882 Modern Welding Thermal stress relieving: Steel is heated to the desired temper- ature, held at that temperature, and then slowly cooled back to room temperature to relax stress in the metal. Thermite welding (TW): A welding process that uses a burning mixture of powdered metals to join metals. Thermoplastics: Plastics that become soft and capable of being formed when heated, harden when cooled, and can be heated and reformed repeatedly. Thermosetting plastics: Plastics that cannot be reheated and reformed once they are formed and cooled. Three-phase resistance spot welding machines: Resis- tance spot welding machines that draw energy from each of the three phases of the electricity supply. Throat depth: The distance from the center of the elec- trodes to the frame of a spot welding machine. Throat of a fi llet weld: See Actual throat. Throttle: The process of adjusting the fi nal fl ow of gases being fed to a positive-pressure welding torch. TIG: A nonstandard term. See Gas tungsten arc welding. Time-temperature-transformation (T-T-T) diagram: Diagram that shows how time and temperature affect steel transfor- mation when cooling from the austenite phase. Tin-lead phase diagram: A graph that plots solidus and liquidus against temperature and composition for tin- lead solders. Tinning: In soldering, a thin coating of solder placed on the metals to be joined. Tip: End of the torch where the gas burns, producing the high-temperature fl ame. In resistance welding, the elec- trode ends. Titanium: A metal with good weight-to-strength ratio, good high-temperature properties, and excellent corro- sion resistance. T-joint: Joint formed by placing one metal against another at an angle near 90° to form a “T” shape. Toe crack: A crack in the base metal at the toe of a weld. Top view: The view on a working drawing that shows the size and shape of an object’s top side. Torch: The device that delivers the heat for welding and cutting processes. Typically held and manipulated by the welder during manual operations. Torch lighter: A device used to ignite an oxyfuel gas torch. Torch valves: Two needle valves on an oxyacetylene welding torch. One controls the rate of fl ow of the oxygen, while the other controls the rate of fl ow of the acetylene to the torch tip. Torsion: Twisting motion resulting in shear stresses and strains. Touch starting: Starting a gas tungsten arc by touching the electrode to the base metal and then withdrawing it about 1/8″ (3mm). Toughness: Metal’s ability to absorb energy and deform before breaking. Toxic: Term used to describe material that is poisonous and possibly deadly. Transducer: Device used to send and receive sonic (sound) waves during ultrasonic testing. Transferred arc: An arc between the electrode and the base metal. Transgun: Portable spot welding gun that has a built-in transformer. Transistor: A semiconductor device similar to an SCR. It can be turned on or off by a control signal. Transition current: Lowest current level at which spray transfer can occur. Trans ratio: The transformer winding ratio, or number of primary windings compared to the number of secondary windings. Transverse bends: Bend tests that are done with the weld going across the test sample. Transverse face bend: Transverse bend done with the face of the weld on the outside of the bend. Transverse root bend: Transverse bend done with the root of the weld on the outside of the bend. Travel angle: The angle measured from a line perpendicular to the weld axis to the electrode axis in the plane defi ned by the weld axis and the electrode axis. T-T-T diagram: See Time-temperature-transformation diagram. Tube: A hollow cylinder with a relatively thin wall. Tundish: A reservoir that holds liquid steel from a furnace before it fl ows into a mold. Turbulent: Agitated, rough, or roiling. Tuyere: Opening in a blast furnace through which air is forced to support combustion. U Ultimate compressive strength: The maximum compres- sion stress that a material can stand under a gradual and evenly applied load. Ultimate strength: The greatest conventional stress, tensile, compressive, or shear that a material can stand. Ultrasonic: Mechanical vibrations in a frequency above the range of humanly audible sound. Ultrasonic testing (UT): Nondestructive examination method that transmits and receives high-frequency sound waves through the material being examined. Ultrasonic welding (USW): Process using high sound frequencies to produce metal fusion. Ultraviolet rays: Energy waves in the short light wave- length, or ultraviolet spectrum, that come from electric arcs and welding fl ames. Unbalanced wave: Waveform that results when there is more current during one half of the ac cycle than during the other half. Underbead crack: Crack in the base metal near the weld and beneath the surface. Undercut: Depression at the toe of the weld that is below the surface of the base metal. Unguided bend test: A welded piece is clamped in a vise or jig just below the weld and the metal is bent to an angle of about 90°. Uphill: Welding with an upward progression. Uphill welding: Starting a vertical or multiposition weld at the bottom or 6 o’clock position and moving upward to the top of the joint or 12 o’clock position. Upper transformation temperature: The A3 critical temperature.