Steel is one of the most essential and iconic metals on earth. From a combination of iron and carbon arose a robust, versatile and vastly used alloy. From buildings, infrastructure, water vessels, automobiles, machinery, appliances to simple utensils like forks and spoons, its applications seem to have no bounds. This is due to the numerous desirable properties which steel has. One of these properties is hardness, the ability of a material to resist deformation induced by indentation, impact, or abrasion. However, the natural hardness of steel is not always sufficient for specific engineering applications, such as load-bearing structures and engine parts. This is why methods have been developed to increase the hardness alongside other properties of steel significantly. These methods are known as steel hardening.
Steel hardening is usually carried out on finished products and not on raw materials. In CNC machining, steel hardening is a post-machining process carried out on machined parts. This is done this way for a number of reasons. First, it is not economical to harden a whole block of steel, since a large percentage of it will be removed in the machining process. Also, hardened steel is far more difficult to machine, as the hardness of the workpiece makes tool penetration more difficult.
Internal structures of steel and their hardness
Not all steels we see have the same composition. Precisely, there are different steel compositions for various purposes. The difference in steel boils down to their internal structures. As the need for stronger load-bearing metals increased, it became necessary to harden steel. Steel in its most basic form has relatively little strength and hardness. However, a modification of its internal structures, yields impressive results in its strength and hardness. Steel hardening simply involves processes designed to favour the formation of a particular internal structure over another. The internal structures of steel include:
Martensite
This is the hardest form of steel internal crystalline structure. The rapid cooling of austenitic iron form martensite. Due to its fast cooling rate, carbon is trapped in a solid solution causing the part to harden. It is extremely hard and brittle. Martensite has a needlelike acicular microstructure which appears as lenticular plates or platelets which divide and subdivide the grains of the parent phase, always touching but never crossing one another. This structure occurs in a whole lot of alloy systems, including Fe-C, Fe-Ni-C.
Austenite
Austenite is the next hardest steel internal structure after martensite. It refers to iron alloys in which the iron is gamma-iron. It usually occurs below 1500ºC and above 723ºC.
Pearlite
Pearlite is different from martensite, as the pearlite structure forms from slow cooling. It is a lamellar arrangement of ferrite and cementite. At 723ºC the gamma-iron transforms from its FCC structure to alpha-iron, forcing iron carbide (cementite) out of the solution.
Methods of Steel Hardening
There are various methods of carrying out steel hardening. These methods may be thermal, mechanical, chemical, or a combination of two or more of these. Thermal hardening processes are the most common steel hardening methods. They typically involve three primary stages, which are heating steel, holding it at a particular temperature, and cooling. The first stage generally involves heating the metal to a very high temperature enough to induce structural changes internally. This also makes it easier to work on the metal like changing its shape. The various methods of steel hardening are:
Cold Working
Cold working typically alters the properties of steel or metals. This method of steel hardening is simply the deformation of a metal at a temperature below its melting point. Properties like yield strength, tensile strength and hardness increase, while plasticity and the ability of the material to deform decrease. Strain hardening, which results from the accumulation and entangling of dislocations during plastic deformation is an essential mode of strengthening elements. Though about 90% of energy during cold working is dissipated as heat, the remainder is stored in the crystal lattice, thereby increasing its internal energy.
Solid Solution Alloying Hardening
Solution hardening is the addition of an alloying element to the base metal to create a solid solution. Upon solidification, the metal hardens due to the presence of the alloy atoms in the crystal lattice of the base metal. The size difference between the atoms of the solute and the solvent affect the effectiveness of solid-solutioning. If the solute atom is larger than the solvent atom, compressive strain fields ensue. On the other hand, if the solvent atom is larger than the atoms of the solute, tensile strain fields occur. Solute atoms which distort the lattice into a tetragonal structure cause rapid hardening. An obvious example is the effect of cementite in steel.
Quenching and Tempering
In quenching, also referred to as martensitic transformation, steel is heated above the critical temperature into the austenite range, held at this temperature, and then rapidly cooled or more often, quenched in water, oil or molten salt. For hypoeutectoid steels, the temperature for heating is 30-50ºC above the limit of austenite solubility line. For hypereutectoid steels, the temperature is above the eutectoid temperature. Quenching brings about martensitic transformation, which considerably hardens the steel. The hardened steel is, however, very brittle. Therefore, it is necessary to carry out tempering to relieve internal stresses and reduce brittleness. Maximum hardness is obtainable when the cooling rate in quenching is rapid enough to ensure full martensite transformation
Surface (Case) Hardening
As the name implies, case hardening creates a hard surface, necessary to resist wear in applications such as crankshafts, bearings and the like. This method of steel hardening, generally involves one of three approaches:
Induction and Flame Hardening
This is a differential heat treatment of the surface. The surface is quickly heated to prevent the centre of the material from being affected. The material is then undergoes much more rapid quenching. This way, a high level of martensite develops on the surface.
Diffusion Hardening (Nitriding)
This involves a compositional alteration of the surface zone. Fine particles are dispersed by allowing selected gases to react with and diffuse into the steel. In this process, steel is heat-treated to obtain a tempered martensitic structure. It is then exposed to an atmosphere of ammonia at around 550ºC for 12-36 hours. Small alloying elements like Al or Crenhance the formation of a fine dispersion of nitrides, which remarkably increase the surface hardness and wear resistance. This composition of nitrides is much more superior to martensite in respect of hardness.
Carburizing
This involves exposing the steel to a carbonaceous atmosphere at a high temperature. The carbonaceous atmosphere can be generated from high-quality coal or disassociated natural gas. The carbon atoms diffuse into the subsurface of the metal resulting in a high carbon case which upon subsequent quenching creates a hard wear-resistant martensitic surface.
Steel Hardness Testing
Hardness does not have one particular unit of measurement. Rather, it is described using index numbers. There are various hardness test and the index used to describe the hardness of a material, depends on the test used. Some common hardness tests are:
Brinell Hardness Test
In this test, a steel ball of known diameter is applied as the load on the surface of the material. The Brinell Hardness Number (BHN) is then calculated using the formula in the table below. The diameter of the resulting impression is measured; together with the diameter of the steel ball, the BHN is calculated.
Vickers Hardness Test
In the Vickers hardness test, a square-based diamond pyramid is the load. This load is applied on the surface of the material for about 30 seconds. The area of the pyramidal impression is calculated and is then used to calculate the hardness of the metal.
Knoop microhardness test
This hardness test is specifically for thin sheets or very brittle material. A pyramidal diamond point creates a very small indentation on the material. Next, the indentation made is studied using a microscope and used to calculate the hardness of the material.
Rockwell Hardness Test
The Rockwell hardness was developed to measure the difference in hardness of steel before and after heat treatment. The penetrator can either be a steel ball or a diamond spheroconical penetrator. The hardness is measured by determining the depth of penetration into the material. Two loads are normally applied. A minor load to cause an initial impression and a major load to cause the main penetration.
| Test | Indenter |
| Brinell | 10mm sphere of steel or tungsten carbide |
| Vickers | Diamond pyramid |
| Knoop microhardness | Diamond pyramid |
| Rockwell | Diamond cone |
Steel grades that can be hardened
The American Iron and Steel Institute (AISI) categorize steel into four main groups:
- Carbon steel
- Alloy steels
- Stainless steels
- Tool steels
The basic elements for steel are iron and carbon. However, the varying amounts of carbon and other alloying elements determine the properties of each grade. The carbon content of any steel determines its hardenability as well as its maximum attainable hardness. This is especially true for quenching, as carbon encourages martensite formation.
Carbon steel (UNS G10050-G15900, DIN 1.0xx)
Carbon steels are alloys of iron, containing up to 2% carbon. They often contain trace amounts of alloying elements that enhance certain properties. Based on the actual amount of carbon contained, carbon steel can further be classified as low carbon steel, medium carbon steel, and high carbon steel.
Low carbon steel
Also known as mild steel, this contains 0.08 – 0.35% carbon. Because of their low carbon content, low carbon steels do not undergo steel hardening by quenching. However, they can be hardened by case hardening.
Medium carbon steel
This steels contain 0.35% – 0.5% carbon. They are stronger than low carbon steels, but are more difficult to work with. Medium carbon steels readily undergo hardening through quenching. When alloyed with traces of manganese, their hardenability increases. Medium carbon steels are also case hardened for applications where wear resistance is critical, such as in crankshafts.
High carbon steels
High carbon steels contain above 0.5% carbon. These kinds of steels are very hardenable due to the high carbon content. They are typically hardened through quenching. However, this makes them quite brittle, hence, tempering is required.
Alloy steels (UNS G13300-G98500, DIN 1.2xxx)
In addition to carbon content, chemical composition is another factor that affects the hardenability of steels. Alloy steels contain varying amounts of copper, nickel, manganese, boron, and vanadium. These steels are very hardenable through quenching. This is because the alloying elements delay austenite decomposition, thus forming martensite readily in alloy steels. Solid solution hardening is also an effective, common way of hardening alloy steels.
Stainless steels (UNS S00001-S99999, DIN 1.4xxx)
Stainless steels are steels that contain 10 to 20% chromium as the main alloying element. They are very resistant to corrosion and erosion. Based on their structure and composition, stainless steels can be classified as:
Austenitic
Austenitic steels typically contain iron, 18% chromium, 8% nickel, and less than 0.8% carbon. They are the most widely used type of stainless steels. Austenitic steels are non-magnetic and non-heat treatable. However, they readily undergo hardening through cold working.
Ferritic
These steels typically contain less than 0.1% carbon, 12-17% chromium, and trace quantities of nickel. Ferritic steels are magnetic but cannot be hardened through heat treatment. Cold working is an effective method of hardening them.
Martensitic
Due to their internal structures, martensitic steels are quite hard. These steels contain up to 1.2% carbon in addition to 12-17% chromium. Because of their relatively high carbon content, martensitic steels readily undergo hardening by heat treatment.
Duplex
Duplex steels have both ferritic and austenitic microstructures. These steels are hardened through heat treatment or surface hardening.
Precipitation hardening
Precipitation hardening steels are stainless steels containing chromium, nickel and other alloying elements such as copper, aluminium and titanium. These alloying elements allow the Stainless Steel to undergo hardening by solution and ageing heat treatment. They may be austenitic or martensitic.
Tool steels ( UNS T00001-T99999; DIN 1.23xx, 1.27xx, 1.25xx)
As the name implies, tool steels are regularly employed in the manufacture of tools such as cutting and drilling tools. They typically contain tungsten, cobalt, vanadium, and molybdenum. These tools can be hardened through cold working and also through heat treatment such as quenching.
Types of steel and their most suitable method of hardening
| Type of steel | Quenching or ageing | Case hardening | Solution hardening | Cold working |
| Low carbon steel | ✔ | |||
| Medium carbon steel | ✔ | ✔ | ||
| High carbon steel | ✔ | |||
| Austenitic steel | ✔ | |||
| Ferritic steel | ✔ | |||
| Martensitic steel | ✔ | |||
| Duplex steel | ✔ | ✔ | ||
| Precipitation hardening steel | ✔ | |||
| Alloy steel | ✔ | ✔ | ||
| Tool steel | ✔ | ✔ |