One of the very popular tool steels is known as D2. This steel is considered to be a high-carbon, high-chromium, cold-work tool steel.
The steel contains approximately 2.00% carbon and 5.00% chromium, which will assist in forming large volumes of secondary chromium carbides. This occurs as a result of the precipitation of the carbides during the tempering procedure and gives the steel very high wear-resistance characteristics.
The D-series tool steel was developed originally as a possible alternative group of steels to the high-speed steels that would make tool-making less costly. However, it was not a successful alternative to high-speed steels because it was unable to sustain its hardness during high-speed machining. The cutting edge of the tool simply tempered itself back to the point of premature failure.
While they do not exhibit good corrosion resistance such as might be expected of a martensitic stainless steel, they do offer good surface oxidation resistance. The D series, in general, do not respond well to normalizing because the group has some air-hardening characteristics.
This series of blogs will focus on the cold-work steel of D2 only, particularly its analysis, thermal treatment (austenitize, quench and double temper), troubleshooting and applications.
D2 Steel Analysis
The nominal analysis of D2 cold-work tool steel is generally seen in the analysis region shown below:
- Carbon = 1.40-1.60% (nominal at 1.50%)
- Silicon = 0.50-0.60% (nominal at 0.50%)
- Manganese = 0.50-0.60% (nominal at 0.50%)
- Chromium = 11.00-13.00% (nominal at 12.00%)
- Molybdenum = 0.70-1.20% (nominal at 1.00%)
- Nickel = 0.30% max (nominal at 0.30%)
- Vanadium = 1.10% max (nominal at 1.10%)
D2 is generally forged at a controlled low temperature and is not allowed to exceed 2100°F (870°C). Conversely, the steel should not be forged below 1600°F (870°C). Care should be given to the slow cooling of the forged D2 as it will tend to air harden on cooling, followed by annealing. The steel should not be normalized because of its air-hardening characteristics.
Care should also be given to the annealing temperature selection. Do not exceed a temperature range for annealing higher than 1650°F (900°C). In addition to the annealing temperature selection, care must be taken on the cooling rate of the steel. This is because of the high carbon and the high chrome, which will assist in the formation of martensite from austenite if the cooling rate is too fast.
Next time, we will discuss thermal processing.