Slide 1General Engineering
petrochemical plants
Railways - Coaches, locomotives, wagons
Food processing - Dairy, brewery, cooking etc.
Mr. R.D.Pennathur
Welding is mostly done for fabrication of metals and alloys
The final properties of the welded assembly will depend on the metallurgical structure of the parent metal and the weld.
All welding processes involve heating and cooling of the components being welded
Thus to ensure a satisfactory welded component, it is necessary to understand metallurgical structures and how they and the weld thermal cycle, determine the properties of the weld joint.
Mr. R.D.Pennathur
Adhesive Bonding
Mr. R.D.Pennathur
Welding Metallurgy
Prior to welding as a useful fabrication, riveting was extensively used for joining
Mr. R.D.Pennathur
Mr. R.D.Pennathur
Extra manufacturing steps
Stress concentration!
Mr. R.D.Pennathur
Mr. R.D.Pennathur
Mr. R.D.Pennathur
Mr. R.D.Pennathur
No interface
Less weight
Suitable for different joint types
Cost effective
Mr. R.D.Pennathur
Welding Metallurgy
Is it totally free of disadvantages?
No, but advantages often outweigh such demerits- faster, safer, cost effective…
Mr. R.D.Pennathur
Welding Metallurgy
Temperatures as high as 1000 to 1600 deg C!
Mr. R.D.Pennathur
Mr. R.D.Pennathur
Welding Metallurgy
Distortion
Mr. R.D.Pennathur
Welding causes
Melting at the interface- fusion zone or weld zone (WZ)
Adjoining regions experience temperatures up to but not exceeding MP, called Heat Affected Zone (HAZ)
Mr. R.D.Pennathur
Dendrites in fusion zone
Is grain size change/orientation only the change? Does not ‘heat treatment’ take place? Yes!
Mr. R.D.Pennathur
Intended properties are predictable/ achievable
Mr. R.D.Pennathur
Time at temperature also varies
Heating and cooling rates vary widely due to the above
High level of unpredictability
Heat input
T0 is preheat temperature, ºC
H is Heat Input, kJ/mm
Mr. R.D.Pennathur
Heat Input During Welding
Is calculated from the Arc energy divided by the welding speed
Arc voltage X Welding current
----------------------------------------------- kJ / mm
For other welding process divide by following factors
SAW ( single wire ) - 0.8
Under such conditions, what is the response of base material?
Chemical composition
Mechanical properties
Prior microstructure
Hence, it is time to understand the principles underlying heat treatment!
Mr. R.D.Pennathur
geometric arrangements/ patterns that get repeated
in all three directions. These are called crystal structures
Mr. R.D.Pennathur
Single Crystal
Unit Cell
Mr. R.D.Pennathur
Crystal boundary or Grain boundary
In these regions there exists a film of metals, some three atoms thick, in which atoms do not conform to any pattern
This crystal boundary is of amorphous nature
Metallic bond acts within and across the crystal boundary and therefore not necessarily an area of weakness
Impurity atoms has got tendency to segregate at grain boundary or crystal boundary.
Depending on the nature of impurity atom they may strengthen or weaken the boundary
Mr. R.D.Pennathur
Defects in Metals - Dislocations
Any real crystal always has defects in its structure and deviates from perfect periodicity
These defects are called Lattice defects / Lattice imperfections / Dislocations
Metals and alloys get deformed when dislocations are forced to move by the application of force
Any solute atom, phase or inter-metallic that resists the flow of dislocations are the strengthening agents in any alloy system
Mr. R.D.Pennathur
Structural Changes
More than one structure within the solid state
It is pertinent to discuss only STEEL here since it is the most important industrial alloy extensively used for welding
Mr. R.D.Pennathur
Body centered cubic crystal (BCC)-Structure of iron at RT- α iron up to 910 deg C
Face centered cubic crystal (FCC)‏-
High temperature structure of iron-γ iron between 910 deg and 1400deg C
Reverts back to BCC (δ iron) between 1490 and 1530 deg C
Melts at 1530 deg C
Body centered tetragonal (BCT)
Structure of martensite - metastable
*
Steel is an alloy with principally carbon and other elements.
How do these elements participate in the crystal structure?
*
Principles of Heat Treatment
Two ways by which the atoms of the element can participate without destroying the arrangement of crystal.
Either they can substitute the iron atoms in their equilibrium sites or can settle in the “gaps” between the iron atoms.
Substitutional and interstitial solid solutions
*
What happens if it is limited?
What happens beyond its solid solubility?
Alloying elements beyond solubility limits are present as different phases/compounds/precipitates
The limit varies with temperature
Mr. R.D.Pennathur
Principles of Heat Treatment
What changes to these arrangements happen when we start heating the steel?
Are there any re-arrangements? What is the extent to which solid solution can take place? What happens beyond that?
A Phase diagram or Equilibrium diagram explains all these.
*
STEEL
The uniqueness of steel as the most widely used engineering material depends on its ability to get heat treated to different levels of strength and toughness!
Heat treatment itself is made possible because of two factors:
Different crystal structures in solid state
Solubility variation of these structures
Mr. R.D.Pennathur
Mr. R.D.Pennathur
Mr. R.D.Pennathur
Temperature below A3:
Pearlite transformed to Austenite, A3 temp is not exceeded, hence not all ferrite transforms to Austenite.  On cooling, only the transformed grains will be normalized. 
Mr. R.D.Pennathur
On cooling all grains will be normalized.
Temperature significantly exceeds A3 line permitting grains to grow. 
On cooling, ferrite will form at the grain boundaries, and a coarse pearlite will form inside the grains. 
A coarse grain structure is more readily hardened than a finer one, therefore if the cooling rate between 800°C to 500°C is rapid, a hard microstructure will be formed –(brittle fracture may occur in this region)
Mr. R.D.Pennathur
Microstructure & Hardness Of HAZ In Steel
Preheating helps reduce hardness of HAZ by extending time it spends between 800-500deg C
Mr. R.D.Pennathur
rate of cooling. But, do we get such condition
in heat treatment or welding? Far from that!!
Carbon < 0.80%
Carbon 0.80%
Carbon > 0.80%
Mr. R.D.Pennathur
Heat treatment enables different structures to be obtained from the same material
Figure 2 Microstructure of medium carbon steel
resulting from normalizing heat treatment, showing
ferrite and pearlite
steel resulting from water quenching
*
Heat Treatment
Formed on rapid cooling below Ms temperature
Tempered Martensite :
However has a good combination of strength and toughness and is a useful structure and is developed by re-heating martensite
Hardness depends on carbon content of steel
Carbon %
0.1
0.2
Bainite
Formed in alloyed steels when austenite is cooled rapidly passed the nose of the C-curve .
Extremely fine mixture of ferrite + carbide but not lamellar like pearlite
Formed between 500 – 220 C Upper Bainite or lower Bainite depending on temp.
Has higher hardness and toughness than pearlite
Mr. R.D.Pennathur
How is the HT Discussion Relevant for Welding?
Martensite is the hardest condition of any steel
Primarily used for high strength and wear resistance, but lacks toughness
Fabrication requires good formability- derived from ductility- hence lower carbon steels are used
Fabricated structures require not wear resistance but good toughness
Mr. R.D.Pennathur
Fine dispersion of alloy carbides results in strengthening by precipitation hardening
Small amounts of carbide forming elements eg. Nb, V, Ti etc added Total amount 0.20% max as such called Micro-alloyed steels
Controlled rolling at low finish roll temperatures results in very fine grain size ASTM 12 – 14. Also improves strength.
Range of medium and high tensile steel developed to give improved strength and toughness without impairing weldability. Covered by IS:8500 - 1991
Gives comparatively lower elongation but better toughness than low alloy HSLA steels
Properties : UTS 600 – 650 MPa
YS 400 – 500 MPa
Grade / Trade name
TISTEN 60
Introduction of welded structures implied
High heat input of the welding arc / heat source and influence of arc atmosphere
Solidification of the molten filler metal and fused portion of base metal into a separate weld zone
Parent metal on both sides of the weld affected by the weld thermal cycle – Heat affected zone ( HAZ )
Metallurgical effects on both reheating and cooling
Introduction of higher strength steels to reduce weight and cost of structure
Alloying elements added to develop strength
Lead to more complex metallurgical changes
Mr. R.D.Pennathur
Measured by Charpy test
Charpy values are specified for welds requiring good toughness – RT as well as at subzero temperatures
Mr. R.D.Pennathur
How is the HT Discussion Relevant?
Since low carbon steels have poor hardenability, martensite seldom forms during welding
But when hardenable steels have to be used, precautions have to be used during welding
Mr. R.D.Pennathur
Affects toughness, possibility of brittle failure
Susceptible to hydrogen cracking at vulnerable regions of HAZ in toe and under bead; if hardenable steel is to be welded precautions to be taken-
Preheat to slow down the cooling rate
Post heat to temper the martensite
Mr. R.D.Pennathur
Hydrogen Cracking
Mr. R.D.Pennathur
3 factors causing Hydrogen induced cold cracking
A brittle martensitic micro-structure produced by rapid cooling in HAZ area heated above A1 line
Presence of Hydrogen from the welding process
Presence of contractional and residual stresses
Mechanism
Hydrogen absorbed by the weld pool diffuses to the fusion zone and HAZ as the weld solidifies and cools
Forms pockets of molecular hydrogen which exerts additional stress on the susceptible microstructure
In combination with existing stresses causes cracking generally in HAZ but can also take place in multi-pass welds
Mr. R.D.Pennathur
Formation of hard microstructure
Cooling rate - Combined thickness of joint
- Heat input of process
- Degree of preheat if any and inter-pass temp
Chemical composition expressed in terms of carbon equivalent
C.E. is the measure of the susceptibility of the material to form a
hard microstructure ( martensite )
Thus Carbon Equivalent has become synonymous with Weldability of a steel
C.E. = %C + % Mn / 6 + % (Cr + Mo + V ) / 5 + % (NI + Cu) / 15
Mr. R.D.Pennathur
Weldability
Steels with Carbon Equivalent (C.E.) value less than 0.40% have good weldability
This means that no detrimental hard microstructures result in WZ and HAZ
Such steels do not require any pre or post heating
C.E.> 0.40% require either pre or post heating or both.
Mr. R.D.Pennathur
Why Preheating?
The cooling rate, particularly from 800 deg to 500 deg C (ΔT800-500), is important that decides the microstructure, from TTT diagrams
The nose of the TTT curve is shifted towards right for hardenable steels and hence harder microstructures tend to form
Mr. R.D.Pennathur
Post Heating
When martensite cannot be avoided, post heating is carried out
PWHT reduces the brittleness by tempering the martensite
Mr. R.D.Pennathur
Butt welds & corner welds of unequal thickness
Av of T1 over 75 mm + T2
Fillet welds – T1 + T2 + T3
Directly opposed simultaneous fillet welds – T1 + T2 + T3 / 2
Two rods - D1 + D2 / 2
Mr. R.D.Pennathur
Hydrogen Levels For Different Processes And Consumables
Scale A : Above 15 ml / 100 gm diffusible hydrogen content in weld – Rutile electrodes, LH electrodes which have been exposed to moisture
Scale B : 10 – 15 ml / 100 gm diffusible hydrogen content - LH electrodes redried at 250 C
Scale C : 5 – 10 ml / 100 gm diffusible hydrogen content – Gas Metal arc welding ( MIG ) process, LH electrodes redried at 350 C
Scale D : below 5 ml / 100 gm diffusible hydrogen content – Gas Tungsten Arc welding ( TIG ) process, LH electrodes re-dried at 450 C
Mr. R.D.Pennathur
Given a steel of known composition or C.E.
Upto what combined thickness can be welded with normal rutile electrodes, without danger of HAZ cracking
Upto what thickness can be welded using Low Hydrogen electrodes
Upto what thickness can be welded using Low Hydrogen electrodes properly re-dried as per manufacturers recommendations
Above what thickness pre-heat is required and degree of pre-heat.
Is it necessary to impose any restrictions on heat input by the welding process and parameters used
Mr. R.D.Pennathur
Combined Influence Of Base-metal Thickness And Carbon Content On Weldability
Greatest single thickness of carbon steel base metal
Highest carbon content of carbon steel base metal %
No Preheat & PWHT required
Only Preheat is required
Both Preheat & PWHT required
Weldability
Weldability is defined as the capacity of a metal to be welded under the fabrication conditions imposed, into a suitable designed structure, and to perform satisfactorily in the intended service
Weldability is the ease with which a metal can be welded to give the required service
Weldability is the number of problems you face to weld a material
Macrograph of a weld joint & HAZ
Mr. R.D.Pennathur
Mr. R.D.Pennathur
WELDABILITY PROBLEMS
-- micro-fissuring
- Liquation cracks
- In the HAZ
Reduced corrosion resistance
HAZ cracking
Delayed cracking
Hot cracking
Solidification cracking
Centerline cracking
Due to high S & P levels which produce low melting films at grain boundaries
Reduced by higher Mn content
Mr. R.D.Pennathur
Solidification Cracking
Steels having unfavourable Mn-S ratio are prone to such cracking.
Mr. R.D.Pennathur
Lamellar Tearing
Is generally associated with welding of fairly large highly restrained structures
Occurs predominantly in plate material
Due to presence of non – metallic inclusions
Difficult to detect by NDT techniques. Maybe assessed by STRA of tensile test in short transverse direction
Cracks can occur in parent plate / HAZ and generally run parallel to the plate surface
Mr. R.D.Pennathur
Lamellar tearing
Lamellar tearing near a C-Mn steel weld
Prevention: Use joint designs that minimise transverse constraint & butter with a softer layer
Mr. R.D.Pennathur
Chromium in solid solution gives corrosion resistance
If slow cooled from 950 to 400 deg C, Cr23 C6 precipitates and segregates to grain boundaries depleting the matrix of Cr, particularly close to GB- sensitization
Under corrosive environs, the GB gets attacked
Mr. R.D.Pennathur
Sensitization in SS Welding
Normally SS is quenched from high temperature to retain the Cr23C6 in solid solution- called ‘solution annealing’
The treatment is carried out at 1050 deg C in vacuum or hydrogen atmosphere
If SS is welded and allowed to slow cool, Cr23C6 precipitates
If assembly permits, it can be resolutionized
Mr. R.D.Pennathur
Boundaries show Cr23 C6 precipitation.600X
Intergranular corrosion of sensitized SS.500X
Corroded Heat Exchanger Tube
Sensitization in SS Welding
If it is not possible, use low carbon or extra low carbon SS grades, to prevent Cr23 C6 formation
Alternately use stabilized grades having Nb or Ti
These with better affinity for C forms the respective carbides which h precipitate within the grains and Cr is not affected
Mr. R.D.Pennathur
We are one of the renowned manufacturers of various grades of welding consumables which are second to none in terms of quality. We have the most modern manufacturing facility equipped with latest sophisticated machinery at Pondicherry, India
We are manufacturing a vast range of Shielded Metal Arc Welding Electrodes and Flux Cored Arc Welding Electrodes. We also supply tested GMAW and GTAW welding consumables for all the applications.
Our hardworking team would always be interested in any opportunity to cater your requirement of welding consumables.
Currently we are supplying massive quantity of welding consumables to many world class EPC companies in various range of welding consumables including Carbon steel, Low Temperature Carbon Steel, Stainless Steel, Low alloy steel etc.
If at all the need arises for a special consumable which is not in our arsenal, our R&D team is fully equipped in developing special electrodes to meet Service Requirements, Impact Test Requirements with strict chemistry controls and as welded hardness test criteria.
We built quality and consistency in our consumables. Our manufacturing facility always had the market pulse to meet its demands and fast track delivery requirements without compromising quality and consistency.