Welding Metallography - Ferrous Metals.pdf - Ksiazki IWE - rajmundos9

Published by Buehler

Volume 4, Issue 3

USING MICROSTRUCTURAL ANALYSIS TO SOLVE PRACTICAL PROBLEMS

Welding Metallography - Ferrous Metals

Introduction

Welding is a very important joining process

and has been used extensively for at least the

past 60 years. Like most processes, there is a

need to control the process and insure a high

quality end result. Welds are no exception

and over the years there have been many

spectacular failures of welded structures that

emphasize this need, e.g., Liberty ship and T2

tanker failures in WWII. Many procedures

involving non-destructive and destructive

tests are used to study weldments. Metallo-

graphic examination can be performed in the

field by grinding and polishing a spot on the

surface of a weld, its heat affected zones or

nearby base metal (the metal being joined

that was unaffected by the heat of the weld-

ing process). This is a reasonably non-destruc-

tive evaluation. However, destructive exami-

nation, where a specimen is removed from

either the welded assembly or test coupons,

is quite commonly preformed. Test coupons

are often used to qualify the welder and

ensure that the techniques and materials cho-

Written by:

George Vander Voort

Director, Research

and Technology,

Buehler

sen will produce a weld with acceptable

soundness and mechanical properties. Post

mortems of failed weldments are also exam-

ined metallographically using sections re-

moved from the welded assembly, generally

after non-destructive examinations are com-

pleted.

Welding Process

There are a great many processes that have

been developed to produce welded joints.

Most people have seen the stick-electrode

process that can be done in the field. But, this

is just one of many welding processes.

Although welding is a comparatively new

technology, forge welding vastly predates all

other methods as it dates from the earliest

days of metalworking. Aside from forge weld-

ing, the other processes date from the 20th

century, particularly since 1940. There are

both gas welding and cutting processes using

an oxyacetylene flame; resistance welding

processes, such as spot welding, induction

welding, flash welding; arc welding processes

Figure 1. Basic terms related to a fillet weld.

such as gas-tungsten-arc (GTA) and metal-inert gas (MIG),

covered electrode processes (stick electrode), submerged-arc

welding, electroslag welding, electron beam and laser weld-

ing, as well as friction welding. Many of these processes have

been further modified in a variety of ways. Some of these

processes use filler metals, generally of somewhat different

composition than the base metal to produce higher strength

in the weld. Others use no filler metal, relying only upon the

melting of the base metal to produce the joint.

Weld Terminology

Figure 1 shows a schematic illustrating the basic features of a

fusion weld. There are three main regions: the base metal, the

heat-affected zone (HAZ) and the weld metal. Regardless of

the welding process, substantial heat is generated in welding

and melting occurs. The heat input can vary greatly with the

welding process used and is influenced by other factors, such

as the thickness of the pieces being joined. The welded joint,

or weld “nugget” is a casting. When wrought metal is welded,

there is a temperature gradient, going from the nugget into

the unaffected base metal, from above the melting point of

the metal or alloy to ambient temperature. This temperature

gradient can produce many effects depending upon the met-

als or alloys being joined. Using steels as an example, the weld

nugget was created by molten metal, in many cases filler

metal, that was heated in the arc until it melted. Solidification

can occur under different cooling conditions, depending upon

the heat input, whether or not pre-heating or post-heating

practices are used, depending upon the mass of the pieces,

ambient temperature, and so forth. Naturally, there is a fusion

line, the boundary between the cast nugget and the non-

melted base metal. Below the fusion line, the temperature

gradually drops to ambient. If the part is made from steel, the

heat-affected zone (between the fusion line and the unaffect-

ed base metal), or at least part of the heat-affected zone, will

be fully austenitic due to temperatures above the upper criti-

cal, A C3 , of the steel. The grains closest to the fusion line will be

the largest in size. At lower temperatures, the grain size can

be quite small due to recrystallization and nucleation of new

fine grains that may, or may not, grow substantially depend-

ing upon the temperature that they experience after nucle-

ation. Depending upon the way the steel was deoxidized,

columnar grains may be seen. In the region of the HAZ that

was heated into the two phase

Figure 2. Terminology for describing cracks in welds (AWS A3.0: 2001

[1]).

Examination Procedure

The metallographer is often requested to examine a welded

joint. To do this, they must cut out one or more specimens to

sample the structure of the weld, heat-affected zone and

adjacent base metal. Naturally, it is most convenient if all

three regions can be 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 by the electroslag

process, the weld nugget alone can be quite large. Even here

it is possible to prepare entire cross sections through the

welds, although it is not as simple to do so as for smaller

welds. The specimen is examined in the as-polished condition

for voids of different types, such as porosity from gas evolu-

tion or shrinkage cavities, cracks that may be present in either

the weld metal or the heat affected zone, regions where the

weld did not exist (lack of fusion or lack of penetration) and for

nonmetallic inclusions associated with the welding operation,

chiefly slag-type in nature, in the weld or between weld pass-

es (for a multi-pass weld).

field, the transformation

on cooling may be quite different. For areas heated below the

lower critical temperature, A C1 , the original structure may be

tempered or may start to spheroidize. Because the filler metal

is a different composition than the base metal, and some

melting of the base metal occurs, the composition will vary

through the weld to the fusion line. With variations in the

phases or constituents and their grain size in the weld nugget

and heat affected zone, we can expect to see hardness varia-

tions across these gradients.

Obtaining and Preparing Weld Specimens

In some cases, the welded structure is large and, in the case of

a field failure, a section must be removed by flame cutting.

This process produces a substantial damaged zone adjacent

to the cut, perhaps as wide as 10 – 15mm. When the section

gets to the laboratory, the damaged cut region must be

removed by a less-damaging cutting method, such as band

sawing or abrasive sectioning. Then, the metallographer will

cut out coupons using a laboratory abrasive cut-off saw that

introduces less damage than production manufacturing

equipment. Weld samples often tend to be large and irregular

in shape. Many will not fit within a standard 1-, 1.25-, 1.5- or

2- inch (25-, 30-, 40- or 50mm) diameter mold for compres-

sion molding. In such cases, the metallographer often builds

a mold using bent sheet metal, coated perhaps with a mold

release agent, places the specimen inside this mold (after the

mold is glued to a suitable base plate), and encapsulates the

specimen with epoxy resin. After it has cured, the specimen

can be ground and polished using a wide variety of semi-auto-

mated equipment. Figure 3 shows an example of a large weld

Cracks may be detected in the weld nugget or in the heat-

affected zone. Figure 2 shows a schematic illustrating com-

mon terminology for cracks and voids. Use of correct termi-

nology to describe cracks is very important. Many of the

cracks are described based upon their location - crater cracks,

root cracks, and heat-affected zone cracks are a few exam-

ples. Sometimes cracks are described based on their orienta-

tion with respect to the welding direction – longitudinal and

transverse cracks being obvious cases. They may also be

described by the nature of the problem that caused the crack

– hydrogen-induced cracks, stress-relief cracks, etc.

Reference 1 is a great source of information regarding weld-

ing terminology.

mounted in a custom made mold using an epoxy resin. If care

is taken in cutting the specimen, so that the cut face is flat and

a minimum amount of damage is introduced, then rough

grinding time can be minimized. If rough grinding must be

extensive, either to remove cut surface roughness, or to

obtain a flat surface across the specimen, then it may be advis-

able to perform the grinding step with the ApexHercules ™ H

disc and a coarse diamond size, e.g., 45mm MetaDi ®

Supreme Diamond Suspension . This has a very high removal

rate and is an excellent procedure for obtaining superior flat-

ness.

(must be de-bondable). This requires that the weight be bal-

anced across the holder face so that the head does not vibrate

or chatter. But, using this approach, specimens larger than

can be placed into large cut-out holes in the specimen holder

(Figure 4), can be prepared.

Grinding and polishing cycles for welds differ little from pro-

cedures for the non-welded metals and alloys. The chief dif-

ference may be the need to polish an area that is larger than

normal and the fact that the hardness can vary across the

specimen. The fact that part of the specimen is a casting while

Another option with rather large specimens is to cut out a disc

through the weld less than 12mm thick. If a machine shop is

available, the opposing sides can be ground flat and parallel

using a Blanchard or other type grinder. Then, the discs can be

attached to a blank specimen holder (a holder without the

holes in it for specimens) using double-sided tape or glue

Figure 4. Example of a 7 inch (178mm) diameter specimen holder (for a 12

inch (300mm) diameter platen) with non-conventional openings for large

rectangular specimens (1.375 x 3 inch or 35 x 76mm)

the balance is wrought generally does not affect the prepara-

tion process. Two generic practices relevant for many

commonly welded ferrous alloys are presented in Tables 1 and

2. The reader is directed to the Buehler web site,

http://www.buehler.com , for preparation methods for a wide

variety of engineering metals and alloys.

Figure 3. Example of a large weld encapsulated in epoxy using a custom-

made mold.

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Table 1. Generic Method for Preparing Ferrous Weld Specimens

Base Speed

Time

Surface

Abrasive/ Size

Load Lb. (N)

(rpm)/Direction

(min)

CarbiMet ®

120/P120-, 180/P180-

6 (27)

240-300

Until

waterproof

or 240/P280-grit SiC

Comp.

Plane

paper

water cooled

UltraPol ™

m MetaDi ® Supreme

6 (27)

120-150

cloth

diamond suspension

Comp.

TexMet ® 1000

m MetaDi Supreme

6 (27)

120-150

or TriDent ™

diamond suspension

Comp.

cloth

MicroCloth ® or

m MasterMet ®

~ 0.05

6 (27)

120-150

ChemoMet ®

colloidal silica or

Contra

MasterPrep ™ sol-gel

alumina suspensions

cloths

Table 2. Generic Method for Preparing Ferrous Weld Specimens Using a Rigid Grinding Disc

Base Speed

Time

Surface

Abrasive/ Size

Load Lb. (N)

(rpm)/Direction

(min)

ApexHercules ™ H

m MetaDi Supreme

6 (27)

120-150

Until

disc

diamond suspension

Comp.

Plane

ApexHercules ™ H

m MetaDi Supreme

6 (27)

120-150

disc

diamond suspension

Comp.

TexMet ® 1000 or

m MetaDi Supreme

6 (27)

120-150

TriDent ™ cloth

diamond suspension

Comp.

MicroCloth ® ,

m MasterMet ®

~ 0.05

6 (27)

120-150

NanoCloth ™

colloidal silica or

Contra

ChemoMet ®

MasterPrep ™ sol-gel

cloth

alumina suspensions

If the specimen is particularly difficult to prepare, it may be best

to add a 1

structure, segregation and residual strain and deformation.

However, these etchants are not widely used. Their use does

require a very well prepared specimen for good results. But this

level of perfection is easily achieved with modern equipment

and consumable products. Figure 5 shows an example of the

superiority of color etching over standard etchants in revealing

the grain structure of a low-carbon steel weld. Both etchants

revealed the as-cast structure of the weld metal, but the color

etch was vastly superior in revealing the grain structure in the

heat affected zone and base metal.

m diamond step to the procedure, in the same man-

ner as the 3

m diamond step, but for 3 minutes. For a holder

with large specimens, the times may need to be increased. As an

alternative, steps 1 and 2 could be performed using the

ApexHercules ™ H or S rigid grinding discs (RGD). The H disc has

a higher removal rate and can be used to prepare all ferrous

alloys, although when the hardness gets below about 200 HV,

the S disc is preferred. Such a procedure is shown in Table 2.

Again, if the materials are difficult to prepare, a 1

m diamond

step can be added following the same approach as for the 3

Figure 6 shows two low-magnification views of a fusion weld in

AISI/SAE 1006 carbon steel etched with 2% nital and with

Klemm’s I tint etch. Nital does a good job, Figure 6a, revealing

the transition from base metal to heat affected zone to weld

metal. Note the black spots in the heat affected zone; these will

be shown at a higher magnification revealing that they are fine

pearlitic regions due to transformation of small regions that

transformed to austenite, and contained all of the carbon in that

area. These spots are not as easily observed in the color etch,

Figure 6b. On the other hand, the color etch revealed the varia-

tion in grain size and shape far better than nital. Figure 7 shows

the microstructure of the weld (a), heat affected zone (b) and

base metal (c). Note that the base metal contained ferrite and

grain boundary cementite films. In the heat-affected zone,

where the black spots were observed, the temperature was high

enough to locally dissolve the cementite into austenite, but

there was insufficient time for the carbon content to become

uniform. With cooling, these carbon-rich areas transformed to a

pearlitic-like constituent. The weld structure is typical of a cast

low-carbon ferritic grain structure with an acicular appearance

and fine dispersions of cementite.

diamond step, except for a 3 minute duration.

Examination of Welds

Examination should always be performed after polishing and

before etching to detect voids, cracks and inclusions. Then, the

metallographer will etch the specimen to study both the

macrostructure and the microstructure using an etchant appro-

priate for the alloy. In some cases, the weld metal is of suffi-

ciently different composition that an etchant chosen to etch the

base metal and heat affected zone will not reveal the weld metal

structure, and vice versa, or one area may be badly overetched.

If the specimen has been polished, the macrostructural details

are usually adequately revealed by the etchant used to reveal

the microstructure. In some studies, the metallographer will

macroetch the specimen after grinding and study the

macrostructure. This specimen is not suitable for microstructur-

al examination, unless polished.

Most etchants used to reveal the microstructure of welds are

standard general-purpose etchants. After examination with

such an etch, it may prove to be valuable to use a color etching

technique, as these can be far more sensitive for revealing grain

Figure 5. Example of a welded low-carbon steel etched with 2% nital (top) and Klemm’s I (bottom) showing the clear superiority of

color etching in revealing the grain structure. The as-cast weld metal is shown at the far left and the base metal is shown at the far

right, above the

m bar. In between, we see the heat affected zone starting with coarse irregular grains adjacent to the fusion line,

progressing to finer more uniformly shaped grains, then to columnar grains and finally to the very fine-grained equiaxed ferrite

grains in the base metal.

If a weld is given a post heat treatment, which does

happen occasionally, the grain structure will be

refined and the heat-affected zone will not be visi-

ble. As an example, Figure 8 shows the microstruc-

ture of Trimrite martensitic stainless steel (Fe –

0.23%C – 14.25%Cr – 0.65%Ni – 0.7%Mo) that was

GTA welded and then heat treated (843 °C, air

cooled, 788 °C, air cooled). Figure 8a shows the

specimen etched with Vilella’s reagent. The cast

structure of the weld (255 HV) is clearly seen, but

there is no heat-affected zone. The base metal (197

HV) is fine grained. Figure 8b, produced using

Beraha’s sulfamic acid etch (100 mL water, 3 g

potassium metabisulite, 2 g sulfamic acid and 1 g

ammonium bifluoride), reveals the weld nicely and

shows a grain size transition from the fusion line to

the base metal where the HAZ was located. It is

Figure 7. Microstructure of the weld (top), heat affected zone

(middle) and base metal (bottom) of welded 1006 carbon steel

etched with 2% nital. (Magnification bars are 20

m long).

clear that the heat treatment refined the grain

structure in the HAZ, but it is still coarser than in the

base metal and the color etch reveals the transition

in grain size from the fusion line to the base metal.

This grain size variation was not as apparent with

Vilella’s reagent, at least not at this magnification

(50X).

Figure 6. Low magnification view of a fusion weld in 1006 car-

bon steel revealed using 2% nital (top) and with Klemm’s I

(bottom) in polarized light plus sensitive tint (magnification

bar is 200

m long).

Welding Metallography - Ferrous Metals.pdf

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