Casehardening Stress Relief of Ponderosa Pine

Study was undertaken to determine efficient means of conditioning ponderosa pine to relieve casehardening stresses. Comparative tests revealed that conditioning time could be reduced from 24 hours to 4 hours by cooling lumber prior to conditioning, by injecting water into the steam spray line, and by increasing the quantity of steam. Three water-injection devices, namely, venturi, tee-siphon, and desuperheater, were tested. The test material was 6/4 Shop ponderosa pine, 16 feet long in quantities of 10,000 bd. ft. The stock was dried in cross-circulation kilns to a moisture content of 6 to 10 percent. One stress sample was cut from each of four selected boards located along the outside edge of the test load before the test and marked for control. Six more boards were selected for stress sampling after test. Many tests were made with various sizes of orifice openings in the spray line at several controlled pressures to the orifices. The following factors were tested to determine their effect upon the relief of stresses: 1) steam volume, 2) lumber temperature, 3) water injection, 4) air circulation. Steam flow of 80 to 100 lbs. of steam/hr./Mbf at a pressure of 30 psig near the outlet is an economical quantity of steam use for efficient relief of casehardening stresses. Cooling of lumber reduces the conditioning time to approximately one-half of the time necessary for hot lumber. Mixing of water with high-pressure superheated steam will reduce conditioning time by one-half of the time needed for conditioning hot lumber. Reversing of air circulation during conditioning is required for uniform stress relief.

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