Making an Inconel 625 Custom Wire Basket for Parts Annealing

Annealing is a metallurgical process wherein an object (typically metal or glass) is heated to a high temperature and then allowed to slowly cool. The process can be used for several different reasons, such as to soften a metal for cold working later, to improve the object’s machinability, or to enhance the object’s electrochemical properties.

These changes are the result of the breakdown and subsequent reformation of crystalline lattice structures in the material being annealed. Typically, the material being annealed is made red-hot—meaning it is brought to near-melting temperature.

One client recently approached Marlin Steel to create a custom wire basket for their parts annealing process.

The Challenge of Making a Basket for Extremely Hot Environments

The major challenge in building a basket for any annealing process is making sure the basket can take the incredible temperatures it will be exposed to. For reference, in this client’s particular annealing process, each basket would pass through a belt furnace at 1,200-1,600°F for a continuous 15 to 30 minutes.

While that is below the melting point of most stainless steels (which is usually 2,500°F or more), it is above the maximum recommended use temperature of many stainless alloys. At these temperatures, metals that aren’t specifically formulated for these conditions can suffer numerous problems, such as rapid oxidation or scaling.

However, this also simplifies the process of choosing a metal alloy for the application. Since only a few types of metal can survive prolonged exposure to these temperatures, there are fewer alloys that need to be compared for other characteristics like tensile strength and corrosion resistance.

One of the metal alloys on the short list of metals that were ideal for the client’s annealing process was Inconel 625®.

Why Inconel 625®?

While there are a few other metal alloys that can withstand temperatures in the 1,200-1,600°F range, Inconel 625® has some other properties that helped to make it ideal for the client’s particular application.

First off, the ultimate tensile strength of Inconel 625® is 13.3 ksi (13,300 psi) at temperatures of 2,000°F. The client’s specified maximum load weight would be 10 kilograms (22.0462 pounds). This meant that the metal would be more than strong enough to hold the load through the hottest parts of the oven because:

1. 2,000°F is well above the hottest temperatures the basket would be exposed to;
2. The ultimate tensile strength per inch of material was well in excess of the weight that the basket would hold; and
3. The conveyor belt would support the majority of the basket’s weight.

In short, it was unlikely that the basket would lose shape from the weight and heat if it was made out of Inconel 625®.

Another benefit of Inconel 625® is that it has strong corrosion resistance properties. It is known to resist exposure to alkaline environments, salt water, and open air exposure without issue. Although the client’s specific application didn’t call for the use of chemical cleaners, there was a potential for secondary exposure in the factory environment, so this basket design could benefit from the added resistance.

Additionally, as with most Inconel® brand alloys, 625 possesses an extreme resistance to scaling and oxidation at high temperatures—making it ideal for prolonged, repeated use in furnace conditions.

These properties would keep the basket in ideal condition for a long period of time, helping to minimize the total cost of ownership by preventing the need for constant reorders.

The Design of the Basket

For the most part, the basket simply needed to hold a series of metal discs in place throughout the annealing process. The discs would vary in size from 1.535” in diameter and 0.1” thick to 1.523” in diameter and 0.203” thick. So, the diameter of each part might not vary much from one to the next, but the thickness could vary greatly.

The desired load capacity specified by the client was about 60 parts per load in a basket that would be 12.25” long, 5.5” wide, and no more than 0.5” tall to fit the 6” wide, 2” tall opening of the belt furnace with ease.

The client’s suggested design called for 60 individual spots in the basket, with each spot being 0.25” wide and 1.75” long to accommodate the largest of the discs with room to spare. This left a bit of extra room in the design according to the customer’s specifications—with 10 0.25”-wide spaces, that would only take 2.5” of the 5.5” width of the basket (not counting the wires themselves). At 1.75” long, 6 lengths would only take 10.5” of the total basket length (not counting wires). So, with nearly 3” of leeway for width, and 2” of leeway for length, Marlin’s engineers had plenty of room to add features like extra-thick, sturdy wires or handles.

In fact, the basket could have individual wires up to 0.27” thick and still be comfortably within the dimensions requested by the client.

Outside of the furnace, the basket would be carried manually by workers wearing extra-thick protective gloves. While handles weren’t a requirement from the client, they would make the baskets easier to transport from the furnace to the next process in line without being dropped by accident.

Verifying the Design Concept Prior to Manufacture

The tricky part was that, if handles were added, the rest of the wires running parallel to them would need to be made thinner to make room for them. And, if the wires were made too thin, then that could compromise the basket’s ability to retain its shape under the weight of the discs at peak furnace temperatures.

So, to verify whether adding handles to the client’s design would be feasible, Marlin’s engineering team ran a series of physics simulations using finite element analysis (FEA) software. These tests would let Marlin’s engineers determine if the baskets could have handles added to them without compromising their integrity in the high-temperature application used by the client.

In the simulation, the basket was tested both with and without handles to see how well each version would hold up to the client’s use conditions. The thicker-wired “plain” version with the thickest possible wires was shown to be capable of withstanding the process, so then engineers then tested the “handled” version with thinner wires.

Aside from verifying whether or not adding handles would be a viable option for the basket, the simulations also helped to determine the minimum wire thickness that the basket could have and still meet the client’s needs—preventing a waste of material.

Learn more about the properties of different metals by checking out our stainless steel properties sheet at the link below! Or, if you have more questions about Marlin’s custom wire baskets, please contact us.