• J-EDGE SHAPE

    An edge shape formed by the combination of a bevel with a bevel radius. 

  • J-GROOVE WELD

    A type of groove weld. See GROOVE WELD. 

  • JAWS, Electrode Holder

    The part of an electrode holder which grips the welding electrode. The electrode holder jaws are usually made of a hard copper alloy. 

  • JIG

    The terms jig and fixture have essentially the same meaning. Jigs, or fixtures, are designed to hold pieces of an assembly in correct relationship during welding, and to expedite removal of the completed parts after welding. Sometimes simple jigs, toggle clamps, C-clamps or wedges are sufficient to hold the alignment. In a manufacturing setting, elaborate jigs designed to hold large sheet or plate metal might be required.

    In industrial production of welded parts, close dimensional control and correct alignment are critical when planning for high rates of production. Specifications must be precisely followed to produce parts that are interchangeable and readily assembled.

    In repair work it is particularly necessary to hold the parts in alignment to bring the broken item to its original shape, especially if it is part of an assembly.

    Jig design requires mechanical ingenuity and a knowledge of the laws of expansion and contraction of metal. When steel is heated to a welding temperature, it has very little strength and ductility. For this reason a crack or tear is very easily started by any stress due to warping contraction. In using a jig, the several parts required for a welded assembly are cut to length and fitted so that there is only a small clearance between the abutting members. This clearance should be as uniform as possible, for example, when a truss member is welded into the sidewall of a tube, the end of the member should be milled to fit the contour of the tube.

    For arc or oxyfuel gas welding of thin sheet metal ranging from, for example, 10 gauge (3.6 mm [0.141 in.]) and thinner sections, the need to use a jig is more critical than when welding the heavier plate metals. As a general rule, the thinner the sheet metal, the greater the need for a jig. Greater changes occur in the edge contours of thin sheets matched up for butt welding than in heavier sheet or plate metal. The jig must provide a means to control warping and edge movements by absorbing heat or forcibly restraining the parts to some degree.

    Some welding jigs are designed to hold the parts in a level position convenient for welding, with capability of rotation in a horizontal or vertical plane.

    Tack-welding jigs are used in laying transmission pipelines to assure concentricity of the adjoining pipe ends and good alignment. They are essentially welding jigs, which are removed immediately after the tack-welds have been made. The fact that the pipe ends no longer require the support of the jig during the welding operation means that the function of the jig has been transferred to the tack-welds.

    Jig Design

    Simplicity should be the first consideration in the design and construction of the shop-made fixture. Sometimes the design can accommodate set-up and welding in the same fixture.

    Convenience in reaching the welded surface and visibility are two important factors. For arc welding, the design usually includes copper backing bars with machined grooves to permit complete penetration of the weld metal. The grooves should be extremely shallow (0. 4to 0.8 mm [0.015 to 0.030 in.]), and comparatively narrow (4.5 to 6.4 mm [0.18 to 0.25 in.]), and should not be square cornered.

    Allowances for heat control must be made to prevent misalignment, buckling or overlapping of the parts. The jig should be constructed so that it carries heat away from the weld. Clamping pressures will largely depend on the type of structure being welded.

    See also FIXTURE and POSITIONER. 

  • JOINT

    The junction of members or the edges of members that are to be joined or have been joined. Of the many types of joints, the most common are edge, butt, lap and tee. 

  • JOINT BRAZING PROCEDURE

    The materials, detailed methods, and practices employed in the brazing of a particular joint. 

  • JOINT BUILDUP SEQUENCE

    A nonstandard term for CROSS-SECTIONAL SEQUENCE. 

  • JOINT CLEARANCE, Brazing and Soldering

    The distance between the faying surfaces of a joint. In brazing, this distance is referred to as that which is present before brazing, at the brazing temperature, or after brazing is completed. 

  • JOINT DESIGN

    The shape, dimensions, and configuration of the joint. 

  • JOINT EFFICIENCY

    The ratio of strength of a joint to the strength of the base metal, expressed in percent. 

  • JOINT FILLER

    A metal plate inserted between the splice member and thinner joint member to accommodate joint members of dissimilar thickness in a spliced butt joint.   See Figure J- 1. 

  • JOINT GEOMETRY

    The shape and dimensions of a joint in cross section prior to welding. 

  • JOINT OPENING

    A nonstandard term for ROOT OPENING. 

  • JOINT PENETRATION

    The distance the weld metal extends from the weld face into a joint, exclusive of weld reinforcement. See GROOVE WELD SIZE.

    Joint penetration is the depth of fusion of a weld from the original surface of the base metal to the point where fusion ends.

    For the weld to be acceptable it is necessary that the base metal and filler metal be completely fused together to that point. A weld may be made with partial penetration, where a gap or notch exists at the root of the weld, or complete penetration where fusion is complete from top to bottom.

    Complete joint penetration is normally required in welds and when it cannot be obtained from one side in one pass, several passes are used with grooved joint preparation or weld passes are made from the root surface, or both.

    In a square butt joint, joint penetration and root penetration are the same. In a groove weld, root penetration is the distance from the bottom of the groove to the point where fusion ends.

    Through penetration and complete fusion and bonding of the metal are essential for successful sound welds. Through penetration is not easily accomplished in square butt joints over 6 mm (1/4 inch) thickness by most arc welding processes. A groove or bevel joint preparation is used to achieve complete penetration. A gap between two plates with square edges may help attain penetration but oxides formed on the edges could prevent complete metallurgical bonding near the root of the joint. Full penetration can be attained in square butt joints in thicker plates by the electron beam, laser and plasma arc processes. 

  • JOINT RECOGNITION

    A function of an adaptive control that determines changes in the joint geometry during welding and directs the welding equipment to take appropriate action. See JOINT TRACKING and WELD RECOGNITION

  • JOINT ROOT

    That portion of a joint to be welded where the members approach closest to each other. In cross section, the joint root may be either a point, a line, or an area. See Figure J-2. 

  • JOINT SPACER

    A metal part, such as strip, bar or ring, inserted in the joint root to serve as a backing and to maintain the root opening during welding.  See Figure J-3. 

  • JOINT TRACKING

    A function of an adaptive control that determines changes in joint location during welding and directs the welding machine to take appropriate action. See JOINT RECOGNITION and WELD RECOGNITION.

  • JOINT TYPE

    A weld joint classification based on five basic joint configurations such as a butt joint, corner joint, edge joint, lap joint and T-joint. 

  • JOMINY TEST

    A laboratory test procedure developed by W. Jominy in 1938 for determining the hardenability of steels and other ferrous alloys. The test, usually called the End Quench Test (ASTM A255), is the most common method of determining hardenability, the relative ability of a steel to form martensite when quenched from a temperature above the upper critical temperature.

    In the test procedure, a sample of a particular steel is heated to the correct quenching temperature, assuring that the surface is protected from oxidation. After heating, the sample is quenched. The quenching water jet impinges on the end of the sample and this area is cooled very rapidly. Since the heat must travel by conduction from the sample to the quenched end, the top portion of the sample will cool very slowly. Different rates of cooling, therefore, will occur all along the sample.

    The hardness of the steel at different rates of cooling is indicated by Rockwell C (HRC) hardness readings, starting at 1.6 mm (1/16 in.) from the hardened end and at 1.6 mm (1/16 in.) intervals for a distance of SO mm (2 in.).

    The sample consists of a piece 10 cm (4 in.) in length. It is 25 mm (1 in.) round for a distance of 9.8 cm (3.875 in.), with a flange approximately 2.8 cm (1.125 in.) in diameter and 0.4 mm (0.015 in.) thick on one end. After the sample has been quenched, the next step is to grind a flat about 0.4 mm (0.015 in.) deep along the entire length of the sample to remove the carburized surface. It is on this flat area that the Rockwell C hardness readings are taken. The data are normally plotted as hardness (HRC) versus distance from the quenched end at which a certain hardness (such as HRC SO) is observed that may be used as an indication of hardenability.

    If the hardness in the coarse-grained region of the heat-affected zone (HAZ) of a weld in a steel is matched with the same hardness on a Jominy bar of the same steel, then the cooling rates at these two positions (one in the HAZ and the other on the Jominy bar) are the same. The cooling rates at various positions along the Jominy bar are measured and tabulated. Further, the HAZ cooling rates for various welding conditions (plate thickness, joint design, initial plate temperature, current. voltage and travel speed) are

    measured and tabulated. Thus it is possible to select conditions that avoid the formation of brittle martensite during the arc welding of a particular steel.  Additionally, in lower-carbon quenched-and-tempered steels, conditions can be selected so that a tougher martensite forms in the heat-affected zone. 

  • JOULE

    A unit of electrical work. It is a current of one ampere flowing through a resistance of one ohm for one second, i.e., one joule is equal to one watt.