Welding characteristics of austenitic stainless steel
1, Prone to hot cracks:
(1) Try to make the weld metal was biphasic, ferrite content control in 3-5% or less. Because ferrite can dissolve a large amount of harmful S, P impurities.
(2) Try to use alkaline coating of high-quality welding electrodes to limit the weld metal S, P, C and other content.
2, Intergranular corrosion:
According to the theory of poor chromium, weld and heat affected zone in the heating to 450-850 ℃ sensitized temperature zone in the grain boundary precipitation of chromium carbide, resulting in poor chromium grain boundaries, not enough to resist the degree of corrosion.
(1) The use of low-carbon or ultra-low carbon welding consumables, such as A002, etc .; using titanium, niobium and other stabilized elements of the electrode, such as A137, A132 and so on.
(2) From the welding wire or welding wire into the weld a certain amount of ferrite forming elements, the weld metal into austenite + ferrite biphase structure, (ferrite generally controlled at 4-12%) The
(3) To reduce the welding pool overheating, use a smaller welding current and faster welding speed, speed up the cooling rate.
(4) The resistance to intergranular corrosion requirements of the weld pieces of high stability after annealing treatment.
3, Stress corrosion cracking:
Stress corrosion cracking is the phenomenon of delayed cracking caused by tensile stress in welded joints under specific corrosive conditions. The stress corrosion cracking of the austenitic stainless steel welded joint is a serious failure form of the welded joint, which is manifested as brittle failure without plastic deformation.
Stress corrosion cracking prevention measures:
(1) The rational development of forming processing and assembly process, as far as possible to reduce the cold deformation, to avoid forced assembly, to prevent the assembly process caused by a variety of scars (a variety of assembly scars and arc burning marks will become SCC crack source, Corrosion pits).
(2) A reasonable choice of welding consumables: weld and base metal should have a good match, does not produce any bad tissue, such as grain coarsening and hard brittle martensite;
(3) To take the appropriate welding process: to ensure that the weld shape is good, do not produce any stress concentration or pitting defects, such as undercut; take a reasonable welding sequence, reduce the residual stress level welding;
(4) To eliminate stress treatment: post-weld heat treatment, such as after welding annealing or annealing; difficult to implement the heat treatment using hammer hammer or shot peening.
(5) Production management measures: control of impurities in the medium, such as O2, N2, H2O in liquid ammonia medium; H2S in liquefied petroleum gas; O2, Fe3 +, Cr6 + in chloride solution; corrosion treatment: Coating, lining or cathodic protection; add corrosion inhibitor.
4, The low temperature of the weld metal embrittlement:
For austenitic stainless steel welded joints, the use of low temperature, the weld metal toughness is the key issue. At this time, the presence of ferrite in the weld structure always deteriorates the low temperature toughness.
Preventive measures: A single austenitic weld is obtained by selecting pure austenitic welding consumables and adjusting the welding process.
5, The welding of the σ phase embrittlement:
Weldments in a certain period of time after the high temperature heating in the weld in the precipitation of a brittle σ phase, resulting in the entire joint brittle, plastic and toughness decreased significantly. The precipitation temperature range of the σ phase is 650-850 °C. During the high temperature heating process, the σ phase is mainly composed of ferrite. The longer the heating time, the more precipitated the σ phase.
(1) Limit the weld metal ferrite content (less than 15%); the use of superalloy welding materials, that is high nickel welding consumables.
(2) The use of small norms to reduce the weld metal at high temperature residence time;
(3) The precipitated σ phase is subjected to solution treatment when the conditions permit, so that σ phase into the austenite.