MARKETING MATTERS CAST TIP
Aluminum Heat Treating Tips T
STEVE ROBISON, AFS SENIOR TECHNICAL DIRECTOR
emperature uniformity is critical to achieve proper heat treatment and desired
mechanical properties of aluminum castings. Improper placement of the castings in the furnace or inadequate furnace controls and temperature measurement can lead to inconsis- tent heat treating, variable mechanical properties and potential casting distortion. For example, the recommended
heat treat temperatures and times for aluminum alloy A356-T6 sand
castings are: · Solution heat treatment at 1,000F (538C) for 12 hours, then quench in hot water (150-212F/66-100C).
· Precipitation treatment age at 310F (154C) for 2-5 hours.
Temperature Measurement In many heat treating furnaces,
the only controls are the internal furnace temperature (which can be measured with a single thermocou- ple reading) and the timer, leaving no means to measure the actual time castings reach the desired tempera- ture or to check for uniform heating. In the case of A356-T6 alumi-
num castings, a solution time of 12 hours begins when the control thermocouple in the oven reaches 1,000F (538C), assuming all cast-
ings in the furnace reach the tem- perature and are solution-treated at that same temperature for the full 12 hours. If some of the castings do not reach the desired temperature or are not at it for the full 12 hours, inad- equate heat treatment would occur. If the control thermocouple mea- sures the temperature in the furnace chamber atmosphere rather than the actual casting temperature, it cannot be assumed all of the castings (especially those in the middle of the basket) have reached the proper solution temperature and are receiv-
Racking castings allows for proper airflow and separation to ensure consistency.
ing enough heat to assure adequate solution heat treatment. A better solution for commercial
castings is to obtain temperature readings from thermocouples placed throughout the basket and set- ting furnace timers to make sure the section of the basket with the least heat is meeting your minimum heat treating standards (Fig. 1a). This information applies primar- ily to commercial castings. Military and aerospace castings are usually governed by NADCAP specifica- tions that are specific about furnace temperature zoning and control.
Racking All castings should be placed in
the furnace so they receive consis- tent heating. Castings placed into the heat treat furnace lumped into a basket will not all receive the same amount of heat, and the ones in the middle may not reach appropriate temperature as quickly as those on the outer edges. A better solution is to place the cast-
ings on racks so heat can flow between them and provide a more even means of heating (Fig. 1b). Tis procedure may potentially slow down the entire heat treating process (perhaps heating fewer castings per batch), but more consistent and even heating stands to increase the effectiveness of the heat treatment pro- cess and, consequently, casting quality and consistency. An important racking consid-
Fig. 1a. For a more accurate temperature reading in commercial castings, take readings from thermocouples placed throughout the basket, rather than at the sides. Fig. 1b. Placing castings on racks can allow heat to flow more evenly.
eration involves allowing for the thermal expansion and decreased strength of the component at solution heat treat temperatures. Aluminum castings expand at nearly twice the thermal expansion rate as the steel rack that supports and con- strains them. When packed tightly, the castings can impinge on each other or against a steel rack. Because the castings are heated to a high temperature, they are ductile. This impingement can create localized distortion even before they enter the quench water.
January 2013 MODERN CASTING | 51
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