Joint Preparation for Repair of Cracks

Engineering Data

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These cracks can propagate in service and lead to ultimate failure of the part. It is necessary to adhere strictly to recognized good welding practice regarding the handling, storage and use of low hydrogen electrodes. To avoid underbead cracks, remove all oil or grease or other contaminates from the surface and be sure the steel is dry.

Preheat to the required temperature. Use only dry electrodes taken from the electrode oven. After exposure to the air, the electrodes must be returned to the oven. Time limit outside the oven is four hours for E7018 and two hours for E8018-C1 electrodes.

Repair Welding Of Cracks

Joint Preparation for Repair of Cracks

Remove the entire crack by arc air-gouging or grinding. Prepare a V-groove of approximately 4560o included angle for rewelding. If cracked through the full thickness and if it is possible to make the repair from both sides, a double V preparation is preferred. When welding from both sides, back-gouging for complete penetration is always recommended.

When it is not physically possible to back-gouge and weld the second side, an alternate approach must be taken. Sometimes it is possible to gouge through to completely remove the crack, then fit a backup bar on the underside to facilitate making a complete penetration weld.

NOTE: The backup bar must fit tightly to the underside otherwise poor welding conditions conducive to cracking may occur.

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Engineering Data

When it is not possible to fit a backup due to limited accessibility to the underside, it is advisable to remove a portion of the defective plate and weld in a new piece, using back up bars on all sides to ensure complete penetration welds.

A further alternative is to gouge through, removing the crack, and then weld small stringer beads to close the gap and prepare a groove for a repair weld. In this case, the repair weld will be essentially full penetration, but will be less than perfect on the under side. The other methods are preferred in the order described, that is, welding both sides, welding against a backup bar, or completely replacing a section of plate.

Preheat

When preheating, apply heat to a broad, general area surrounding the repair location. A soaking heat that penetrates the material through its full thickness is preferred to high surface temperature quickly applied. Electrical resistance or radiant heating is suggested since the heat source can be left operating during welding to maintain the minimum preheat temperature. Preheat temperatures up to 400oF, depending upon the steel analysis, are adequate under most conditions for major repairs. As previously stated, preheat temperatures are determined by the type of steel in the structure. Temperatures should be measured with temperature indicating crayons.

Engineering Data

Welding Technique

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Maximum electrode sizes are 1/4 inch for flat position, 3/16 inch for horizontal position and 5/32 inch for vertical and overhead positions. Use a split weave as soon as bead width is large enough to accommodate beads side by side. Throughout all welding be certain thorough fusion with the base metal and adjacent weld beads is obtained on all weld passes. Clean off all slag between passes and all craters before starting the next electrode.

Inspect closely for cracks, undercut or overlap of beads, and gouge out or grind flaws where they occur before continuing. It is very important on highly loaded parts, particularly members subject to cyclic or dynamic stresses, to obtain sound repair welds.

The presence of any stress risers on the surface of a part is detrimental to fatigue life and can result in future cracking. Therefore, it is essential that the finished repair be smooth and well blended into the base metal. Repair weld any undercuts, grind off overlapped weld beads and blend out any notches or gouges. The best condition in a repair weld location is provided by grinding the repair smooth and flush with the surface of the base metal. Final grinding direction is to be identical with the direction of applied load on the structure. Refer to the figure.

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Engineering Data

Repair Welding Of Broken Parts

All recommendations given for repair of cracks apply to repair of broken parts, with additional corrections. Depending on the size and cross-section of the part, a specific sequence of welding procedures may be required in making the repair. These techniques include back-step welding sequence, block welding sequence, alternating from side to side, welding simultaneously on opposite sides, etc. All of these precautionary measures are intended to minimize shrinkage stresses and subsequent distortion or cracking during welding. The method to follow should be determined after a careful analysis of the situation and by approaching the problem with common sense. Generally a procedure which has proven successful in previous experience could logically be applied in most cases.

Use of doubling plates, stiffeners or other reinforcements to strengthen a member which has cracked in service, must be carefully considered before that repair measure is decided upon. Additional material added for strengthening alters the configuration and geometry of the member, possibly with a pronounced effect on the fatigue life of that structure. Many times, such attempts at strengthening by adding plates only serves to “chase the crack someplace else.” The stress flow in the part has been altered, creating a location for stress concentration. Attachments requiring fillet welds across a tension member for example, are poor repair methods. A sound repair weld, carefully made and smoothly blended into the base metal on all sides, is preferable to additional reinforcements. Any application of reinforcements requires careful consideration regarding total overall effect on the structure during service, and should be done only after consulting the manufacturer.

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Repair Welding Swing Rack Teeth

The following is a suggested method for the repair of cracked or broken swing rack teeth. The repair method should produce satisfactory results and provide a serviceable swing rack, provided the procedure is carefully followed in all details. The welding procedure is essentially the same for the three conditions shown.

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Engineering Data

Preparation

1. Clean off the swing rack in the area of the tooth to be repaired to remove all traces of grease, oil or dirt. Wash down thoroughly with a solvent cleaner and wipe dry with clean rags. 2. Grind to completely remove the crack and prepare a groove for welding as shown in the figures. 3. Dye penetrant test for complete crack removal. 4. In the case of a broken tooth, grind the fracture surface on the swing rack smooth and round off edges as shown.

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Swing Rack Welding Electrodes

1. All welding is to be done using oven dry E11018-M low hydrogen electrodes. Connect an electrode drying oven as close as possible to the work area. Set the oven temperature at 300oF. As sealed containers of electrodes are opened, place the entire contents of the container in the oven. Remove no more electrodes than can be consumed in one-half hour. If all electrodes are not used in 1/2 hour, return them to the oven. Discard any electrodes which have been wet.

Preheat

1. Two levels of preheat are required for tooth repair, 250oF minimum for butter welding and 175oF minimum for filling in the groove. Temperatures should be measured with temperature indicating crayons. 2. Preheat will be difficult to maintain because of the mass of metal in the swing rack to draw the heat away from the repair area. It is therefore recommended to apply heat to a large section of the swing rack to offset this quench effect. A section covering at least 3 teeth on either side of the tooth being repaired should be heated so that the minimum preheat temperature is obtained at the repair location. It is suggested that electrical resistance heaters be applied behind the teeth as shown, if such physical arrangement is possible. If the repair area cannot be heated to the required minimum temperature by this arrangement, supplementary heat must be applied from the tooth side of the swing rack. Electrical resistance heaters can be applied, or heat provided by oxy-propane torches. Adjacent areas of the swing rack should be covered with heavy insulation to avoid heat loss.

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Engineering Data

NOTE: Contract services are available for field application of resistance heating equipment. Suggested source: Stresstech Box 11 Red Wing, MN 55066 (612) 388-7117 3. During welding, the repair area must be continually checked with temperature indicating crayons to be sure that the minimum temperature is being maintained. Do not weld at temperatures below the specified minimums.

Welding Procedure

1. Refer to the previous butter-welding figures. The purpose of the butter weld layer is to provide a buffer between the weld required to fill the groove and the alloy cast steel. This technique is essential in preventing heat affected zone cracking in the cast steel which otherwise is likely to occur due to the shrinkage of the weld as the groove is filled. Butter welding permits the repair weld to be made at the preheat temperatures specified. Much higher temperature would otherwise be required for crack free welding. 2. Butter welding is done using 1/8 inch diameter E11018-M electrodes at a minimum preheat temperature of 250oF. (See section under WELDING ELECTRODES regarding use of the electrode drying oven.) Horizontal weld beads are applied to completely cover the cavity or surface to be welded. The edges of the butter weld must overlap the adjacent surface about ½ inch. This is to prevent subsequent welding from impinging upon the cast steel. 3. Before starting the butter weld, position a piece of thin mild steel plate under the bottom end of the tooth. This is to provide a shelf for starting the weld and to avoid irregular weld beads at the bottom which would become stress risers to cause cracking when the tooth is loaded in service.

4. Apply the butter weld, progressing with horizontal beads from bottom to top. Use care to keep the butter weld layer as smooth and regular as possible. When finished, remove all slag and grind any high spot which could trap slag when welding to fill the groove. In the case of repairing a broken tooth the prepared tooth segment is butter welded separately before positioning for final welding in place. 5. Filling the groove is done with 1/8 inch or 5/32 inch E11018-M electrodes at a minimum preheat temperature of 175oF. (See section under WELDING ELECTRODES regarding the use of the electrode drying oven.) Weld vertically up using a split layer technique as soon as the groove is wide enough to accommodate 2 or more beads. Refer to the butterweld figures for the approximate weld bead sequence. 6. For replacing a tooth segment as shown, it will be necessary to prepare a tooth profile template. The template must be carefully made to fit the rack teeth as accurately as possible. Use the template to position the tooth segment prior to tack welding. When welding the tooth segment in place alternate welding from side to side to control distortion. Check frequently with the template.

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7. When groove welding is finished check carefully for low spots and fill in as required. Grind the weld smooth and flush with the adjacent tooth surfaces. Use the tooth profile template to check grinding of a welded-on tooth segment. Grinding of the radius at the root of the tooth is very important. Avoid any nicks, gouges or grinding marks in a vertical direction. Grind a smooth radius using small diameter (peanut) grinders. Failure to achieve a smooth, notch-free radius may result in future cracking at the root of the tooth. 8. After the swing rack has cooled to ambient temperature, dye-penetrant test the repaired tooth for soundness.

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Engineering Data

Welding And Cutting Equipment

The welding and cutting equipment needed for general repair welding is listed below.

WELDING MACHINES, Arc - 600 ampere (for Welding and Air Arcing) OXY ACETYLENE TORCHES with Gauges - 150' Long Hoses HEATING TORCHES, Butane (for Boom Welding) ARC AIR ATTACHMENTS (for use with 600 Ampere Welding Machines) WELD RODS - E7018, E8018, E11018 - 1/8", 5/32". 3/16", 1/4" C02 - Welding Grade - 45oF Max. Dew Point OXYGEN - Cutting, and Heating ACETYLENE WELD FLUX CHIPPERS - Pneumatic with Chisels and Spare Parts BLOWERS, Ventilating (for Compartment Welding,) GRINDERS, Wheel - Air Powered w/Wheels TARPAULINS - Fireproof (for Boom Welding Shelters) HIGH TEMPERATURE GLOVES CUTTING GOGGLES, Dark SOAPSTONE MARKERS ANTI-SPATTER COMPOUND TEMPILSTIKS - 200o and 400o

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Stress Relieving & Temperature Measuring Equipment

The equipment recommended for stress relieving, after repair welding, is listed below.

TEMPERATURE MEASURING EQUIPMENT SPEEDOMAX, “W” Multipoint, Potentiometer Recorder POWER REQUIREMENT - 120 Volts, 60 or 50 Hz. RANGE - 0 to 1500oF CALIBRATION - Chromel-Alumel Thermocouple Wire ACCURACY RATING - 0.3% of Electrical Span CHART SPEED - 2" per Hour THERMOCOUPLE SELECTION - Six (6) Points FLUORESCENT LIGHTING MANUFACTURER - Leeds & Northrup Company THERMOCOUPLE EXTENSION WIRE DUPLEX WIRES - Chromel-Alumel - Type K - 16 AWG Stranded LENGTH - Total External Resistance for both Wires including Thermocouple not to exceed 2500 Ohms or 410 Feet. WIRE RESISTANCE - Nominal Resistance, Ohm per Foot at 20oC (66oF) - Chromel - Ohms Alumel - .0683 Ohms. WIRE INSULATION - each Conductor Enamel, Asbestos (Twisted Pair) Overall Asbestos Braid WIRE CODE -Alumel, Negative Wire (Red); Chromel, Positive Wire (Yellow) OVERALL COLOR - Yellow CATALOG No. 16-59-17 POLARITY DISCONNECT CHROMEL-ALUMEL COMPENSATED CONNECTION JACK Color Code (Yellow) - Catalog No. 040419 PLUG Color Code (Yellow) - Catalog No. 040434 CABLE CLAMP - Catalog No. 072513 ADAPTER - Catalog No. 076794 THERMOCOUPLE ELEMENT CHROMEL-ALUMEL - Type K - Swaged One (1) Inch Stripped CHROMEL POSITIVE WIRE Color Code (None) ALUMEL NEGATIVE WIRE Color Code (Red)

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Engineering Data

CATALOG No. 8784-K-1-3-12"-D 1. 588003 Kaopak Flex Heaters 2. 588004 Kaopak Collector Streamer Type 3. Kaopak Blankets 3, 5, or 6 Pocket Size as needed for Size Pipe being Stressed Relieved Thermocouple Assembly, Complete Catalog No. 8784-K-1-3-12"-Q Temperature Indicating Pellets (Tempil° Pellets) Indicating Temperature: 1050°F, 1100°F, 1200°F, and 1250°F. (Several of each temperature)