Mr. David Pye (View Profile)
Pye Metallurgical International Consulting.
D2 Tool Steel
The popular tool steel which is considered to be a High Carbon High Chromium Cold Work tool steel is known by the international designation of D2 and is a very popular tool steel that can be used for many applications.
Courtesy HSS, New Delhi, India
The steel is contains approximately 2.00% carbon and 5.00% chromium which will assist in the forming large volumes of secondary chromium carbides from heat treatment. 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 tool steel was originally developed as a probable alternative to the High Speed Steels which would make tool making less costly.
The net result was that it was not a successful alternative to high speed steels as it was unable to sustain its hardness during high speed machining and high temperature operations. 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 say a martensitic stainless steel, (because of the moderately high chromium content) they do offer good surface oxidation resistance and is used extensively for the manufacture of specialty knife making.
The D series in general do not respond well to normalizing simply because there is some air hardening characteristics of the steel group. Therefore because of the high temperature required for the normalize procedure air hardening is likely to occur as a result of the air cool required in normalizing
This presentation is focusing on the cold work steel of D2:
- Its analysis
- Its thermal treatment (Austenitize, quench and double temper)
- Trouble shooting
D2 Steel Analysis
The nominal analysis of D2 Cold Work Tool steel is seen in the analysis as shown below; (The nominal analysis %values are shown)
Carbon = 1.40% to 1.60% (Nominal at 1.50%)
Silicon = 0.50 % to 0.60% (Nominal at 0.50%)
Manganese = 0.50% to 0.60% (Nominal at 0.50%)
Chromium = 11.00% to 13.00% % (Nominal at 12.00%)
Molybdenum = 0.70$ to3 1.20% % (Nominal at 1.00%)
Nickel = 0.30% Max (Nominal at 0.30%)
Vanadium = 1.1 0% 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 to this, the steel should not be forged below 1600˚F (870˚C).
If the steel is forged at too high a recommended forging temperature, then the following can occur;
- Grain growth
- Excessive surface oxide formation which can result in forging die premature failure. The surface oxide will also act as an insulator.
Care should be given to the slow cooling of the forged D2 as it will tend to air harden on cooling, and followed by annealing. The steel should not be normalized. (Because of its air-hardening characteristic’s)
Care should 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 from of martensite from austenite if the cooling rate is too fast.
D2 Thermal Heat Treatment
The high carbon content of the steel, gives the steel a strong tendency for decarburization at elevated temperatures unless the steel is treated under vacuum or alternative surface protection such as protective atmospheres. Decarburization can be reduced by using any of the methods listed below;
- Wrap in stainless steel foil with a smear of oil on the inside face of the foil so as to burn up oxygen that is entrapped inside of the foil. This will reduce the risk of surface oxidation and reduce the risk of decarburization
- Atmosphere heat treatment. Care must be given to create and maintain equilibrium conditions with the carbo potential of the furnace atmosphere and the carbon content of the steel.
- Salt bath heat treatment which (if neutral) will protect the work surface from both oxidation and decarburization.
- Vacuum heat treatment
In general, the D2 steel does have good oxidation resistance due to the high chromium (5%) content. The steel will polish very well after austenitizing which is often a requirement in the die-casting die manufacturing.
It is mandatory that the steel be very carefully pre-heated. (The steel does not react favorably to thermal shock) This steel has a low heat conductivity value and a poor ability to absorb heat.
If the steel is heated too quickly, then there is a very strong likelihood that the steel could possibly crack during the heat up phase. Therefore ramp and soak is very necessary for pre-heating. (This also applies to heat up for forging). The ramp up time could be adjusted as required. More complex D2 die forms would be pre-heated very slowly.
A suggested procedure is;
- Ramp up to say 500˚ (260˚C), and hold for equalization.
- Ramp to 1200˚F (650˚C, equalize throughout the cross section.
- Ramp to the austenitize temperature at 1790˚F (975˚C) to 1850˚F (1010˚C). Hold at the temperature and soak for 1 minute per 1 mm of maximum cross sectional area, and follow with the quench.
- Do not over soak at the austenitize temperature as grain growth will occur.
- Do not go to too low an austenitizing temperature, as size growth will occur. In addition to which, insufficient carbides will dissolve, resulting in perhaps, a lower hardness value. Conversely, do not select too high an austenitizing temperature, otherwise there is a risk of too many carbides dissolving, which can lead to retained austenite.
The above procedure is the most common practice of austenitizing D2 steel.
These steels (The D series) are very susceptible to retained austenite conditions. This is due to the high carbon and high chromium chemistry of the steel.
Above is shown a suggested Heat Treatment procedure.
Below is shown the Time, Temperature, Transformation cooling curve (Courtesy ASM Heat Treaters Guide). The cooling curve will show the time that nit will take for D2 tool steel on rapid cooling (quench) to reach the Martensite Start line based on the correct and appropriate austenitize temperature selection. It should be further noted (and strongly emphasized) that the TTT curve for D2 (and any other tool steel) is based on a 25 mm maximum cross sectional area. The hardness results and other mechanical properties will react completely differently with great maximum cross sectional areas
It is absolutely necessary to temper the D2 steel as soon after the quench procedure as is practical. Do not let the steel grow cold after quenching, and delaying the tempering procedure.
Be sure that you have a furnace to commence the tempering procedure after the quench operation.
There will be a very serious risk of cracking, particularly if retained austenite is present. The retained austenite will begin its decomposition and transformation to un-tempered fresh martensite. This will exhibit an increase in hardness, as well as a volumetric change (dimensional size change). In other words the steel will, most likely grow in size.
The tempering temperature selection will depend on what the austenitizing temperature was in relation to the as quenched hardness. The higher the austenitizing temperature selection and the longer the time, the more carbon, chromium and remaining alloys will have been taken into solution. This means that more carbides are available at elevated tempering temperatures. The tempering procedure must always be at least two times. The purpose of this is, that if there is any retained austenite, then at least 50% of that austenite will de decomposed at each subsequent temper. This also helps the steel for dimensional stability by the decomposition of the retained austenite
Correctly tempered D2 tool steel microstructure (Courtesy ASM International) Note the globular carbide formation.
D2 trouble shooting
The problem of retained austenite will usually be ‘observed’ by the as quenched hardness test. However the following ‘tree’ shows alternatives methods of determination. Retained austenite is usually indicated by a lower as quenched hardness value. The as quenched hardness value is generally around 63HRC. Thus if the as quenched hardness value is say 56 to 57HRC, then there is a strong possibility that retained austenite is present (it is not though a guarantee that it is present).
Courtesy Pye Metallurgical Consulting.
Probable causes Retained Austenite is;
(Courtesy Pye Metallurgical Consulting)
Probable causes of cracking are:
(Courtesy Pye Metallurgical Consulting)
Overheated D2 tool steel. Note the large carbide formation
(Courtesy British Blades)
In conclusion, it can be seen that to treat the D2 tool steel is very critical heat treatment procedure.
The very critical of the procedure is of course the tempering procedure.
Tempering in the operation that will give the steel its appropriate performance characteristics when the tooling is in operation. It is a mandatory requirement that the D2 steel tempered minimum of 2 times. (If 3 times champ me given, then it is all to the benefit of the performance of the tool is being treated).
One of the primary reasons for the tempering procedure is that if any retained austenite is present, the tempering assessed in the decomposition of that retained austenite. It is a known fact that when tempering, each tempering procedure will decompose any retained austenite by approximately 50%. Therefore the more steel can be tempered, the more the retained austenite will be decomposed.
The D2 tool steel is very sensitive to thermal shock (which can cause cracking or distortion) nor does it might to rapid a shakeup rate to its thermal processing temperature (austenitizing).
It is therefore mandatory that the steel be preheated prior to raising the process temperature, for example, to the austenitizing temperature.