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What You Should Know About Heat Treating Steve Robison, AFS Senior Technical Director


castings. When receiving less than quality castings after heat treat, it is worth checking the procedures at the heat treatment facility. For instance, improper placement of the castings in the furnace or inad- equate furnace controls and tem- perature measurement can lead to inconsistent heat treating, variable mechanical properties and potential casting distortion. For example, the recommended


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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 thermocouple reading) and the timer, leaving no means to measure the actual time castings reach the desired temperature 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), and assumes that all castings in the furnace reach the desired temperature and are solution-treated at that same tem- perature for the full 12 hours. If some of the castings do not reach the desired temperature or are not at temperature for the full 12 hours, inadequate heat treatment occurs. If the control thermocouple mea- sures the temperature in the furnace chamber atmosphere rather than the actual casting temperature, it can-


emperature uniformity is critical to achieve proper heat treatment and desired mechanical properties of aluminum


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-


Racking castings allows for proper airfl ow and separation to ensure consistency.


not be assumed all of the castings (especially those in the middle of the basket) have reached the proper solution temperature and are receiv- 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.


ings on racks so heat can fl ow between them and provide a more even means of heating (Fig. 1b). T is 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 eff ectiveness of the heat treatment process and, consequently, casting quality and consistency. An important racking consider-


ation involves allowing for the ther- mal expansion and decreased strength of the component at solution heat treat temperatures. Aluminum cast- ings expand at nearly twice the ther- mal expansion rate of the steel rack that supports and constrains them. When packed tightly, the castings can impinge on each other or against the steel rack. Because the castings are heated to a high temperature, they are ductile. T is impingement can create localized distortion even before they enter the quench water. 


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 fl ow more evenly.


Jan/feb 2013 | METAL CASTING DESIGN & PURCHASING | 45


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