Options abound when it comes to selecting the preferred type of fixture. In this Technical Tuesday installment, Garrett Gueldenzoph, applications engineer at Rolled Alloys, examines various advantages of wrought versus cast alloys in heat treat operations.
This informative piece was first released in Heat Treat Today’s February 2025 Air/Atmosphere Furnace Systems print edition.
There are various types of heat treating fixtures, such as trays, racks, boxes, and other part holders available in the market. These fixtures are generally made of castings, wrought fabrications, or hybrids.
For heat treaters, it can be challenging to determine which fixture best suits the job. The decision usually involves a combination of cost and design factors. However, many heat treaters tend to only consider the initial cost and overlook the importance of life cycle costs. It is crucial to consider the cost per pound of heat treated product, which is often overlooked but should be an important consideration.
Cast materials and wrought materials each have their own advantages. The pros and cons of each are summarized in Table 1. Cast materials offer a low cost per unit, the ability to incorporate beneficial elements like Cr and C, higher creep strength, and the ability to be cast into complex shapes that are ready to use.
Wrought alloys can be used in thinner sections, are repairable/weldable, resist thermal fatigue better, and have a better surface finish. Using thinner sections can result in a lower-weight fixture and fewer 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 a wrought round bar, is commonly called a bar basket or rod frame basket. This type of basket is either used as is or lined with wire mesh to hold small parts such as hardware in heat treating facilities. Wire mesh liners are inserted on all five sides to prevent these parts from falling into the furnace. Fully cast baskets or wrought-cast hybrid baskets are also used, but they tend to be heavier due to the larger amount of material they require. These types of baskets are used to support heavier loads than the wrought wire bar basket can handle.
A wrought basket has a lower carbon content and a defined grain structure, making it more resistant to sudden changes in temperature compared to cast baskets or hybrids. This allows it to endure multiple quenching and heating cycles. In contrast, cast baskets may develop cracks from frequent temperature changes. The wrought basket remains resilient to thermal shock until a case is accumulated during case hardening operations.
Cast baskets have a higher carbon content and better resistance to deformation under heavy loads. However, they are more susceptible to cracking than wrought baskets. When choosing between the two, the expected service life and cost per pound for heat treatment are the main economic factors to consider.
Trays
Trays are commonly used to support heavier parts. There are three main types of trays; two are traditional designs and one is a newer design (see Figure 2). The first traditional tray consists of a serpentine grid made of snakelike bent pieces bordered by consecutive lengths. The pieces are held together by a threaded round bar with nuts welded to each end. A gap is left at one end between the last straight section and the end nut, allowing for free expansion and contraction of the individual pieces. While the serpentine grid can be made from a relatively thin sheet (11 gauge), higher strength can be achieved by increasing the top-to-bottom grid thickness. The second traditional tray is cast with straight legs connecting to round tubes.
The final tray design features a honeycomb pattern by Duraloy, with relatively thick legs. As a result, this heavy duty grid can support heavier weights compared to the traditional cast grid. These grids are becoming more common in heat treat shops due to their ability to handle significant weight. All three tray designs are depicted in Figure 2.
Design
When designing baskets and trays, it is important to decide how thick the supports should be. Thicker supports can hold more weight, but the furnace capacity should also be taken into account to maximize efficiency.
Optimization
Using a tray with thick support members may not always be the best solution, as the furnace has a weight capacity limit. If the furnace can be run at total capacity, the strength of the fixture is well spent. It is best to use a fixture with the highest utilization, which means having the best possible ratio of part weight to total weight. A fixture that is too small will not allow the furnace to be filled to near capacity, while a fixture that is too heavy will limit the number of parts that can be processed.
Damage
Forklifts are a common cause of basket or fixture failure, especially during case hardening operations. The properties of the fixture material must be considered to prevent failure. For example, cast trays are strong but brittle, while wrought material has good impact resistance.
Custom
The final type of fixture is custom designed. One standard fixture is called a daisy wheel because of its grid-like shape. The decision to use a particular fixture depends on its ability to support parts and its expected lifespan. Cast fixtures tend to split in the joint areas, whereas welded wrought fixtures have more ductility and will not break as quickly in the welds. Stiffeners should be avoided unless some means of movement is provided, as they can cause the material to bend, buckle or crack.
Materials
In the heat treating industry, fixtures and baskets are often made from a versatile alloy called RA330®. This alloy is resistant to oxidation up to 2100°F (1150°C) and has usable creep strength up to 1800°F (980°C). Most steel heat treatment is done below 1750°F (950°C), and many operations are done below 1600°F (870°C). Sigma phase forms in some fixture materials below 1600°F, which makes them brittle at room temperature and prone to failure eve with slight impacts such as forklift hits. But RA330, with 35% nominal nickel, is immune to sigma phase formation, as are nickel alloys with higher nickel content.
RA330 also has good resistance to surface hardening operations like carburizing and nitriding, but carbon and nitrogen can penetrate the protective oxide and diffuse into the base metal over time. Generally, RA330 fixtures last approximately one year in carburizing atmospheres and should last longer in nitriding environments. They may warp from continued use but are resistant to thermal fatigue.
There are other options for wrought materials, but they are often more expensive than RA330. For instance, RA 253 MA® is an alternative with good creep strength and lower cost than RA330. However, due to its lower nickel content, it is subject to sigma phase embrittlement and does not offer much resistance to carburization or nitriding.
If the fixture is used only for neutral hardening in an inert atmosphere or vacuum, then RA 253 MA may be a cost-effective option. On the other hand, RA 602 CA® has performed exceptionally well as a fixturing material for the highest temperature vacuum heat treating operations, up to temperatures just below 2300°F (1260°C). This alloy has one of the highest creep strengths among all potential wrought products.
Despite the other options, RA330 is still the most economical alloy for heat treating fixtures. However, a higher strength alloy may be considered when final heat treat part dimensions are critical and straightness specifications are tight. Other alloys could be considered, but these fixtures would be restricted to that one application.
References
Glasser, Marc. “RA330: Versatile Nickel Based Alloy for Heat Treating.” Industrial Heating, Sept. 2016.
Rolled Alloys. “Cast vs. Wrought.” https://www.rolledalloys.com/resources/cast-vs-wrought/.
Rolled Alloys. “RA 602 CA® Chosen for Heat Treat Baskets for Extreme High Temperature Vacuum Heat Treating.” https://www.rolledalloys.com/wp-content/uploads/2022/07/RA-602-CA-Chosen-for-Heat-Treat-Baskets_nickel-rolled-alloys-metal-supplier.pdf.
About the Author:
Applications Engineer
Rolled Alloys
Garrett Gueldenzoph specializes in stainless steel and nickel alloy welding at Rolled Alloys. He holds a bachelor’s degree in Mechanical Engineering from the University of Toledo and is actively involved in several respected technical organizations, including the American Welding Society (AWS), the American Society for Metals (ASM), and the American Society for Testing and Materials (ASTM). Garrett has a strong passion for aerospace and space-related applications, and he plays a key role in enhancing the company’s technical expertise in this market.
For more information: Contact Garrett at ggueldenzoph@rolledalloys.com.
This article was initially published in Industrial Heating. All content here presented is original from the author.
