Steel Hardening Vs. Hardenability
Hardness and hardenability are not the same thing.
The hardness of steel is a measure of the steel’s ability to resist deformation, such as abrassive wear or impact indentation or simply when a load is applied to the material. The next few paragraphs will summarise and explain what hardenability is and why this is so important to appreciate and understand. It will re-inforce why Spring Steels are such wonderful products, and why some consideration of the grade of Spring Steel supplied or used is prudent.
In general, components made from carbon and/or alloy steel become stronger as they get harder. As a good rule of thumb, as hardness increases, so too tensile strength increases as well. Thus, unsurprisingly hardness is a critical factor to consider in many engineering applications, because of this relationship between hardness and strength. And the ability to harden a metal is a product of its micro-element composition, a necessary yet insufficient condition for achieving the intended mechanical properties.
We have two mechanical property considerations for steel hardness: core/through hardness and surface/case hardness, but before discussing these let's cover an important associated metalurigical process - Decarburization.
Decarburization
Decarburization is a surface degradation phenomenon in the forging and heat treating of steels. Decarburization may be described as a metallurgical process in which
the surface of steel is depleted of carbon, by heating above the lower critical temperature or by chemical action.
This process can happen as an intended side effect, or can be the unintentional outcome of poor heat treatment. Regardless, surface hardness is affected by decarburization.`The amount of carbon contained in a metal influences its hardness, as we have mentioned. During decarburization, the carbon diffuses from the surface of the metal, and in so doing, weakening the metal. This diffusion increases at higher temperatures. When metals are subjected to high temperatures, and the carbon present on the surface reacts with oxygen or hydrogen it then diffuses.The effect of decarburization not only reduces the tensile strength, but also increases the shear strain below the metal surface. Thus fatigue resistance is decreased while the rate of deformation growth and wear rate will increase.
This process does not have its benefits. Indeed, it is an essential tool in heat treatment, especially where stainless steel and gas nitriding are concerned.
Carburization
Converesely, carburization is the formation of metal carbides in a material as a result of exposure to an atmosphere containing carbon. In some cases, materials are intentionally carburized to impart a high surface hardness and wear resistance. In many instances, QCP dictate a maximum hardness which is set at a value which will produce a tensile strength just below the maximum acceptable tensile strength of the particluar component. An upper limit is required for the hardness as many steels become brittle when a certain hardness is exceeded.
Resistance to carburization generally centers around two mechanisms:
- the establishment of an effective barrier on the surface of the materials that limits the ingress of carbon into the material.
- using alloying elements which are strong carbide formers thus bonding the carbon in the material
In the first instance, the barrier is usually a Cr2O3 layer on the material. This is accomplished through the use of Cr as an alloying element. Secondly alloying additions of Ni also help by reducing carbide diffusion in the material. Si additions have been found to be beneficial in enhancing these alloys through formation of a protective Si2 layer. Intentional carburization to impart a high surface hardness is a well understood heat treatment process and achievble outcome for even themildest of steels
What's the point?
PhD's and years of study are possible in this subject, it can be intimidatingly complex. But the key take aways are these:
- In some applications choice of Steel Grade is vitally important!, lives may well depend on it.
- The final mechanical properties of fabricated steel components are dependent on the core cross-section and not merely its surface properties.
- The micro element composition of alloys and carbon identifying the grade has everything to do with the design outcome intended, but is still insuficient in itself
- The manner in which heat treatment is applied, and the specific heating, quenching, tempering cycles and techniques is critically important.
Through/Core Hardening vs Surface/Case Hardening
By now you are aware that a component can be hardened in such a manner that only the outer surface is sufficiently hard. But as stated previousy, there is a good correlation between core hardness and tensile strength. Without the core reaching the design hardness the component will fail. Most low carbon, ductile steels can be case hardened using the technique descibed above. Don't be fooled!!
Hardenability, finally!
As opposed to hardness, this is the ability of the steel to achieve a hardness value at a particular depth beneath the surface. Refer to our Introduction to Heat Treatment page for more, but essentially this is really the essence of steel heat treatment: The rate at which cooling of sufficiently heated steel takes place has a profound influence on the steel's room temperature microstructures and therefore its mechanical properties. The rate of cooling at the surface of a metal is far quicker than that achieved "deeper" in the material. Mechanical properties (such as hardness), are a product of the atomic micro structure products derived from cooling, these micro structure products are also heavily influenced by the element and chemical composition of the steel.