Welding Austenitic, Ferritic, Martensitic, Duplex, and Precipitation-Hardenable Stainless Steel

Learning to weld Austenitic, Ferritic, Martensitic, Duplex, and Precipitation-Hardenable Stainless Steel can improve your companies welding processes. 

• The stainless properties of stainless steels are primarily due to the presence of chromium in quantities greater than roughly 12 weight percent.
• This level of chromium is the minimum level of chromium to ensure a continuous stable layer of protective chromium-rich oxide forms on the surface.
• In commercial practice, however, some stainless steels are sold containing as little as 9 weight percent chromium and will rust at ambient temperatures.
• Stainless steels are subject to weld metal and heat affected zone cracking, the formation of embrittling second phases and concerns about ductile to brittle fracture transition.
• The prevention of cracking or the formation of embrittling microstructures is another main concern when welding or fabricating stainless steels.
• The austenitic stainless steels were developed for use in both mild and severe corrosive conditions. Austenitic stainless steels are used at temperatures that range from cryogenic temperatures, where they exhibit high toughness, to elevated temperatures, where they exhibit good oxidation resistance.
• A concern, when welding the austenitic stainless steels, is the susceptibility to solidification and liquidation cracking. Cracks can occur in various regions of the weld with different orientations, such as centerline cracks, transverse cracks, and microcracks in the underlying weld metal or adjacent heat-affected zone (HAZ).
• These cracks are primarily due, to low-melting liquid phases, which allow boundaries to separate under the thermal and shrinkage stresses during weld solidification and cooling.
• Even with these cracking concerns, the austenitic stainless steels are generally considered the most weldable of the stainless steels.
• Ferritic stainless steels comprise approximately half of the 400 series stainless steels. These steels contain from 10.5 to 30 weight percent chromium along with other alloying elements, particularly molybdenum.
• Ferritic stainless steels are noted for their stress-corrosion cracking (SCC) resistance and good resistance to pitting and crevice corrosion in chloride environments, but have poor toughness, especially in the welded condition.
• Martensitic stainless steels are considered to be the most difficult of the stainless steel alloys to weld. Higher carbon contents will produce greater hardness and, therefore, an increased susceptibility to cracking.
• In addition to the problems that result from localized stresses associated with the volume change upon martensitic transformation, the risk of cracking will increase when hydrogen from various sources is present in the weld metal. A complete and appropriate welding process is needed to prevent cracking and produce a sound weld.
• Duplex stainless steels are two phase alloys based on the iron-chromium-nickel system. Duplex stainless steels usually comprise approximately equal proportions of the body-centered cubic (bcc) ferrite and face-centered cubic (fcc) austenite phases in their microstructure and generally have a low carbon content as well as, additions of molybdenum, nitrogen, tungsten, and copper.
• The specific advantages offered by duplex stainless steels over conventional 300 series stainless steels are strength, chloride stress-corrosion cracking resistance, and pitting corrosion resistance.
• Precipitation-hardening (PH) stainless steels are iron-chromium-nickel alloys. They generally have better corrosion resistance than martensitic stainless steels.
• The high tensile strengths of the PH stainless steels is due to precipitation hardening of a martensitic or austenitic matrix.
• Copper, aluminum, titanium, niobium (columbium), and molybdenum are the primary elements added to these stainless steels to promote precipitation hardening.
• It is important to understand the microstructure of the particular type of alloy being welded. Some of the PH stainless steels solidify as primary ferrite and have relatively good resistance to hot cracking.
• In other PH stainless steels, ferrite is not formed, and it is more difficult to weld these alloys without hot cracking.

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