Zavarivanje svornjaka

8/13/2019 Zavarivanje svornjaka

1/13

Harry A. ChambersConsultantNelson Stud Welding, Inc.Elyria, Ohio

The embedment properties of stud welded anchors have been thesubject of many testing programs worldwide. Currently, designprovisions for cast-in-place anchorages are included in the 2000edition of the International Building Code (IBC 2000), and these will

be introduced along with design provisions for post-placed anchorsinto the 2002 edition of the ACI Building Code (ACI 318-02). In all ofthe testing programs, performance and failure modes of the as-weldedstuds with respect to groups, edge conditions, tension, shear, andcombined loadings have been the central theme of the reports. Studwelding principles and practices necessary for obtaining consistentstud weld quality and anchorage performance have received littleattention. The purpose of this article is to present the fundamentalprinciples of stud welding and implementing practices so that the usermay be confident in the ensuing welding results and performance ofthe finished product.

E lectric arc stud welding wasinvented just prior to WorldWar II and developed out ofa necessity to attach wood plankingto naval aircraft carriers. Threadedstuds could be placed on the exte-rior side of steel deck plates by usingonly one welder rather than drillingholes through the plate, a very slow

and labor intensive process. Since that

time, the use of studs has increasedenormously in the appliance, automo-tive, and construction industries.

Today, stud welding is widely usedin the construction industry. There aremany different stud welded productsthat are commonly used in the manu-facture of precast/prestressed com-ponents, including threaded, headed,

and deformed bars. These products are

Principles and Practicesof Stud Welding

46 PCI JOURNAL

STATE-OF-THE-ART PAPER


2/13

September-October 2001 47

welded to steel plates and other shapesthat are embedded in precast/pre-stressed concrete products for struc-tural connections (see Fig. 1). 1

The behavior of these embed-ments is critical to the performance ofeach structural element. Because thestrength and integrity of the finishedstructural system is vital to the overall

safety of the structures occupants andthe general public, engineers must givefull attention to the planning, design,and execution of these connections.

LITERATURE REVIEWTest results that describe the perfor-

mance of headed studs and deformedbars made from cold deformed wire asembedments in concrete are availablein various publications. 2-6 References

2 and 3 describe the ability of studs todevelop full strength welds and dis-cuss the fact that, in some cases, somewelds were less than full strength. Ref-erences 4 and 5 document embedmentshear and tension tests of deformedbar anchors where no weld failuresoccurred. Reference 6 summarizes theresults of extensive testing and studiesfrom many sources on the performanceof stud welded anchors and other typesof anchorage devices.

STUD WELDING FAILURESWhen the proper operation of stud

welding equipment is combined withgood quality control and inspectionprocedures, full strength welds canbe obtained consistently and result inoptimal performance of the embeddedanchorages. In the authors experienceand in discussions with other expertsin the stud welding industry, the rootcauses for weld or stud failures canusually be attributed to one or more ofthe following factors:

Unacceptable base plate materialor plate surface condition

Inappropriate weld settings Malfunctioning or obsolete equip-

ment Little or no formal training for

stud welding operators Lack of quality control and in-

spection proceduresThis paper addresses these problems

by explaining the fundamentals of the

process and highlighting key issuesin the assurance of quality in the studwelding process. The first section dis-cusses basic principles, and the secondpart presents the recommended stepsfor producing quality stud welds andpreventing weld failures.

THE STUD WELDING PROCESSElectric arc stud welding involves

the same electrical, mechanical, andmetallurgical principles found in anyother arc welding process. In stud

welding, the power source and studwelding control system are set to con-trol the amperage and the arc dura-tion or time. The welding gun has atrigger-activated circuit to initiate theweld and a lifting mechanism to drawthe stud away from the base materialand initiate the welding arc. The gunincludes a stud-holding chuck, twolegs, a foot piece, and a ferrule grip tohold the ceramic ferrule (also calledan arc shield) (see Fig. 2).

The sequence of operations formaking a stud weld is illustrated in

Fig. 1. Typical connection using deformed bar anchor studs in a precast/prestressedconcrete tee. Source: Nelson Stud Welding, Inc., Reference 1.

Fig. 2. Stud welding gun. Source: Nelson Stud Welding, Inc.

Precast/Prestressed Tee

Strand Hold Down Stud

Deformed

Bar Studs

Steel InsertPlate

FerruleGripLeg

Ferrule

Foot

PlungeStud

Chuck


3/13

48 PCI JOURNAL

Fig. 3. 7 The fasteners for electric arcstud welding have a special shapeand flux on the end of the stud that isto be welded. This flux initiates andimproves starting and stabilizing thewelding arc. It also serves to deoxidizethe molten weld metal for a sound,low-porosity weld zone to produce afull penetration weld.

The ceramic ferrule confines theweld arc and heat to a specific area ofthe base material and holds the moltenmetal in place to provide the uniformweld flash. The term weld flash is usedinstead of fillet because the weld zonecomprises a mixture of melted mate-rial from the stud end and expelledfrom the base plate material, ratherthan a weld made from a separate fillermaterial that is deposited.

A completed weld cross section is

shown in Fig. 4, with the various areasof the weld cross section identified.

STUD MATERIALSStud and base plate materials must

be compatible with the stud weldingprocess. Suppliers of both materialscan provide physical and chemicalcertification on the products they sup-ply and these should be requested onall orders as part of good quality con-trol recordkeeping. Studs of all stylesare available in low carbon steel and

Fig. 3. Sequence of stud welding operations.A. A stud and ferrule are loaded into the gun, and the gun properly positioned against the base plate.B. The gun is pushed against the base material taking up the plunge, or stud length available for burn off against the gun spring

pressure. The trigger button is depressed to start the fully controlled automatic sequence. This sequence consists of initiating theweld current, and lifting the stud to create an arc by energizing the gun solenoid.

C. The arc duration time is completed and the stud plunged into the molten pool by means of a spring when the gun solenoid isde-energized and turning off the weld current at the end of the weld cycle.

D. The weld is completed and the gun is lifted off the stud and the ceramic ferrule removed. The stud is inspected for weld quality.

Fig. 4. Typical stud weldcross section. Source:

Nelson Stud Welding, Inc.Weld Zones: (A) Heat-

unaffected stud material,(B) Stud heat affected zone

(HAZ), (C) Cast zone,(D) Base material HAZ,

(E) Heat-unaffected basematerial.

When performed properly,the stud weld is stronger

than both the stud materialand base plate material,and failure will occur at

the ultimate steel strengthin the stud shank or in thebase plate, rather than in

the weld.


4/13


various grades of stainless steel. Com-monly used studs in the precast andconstruction industries are governedby the following American WeldingSociety (AWS) codes:

Low carbon steel studs: ANSI/ AWS D1.1-00, Structural WeldingCodeSteel. 8 Studs are required tomeet ASTM A 108 9 chemical speci-fications for Grades C-1010 throughC-1020 with the mechanical propertyrequirements listed in Table 1. Threestud classifications are identified asfollows (see Table 1):

Type A studs are general purposestuds, of any type or size, usedfor purposes other than embed-ment or composite design con-struction. These include 1 / 4 in.and 3 / 8 in. (6 and 10 mm) headed

anchors. Type B studs are headed, bent, or

some other configuration, 1 / 2 in.(13 mm) or larger in diameter,used for concrete embedmentor as an essential component incomposite design construction.

Type C studs are ASTM A 496 10 deformed wires of any diameter,used for concrete embedmentpurposes.

Stainless steel studs: ANSI/AWSD1.6-99, Structural Welding CodeStainless Steel. 11 Studs are requiredto meet ASTM A 493 12 or ASTMA 276 13 chemical specifications forGrades XM-7, 304, 305, 309, 310,or 316, or the low carbon versionsof these alloys with the mechanicalproperty requirements listed in Table2. The properties listed apply to studTypes A and B as described in Table1. This code does not contain stainlesssteel Type C, deformed bar studs.

Acceptable base plate materials arelisted in ANSI/AWS D1.1, Table 3.1,Groups I and II. These include mostof the common low carbon structuralsteels such as A36, A709, A992, andA516. Both the common carbon steelmaterials in Table 3.1 of ANSI/AWSD1.1 and the stainless steel materialslisted in ANSI/AWS D1.6, Section7.2.6, that meet ASTM A 276 are ac-ceptable base materials for stainlesssteel stud welds.

Table 1. Minimum mechanical property requirements for studs. 8

Type A Type B Type CProperty AWS D1.1 AWS D1.1 ASTM A 496 5

Tensile strength 61,000 psi 65,000 psi 80,000 psi(UTS) (420 MPa) (450 MPa) (552 MPa)Yield strength 49,000 psi 51,000 psi Not specified(0.2 percent offset) (340 MPa) (350 MPa)Yield strength Not specified Not specified 70,000 psi(0.5 percent offset) (485 MPa)Elongation (percent in 2 in.) 17 20 Not specified

Elongation (percent in 5 x diameter) 14 15 Not specifiedPercent area reduction 50 percent 50 percent Not specifiedNote: 1 in. = 25.4 mm; 1 psi = 0.006895 MPa.

STUD WELDING EQUIPMENT

Stud welding has been in continuoususe for over 60 years. Since the early1900s, there have been many changesin the equipment used in the process,but the principles remain the same.In the basic process, DC power froma self-contained generator or trans-former/rectifier is passed through astud welding control system. The con-trol system is set at a determined time,voltage, and amperage to initiate thearc, which forms the molten pool. Theremaining unmelted stud shank is thenplunged into the molten metal pool.

In the early years of stud welding,the power source was separated fromthe control system. Power came frommotor generators, battery banks, and

transformer rectifiers. Today, the con-trol system is typically incorporatedinto the power source, and transformer

Tensile strength 70,000 psi(UTS) (490 MPa)Yield strength 35,000 psi

(0.2 percent offset) (245 MPa)

Elongation (percent in 2 in.) 40

Table 2. Minimum mechanicalproperty requirements for stainlesssteel studs. 11

Note: 1 in. = 25.4 mm; 1 psi = 0.006895 MPa.

Fig. 5. Transformer/rectifier powered stud welding systems. Source: Nelson StudWelding, Inc.

rectifiers are now the predominantsources of power (see Fig. 5). Systemsare available for welding studs withdiameters ranging from 1 / 8 to 1 1 / 4 in. (3to 32 mm).

In recent years, the stud welding in-dustry has been transformed from me-chanical to solid state welding equip-ment with closed-loop controls thathave contributed significantly to sim-pler weld setups and better weld qual-ity. These solid state controls provide


5/13

50 PCI JOURNAL

excessive splatter and high or unevenfillet formation. Plunge is a physicalmeasurement set and measured with

the stud and ceramic ferrule in placeon the stud gun (see Fig. 2).

Lift is the distance the gun pullsthe stud away from the base material.Before the weld is started, the stud andbase metal are in contact. Lift createsan air gap that the electric current mustbridge. The current flow across the re-sistance of this gap creates the arc heatto melt the stud and base material.

If no gap exists, the current will notcreate sufficient heat to fuse the metal.

A short lift may allow the moltenmetal to bridge the arc gap, resultingin a cold weld. An excessively longlift increases the chance of havingarc blow and welds that are bondedon only one side of the fillet. Lift isphysically set on the stud gun and ismeasured when the stud weld is initi-ated. Lift should be set and measuredby placing the stud and ferrule on anonconductive surface and initiatingthe weld cycle so that an actual molten

weld is not made. Time is the duration of the weld.On thin base material, a shorter timeand higher amperage can be used toachieve sufficient heat and preventmelting through the base material. Onsome base materials, a longer time andlower amperage improve the ductilityof the weld zone. Weld time is set onthe time setting indicator of the con-trol system.

Amperage is a measure of the cur-rent from the power source that flowsacross the air gap created by the lift.

verification measurement of both weldtime and current.

Current controls can adjust for weld

cable resistance, compensate for in-coming power fluctuations, and pro-vide system shutdown in the event ofvariations from the set weld param-eters. As another example, weldingguns with potentially erratic pneumaticweld plunge dampeners have been up-graded to very reliable, self-contained,sealed hydraulic dampener controls.

STUD WELDING SETUP

An understanding of the settings and

adjustments and their relationship withweld quality is needed to ensure con-sistent stud welding results. The fol-lowing definitions of specialized studwelding terms will make the narrativeeasier to understand.

Plunge is the length of stud thatprotrudes beyond the ferrule. Thisportion of the stud is available to beburned off, or melted, to develop theweld fillet. A short plunge may cause

Downhand welding Overhead welding Vertical welding Diameter Area Time Time Time in. mm sq in. Amp Sec Cycles Lift Plunge Amp Sec Cycles Lift Plunge Amp Sec Cycles Lift Plunge

0.187 4.9 0.0276 300 0.15 8-9 0.062 0.125 300 0.15 8-9 0.062 0.125 300 0.15 8-9 0.062 0.125 0.250 6.4 0.0491 450 0.17 10-12 0.062 0.125 450 0.17 10-12 0.062 0.125 450 0.17 10-12 0.062 0.125 0.213 7.9 0.0767 500 0.25 15 0.062 0.125 500 0.25 15 0.062 0.125 500 0.25 15 0.062 0.125 0.375 9.5 0.1105 550 0.33 20 0.062 0.125 550 0.33 20 0.062 0.125 600 0.33 20 0.062 0.125 0.437 11.1 0.1503 675 0.42 25 0.062 0.125 675 0.42 25 0.062 0.125 750 0.33 20 0.062 0.125 0.500 12.7 0.1964 800 0.55 33 0.062 0.125 800 0.55 33 0.062 0.125 875 0.47 28 0.062 0.125

0.625 15.9 0.3068 1200 0.67 40 0.093 0.187 1200 0.67 40 0.093 0.187 1275 0.60 36 0.062 0.187 0.750 19.1 0.4418 1500 0.84 50-55 0.093 0.187 1500 0.84 50-55 0.093 0.187 1700 0.73 50 0.093 0.187 0.875 22.2 0.6013 1700 1.00 60-65 0.125 0.250 1700 1.00 65 0.125 0.250 Not Recommended 1.000 25.4 0.7854 1900 1.40 85 0.125 0.250 2050 1.40 85 0.125 0.250 Not Recommended

Table 3. Stud welding setups for mild and stainless steel studs welded to mild and stainless steel base materials. 14

Note 1: These welding parameters should be considered as an initial setup guide for the stud diameter and position being welded. They should produce satisfactory results, butshould not be considered so restrictive that they prevent modification based on physical and visual test evaluations of the finished welds as outlined in AWS D1.1 Structural Weld-ing Code Steel, AWS D1.6 Structural Welding Code Stainless Steel and/or AWS C5.4 Recommended Practices for Stud Welding, which would assist in finalizing the weldsetup based on the conditions and equipment at the production welding site.Note 2: 1 in. = 25.4 mm; 1 sq in. = 645 mm 2.

Fig. 6. Typicaltension

test fixture(Reference 8).

Slotted Fixtures toHold Stud Head andSpecimen Plate


6/13


Increasing the amperage increases theweld heat. As with the time setting, ahigher amperage setting is needed forlarger stud sizes. Amperage is also seton the control systems current settingindicator.

Alignment is the proper center-ing the stud in the ceramic ferrule sothat the stud does not contact the ce-

ramic ferrule during lift and plunge,which can cause friction or bindingbetween the stud and ferrule. Bind-ing can inhibit both stud lift and studplunge sufficiently so that there is lessthan sufficient stud melting and lessthan full penetration into the moltenweld pool, resulting in a less than fullstrength weld.

Table 3 contains suggested weld set-tings. 14 These settings are suggestedstarting points for obtaining a final

setup. The final setup is verified byvisual inspection, after weld measure-ments and physical testing to confirmweld quality. Other factors such aswelding system grounding, base platecomposition, and cable connectionscan influence the weld settings.

Stud welding manufacturers are re-quired by AWS codes 8,11 to qualifytheir standard stud weld base diametersby 60 stud tests of each stud diameterwith the current at plus or minus 10percent from optimum [except in thecase of 7 / 8 and 1 in. (22 and 25 mm)diameters, which are plus or minus 5percent]. The tests require bending andtensile testing all of the studs to failurewithout failure in any weld. Currentrange variations in the test provide anindication of the acceptability of thewelds under conditions that can occurin actual shop or field welding produc-tion.

Figs. 6, 7, and 8 illustrate typicalfixtures used for stud base tests, whichinclude the alternating 30-degree bendtest at a distance of 2 in. (51 mm) fromthe stud weld. 8,11 Fig. 9 shows accept-able and unacceptable weld failures. 8

STUD WELDING PRACTICEProper implementation of the

practices presented in this sectionwill result in successful stud weld-ing products. As in other welding andfabricating methods, there are somegeneral guidelines to consider when

Fig. 7. Bend testing device (Reference 8).

Fig. 8. Suggested type of device for qualification testing of small studs (Reference 8).

Notes:

1. Fixture holds specimen and stud is bent 30o

alternatelyin opposite directions. 2. Load can be applied with hydraulic cylinder (shown)

of fixture Adapted for use with tension test machine.


7/13

52 PCI JOURNAL

determining the basics of good prac-tice. Among these are the followingconsiderations:

Weld Plate Thickness A platethickness that is at least 0.33 timesthe stud shank diameter will developthe full steel tensile and shear capac-ity of the stud. It is possible, however,that this ratio of stud diameter to platethickness will cause unacceptable dis-tortion and bending when high loadsare applied. A minimum plate thick-ness of one-half the stud diameter isoften specified and is recommendedin the PCI Design Handbook. 15 Platebending should always be a consider-ation in any embedment plate design.

Weld Plate Cleanliness The areawhere the stud is to be welded (weldspot) should be as clean as possible.The spot where the ground clamp isfastened should also be cleaned onboth sides of the plate so that a goodcurrent path is established.

The presence of light rust or lightmill scale is not usually detrimental.

Heavy mill scale or heavy, flaky rustshould be removed, as well as anydeleterious coating such as heavy oil,paint, galvanizing, grease, and mois-ture. Weld and ground spots can becleaned very quickly with an abrasivewheel, wire brush or wheel, drill burr,or end mill.

Note that solid grinding wheels orabrasive discs do not remove zinc gal-vanized plating very well. The grind-ing disc or wheel fills with zinc anddoes no more than spread the zincplating around, making the weld spotlook shiny and clean. Open pore abra-sive discs and grinding burrs are bet-ter options for removing galvanizing.When zinc galvanizing is used to coatstuds, the coating should be removedfrom the weld end by the stud manu-facturer before the studs are shippedand welded.

Galvanizing Even though zincgalvanizing is electrically conductive,studs should not be welded to a galva-nized plate. Zinc is a contaminant that

affects the weld metallurgy, causingbrittle welds. Galvanizing should bedone after, rather than before, the baseplate has been welded to the stud.

The effects of hydrogen embrittle-ment should be considered when hotdip galvanizing is used. Hydrogen em-brittlement can have several points oforigin, which can cause serious brittle-

ness in the weld stud shank when studsare bent to a very tight bend diameter.Causes of hydrogen embrittlement areimproper pickling of the finished weldplate, either by pickling too long ornot rinsing thoroughly after pickling;plating too long or at too high a tem-perature; and using a stud that is ofa much higher strength than the baseplate material strength.

To prevent embrittlement in thebend area of bent bar studs, the bend-

ing radius in studs to be zinc platedshould be four to six times the stud di-ameter as provided by the ACI Build-ing Code. 16

Edge Distance Studs should beplaced no closer to a base plate freeedge than the stud diameter plus 1 / 8 in.(3 mm) to the edge of the stud base.This distance should be at least 1 to11 / 2 in. (25 to 38 mm) from each freeedge.

Grounding/Arc Blow Edge dis-tance and ground placement can influ-ence weld quality due to arc blow; thatis, the welding arc is electromagneti-cally deflected toward the groundingpoint or toward the larger mass of thebase plate configuration being welded.Fig. 10 shows typical ground and edgeeffect patterns due to arc blow. 17

These effects are less noticeablewhen large masses of steel are present,such as in large beams, but relativelysmall embedment plates can presentdifficulties. Typically, there is a lackof weld fillet on the periphery of thestud opposite the direction of the arcblow that can adversely affect weldstrength and quality.

As an example, a welding platentable may be grounded at all four cor-ners by four separate ground wiresbolted to the table and joined to thewelding system single ground wireby a bolted connection. This turns theentire platen into a grounded mass andarc blow on a small plate set onto thetable is minimized. On longer, rect-

Fig. 9. Typical acceptable stud failures and typical unacceptable weld failures.

Note: Fracture in weld nearstud. Fillet remains on plate.

Note: Fracture through flash.Fillet torn from plate.


8/13


angular plates, a double ground (oneat each end on opposite edges of theplate) provides a good current flowpattern.

Ground connections may be screwtype C clamps, fast action springclamps, or lever action hold-downclamps mounted to the welding tableand connected to the stud welding

system ground cables. All of thesetypes have proven successful. Fre-quently, a copper or steel plate largerthan the base plate to be welded, witha center ground bolted to the bottom,will eliminate or minimize the effectsof arc blow.

Weld splatter, weld berries, and fer-rule pieces should be removed fromthe weld plates so that the surfacecontacts the weld plate cleanly andevenly. This will ensure good electri-

cal contact and current flow. It maytake some time and a few trials to es-tablish a good current flow path forthe typical base plate configuration.When proper grounding is establishedand arc blow is minimized, the resultis a consistent high quality weld witha low rate of rejection and decreasedrepair costs.

Ceramic Ferrules These piecesare also called ceramic arc shields.Ferrules serve several functions.First, they contain the pool of moltenmetal and form it into the fillet (flash)formed around the periphery of thestud. Second, they control to a greatextent the amount of arc brightness andthe quantity of sparks expelled duringthe weld. Third, they are designed withspecific vent patterns so that when thearc is initiated, the flux in the stud endis consumed and deoxidizes the weldzone by expelling weld gasses throughthe vents, thus preventing oxygen fromentering the weld area.

Ferrules are also available in a widevariety of configurations to allow studsto be welded to round tubing and bars,plate edges, channels, struts, rectangu-lar and square structural tubing edges.These various configurations alsoallow for a number of weld positionsother than downhand.

It is important to keep the ferrulesdry. If they absorb too much moisture,the weld instantly turns the moistureinto steam with a substantial amountof molten metal expelled. In addi-

tion to the possible danger from thisexplosion, the weld is made very

porous and weak. Ferrules that havebeen wet or have absorbed moisturecan be dried by heating them to 250F(121C) until the moisture is gone.

Ferrule cartons are marked with awarning that they contain silica, a pos-sible health hazard. Because the fer-rules are made from green fireclaywith binders and fired at high tem-peratures, there is no free silica mate-rial in respirable sizes released duringthe weld. If simply broken free of theweld, they are basically an inert, inor-ganic material, such as fired aggregateor rock, and may be disposed of easilyand safely. It would take an enormousamount of ferrule breakage or grindingto produce a dangerous health hazardto the stud welding operator or thosenearby.

Position Welding Studs of allweld base configurations and di-ameters can be easily welded in thedownhand position. As a general rule,studs up to 3 / 4 in. (19 mm) in diametercan be welded to the weld plate in avertical position with consistent fullstrength results. Special ceramic fer-rules are used with studs 5 / 8 in. (16mm) and larger when welding to thevertical position of the plate. Thereis a special ceramic ferrule for weld-ing 7 / 8 in. (22 mm) diameter studs tothe vertical position of the plate, butwelding this diameter with the baseplate vertical requires very carefullycontrolled conditions.

Welding overhead can also be done

with all stud diameters. Naturally, theoverhead position causes an increased

amount of welding sparks to fall dur-ing welding and suitable operator pro-tection is needed. There are spark re-tention accessories available from thestud manufacturer. Welding positionsare shown in Fig. 11. 11

Stainless Steel Studs Welded toCarbon Steel Base Plates Fullstrength welds are made when stan-dard carbon steel studs are welded toeither approved stainless steel or car-bon steel base plate materials. Simi-larly, welds made with stainless steelstuds to stainless or carbon steel baseplate materials develop full stud steelcapacity in tension or shear. However,in cases where stainless steel studs arewelded to carbon steel plates and areto be subject to repetitive or cyclicloads, stress corrosion failure in theweld can occur, initiated by carbideprecipitation during the weld and sub-sequent intergranular cracking.

As an example, on one project,stainless threaded studs were weldedto low carbon structural steel to securecounterweight vibration dampers ex-posed to open weather conditions. Theconstant movement of the dampenerweights and repetitive loading cycleson the studs produced cracking, corro-sion and failure in the stud welds.

It is good practice to specify that thestainless studs to be used under suchconditions are either annealed aftermanufacture or made from annealedin-process stainless steel with lessthan 90 Rockwell B Hardness in the

Fig. 10. Effect of (a) Ground connection, and (b) Workpiece on distortion ofmagnetic field (Reference 17).


9/13

54 PCI JOURNAL

finished condition. This minimizes thechance of weld cracking and failures.

Material Selection and Verification

Studs are made from steel suppliedto the stud manufacturer by qualityapproved steel suppliers. Quality as-surance procedures require that thesteel supplier provide certified milltest reports (CMTRs) for each heatand diameter of steel supplied. TheseCMTRs certify compliance to weld-ing code specified material grades andchemistry. These reports are requestedat the time of purchase from the studmanufacturer and are part of their cer-tification package on every shipmentof studs.

At the time of manufacture, studheats are also tested to determinewhether their mechanical propertiesare in compliance with welding coderequirements. A certificate of compli-ance (COC) for each stud shipment

can be made at the time of shipmentto verify that chemical and physicalproperties comply with the specifica-tions of the engineer.

A COC can certify compliance withtypical steel specifications such asASTM A 108 9 and A 276, 13 as well asvarious welding codes such as AWSD1.1, 8 AWS D1.6, 11 CSA W59, 18 andISO 13918. 19 Similarly, the weld platefabricator should require certified milltest reports from their steel vendors toverify compliance to welding code-ap-proved materials.

As mentioned previously, weldingcodes require stud manufacturers toweld test and qualify their stud basediameters and materials. These testsmust be certified by an independenttesting laboratory, which will eitherconduct or witness the testing. Alongwith the stud COCs and CMTRs forboth studs and weld plates, the studweld base qualifications should bekept on file with the weld plate fabri-

cator to verify compliance with qualityprograms and welding codes.

Stud Welding at Low Tempera-ture Studs should not be weldedwhen the base plate temperature isbelow 0F (-18C) or when the sur-face of the base plate is wet, coveredwith frost, or exposed to rain or snow.When the base plate is below 50F

(10C), impact testing of the weldedstud, such as with a hammer blow,should not be done. Instead, the studshould be tested by bending it slowlywith a hollow pipe or other bendingdevice. A brittle failure by impact test-ing at low temperatures in the weld orin the base metal is quite common.

Tension and shear tests done at tem-peratures as low as -40F (-40C) onstuds welded at -40F have shown noloss in weld strength. 20 It is the impact

testing that causes the weld failure.Whenever possible, welding and test-ing studs at low temperatures shouldbe avoided; if indeed it is necessary,the bend test should be done with apipe instead of a hammer.

Training and Qualification of the StudWelding Operator

The following section discussestraining of operators and process qual-ification.

Operator Training Introduc-tory training of operators in the studwelding process is the first step insuccessful production stud welding.This familiarizes the operators withthe general principles of the process,proper equipment setup, weld setupfor the studs, general guidelines, andinspection techniques. Trained and ex-perienced representatives of stud andequipment manufacturers can providespecifications, guidelines, and otherliterature as part of a complete pro-gram that will lead to formal qualifica-tion of both the operator and the studwelding process.

Process Qualification Stud weld-ing is a prequalified process uniqueamong the many welding processesdue to the many millions of studs thathave been successfully welded usingthe process. When studs are weldedin the downhand position to approvedbase plate materials, only two studsare required to be welded and tested.

Fig. 11. Positions for stud welding (Reference 11).


10/13


The test consists of both a visual andphysical inspection.

The physical inspection requiresbending the studs 30 degrees from thevertical by hammering the stud on theunwelded end or bending it with apipe or other device. The pipe shouldbe placed 2 in. (51 mm) above the studweld. The visual inspection verifies

that the stud flash is complete aroundthe stud periphery, and that the after-weld height of the stud is appropriatefor the diameter being welded.

If the physical and visual inspectionare each satisfactory, then both theprocess and the operator are consid-ered qualified for those stud diameterswelded downhand. This qualificationapplies as long as there are no changesmade in the studs, settings, equipment,welding cables, or ceramic ferrules, or

from an approved to a non-approvedbase material qualified by the test. Thetwo-stud test is required at the startof every production period, such as ashift change or operator change.

Studs welded to a non-approvedbase plate material or in a positionother than downhand, such as to avertical or overhead base plate, orthe fillet or heel of an angle, must beapplication qualification approved.Qualification approval consists ofwelding ten of each stud style anddiameter to be used in production tothe positions and base materials tobe used in production, with the sameequipment, settings, welding cablesand ceramic ferrules to be used in theactual production welding.

All ten studs of each diameter andin each position to be qualified mustbe tested to failure by tensile, bend, ortorque test, or a combination of these.They must also meet visual inspectionrequirements. For all studs tested, fail-ure must be in the stud shank or baseplate material, rather than in the studweld. Note that failure is allowed inthe base plate material.

This provision is acceptable onlywhen it is known the base plate mate-rial may be of a composition, strengthor thickness that will not fully developthe stud weld strength, but the strengthand weld results are acceptable for theend use intended. Such a failure is notacceptable for embedment plates usedfor structural connections nor for at-

tachments requiring full strength andductility. The successful completion ofthe specific application qualificationtests qualifies both the operator andthe process.

Qualification documentation for theoperator and the process should bepart of the overall recordkeeping of theplate fabrication facility. This docu-mentation can be completed by firstestablishing a Welding Procedure Spec-ification (WPS) and Procedure Qualifi-cation Record (PQR) for the stud diam-eters, equipment, settings, cable lengths,welding positions, weld plate materials,and ceramic ferrules that are to be usedin the weld plate fabrication.

Once the welding procedure speci-fications are completed and docu-mented, the operators set up and per-

form the welding procedure using theWPS guidelines. The operator weldstwo studs of all diameters downhandand ten studs for all other positions.The welds are visually inspected andtested with the testing certified by thewelding supervisor or trainer. The op-erator is certified by name using thesame form as a Welder QualificationRecord (WQR).

These records are retained alongwith material certifications and pro-duction inspection records as part ofthe documentation files for review byinterested parties including customers,quality certification agencies, the engi-neer of record, and the owners repre-sentative. A sample WPS/PQR/WQRform for stud welding is provided inAWS D1.1. 8

Fig. 12. Visual inspection of stud weld appearance. Source: Nelson Stud Welding, Inc.


11/13

56 PCI JOURNAL

Inspection Guidelines

Visual Weld Inspection A properrelationship between the lift, plunge,time, and amperage is needed to ob-tain good weld results. The length re-duction or burn-off and the weld fil-let appearance are determined by theweld settings. Referring to Fig. 12,visual weld inspection consists of in-terpreting the appearance of the weldfillet, and is normally a very accuratemethod if certain guidelines are fol-lowed.

A good weld is characterized by: Even flash (fillet) formation A shiny, bluish hue to the flash

surface A slight flow or bend of the flash

metal into the base material Good flash height Consistent after-weld length Full wetting i.e., flash around

the stud periphery without under-cut

A cold weld, which requires moretime, amperage, or both, is indicatedby:

Low flash (fillet) height Incomplete flash formation A dull gray cast to the flash sur-

face Stringers of flash metal forming

spider legs

A hot weld, which is made withtoo much time, amperage, or both, isdistinguished by:

Excessive splatter A washed-out flash (fillet) Undercutting of the stud Burn through the base material

Stud hang-up can result from toomuch lift, poor alignment of the studrelative to the work piece, and bind-ing during lift and plunge caused by

poorly centering the stud in the ce-ramic ferrule. Stud hang-up is distin-guished by:

Very low or no flash (fillet) Severe undercut in the weld No penetration of the weld into

the base plate Insufficient stud burn-off (after

weld height is too long)

Burn-Off Stud Length Reduc-tion In most arc welding processes,the weld fillet metal comes from a

stick electrode or a spool of welding

Stud diameter Length reduction 3 / 16 1 / 2 in. 3 / 32-1 / 8 in. (2.4-3 mm) 5 / 8 7 / 8 in. 5 / 32-3 / 16 in. (4-5 mm) 1 in. and over 3 / 16-1 / 4 in. (3-6 mm)

Table 4. Stud burn-off lengths. 21

Note: 1 in. = 25.4 mm.

Unsatisfactory pre-production or pro-

duction inspections and tests shouldbe brought to the welding supervisorsattention and corrections made. Thisshould be accompanied by additionaltests with fully satisfactory inspectionand test results before proceeding withfurther welding.

At regular intervals during produc-tion welding, the studs welded sincethe previous testing interval shouldbe inspected visually. If the visual in-spection shows a full periphery weld

flash without undercut and satisfactoryafter-weld length, welding may con-tinue. If the visual inspection shows alack of fillet or insufficient weld burn-off, the questionable studs should bemarked and appropriate supervisorypersonnel notified.

In accordance with codes, the con-tract documents, or the quality assur-ance inspection criteria in use, thestuds without a full peripheral fillet butwith a satisfactory after-weld lengthmay be tested by bending 15 degreesin the direction opposite the lack offillet, or repaired with a hand weldby adding a minimum fillet weld asrequired in Table 5. 8 The repair weldshall extend at least 3 / 8 in. (10 mm)beyond each end of the discontinuitiesbeing repaired.

If any tested stud fails the bend testor if there is continued and frequentevidence of insufficient stud burn-off,or incomplete fillet, production mustbe halted and appropriate supervisorypersonnel notified. The welding vari-ables should be checked, the necessaryadjustments made and the process andoperator qualification procedures re-peated with satisfactory results beforecontinuing with further productionwelding.

In many cases, the fabrication ofstud welded embedment plates may besubcontracted to an outside supplier.The weld plate fabricators inspec-tion procedure and inspection resultsshould be made available to the pre-

wire. In stud welding, a portion of thestud itself is the source of the filletmetal. The stud material that is meltedto develop the fillet is called burn-off.The difference in length between awelded stud and the original length isthe burn-off length.

The burn-off length is a very goodmeasure of weld quality, because the

burn-off is determined by the weld set-tings of time, current, lift, and plunge.Proper burn-off also indicates thatthere was no binding or hang-up dur-ing the plunging motion of the gun.

The most convenient method ofchecking burn-off is to stand an un-welded stud upside down (load endup) next to a welded stud to comparethe length difference to the stud base not to the end of the flux load. After-weld height can also be checked with

a sliding carpenters level or square.Table 4 shows typical burn-off lengthreductions when welding to the bareplate. 21

Physical Weld Inspection At thebeginning of the day, shift change, orany change of operator, equipment,position, or settings, two studs arewelded according to qualified settingsduring pre-production testing. Follow-ing satisfactory visual inspection, theyare bent 30 degrees (or torque tested inthe case of threaded studs). Studs thatare bent may be straightened to theoriginal axis and used in production.However, studs should not be heatedduring bending nor straightening with-out approval by the engineer of record.

Torque testing is performed to aproof load level slightly lower than thenominal stud yield to prevent perma-nent distortion of the threads. Torquetest proof load requirements are foundin AWS D1.1 or D1.6. The torque teststuds may be used in production.

The stud welding operator is re-sponsible for pre-production setup andtesting. The operator shall weld twostuds to a production weld plate, orto a piece of material similar to theweld plate in material composition andwithin 25 percent of the productionweld plate thickness.

Inspections during production arealso the responsibility of the operator.Pre-production and production inspec-tion test results should be recorded andapproved by the welding supervisor.


12/13

cast/prestressed producer. Further, theproducer should implement an incom-ing inspection procedure with accep-tance, rejection and corrective actioncriteria and corresponding records.

An excellent welding inspectionprocedure can be found in PCIs Man-ual for Quality Control for Plants andProduction of Architectural PrecastConcrete Products. 22

Equipment Service and Repair

The following section discussessafety precautions to be taken in theevent of malfunctioning equipmentand stud welding operations.

Malfunctioning/Obsolete Equip-ment and Maintenance Older studwelding units with mechanical contac-tors and relays powered by a separatemotor generator or transformer recti-fier are obsolete. These systems lackread-out displays for time and current,current compensation, fault protection,and shut down, which allow controlledquality welds. Spare parts for thesesystems are not readily available.

Newer equipment is solid state, buteven a new system can suffer break-downs. Welding cable connections candeteriorate, wiring connections canbecome brittle and fail, and the ac-cumulation of heavy dust or moisturecan cause the solid-state componentsto overheat and short out. Parts in thewelding gun can suffer wear or break-age, affecting the consistency of pro-grammed weld setup parameters.

It is good practice to regularly cleanthe welding systems and to inspectand replace worn or defective parts.Trained representatives of the equip-ment and stud supplier are usuallyavailable to assist in evaluating theequipment and cleaning guns or sug-gesting which components may needto be replaced.

Table 5. Minimum fillet weld sizes for studs. 8* Stud diameter Minimum size fillet in. mm in. mm 1 / 4 through 7 / 16 6.4 through 11.1 3 / 16 5 1 / 2 12.7 1 / 4 6 5 / 8, 3 / 4, 7 / 8 15.9, 19.0, 22.2 5 / 16 8 1 25.4 3 / 8 10

Note: 1 in. = 25.4 mm.* Welding shall be done with low hydrogen electrodes 5 / 32 or 3 / 16 in. in diameter except that a smaller diameterelectrode may be used on studs 7 / 16 in. diameter or under or for out-of-position welds.

Stud Welding Safety As withany welding process, common senseshould be applied for the safety ofthe stud welding operator and weld-ing station facilities. The welding areashould be kept clear of any flamma-ble materials, gases, and falling andtripping hazards. The operator shouldalso wear welding safety glasses withNo. 3 tinted lenses, protective gloves,

welding aprons, and other protectiveclothing as needed for the setup of thewelding table and welding position.For example, a welding table at waistheight may subject the operator tosparks from the weld process, in whichcase a welding apron should be used.

Welding cables should be regularlyinspected for such things as loss ofinsulation and exposed connections.Any necessary repairs should be madewithout delay. Finished weld platesmust be handled carefully as they canbecome very hot during or shortlyafter welding is completed. The workarea should be adequately ventilatedso that welding fumes are exhaustedfrom the area.

Welding arc brightness is minimizeddue to the ceramic ferrule, but contin-ual observation of the weld should beavoided and tinted lenses are recom-mended. The sparks that are caused bystud welding are usually minimal anddo not carry for a long distance, butworkers should be at least 5 ft (1.52m) away from each other. These safetypractices and other precautions areoutlined in ANSI/ AWS Z49.1. 23

CONCLUSIONS ANDRECOMMENDATIONS

Stud welding is a long recognizedand practiced welding method. In anygiven year and throughout the world,over 100 million stud welds of alltypes are made with the requirement

for development of full weld strength.Quality welds are critical to the per-formance of finished structural mem-bers, attachments, and connections.Whether the user employs the weldingprocedure in their in-plant fabricationshop or purchases the finished weldplates from a subcontractor, the prin-ciples and practices discussed in this

article should be followed. The fol-lowing summary recommendations aregiven for achieving consistent quality.

Stud and Weld Plate Materials1. Studs should be manufactured

from materials acceptable for the studwelding process as approved in theAWS welding codes. The stud manu-facturer should be required, by specifi-cations from the purchaser, to providecertified material test reports on thematerial used to make the studs. Themanufacturer should also be requiredto provide certificates of complianceverifying that the shipped studs meetthe AWS codes and engineering speci-fication requirements provided by thepurchaser in the purchase documents.

2. Base plate materials should alsomeet the requirements for AWS codeapproved materials for use with studwelding and appropriate certificationsshould be required for the base materi-

als from the steel manufacturer or theplate supplier. In both cases, the steelcertification documents are part of thefabricators quality control system andshould be maintained in their recordsand be included in the purchasers orusers documentation package.

Welding and Inspection

1. Stud welding training, equipmentsetup, welder qualification, and uniqueapplication qualification instructionare available from stud and equipmentsuppliers, and fabricators should availthemselves of these services. The re-cords for such training can be incorpo-rated into an appropriate quality assur-ance program.

2. Inspection of pre-productionwelds and continuous and documentedresults of both visual and physical in-spection of finished welds is furtherassurance of acceptable weld quality.

Stud Welding Equipment



13/13

and Safety

1. Equipment for stud welding hasgrown much more reliable than in pastyears, and improvements in the tech-nology are ongoing. Obsolete equip-ment may not function properly andshould be considered for replacement.All pieces of operating equipment aresubject to wear or possible failure.

Equipment must be regularly inspected,repaired, or replaced as needed.

2. Welding safety for both opera-tors and other personnel in the weld-ing area is a matter of common sense.Publications are available with guide-lines for safety. Safety increases pro-ductivity.

ACKNOWLEDGMENTThe author wishes to express his

appreciation to the PCI JOURNALreviewers for their thoughtful and con-structive comments.

1. Stud Welding Applications: Concrete Connections , NelsonStud Welding, Inc., Elyria, OH, 1988.

2. Anderson, Neal S., and Meinheit, Donald F., Design Criteriafor Headed Stud Groups in Shear: Part 1 Shear Capacity andBack Edge Effects, PCI JOURNAL, V. 45, No. 5, September-October 2000, pp. 36-75.

3. Strigel, Roberta M., Pincheira, Jos A., and Oliva, MichaelG., Reliability of 3 / 8 in. Stud-Welded Deformed Bar AnchorsSubject to Tensile Loads, PCI JOURNAL, V. 45, No. 6, No-vember-December 2000, pp. 72-82.

4. Wiewel, Harry, Load Capacity of Nelson D2L Deformed BarAnchors Installed in Concrete, Nelson Stud Welding, Inc.,Elyria, OH, September 1986.

5. Wiewel, Harry, Report of Tests Conducted on Nelson D2LDeformed Bar Anchors Installed in Stone Aggregate Concrete,Nelson Stud Welding, Inc., Elyria, OH, December 1994.

6. ACI Committee 355, State-of-the-Art Report on Anchorageto Concrete, ACI 355.1R-91, American Concrete Institute,Farmington Hills, MI, 1991.

7. Welding Handbook , Eighth Edition, V. 2, Welding Processes,Chapter 9, Stud Welding, American Welding Society, Miami,FL, 1991, pp. 300-327.

8. ANSI/AWS D1.1-00, Structural Welding Code Steel , Ameri-can Welding Society, Miami, FL, 2000.

9. ASTM A 108-99, Standard Specification for Steel Bars, Car-bon, Cold Finished, Standard Quality, American Society forTesting and Materials, West Conshohocken, PA, 1999.

10. ASTM A 496-97a, Standard Specification for Steel Wire,Deformed, for Concrete Reinforcement, American Society forTesting and Materials, West Conshohocken, PA, 1999.

11. ANSI/AWS D1.6-99, Structural Welding Code StainlessSteel , American Welding Society, Miami, FL, 1999.

12. ASTM A 493-82a, Standard Specification for Stainlessand Heat Resisting Steel for Cold Heading and Cold Forg-

ing, American Society for Testing and Materials, West Con-shohocken, PA, 1999.

13. ASTM A 276-00, Standard Specification for Stainless SteelBars and Shapes, American Society for Testing and Materi-als, West Conshohocken, PA, 2000.

14. Training Program, Nelson Stud Welding, Inc., Elyria, OH,

2000.15. PCI Design Handbook: Precast and Prestressed Concrete ,

Fifth Edition, MNL 120-99, Precast/Prestressed Concrete Insti-tute, Chicago, IL, 1999.

16. ACI Committee 318, Building Code Requirements for Struc-tural Concrete (ACI 318-99) and Commentary (ACI 318R-99), American Concrete Institute, Farmington Hills, MI,1999.

17. Baeslack, III, W. A., Fayer, III, G., and Jackson, C. E., Qual-ity Control in Arc Stud Welding, The Welding Journal , No-vember 1975, pp. 789-797.

18. CSA W59-89, Welded Steel Construction (Metal Arc Weld-ing), Canadian Standards Association, Rexdale, Ontario,Canada, 1989.

19. ISO 13918:1998, WeldingStuds and Ceramic Ferrules forArc Stud Welding, International Organization for Standard-ization, Geneva, Switzerland, 1998, 21 pp.

20. Kennedy, D. J. Laurie, Stud Welding at Low Temperatures,Canadian Journal of Civil Engineering , V. 7, 1980, pp. 442-454.

21. ANSI/AWS C5.4-94, Recommended Practices for Stud Weld-ing, American Welding Society, Miami, FL, 1994.

22. Manual for Quality Control for Plants and Production of Ar-chitectural Precast Concrete Products , Third Edition, MNL117-96, Precast/Prestressed Concrete Institute, Chicago, IL,1996.

23. ANSI/AWS Z49.1-94, Safety in Welding, Cutting and AlliedProcesses, American Welding Society, Miami, FL, 1994.

REFERENCES

Zavarivanje svornjaka

Documents

Transcript of Zavarivanje svornjaka