Hardenability – Testing, Influential Factors
The hardenability of steel or another metal alloy is an important characteristic for many manufacturing processes. Welding in particular is influenced by the hardenability of a material, as it is inversely proportional to the weldability of a material.
Hardness and hardenability – What is the difference
Although they derive from the same word origin, hardness and hardenability have very different meanings. In short, while hardness is a property of steel, hardenability describes a capability of metal.
Hardness
Hardness is a property of steel and plays an important role in many processes. In simple terms, hardness is the resistance that a material offers to a harder material when penetrated. Hardness can be determined using various methods, with Brinell, Rockwell and Vickers being the best-known hardness tests.
Hardness is measured as an index and not in units. The higher the hardness index, the more resistant the surface of the material. This means that operations such as cutting and machining are more difficult to perform.
Hardenability
A steel is only hardenable from a carbon content of > 0.25%. However, this hardenability only refers to hardening through structural transformation. During heat treatment, the material is brought to austenitizing temperature and austenitized. It is then quenched from this temperature using a suitable quenching medium. Hardenability includes hardenability and age-hardenability. Hardenability ultimately refers to the ability of a material to form martensite during quenching. However, there are also other processes to increase hardness, such as BORINOX® for hardening stainless steel, NICKELCOAT® or nitriding.
How to test the hardenability of steel?
One of the most common methods for testing the hardenability of steel is the Jominy quenching test in accordance with DIN EN ISO 642, which was named after the metallurgist Walter E. Jominy, who developed it.
The procedure for the Jominy test is briefly summarized as follows: A normalized, round metal rod of standard-compliant size is austenitized. The holding time is 30 – 35 min. Decarburization must not take place. After heat treatment, the sample is quenched at one end with water at room temperature. The cooling rate varies with the length of the bar. While it is highest at the quenched end, the cooling rate decreases with increasing distance. After cooling, a flat surface is ground on the sample and the hardenability is determined by measuring the hardness (HRC) along the bar. The further the hardness has increased from the quenched end, the higher the hardenability.
The hardenability can be measured using the Jominy tables (H-band). These tables predict the hardness of a product from the surface to the core.
In addition to the Jominy test, there are other methods for determining the hardenability of metals, such as the Grossmann test.
The 4 factors influencing hardenability
The hardenability of steel is influenced by four main factors:
- Grain size
- Austenitisation temperature and time
- Carbon content of the steel
- Alloying elements in the steel.
Grain size limit
The hardenability of steel correlates positively with the grain size. This means that it increases with increasing grain size. Coarse grains have detrimental effects such as increased brittleness and a higher tendency to quenching cracks. This method of influencing hardenability is therefore avoided under normal circumstances.
Austenitisation temperature and time
In order to positively influence the hardenability of steels, the alloying elements must be brought into solution. In addition, the steel must be heated to the correct austenitising temperature and exposed to this temperature for a sufficiently long time so that a homogeneous microstructure is formed. Caution is required with this process, as an excessively long holding time leads to a coarse austenite grain.
Carbon content
The carbon content affects both the hardness and the hardenability of the steel. Carbon has the property of increasing hardenability when its content is up to 0.77 %. Above this level, however, hardenability decreases.
Alloying elements
Alloying elements reduce the diffusion rate of carbon, thereby reducing the critical cooling rate required for martensite formation. This increases the stability of the austenite, which results in an increase in hardenability. However, undissolved inclusions such as nitrides or non-metallic inclusions reduce the hardenability of steel.
