1、An Approved Continuing Education Provider PDH Course S150 Structural Steel Welding Semih Genculu, P.E. 2007 PDH Center 2410 Dakota Lakes Drive Herndon, VA 20171-2995 Phone: 703-478-6833 Fax: 703-481-9535 www.PDHwww.PDH PDH Course S150 www.PDHonline.org Semih Genculu Page 2 of 18 Structural Steel Wel
2、ding Semih Genculu, P.E. Arc welding requires striking a low-voltage, high-current arc between an electrode and the workpiece (base metal). The intense heat generated with this arc melts the base metal and allows the joining of two components. The characteristic of the metal that is being welded and
3、 the joint type (i.e. groove, fillet, etc.) dictates the welding parameters and the procedure that needs to be followed to obtain a sound weld joint. Typical Arc Welding Processes: Shielded metal arc welding (SMAW): Shielded metal arc welding, which is also known as stick welding, is the most widely
4、 used process. The arc is struck between the metal to be welded and a flux coated consumable electrode. The fluxes are mostly made from mineral components and cover the hot weld deposit and protect it from the environment. The solidified glassy product, slag should be removed by chipping or with a w
5、ire brush. Gas metal arc welding (GMAW): This process is also referred to as metal inert-gas (MIG) welding uses an uncoated continuous wire. The weld area is shielded from contamination by the gas that is fed through the welding torch. The mode of metal transfer (spray, globular, short-circuiting, p
6、ulsed-arc) is varied by adjusting the amperage and the shielding gases used depending on the welding position and the type of joint. www.PDH PDH Course S150 www.PDHonline.org Semih Genculu Page 3 of 18 Flux-cored arc welding (FCAW): The shielding gases and slag are provided by the decomposing flux t
7、hat is contained within the electrode. Auxiliary shielding is also used in certain instances where deeper penetration is needed. Gas tungsten arc welding (GTAW): Also known as tungsten inert-gas (TIG), the process uses a non-consumable electrode. The shielding gas is again fed through the welding to
8、rch. Welding may be accomplished without the addition of filler metal, which is advantageous especially for thin walled parts. Shielding gases: The primary purpose of the shielding gas is to protect the molten weld from contamination and high temperature oxidation by the surrounding atmosphere. Alth
9、ough plain inert gases may not be suitable for all applications, mixtures with reactive gases (i.e. oxygen, nitrogen, hydrogen and carbon dioxide) in controlled quantities will produce stable and relatively spatter-free metal transfer. Argon: Argon by itself is frequently used for MIG welding of non
10、ferrous metals. A mixture of argon and oxygen or argon and carbon dioxide is usually preferred for ferrous metals. The high-density arc that is created by argon permits the energy to go into the work piece as heat resulting in a narrow bead width with deep penetration. Helium: has higher thermal con
11、ductivity and arc voltage than argon, which causes it to produce broader weld beads. Because helium is a very light gas, higher flow rates must be used for effective shielding. This characteristic is beneficial in overhead welding. Carbon dioxide: is widely used for steels. Higher welding speed, bet
12、ter joint penetration and sound deposits with good mechanical properties can be achieved. Carbon dioxide is not an inert gas as the argon and helium and breaks down into carbon monoxide and free oxygen under the heat of the arc. The oxygen is used to superheat the weld metal transferring across the
13、arc. Arc welding defects: Most welds contain defects (porosity, cracks, slag inclusions, etc.). The question is to determine if they are significant considering the application. Typically, the applicable codes or standards specify the maximum allowable limits of these types of defects in a weld base
14、d on the application. Sometimes discontinuities that may not affect mechanical properties may reduce corrosion performance. The properties of the heat-affected zone (HAZ) are one of the significant factors to consider when evaluating the soundness of the weld joint. The HAZ may be considered as a di
15、scontinuity because of the metallurgical alterations as a result of the welding heat, which causes very rapid heating and cooling rates. Grain growth, phase transformations (i.e. brittle untempered martensite which can form depending on the cooling rate and the chemical composition of the base mater
16、ial), formation of precipitates or overaging (loss of strength in precipitation-hardened alloys) all has a drastic effect on the properties of the HAZ. It is possible to improve the weld zone properties by controlling the www.PDH PDH Course S150 www.PDHonline.org Semih Genculu Page 4 of 18 cooling r
17、ate. This may be accomplished by slowing the cooling rate down either by increasing the heat input or preheating (i.e. heating the metal up before welding). Porosity: Gas pockets are formed in the weld metal when they are entrapped during solidification. Molten steel readily absorbs hydrogen, carbon
18、 monoxide and other gases to which it is exposed. Since these are not soluble in solid metal, they are expelled as the metal solidifies. Standard shielded arc electrodes with organic coating such as E6010 produce an atmosphere around the arc that contains hydrogen, a notable contributor to porosity.
19、 When using such electrodes, welding should be done slowly to allow the gases time to escape since too high of a travel speed causes rapid solidification of the weld metal leading to porosity. Weld joint cleanliness is also crucial in avoiding porosity since moisture, oil, paint, or rust on the base
20、 metal may also cause porosity by introducing oxygen or hydrogen into the weld metal. Employing some minimum preheat temperature is often useful to remove condensation. It is also necessary to maintain the fluxes and the coated electrodes dry to avoid moisture pick-up. They are typically kept in an
21、oven at approximately 250F, or if the hermetic seal is broken on the containers then the consumables (e.g. welding rods) should be baked at higher temperatures to drive off the moisture and restore the low hydrogen characteristics. Common causes and remedies of porosity are listed below along with a
22、 macrograph of a fillet weld containing porosity. An illustration of a groove weld which exhibits cluster porosity is also included with its corresponding radiograph. Porosity: gas pockets or voids that are found in welds Causes Remedies Excessive hydrogen, nitrogen or oxygen in welding atmosphere U
23、se low hydrogen welding process, filler metals high in deoxidizers, increase shielding gas flow High solidification rate Use preheat or increase heat input Dirty base metal Clean joint faces and adjacent surfaces Dirty filler wire Use clean wire and store fillers in a clean area Improper arc length,
24、 welding current or electrode manipulation Modify welding parameters and techniques Volatilization of zinc from brass Use copper-silicon filler, reduce heat input Porosity: gas pockets or voids that are found in welds Causes Remedies Galvanized Steel Use E7010 electrode and manipulate the arc heat t
25、o volatilize the galvanizing (zinc) ahead of the molten weld pool Excessive moisture in electrode covering or on joint surfaces Use recommended procedures for baking and storing electrodes High sulfur base metal Use electrodes with basic slagging reactions www.PDH PDH Course S150 www.PDHonline.org S
26、emih Genculu Page 5 of 18 Slag inclusions: The oxides or other nonmetallic inclusions that become entrapped in the weld metal. They may be caused by contamination or inadequate cleaning between weld passes. The slag derived from fluxes employed during welding needs to be cleaned between weld passes
27、(in multi-pass operations) using a chipping hammer or a wire brush. The macrograph below illustrates a successful multipass weld joint. www.PDH PDH Course S150 www.PDHonline.org Semih Genculu Page 6 of 18 Tungsten inclusions: In the TIG process, the touching of the electrode to the weld metal may ca
28、use transfer of the tungsten particles into the weld metal. These inclusions are detected by x-ray and show up as bright particles since they are much denser than the steel. An example is shown below where the x-ray revealed the tungsten inclusions. www.PDH PDH Course S150 www.PDHonline.org Semih Ge
29、nculu Page 7 of 18 Incomplete fusion/penetration: Although these terms are sometimes used interchangeably, lack of fusion occurs when the weld and base metal fail to adequately fuse together. It can also be encountered between weld passes. It may be caused by not raising the temperature of the base
30、metal or previously applied weld metal to the melting point or failure to remove the slag or mill scale. Lack of penetration is typically due to inadequate heat input for the particular joint that is being welded and is usually seen at the sidewalls of a weld joint, between weld passes or at the roo
31、t of the weld joint. The shielding gas can also influence the penetration; typically helium is added for nonferrous metals and carbon dioxide is added for ferrous metals (to argon) to increase penetration. The first macrograph below shows an acceptable single pass fillet weld profile with adequate b
32、ase metal penetration and root fusion. The second macrograph shows lack of penetration to the root in a double welded joint, and the third macrograph illustrates lack of penetration to one of the members. The final illustration shows the sketch of another variation of incomplete root penetration and
33、 its appearance on an x-ray film (radiograph). www.PDH PDH Course S150 www.PDHonline.org Semih Genculu Page 8 of 18 Undercut: This occurs when a groove that is formed adjacent to the weld as a result of the melting of the base metal remains unfilled. An example is shown in the macrograph below (at t
34、he toe of the fillet weld) along with the appearance of this type of defect on a radiograph of a groove weld. www.PDH PDH Course S150 www.PDHonline.org Semih Genculu Page 9 of 18 Weld profile: The profile of a finished weld may have a considerable effect on the performance under dynamic loading cond
35、itions. Overlap, excessive reinforcement or mismatch can provide stress concentration points where fatigue cracks can initiate. Typical unacceptable butt and fillet weld profiles are shown below along with an example of poor fit-up (mismatch): www.PDH PDH Course S150 www.PDHonline.org Semih Genculu
36、Page 10 of 18 Macrograph showing poor fit-up (butt weld) Arc strikes: They are caused by the unintentional melting of the base metal outside the weld deposit area by the welding arc. It can create localized hard or soft spots, cracking or undercut. Another welder-induced defect is weld spatter. It u
37、sually occurs when excessive welding current, long arc or welding voltage is used. Below macrograph shows arc strikes near a fillet weld. www.PDH PDH Course S150 www.PDHonline.org Semih Genculu Page 11 of 18 Cracks: Cracks are the most serious type of weld defects that can lead to catastrophic failu
38、res in service. There are many different types of cracks. One way of categorizing them is as surface or subsurface cracks. Another way would be as hot (which occur during or immediately after the weld is made) or cold (cracks that occur after the weld has cooled to room temperature-sometimes within
39、hours or days). In general, weld or heat- affected zone cracks indicate that the weld or the base metal has low ductility and that there is high joint restraint. Many factors can contribute to this condition such as rapid cooling, high alloy composition, insufficient heat input, poor joint preparati
40、on, incorrect electrode type, insufficient weld size or lack of preheat. Some common causes and remedies are given in table below. Cracks: Hot and cold cracks or microfissures can form in the weld or the base metal Causes Remedies Highly rigid joint Preheat Relieve residual stresses mechanically Min
41、imize shrinkage stresses using backstep sequence (a longitudinal sequence in which weld passes are made in the direction opposite to the progress of welding) Excessive dilution (change in chemical composition of a weld deposit caused by the admixture of the base metal) Change welding current and tra
42、vel speed Weld with covered electrode negative; butter the joint faces prior to welding (buttering is depositing surfacing metal to provide metallurgically compatible weld metal to the subsequent weld passes) Poor fit-up Reduce root opening Small weld bead Increase electrode size, raise welding curr
43、ent, reduce travel speed High sulfur base metal Use filler metal low in sulfur Excessive distortion Change to balanced welding on both sides of joint Crater cracking Fill crater before extinguishing the arc High residual stresses Redesign weldment, change welding sequence, apply intermediate stress
44、relief High hardenability Preheat, increase heat input, heat treat without cooling to room temperature Photograph illustrating crater cracking resulting from abrupt weld termination in an aluminum weld (MIG process) www.PDH PDH Course S150 www.PDHonline.org Semih Genculu Page 12 of 18 The effect of
45、carbon equivalent: The carbon equivalent (C.E.) may be considered as the main factor in estimating preheat need. Generally, the higher the carbon content of a steel, the greater the tendency to form a hard and brittle HAZ. This necessitates the use of preheat and low hydrogen electrodes. Carbon, how
46、ever, is not the only element that influences hardenability. Other elements in steel also are responsible for the hardening and loss of ductility that occur with rapid cooling. One of the various empirical formulas used to determine carbon equivalent is given in the Structural Steel Welding Code (AW
47、S D1.1) as follows: %C.E. = %C + % (Mn+Si)/6+ % (Cr+Mo+V)/5 + % (Ni+Cu)/15 The approximate recommended preheat temperatures based on C.E. are: For up to 0.45%preheat is optional 0.45-0.60%200-400F Over 0.60%.400-700F Usually a steel that requires preheating must also be kept at this temperature betw
48、een weld passes. The heat input of the welding process is adequate to maintain the required interpass temperature on most weldments. On massive components this may not be the case and torch heating between passes may be required. Since the purpose of preheating is to reduce the quench rate, the same
49、 slow cooling rate must be accomplished for all passes. Besides the widely used carbon equivalent criteria, the following factors should also be considered when determining the need for preheat/post weld heat treat: code requirements, section thickness, restraint, ambient temperature, filler metal h
50、ydrogen content and previous cracking problems. AWS Classifications: The American Welding Society numbering system tells a lot about the properties and usability of the electrode. Using the stick electrode numbering system as a representative example: The prefix “E” designates an arc-welding electro
