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Heat Treating - A Tool to Unlock Value
By Rick Sabol | August 17, 2007
The Crucible, the flagship publication of the Non-Ferrous Founders’ Society has published the following article “Heat Treating - A Tool to Unlock Value” Co-Authored by Ralph Marshall and Chris Pilko of Nabertherm Inc. in the July /August 2007 Edition.
Adding a heat treatment operation to your foundry could allow you to save costs and offer more services to your customers.
What have you done to make your products a better value for your customers today? Or, if you prefer: What has your competition done to make their products a better value for your customers today? We all know in today’s fast-paced global economy, the easy sales are over. Customers will switch vendors whenever they can find a better value for their money.
Talladega Pattern & Aluminum Works in Talladega, Alabama realized that they could increase the value to their customers by bringing their heat treating operation in-house. With a single furnace and quench-tank, they are able to deliver heat treated castings in two days. Their rapid turnaround time has made Talladega Pattern indispensable to their customers.
Talladega Pattern & Aluminum is a green sand foundry that was started in 1951. Their primary business is small run aluminum and brass castings in a variety of industries. They have pallet lines using jolt squeeze machines and two running floors. They are now in the process of adding a new muller system as well as another pallet line.
Talladega Pattern and Aluminum Works started out as a pattern shop. As the marketplace became more competitive, they integrated into a full time foundry and phasing out the pattern shop. In 1990, they began doing their heat treating in house. In 2006 they upgraded their heat treating operation from an older gas-fired furnace to a new Nabertherm 75 cubic foot car bottom furnace. They use this furnace to perform both the solution heat treatment and the artificial ageing.
Aluminum Heat Treatment Basics:
Certain alloys of Aluminum can have their properties improved with heat treatment. Most alloys containing copper or silicon including many in the 200, 300 and 700 series of alloys can have their properties improved with a heat treatment. This is due to the metallurgy of the alloy system that allows the individual alloying elements to segregate from or react with the surrounding aluminum matrix in different ways.
In the heat treating of an aluminum alloy, there are three different common processes that can be done: Solution Heat Treatment, Quenching, and Ageing. These processes can be combined in different combinations called temper designations (see Heat Treat Temper Designations for 356.0 Aluminum below). In this case, we will look at the most common, the T6 temper. T6 indicates that the metal is to be solution heat treated, quenched, and then artificially aged.
Alloys that benefit from solution heat treatment are those that, when cooled slowly, allow the individual alloying components to segregate into pockets within the base metal matrix. The goal of solution heat treatment is to take the metal, and distribute all of the alloying elements evenly throughout the metal. To perform a solution heat treatment, the alloy must be heated close to its melting temperature, and then held there for a long enough time to allow all the alloying elements to diffuse evenly throughout the matrix.
For 356.0 aluminum, solution heat treatment is performed at 1000°F, only 40°F below the melting range for the alloy. The metal must be held at temperature for 4-12 hours, depending upon the thickness of the casting. As you can imagine, temperature control in this process is critical: If the temperature is too low, the diffusion will not take place, and the metal will not develop its full properties. If the temperature is too high, your castings will begin to melt resulting in inferior physical properties, and usually a severely blistered surface.
A solution heat treatment is followed by a quench that cools the metal quickly and does not allow the alloying components to segregate again. For most aluminum alloys, quenching is done by submerging the casting in nearly boiling water. The temperature of the water is critical: If the water is too cold the casting will cool too quickly and may crack. If the water is too hot, a thick layer of insulating steam will form around the casting, and allow the metal to cool too slowly.
Also important is the delay time between when the casting is removed from the furnace and when it is fully immersed in the quench tank. The delay time is dependent on the thickness of the thinnest section of the casting. Some standards bodies require that the casting be completely submerged in as few as 3 seconds.
Unlike with steels where a heat treated and quenched steel is very hard and brittle, solution heat treated and quenched aluminum is still soft. To harden the aluminum, an ageing process is required to allow some of the individual constituents to precipitate out of the matrix and segregate along the grain boundaries. This process can also be called precipitation hardening.
Highly alloyed compositions, especially those with more than 3% Cu, will age spontaneously at room temperature over the course of several days to weeks. These alloys are called “naturally aged”. Other than this waiting period, no additional treatment is required. The 356.0 alloy has less than 1% Cu, so it will not age naturally, and to develop full strength, it needs to be artificially aged for several hours in a separate heat treatment step. Like solution heat treatment, temperature control is critical in the ageing step: Too little heat or time, and the hardness will not develop. Too much heat, and the alloy can “overage” and become harder than desired. In fact, there is a separate temper , T7, for these overaged alloys.
Some alloys, including 356.0, can be aged without a solution heat treatment and develop properties that are superior to the as-cast properties. For instance, only ageing 356.0 results increases the tensile strength by 20%. This is a T5 temper.
At Talladega Pattern, as the castings are stripped from the mold, they are loaded into a steel basket. At the end of the shift, this 3 x 7 foot basket is lifted with an overhead crane and set on the rolling furnace car base. The car is rolled into the furnace, and the solution heat treatment cycle is allowed to run overnight. In the morning, the hot furnace is opened, and the basket is quickly moved into the quench tank.
While the parts are quenched, and allowed to dry, the furnace is cooled to the ageing temperature. Once there, the basket of parts is returned to the furnace and artificially aged for the remainder of the work day. Near the end of the day, the basket of parts, now with a T6 or T7 temper is removed from the furnace, and a new basket of parts is charged.
Talladega Pattern’s Nabertherm furnace is equipped with a modern electronic programmer that allows several different programs to be stored so that the appropriate set temperature for each alloy and each heat treatment can be selected at the push of a button. Mr. Joey Peters, the President of Talladega Pattern and Aluminum Works has been pleased with the results of this new furnace and how it has helped their process. According to Peters the new furnace “substantially reduced our energy costs from the heat treatment step, and allowed us to ship more product.”
Many casting shops have found that heat treatment can be a way to increase their value proposition to their customers, especially for parts that are already require heat treatment. When looking at all the costs for a third-party to heat treat a part: extra inventory, transportation, and their costs and overhead; moving a heat treating operation in house can make this payback compelling.
Heat Treat Temper Designations for 356.0 Aluminum
F, As fabricated
Cast and allowed to cool naturally.
(Tensile strength = 19 ksi)
T51, Cooled and artificially aged
Cast and cooled naturally and stress relieved by stretching.
Artificially age at 440°F for 7-9 hours.
(Tensile strength = 23 ksi)
(Yield strength = 16 ksi)
T6, Solution heat treated, quenched and artificially aged
Solution heat treated at 1000°F for 4-12 hours.
Quench in water at 150 °F.
Artificially age at 310 °F for 2-5 hours.
(Tensile strength = 30 ksi)
(Yield strength = 20 ksi)
T7, Solution heat treated, quenched and overaged
Solution heat treated at 1000°F for 4-12 hours
Quench in water at 150 °F
Artificially age at 400 °F for 3-9 hours
Times New Roman”>(Tensile strength = 31 ksi)
(Yield strength = 29 ksi)
From theMetals Handbook, Desk Edition 2nd edition. Published by ASM International.
Topics: Foundry, Heat Treatment, News |
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