Guide to Physical Weld Testing

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Guide to Physical Weld Testing

The tests described below have been developed to check the
skill of the welding operator as well as the quality of the weld metal
and the strength of the welded joint for each type of metal used in
ordnance material. Many tests detect defects not visible to the naked eye.

Destructive Tests

Some of these tests, such as tensile and bending tests, are
destructive, in that the test Specimens are loaded until they fail, so
the desired information can be gained.

Destructive Tests are in two categories:

  • Workshop based tests
  • Laboratory tests (corrosive, chemical, microscopic, macroscopic/magnifying glass)

Non-destructive Tests (NDT)

Other testing methods, such as
the X-ray and hydrostatic tests, are not destructive (NDT). This type of testing is also referred to as NDE or nondestructive examination and NDI or nondestructive inspection. The goal of these methods is to exam the welds without causing any damage.

Each weld physical testing approach is described below.

Destructive Physical Weld Testing

Acid Etch Test

This type or physical weld testing is used to determine the soundness of a weld. The acid
attacks or reacts with the edges of cracks in the base or weld metal and
discloses weld defects, if present. It also accentuates the boundary
between the base and weld metal and, in this manner, shows the size of
the weld which may otherwise be indistinct. This test is usually
performed on a cross section of the joint.

Solutions of hydrochloric acid, nitric acid, ammonium per sulfate,
or iodine and potassium iodide are commonly used for etching carbon and
low alloy steels.

Guided Bend Test

The quality of the weld metal at the face and root of the welded joint,
as well as the degree of penetration and fusion to the base metal, are
determined by means of guided bend tests. It also shows the efficiency of the weld.

This type of physical weld testing is made in a jig (fig 13-1).
These test specimens are machined from welded plates, the thickness of
which must be within the capacity of the bending jig. The test specimen
is placed across the supports of the die which is the lower portion of
the jig. The plunger, operated from above by a hydraulic jack or other
device, causes the specimen to be forced into and to assure the shape of
the die.

To fulfill the requirements of this test, the specimens must
bend 180 degrees and, to be accepted as passable, no cracks greater than
1/8 in. (3.2 mm) in any dimension should appear on the surface. The
face bend tests are made in the jig with the face of the weld in tension
(i.e., on the outside of the bend) (A - fig 13-2). The root bend tests are made with the root of the weld in tension (i. e., on outside of the bend) (B - fig 13-2). Guided bend test specimens are also shown the in figure 13-3.

Guided Bend Test Jig (Figure 13-1)

Notes:

  • T=Test Plate Thickness
  • Hardened Rolls May be used on shoulders if desired
  • Specific dimensions for 3/7 plate
  • All dimensions shown are in inches

Guided Bend Test Specimens (Figure 13-2)

Guided Bend and Tensile Strength Test Specimens (Figure 13-3)

Free Bend Test

The free bend physical weld testing approach has been devised to measure the ductility of the weld
metal deposited in a weld joint. A physical weld testing specimen is machined from the
welded plate with the weld located as shown at A, figure 13-4.

Each corner lengthwise of the specimen shall be rounded in a radius not
exceeding one-tenth of the thickness of the specimen. Tool marks, if
any, shall be lengthwise of the specimen. Two scribed lines are placed
on the face 1/16 in. (1.6 mm) in from the edge of the weld. The distance
between these lines is measured in inches and recorded as the initial
distance X (B, figure 13-4).
The ends of the test specimen are then bent through angles of about 30
degrees, these bends being approximately one-third of the length in from
each end. The weld is thus located centrally to ensure that all of the
bending occurs in the weld.

The specimen bent initially is then placed
in a machine capable of exerting a large compressive force (C, figure 13-4) and bent until a crack greater than 1/16 in. (1.6 mm) in any dimension
appears on the face of the weld. If no cracks appear, bending is
continued until the specimens 1/4 in. (6.4 mm) thick or under can be
tested in vise. Heavier plate is usually tested in a press or bending
jig.

Whether a vise or other type of compression device is used when
making the free bend test, it is advisable to machine the upper and
lower contact plates of the bending equipment to present surfaces
parallel to the ends of the specimen (E, figure 13-4). This will prevent the specimen from slipping and snapping out of the testing machine as it is bent.

Free Bend Test of Welded Metal (Figure 13-4)

After bending the specimen to the point where the test bend is
concluded, the distance between the scribed lines on the specimen is
again measured and recorded as the distance Y. To find the percentage of
elongation, subtract the initial from the final distance, divide by the
initial distance, and multiply by 100 (figure 13-4).
The usual requirements for passing this test are that the minimum
elongation be 15 percent and that no cracks greater than 1/16 in. (1.6
mm) in any dimension exist on the face of the weld.

The free bend test is being largely replaced by the guided bend test where the required testing equipment is available.

Back Bend Test

The back bend test is a type of physical weld testing that is used to determine the quality of the weld metal
and the degree of penetration into the root of the Y of the welded butt
joint. The specimens used are similar to those required for the free
bend test
except they are bent with the root of the weld on the tension side, or
outside. The specimens tested are required to bend 90 degrees without
breaking apart. This test is being largely replaced by the guided bend
test
.

Nick Break Test

The nick break test has been devised to determine if the weld metal of a
welded butt joint has any internal defects, such as slag inclusions,
gas pockets, poor fusion, and/or oxidized or burnt metal. The specimen
is obtained from a welded butt joint either by machining or by cutting
with an oxyacetylene torch. Each edge of the weld at the joint is
slotted by means of a saw cut through the center (figure 13-5). The piece thus prepared is bridged across two steel blocks (figure 13-5)
and stuck with a heavy hammer until the section of the weld between the
slots fractures.

The metal thus exposed should be completely fused and
free from slag inclusions. The size of any gas pocket must not be
greater than 1/16 in. (1.6 mm) across the greater dimension and the
number of gas pockets or pores per square inch (64.5 sq mm) should not
exceed 6.

Nick Break Test (Figure 13-5)

Another break test method is used to determine the soundness of fillet
welds. This is the fillet weld break test. A force, by means of a press,
a testing machine, or blows of a hammer, is applied to the apex of the V
shaped specimen until the fillet weld ruptures. The surfaces of the
fracture will then be examined for soundness.

Tensile Strength Test

Tensile Weld Strength Testing

Tensile weld test performed in workshop is a type of physical weld testing device

This type of physical weld testing is used to measure the strength of a welded joint. A portion
of a to locate the welded plate is locate the weld midway between the
jaws of the testing machine (figure 1306).
The width thickness of the test specimen are measured before testing,
and the area in square inches is calculated by multiplying these before
testing , and the area in square inches is calculated by multiplying
these two figures (see formula, figure 13-6).

The tensile physical weld testing specimen is then mounted in a machine that will exert
enough pull on the piece to break the specimen. The testing machining
may be either a stationary or a portable type. A machine of the portable
type, operating on the hydraulic principle and capable of pulling as
well as bending test specimens, is shown in figure 13-7.

As the specimen is being tested in this machine, the load in pounds is
registered on the gauge. In the stationary types, the load applied may
be registered on a balancing beam. In either case, the load at the point
of breaking is recorded. Test specimens broken by the tensile strength
test are shown in figure 13-3.

Tensile Strength Test Specimen and Test Method (figure 13-6)

Portable Tensile Strength and Bend Testing Machine (Figure 13-7)

The tensile strength, which is defined as stress in pounds per
square inch, is calculated by dividing the breaking load of the test
piece by the original cross section area of the specimen. The usual
requirements for the tensile strength of welds is that the specimen
shall pull not less than 90 percent of the base metal tensile strength.

The shearing strength of transverse and longitudinal fillet welds
is determined by tensile stress on the test specimens. The width of the
specimen is measured in inches. The specimen is ruptured under tensile
load, and the maximum load in pounds is determined. The shearing
strength of the weld in pounds per linear inch is determined by dividing
the maximum load by the length of fillet weld that ruptured. The
shearing strength in pounds per square inch is obtained by dividing the
shearing strength in pounds per linear inch by the average throat
dimension of the weld in inches. The test specimens are made wider than
required and machined down to size.

Non-destructive Tests

Hydrostatic Test

This is a nondestructive type of physical weld testing used to check the quality of welds on
closed containers such as pressure vessels and tanks. The test usually
consists of filling the vessel with water and applying a pressure
greater than the working pressure of the vessel. Sometimes, large tanks
are filled with water which is not under pressure to detect possible
leakage through defective welds. Another method is to test with oil and
then steam out the vessel. Back seepage of oil from behind the liner
shows up visibly.

Magnetic Particle Test

This is a physical weld testing or inspection method used on welds and parts made of
magnetic alloy steels. It is applicable only to ferromagnetic materials
in which the deposited weld is also ferromagnetic. A strong magnetic
field is set up in the piece being inspected by means of high amperage
electric currents.

A leakage field will be set up by any discontinuity
that intercepts this field in the part. Local poles are produced by the
leakage field. These poles attract and hold magnetic particles that are
placed on the surface for this purpose. The particle pattern produced on
the surface indicates the presence of a discontinuity or defect on or
close to the surface of the part.

X-Ray Testing

This is a radiographic physical weld testing method used to reveal the presence and
nature of internal defects in a weld, such as cracks, slag, blowholes,
and zones where proper fusion is lacking. In practice, an X-ray tube is
placed on one side of the welded plate and an X-ray film, with a special
sensitive emulsion, on the other side. When developed, the defects in
the metal show up as dark spots and bands, which can be interpreted by
an operator experienced in this inspection method.

Porosity and
defective root penetration as disclosed by X-ray inspection are shown in figure 13-8.

Internal Weld Defects Disclosed by X-ray Inspection (Figure 13-8)

Instructions for handling X-ray apparatus to avoid ahrm to operating personnel are found in the "American Standard Code for the Industrial Use of X-rays"

Gamma Ray Test

This test is a radiographic physical weld testing and inspection method similar to the X-ray method described in the paragraph on acid etch testing,
except that the gamma rays emanate from a capsule of radium sulfate
instead of an X-ray tube. Because of the short wave lengths of gamma
rays, the penetration of sections of considerable thickness is possible,
but the time required for exposure for any thickness of metal is much
longer than that required for X-rays because of the slower rate at which
the gamma rays are produced. X-ray testing is used for most
radiographic inspections, but gamma ray equipment has the advantage of
being extremely portable.

Flourescent Penetrant Test (Dye Test)

Weld Dye Penetrant Tests

Types of Weld Dye Penetrant Tests

Fluorescent penetrant inspection is a nondestructive physical weld testing method by
means of which cracks, pores, leaks, and other discontinuities can be
located in solid materials. It is particularly useful for locating
surface defects in nonmagnetic materials such as aluminum, magnesium,
and austenitic steel welds and for locating leaks in all types of welds.
This method makes use of a water washable, highly fluorescent material
that has exceptional penetration qualities.

This material is applied to
the clean dry surface of the metal to be inspected by brushing,
spraying, or dipping. The excess material is removed by rinsing, wiping
with clean water-soaked cloths, or by sandblasting. A wet or dry type
developer is then applied. Discontinuities in surfaces which have been
properly cleaned, treated with the penetrant, rinsed, and treated with
developer show brilliant fluorescent indications under black light.

Advantages of this physical weld testing method:

  • Good for ferrous and non ferrous metals
  • Low cost
  • Easy to apply and interpret
  • Minimal training

Disadvantages:

  • Might miss problems below the surface
  • Can't work on porous materials

Types of Dye:

  • Type A: Fluorescent that emits visible light when viewed using a black light
  • Type B: Brightly colored dye that can be inspected in regular light. Simple to use and good for testing in the field.

Hardness Tests

Hardness may be defined as the ability of a substance to resist
indentation of localized displacement. Simply said, resistance to indentation, wear and abrasion. The hardness test usually applied
is a nondestructive test, used primarily in the laboratory and not to
any great extent in the field. Hardness tests are used as a means of
controlling the properties of materials used for specific purposes after
the desired hardness has been established for the particular
application.

A hardness test is used to determine the hardness of weld
metal. By careful testing of a welded joint, the hard areas can be
isolated and the extent of the effect of the welding heat on the
properties of the base metal determined.

Hardness Testing Equipment

File Test:

The simplest method for determining comparative
hardness is the file test. It is performed by running a file under
manual pressure over the piece being tested. Information may be obtained
as to whether the metal tested is harder or softer than the file or
other materials that have been given the same treatment.

Hardness Testing Machines:

There are several types of hardness testing
machines. Each of them is singular in that its functional design best
lends itself to the particular field or application for which the
machine is intended. However, more than one type of machine can be used
on a given metal, and the hardness values obtained can be satisfactorily
correlated. Two types of machines are used most commonly in laboratory
tests for metal hardness: the Brinell hardness tester and the Rockwell
hardness tester.

  • Brinell Hardness Tester

    In the Brinell tests, the
    specimen is mounted on the anvil of the machine and a load of 6620 lb
    (3003 kg) is applied against a hardened steel ball which is in contact
    with the surface of the specimen being tested. The steel ball is 0.4 in.
    (10.2 mm) in diameter. The load is allowed to remain 1/2 minute and is
    then released, and the depth of the depression made by the ball on the
    specimen is measured.

    It should be noted that, in order to facilitate the determination of
    Brinell hardness, the diameter of the depression rather than the depth
    is actually measured. Charts of Brinell hardness numbers have been
    prepared for a range of impression diameters. These charts are commonly
    used to determine Brinell numbers.

    The resultant Brinell hardness number is obtained
    by the following formula:

  • Rockwell Hardness Tester

    The principle of the Rockwell tester is essentially the same as the
    Brinell tester. It differs from the Brinell tester in that a lesser load
    is impressed on a smaller ball or cone shaped diamond. The depth of the
    indentation is measured and indicated on a dial attached to the
    machine. The hardness is expressed in arbitrary figures called "Rockwell
    numbers." These are prefixed with a letter notation such as "B" or "C"
    to indicate the size of the ball used, the impressed load, and the scale
    used in the test.

Other tests are Vickers diamond pyramid and Scleroscope.

Vickers Hardness Weld Tester

Magnaflux Test

This is a rapid, non-destructive physical weld testing method for locating defects at or
near the surface of steel and its magnetic alloys by means of correct
magnetization and the application of ferromagnetic particles.

Basic Principles

For all practical purposes, magnaflux
inspection may be likened to the use of a magnifying glass as a physical weld testing method. Instead of
using a glass, however, a magnetic field and ferromagnetic powders are
employed. The method of magnetic particle inspection is based upon two
principles: one, that a magnetic field is produced in a piece of metal
when an electric current is flowed through or around it; two, that
minute poles are set up on the surface of the metal wherever this
magnetic field is broken or distorted.

When ferromagnetic particles are brought into the vicinity of a
magnetized part, they are strongly attracted by these poles and are held
more firmly to them than to the rest of the surface of the part,
thereby forming a visible indication.

Eddy Current (Electromagnetic Testing)

Magnetic Particle Weld Tester

Magnetic Particle Testing is Mainly for surface defects and ferrous metals

Eddy current (electromagnetic) testing is a nondestructive test
method based on the principle that an electric current will flow in any
conductor subjected to a changing magnetic field. It is used to check
welds in magnetic and nonmagnetic materials and is particularly useful
in testing bars, fillets, welded pipe, and tubes. The frequency may vary
from 50 Hz to 1 MHz, depending on the type and thickness of material
current methods. The former pertains to tests where the magnetic
permeability of a material is the factor affecting the test results and
the latter to tests where electrical conductivity is the factor
involved.

Nondestructive physical weld testing by eddy current methods involves inducing
electric currents (eddy or foucault currents) in a test piece and
measuring the changes produced in those currents by discontinuities or
other physical differences in the test piece. Such tests can be used not
only to detect discontinuities, but also to measure variations in test
piece dimensions and resistivity. Since resistivity is dependent upon
such properties as chemical composition (purity and alloying), crystal
orientation, heat treatment, and hardness, these properties can also be
determined indirectly. Electromagnetic methods are classified as
magnetoinductive and eddy current methods. The former pertains to tests
where the magnetic permeability of a material is the factor affecting
the test results and the latter to tests where electrical conductivity
is the factor involved.

One method of producing eddy currents in a test specimen is to
make the specimen the core of an alternating current (ac) induction
coil. There are two ways of measuring changes that occur in the
magnitude and distribution of these currents. The first is to measure
the resistive component of impedance of the exciting coil (or of a
secondary test coil), and the second is to measure the inductive
component of impedance of the exciting (or of a secondary) coil.
Electronic equipment has been developed for measuring either the
resistive or inductive impedance components singly or both
simultaneously.

Eddy currents are induced into the conducting test specimen by
alternating electromagnetic induction or transformer action. Eddy
currents are electrical in nature and have all the properties associated
with electric currents. In generating eddy currents, the test piece,
which must be a conductor, is brought into the field of a coil carrying
alternating current. The coil may encircle the part, may be in the form
of a probe, or in the case of tubular shapes, may be wound to fit inside
a tube or pipe. An eddy current in the metal specimen also sets up its
own magnetic field which opposes the original magnetic field. The
impedance of the exciting coil, or of a second coil coupled to the
first, in close proximity to the specimen, is affected by the presence
of the induced eddy currents. This second coil is often used as a
convenience and is called a sensing or pick up coil. The path of the
eddy current is distorted by the presence of a discontinuity. A crack
both diverts and crowds eddy currents. In this manner, the apparent
impedance of the coil is changed by the presence of the defect. This
change can be measured and is used to give an indication of defects or
differences in physical, chemical, and metallurgical structure.
Subsurface discontinuities may also be detected, but the current falls
off with depth.

Acoustic Emission Testing

Acoustic Weld Tester

One acoustic method is to strike the welded object and determine weld quality based on the tone.

Acoustic emission testing (AET) physical weld testing methods are currently considered
supplementary to other nondestructive testing methods. They have been
applied, however, during proof testing, recurrent inspections, service,
and fabrication.

Acoustic emission testing consists of the detection of acoustic
signals produced by plastic deformation or crack formation during
loading. These signals are present in a wide frequency spectrum along
with ambient noise from many other sources. Transducers, strategically
placed on a structure, are activated by arriving signals. By suitable
filtering methods, ambient noise in the composite signal is notably
reduced. Any source of significant signals is plotted by triangulation
based on the arrival times of these signals at the different
transducers.

Ferrite Testing

Effects of Ferrite Content

Fully austenitic stainless steel
weld deposits have a tendency to develop small fissures even under
conditions of minimal restraint. These small fissures tend to be located
transverse to the weld fusion line in weld passes and base metal that
were reheated to near the melting point of the material by subsequent
weld passes. Cracks are clearly injurious defects and cannot be
tolerated. On the other hand, the effect of fissures on weldment
performance is less clear, since these micro-fissures are quickly
blurted by the very tough austenitic matrix. Fissured weld deposits have
performed satisfactorily under very severe conditions. However, a
tendency to form fissures generally goes hand-in-hand with a tendency
for larger cracking, so it is often desirable to avoid fissure-sensitive
weld metals.

The presence of a small fraction of the magnetic delta ferrite
phase in an otherwise austenitic (nonmagnetic) weld deposit has an
influence in the prevention of both centerline cracking and fissuring.
The amount of delta ferrite in as-welded material is largely controlled
by a balance in the weld metal composition between the ferrite-promoting
elements (chromium, silicon, molybdenum, and columbium are the most
common) and the austenite-promoting elements (nickel, manganese, carbon,
and nitrogen are the most common). Excessive delta ferrite, however,
can have adverse effects on weld metal properties. The greater the
amount of delta ferrite, the lower will be the weld metal ductility and
toughness. Delta ferrite is also preferentially attacked in a few
corrosive environments, such as urea. In extended exposure to
temperatures in the range of 900 to 1700°F (482 to 927°C), ferrite tends
to transform in part to a brittle intermetallic compound that severely
embrittles the weldment.

Portable ferrite indicators are designed for on-site use. Ferrite
content of the weld deposit may indicated in percent ferrite and may be
bracketed between two values. This provides sufficient control in most
applications where minimum ferrite content or a ferrite range is
specified.

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