The alloying element of tungsten is a very strong complex carbide former. As the tungsten alloy percentage is increased, however, the hardenability will begin to decrease. The nominal maximum percentage concentration with tungsten would be in the region of 8-10%. After that, the hardenability will begin to decrease dramatically.
The precipitation of the carbides will occur at the multiple tempering procedure and will precipitate out as finely dispersed complex carbides (dependent of course on the tempering temperature selected).
In addition, the tempering procedures to produce the finely dispersed carbides will initiate (again, temperature dependent) to produce a fine grain (grain size 8+).
Titanium is generally present only in small quantities in secondary hardening tool steels. This is because it is perhaps one of the strongest carbide-forming elements. It will assist in the retention of austenite in high chrome tool steels, which can be problematic as retained austenite is unstable insomuch as it will progressively transform into fresh untempered martensite. If this occurrence happens, then there will be an increase in the hardness value along with a volumetric change due to the austenite transforming its lattice structure from face-centered cubic (FCC) into body-centered tetragonal (BCT).
In order for the complex carbides to form, it is necessary to ensure full solutionizing of the alloying elements. Therefore, it is necessary that most of the tool steels have a high austenitizing temperature around the 1000°C (1830°F) region. This includes high-speed steels (T and M Series), dimensionally stable tool steels (D Series) and high-temperature operating tool steels (hot work; H Series).
It will also be seen that all of the highly alloyed secondary hardening steels have a high Ms (martensite start) temperature on the appropriate Time-Temperature-Transformation diagram when quenched after austenitizing.
It must be strongly emphasized at this point that the tempering operation (multiple tempers are absolutely critical to the performance of the high-alloyed tool steel) must be carried out appropriately and with good temperature uniformity within the tempering furnace.
What is happening during the multiple tempering procedure is rather complex. To simplify, what is occurring is that steel is heated during the tempering procedure as follows:
- As the temperature is increased, there is a slight increase in the as quenched hardness value.
- This is followed by a decline in hardness as the freshly formed martensite begins to transform from it body-centered-tetragonal lattice (BCT) into a body-centered-cubic lattice (BCC).
- This transformation is accompanied by a reduction in the as-quenched hardness value.
- As the tempering temperature increases, some cementite forms and the hardness decreases. Precipitation occurs from 400-570°C (750-1060°F) with different carbides forming at specific temperatures.
- This is what is known as secondary hardening. Typically, multiple tempers are required to ensure full carbide precipitation has occurred. It is generally considered to require at least two tempering procedures. This is because whatever retained austenite (untransformed austenite) might be present is being decomposed during the first temper into fresh martensite, and the subsequent tempers are tempering of the newly formed fresh martensite for the previous retained austenite.
- It is generally considered that with each successive temper given, approximately 50% of the retained austenite will transform into fresh untempered martensite, which requires tempering. Therefore, a minimum of two tempers will be necessary
It is hoped that the above is both understandable and practical.