chapter 5c

88 Welding Technology

WELD 1

i\SIDE VIEW

FRAME DISTORTEDAFTER BEAD #1

— (A) Distortion after the first weld.

WELD 1

*j SIDE VIEW J

FRAME TURNEDOVER AFTERFIRST WELD

BEAD APPLIED

(B) Frame turned over for the second weld.

WELD 2

WELD 1 -*-— (C) After the second weld, the frame is straighter.

5-71. Distortion created by welding channel irontogether to make box frames.

CORRECT EDGE PREPARATION

AND GOOD FITUP

A 60 MAX. L

\/1/32" TO 1/16" MAX -|

REDUCE BEVELANGLE WITH LARGE

ROOT OPENINGS

"U" PREPARATION

USE DOUBLE-VPREPARATION

30f

VUX

5-72. Correct edge preparation and good fitup.

SHRINKAGE WELDALONG

NEUTRALAXIS

T JOINT (FILLET WELDED)

5-73A. The moment arm is the distance between thecenter of the weld and the neutral axis.

SHRINKAGE ABOUTNEUTRAL AXIS

NEUTRALAXIS

STEEL

SUBMERGEDARC WELDS

T JOINT MOMENTARM

Increased Welding Speed

5-73B. Deep penetration of the arc places the weldcloser to the neutral axis and reduces the moment arm.

The volume of base metal adjacent to theweld affects distortion. Slower welding speedscreate a larger heat affected zone. Faster weld-ing speeds reduce the time the metal has toabsorb heat. The larger the area of cool parentmetal, the less distortion, Fig. 5-74.

While welding sheet metal, you willnotice that more of the nearby metal isaffected due to higher conductivity. This, com-bined with the fact that thinner sheet metal isless rigid than thicker plate, explains whysheet metal always distorts easily.

Welding Designs and Defects 89

ADJACENTBASE METAL

HEAT AREA (B) HEAT AREA (B)

5-74. Distortion is reduced by having a smaller heat areaor a larger, cool, base metal area.

MINIMUM NUMBER OF PASSES

WRONG RIGHT

5-75. Using the fewest number of passes helps preventdistortion.

Single Rather Than Multiple Passes

A weld made with more passes createsgreater distortion than one made with fewerpasses because of the additional heat, Fig. 5-75.

Proper Alignment

Various methods are used for pullingplate edges into alignment and maintainingthis alignment during welding, Fig. 5-76. Onesuch method is by using various types of jigs.The most widely used technique involves

small clips which are tack welded to the edgeof one of the plates, Fig. 5-77. Wedges are thendriven between the clips and the second plate,in order to bring both plate edges into align-ment. After the plates have been weldedtogether, the wedges are removed and the clipsare knocked off with a hammer.

BACK-TO-BACK CLAMPING

ZJ D D DS3 Da

5-76. Back-to-back clamping maintains alignment.

V PREPARATION

AREASTO BE

WELDE:

PL/

I

OS)CLIP C

KtS

tSRt

CLIP B

(£&s$i>

CLIP A

VTE #2

—

-49

r^

(B5PLATE #1

CLIP A

•*-

PLATE #2 \ / LTACK

WELDS

a

PLATE#1

V PREPARATION

5-77. Clips are tack welded along the length of plate # 1.Wedges are placed in the opening to keep the platesaligned.

A second type of jig is a strong-back. Thisconsists of a steel plate fitted with bolts, Fig. 5-78. The bolts are tack welded to the parentmetal and can be adjusted to apply correctamounts of pressure.

Strong backs can be made from I-beams,channel iron, angle iron, or even railroad iron.The welder should, however, use a strongback just rigid enough to hold the weldment inthe proper position.


NUT

\STEEL

SPACERSXNUTS STEEL

SPACERS

jftf

,NUT

/ \ STRONG BACK STEEL PLATE / \

5-78. Using a strongback to preventdistortion.

Peening

Another method occasionally used tocontrol distortion is peening. Since the weldarea contracts, peening, or light hammering,tends to expand it, Fig. 5-79. However, thisexpansion occurs only near the surface.

Expanding the weld metal by peening ismost effective at higher temperatures wherethe metal's yield strength is lower. Unfortu-nately, most distortion problems occur atlower temperatures. For this reason, peeningis not always effective. Another disadvantageof peening is that it work-hardens the metal'ssurface.

Flame Shrinkage

Fig. 5-80 illustrates how controlled expan-sion and contraction are applied usefully inflame shrinkage, often referred to as flamestraightening. In (A), a gas torch flame is heat-ing a center spot on a distorted steel plate. Thespot heats up rapidly and must expand. But, asindicated by (B), the surrounding cooler plateprevents the spot from expanding along theplane of the plate. The only alternative is forthe spot to expand abnormally through itsthickness. When it cools, it tends to contractuniformly in all directions (C). When carefullydone, spot heating gives a planned shrinkage,useful for correcting distortion caused byheating, cooling, or bending.

PEENING j^*HAMMER mi

5-79. Relieving stress by peening.

A high temperature is not required whenflame shrinking, but a large torch should beused. When a length of metal is to be flameshrunk, such as along the edge of a twistedpanel or the flange of a beam, the torch may bemoved progressively along the length. Or,selected spots may be heated and allowed tocool with intermediate checking, Fig. 5-81.

A buckled panel, as shown in Fig. 5-82,has too much weld material in the centralarea, and flame shrinking will straighten thepanel. If the part of an assembly to be shrunkis restrained, which will be the case in manywelds, too much heating could cause thedevelopment of locked-in stresses. It is goodpractice to proceed cautiously when flameshrinking, allowing the metal to cool andchecking the results.


(A)

PLATE -(C)

(B)

SURROUNDING PLATERESISTS EXPANSION OF

SPOTS WHICH HAS BEEN HEATED

TORCH

COOL PLATERESTRAINSEXPANSION

RESTRAINT CAUSED ABNORMALEXPANSION IN THICKNESS

UPON COOLING WELD TENDS TOSHRINK TO A SMALLER SECTION

7IF PLATE IS RESTRAINED

SHRINKAGE STRESSES DEVELOP

5-80. The flame shrinking procedure. (Lincoln Electric Co.)

SHADED AREATO BE HEATED

POINT OF BEND

RESTRAINEDEXPANSION

POINT OF BEND

5-81. Flame shrinkage will help straighten the T iron.

A water spray is sometimes used to speedthe cooling. This may be accomplished, asshown in Fig. 5-83, by inserting a Y fitting intoa compressed air line and running a rubberhose from the fitting to a pail of water. Whenthe valve is opened, the rush of air past theopening in the fitting draws some water intothe air stream, creating an atomized spray.When the spray strikes the hot plate, it turnsinto steam and absorbs a substantial amountof heat. One pound of water will absorb142 Btus [35.67 kcal] of heat as it is raised from70 to 212°F [21 to 100°C] and another 970 Btus[243.7 kcal] in vaporizing into steam. Because

of the vaporizing heat required, cooling israpid, even with only a small amount of water.Since all of the sprayed water is vaporized, thework remains dry.

Using Proper Weld Techniques

Do not overweld. Excessive weld metalincreases distortion due to increased heat anda pull within the weld metal which may bestronger than the base metal, Fig. 5-84.

Intermittent welding can also reduce dis-tortion because the areas of heat conductivityare broken up, Fig. 5-85. Intermittent weldsmay be either parallel or staggered.

The backstep method, shows in Fig. 5-86is another method of reducing distortion. Dur-ing backstepping, the areas of expansion arebroken up by cooler areas, and they tend toneutralize one another.

Automatic welding is preferred to manualwelding because it gives deeper penetration.

Chill Strips

Heavy metal strips called chill strips maybe used in conjunction with backers. Chillstrips help prevent distortion beyond the weldarea by absorbing the heat due to conduction,Fig. 5-87.


PANELS WITH STIFFENERS-

EXCESS METAL

5-82. A buckled panel and the areasof excess metal to be heated. (LincolnElectric Co.)

Y -FITTING

VALVE

AIR HOSE

WATER ANDAIR SPRAY

COMES OUT HERE

INTERMITTENT WELDING

WELD POSITION

5-83. Using an air hose to spray water for metalshrinkage. (Lincoln Electric Co.)

DO NOT OVERWELD

EXCESSIVE WELD METAL/INCREASES DISTORTION

5-84. Overwelding increases distortion.

5-85. Distortion can be reduced by intermittent welds.

Since chill strips are held in place byclamps, any weld with chill strips will haveless opportunity to warp. Alignment of all thepieces while welding is also assured.

Fixtures have been made with chill stripsfor special purposes, such as production of

ELECTRODE ARC STARTINGSEQUENCE ROOT GAP

5-86. The operator welds from right toleft but moves farther right each time tostart each short weld.


HOLDDOWNCLAMP

5-87. Chill strips are used to absorb some of the heat.

tanks and containers, using either light alumi-num, stainless steel, or other materials.

Planning the Welding Sequence

Weld minor assemblies first. This reducesthe chances of severe distortion occurringlater during the final assembly, Fig. 5-88. Alsoweld the more ductile sections first so any dis-tortion can be corrected before final assembly.

PART #1 PART #2- PART #3

5-88. Final assembly is done after minor assemblies toprevent major distortion.

Discussion Questions

1. Name and describe the parts of a weld joint.2. What is the difference between a bead weld

and a weave weld?3. Describe a fillet weld.4. What is the purpose of a fixture?5. What factors must be taken into considera-

tion when selecting a weld joint?6. Name as many weld defects as you can.

chapter 5c

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