Vacuum Furnace Hardening of Very Large H13 Dies
The use of vacuum furnaces to heat treat dies for the die-casting industry in the 1980s and 1990s had primary objectives of reducing distortion and obtaining a nice surface finish with no post cleaning combined with easy process control. Minimizing distortion saved money on post machining, especially on large H13 hot-work tool steel die inserts. The downside was that low distortion was mostly realized through a very slow gas quench (<30 F, or 17 C/min), which consequently resulted in the precipitation of grain boundary carbides, leading to shorter die life due to reduced impact toughness (Fig. 1).
The North American Die Casting Association (NADCA) and many leading companies in the die casting industry issued several papers with the recommendation of a minimum surface quench speed of 50 F/min (28 C/min). Through the selection of high quality die material and the best heat treating sources together with the creation of heat treat specifications like GM Powertrain's (GMPT) DC-9999-1 in 1995 and Ford's AMTD DC2010 in 1999, the North American automotive industry saved millions of dollars in die costs.
Cooling speed and distortion was precisely controlled on die inserts by better placement of thermocouples and an interruption in quenching (isothermal hold). However, for very large dies, quench rates still fell below 50 F/min. In addition, distortion was high and there was concern of die cracking due to excessive temperature differences between the core and the surface that occur during quenching. Cooling holes often had to be machined into the hardened part and material stock was sometimes not sufficient.
Advancements in vacuum furnace technology
To meet industry demand for fast, uniform quenching of large dies, Ipsen developed its 12-bar TurboTreater with a 360-degree nozzle field and convection-assisted heating. The company performed many tests on the surface of a 16 in. (406 mm) H13 cube (Fig. 2) in accordance to the GMPT DC-9999-1 specification, resulting in an average quenching speed of 140 F/min (81 C/min) in furnaces with work zones of 36 in. W x 36 in. H x 49 in. D (910 x 910 x 1,220 mm). The 12-bar TurboTreater meets the North American die heat treatment industry preference for furnaces that can rapidly and uniformly quench die inserts up to 10,000 lb (4,545 kg), such as those shown in Fig. 3, typically using a round hot zone with convection heating, and no actuated parts in the hot zone like bungs or dampers for reliability and maintenance purposes.
Next generation vacuum furnace
To minimize distortion further, Ipsen developed its Super TurboTreater, which can handle larger load sizes, and includes the latest technologies for superior processing of dies, tools and parts with complicated geometry. The furnace offers up to 15 bar quenching pressure, directional controlled cooling and isothermal hold, reliable water-cooled motor (Fig. 4) with LCP (low current power) start and convection-assisted heating using Flapper Nozzles(tm).
The cooling gas flow is programmable for various geometric loads and part sizes (Fig. 5), providing both better control to achieve higher part hardness and a faster quench, while minimizing distortion. The combination of high quenching pressure, directional cooling, and water-cooled motor allows hardening of heavy loads with significant gains in part quality. LCP-start reduces motor start up time and cooling time, while greatly reducing energy costs during the peak demand of starting the motor.
The first SuperTurbo vacuum furnace was installed in spring of 2002 at a large commercial heat treater in Germany, who needed to improve efficiencies in cycle time and energy costs, as well as to meet the highest quality demands on heat treating large dies at a competitive market price. The heat treater exceeded GMPT specification requirements for metallurgical properties and part distortion for processing H13 dies, and also reduced energy costs, improved part quality and improved heat treating efficiency.
The combination of convection-assisted heating and Flapper Nozzle can achieve a 33% reduction in cycle time while maintaining precision control even in the most demanding applications. The Flapper Nozzle is a simple, reliable cooling gas injection port (Fig. 6), which requires no complex linkage or actuation mechanism. The design reduces heat loss from the hot zone while improving temperature uniformity during heating. Convection heating has been demonstrated to dramatically reduce cycle time, especially for large cross sections and dense loads. These technologies also decrease energy consumption, saving both time and money in addition to reducing maintenance concerns and expenses.
Heat treatment of a large die
The heat treatment of very large dies poses the great challenges to the heat treater, because the cooling rate is limited by the thermal conductivity of the bulk material. This results in significant temperature differentials between core and surface during quenching, which increase the risk of distortion or even cracking. An alternative of using an assembly of several smaller die segments presents problems of water cooling during operation, and castings made in this arrangement show a tendency in crash tests to fail at the merging lines of the dies. This led to the interest in one-piece large dies.
The die used to illustrate heat treating results is a proprietary design, and is one part of a ten-piece set of aluminum casting dies for a newly developed passenger car. The 1,660 mm x 1,550 mm x 465 mm (65 x 61 x 18 in.), 5,600 kg (12,320 lb) die was machined out of a 9,800 kg (22,000 lb) forged 1.2343 (AISI H11) electroslag remelted block.
The part was heated up uniformly in steps by means of convection heating to a temperature of 750 C (1380 F), then heated further using the radiation of flat bar graphite elements to an austenitizing temperature of 1000 C (1830 F). After a sufficient soak time, the part was uniformly quenched using 15-bar nitrogen and alternating gas flow patterns, controlled by the cooling distribution on the thermocoupled part. At 400 C (750 F), cooling was interrupted to an isothermal hold stage to allow the temperature of the massive core to equalize with the cooler surface to avoid die distortion or cracking (Fig. 7).
An 80 mm diameter by 120 mm (3.15 by 4.72 in.) long test bar machined from the ingate section of a 5.7-metric ton H11 die insert was used to determine tensile strength, impact values and microstructure. Hardness and microstructure were acceptable and impact values for the surface and core were 220 to 280 J (162 to 206 ft lbf), exceeding the required 80 to 150 J (59 to 110 ft lbf). Microstructures are shown in Fig. 8 and corresponding properties are shown in Table 1.
Distortion was less then 2 mm (0.08 in.) on every part of the die, so the additional 5 mm (0.2 in.) stock on the material was more than sufficient. The die has been in operation since summer 2003 with good results.
Conclusions and future outlook
Over the past ten years, die life has been increased with the use of faster quenching speed through high pressure gas quenching, often above 10 bar. The use of very large die cast tooling in the automotive industry with part weight over 3 metric tons will increase as aluminum cast parts are increasingly used to lower the manufacturing cost to produce lighter weight automobiles. The use of new modified hot work steels with reduced silicon content, higher cleanliness and uniformity combined with the use of the latest vacuum furnace technology for heat treatment can further minimize distortion (even on very large dies) and significantly increase die life. IH