Strong, hard and tough: the many ways nickel-containing alloy steels deliver

Alloy steels include a wide variety of iron-based materials. Nickel content ranges from very low, ~0.3% in some alloy steels, and up to as much as 20% in maraging steels. Each alloy is designed for some combination of greater strength, hardness, wear resistance or toughness than plain-carbon steels. They are typically used in equipment that delivers power, forms and cuts metal, or are used at low temperatures where carbon steels lack adequate toughness. For simplicity, alloy steels can be divided into several types, with specific properties for specific end uses. Nickel alloy steels are essential in the construction of tools and machinery that enable industry to make other tools and machinery.

Typical chemical composition of some notable nickel-containing alloy steels

Hardenable low alloy steel

These steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels. This is achieved by the addition of alloying elements such as nickel, chromium, and molybdenum, followed by a quench (rapid cooling) and temper heat treatment. These elements, when dissolved in austenite prior to quenching, increase hardenability. Nickel complements the hardening effect of chromium and molybdenum and is important in providing toughness to the hard-martensitic microstructure that results from the quench and temper heat treatment.

Comparing typical mechanical values for AISI 4340 in the annealed and quench and tempered condition to AISI 1045 carbon steel

Tool steel

Tool steel is a term applied to a variety of high-hardness, abrasion-resistant steels used for applications, such as dies (stamping or extrusion), cutting or shearing, mould making, or impact applications like hammers (personal or industrial). Their heat treatment is similar to hardenable low alloy steels.

Air-hardened tool steels reduce distortion caused by the rapid water quenching and they possess a balance of wear resistance and toughness.

Plastic mould tool steels are low carbon steels that are shaped and then carburised, hardened and then tempered to a high surface hardness, which makes them ideal for injection moulds and die-casting dies.

High strength low alloy (weathering steel)

The finer grain structure of these steels results in increased strength compared to plain carbon steels. This finer grain is achieved by influencing transformation temperatures so that the conversion of austenite to ferrite and pearlite occurs at a lower temperature during air cooling. At the low carbon levels typical of HSLA steels, elements such as silicon, copper, nickel, and phosphorus are particularly effective for producing fine pearlite.

The addition of chromium, copper and nickel produce a stable rust layer that adheres to the base metal and is much less porous than the rust layer that forms on ordinary structural steel. The result is a much lower corrosion rate allowing these steels to be utilised uncoated.

The table below shows the difference in mechanical properties for ASTM A36 carbon structural steel and ASTM A588 Grade C high-strength low-alloy structural steel.

The difference in mechanical properties for ASTM A36 carbon structural steel and ASTM A588 grade C high-strength low-alloy structural steel

Nickel steel

Ferritic steels with high nickel content, typically greater than 3%, find extensive use in applications involving exposure to temperatures from 0 °C to -196 °C. Such applications include storage tanks for liquefied hydrocarbon gases, as well as structures and machinery designed for use in cold regions. These steels utilise the effect of nickel content in reducing the impact transition temperature, thereby improving toughness at low temperatures.

In carbon and most low-alloy steels, as the temperature drops below 24 °C (75 °F), strength and hardness increase, while tensile ductility and toughness decrease. Nickel improves low-temperature toughness, as illustrated by the charpy impact results in Figure 1.

First applied in liquid oxygen containment vessels in 1952, 9% nickel steel has since mainly been used for the inner shell of LNG tanks. It is selected instead of austenitic stainless steels, due to the combination of high strength and reliable fracture toughness at very low temperatures down to -196 °C.

Maraging steel

Maraging steels are low carbon Fe-Ni alloys, containing ~18% nickel and additionally alloyed with cobalt, molybdenum, titanium and other elements. These alloys are quenched to martensite followed by a precipitation hardening heat treatment at 480-500 °C, which promotes precipitation of intermetallics such as Ni3Mo and Ni3Ti. 

These steels possess high fracture toughness, and their impact-fatigue strength indicates that they are useful for repeated impact loading situations, such as in electro-mechanical components. The relatively low heat treatment temperature results in much less distortion than the quenching of hardenable low alloy steels, making them desirable for long, thin parts. Though the amount of nickel used in these alloy steels is less significant than in stainless steel production, their variety is extensive and industrially they are important enablers.

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