metallography of welds - asm international.pdf

7/21/2019 Metallography of Welds - ASM International.pdf

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Welding is an important joining tech-nology, and is highly dependent on

the process choice, consumablesused, operating parameters, and operator pro-ficiency. Thus, inspection procedures, bothnondestructive and destructive, are required tocontrol the process and guarantee quality. Met-allographic examination is a key tool in the de-structive examination of weldments, both as aprocess control tool and as a post-mortem ex-amination of failed components. Macrostruc-ture must also be examined, which can be doneon sections after grinding or polishing.Macrostructural examination is used to learnabout the weld geometry, the depth of weldmetal penetration, the magnitude of the heat-affected zone, and to detect cracks and voids.Microstructural examination is used to deter-mine the mode of cracking and the crackingmechanism and to identify phases or con-stituents in the weld metal, heat-affected zone,and base metal including nonmetallic inclu-sions, as related to governing specifications, fit-ness for service, or cause of failure.

Many processes have been developed toproduce welded joints such as the commonlyused stick-electrode process that can be donein the field. Although welding is a compara-tively new technology, forge welding predates

by a large margin all other methods, as it datesfrom the earliest days of metalworking.

Brazing also has its roots in antiquity,mainly with jewelry production. Aside fromforge welding, the other processes date fromthe 20th century, particularly since 1943 wheninert-gas shielded welding was invented. Resist-ance welding of carbon steel was well estab-lished by about 1920, as it is less sensitive to

metallurgical problems associated with fusionwelding. Today, available processes include gas

welding (and cutting) using an oxyacetyleneflame; resistance welding, such as spot welding,induction welding, and flash welding; arc weld-ing processes, such as gas-tungsten-arc (GTA),metal-inert gas (MIG), covered-electrodeprocesses (stick electrode), submerged-arcwelding, electroslag welding, electron beamand laser welding; and various friction weldingprocesses. Many of these processes have beenfurther modified in various ways. Some of theseprocesses use filler metals, generally of some-what different composition than the base metalto produce higher strength in the weld. Othersuse no filler metal, relying only on the meltingof the base metal to produce the joint.

Weld terminology

Three main regions in welds are the basemetal, the heat-affected zone (HAZ), and theweld metal (Fig. 1). Aside from the forge weld-ing process, substantial heat is generated inwelding, and melting occurs. The heat inputcan vary greatly with the welding process usedand is influenced by other factors, such as thethickness of the pieces being joined. Thewelded joint, or weld nugget, is essentially acasting. When wrought metal is welded, there

is a temperature gradient, going from thenugget into the unaffected base metal, fromabove the melting point of the metal or alloy toambient temperature. This temperature gradi-ent can produce many effects depending on themetals or alloys being joined. Using steels as anexample, the weld nugget is created by moltenmetal (in many cases filler metal) that is heatedin the arc until it melted.

Metallographyof WeldsGeorge F.

Vander Voort,

FASM*Consultant

Struers Inc.

Westlake, Ohio

Metallographic

examination of

weldments

reveals macro-

structural

and micro-

structural

features

clearly when

good

metallographic

preparation

and the right

etchant are

used.

*Member of ASMInternational andmember, ASM HeatTreating Society and

InternationalMetallographicSociety Fig. 1 Weldment terminology for a butt weld (left) and a fillet weld (right). Source: AWS A3.0: 2001.

Weld Zone

Weld Metal

Fusion Zone

Heat-affected zoneWeld root

Fusion penetration

Parent metal

Weld zoneWeld metal

Fusionpenetration

Weld junction

Fusion zoneWeld root

Heat-affected zone


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ADVANCED MATERIALS & PROCESSES JUNE 201120

Solidification can occur under different cooling condi-tions, depending on the heat input, whether or not pre-heating or post-heating practices are used, depending onthe mass of the pieces, ambient temperature, and so forth.Consequently, there is a fusion line; the boundary betweenthe cast nugget and the nonmelted base metal. Below thefusion line, the temperature gradually drops to ambient.For a carbon or alloy steel, the heat-affected zone (betweenthe fusion line and the unaffected base metal), or at leastpart of the heat-affected zone, will be fully austenitic dueto temperatures above the upper critical,Ac3, of the steel.The grains closest to the fusion line will be the largest insize. At lower temperatures, the grain size can be quitesmall due to recrystallization and nucleation of new finegrains that may or may not grow substantially dependingon the temperature that they experience after nucleation.If a carbon steel was not deoxidized with aluminum,

columnar grains might be seen. In the region of theHAZ that is heated into the two-phase + field,the transformation on cooling could be quite differ-ent. For areas heated below the lower critical tem-perature, Ac1, the original structure might betempered or might start to spheroidize. Because thefiller metal is a different composition than the basemetal, and some melting of the base metal occurs,the composition will vary through the weld to thefusion line. With variations in the phases or con-stituents and their grain size in the weld nugget and

heat affected zone, we can expect to see hardness varia-tions across these gradients.

Cracks might be detected in the weld nugget or in theheat-affected zone. Figure 2 shows a schematic illustratingcommon terminology for cracks and voids. Use of correct

terminology to describe cracks is very important. Many ofthe cracks are described based on their locationcratercracks, root cracks, and heat-affected zone cracks are a fewexamples. Sometimes, cracks are described based on theirorientation with respect to the welding direction, such aslongitudinal and transverse cracks. They may also be de-scribed by the nature of the problem that caused the crack;for example, hydrogen-induced cracks, stress-relief cracks,etc. Reference 1 is a good source of information regardingwelding terminology.

Macrostructural examination

The logical progression in examination is to examine

the macrostructure before the microstructure. To do this,a section must be taken from the weldment, almost alwaystransverse to the welding direction. The section can besmooth ground to a reasonably good finish using produc-tion machining equipment or metallographic preparationequipment. The author usually avoids using an endlessbelt grinder, as the metal plate under the grinding beltoften wears non-uniformly with use, which leads to non-planar surfaces. Grinding to the equivalent of a 600-gritSiC surface finish is adequate.

Macroetchants for steels cover a range of aggressive-ness. The classic 1:1 HCl:H2O hot-acid etch at 160 to180F

(a)

(b)

Legend1 Crater crack2 Face crack

3 Heat-affected zone crack4 Lamellar tear6 Root crack7 Root surface crack8 Throat crack9 Toe crack

10 Transverse crack11 Underbead crack12 Weld-interface crack13 Weld-metal crack

Fig. 2 Terminology for describing cracks in welds. Source: AWS A3.0:2001.

Fig. 3 Macrostructure of three GMA welds in a structural steel with a heatinput of 45 kJ/in. and atmospheres of 100% CO2, argon plus 25% CO2, andargon plus 2% O2. Etched with aqueous 10% nitric acid.

Fig. 4 Macrostructure of submerged arc welds in structural steel withheat inputs of 90, 60, and 30 kJ/in. (left to right); etched with aqueous 10%nitric acid.

100% CO2 Ar-25% CO2 Ar-2% CO2

90 kJ/in 60 kJ/in 30 kJ/in


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ADVA NCED M ATER IALS & PROCESSES JUNE 2011 21

(70-80C) for 15 min-utes, or more (ASTME 381) can be used,but cold acid etchingis far more conven-ient. For steels, aque-ous 10% HNO3 or 10%ammonium persul-fate have beencommonly used.Both are reason-ably safe to use. It isimportant to clean theground surface properly beforeimmersing the specimen in these solutions. Gentle swab-bing can also be performed. Etching proceeds until themacrostructure is revealed with reasonable contrast anddefinition. The specimen is washed under running warmwater, the water is displaced by squirting ethanol on thesurface (avoid using methanol as it is a cumulative poisonfor humans), and dried using either compressed air or awarm air blast.

Figures 3 and 4 are examples of macroetched low-carbon structural steels welded using either differentprotective gas atmospheres or different heat inputs

showing the influence of these variables upon the shapeof the weld nugget, the depth of weld penetration, andthe depth of the heat-affected zones. The struc-tures were revealed using aqueous 10% nitricacid at room temperature. This technique is

very useful for revealing weldment macrostruc-ture. Figure 5 shows a device called the WeldingExpert (Struers Inc., www.struers.com) used tomeasure key weld macrostructural parameters.An example of results for a typical fillet weldbetween two sections of carbon steel, groundto a 600-grit SiC finish and macroetched using10% HNO3 is shown in Fig. 6. Using the Weld-

ing Expert, vertical and horizontal lines aredrawn in to define the edges of the horizontaland vertical legs of the weld, producing thick-ness measurements of 7.1 and 4.9 mm, respec-tively. Then, lines are drawn to the deepestpenetration of the nugget into the horizontaland vertical legs, yielding 3.0 and 0.5 mm, re-spectively, for penetrations of 42 and 10%, re-spectively. It also is possible to draw horizontaland vertical lines to the depth of each heat-af-fected zone, or make other measurements, in-cluding angles.

Microstructural examination

Metallographers often are asked to examine a weldedjoint. This requires cutting out one or more specimens tosample the structure of the weld, heat-affected zone, andadjacent base metal. It is convenient if all three regions canbe contained within a single specimen. In many cases,welds are small enough to do this easily. But, in some cases,such as heavy plate welded using the electroslag process,the weld nugget alone can be quite large. Even here, it ispossible to prepare entire cross sections through the welds,although it generally requires robust, large-scale

grinder/polishers, such as the Abra System (Struers Inc.;www.struers.com). The specimen is examined in the as-

Fig. 6 An example of weld penetration depth measurements using theWelding Expert.

Fig. 5 To make measurements ofweldment features using the WeldingExpert, an etched surface is placed

against the glass slide (as shown),the macrostructure is

imaged on a computerscreen, and themeasurements are made.

4.9 mm

0.5 mm 10%

3.0 mm

42%

Notes: For hard steels, MD-Allegro can be substituted for the SiC step. Alternatively, anMD-Piano 120 or 220 disc can be used rather than SiC. For soft steels or for ferritic andaustenitic stainless steels, i t may be desirable to add a 1-m diamond step (similar to the3-m step) for best results.

TABLE 1 METALLOGRAPHIC PREPARATION METHOD

FOR STEELS

7.1 mm

Step Surface DP DP OP

MD-Nap

OP-S or OP-AN

0.04 or 0.02 m

120-150

25

3

MD-Dac

DP-suspension

3 m

Green or red

120-150

25

5

MD-Dur

DP-suspension

9 m

Green or red

120-150

25

5

SiC paper

SiC

120-220

Water

240-300

25

Until planar

Surface

Abrasive

GritGrain size

Lubricant

(rpm)

Force, N

Time, min


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ADVANCED MATERIALS & PROCESSES JUNE 201122

polished condition for different types of voids, such asporosity from gas evolution or shrinkage cavities, cracksthat may be present in either the weld metal or the heat af-fected zone, regions where the weld did not exist (lack offusion or lack of penetration) and for nonmetallic inclu-

sions associated with the welding operation (chiefly slag-type in nature) in the weld or between weld passes (for amulti-pass weld).

Grinding and polishing cycles for welds differ littlefrom procedures for non-welded metals and alloys. The

major differences are that it might be

necessary to polish an area that is largerthan normal and that the hardness canvary across the specimen. That part ofthe specimen is a casting while the bal-ance is wrought generally does not affectthe preparation process. A practice suit-able for many commonly welded ferrousalloys is presented in Table 1. Preparationmethods for a wide variety of engineer-ing metals and alloys are available from

various sources, such as Struers Inc. Themethod in Table 1 can be easily modifiedto prepare Al, Cu, and Ni alloys.

A polished specimen, when etchedusing a suitable reagent for the alloy, re-

veals both the macrostructure and themicrostructure. In some cases, the weldmetal may be of a sufficiently differentcomposition that an etchant chosen toetch the base metal and heat affectedzone will not reveal the weld metal struc-ture, or vice versa. Most etchants used toreveal the structure of welds are standardgeneral-purpose etchants. After exami-nation using such an etch, it might proveto be valuable to use a color etching tech-nique, as these can be far more sensitive

for revealing grain structure, segregation,and residual strain and deformation.However, these etchants are not widelyused. Their use requires a very well pre-pared specimen for good results. Thislevel of perfection is easily achieved using

Fig. 7 Montage showing the structure of a large weld in a carbon steel revealed by 2% nital(top) and by Klemms I (bottom) reagent (polarized light plus sensitive tint). The HAZ is largerthan usual. Note that the grain size and shape change dramatically from the fusion line (red

arrows at the top and white arrows at the bottom) to the base metal. The yellow arrows showthe transition from columnar grain growth (for temperatures between Ac1and Ac3 (yellow

arrows) of a non-Al killed steel) and fine equiaxed grains for temperatures < Ac1and forirregular, coarse equiaxed grains as the temperature increase above the Ac3.

Fig. 8 Grain structure of a friction buttweld in Type 439 ferritic stainless steeletched using Berahas sulfamic acid

reagent (No. 4) and viewed using brightfield illumination at 50. The arrows

point to faint strain lines in theweldment.

Fig. 9 Microstructure of a spot weld in a deep-drawing quality C-Mn sheet steelrevealed using Klemms I reagent and polarized light plus sensitive tint.

200 m

Centerof weld

HAZ

Base

Weld HAZ

Base

200 m

A

c1HAZ Ac3

Weld Base

200 m


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ADVA NCED M ATER IALS & PROCESSES JUNE 2011 23

modern equipment and consumable products. Figure 7shows an example of the superiority of color etching overstandard etchants in revealing the grain structure of a low-

carbon steel weld. Both etchants, nital and Klemms I[2], re-vealed the as-cast structure of the weld metal, but theKlemms color etch is vastly superior in revealing the grainstructure in the heat-affected zone and base metal.

Ferritic stainless steels are rather difficult to etch, evenwhen not welded. Figure 8 shows a friction butt weld (nofiller metal used) between bars of Type 439 stainless steel

where Berahas No. 4 sulfamic acid etch [2] was used toclearly reveal the weld structure, heat-affected zone, andbase structure. The arrows point to strain lines created bythe frictional pressure.

Figure 9 shows an electrical-resistance (spot) weld be-tween two sheets of C-Mn deep-drawing sheet steel using

Klemms I. Note the directional solidification pattern re-vealed fully by the color etch. In spot welding, the perma-nent electrodes are water cooled and the contact surfacesshould not be melted. Melting should only occur betweenthe two faying surfaces; that is, the contact region betweenthe two sheets. Figure 10 shows an example of a spot weld(weld metal only) between two 350-MPa HSLA steelsheets using Berahas sulfamic acid reagent No. 1. Both ex-amples were imaged using polarized light plus a sensitivetint filter to enhance coloration. Figure 11 shows a frictionstir weld in 2519 aluminum alloy with the grain structure

beautifully revealed using Wecks reagent for Al[2]. Note thedark zone along the top edge due to the deformation pro-

duced by the friction stir tool. Figure 12 shows a brazeusing Ni-base Nicrobraze LM between K-Monel (left) andType 304 austenitic stainless steel (bottom). Note theheavy carbide precipitation in both heat-affected zones.

Conclusions

With good metallographic preparation, and the choiceof the best etch, metallographic examination of weldmentsreveals macrostructural and microstructural featuresclearly with excellent image contrast. There is no substi-tute for a high-quality light microscope and a good imag-ing system. For macrostructural examination, color

etchants often provide results far superior to what can beaccomplished using standard black and white etchants.Weld macrostructural features can be very important in

judging the suitability of a weld for service, for determin-ing conformance to specifications, and for diagnosing fail-ures when they occur.

References

1. Standard Welding Terms and Definitions, AWS A3.0: 2001,The American Welding Society, Miami, Fla.2. G.F. Vander Voort, ASM Handbook Vol 9: Metallographyand Microstructures; Color Metallography, ASM Interna-tional, Materials Park, Ohio, p 505, 2004.

For more information: George Vander Voort is a consultantfor Struers Inc., Westlake, OH 44145. He can be reached at847/623-7648; email: [email protected].

Fig. 10 Microstructure of a spot weld (weld only) madebetween two 350-MPa HSLA steel sheets revealed by Berahassulfamic acid reagent No. 1 and viewed using polarized lightplus sensitive tint.

Fig. 11 Microstructure of a friction stir weld in 2519 aluminum alloy

(Al - 5.8% Cu - 0.3% Mn - 0.3% Mg - 0.06% Ti - 0.1% V - 0.15% Zr)etched using Wecks reagent and viewed using polarized light plus sensitivetint.

Fig. 12 Braze using Nicrobraze LM (Ni-7Cr-3Fe-3.1B-4.5S)between K-Monel (left) and Type 304 stainless steel (below).Note carbide precipitation (sensitization) in the heat affected

zones. Etch: 14 mL water, 100 mL HCl, 3 mL HNO3, and 20 gFeCl3 dissolved in 20 mL water.

250 m

200 m

Weld Heat-affected zone

100 m

metallography of welds - asm international.pdf

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