The embrittlement phenomenon in stainless steel welded joints

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The phenomenon in which the impact toughness of stainless steel welds decreases after being heated at high temperatures for a period of time is known as embrittlement. ⑴Brittleness at 475°C: In duplex weld metal containing a high amount of ferrite phase (over 15%–20%), plasticity and toughness are significantly reduced upon heating to 350–500°C, resulting in increased brittleness. Since the embrittlement rate is fastest at 475°C, it is referred to as “475°C brittleness”. The more ferrite there is, the more severe this embrittlement becomes. Welds that have become brittle at 475°C can have this brittleness eliminated by quenching at 900°C. ⑵σ-phase embrittlement: When stainless steel weldments are used over a long period at temperatures ranging from 375 to 875°C, an FE-Cr intermetallic compound is formed, known as the “σ-phase”. The σ-phase is hard and brittle; when its hardness exceeds 68HRC, the impact toughness of the weld decreases sharply due to the precipitation of this σ-phase, a phenomenon known as \"σ-phase embrittlement\". It is generally believed that the σ-phase arises from the transformation of ferrite, and when the mass fraction of ferrite exceeds 5%, the σ-phase forms rapidly. Therefore, for stainless steel materials used at high temperatures, in order to prevent the formation of the σ phase, it is necessary to control the ferrite content. To eliminate the formed σ-phase and restore the toughness of the welded joint, it can be heated to 1000–1050°C and then rapidly cooled. The σ phase generally does not form in the welds of 1Cr18Ni9Ti stainless steel. ⑶Fracture along the weld line: When stainless steel welded parts are used at high temperatures for an extended period of time, brittle fracture occurs in the areas surrounding the weld line, within a few grains; this phenomenon is known as fracture along the weld line. Adding Mo to steel can improve its resistance to brittle fracture.