stress concentration or pitting defects such as undercut; take reasonable welding sequence, reducing the welding residual stress level; ( 2) a reasonable choice of welding consumables, weld and base metal should be a good match, it does not produce any adverse tissue, such as grain coarsening and brittle martensite; (3) eliminate the stress treatment: post-weld heat treatment, such as welding After fully annealed or annealed; when it is difficult to implement heat treatment using shot peening or the like.

4, low temperature embrittlement of the weld metal.

For austenitic stainless steel welded joints at low temperature using plastic weld metal toughness is the key issue. At this time, there is a weld in ferrite is always the low-temperature toughness. Usually to prevent low-temperature catalytic single weld metal can be obtained by a method of austenitic welds use of pure austenitic welding consumables and welding process adjustment.
5, σ phase embrittlement of welded joints.

Weldment will precipitate a brittle σ in the weld after being subjected to a certain time of high temperature heating phase, leading to the joint embrittlement, ductility and toughness decreased significantly. σ phase precipitation temperature range of 650-850 ℃. In the high-temperature heating process, σ phase mainly made of ferrite transformation. The longer the heating time, σ phase precipitation more.
    Preventive measures: (1) limit the weld metal ferrite content (less than 15%), the use of super-alloyed welding material, namely high nickel welding consumables; (2) the use of small practices to reduce the weld metal at high temperature residence time; and (3) have been precipitated σ phase when conditions permit the solution treatment, so that σ integration into the austenite phase.

Second, the austenitic stainless steel welding rod selection principle

Stainless steel is mainly used for corrosion resistance, but also serves as a heat-resistant steel and cold steel. Therefore, when welding stainless steel, welding performance must be consistent with the first use of stainless steel, followed by stainless steel welding rod must be selected according to the base material and working conditions (including temperature and exposure to media, etc.). Combined with stainless steel welding process prone to problems as well as preventive measures, welding general selection principles are the following:
1, in general, the selection of the base material can reference electrode material, the choice of the base material of the same or similar ingredients electrode. Such as: A102 correspond 0Cr19Ni9, A137 and other correspondence 1Cr18Ni9Ti.
2, austenitic stainless steel weld metal should ensure that the mechanical properties. This can be verified by the welding procedure qualification.
3, due to the carbon content can have a great influence on the corrosion resistance of stainless steel, therefore, generally used in the carbon content of the deposited metal is not higher than the base material of stainless steel electrode. Such as 316L A022 welding rod must be selected.
4, for the high temperature heat-resistant stainless steel (austenitic heat-resistant steel), the choice of electrode should be able to meet the main thermal cracking temperature performance properties of the weld metal and welded joints.
(1) Cr / Ni≥1 austenitic heat-resistant steel, such as 1Cr18Ni9Ti, etc., are commonly used austenitic - ferritic stainless steel electrode to weld metal ferrite containing 2-5% is appropriate. When the ferrite content is too low, the weld metal crack resistance is poor; if too high, the long-term use at high temperatures or heat treatment is easy to form σ phase embrittlement, causing cracks. Such as the A002, A102, A137. In some special cases, may require a fully austenitic weld metal, it can be used such as A402, A407 welding rod.
(2) to Cr / Ni <stable austenitic heat-resistant steel 1, as Cr16Ni25Mo6 etc., should generally be at the same time ensure that the weld metal and the base material having a substantially similar chemical composition, increasing the weld metal Mo, W, Mn content and other elements, so ensure that the weld metal heat resistance while improving weld crack resistance. Such as the use of A502, A507.
5, for working in a variety of corrosive media corrosion resistance of stainless steel, the media and working temperature should be selected electrode, and to ensure its resistance to corrosion (welded joints made of corrosion test).
(1) For operating temperatures above 300 ℃, there is a strong corrosive media, shall contain the elements Ti or Nb stabilized or ultra-low carbon stainless steel electrode. Such as the A137 or A002 and so on.
(2) The medium contains sulfuric acid or hydrochloric acid, often used with Mo or Mo and Cu-containing stainless steel electrode, such as: A032, A052 and so on.
(3) the work of the media only to avoid corrosion corrosive weak or contamination of stainless steel equipment, can be used free of Ti or Nb stainless steel electrode. To ensure the resistance to stress corrosion of the weld metal, ultra-alloyed welding consumables, namely weld metal corrosion resistant alloy elements (Cr, Mo, Ni, etc.) content is higher than the base metal. Such as the use 00Cr18Ni12Mo2 type of welding materials (such as A022) welding 00Cr19Ni10 weldment.
6, work at low temperatures for austenitic stainless steel, should ensure that the use of welded joints at a temperature low temperature impact toughness, so the use of pure austenitic electrodes. Such as A402, A407.
7, can also be used nickel-based alloy electrode. Such as the use of 9% Mo Weld Mo6 nickel based super austenitic stainless steel.
   

In summary, the austenitic stainless steels has its unique characteristics, the austenitic stainless steel welding electrode selection is particularly noteworthy that only the use of different welding methods and different welding materials depending on the material and working conditions in order to to achieve the desired weld quality.

 
Reference:

(1) "stainless steel" editor Zhang Qishu block Yaoting Machinery Industry Press, 1997
(2) "Welding Procedure Qualification Handbook," edited Chenyu Chuan Machinery Industry Press, 1996

(3) "hot corrosion resistant stainless steel and alloy welded 100 asked," edited by Lee Kei Fukuo Meet Xuechun Ming Chemical Industry Press 1985

(4) "austenitic stainless steel stress corrosion analysis" Zhang Guohua Li Jing high welding technology Vol.31 NO.6 Dec.2002.
(5) "operating difficulties austenitic stainless steel pipe welding of" Guo Qiang welding 2003