Nickel in carbon steel
Nickel is an important and widely used constituent in alloy steel.
It metallurgically acts as a:
- solid solution strengthener
- mild hardenability agent
- promoter of high toughness especially at low temperatures
Metallurgical effects of Nickel
In structural steels nickel alloying comes into play especially when heavy gages or very high strength are required. By extending the austenite phase field and particularly lowering the austenite-to-ferrite transformation temperature nickel retards the early formation of polygonal ferrite grains that subsequently can grow to rather large grain size under low cooling rates. When accelerated cooling is applied, nickel improves the hardenability. In other words, the critical cooling rate for achieving strong bainitic or martensitic microstructures is reduced by nickel alloying. Yet even for the polygonal ferritic microstructure, typically present in medium strength structural steels, the transformation retarding effect caused by nickel produces finer grain size which in turn results in a favorable combination of increased strength and simultaneously improved low-temperature toughness. The magnitude of achievable improvements in function of the nickel alloy content is exemplarily shown in Figure 1. Although grain refinement is typically obtained by microalloying in combination with controlled rolling it might be challenging to provide sufficiently high rolling reduction when producing heavy gaged plate. Consequently, the occurrence of larger grain sizes especially in the plate center is often unavoidable. The grain refining effect of nickel, on the contrary, does not require a specific rolling reduction therefore making its use particularly suitable for heavier gaged products. A refined grain size with a more homogeneous distribution over the entire thickness can thus be achieved.
Evolution of strength and low-temperature toughness improvement in function of the nickel addition based on a normalized plate steel alloy S355N.
Nickel promotes through-hardenability during quenching. Adding 2% nickel to a given steel alloy increases the hardenable thickness by more than three times. Yet there is more to higher nickel additions than just hardenability. Beyond the indirect effect of lowering the DBTT via grain refinement (see above), nickel directly contributes to excellent low temperature toughness. This intrinsic nickel functionality is related to improving the mobility of dislocations within the iron lattice - a phenomenon that becomes particularly relevant at very low temperatures. The well-known cryogenic steels exploit this effect using nickel contents in the range of 7% to 9% thus reducing the DBTT to below -200°C. Additional strengthening mechanisms complementing to grain refinement are needed to produce ultra-high strength Q&T structural steels. These strengthening mechanisms, however, are detrimental to low temperature toughness. Therefore, moderate nickel additions are employed for compensating this loss of toughness allowing to adjust the DBTT in accordance with the lowest prevailing operational temperature.
Effect of nickel on ductile-to-brittle transition behavior in a S460M plate steel (grain refinement originates from Nb microalloying in combination with TMCP; Ni provides intrinsic toughness effect).