Heat Treatment: The Finishing Touch to Iron Castings

As far as foundry customers are concerned, the more seamless the process, the better. Meeting their timeliness and quality expectations doesn’t hurt, either. State Line Foundries delivers on all counts when castings require heat treatment. 

“Heat treating isn’t something everyone gets excited about, but it’s important,” acknowledges State Line president Jesse Milks. It’s often a casting’s final touch. It can make the iron harder, softer or more flexible, as needed.

State Line can handle the heat treatment in-house or, when a unique situation arises, send the casting to a trusted outside supplier. Either way, the practice remains seamless as State Line manages all elements and delivers the finished castings to its customers.

Types of heat treatment

The heat treatments commonly utilized by State Line fall into three categories: stress relief, annealing and austempering.

Stress relief

“When you’re making an iron casting that has some size to it or odd geometries, stresses can build up in areas of the casting if it’s removed from the sand mold shortly after the solidification phase,” Milks details. These stress points can cause fractures or distortion issues if they’re not addressed.

The stress relief method of heat-treating adds heat to the cooling process. This ensures that the casting cools more slowly, which alleviates stress on its radii or uniquely shaped areas. 

“The stress relief is not going to change the strength or hardness a whole lot,” Milks states. “Instead, it homogenizes the microstructure of the metal so you don’t have pent-up stresses within it.”

Stress relief is commonly used with gray and ductile iron. The temperature of this heat treat process typically maxes out at about 1,000º F.

In lieu of stress relief in a heat treat furnace, some OEMs suggest or allow for “in-mold” stress relief.  This involves leaving the casting in the sand mold for a longer period of time – such as 24 hours for castings over 1,000 pounds.  

Annealing

Annealing is a heat treatment that relies on a hotter cycle, typically 1,300º F to 1,400º F. “It alters the microstructure of the iron,” Milks explains. “It reduces the hardness of the casting but improves its ductility.”

Essentially, the casting becomes softer and more flexible as it slowly cools. Annealing is often done to castings that have specifications for elongation – simply put, the castings have requirements for the amount of pressure they must endure without breaking.

Some OEMs like annealing because a softer casting is easier to machine. “In the machining world, there’s a happy medium between too soft and too hard,” Milks explains. “If it’s too hard, it’s difficult to machine. If it’s too soft, it galls or clogs.”

Austempering

Austempering is a type of heat treatment that hardens standard iron castings by transforming their microstructure.

Austempering starts like a traditional casting process, with parts heated to between 1,550º F and 1,750º F. Then the departure from traditional casting begins.

With austempering, the heated parts are soaked in a molten salt bath at temperatures between 450º F and 750º F. This causes them to cool much slower than normal, sometimes for hours. This enables a uniform transformation of the iron’s microstructure, preventing the distortion that traditional cooling can cause.

Austempered castings are often used in high-wear applications, such as agricultural tilling and ground-engaging tools for the construction industry.

In-house work

State Line handles two of these three heat-treating methods – stress relief and annealing – internally.

“When you break down heat treating to its simplest form, it’s controlling the temperature of an oven over time with castings inside,” Milks points out.  Like everything else, technology has greatly improved the quality, efficiency and ease of heat-treating components.  

Yet it’s most certainly not a risk-free proposition. Improper treatment can cause castings to distort or transform properties to unacceptable levels.

“Like making castings, for heat treating you put as much science into it as you can, but there also is some know-how on the part of the heat-treater,” Milks stresses. “You need to understand casting wall thicknesses and the impact of heat treatment on them.”

Good heat treaters can learn a lot by reviewing prints and focusing on wall thicknesses and radius sizes. 

“Temperature and time are the main variables,” he adds. “You need to hold a certain temperature for a certain amount of time to get the results you require.”

Outsourced heat treating services

Castings that require austempering are sent to outside sources. The salt bath and other parts of the approach are “a little bit of a secret sauce,” Milks explains. State Line prefers to let those experienced in austempering take on the work.

“We have multiple suppliers for the unique and specialty processes,” Milks says. “They are certified and very capable of handling those needs.”

Adding value to the heat-treating process

State Line helps its customers find the right answers by advising if heat treatment is needed and when it is, whether it should be processed internally or outsourced.

State Line’s experience with outside sources ensures a high-quality heat treater is selected and the proper instructions are communicated. Castings are kept in-house whenever possible. 

“The biggest benefit is lead time,” Milks declares. “If you send a casting out for annealing, the lead time may be upwards of a week. When we do it here, our lead time is a day or two … or even 12 hours. We can be ready to ship the next day.”

Lead times are crucial when developing prototypes. They also matter on production runs. “It’s just handy to have that service in-house, so we can get the castings out the door faster,” he adds.

The techniques used by State Line “aren’t overly sophisticated,” but they still bring a significant amount of value to the foundry’s customers. “It’s controlling your destiny,” Milks asserts. “Our customers aren’t waiting on anyone else when we handle these jobs in-house.”

And, of course, State Line’s experience does pay off. “Heat treatment is an everyday thing for us,” he concludes.

Contact State Line today to discuss your iron casting heat treatment needs.