INSPECTION AND
TESTING OF WELDED SPECIMEN
INTRODUCTION :
To produce quality weld joints, it is necessary to keep an eye on
what is being done in three different stages of the welding.
Ø Before
welding such as cleaning, edge preparation, baking of electrode etc. to
ensure sound and defect free weld joints.
Ø During
welding various aspects such as manipulation of heat source, selection
of input parameters (pressure of oxygen and fuel gas, welding current, arc
voltage, welding speed, shielding gases and electrode selection) affecting the
heat input and so melting, solidification and cooling rates besides protection
of the weld pool from atmospheric contamination.
Ø After
welding steps, if any, such as removal of the slag, peening, post welding
treatment.
Selection of
optimal method and parameters of each of above steps and their execution
meticulously in different stages of production of a weld joint determine the
quality of the weld joint. Inspection is mainly carried out to assess ground
realties in respect of progress of the work or how meticulously things are
being implemented.
Testing helps to:
a) Assess the suitability of the weld joint
for a particular application and
b) To take
decision on whether to go ahead (with further processing or accept/reject the
same) at any stage of welding and
c) Quantify the
performance parameters related with soundness and performance of weld joints.
Testing methods of the
weld joint are broadly classified as :
Ø destructive
testing and
Ø Non-destructive
testing.
Destructive testing methods damage the test
piece to more or less extent. The extent of damage on (destructive) tested
specimens sometime can be up to complete fracture (like in tensile or fatigue
testing) thus making it un-useable for the intended purpose while in case of
non-destructive tested specimen the extent of damage on tested specimen is
either none or negligible which does not adversely affect their usability for
the intended purpose in anyways.
DESTRUCTIVE TESTING:
There are mainly three types of destructive testing they are :
Ø Tensile test
Ø Bending test
Ø Hardness test
TENSILE TEST :
Tensile properties of the weld joints namely yield and
ultimate strength and ductility (%age elongation, %age reduction in area) can
be obtained either in ambient condition or in special environment (low
temperature, high temperature, corrosion etc.) depending upon the requirement
of the application using tensile test which is usually conducted at constant
strain rate (ranging from 0.0001 to 10000 mm/min).
Tensile properties of the weld joint are obtained in two
ways :
a) Taking specimen
from transverse direction of weld joint consisting base metal heat affected zone-weld metal-heat affected
zone-base metal and
b) All weld metal
specimen
Figure : Schematic of tensile specimens from
a) Transverse section of weld joints and b) all weld specimen
BEND TEST :
Bend test is one of the most important and commonly used
destructive tests to determine the ductility and soundness (for the presence
porosity, inclusion, penetration and other macro-size internal weld
discontinuities) of the weld joint produced using under one set of welding conditions. Bending
of the weld joint can be done from face or root side depending upon the purpose
i.e. whether face or root side of the weld is to be assessed. The root side
bending shows the lack of penetration and fusion if any at the root. Further,
bending can be performed using simple compressive/bending load and die of
standard size for free and guided bending respectively . Moreover, free bending
can be face or root bending while guided bending is performed by placing the
weld joint over the die as needs for bending is better and controlled condition.
For
bend test, the load increased until cracks start to appear on face or root of
the weld for face and root bend test respectively and angle of bend at this
stage is used as a measured of ductility of weld joints. Higher is bend angle
(needed for crack initiation) greater is ductility of the weld. Fracture
surface of the joint from the face/root side due to bending reveals the
presence of internal weld discontinuities if any
Figure : bending
test equipment
HARDNESS TEST :
All methods of hardness testing are based on the principle
of applying the standard load through the indenter (a pointed object) and
measuring the penetration in terms of diameter/diagonal/depth of indentation.
High penetration of an indenter at a given standard load suggests low hardness.
Various methods of hardness testing can be compared on the
basis of following three criteria as below
1) Type of indenter,
2) Magnitude of load and
3) Measurement of indentation
ROCKWELL HARDNESS TEST :
Stanley P.
Rockwell invented
the Rockwell hardness
test. He was
a metallurgist for a large ball
bearing company and he
wanted a fast
non-destructive way to
determine if the
heat treatment process
they were doing on the
bearing races was successful. The only
hardness tests he had available at time were Vickers, Brinell and Scleroscope..
Types of the Rockwell Test :
There are two types of Rockwell tests:
1. Rockwell:
the minor load is 10 kgf , the major load is 60, 100, or 150 kgf.
2.
Superficial Rockwell: the minor load is 3 kgf and major loads are 15,
30, or 45 kgf.
In both tests, the indenter may be either a diamond
cone or steel ball, depending upon the characteristics of the material being
tested.
Figure
: Increasing depth of penetration in the Rockwell test
BRINELL HARDNESS TEST :
Dr. J. A. Brinell invented the
Brinell test in Sweden in 1900. The oldest of the hardness test methods in
common use today, the Brinell test is frequently used to determine the hardness
of forgings and castings that have
a grain structure
too course for
Rockwell or Vickers testing.
Therefore, Brinell tests are
frequently done on large parts. By varying the test force and ball size,
nearly all metals can be tested using a Brinell test. Brinell values are
considered test force independent as long as the ball size/test force
relationship is the same.
Brinell Test
methods are defined in the following standards:
Ø ASTM
E10 ; ISO 6506
Brinell Test Method:
All Brinell tests use a carbide ball indenter. The
test procedure is as follows:
Ø The
indenter is pressed into the sample by an accurately controlled test force.
Ø The
force is maintained for a specific dwell time, normally 10 - 15 seconds.
Ø The average
of the two
diagonals is used in
the following formula
to calculate the
Brinell hardness.
The Brinell number, which normally ranges from HB 50
to HB 750 for metals, will increase
as the sample
gets harder. Tables
are available to make the calculation simple.
VICKERS TEST :
The Vickers hardness test is based on the same
principle as the Brinell test, except the indenter is a diamond pyramid with
square base. The angle between the faces of pyramid is 1360as shown
in Figure. The Vickers Hardness Number (VHN) of materials is obtained by
dividing the applied force P, in kgf, by the surface of the pyramidal
depression yielding the relationship
Figure
:vicker’s hardness indentation
IMPACT TESTING:
INTRODUCTION :
A metal may be very hard (and therefore very string
and yet be unsuitable for applications in which it is subjected to sudden loads
in service. Materials behave quite differently when they are loaded suddenly
than when they are loaded more slowly as in tensile testing. Because of
this fact, impact
test is considered
to be one
of the basic
mechanical tests (especially
for ferrous metals).
The term brittle fracture is used to describe rapid
propagation of cracks without any excessive plastic deformation at a stress
level below the yield stress of the material. Metals that show ductile behavior
usually can, under
certain circumstances, behave
in a brittle
fashion. The stress needed to
cause yield rises as the temperature falls. At very low temperatures, fracture
occurs before yielding
Figure
: charpy impact pendulum test
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