The question of whether nickel is harder than steel is complex because “steel” is a vast family of iron alloys, and “hardness” can be measured in several ways. Both nickel and steel are foundational materials in modern engineering, but their distinct compositions and processing methods lead to a wide range of mechanical properties. A comparison requires understanding what hardness means and how these materials are engineered to exhibit different levels of it.
Defining Material Hardness
In materials science, hardness is defined as a material’s resistance to localized permanent deformation, including resistance to scratching, abrasion, and indentation. This resistance to plastic deformation indicates a material’s durability and wear resistance. Engineers rely on standardized testing methods to quantify this property and allow for accurate comparison.
The most common industrial method is indentation testing, using scales like Rockwell, Brinell, or Vickers. These tests involve pressing a geometrically defined indenter—such as a hardened steel ball or a diamond pyramid—into the material’s surface under a specific load. The resulting size or depth of the indentation dictates the material’s hardness value.
Pure Nickel Versus Common Steel
Commercially pure nickel is significantly softer than even the most basic grades of steel. Pure nickel, such as Nickel 200, is known for its high ductility and relatively low hardness, making it easily shaped and worked. It offers excellent corrosion resistance and electrical properties, but it is not intended for high-wear structural applications.
In contrast, common low-carbon steel, often called mild steel, gains a notable advantage in hardness due to the presence of carbon. Even with carbon content below 0.3%, the resulting iron-carbon microstructure provides a baseline strength that exceeds pure nickel. Mild steel exhibits a higher resistance to indentation than pure nickel because the carbon atoms interfere with the movement of dislocations within the iron’s crystal lattice, which are the defects that allow metal to deform.
The Impact of Alloying and Heat Treatment on Steel
The hardness of steel is not fixed; it is one of the most variable and controllable properties. The primary factor influencing this is the percentage of carbon, which can range from less than 0.05% in mild steel to over 2% in some cast irons. As carbon content increases, the potential for achieving extreme hardness through heat treatment rises.
When steel with sufficient carbon is heated to high temperatures and then rapidly cooled (quenching), the microstructure transforms into a metastable phase called martensite. This martensitic structure is exceptionally hard and brittle because the rapid cooling traps the carbon atoms, severely straining the iron crystal structure and preventing deformation. Tempering is often applied afterward, involving reheating the steel to a lower temperature to reduce brittleness while retaining high hardness.
Steels engineered for extreme wear resistance, such as tool steels, can reach Brinell hardness values far greater than any form of pure nickel. For example, a hardened tool steel can easily exceed 600 Brinell, whereas a soft steel might be around 279. This difference highlights how the engineered iron-carbon system allows steel to be tailored for maximum hardness and strength through the combination of carbon content and controlled thermal processing.
Specialized Nickel Alloys and Their Strength Profiles
The comparison shifts when moving from pure nickel to specialized nickel-based alloys. Nickel serves as the matrix for superalloys, which are highly engineered materials designed for performance in extreme environments. These alloys incorporate elements like chromium, molybdenum, cobalt, and titanium, which transform the metal’s mechanical properties. The presence of these alloying elements hinders the movement of crystal defects, significantly increasing strength and hardness.
While these alloys are prized for their exceptional corrosion resistance and stability at high temperatures, their specialized compositions also result in remarkable mechanical strength. Alloys such as Inconel and Hastelloy are manufactured to maintain integrity in environments where even high-strength steels would fail due to softening or oxidation. For instance, Inconel 718, a nickel-chromium superalloy, is known for its high-temperature strength and can rival or exceed the strength and hardness of many high-strength steels, particularly when exposed to significant heat.
These specialized nickel alloys achieve their superior properties through mechanisms like solid solution strengthening and precipitation hardening, where tiny, hard particles are dispersed throughout the material’s microstructure. Therefore, while pure nickel is soft, its engineered alloys can be significantly harder and stronger than most common steels, especially when the application demands high performance far above room temperature.