Welding engineering and Metallurgy

Mechanical engineering, 8th Sem

Roll no.-1520022
QUIZ TEST No. 3
Q.1 Give Classification of Stainless Steels.
Stainless steels
Stainless steel, any one of a family of alloy steels usually containing 10 to 30 percent
chromium. In conjunction with low carbon content, chromium imparts remarkable resistance
to corrosion and heat. Other elements, such as nickel, molybdenum, titanium, aluminium,
niobium, copper, nitrogen, sulphur, phosphorus, or selenium, may be added to increase
corrosion resistance to specific environments, enhance oxidation resistance, and impart
special characteristics.
There are more than 100 grades of stainless steels. The majority are classified into five
major groups in the family of stainless steels:

 Austenitic- Austenitic steels, which contain 16 to 26 percent chromium and up to 35

percent nickel, usually have the highest corrosion resistance. They are not hardenable
by heat treatment and are nonmagnetic. The most common type is the 18/8, or 304,
grade, which contains 18 percent chromium and 8 percent nickel. Typical applications
include aircraft and the dairy and food-processing industries.

 Ferritic- Standard ferritic steels contain 10.5 to 27 percent chromium and are nickel-
free; because of their low carbon content (less than 0.2 percent), they are not
hardenable by heat treatment and have less critical anticorrosion applications, such as
architectural and auto trim.

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 Martensitic- Martensitic steels typically contain 11.5 to 18 percent chromium and up

to 1.2 percent carbon with nickel sometimes added. They are hardenable by heat
treatment, have modest corrosion resistance, and are employed in cutlery, surgical
instruments, wrenches, and turbines

 Duplex- Duplex stainless steels are a combination of austenitic and ferritic stainless
steels in equal amounts; they contain 21 to 27 percent chromium, 1.35 to 8 percent
nickel, 0.05 to 3 percent copper, and 0.05 to 5 percent molybdenum. Duplex stainless
steels are stronger and more resistant to corrosion than austenitic and ferritic stainless
steels, which makes them useful in storage-tank construction, chemical processing,
and containers for transporting chemicals.

 Precipitation-hardening- Precipitation-hardening stainless steel is characterized by

its strength, which stems from the addition of aluminum, copper, and niobium to the
alloy in amounts less than 0.5 percent of the alloy’s total mass. It is comparable to
austenitic stainless steel with respect to its corrosion resistance, and it contains 15 to
17.5 percent chromium, 3 to 5 percent nickel, and 3 to 5 percent copper. Precipitation-
hardening stainless steel is used in the construction of long shafts

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Q2. Types of ARC Welding

SMAW (Shielded Metal Arc Welding): The electric arc is generated by touching the tip of a
coated electrode against the work piece. The electrodes are in the shape of a thin long stick
hence, it is also called as stick welding. This electrode is coated with flux as it deoxidizes and
provides a shielding gas in the weld area when heated at high temperatures as soon as the arc
forms. This helps shield the weld from the nitrogen and oxygen in the atmosphere. The setup
is connected to an AC or DC power supply which supplies the required current and voltage
depending on the work piece and the job requirements.

Fig no. 1 – SMAW (Shielded Metal Arc Welding)

Advantages:
 Simple, portable and inexpensive equipment
 Versatile and welding in all positions
 Equipment-simple and light
Limitations:
 Low productivity
 Automation not possible
 Fumes make the process control difficult

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GMAW (Gas Metal Arc Welding): In this process, an electric arc is produced between the
consumable metal wire electrode (anode) and the work piece (cathode). A constant voltage,
direct current power source is most commonly used here. This completes the circuit and
electric arc is produced. The arc continuously melts the wire electrode as it is fed into the
weld puddle which is shielded by the flow of gases such as helium, argon, carbon dioxide,
etc. The consumable bare wire is fed automatically through a nozzle into the weld area.

Fig no. 2 – GMAW (Gas Metal Arc Welding)

Advantages:
 High filler metal deposition rates
 Cost effective equipment
 Visible arc
Limitations:
 Higher degree of welder skill
 Comparatively more spatters
 High maintenance cost of equipment

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 FCAW (Flux Cored Arc Welding): FCAW process is almost similar to GMAW
welding except that it uses a tubular wire containing flux inside it. This process
requires a continuously fed consumable tubular electrode containing flux along with a
constant voltage power supply. An externally supplied shielding gas may be used, but
often the flux itself is relied upon to generate the necessary protection from the
atmosphere producing both gaseous protection and a liquid slag protecting the weld.
This method was developed as an alternative to SMAW and is widely used in
construction because of its high welding speed and portability.

Fig no. 3 – FCAW (Flux Cored Arc Welding)

Advantages:
 High welding speed
 High deposition rates.
Limitations:
 Higher degree off welder skill
 Higher equipment cost of equipment/torch.

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SAW (Submerged Arc Welding): In SAW, the arc and the whole welding zone is covered
by a layer of granular fusible flux which protects the weld area and the arc zone by
submerging it inside the blanket of flux. This prevents the evolution of sparks, spatter and
fumes and also acts as a thermal insulator which allows for the deeper penetration of heat into
the work piece. SAW is used for welding medium carbon steels, heat- resistant steels,
corrosion resistant steels and many high strength steels but is limited to weld in the flat or
horizontal position only.

Fig no. 4 – SAW (Submerged Arc Welding)

Advantages:
 High productivity due to high deposition rate
 Negligible smoke/fumes
 Lesser skill required.

Limitations:
 Flux is subjected to contamination which can cause weld porosity
 Not suitable for lower thickness (< 6mm)
 Costly equipments and higher maintenance.

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GTAW (Gas Tungsten Arc Welding): GTAW is also known as TIG welding (Tungsten
Inert Gas). The filler metal is similar to the base metal being used and is provided from an
external filler wire being supplied. The tungsten electrode is not consumed in this operation
and the shielding gas used is usually argon or helium or a mixture of it. GTAW is especially
suited for thin metals. Either AC or DC power supply can be used here, but for magnesium
and aluminium we usually prefer the former as its cleaning action removes the oxides and
impurities out of the weld area. The cost of inert gases makes this process more expensive
than SMAW, but it provides very high quality welds. Close-fit joints can be welded using
GTAW even without a filler metal.

Fig no. 5 – GTAW (Gas Metal Arc Welding)

Advantages:
 No Sparks, fumes and smoke
 High quality and precision
 No chip back required

Limitations:
 Very low productivity
 High degree of skill required
 High equipment cost

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Q.3 Non Destructive Testing Seen on Shop Floor.

 Dye Penetrant Test:
Below are the steps for dye penetrant test:
 Pre Cleaning: The test surface is cleaned to remove any dirt, paint, oil, grease or any
loose scale that could either keep penetrant out of a defect, or cause irrelevant or false
indications. Cleaning methods may include solvents, alkaline cleaning steps or media
blasting. The end goal of this step is a clean surface where any defects present are
open to the surface, dry, and free of contamination.
 Application Of Penetrant: The penetrant is then applied to the surface of the item
being tested. The penetrant is usually a Brilliant colour mobile fluid with very low
surface tension and capillary action. The penetrant is allowed "dwell time" to soak
into any flaws (generally 5 to 30 minutes). The dwell time mainly depends upon the
penetrant being used, material being tested and the size of flaws sought.
 Excess Penetrant Removal: After excess penetrant has been removed, a white
developer is applied to the sample. Several developer types are available, including:
non aqueous wet developer, dry powder and water-soluble. Choice of developer is
governed by penetrant compatibility (one can't use water-soluble developer with
water-washable penetrant) and by inspection conditions.
The developer draws penetrant from defects out onto the surface to form a visible
indication, commonly known as bleed-out. Any areas that bleed out can indicate the
location, orientation and possible types of defects on the surface

Fig no 6 – Dye Penetrant Test

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Radiographic Testing:

 The basic principle is that the radioactive penetrating rays like gamma, cobalt, X-rays
are made to pass through the weld in which the defect is to be found out and the rays
are captured back on a photographic screen resulting in the internal structure of the
weld.
 Energy absorbed by the material depends upon the density and thickness of the
material at that point.
 The defects appear as dark spots in the film. The areas where less energy is absorbed
appear as the lighter areas than the ones which have absorbed relatively more.
 Although the test is slow and expensive, it can detect inclusions, voids and
disbondings in the internal structure of the weld. Workers are susceptible to health
hazards and must be equipped with safety equipment while handling the radiations.

Ultrasonic Testing:

 This test uses ultrasonic waves with a frequency of around 500 kHz to 20 MHz to
detect any defects below 3mm of surface of the weld.
 The probe called the transducer sends out the waves onto the weld area which get
reflected by the back wall or due to the defects present inside the weld. These
reflected waves are either received by the same probe or a different probe.
 The transducer works by converting the electrical energy into mechanical vibrations
and vice versa. The results of the test are displayed on a digital monitor. If there is a
defect, there will be an indication of a different amplitude on the digital monitor.
 Initially, the surface needs to be cleaned and an oil or grease like material must be
applied on the surface so that there is sufficient contact area between the transducer
and the surface otherwise there will be a false indication due to a film of air being
formed in between the transducer and the surface thereby depicting a change in the
medium.
The defects found out from this test are cracks, voids, disbonds and inclusions.

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There are two types of transducers:-

o Single Element or Direct:- This sends the waves perpedicular to itself and
recieves back the waves to show the results. The same probe acts as the
emitter and the reciever.
o Dual Element or Indirect:- This sends the waves at an angle to itself so as to
also calculate the dimensions of the defects. Two different probes act as the
emitter and the reciever.
The advantages of this test are that it is safe from health hazards, the results are reliable and
the job is not exposed to any damaging chemicals or oxidants.

Fig no. 7 – Radiographic Testing

Fig no. 8 – Ultrasonic Testing

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Positive Material Identification (PMI):

 PMI (Positive Material Identification) testing is the analysis of materials to determine

the chemical composition of a metal or alloy on a particular surface.
 Knowing the exact composition and grade of an alloy enables one to match alloy
specifications that are chosen for their specific properties such as heat resistance,
corrosion resistance, durability, etc.
 Stainless steel grades differ by chemical composition. The key differences between
the various grades are mainly the amount of Fe (iron), Cr (chromium), Ni (nickel),
Mo (molybdenum), and Cu (copper).
 Here, some energy is produced which is incident upon an object, and then another
lower energy is emitted from that object. For example, this original energy is an x-ray
beam with enough energy to cause electrons to be knocked out of their orbits in the
atoms in the stainless steel sample.
 These atoms then emits another energy that is lower than the energy of the primary x-
ray beam. The energy emitted from the sample is a result of an electron-losing energy
when the atoms move from a higher energy state to a lower energy state in the given
sample.
 This lower energy is then absorbed by the machine in order to find out an accurate
composition of the material being tested.

Fig no. 9 – PMI (Positive Material Identification)

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Q.4 Enlist Problems encountered while welding Ni Based Alloy

1 Porosity

Pure Nickel and Monel alloys are subjected to porosity if the weld metal does not contain any
nitride forming and deoxidizing elements. Filler rods and electrodes for the arc welding of
these alloys are designed to give weld deposits containing Aluminum, Titanium, Niobium or
combination of there of Aluminum and Titanium forms a stable compound with nitrogen and
oxygen, and by avoiding the atmospheric contamination, if necessary preventing access of air
to the weld bead.

2 Solidification cracking

Hot shortness of Nickel and Nickel alloys may be caused by contamination with sulphur,
lead, phosphorus and number of low melting elements such as Bismuth.

These contaminants form intergranular films of sulfide and phosphates that cause severe
embrittlement at elevated temperature. Hot cracking of weld metal may result from such
contamination, but more frequently it occurs in heat-affected zone of weld and is caused by
intergranular penetration of contaminants from the metal surface. Sulphur is a common
constitute of cutting oil used in machining therefore it is frequently present on metal surface
of nickel that has been is service and require weld repair.

3 Iron dilution

Most nickel alloy weld metals can accept a substantial amount of Iron dilution, but the
dilution limit generally varies with the welding process and sometimes with the heat
treatment. Figure 2.2 shows the limits of iron dilution in four types of weld metal deposited
by commonly used arc-welding process. These limits are based on practical experience rather
than metallurgy.

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Figure 10: Acceptable limits of iron dilution for Ni alloys

Acceptable limit for Ni-Cu weld metal vary greatly, depending on the welding process. With
SMAW, Iron dilution of up to 30% usually tolerated, SAW weld metal should not be diluted
more than 25%, With GMAW and GTAW are less tolerant of Iron dilution especially if the
weld is to be thermally stress relieved.

The max limits are not closely defined, but conservative guidelines for iron dilution in a
welded joint 10 % and 5% after Post weld heat treatment.

The weld metal composition in the copper-Nickel-Iron alloy system that are not susceptible
hot cracking are shown in Figure 2.3 the dilution of Cu –Nil filler metal by iron or chromium
or combination of two, should be limited to 5%.The selection and composition of filler metal
is based on this limits only.

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Figure 11: Hot cracking susceptibility of Iron diluted Ni-Cu weld deposit

Excessive Iron dilution usually does not results in corrosion problem ,because multilayer
overlays are most often specified by final layer permissible iron content, however most
overlay procedures are qualified by bend test, and excessive dilution can cause failure of such
tests. So the control of the %Fe in the final weld metal is critical from process point of view.
High dilution can be maintained by maintaining higher percent of bead overlap.

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Q.5 Ferrite number control in Austenitic Stainless Steel Welding

Ferrite is defined as the solid solution of carbon in iron. What we perceive from ferrite
number in welding? Ferrite number is the amount of retained delta ferrite into the weldment
of austenitic stainless steels, which may support mechanical and corrosion properties (if
present within the acceptable range). Ferrite number approximates 5-20. Lower limit is
required for protection from aggressive corrosion while the higher limit is required to assure
the minimum solidification cracking in weld deposits.

Stainless Steel Austenitic

Stainless steel family includes three types; austenitic, martensitic and ferritic. Austenitic
stainless steel is known for best welding properties and excellent corrosion resistance. It has
face centered cubic structure with the alloying elements of chromium, nickel, manganese,
carbon, nitrogen, chromium, molybdenum. Their strength can be increased when
applied for cold working. Unlike other carbon steels they don’t undergo martensitic
formation on quenching and similarly not subjected to cold cracking. However they can be
subjected to hot cracking which is the concerning problem. On the basis of alloying elements
it is classified as full austenitic or austenitic ferritic. Elements nickel, carbon, manganese and
nitrogen constitutes austenite while the chromium, silicon, niobium, molybdenum form
ferrite.

Solidification modes

The solidification modes are classified on the basis of austenite and ferrite phases. The modes
are named as A (austenitic), AF (austenitic + ferrite), FA (ferrite + austenite) and F (ferrite).
These modes depends on Crreq / Nireq ratio. Lower ratio will favor the formation of A mode
while the higher ratio will favor F mode. In AF mode austenite is primary phase while in FA
mode ferrite is the primary phase.

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Problems

Weld deposit with less ferrite number will lead to hot cracking while on the other hand more
ferrite content can also lead to other mechanical problems. Lower amount is required to
protect the weldment from corrosive media.

Factors affecting the ferrite content

Cooling rate of weld metal, is the major one that controls the ferrite number in austenitic
stainless steel weld deposits. So the selection of electrode diameters, arc length, amperages
(current) and heat treatments are the controlling parameters that influence the ferrite number.
As we know the common practice is to select the current according to the electrode diameter.
Right selection of all above mentioned parameters assists in achieving controlled cooling rate
which is required to obtain desired ferrite content.

Measurement of ferrite number

Following are the common methods which are being utilized for the evaluation of ferrite
number into the weldment.
 Magnetic testing
 Eddy current testing
 Use of Severn gage
 Use of Magne gage
 Use of Ferrite meter

Ferrite Control
Special electrodes are being used called controlled ferrite consumables in order to achieve
required amount of delta ferrite into the weld metal. As it is stated above cooling rate has an
amazing effect on ferrite number. As the cooling rate decreases, ferrite content tends to
reduce while on the other hand cooling rate increases, the ferrite content tends to increase and
(appear as fine particles into the microstructure).

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We should know that an acceptable range of ferrite is required

for particular service conditions when
 High toughness is required at cryogenic temperature
 Low magnetic permeability
 Protection from corrosive media.

However the factors like multipass welding, heat input, penetration and cooling rate are the
keys for controlling ferrite number in austenite welds.

Heat Treatment

Some post weld heat treatments may embrittle the ferrite in microstructure . Delta ferrite has
body centered cubic structure. Heat treatment within 1040-1100 C and holding for 10 minutes
can dissolve the delta ferrite. Hence reducing the ferrite content up to 7-8%.
Heat treatment of austenite welds by annealing has a damaging effect by increasing delta
ferrite content that may lead to the brittleness in weld. However the decomposition –
transformation depends on the annealing temperature and time.

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