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Nickel is an important alloying element in carbon steels, as it significantly enhances hardenability and toughness, particularly at low temperatures. It plays a key role in steel grades for demanding applications such as energy, mining, and defense. While most carbon steels contain less than 1 wt% Ni, cryogenic steels may include up to 9 wt%. In plate production—via normalizing, controlled rolling, or quenching and tempering (Q&T) nickel interacts with processing conditions and other alloying elements, influencing microstructure and mechanical properties. Its effects are discussed in this contribution for steels with yield strengths from 350 to 1500 MPa.
For heavy plates in offshore constructions with demanding toughness requirements (350–460 MPa), typically niobium-microalloyed HSLA steels, nickel additions up to ~1 wt% expand the austenite phase field and delay the austenite-to-ferrite transformation. This leads to increased undercooling, promoting ferrite nucleation and refining the microstructure. At slow cooling rates, the delay also enhances precipitation strengthening, compensating for reduced grain refinement.
In ultrahigh-strength structural steels for highly stressed constructions, produced by reheat quenching or direct quenching, sufficient hardenability is required to achieve martensite in thick plate centers. Nickel additions to Mo- or Mo–B alloy systems significantly improve hardenability, especially at low cooling rates, while also increasing toughness—an advantage for mining and defense applications.
For cryogenic applications, like LNG-applications, so-called „9% Ni steels“ provide a cost-effective alternative to austenitic steels. In addition to solid solution softening by nickel alloying primarily accounting for the improvement of low temperature toughness, the Q&T treatment results in a very fine grain structure. Nickel partitioning stabilizes a small fraction of retained austenite during tempering at temperatures in the two-phase region between the Ac1 and Ac3 temperatures. An optimized retained austenite content results in exceptionally high toughness at temperatures below −100 °C.