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Cold-work tool steels which include D2, D3, D4, D5, and D7 steels are high-carbon, high-chromium steels. Apart from D3 steel all group D steels have 1% Mo and are air hardened. Type D3 steel is oil-quenched; though small sections can be gas quenched after austenitization using vacuum. As a result, tools made with type D3 steel tend to be brittle during hardening. Type D2 steel is the most commonly used steel among the group D steels. The D3 steels contain 1.5 to 2.35% of carbon and 12% of chromium.
AISI D3 Cold Work Steel has high strength, good hardenability and wear resistance. D3 is suitable for applications such as complex blanking and forming tools for long runs and for hard and abrasive materials.
The following table shows the chemical composition of D3 steel:
Izod impact unnotched
Hardness in the range 58-64 HRC.
Heat treatment temperatures, including the rate of heating, cooling, and soaking times will vary due to factors such as the shape and size of each steel component. Other considerations during the heat treatment process include the type of furnace, quenching medium, and workpiece transfer facilities.
It is preferable to heat the tools in a controlled atmosphere. If this is not possible, pack hardening is recommended. A reducing atmosphere is desirable. Preheat the component to 750-800°C. and allow soaking at this temperature. Raise to the hardening temperature of 950-980°C. Soak thoroughly at the temperature for thirty minutes per 25mm of the ruling section, then cool or quench accordingly.
Double tempering is recommended. Tempering should be done with the least possible delay after hardening, preferably when the tools still hand warm. Select a suitable tempering temperature, bearing in mind the service requirements. Heat slowly and uniformly. When the component has reached the desired temperature, soak for at least one hour per 25mm of thickness. The second tempering should be a repetition of the first.
Physical properties of D3 tool steels are given below:
7.7 x 1000 kg/m3
The following table shows the thermal properties of D3 steels.
12 x 10-6/ºC
Pre-heat at 900-950°C then raise temperature to 1050-1100°C. Soak until uniformly heated. D3 tool steel is relatively hard at elevated temperatures. Therefore, initial hammer blows must be light and the temperature must not be allowed to fall below 1020°C until the metal begins to flow. Final forging should not be done below 900°C.
When tools are heavily machined, ground, or otherwise subjected to cold work, the relief of internal strains is advisable before hardening to minimize the possibility of distortion. Stress relieving should be done after rough machining. To stress relief, heat the steel component to 600-650°C. Soak well and cool in the furnace or in the air. The tools may then be finish machined before hardening.
After normalizing hardness is 55 HRC given on the Rockwell testing machine. It shows after normalizing the specimen becomes harder than annealing specimen. This is due to the formation of a greater no. of Un-dissolved carbide particles so the specimen becomes brittle.
D3 is supplied in the annealed and machinable condition. Re-annealing will only be necessary if the steel has been forged or hardened by the toolmaker. To anneal, heat slowly and uniformly to 900°C. Soak for three to four hours and allow to cool in the furnace to room temperature. Re-heat to 800-1040°C and again soak for three to four hours. Allow cooling in the furnace to room temperature.
45-50% of a 1% carbon steel
Due to the high risk of crack formation welding of D3 tool steel should be avoided, if possible. When however welding is essential, the following serves as a guide:
A. Welding of soft annealed D3 tool steel
· Preheat to 300 – 500°C
· Weld at 300-500°C
· Immediately stress relieve
Electrode: Cr-Mo alloy electrode for welding structural steel.
B. Welding in connection with hardening of soft annealed D3 tool steel.
· Heat to austenitizing temperature.
· Cool to approx. 500°C
· Weld at approx. 500°C
· Cool to approx. 100°C in the same way as at normal hardening, then temper to desired hardness. Double tempering will reduce the risk of crack formation. Electrode: Hard-facing electrode.
C. Repair welding of D3 tool steel is hardened and tempered in condition. · Preheat to the tempering temperature (min. 200°C) previously used. · Weld at tempering temperature. · Heat immediately to tempering temperature, but max. 300°C. Soaking time 3 hours. · Cool in the air to approx. 80°C. · Heat to tempering temperature. Soaking time 2 hours. Electrode: Hard-facing electrode.