M1_4 Materials and weld metallurgy-1.pdf

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M1.4 Materials and weld metallurgy

Welding Metallurgy definitions:AlloyHaving two or more chemical elements of which at least one is an elemental metal.Alloying ElementAn element added to a metal to change the properties of the parent metal

PhaseA physical condition of the arrangement of atoms in a crystal. eg, ice is a phase of water.FerriteIron with 0.02% dissolved carbon.CementiteA very hard intermetallic compound consisting of 6.7% carbon and the remainder iron, itschemical symbol is Fe3C. Cementite is very hard, but when mixed with soft ferrite layers itsaverage hardness is reduced considerably. Slow cooling gives course perlite; soft easy tomachine but poor toughness. Faster cooling gives very fine layers of ferrite and cementite;harder and tougher Fe3C also known as Iron Carbide.PearliteA mixture of alternate strips of ferrite and cementite in a single grain. Softer than most other

microstructures. Formed from austenite during air cooling from austenite. The name for thisstructure is derived from its mother of pearl appearance under a microscope. A fully pearliticstructure occurs at 0.8% Carbon. Further increases in carbon will create cementite at thegrain boundaries, which will start to weaken the steel.

Ferite = Light AreasCementite = Dark Areas

Figure 1 - Pearlite

AusteniteThe first phase formed as liquid steel freezes. Austenite is a metallic non-magnetic allotropeof iron or a solid solution of iron, with an alloying element. In carbon steel, austenite existsabove the critical eutectoid temperature of 727 °C; other steel alloys have different eutectoidtemperatures.MartensiteA supersaturated solid solution of carbon in iron. Carbon atoms trapped in an iron crystal. Ifsteel is cooled rapidly from austenite, the F.C.C structure rapidly changes to B.C.C leavinginsufficient time for the carbon to form pearlite. This results in a distorted structure that has

the appearance of fine needles. There is no partial transformation associated withmartensite, it either forms or it doesn’t. Only the parts of a section that cool fast enough willform martensite; in a thick section it will only form to a certain depth, and if the shape iscomplex it may only form in small pockets. The hardness of martensite is solely dependanton carbon content, it is normally very high, unless the carbon content is exceptionally low. Itis the hardest and strongest of the microstructuresBainiteSame as martensite but considerably less carbon is trapped. Forms from austenite if rate ofcooling is in sufficient. Strength and hardness is between martensite and pearlite.Cold FormingForming a metal at or near room temperatures using high pressures.Ductility

The ability of a material to be plastically deformed without fracturing.Fracture ToughnessThe ability of a material at a given temperature to resist further crack propagation, once acrack has started.



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HardnessThe ability of a material to resist plastic deformation. The common measurement systemsare Rockwell, Brinell, Vickers and Knoop.HSLA SteelHigh Strength Low Alloy SteelImpact Toughness

The ability of a material to resist fracture under an impact.I.T. DiagramIsothermal TransformationInclusionsImpurities in a metal. Ie MnS (Manganese-sulfide)Mechanical PropertiesTensile strength, yield strength, and hardnessMetallographAn inverted microscope using indirect lighting.MicrohardnessHardness determined by using a microscope to measure the impression of a Knoop orVickers indenter.

MicrostructureThe phases or condition of a metal as viewed with a metallographModulus of ElasticityMeasure of stiffness. Ratio of stress to strain as measured below the yield point.OxidationThe chemical reaction between oxygen and another atomPhysical PropertiesElectrical conductivity, thermal conductivity, thermal expansion and vibration dampeningcapacityPlastic DeformationDeformation that remains permanent after the removal of the load that caused it.Steel

A solid solution of iron and carbonTensile StrengthThe ratio of maximum load to the original cross-sectional area.Yield StrengthThe point at which a material exhibits a strain increase without increase in stress. This is theload at which a material has exceeded its elastic limit and becomes permanently deformed.Cooling time t 8/5 time taken, during cooling, for a weld run and its heat affected zone to pass through thetemperature range from 800 °C to 500 °C.

The Heat input

In arc welding, energy is transferred from the welding electrode to the base metal by anelectric arc. When the welder starts the arc, the base and the filler metal are melted andcreate the weld. This melting is possible because a sufficient amount of heat (energytransferred per time unit) supplied. Heat input is the relative measure of the energytransferred per unit length of weld.It is one of the most important characteristic related to welding because it influences thecooling rate, which may affect the mechanical properties and metallurgical structure of theweld and the HAZ. For many steels, abrupt cooling from the heat of welding needs to beavoided, because of the risk of hardening or cold cracking. Depending on the type ofmaterial, thickness of material and heat input, preheating and the maintenance of an upperor lower interpass temperature are normally required.

The heat input during welding can be viewed as a main influencing factor on the properties offerritic and ferritic-austenitic stainless steel welds in particular.



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The heat input value may be calculated as follows:

whereQ is the heat input in kJ/mm;

k is the thermal efficiency;U is the arc voltage, measured as near as possible to the arc, in V;I is the welding current, in Av is the travel speed in mm/s.

Thermal efficiency factor k for welding processes

WeldingProcess No

Process Factor k

121 Submerged arc welding with wire electrode 1,0111 Metal-arc welding with covered electrode 0,8131 Metal-arc inert gas welding 0,8135 Metal-arc active gas welding 0,8114 Flux-cored wire metal-arc welding without gas shield 0,8136 Flux-cored wire metal-arc welding with active gas shield 0,8137 Flux-cored wire metal-arc welding with inert gas shield 0,8138 Metal-cored wire metal-arc welding with active gas shield 0,8139 Metal-cored wire metal-arc welding with inert gas shield 0,8141 Tungsten inert gas arc welding 0,615 Plasma arc welding 0,6

(table from EN 1011-1)

Heat input can not be measured directly. It has to be calculated from the measured values of

arc voltage, current and travel speed.

Arc VoltageIn determining the arc voltage (U), the voltage should be measured as close to the arc aspossible, as opposed to the value displayed on the welding machine voltmeter. Measuringthe voltage across the arc provides the actual voltage drop across the welding arc. Thewelding equipment voltmeter reading is always higher than the arc voltage due to theresistance of the welding cables. The equipment voltage, therefore, can be used only forapproximate calculations and, in the case of significant voltage drops, may lead to heat inputcalculation errors.

Current

The welding current (I) is measured with either an inductance meter (tong meter) or a shuntwith appropriate metering equipment. The current is never fixed with respect to time,especially on a microsecond level. The current is also a function of the arc length, which isdependent on the welder's skill. The current used in heat input calculations is, normally, anaverage value.

Travel SpeedThe travel speed (v) is the travel speed of the arc measured in millimeters per second. Onlythe forward progress contributes to the travel speed. If a weaving technique is used, only theforward speed counts, not the oscillation rate. For vertical welding, the upward or downwardspeed of the arc is used. The travel speed must be measured in terms of minutes and notseconds in order to have a balanced value in the heat input equation. When the travel speedis measured, the arc should be established for an amount of time that will produce anaccurate average speed. A continuous welding time of 30 seconds is suggested. The travelspeed accuracy for manual or semi-automatic welding is dependent on the welder.



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Weld Size relation to Heat InputThe cross-sectional area of a weld is, generally, proportional to the amount of heat input.This is important because as more energy is supplied to the arc, more filler metal and basemetal will be melted per length unit, resulting in a larger weld bead.

Cooling Rate relation to Heat InputThe effect of heat input on the cooling rate is similar to that of the preheat temperature. As

the heat input or the preheat temperature increases, the rate of cooling decreases, for agiven base metal thickness. These two variables interact with other material variables suchas material thickness, specific heat, density, and thermal conductivity in order to influencethe cooling rate.The cooling rate is a primary factor that determines the final metallurgical structure of theweld and heat affected zone (HAZ), and is especially important in case of heat-treated steels.In case of welding quenched and tempered steels, for example, slow cooling rates will resultfrom high heat inputs. This may soften the material adjacent to the weld, reducing thedesigned load-carrying capacity of the connection.All of the mechanical properties show a relationship to heat input, that is, the mechanicalproperties only increases or decreases with increasing heat input.When heat input control is a contract requirement, and if the procedure used in production

has a corresponding heat input that is 10% or greater than that recorded in the ProcedureQualification Record (PQR), then the qualified WPS must be requalified. This is primarily dueto concerns regarding the potential alteration of the weld metal and HAZ mechanicalproperties. If the Production Procedure Method is used, the heat input can only deviate fromthe PQR by the following: an increase of up to 10% or a decrease not greater than 30%.When quenched and tempered steels are to be welded, the heat input, and the minimumpreheat and maximum interpass temperatures, shall conform to the steel producer's specificwritten recommendations. If high heat input energies are used at welding, the HAZ can besignificantly weakened due to high temperatures and slower cooling rates. However, this notapplies to all quenched and tempered steels.

The study of all steels microstructures need to start with the metastable iron-carbon (Fe-C)

binary phase diagram (Figure 2). This phase diagram provides an invaluable foundation forthe knowledge of carbon and alloy steels, as well as for a number of various heat treatmentsthose steels are usually subjected to (hardening, annealing, etc).

Figure 2. The Fe-C phase diagram shows the phases which are expected at metastableequilibrium, for different combinations of carbon and temperature values.



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At the low-carbon part of the metastable Fe-C phase diagram, we distinguish ferrite (alpha-iron), which can at most dissolve 0.02 % C at 738 °C, and austenite (gamma-iron), which candissolve 2.08 % C at 1154 °C. The much larger phase field of gamma-iron (austenite)compared with that of ferrite indicates clearly the considerably grater solubility of carbon inaustenite, the maximum value being 2.08 % at 1154 °C. The hardening of carbon steels, aswell as many alloy steels, is based on this difference in the solubility of carbon in ferrite andaustenite. At the carbon-rich side of the metastable Fe-C phase diagram we find cementite

(Fe3C).

Figure 3 –Structures of the FE-C diagram

The steel portion of the Fe-C phase diagram covers the range between 0 and 2.08 % C.The cast iron portion of the Fe-C phase diagram covers the range between 2.08 and 6.67 %

C.

The steel portion of the metastable Fe-C phase diagram can be subdivided into threeregions: hypoeutectoid (0 < % C < 0.68 %), eutectoid (C = 0.68 %), and hypereutectoid (0.68< % C < 2.08 %).

A very important phase change in the metastable Fe-C phase diagram occurs at 0.68 % C.The transformation is eutectoid, and its product is called pearlite (ferrite + cementite):

Some important boundaries at single-phase fields have been given special names. Theseinclude:• A1 — The so-called eutectoid temperature, which is the minimum temperature for austenite.• A3 — The lower-temperature boundary of the austenite region at low carbon contents; i.e.,the gamma / gamma + ferrite boundary.• Acm — The counterpart boundary for high-carbon contents; i.e., the gamma / gamma +Fe3C boundary.

Sometimes the letters c, e, or r are included:• Accm — In hypereutectoid steel, the temperature at which the solution of cementite inaustenite is completed during heating.• Ac1 — The temperature at which austenite begins to form during heating.• Ac3 — The temperature at which transformation of ferrite to austenite is completed duringheating.

• Aecm, Ae1, Ae3 — The temperatures of phase changes at equilibrium.• Arcm — In hypereutectoid steel, the temperature at which precipitation of cementite startsduring cooling,.• Ar1 — The temperature at which transformation of austenite to ferrite or to ferrite plus



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cementite is completed during cooling.• Ar3 — The temperature at which austenite begins to transform to ferrite during cooling.• Ar4 — The temperature at which delta-ferrite transforms to austenite during cooling.

If alloying elements are added to an iron-carbon alloy (steel), the position of the A 1, A3, andAcm boundaries, as well as the eutectoid composition, are changed. In general, the austenite-stabilizing elements (e.g., nickel, manganese, nitrogen, copper, etc) decrease the A1

temperature, whereas the ferrite-stabilizing elements (e.g., chromium, silicon, aluminum,titanium, vanadium, niobium, molybdenum, tungsten, etc) increase the A1 temperature.

The carbon content at which the minimum austenite temperature is attained is called theeutectoid carbon content (0.68 wt. % C in case of the metastable Fe-C phase diagram). Theferrite-cementite phase mixture of this composition formed during slow cooling has acharacteristic appearance and is called pearlite and can be treated as a microstructural entityor microconstituent. It is an aggregate of alternating ferrite and cementite lamellae thatcoarsens (or "spheroidizes") into cementite particles dispersed within a ferrite matrix afterextended holding at a temperature close to A1.

Finally, we have the martensite start temperature, Ms, and the martensite finish temperature,

Mf:

• Ms — The highest temperature at which transformation of austenite to martensite startsduring rapid cooling.• Mf — The temperature at which martensite formation finishes during rapid cooling.

The time-temperature transformation curves correspond to the start and finish oftransformations which extend into the range of temperatures where austenite transforms topearlite. Above 550 C, austenite transforms completely to pearlite. Below 550 C, bothpearlite and bainite are formed and below 450 C, only bainite is formed. The horizontal lineC-D that runs between the two curves marks the beginning and end of isothermaltransformations. The dashed line that runs parallel to the solid line curves represents the

time to transform half the austenite to pearlite. Below we have listed some simple examplesas an exercise at other temperatures that result in different phase transformations and hencedifferent microstructures.

Figure 4 - Carbon equilibrium diagram

Note: the carbon equilibrium diagram shown above it will be heavily distorted, in practice,due of the rapid heating values and cooling rates, due to the welding process.



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Figure 5 – Mixtures of constituents

a) Mixture of ferrite and pearlite grains; temperature below A1, therefore microstructure notsignificantly affected.

b) Pearlite transformed to Austenite, but not sufficient temperature available to exceed theA3 line, therefore not all ferrite grains transform to Austenite. On cooling, only thetransformed grains will be normalised.

c) Temperature just exceeds A3 line, full Austenite transformation. At cooling all grains willbe normalized.

d) Temperature significantly exceeds A3 line permitting grains to grow. On cooling, ferrite willform at the grain boundaries, and a course pearlite will form inside the grains. A course grainstructure is more readily hardened than a finer one, therefore if the cooling rate between800°C to 500°C is rapid, a hard microstructure will be formed. This is why a brittle fracture ismost likely to propagate in this region.

There are two main types of transformation diagram that are helpful in selecting the optimumsteel and processing route to achieve a given set of properties.

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Time Temperature Transformation (TTT) diagrams- Continuous Cooling Transformation (CCT) diagrams

1. Time Temperature Transformation (TTT) diagrams: measure the rate of transformationat a constant temperature. In other words a sample is austenitised and then cooled rapidly toa lower temperature and held at that temperature whilst the rate of transformation ismeasured. The phases finally formed in a heat affected zone during cooling or subsequentheating depend upon time and temperature. TTT diagram shows the time required fortransformation to various phases at constant temperature, and, therefore, gives a usefulinitial guide to likely transformations. A large number of experiments is required to build up acomplete TTT diagram.In Figure 1 the area on the left of the transformation curve represents the austenite region.

Austenite is stable at temperatures above the lower critical temperature (LCT) but unstablebelow LCT. Left curve indicates the start of a transformation and right curve represents thefinish of a transformation. The area between the two curves indicates the transformation ofaustenite to different types of crystal structures. (Austenite to pearlite, austenite tomartensite, austenite to bainite transformation.)



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Figure 6 - TTT Diagram

When austenite is cooled to temperatures below LCT, it transforms to other crystalstructures. A specific cooling rate may be chosen so that the transformation of austenite canbe 50 %, 80% or 100 %. If the cooling rate is very slow such as for the annealing process,the cooling curve passes through the entire transformation area and the end product of thisthe cooling process becomes 100% Pearlite. If the cooling curve passes through the middleof the transformation area, the end product is 50 % Austenite and 50 % Pearlite, whichmeans that at certain cooling rates we can retain part of the Austenite, without transforming itinto Pearlite.

If a cooling rate is very high, the cooling curve will remain on the left hand side of theTransformation Start curve. In this case all Austenite will transform to Martensite. If there is

no interruption in cooling the end product will be martensite.

Figure 7 - TTT Diagram and microstructures obtained by different types of cooling rates



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Time-Temperature-Transformation (TTT) diagram for two steels: one with 0.4% wt. C (redline) and one with 0.4% wt. C and 2% wt. Mn (green line). P = pearlite, B = bainite and M =martensite.

Figure 8 – TTT diagrams for 2 steels

2. Continuous Cooling Transformation (CCT) diagrams: measure the extent oftransformation as a function of time for a continuously decreasing temperature.Since in welding, the heat affected zone structure is usually formed under conditions of(rapid) continuous cooling, the constant temperature basis of TIT diagram becomesunrepresentative for welds. More relevant information can, thus, be obtained from a CCTdiagram in which phase changes are tracked for a variety of cooling rates.

Plotting actual cooling curves on such a diagram will show the types of transformationproduct formed and their proportions. In other words a sample is austenitised and thencooled at a predetermined rate and the degree of transformation is measured.A large number of experiments is required to build up a complete CCT diagram.

Figure 9 - TTT diagram for a 0.12% C-0.85% Mn-0.3% Si-1.4% Ni-0.7% Cr Steel



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An important factor related to the steels weldability is the carbon equivalent value. Theequivalent carbon content of a steel alloy refers to method of measuring the maximumhardness and the weldability of the alloy based on the chemical composition of the alloy.Higher concentrations of carbon and other alloying elements tend to increase the hardnessand decrease the weldability of the material. Each of these materials tends to influence the

hardness and weldability of the steel to different magnitudes, however, making a method ofcomparison necessary to judge the difference in hardness between two alloys made ofdifferent alloying elements.The equivalent carbon content is the most common such standard, but others exist, such asthe equivalent nickel content and the equivalent chromium content (usually used togetherand in conjunction with the Schaeffler-Delong diagram and considered more accurate formeasuring weldability).For carbon steel specifications, it is specified as the maximum carbon content shall be 0.25%and the maximum carbon equivalent (CE) shall be 0.43%, where:

CE=C%+Mn%/6+(Cr% + Mo% +V%) / 5 + (Ni% + Cu%) / 15.

For this equation, weldability is defined as:

Carbon equivalent (CE)Weldability

Up to 0.35 Excellent

0.36–0.40 Very good

0.41–0.45 Good

0.46–0.50 Fair

Over 0.50 Poor

The carbon equivalent is a measure of the tendency of the weld to form martensite oncooling and to suffer hydrogen cracking.

Generally: The higher the CE value, the greater the risk of hydrogen cracking.When the carbon equivalent is between 0.40 and 0.60 weld preheat may be necessary.When the carbon equivalent is above 0.60, preheat is necessary, postheat may benecessary, and techniques to control the hydrogen content of the weld are required.For stainless steels and welds between carbon steels and stainless steels, the equivalentnickel content and the equivalent chromium content (usually used together and inconjunction with the Schaeffler-Delong diagram) are considered more accurate formeasuring weldability.As a student in weld metallurgy we should now that Ferrite is important in avoiding hotcracking during cooling from welding of austenitic stainless steels. 'Constitution diagrams' areused to predict ferrite levels from the composition by comparing the effects of austenite andferrite stabilizing elements. The Schaeffler and Delong diagrams are the original method-s of

predicting the phase balances in austenitic stainless steel welds.A 'nickel equivalent' is calculated for the austenite stabilizing elements and a 'chromiumequivalent' ferrite stabilizing elements. These are used as the axes for the diagrams, whichshow the compositional equivalent areas where the phases austenite, ferrite, martensite (andmixtures of these) should be present. Although intended to show the phase balance of weldfillers, these diagrams can also be used to illustrate the phase balance of the 'parent'material. There are different diagrams for different alloy systems.

The Schaeffler DiagramThe nickel and chromium equivalents use the formulae:Ni (eq) = Ni + (30 x C) + (0.5 x Mn)Cr (eq) = Cr + Mo + (1.5 x Si) + (0.5 x Nb)

This gives a diagram that is useful for the austenitic steels, except those with Nitrogenadditions. This the main disadvantage of this diagram since Nitrogen, which is a very strongAustenite former. The Schaeffler diagram, identifying the phase boundaries is shown below.



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Figure 10 - Schaeffler Diagram for stainless steel

When a weld is made using a filler wire or consumable, the weld will be formed thorugh amixture consisting of approximately 30% parent metal and 70% filler metal alloy (percentagedepends on welding process, type of joint and welding parameters, as discusses previously).

The Delong DiagramRefines the Schaffler diagram by taking account of the strong austenite stabilising tendencyof nitrogen. The chromium equivalent is unaffected but the nickel equivalent is modified to:

Ni (eq) = Ni + (30 x C) + (0.5 x Mn) + (30 x N)

The diagram, identifying the phase boundaries is shown below. It shows the ferrite levels in

bands, both as percentages, based on metallographic determinations and as a ferritenumber 'FN', based on magnetic determination methods.



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Figure 11 - DELONG Diagram for stainless steel

The purpose of this chapter is also to present in general the imperfections that mayoccur for the main welding processes used in practice. Actually this problem is in a closerelation with the welding equipment and the welder’s performance.

Below will be presented the main possible causes of imperfections, in relation withthe used welding process:

1. SHIELDED METAL ARC WELDING (SMAW)Some problems that may be encountered and possible remedies are listed in the

following table:PROBLEM PROBABLE CAUSE REMEDY

1. Lack of fusion Insufficient AmperageTravel speed too high

Increase amp settingsReduce travel speed

2. Burn-thru Excessive AmperageArc length too shortTravel speed to slowRoot opening too wide

Reduce amp settingMaintain correct. arc lengthIncrease travel speedReduce root opening, use abackup material

3. Inclusions Insufficient AmperageExcessive arc lengthUneven oscillations and/ortravel speedDirty plate

Increase amp settingsMaintain correct. arc lengthMove electrode uniformly

Remove rust, grease, paint, etc.

4. Porosity Dirty plateExcessive amperageExcessive arc length

Remove rust, grease, paint, etc.Lower amp settingMaintain correct arc length

5. Undercut Excessive arc lengthImproper electrode angle

Travel speed too high

Maintain correct. arc lengthDirect electrode more into areaof undercutReduce travel speed



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Excessive amperage Lower amp setting6. Overlap Improper electrode angle

Travel speed too slowLower electrode angleIncrease travel speed

7. Cracking Bend too small or too concaveFailure to fill craters

Wet or dirty plate

Wet or dirty electrode

Reduce travel speedCircle electrode at end of bead,re-strike to fill as requiredDry or clean plate as needed

Use only dry and cleanelectrodes8. Excess spatter Excessive amperage (fine

sized spatter)Excessive arc length (largesized spatter)

Lower amp setting

Maintain correct arc length

9. Rough appearance Oscillations spaced too farapartImproper travel angle

Use more oscillations

Reduce travel angle

10. Fingernailing (offlux)

Flux coating cracked orchippedFlux coating not concentric

with rod

Use undamaged electrode

Exchange for quality electrode

Observation: Cracking in welded joints can be classified in cold and hot cracking.The location of cracks in a welded joint may occur in the weld metal, base metal, or

in both.Cold cracking appears due the existing hydrogen content in the welded joint.In the next table we presented the relation between hardness, martensit content

and the cracking susceptibility of the welded joint: for the case of a unalloyed steel.

Maximum hardness in HAZ Maximum martensit content Remarks

>450 HV 10 > 70% very probable appearing of

cracks>380HV 10 < 450 HV 10 >50 % <70 % possible appearing of cracks>280 HV 10 <350 Hv 10 < 50 % >30 % without cracks

<280 HV 10 < 30 % no need of postheating

Hot cracking is a function of the chemical composition. The appearing of this type ofcracking is due the constituents in the welded metal who have a relatively low meltingtemperature and which accumulate at the grain boundaries during solidification.

If cracking is observed during welding the cracks should be removed prior to furtherwelding because the cracks can continue into the newly deposited weld metal.

Solutions to cracking problems are including also:

• changing the base metal;• changing the filler metal;• changing the welding technique/procedure.

2.TUNGSTEN INERT GAS WELDING (GTAW)

Welding Problems and RemediesNumerous welding problems may develop while setting up or operating a GTAW

operation. Their solution will require careful evaluation of the material, the fixturing, thewelding equipment, and the procedures.

Tungsten Inclusions

One discontinuity found only in gas tungsten arc welds is tungsten inclusions.Particles of tungsten from the electrode can be embedded in weld when improper weldingprocedure are used with GTAW process. Typical causes are the following:

♦ Contact of electrode tip with molten weld pool



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♦ Contact of filler metal wit hot tip of electrode♦ Contamination of electrode tip by spatter from the weld pool

♦ Exceeding the current limit for a given electrode size or type

♦ Extension of electrodes beyond their normal distances from the collate (as withlong nozzles) resulting in overheating of the electrode

♦ Inadequate tightening of the holding collate or electrode chuck

♦ Inadequate shielding gas flow rates or excessive wind drafts resulting in oxidationof the electrode tip

♦ Defects such as splits or cracks in the electrode

♦ Use of improper shielding gases such as argon-oxygen or argon-CO2 mixturesthat are used for gas metal arc welding

Corrective steps are obvious once the causes are recognized and the welder isadequately trained.

Lack of Shielding GasDiscontinuities related to the loss of inert gas shielding are tungsten inclusions

previously described, porosity, oxide films and inclusions, incomplete fusion, and cracking.The extent to which they occur is strongly related to the characteristics of the metal being

welded. In addition, the mechanical properties of titanium, aluminum, nickel, and high-strength alloys can be seriously impaired with loss of inert gas shielding. Gas shieldingeffectiveness can often be evaluated prior to production welding by making a spot weld andcontinuing gas flow until the weld has cooled to a low temperature. A bright, silvery spot willbe evident if shielding is effective.

Some problems that may be encountered and possible remedies are listed in thefollowing table:

PROBLEM PROBABLE CAUSE REMEDY

1. Porosity

Entrapped gas impurities

(hydrogen, nitrogen, air,water vapor

Defective gas hose or loosehose connectionsOil film on base metal

Blow out air from all lines before

striking arc; remove condensedmoisture from lines; use welding grade(99.99%) inert gasCheck hose and connections for leaks

Clean with chemical cleaner not proneto break up in arc; (! Do not weld whilebase metal is wet)

2. Tungstencontamination ofworkpiece

Contact starting withelectrodeElectrode melting andalloying with base metal

Touching tungsten tomolten pool

Use high frequency starter; use copperstriker plateUse less current or larger electrode;use thoriated or zirconium-tungsten

electrodeKeep tungsten out of molten pool

3.GAS METAL ARC WELDING (GMAW)

Problems and corrections:Hydrogen EmbrittlementAn awareness of the potential problems of hydrogen embrittlement is important,

even though it is less likely to occur with GMAW, since no hygroscopic flux or coating isused. However other hydrogen sources must be considered. For example, shielding gas

must be sufficiently low in moisture content. This should be well controlled by the gassupplier, but may need to be checked. Oil, grease, and drawing compounds on the electrodeor the base metal may become potential sources for hydrogen pick-up in the weld metal.Electrode manufacturers are aware of the need for cleanliness and normally take special



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care to provide a clean electrode. Contaminants may be introduced during handling in theuser’s facility. Users who are aware of such possibilities take steps to avoid seriousproblems, particularly in welding hardenable steels. The same awareness is necessary inwelding aluminum, except that the potential problem is porosity caused by relatively lowsolubility of hydrogen in solidified aluminum, rather than hydrogen embrittlement.

Oxygen and Nitrogen Contamination

Oxygen and Nitrogen Contamination are potentially greater problems than hydrogenin the GMAW process. If the shielding gas is not completely inert or adequately protective,these elements may be readily absorbed from the atmosphere. Both oxides and nitrides canreduce weld metal notch toughness. Weld metal deposited by GMAW is not tough as weldmetal deposited by gas tungsten arc welding. It should be noted here, however, that oxygenin percentages of up to 5 percent and more can be added to the shielding gas withoutadversely affecting weld quality.

CleanlinessBase metal cleanliness when using GMAW is more critical than with SMAW or submergedarc welding (SAW). The fluxing compounds present is SMAW and SAW scavenge andcleanse the molten weld deposit of oxides and gas-forming compounds. Such fluxing slags

are not present in GMAW. This places a premium on doing a thorough job of preweld andinterpass cleaning. This is particularly true for aluminum, where elaborate procedures forchemical cleaning or mechanical removal of metallic oxides, or both, are applied.

Weld DiscontinuitiesSome of the more common weld discontinuities that may occur with the GMAW

process are listed in the following paragraphs.

UndercuttingThe following are possible causes of undercutting and their corrective actions

POSSIBLE CAUSES CORRECTIVE ACTIONS

1. Travel speed to high Use slower travel speed2. Welding voltage too high Reduce the voltage

3. Excessive weldingcurrent

Reduce wire feed speed

4. Insufficient dwell Increase dwell at edge of molten weld puddle5. Torch angle Change torch angle

PorosityThe following are the possible causes of porosity and their corrective actions:


1. Inadequate shielding gascoverage

Optimize the gas flow. Increase gas flow to displace allair from the weld zone. Decrease excessive gas flow toavoid turbulence and the entrapment of air in the weldzone. Eliminate any leaks in the gas line. Eliminatedrafts (from fans, open doors, etc.) blowing into thewelding arc. Eliminate frozen regulators in CO2 weldingby using heaters. Reduce travel speed. Reduce nozzle-to-work distance. Hold gun at the end of weld untilmolten metal solidifies.

2. Gas contamination Use welding grade shielding gas.3. Electrode contamination Use only clean and dry electrode.4. Worpiece contamination Remove all grease, oil, moisture, rust, paint, and dirt

from work surface before welding. Use more highlydeoxidizing electrode.

5. Arc voltage too high Reduce voltage



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6. Excess contact tube-to-work distance

Reduce stick-out

Incomplete fusionThe reduced heat input common to the short circuiting mode of GMAW results in low

penetration into the base metal. This is desirable on thin gauge materials and for out-of-position welding. However, an improper welding technique may result in incomplete fusion,

especially in root areas or longer groove faces.The following are the possible causes of incomplete fusion and their correctiveactions:


1. Weld zone surfaces not free offilm or excessive oxides

Clean all groove faces and weld zone surfaces ofany mill scale impurities prior to welding.

2. Insufficient heat input Increase the wire feed speed and the arcvoltage. Reduce electrode extension.

3. Too large a weld puddle Minimize excessive weaving to produce a morecontrollable weld puddle. Increase the travelspeed.

4. Improper weld technique When using a weaving technique, dwellmomentarily on the sidewalls of the groove.Provide improved access at root of joints. Keepelectrode directed at the leading edge of puddle.

5. Improper joint design Use angle groove large enough to allow accessto bottom of the groove and sidewalls with properelectrode extension, or use a “J” or “U” groove.

6. Excessive travel speed Reduce travel speed.

Incomplete joint penetrationThe following are the possible causes of incomplete joint penetration and their

corrective actions:


1. Improper joint preparation Joint design must provide proper access to thebottom of the groove while maintaining properelectrode extension. Reduce excessively largeroot gap in butt joints, and increase depth ofback gouge.

2. Improper weld technique Maintain electrode angle normal to work surfaceto achieve maximum penetration. Keep arc onleading edge of the puddle.

3. Inadequate welding current Increase the wire feed speed (welding current).

Excessive Melt-ThroughThe following are possible causes of excessive melt-through and their corrective

actions:


1. Excessive heat input Reduce wire feed speed (welding current) andthe voltage. Increase the travel speed.

2. Improper joint penetration Reduce root opening. Increase root facedimension.

Weld Metal Cracks

The following are possible causes of weld metal cracks and their corrective actions:


1. Improper joint design Maintain proper groove dimensions to allow



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deposition of adequate filler metal or weld crosssection to overcome restraint conditions.

2. Too high a weld depth-to widthratio

Either increase arc voltage or decrease thecurrent or both to widen the weld bead ordecrease the penetration.

3. Too small a weld bead(particularly fillet and root beads)

Decrease travel speed to increase cross sectionof deposit.

4. Heat input too high, causingexcessive shrinkage anddistortion

Reduce either current or voltage, or both.Increase travel speed.

5. Hot-shortness Use electrode with higher manganese content(use shorter arc length to minimize loss ofmanganese across the arc). Adjust the grooveangle to allow adequate percentage of filler metaladdition. Adjust pass sequence to reducerestrain on weld during cooling. Change toanother filler metal providing desiredcharacteristics.

6. High restraint of the joint

members

Use preheat to reduce magnitude of residual

stresses. Adjust welding sequence to reducerestraint conditions.

7. Rapid cooling in the crater at theend of the joint

Eliminate craters by backstepping technique.

Heat Affected Zone CracksCracking in HAZ is almost always associated with hardenable steels.


1. Hardening in the heat-affectedzone

Preheat to retard cooling rate.

2. Residual stresses too high Use stress relief heat treatment.3. Hydrogen embrittlement Use clean electrode and dry shielding gas.

Remove contaminants from the base metal. Holdweld at elevated temperatures for several hoursbefore cooling (temperature and time required todiffuse hydrogen are dependent on base metaltype).

4.FLUX CORED ARC WELDING (FCAW)

Problems and corrections:


PROBLEM PROBABLE CAUSE REMEDY

1. Porosity

Low gas flow Increase gas flowmetter setting cleanspatter clogged nozzle.

High gas flow Decrease to eliminate turbulence

Excessive wind drafts Shield weld zone from draft/wind

Contaminated gas Check gas sourceCheck for leak in hoses/fittings

Contaminated base metal Clean weld joint faces

Contaminated filler wire Remove drawing compound on wireClean oil from rollersAvoid shop dirtRe-dry filler wire



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Insufficient flux in core Change wireExcessive voltage Reset voltageExcess electrode stick out Reset stickout & balance current

Insufficient electrode stick out(self-shielded electrodes)

Reset stickout & balance current

Excessive travel speed Adjust speed

2. Incompletefusion orpenetration

Improper manipulation Direct electrode to the joint root

Improper parameters Increase currentReduce travel speedDecrease stickoutReduce wire sizeIncrease travel speed (self-shieldedelectrodes)

Improper joint design Increase root openingIncrease root face

3. Cracking

Excessive joint restrain Reduce restraintPreheatUse more ductile weld metalEmploy peening

Improper electrode Check formulation and content of theflux

Insufficient deoxidizers orinconsistent flux fill in core

Check formulation and content of theflux

5.SUBMERGED ARC WELDING (SAW)

Problems and corrections:Porosity problemsSubmerged arc deposited weld metal is usually clean and free of injurious porosity

because of the excellent protection offered by the blanket of molten slag. When porositydoes occur, it may be found on the weld bead surface or beneath a sound surface. Variousfactors that may cause porosity are the following:

♦ Contaminants in the joint

♦ Electrode contamination♦ Contaminants in the flux

♦ Insufficient flux coverage♦ Entrapped flux at the bottom of the joint

♦ Segregation of constituents in the weld metal

♦ Excessive travel speed♦ Slag residue from tack welds made with covered electrodes

As with other welding processes, the base metal and electrode must be clean anddry. High travel speeds and associated fast weld metal solidification do not provide time forgas to escape from the molten weld metal. The travel speed can be reduced, but othersolutions should be investigated first to avoid higher welding costs. Porosity from coveredelectrode tack welds can be avoided by using electrodes that will not leave a porosity-causing residue.

Cracking ProblemsCracking of welds in steel is usually associated with liquid metal cracking (center

bead cracking). This cause may be traced to the joint geometry, welding variables, orstresses at the point where the weld metal is solidifying. This problem can occur in both butt

welds and fillet welds, including grooves and fillet welds simultaneously welded from twosides.One solution to this problem is to keep the depth of the weld bead less than or equal

to the width of the face of the weld. Weld bead dimensions may best be measured by



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sectioning and etching a sample weld. To correct the problem, the welding variables or the joint geometry must be changed. To decrease the depth of penetration compared to thewidth of the face of the joint, the welding travel speed as well as the welding current can bereduced.

Cracking in the weld metal or the heat-affected zone may be caused by diffusiblehydrogen in the weld metal. The hydrogen may enter the molten weld pool from flux, greaseor dirt on the electrode or base metal. Cracking due to diffusible hydrogen in the weld metal

is usually associated with low alloy steels and with increasing tensile and yield strengths. Itsometimes can occur in carbon steels. There is always some hydrogen present in depositedweld metal, but it must be limited to relatively small amounts. As tensile strength increases,the amount of diffusible hydrogen that can be tolerated in the deposited weld decreases.

Cracking due to excessive hydrogen in the weld is called delayed cracking; it usuallyoccurs several hours, up to approximately 72 hours, after the weld has cooled to ambienttemperature. Hydrogen will diffuse out of the base metal at elevated temperatures (aboveapproximately 1000C) without resulting in cracking. It is at ambient temperatures thathydrogen accumulated at small defects in the weld metal or base metal results in cracking.

To keep the hydrogen content of the weld metal low:

♦ Remove moisture from the flux by baking in an oven (follow manufacturer’srecommendations).

♦ Remove oil, grease, or dirt from the electrode and base material.♦ Increase the work temperature to allow more hydrogen to escape during the

welding operation. This may be done by continuing the “preheat” until the seamis completely welded, or by postheating the weld joint for several hours beforeletting it cool to ambient temperature.

6.ELECTROSLAG WELDING


Location Problem Causes Remedy

Weld 1. Porosity Insufficient slag depthMoisture, oil, or rustContaminated or wet flux

Increase flux additionsDry or clean workpieceDry or replace flux

2. Cracking Excessive welding speedPoor form factor

Excessive center-to-centerdistance between electrodesor guide tubes

Slow electrode feed rateReduce current; raise voltage;decrease oscillation speedDecrease spacing betweenelectrodes or guide tubes

3. Non-metallicinclusions

Rough plate surfaceUnfused nonmetallics fromplate laminations

Grind plate surfacesUse better quality plate

Fusion line 4. Lack offusion

Low voltageExcessive welding speedExcessive slag depth

Misaligned electrodes orguide tubesInadequate dwell timeExcessive oscillation speedExcessive electrode to shoedistanceExcesive center to center

distance between electrodes

Increase voltageDecrease electrode feed rateDecrease flux addition; allowslag to overflowRealign electrodes or guidetubesIncrease dwell timeSlow oscillation speedIncrease oscillation width oradd another electrodeDecrease spacing between

electrodes



5. Undercut Too slow welding speedExcessive voltageExcessive dwell timeInadequate cooling of shoes

Poor shoe designPoor shoe fit-up

Increase electrode feed rateDecrease voltageDecrease dwell timeIncrease cooling water flow toshoes or use larger shoeRedesign groove in shoeImprove fit-up; seal gap with

refractory cement damHeat-affected

zone

6. Cracking High restraintCrack-sensitive materialExcessive inclusions in plate

Modify fixturingDetermine cause of crackingUse better quality plate

7.OXYFUEL-GAS WELDING

Problems and corrections:The appearance of a weld does not necessarily indicate its quality. If imperfections

exist in a weld, they can be grouped into two broad groups: those that are apparent to visualinspection and those that are not. Visual examination of the underside of a weld will

determine whether there is complete penetration and whether there are excessive spatter ofmetal. Inadequate joint penetration may be due to insufficient beveling of the edges, too thicka root face, too high a welding speed, or poor torch and welding rod manipulation.

Oversized and undersized welds can be observed readily. Weld gauges are used todetermine whether a weld has excessive or insufficient reinforcement. Undercut or overlap atthe sides of the welds can usually be detected by visual examination.

Although other imperfections, such as incomplete fusion, porosity, and cracking,may not be externally apparent, excessive grain growth and the presence of hard spotscannot be determined visually. Incomplete fusion may be caused by insufficient heating ofthe base metal, too rapid weld travel, gas or dirt inclusions. Porosity is a result of entrappedgases, usually carbon monoxide, which may be avoided by careful flame manipulation andadequate fluxing where needed. Hard spots and cracking result from the metallurgicalcharacteristics of the weldment.

M1_4 Materials and weld metallurgy-1.pdf

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