Vacuum heat treatment

1. Preheating and stepped heating to 1030 °C, so the whole die austenitizes evenly.
2. Slowing the heating rate at two points, so the surface and core temperatures can even out without unwanted thermal stress.
3. Quenching in nitrogen gas at a controlled pressure, for the right cooling rate.
4. Three tempers, tracked with contact thermocouples, to reduce residual stress and stabilize the mechanical properties.

The heat treatment followed ASTM B637.

Solution treatment: the parts were heated to a high temperature in the vacuum furnace to dissolve the γ′ phase into the matrix, giving a uniform structure before ageing.

Ageing: the parts were then held for a long time at an intermediate temperature, so that γ′ precipitates as fine, evenly dispersed particles. These precipitates give the alloy its hardness, creep resistance and stability at high temperature.

Dies that are thin compared with their length and width tend to distort. For accurate dimensions with minimal distortion, such a die is best stress relieved at 600–650 °C after rough machining; this die was stress relieved too.

To control distortion and reduce the risk of cracking, the die was heated in steps, with holds at 650 °C and 850 °C; the hold at 650 °C was about twice the usual length. The austenitizing temperature was chosen at the most suitable point of the range, and the die was held there for 30 minutes, with its surface and core temperatures monitored throughout.

The next key point was controlling the quench rate around the martensite-start temperature (Ms), which the Schmetz vacuum furnace can monitor.

The die was tempered twice, for 3 hours each, at 525 °C and 600 °C, and delivered at 48–50 HRC.

These dies, in 1.2344 hot-work and 1.2379 cold-work tool steel, were heat treated in the Schmetz vacuum furnace. The forming die is extremely sensitive to dimensional change, so flexible thermocouples were placed on the parts with great care, to record the real part temperature during heating and cooling and confirm that the cycle run matched the cycle designed.

Heating was done in steps, with holds at 600 °C and 850 °C, so the microstructure changed gradually and evenly and thermal stress stayed to a minimum. After the hold at the austenitizing temperature of 1030 °C, the parts were quenched with nitrogen gas.

The Schmetz furnace can blow the cooling gas onto the load from two directions, giving a controlled quench in the chosen direction. For tighter dimensional control, the quench intensity was varied at the critical temperatures, following the CCT diagrams of these steels.

After hardening, the hot-work dies were tempered at 525 °C, and the cold-work die was given a mild temper to prepare it for sub-zero treatment at −80 °C. In all, the hot-work dies were tempered three times, and the cold-work die was tempered three times and sub-zero treated.

1.2379 is a high-chromium cold-work tool steel with high hardness and excellent wear resistance after heat treatment. The steps in the vacuum furnace were:

1. Preheating
2. Austenitizing in vacuum, in the usual range of 1020–1040 °C
3. Fast quenching with nitrogen gas, to form martensite
4. Sub-zero (cryogenic) treatment to transform the retained austenite
5. Tempering in several stages (usually three) to reach the specified hardness

These dies were hardened in the vacuum furnace following standard heat-treatment practice. They were heated in steps, with holds at 650 °C and 850 °C, up to the austenitizing temperature of 1030 °C, and after a sufficient hold they were quenched with nitrogen gas, so that the transformation to martensite was as complete as possible.

The dies were then tempered twice, at 525 °C and 610 °C, following the designed cycle. Thermocouples tracked the surface and core temperatures, so the cycle could be adjusted if needed.

We had also supplied the steel for these dies.

Stepped heating with holds at 650 °C and 850 °C, a hold at the austenitizing temperature of 1030 °C, a nitrogen gas quench and two tempers at 525 °C and 610 °C, with thermocouples tracking the surface and core temperatures throughout.

We had also supplied the steel for these dies.

These cylinders were hardened in the vacuum furnace together with other parts in the same steel, austenitized at about 1030 °C and then tempered. Thermocouples attached to the parts tracked their real temperature through hardening and tempering.