Vacuum Austenitize & Gas Quenching – DH21 should first be preheat at a
rate of less than 220° C (400° F) per hour and held at 800° C (1,475° F) to 850°
C (1,560° F) until the tools are uniformly heated from surface to core to within
60° C (100° F) and then increased to 1,030° C ±10° C (1,885° F±18° F) to
austenitize otherwise known as soaking the tool.
When distortion in heating is concerned for large or complicated geometry
parts, a double pre-heating is recommended: The first pre-heat is at 550° C
(1,020° F) to 650° C (1,200° F) and the second at 800° C (1,470° F) to 850° C
(1,560° F). The temperature should be allowed to equalize from the surface to
the center of the part in each step before proceeding up to the austenitizing
temperature.
Once the part reaches 1030° C (1,885° F) and is fully equalized in
temperature, the holding time is 30 minutes. In cases where it is difficult to
measure the temperature of the center of the part, the holding time from when
the atmosphere reaches to 1030° C (1,885° F) should be as shown in the Table
considering the delay of the rise in the center temperature. As the time
required for equalization of the temperature is ruled by furnace size, loading
weight, heating method and so on, holding time is allowed to justify by the
recommendation in the Table.
After austenitizing, the part will need to be quenched in inert high pressure
gas (generally in nitrogen) to rapidly cool with at least 5 times atmosphere
pressure (5 bar) or high velocity equivalent, however 10 bar pressure is
recommended. The cooling rate at the center of the part should be at least 5° C
(9° F) per minute in the temperature range of 500° C (930° F) to 200° C (390° F)
to obtaining suitable toughness.
An interrupted quench is preformed by temporarily halting the cooling process
between 450° C and 400° C (840° F and 750° F) to allow the temperature
throughout the part to equalize within 110° C (200° F) before continuing the
rapid quench. This interrupted quench is recommended to reduce the distortion in
quenching. The part should be immediately tempered once it has reached 65° C to
(150° F) to 45° C (120° F).
Using Thermocouples - It is important that efforts be taken to measure
temperatures at the center of the part throughout the heat treat process.
Inserting a thermocouple in the center of the part insures that the entire part
has reached the desired temperature before proceeding to the step in the heat
treat process. The initial design should allow for the insertion of a
thermocouple when possible. Water lines often make excellent locations for
thermocouples. Be sure to pack thermocouple holes with a refractory fiber
material to help prevent false readings. If the geometry of the part does not
accommodate such a feature, it is recommended that a dummy part of similar shape
and mass with a thermocouple mounted in the center be used as a temperature
control.
There are two ways to use this control information. The first way is to run
the part side by side with the dummy part using it to measure the temperature at
the center of the part throughout the heat treat process. Realizing that this
may not be practical when heat treating large parts, the second way is to cycle
the dummy part through the heat treating process while documenting the times it
takes for the tool to reach the desired temperatures and use that data to run
the actual part. Thermocouples are useful in both the hardening and tempering
processes.
Other Quenching Methods - As an alternative to vacuum furnace with gas
quench, an atmosphere controlled furnace can be used for austenitizing. The
holding time is recommended to be a little longer than that of a vacuum furnace.
In this case salt bath or oil bath is used as quenching media to obtain
quenching rate higher than 3° C per minute in the range of 500° (930° F) to 200°
C (390° F).
Generally, the quench rate should be as rapid as possible to obtain optimum
balance of strength and toughness. Quenching too fast, however, may cause
distortion and cracking. In controlling the cooling rate by vacuum furnace
quenching, it must be considered not only gas pressure, but also gas flow, its
rate, heat exchanger efficiency, gas running path and so on.
Tempering – DH21 requires at least two tempers. however triple
tempering is recommended for large parts. Preferably, tempering is performed in
air convection or atmosphere type furnace however can also be vacuum tempered
when aided by the use of thermocouples and special care is taken to monitor
temperatures.
The first tempering temperature is 550° C (1,022° F) to 650° C (1,200° F).
580° C (1,075° F) to 600° C (1,110° F) is generally applied for die casting
molds. Second tempering is carried out at 550° C (1,022° F) to 650° C (1,200° F)
depending on the required hardness. If the resulting hardness is higher than
specified, hardness, it can be decreased by a third tempering. When the hardness
meets the specification by second tempering, the third tempering is done at 30°
C (54° F) to 50° C (90° F) lower than that of the second.
Holding time after atmosphere temperature reaches aimed temperature is 60
minute per 25mm (1 inch) in thickness in cross section with a minimum tempering
time of 2 hours.
Note: As with any tool steel, actual heat treat results may vary depending
on the circumstances of each application. Therefore the information pertaining
to this document is given as a starting point and may require modification based
on the size of a given load, fixturing, and equipment capabilities therefore
some adjustments in the process may need to be developed and fine tuned base on
you combination of circumstances to achieve optimum. Empirical data from heat
treating other hot work grades of similar size in specific equipment can be
helpful in developing optimum heat treat process for DH21.
Annealing and re-hardening – If re-hardening is needed, it is
necessary to first re-anneal the part.
