Chapter 6 Advanced Shielded Metal Arc Welding
What effect does changing from a trailing angle to a leading angle have on a weld?
As the trailing angle is decreased making the electrode flatter to the plate, penetration depth, and burn through decreases.
Back gouging
Back gouging is a process of cutting a Groove in the back side of a joint that has been welded. That gouging can Insure One hundred percent Fusion at the root and remove discontinuities of the root pass.
How can small gaps between the weld plate and backing strip be closed?
By placing the assembled test plates on an anvil and striking the tack weld with a hammer this will close up the gap by compressing the tack welds.
Why should a filler weld pass not have deep penetration?
Deep penetration will slow the rate of build up in the joint.
How can a wide gap in a joint be closed by welding?
Make stringer beads on both sides of the joint until the gap is closed.
Why are some backing strips removed from the finished weld?
May cause it to fail due to concentrations of stresses along the backing strips.
What can happen if the molten pool becomes too large on a root weld?
Metal may drip through the keyhole, forming an icicle on the back side of the plate. Extremely large molten weld pool sizes can cause a large hole to be formed or cause burn through
Post Heating
Post heating is the application of heat to the metal after welding. Post heating is used to slow the cooling rate and reduce hardening.
Pre-heating
Pre-heating is the application of heat to the metal before it is welded. This process helps to reduce cracking, hardness, Distortion, and stresses by reducing the thermal shock from the weld slowing the cooldown rate. Pre-heating is most often required on large, six plates, when the plate is very cold; on days when the temperature is very cold; when small diameter electrodes are used; on high carbon our maganese steels; on complex shapes; or with fast welding speeds.
Other than penetration why would thick butt joints be grooved on both sides?
Requires less weld metal and has less distortion.
Poor Fit-Up
Requires some special skills. The wilder must be able to read the molten weld pool correctly to make needed changes in the amperage, current, electrode movement, electrode angle,and timing.
Why is it very important to make a weld with a good root surface?
Save both time and resources. Must be produced adequately, without the benefit of being able to clean and repair the back side.
Sketch four of the standard grooves used for welded joints
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Filler pass
Filler passes are made with Stringer beads or weave beads. They should overlap enough so that the finished bead is smooth. Stringer beads usually overlap about 50% and weave beads overlap approximately 25%. Each weld bead must be cleaned before the next bead it started. Slag left on the plate between welds cannot be completely burned out because filler welds should be made with a low amperage setting. Deep penetration will slow the rate of buildup in the joint. Deeply remelting the previous weld metal may weaken the joint. All that is required of a filler weld is that it be completely fused to the base metal.
Groove welds
Groove weld on 1/2 inch thick plates are the most common test plates given to new welding applicant for employment. Vertical and overhead positions.
Groove welds
Groove weld on 1/2 inch thick plates are the most common test plates. Used by companies as required employment. Vertical and overhead positions test.
Why are backing tape used on some joints?
Increase penetration and prevent burn through.
What benefit would there be to the root pass if the electrode holder were rocked back and forth while keeping the electrode tip in the joint?
It helps control the bead shape. It allows the arc force to better tie on the root to the base metal.
Why is it important to have a good cover pass?
Keeping the cover pass uniform and neat looking is important. Most welds are not tested and often the inspection program is only visual.
Inter pass temperature
Temperature of the metal during welding. Given as a minimum and maximum.
Post Heating
The application of heat to the metal after welding. Post heating is used to slow the cooling rate and reduce hardening.
What might cause the bright color on the flux as a weld cools?
The brighter color is caused by the slower cooling of the slag, because there is less fused metal to conduct the heat away quickly.
What is the purpose of the hot pass?
The fastest way to clean out trap slag and make the root pass more uniform. The hot pass uses a higher than normal amperage setting and a fast travel rate to reshape the bead and burn out the trapped slag.
The groove
The groove may be ground,flame cut, plasma cut, gouged, or machined on the edge of the plate before or after the assembly. Bubbles and the groups are best if they are cut before the parts are assembled. J grooves and you girls can be cut either before or after assembly. The lap joint is seldom prepared with a Groove because little or no strength can be gained by grooving this joint the only advantage to grooving the lap joint design is to give additional clearance.
Cover Pass
The last weld bead on a multipass weld is known as the cover pass. Recover past may use a different electrode weave, or it may be the same as the Philippines. Keeping the cover past uniform and neat looking as important. Most Wells are not tested, and often inspection program is only visual. The appearance might be the only factor used for accepting or rejecting welds. The Cover pass should be free of any visual defects such as undercut, overlap, porosity, or slag inclusions. It should be uniform in width and reinforcement. A cover pass should not be more than one-eighth inch wider than the groove opening.
Plate Preparation
The preparation of the plate is usually in the form of a weld groove. The groove can be cut into one side or both sides of the plate, and it may be cut into either side one plate are both plates of the joint. The type, Depth, angle, and location of the groove are usually determined by a code standard that has been qualified for the specific job
Correct weld specimen preparation
The specimens may be cut out of the test weldment using an abrasive discs, by sawing, or by cutting with a torch. Flame test specimens must have the edges ground our machine smooth after cutting. All Corners must be rounded to a radius of 1/8 inch maximum, and all grinding R Machining marks must run lengthwise on the specimen. The weld must pass both the face and root bins to be acceptable. After bending, there can be no single defects larger than 1/8 inch, and the sum of all defects larger than 1/32 inch but less than 1/8 inch must not exceed a total of 3/8 inch for each Bend specimen. An exception is made for cracks that start at the edge of the specimen and do not start at a defect in the specimen.
Inter pass temperature
The temperature of the metal during welding. The temperature is given as a minimum and maximum.
Restarting a weld bead
The welding bead will need to be restarted after a welder stops to change the electrodes. When a weld bead is nearing completion, it should be tapered so that when it is restarted the build-up will be more uniform. To taper a weld bead, the travel rate should be increased just before welding stops. A one-quarter inch paper is all that is required. The taper allows the new weld to be started and the depth of the penetration reestablished without having excessive buildup. The slag should always be chipped and the weld crater should be cleaned each time before restarting the weld. The Arc should be restarted and the joint ahead of the weld.
Poor Fit-Up
The wilder must be able to read the molten weld pool correctly to make needed changes in the amperage, current, electrode movement, electrode angle,and timing.
What determines the acceptance or rejection of a bead specimen?
There can be no single defects larger than 1/8 inch and the sum of all defects larger than 1/32 inch but less than 1/8 must not exceed a total of 3/8 inch for each bend specimen.
Why should some weldments be pre-heated before welding starts?
This process helps to reduce cracking, hardness, distortion, and stresses by reducing the thermal shock from the weld and slowing the cooldown rate.
Why are some weld joints grooved?
To allow deeper penetration into the joint of the weld impaired strength.
What is post heating used for?
To slow the cooling rate and reduce hardening
What are the two common methods of making a root pass on an open Root joint?
Use only on joints with little or no root gap. This method requires a high amperage and short arc length. The arc length is so short that the electrode flux may drag along on the edges of the joint. The setup for this method must be correct for it to work. The other method can be Use on joints with wide narrow or varying root gaps. The electrode is moved in and out of the molten weld pool as the weld progresses along the joint.
Acceptance criteria for face beads and root beads
Visual inspection looks to see that the weld is uniform in width and reinforcement. There should be no Arc strikes on the plate other than those on the weld itself. The weld must be free of both incomplete fusions and cracks. The Joint penetration must be either 100% or as required by the specifications. The weld must be free of overlap, and undercut must not exceed either 10% of the base metal are 1/32 inches, whichever is less
List the things that a weld must be inspected for before it is ground for Bend testing.
Weld is uniform in width and reinforcement. No Arc strikes on the plate other than those in the weld itself. Free of both incomplete fusion and cracks. Joint penetration must be 100% or as required by the specs. Free of overlaps and undercut must not exceed either 10% of the base metal.
What technique can be used to make restarting weld beads easier and more uniform?
When a weld bead is nearing completion, it should be tapered so that when it is restarted the build-up will be more uniform.
What can be done to increase the amount of high-strength welding electrode in the final weld if the root weld was made with a low alloy electrode?
When high-strength, low alloy welding electrodes are used grinding is important to remove most of the most of the low-strength weld deposit. This will leave one hundred percent of the high strength.
What can watching the back edge of a weld pool help you determine?
You can change the electrode angle to control penetration and burn through. As the trailing angle is decreased making the electrode flatter to the plate penetration depth and burn through decreases, because both the art force and heat are directed away from the bottom of the joint back toward the weld. Surface tension holds the metal in place, and the mass of the lead quickly cools the molten weld pool holding it in place. Increasing the electrode angle toward the perpendicular will increase penetration depth and possibly cause more burn through. The arc force and heat focus on the gap between the plates will push the molten metal through the joint.
