Considerations for Choosing the Optimum Fixtures
There are many types of heat-treating fixtures, trays, racks, boxes and other part holders in the market. They are generally castings, wrought fabrications or hybrids.
How is the heat treater supposed to decide which fixture is best? There is no easy answer. It is usually a combination of cost and design. Often, only initial cost is considered, and life-cycle costs are overlooked. Cost per pound of heat-treated product is a concept that gets almost no thought, but it should be a very important consideration.
There are distinct advantages of cast materials over wrought, and there are also distinct advantages of wrought materials over cast. Table 1 summarizes the pros and cons of each.
The most commonly referenced advantages of cast materials are a low cost per unit, the ability to add more of certain beneficial elements such as Cr and C, higher creep strength and the ability to be cast into complex shapes ready to be used.
Wrought alloys, on the other hand, can be used in much thinner sections, are repairable/weldable, resist thermal fatigue better and have better surface finish. The ability to use thinner sections can mean a lower-weight fixture and less BTUs to heat the fixture.
Baskets: Wrought and Cast
Baskets are one of the most common heat-treating fixtures. A typical basket is shown in Figure 1. This simple basket, made entirely from wrought round bar, is commonly referred to as a wire-bar basket. This type of basket is used extensively in heat-treating facilities. For small parts like hardware, wire-mesh liners are inserted on all five sides to prevent parts from dropping into the furnace. Facilities will also use fully cast baskets or wrought-cast hybrid baskets.
Cast baskets and hybrids would require more material and therefore result in a heavier fixture. They would be used to support heavier loads than the wrought wire-bar basket can handle. The wrought basket with lower carbon and a grain structure has good thermal-shock resistance, which means it can be quenched and heated many times, whereas the cast basket will eventually crack from the thermal cycling.
The wrought basket will remain shock-resistant until a case is built up in case-hardening operations. The cast baskets, with their higher carbon, have better creep strength and therefore keep their shape better. However, they will show signs of cracking much sooner than wrought. The economics of expected service life and cost per pound to heat treat will be the key factors of the decision.
Trays often support heavier parts. This article discusses two traditional types of trays and a newer-design tray. The traditional tray is cast, consisting of straight legs connecting to round tubes.
A serpentine grid has snakelike bent pieces bordered by straight lengths, all held together with threaded round bar with nuts welded to each end. A gap is allowed at one end between the last straight section and the end nut so that the individual pieces can expand and contract freely without constraint. The serpentine grid can be fabricated from relatively thin sheet
(11 gauge). Higher strength can be achieved by increasing the top-to-bottom grid thickness.
The final tray design is a honeycomb pattern made by Duraloy where each leg is relatively thick. As a result, this heavy-duty grid can hold heavier weights than the traditional cast grid. These grids are being seen in more heat-treat shops as a result of their ability to hold a lot of weight. These three trays are all shown in Figure 2.
In both baskets and trays, an important design decision must be made about how thick the supports should be. The thicker the supports, the more weight the fixture should be able to hold.
To get the most out of a fixture, however, consideration must be given to furnace capacity. Use of a tray with thick support members is not necessarily the best answer because a furnace has a weight-capacity limit. A fixture that is strong enough to support a lot of weight might not be able to do so because the weight of the fixture plus the weight of parts it can support is well above furnace capacity.
If you can’t run a full load, the strength of the fixture is wasted. In reality, the optimum situation is to use the fixture with the highest utilization (i.e., best ratio of part weight to total weight as possible). Too small a fixture and the furnace can’t be filled to near capacity; too heavy a fixture and the number of parts is limited. Too many furnace BTUs are being used to heat the fixture.
In many heat-treating shops, the forklift is the number-one cause of basket or fixture failure, especially when case-hardening operations are being performed. The high hardness of the cast tray combined with its cast microstructure and high carbon content tends to embrittle the fixture. It has high strength but low ductility and limited impact resistance. A wrought material will have good impact resistance until enough case is formed to through-carburize the microstructure.
The final type of fixture would be custom-designed. One common fixture is called a daisy wheel because of the shape of the grid. Once again, the use decision is based on the ability to support parts as well as the expected life. Cast fixtures have a tendency to split in the joint areas.
Welded wrought fixtures have more ductility and will not break as quickly in the welds. A wrought fixture is shown in Figure 3. Welds are evident, and most are holding up well. The broken areas can be re-welded.
One can also see stiffeners in this fixture. Stiffeners should be used with extreme caution. Stiffeners are often shielded from the heat, and they tend to heat up and cool down more slowly, creating thermal gradients. The stiffener will then restrict movement of other pieces. Such restrictions will cause material to bend, buckle, crack or a combination thereof. Stiffeners should be avoided unless some means of movement are provided.
In the heat-treating industry, many fixtures and baskets are fabricated from RA330®, which is a very versatile alloy. It exhibits oxidation resistance to 2100°F (1150°C) and usable creep strength to at least 1800°F (982°C). Most steel heat treating is done below 1750°F (954°C), and many operations are done below 1600°F (871°C). Below 1600°F, sigma phase forms in some fixture materials. At room temperature, sigma phase is quite brittle. Even slight impacts, such as hitting with a forklift, can cause failure. RA330 with 35% nominal nickel is immune to sigma-phase formation, as are nickel alloys with even higher nickel content.
RA330 is also resistant to surface-hardening operations like carburizing and nitriding. Over time, carbon and nitrogen will penetrate the protective oxide and diffuse into the base metal. As a general rule, RA330 fixtures will last approximately one year in carburizing atmospheres and should last longer in nitriding environments. They will warp from continued use. While cast fixtures will hold their shape better, they are more prone to crack sooner. Daily liquid quenching will make cast fixtures crack more rapidly. RA330 is resistant to thermal fatigue.
There are other options for wrought materials, but the cost is often higher than RA330. RA253 MA® is a stainless steel with very good creep strength, and it costs less than RA330. Because it is a stainless steel with less nickel, however, it is subject to sigma-phase embrittlement and will not offer much in the way of resistance to carburization or nitriding.
If the fixture will only be used for neutral hardening in an inert atmosphere or vacuum, RA253 MA may be a good, cost-effective alternative. RA602 CA® has performed very well as a fixturing material for the highest-temperature vacuum heat-treating operations to temperatures just below 2300°F (1260°C). This alloy has one of the highest creep strengths of all potential wrought products.
RA330 is still the most economical alloy for heat-treating fixtures. There are going to be occasions where a higher-strength alloy might be considered, particularly where final heat-treat part dimensions are critical and straightness specifications are tight. Other alloys could then be considered, and any such fixtures would be restricted to the one application.
For more information: Contact Marc Glasser at Rolled Alloys, 125 West Sterns Road, Temperance, MI; tel: 800-521-0332, e-mail: email@example.com web: www.rolledalloys.com. Various welding and fabrication manuals found on www.rolledalloys.com/technical-resources/ and from James Kelly’s Heat Resistant Alloys were utilized. 602 CA® is a registered trademark of VDM Metals.
1. “Cast vs. Wrought.” https://www.rolledalloys.com/technical-resources/fabrication-information/cast-vs-wrought/
2. Glasser, M., “RA330: Versatile Nickel-Based Alloy for Heat Treating,” Industrial Heating, Sept. 2016
3. “RA 602 CA® Chosen for Heat Treat Baskets for Extreme High Temperature Vacuum Heat Treating.” https://www.rolledalloys.com/shared/library/case-histories/RA-602-CA-Chosen-for-Heat-Treat-Baskets_CH_US_EN.pdf