Casting answers & advice.

Q Our aluminum castings (alloy 296, permanent mold cast, titanium and boron grain refinement) are "soft" during machining. We think the castings might not be adequately heat-treated to the T-6 condition. What is the correct heat-treat procedure and how long should we age for maximum hardness and machinability?

Foundry Background - Castings are placed in a large basket and heat-treated to a T-6 condition. Current practice is solution heat-treating the castings at 950F (510C) for 8 hr (furnace is equipped with a timer and alarm), quenched in hot water and aged at 350F (177C) for 5.25 hr.

Recommendations - Temperature uniformity is critical to proper heat treatment. Currently, no procedure is in place to measure the furnace environment and check for uniform heating of the castings. The only furnace controls are the internal furnace temperature and the time.

1. Follow heat-treat temperatures and times as listed in Aluminum Casting Technology from AFS. They are:

* T-6 Solution Heat Treatment - Soak at 960F (516C) for 12 hr; quench in hot water at 150-212F (66-100C).

* T-6 Precipitation Treatment - Age at 310F (154C) for 3-6 hr.

2. With only one thermocouple in use, only the internal furnace temperature in the chamber outside the basket, and at only one location, is being measured. It is not known whether the furnace is maintaining consistent heat throughout or what the actual temperature is of the castings deep in the basket. This information is critical to ensuring that all castings are receiving enough heat to assure adequate solution heat treatment. Specifically, the concern is whether the castings in the center of the basket are receiving enough heat for a long-enough period of time. Consider obtaining temperature readings from thermocouples placed throughout the basket and setting furnace timers to ensure that the section of the basket with the least heat is meeting your minimum heat-treating standards.

3. Aging also is effected. As noted in Aluminum Casting Technology, "Age hardening is diminished by insufficient solution temperature." To determine the aging time that will provide optimum machinability, examine a series of test castings. Remove castings from the furnace each hour (or every 30 min) during the aging cycle, and track these castings through machining. This will help establish minimum and optimal age times needed for the desired machinability.

4. Rather than using the current system of filling a basket with castings, consider placing castings on racks so that heat and air can flow through the racks and more easily reach all castings to provide more consistent heating (see illustration below). This practice might slow down the heat-treating process (perhaps fewer castings per batch), but more even heating should increase the effectiveness of the heat treatment and, consequently, casting quality and consistency.

Q Our foundry produces automotive gray iron castings. Recently, we have been experiencing a chronic problem with subsurface pinholes that becomes readily apparent after machining. How do we eliminate this quality problem?

Foundry Background - The foundry's green sand moisture is 2.8% and its final chemical analysis is 3.48 carbon (c), 2.3 sulfur (s) and 0.72 manganese (Mn).

Investigation - After investigating the foundry's core practice, quality control and venting techniques, the casting microstructure in the defect area revealed numerous evenly distributed small holes. These holes were sites of free graphite that were removed during the machining operation - they were not caused by gas. This type of porosity is frequently called "open grain" and its occurrence increases with higher C content and size and quantity of free graphite. Further investigation of the defective sample revealed that the final C in the iron was above the upper limit of the casting specification.

Recommendations - Reduce the C content from 3.48% to the lower limit of the specified range (3.35% in this case). If the porosity defect still persists, the machining practice should be studied and modified. Specifically, the cutting tool condition, machining speed and depth of cut should be analyzed.

Literature - Articles pertaining to this situation are available from the Handbook of Iron Casting and other resources from the AFS Library.

Recommendations are the opinion of the AFS Technical Dept. based on referenced literature and experience. If you need assistance with a technical issue, fax your question to: Casting Answers & Advise, c/o modern casting, at 847/824-7848.