Heat Treating Frames For Military Motorcycles Featuring Christini Motorcycles

Christini Technologies is a Philadelphia-based, manufacturer of custom all-wheel drive motorcycles. Known for manufacturing custom motorcycles for competition racing, Christini was recently awarded a contract to supply the Army with 450 tactical scout bikes to be used in worldwide operations.
Integral to the construction of the motorcycle and the performance of the bike is a special welded aluminum alloy frame. Because the frames are of a welded construction, they must be heat treated to restore the mechanical properties to eliminate the soft zone adjacent to the weld. It is critical for the application that the mechanical properties of the welded aluminum frame are restored to the original requirements so the frames do not prematurely fail during use. Metlab is the vendor of choice for the heat treating.

Steve Christini, owner of Christini Motorcycles explains, “During the manufacturing process of the motorcycle frame, you cut the head tube off then weld the frame. This changes the integrity of the metal. Heat treating brings it back to the correct specifications. Then a finish blast is used for aesthetics. We use Metlab because of the way they do the entire heat treating process. The parts are not contaminated and their process is a very clean process. This enhances the strength and integrity of frame.”

Steve Christini owner of Christini Motorcycles

The material used in the frames is 6061 Aluminum. It is specified due to its strength, heat treatability, comparatively easy machining and weldability. Prior to heat treatment, the frames are cleaned to be free of any grease, oil or other contamination that may affect the parts. The frames are then heat treated to the T-6 condition, which consists of solution treating, or heating the parts to 985°F, holding for one hour at temperature and then immediately water quenching. Then the frames are age hardened at 350°F for 8 hours.
We have a load thermocouple that indicates when the part is up to the appropriate temperature. The soak time is dictated by the maximum section thickness.

After heat treatment, the parts are checked for hardness and conductivity to ensure that they meet the mechanical properties for properly heat treated frames. This process uses a calibrated micro-hardness tester and is carried out by trained quality control personnel.

Furnace temperature uniformity is the key to ensuring uniform mechanical properties, and the furnaces used for processing the frames are uniform within +/- 15°F in the complete work zone.
During the inspection process we make sure the mechanical properties, primarily hardness, fall into the process requirement range:

Reference MIL-H-6088, AMS 2770
Solution Treat Quenched Condition W
Age Harden- T6
80 BHN500 – min; 42 HRB; 78 HR15T, 40-50 Conductivity

Solution Treat (Condition W as quenched, T4-natural age at room temperature)
Age Harden – T6 Condition (Artificially Age Harden)

A visual inspection is conducted to determine if there are any types of defects especially around the integrity of the welds.

It is important to position the frames in the furnace so they do not distort during heat treating and also water quenching. While not the case with this particular application, Metlab heat treats certain parts for some customers who weld braces in place to keep the parts from moving. Also, for some customers where distortion is especially critical, parts are straightened before age hardening.

There is special engineering involved in the heat treat relating to the fixturing to both hold the parts and minimize distortion, as well as minimize transfer time. The parts are placed on special fixtures to keep them upright so not distortion from sagging. No special materials are needed for processing, just special equipment.

Metlab often turn parts around for Christini Motorcycles within a few days after receipt, and have given the company routine Friday to Monday service. Currently over 75 frames have been heat treated and processed by Metlab in 2011.

Mark Podob
Metlab Heat Treating

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