Figure 1. Vacuum furnace (50” diameter) with a load of H-13 dies
Since its inception, vacuum heat-treating (VHT) technology has been continuously developed in response to new metallurgical and manufacturing requirements. VHT’s development stems from the all-electric nature and contained-energy design of the vacuum furnace. The motivation to develop the technology is great because there are no emissions from combustibles, and there is no significant waste-stream produced as a result of the process. When parts are removed from the vacuum furnace, they are free of decarburization and surface contamination. Most importantly to die manufacturers is the advantage of being able to tightly control the thermal profile.
The advantage of VHT is control – more specifically, control of the furnace atmosphere, die temperature, and heating and cooling rates. The vacuum environment in conjunction with advanced control technology enables repeatability and adaptability. The furnace environment and process is fully programmable, yet adaptable. Vacuum furnace technology is commonly engineered for specific and emerging processing applications such as vacuum gas nitriding, vacuum carburizing and various coating processes. It is the marriage of advanced mechanical design, digital controls and processing expertise that has enabled our company to offer improved processing technology, which can lead to improved die life for forging-die manufacturers and users (Figure 1).
Forging-die manufacturers have taken advantage of the benefits of VHT, often motivated by lean manufacturing goals and tight delivery requirements.
DIGITAL CONTROLS
All our furnaces are equipped with state-of-the-art digital controls. These are only as good, however, as the sensors to which they are attached. Solar Atmospheres uses numerous “type S” thermocouples to control each vacuum furnace. These utilize a platinum-rhodium wire to sense the temperature of the furnace and are very expensive. Yet, they are extremely reliable, and it is tough to place a price on the value they add to the process. Additionally, we use at least three “load” thermocouples embedded in either the die itself or a “dummy block” of like cross section and weight to monitor the actual part temperature during every heat treatment. If there is a large disparity between different cross-sections on the same die, thermocouples are placed in those sections to monitor the gradient between them. This is important to consider when heating and quenching the dies. The result is accurate and reliable furnace and part temperature readings that are used to control the process to gain maximum benefits of the vacuum furnace.MINIMIZING DISTORTION
Vacuum hardening and tempering of forging dies – specifically H-13 forging dies – allow the heat-treat processor to minimize distortion and all but eliminate the possibility of cracking caused by thermal treatment. This is most important for dies with thick and thin sections on the same die, dies with abrupt changes in geometry and heavily machined dies. There are two main areas where distortion may be controlled during the hardening process – heating and cooling. Programmable controls enable excellent temperature uniformity throughout all the die sections during the heating and cooling phases, reducing variation and minimizing stresses in the die.
FIXTURING THE DIE
Not to be forgotten is the “artistic” side of VHT. H-13 is a challenging material to heat treat, particularly for large dies when heavy machining has been done to only a portion of the die. In such situations, the “art” of fixturing is extremely important for successful results, and the wisdom derived from decades of experience cannot be understated. Fixturing and die placement in the furnace are critical. In fact, a good setup can take up to 30% of the total processing time. The experienced heat treater understands furnace dynamics and die vulnerabilities.VHT allows a die to be fixtured in the furnace and not moved until the whole process is completed. This is another advantage over conventional processing, where hot parts must be moved to a cooling chamber and cooled. Moving a hot die can lead to distortion caused by movement in the supporting fixturing. In a vacuum furnace, graphite fixtures may be used, and this is a huge advantage over conventional fixturing. The problem with graphite in an air or gas furnace is that it will oxidize and actually burn at temperatures above 600°F, meaning it is useless in the presence of air. The beauty of graphite is its thermal stability. It does not expand and contract like steel or nickel alloys. In fact, graphite barely changes at all. The strength actuallyimproveswith heat, which is the opposite of steel and nickel alloys (Figure 2).
CONTROL OF HEATING RATE
One of the biggest benefits of the vacuum furnace is the ability to control the heating rate and the insertion of “preheats” at strategic points in the heating portion of the process cycle. In a typical air or atmosphere furnace, parts are generally charged at the high temperature needed to “austenitize,” or harden, the material. This creates a huge amount of stress in the die and leads to distortion and cracking. Having the ability to start heating from ambient temperature and control the heating rate allows us to heat the part at a modest enough rate to allow machining stresses to be slowly relieved, minimizing distortion. Conventional heat-treating methods “charge” the die into the furnace, which is already soaking at 1850°F. Thisaddsstress and leads to distortion and cracking. A controlled heat-up cycle, which also utilizes preheats at strategic points in the process, has been very successful in minimizing distortion from heating and machining stresses in the die block.SOAKING
When a die is brought to the transformation temperature (approximately 1875°F), it must be “soaked.” Just like a turkey in the oven, the die must be heated uniformly throughout its entire cross section to assure full metallurgical transformation in all sections of the die. Having the ability to utilize load thermocouples right in the die means getting the right temperature for the right time. Conventional furnaces require movement of the part when loading and cooling, meaning thermocouples would get in the way. It is very difficult to use load thermocouples in a conventional furnace for that reason. Vacuum furnaces are designed to utilize load thermocouples, and it is quick and easy to do so. This allows realization of improved metallurgical properties for extended die life.
Figure 3. Time, Temperature Chart for H-13 showing the steep drop in the tempering curve (Heat Treaters Guide, ASM, 2nd Edition, page 594)
COOLING
Cooling the die is the next important step in the process and is most critical to attaining the desired metallurgical results without distortion. Conventional heat-treating ovens are designed to simply remove the part from the furnace (at 1875°F) and cool it in either still or forced air.Forced-air cooling means putting a fan in front of the die and blowing the room air across it until it is cool. This is hard to control and leads to poor die properties, distortion and cracking. It is also downright dangerous! We employ controlled, gas pressure quenching using dry, high-purity nitrogen. The key to this technology is the ability to slow the cooling rate when a huge gradient exists in the same die. Slowing the cooling rate allows the die to more closely reach equilibrium, which reduces stress from the cooling process. Having embedded thermocouples in the core of thick and thin cross sections allows us to see exactly how much of a gradient exists. It also allows us to control the gradient and produce a higher quality die.
Quenching technology may be the biggest single advancement in vacuum-furnace design. In the past, a vacuum furnace was terribly slow to cool because just turning off the power was the norm. Today’s advanced fan and gas delivery systems have changed the vacuum furnace from a tortoise to a hare, rivaling oil quenching in many situations.
TEMPERING
Tempering is the final part of the process. It is the key to finishing the high-quality job already started during vacuum hardening. The purpose of tempering is to impart to the H-13 the maximum toughness with enough hardness to provide maximum wear resistance at elevated operating temperatures. H-13 must be given two tempers – preferably three – to stabilize the newly formed microstructure and deliver long die life. The atmosphere options include air or vacuum temper. Vacuum is the preferred method for preventing oxidation and scaling, and it offers a product that is clean and visually appealing.The first temper is usually performed between 1000°F and 1050°F, which stabilizes the newly formed martensite and produces full hardness (usually about 54 Rc). The second temper’s primary goal is to attain the lower specified hardness using precise temperature controls. Usually this hardness is about 46-48 Rc. During the temper, temperature uniformity of the die is essential since a difference of 5°F can reduce hardness by five points because of the steep tempering curve inherent with H-13 (Figure 3). The second temper is essential, and a third temper is beneficial.
ION NITRIDING
For the particular problem of material wash-out, VHT offers ion or plasma nitriding. Ion nitriding hardens the surface of the die, minimizing the wearing effect on the die when excess material “washes out” of the die impression during the forging process. Ion nitriding is a low-temperature process (1000°-1100°F) and can be done as a second temper to help forestall this problem.The value of ion nitriding is twofold. First, surface hardness increases (monophase layer) while the core hardness retains its tempered durability. Second, the hardened surface reduces the friction coefficient so that wash-out conditions are minimized. Ion nitriding is a good option when wash-out is a problem and has the added benefit of adding compressive stresses. According to a 1999 article, “Life Enhancement of Hot Forging Dies by Plasma-Nitriding”, by Mehmet, the maximum life of a plasma-nitrided, hot-forging die can be increased by eight times that of the as-received die.
Bruce Craven, Solar Atmospheres’ engineer overseeing ion nitriding, states, “Depending on the quality of the material and the case depth chosen, surface hardness can measure above 60 Rc. A typical rule of thumb is 10-15 Rc points higher than beginning core hardness providing the previous tempering temperature exceeded the nitriding temperature by 50°F or more.”
EXAMPLES
In one of our applications, vacuum heat treating of H-13 dies extended the life of smaller dies from 5,000 forgings to 40,000 at one plant location. This is a rather extreme example, but moving from an in-house air furnace to VHT can be this dramatic. The change resulted from the transition from very limited temperature controls for the hardening and tempering treatment to the precise temperature controls of the vacuum process. Uniform cooling minimized distortion while providing a high-quality microstructure. This application revealed that a third temper contributed to extending die durability due to EDM work after the second temper.Large hot-forging dies used to manufacture hand tools also greatly benefited from VHT and ion nitriding. A shallow die impression made wash-out a continuous problem, so the ion nitriding was a good cure.
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