Steel, Nickel vs Toughness

[Robertmet]

While it's long been known that nickel improves toughness in steel can anyone tell me why ? My searches have found nothing.The only logical reason I can think of is nickel's aversion to carbon.

 

[CoryPad]

Here are a couple of quotes from the ASM Handbook series:

It should be noted that moderate additions of nickel mainly improve low-temperature fracture toughness through resistance to cleavage, and that little effect may be seen at higher temperatures at which microvoid coalescence is the fracture mode.


These steels utilize the effect of nickel content in reducing the impact transition temperature, thereby improving toughness at low temperatures.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[metengr]

In addition to what CoryPad has mentioned, Ni is in solid solution with iron, and in my mind this would appear to increase the cleavage fracture stress of the steel, thus toughening the material (increasing resistance to brittle fracture).

 

[unclesyd]

Adding to the above posts.

Taken from "Modern Steels and Their Properties".
Published by Bethlehelm Steel.

Nickel lowers the critical temperatures of steel, widens the temperature range for successful heat-treatment, retards the decomposition of austenite, and does not form any carbides or other compounds which might be difficult to dissolve during austenitizing. All these factors contribute to easier and more nearly foolproof heat-treatment.

Nickel steels are especially suitable for the case-hardening
process.

Advantages imparted by nickel are not confined to quenched and tempered steels. Nickel is a powerful ferrite strengthener. It also reduces the eutectoid carbon-ratio, hence the structure of unhardened nickel steels contains a higher percentage of perlite than plain carbon steels of comparable carbon content, similarly treated. The perlite is also finer, and therefore stronger and tougher, than the perlite in unalloyed steels, because it forms in a lower temperature range. These factors permit given strength levels to be obtained at considerably lower carbon contents, thereby markedly increasing toughness, plasticity, and fatigue resistance. Nickel steels are therefore highly suited for applications where liquid quenching is not employed, such as high strength structural steels which are used in the as rolled condition, or heavy forgings which are not adapted to quenching and must develop superior properties after not more severe than air-quenching treatments.

For the same reasons low-carbon nickel steels permit higher strengths to be obtained safely in structures which are fabricated by welding. Weldability suffers with an increase in carbon content and it is sometimes preferable, when steel is to welded , to develop extra strength by suitable alloying while maintaining a low carbon content.

 

[Robertmet]

Let me give some examples .In the custom knife business for impact type blades of machetes etc they may choose L-6 or 15N20 ,nickel containing alloys noted for toughness. In large case hardened gears or bearings in applications such as rolling mills seeing high impact they may use 3310 or 9310 for toughness.Many applications 4340 is chosen because of it's toughness.All martensite structures ,all tough but exactly why ???