S. K. Gupta, B.E., C.E.,
FIE., FIIW., MISNT.,
Generation and Computer based Storage of Welding Data on Materials
PARTS OF THE ARTICLE (Each part will be published in different WeldFab Tech Times Issues.)
Part – I : Deals with the Framework of Data Based Management system outlined very briefly.
Part – II : Deals with the Materials used in fabrication and related data.
Part – III : Provides details of the Data content to formation of WPS,PQR and WPQ.
Part – IV : Deals with the data on Welding Processes, Equipment and Consumables.
Part – V : Illustrates and explains how the Welding Management Software works .
Welding is used extensively as a major process in fabrication and manufacturing products ranging from Nano components to massive steel structures. Since the inception of shielded arc welding process in 1907, a proliferation of welding processes and related technologies have been developed now covering a wide range of materials, products and applications in practice.
At present, SMAW, GMAW, FCAW, GTAW, SAW, PAW, FSW, EBW , Resistance Welding etc are considered to be the basic processes, based on which specialized branch processes have been developed and effectively applied.
It is imperative to mention here that along with the process, development, manufacture and application of welding equipment and consumables have multiplied astronomically.
In order to keep up the Quality Standards of the Process and products a large number of Destructive, Nondestructive, Analytical and Statistical Process Control Techniques are in regular practice.
International Standards, Codes and Specifications in conjunction of different National Specifications have been formulated on every aspects of Process, Equipment, Consumables, Operators, Operation and Products.
All the above mentioned areas covering Welding have generated vast number of Information in the form of Data and also continuing to generate equal volume of it every period of time. It is not possible for any individual or even any manufacturing organization to store, access, retrieve and apply useful data to the best advantage. But, individually if we can identify our needs, store methodically and systematically welding data used in industries, updating these to the current standards and use Computer Software as the Welding Management System benefits will show all round Production – Operation Management especially where Welding is used as the main Manufacturing Process.
Welding Data Management – Part – II is the follow up of the Welding Management – Part – I. In this Part different types of materials in common use , their properties like structures, strengths, changes with temperature, density, thermal conductivity which are some of the essential data to be generated, stored and retrieved as and when required in welded fabrication work are detailed.
The essential and vital information are provided below in the form of Tables and Charts.
Metals can be classified into two groups as shown above :
- Ferrous Metals – The Metals which contain Iron —Iron, Steel, Cast Iron, Alloy Steels.
- Non–Ferrous Metals – The Metals which do not contain any Iron — Aluminum, Copper, Tin, Zinc.
Ferrous Metals can be classified into :
- Plain Carbon Steels – The steels containing only Iron and Carbon.
- Alloy Steels – The steels containing Alloying Elements like Chromium, Nickel, Titanium to enhance the properties of the metals.
Plain Carbon Steels can be classified into :
- Low Carbon Steels / Mild Steels – which contain Carbon upto 0.30 %.
- Medium Carbon Steels – which contain Carbon from 0.30 to 0.60 %.
- High Carbon Steels – which contain Carbon more than 0.60 %.
- The carbon is added to the Iron in varying amounts to harden or strengthen the steel. As carbon content increases the hardness and tensile strength increases and the ductility, plasticity, and malleability will decrease.
- The reason the carbon content varies is to produce a family of steels that exhibit the desired characteristics for a given application.
- The American Iron and Steel Institute (AISI) defines carbon steel as:
- “Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.”
- The term “carbon steel” may also be used in reference to steel which is not stainless steel; in this use carbon steel may include alloy steels.
- As the carbon content rises, steel has the ability to become harder and stronger through heat treating, but this also makes it less ductile. Regardless of the heat treatment, a higher carbon content reduces weldability. In carbon steels, the higher carbon content lowers the melting point.
- A very small interstitial atom that tends to fit into clusters of iron atoms. It strengthens steel and gives it the ability to harden by heat treatment. It also causes major problems for welding , particularly if it exceeds 0.25% as it creates a hard microstructure that is susceptible to hydrogen cracking. Carbon forms compounds with other elements called carbides. Iron Carbide, Chrome Carbide etc
Effect of alloying additions
- Alloying elements such as Ni, Cr, Mn, Si, Mo & V shift the nose of the C-C-T curve to the right. Exception Cobalt which shifts it to left
- This is because they slow down growth of pearlite. eg – 0.5% Mo slows growth rate X 100
- Martensite can thus be formed at much slower cooling rates
- In a Ni-Cr-Mo low alloy steel cooling rate of
- 8 deg C / sec – Full martensite
- 3 deg / sec – Bainite + martensite
- 02 deg / sec – Pearlite
Note : Alloy elements do not affect the hardness of the Martensite they only affect the ease with which Martensite forms.
High strength low alloy structural steels
- Carbon in same range as mild steels 0.15 – 0.25%
- Small amounts of alloying elements Mo, Cr, Cu, Ni etc added eg. weathering steels to IS: 11587
- Structure accicular ferrite and bainite or ferrite and tempered martensite
- Sronger and tougher than pearlitic steels with higher strength
- Hardenability is increased which affects weldability
YS 400-700 MPa
UTS 500-800 MPa
Micro – alloyed HSLA steels
- 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 comparitively lower elongation but better toughness than low alloy HSLA steels
- Properties : UTS 600 – 650 MPa
YS 400 – 500 MPa
Elongation 20 – 22 %
Effect of alloying elements
Sulphur can be added up to ~ 0.35 % by weight. It is used in free-cutting steels in combination with manganese additions, to give manganese sulphide inclusions which deform plastically during rolling and cutting; these elongated inclusions promote chip formation and breakup during subsequent machining. They also reduce cutting temperatures and tool wear.
Manganese is used to harden steels and increase its toughness and strength. High manganese content coupled with increased carbon content lowers the ductility and weldability. Consideration of preheat and or postheat techniques usually apply.
May be used in conjunction with other elements to aid in hardening and provide steel with good strength at elevated temperatures. Preheating may be required for welding and they are often heat treated after welding.
Nickel may be used to Increase toughness and impact strength and improve corrosion resistance. Good strength and ductility may be obtained even with lower carbon content. Depending on the amount added special procedures may be necessary when welding.
Chromium helps improve the hardenability of steels and improves wear resistance, heat resistance, and corrosion resistance. Depending on the amount added special procedures may be necessary when welding. Chromium and Chromium Nickel are used in the production of Stainless Steel.
What are Stainless Steels
- Steels containing 11 – 30% Chromium
- The chromium oxide forms a passive layer on the surface which is adherent and regenerative.
- This prevents corrosive attack and gives the steel its ‘stainless’ property.
- Minimum 11% Cr needed to protect against atmospheric corrosion.
- Elements like Ni, Mo, Cu, Nb, Ti etc added to improve mechanical properties and corrosion resistance
- Do not resist corrosion in strongly reducing media.
Types of stainless steels
- (Austenite +Ferrite)
Martensitic stainless steels
- Cr 12 –18%
- C 0.1 – 1.2%
- AISI 410, 420, 440 grades
- Martensitic structure – higher carbon grades used in tempered condition.
Used for cutlery, surgical instruments, steam, gas & hydel turbine blades, ball bearings and races.
Ferritic stainless steels
- Cr 11 – 30%
- C 0.02 – 0.2%
- AISI 405, 430, 446 grades
- Ferritic structure -higher ductility and resistance to SCC & pitting corrosion.
- Used as thin sheet for decorative applications, oxidising corrosive media, heat resisting applications.
Austenitic stainless steels
- Cr 16 – 26%
- Ni 6 – 26%
- AISI 304, 310, 316, 321 & 347 grades
- Austenitic structure gives good weldability with excellent ductility and toughness down to cryogenic temperatures.
- Nickel improves general corrosion resistance
- Widely used for chemical, petrochemical plant, food processing and dairy equipment. Also used for cryogenic plant.
Physical properties of austenitic stainless steels
- Has high coefficient of linear expansion, twice that of carbon steels
- Has poor thermal conductivity, half that of carbon steels
- Results in much higher distortion after welding
- Steps to prevent distortion
– closer tacking
– greater use of jigs and fixtures
– use of balanced and skip welding techniques
Limitations of Standard Stainless Steels
- Low proof stress – 35% of UTS
- Sensitive to stress corrosion in acidic Cl or F ion media above 70 C and also hot caustic
- Sensitive to pitting corrosion in more aggressive acid chloride media Preferential attack on the ferrite phase in weak reducing media ( urea carbamate )
- Inadequate corrosion resistance in stronger reducing media such as hot phosphoric acid or sulphuric acid.
Steel makers have developed new grades of stainless steel to overcome these limitations and meet the requirements of higher operating pressures / temperatures or liquor concentrations demanded by modern chemical and petro-chemical plant.
Duplex stainless steels
- Half the nickel content of austenitic steels
- Cr 18 – 28% Ni 4.5 – 9.0 %
- 50% austenite + 50% ferrite structure
- Almost twice the strength of austenitic steels
- Excellent pitting + SCC resistance
- Used for plant and piping in oil and gas production, corrosive applications to resist chloride ion media. Higher strength structurals.
About Duplex Stainless Steels
- They are extremely corrosion resistant having high resistance to intergranular corrosion. Even in chloride and sulphide environments, they exhibit very high resistance to stress corrosion cracking.
- The super duplex grades are even more resistant to corrosion
- High chromium content gives protection against corrosion, but causes embrittlement at temperatures over about 300°C.
- At low temperatures they have better ductility than ferritic and martensitic grades. Duplex grades can readily be used down to at least -50°C.
- They cannot be hardened by heat treatment. Can however be work hardened.
- Solution treatment or annealing can be done by rapid cooling after heating to around 1100°C.
All composite and fabricated structures work under different environmental and loading conditions such as :
- Static or Dynamic loading
- Concentrated or Distributed loading
- Tension, Compression or Torsional loading
- Combination of above loadings
- At normal temperature
- At sub-zero temperature
- At elevated temperature
- Mechanical Properties of Materials
- ultimate tensile strength
- yield strength
- toughness (Kjc, CVN)
- stress rupture (most lacking in HAZ data)
- Physical Properties of Material –
- wear and abrasion characteristics
- Corrosion rates – Electrode potential
The driving force for the flow of current and thus the corrosion is the difference in electrode potential. The electrode potential of a metal is an indication of the tendency of the metal to dissolve and corrode in a certain electrolyte
The electrode potentials of different metals can be specified in relation to one another in galvanic series for different electrolytes.
Considering the steel-copper example, it will be noted from the Galvanic Table that the copper has a higher potential (is nobler) than plain carbon steel. The steel will be the anode and corrode, whereas the copper will be the cathode and not corrode.
- Additional metallurgical characteristics
- prior history, including thermal treatment or deformation
- Chemical properties
Fracture is classified based on several characteristic features:
|Characteristic||Terms used||Terms used|
|Strain to fracture||Ductile||Brittle|
|Appearance||Fibrous and gray||Granular and bright|
|Crack propagation||Along grain boundaries||Through grains|
FIG. – II. FATIGUE COEFFICIENTS STRESS RANGE
Strength and stiffness.
The words ‘stiffness’ and ‘strength’ both imply a sense of resistance and are both determined by geometry and material properties. However, a strong object may not necessarily be stiff, and vice versa!
Strength ≠ Stiffness
- Strength [N/m2] is the maximum stress that the material can resist before deformation or fracture.
- Stiffness [N/m] is the rigidity or resistance to bending.
ALUMINIUM ALLOYS, PRPOPERTIES AND USES
Aluminium is not just one material, but it gives rise to a family of different groups of alloys whose mechanical properties widely vary from one group to another and also within each group itself. From the point of view of the technological use, the aluminium alloys can be grouped into eight series, according to the American Association classification, the first of the four digits characterizing the main alloying element and the other three the secondary ones.
TABLE – I ALUMINIUM ALLOYS – PROPERTIES & USES
|1XXX||Al||Its elastic limit is very low (f0.2@30 Nmm-2), but its ductility is excellent, being the ultimate elongation et@30 to 40 percent. If the material is cold-worked, the strength can increase up to f0.2@100 Nmm-2, whereas the ductility is drastically reduced (et@3 to 4 percent).||Not used much in Industries
|2XXX||Cu||. When submitted to heat-treatment, elastic limit f0.2 can increase up to 300 Nmm-2, with a sufficient ductility, being et@10 percent. the corrosion resistance of these alloys is not very high and poor weldability||Basically, they are used in aeronautical industry with riveted connections.
|3XXX||Man||These alloys cannot be heat-treated and they have a slightly higher strength than pure aluminium by keeping a very high ductility, which allows very hard cold-forming processes for increasing strength. They are corrosion resistant.||Specific applications are panels and trapezoidal sheeting for roofing systems.|
|4XXX||Si||The properties of these alloys are similar to those of the 3000 series||Not often used, except for welding wires.|
|5XXX||Mag||Even though these alloys cannot be heat-treated, their mechanical properties could be higher than those corresponding to the 1000, 3000 e 4000 series. The strength can be increased when they are cold worked, being the elastic limit f0.2 up to 200 Nmm-2 and the ductility still quite high (et up to 10 percent). The corrosion resistance is also high, especially in marine environment, when the amount of Mg is less than 6 percent. These alloys are often used in welded structures, since their strength is not drastically reduced in the heat-affected zone.||Used especially in marine environment, when the amount of Mg is less than 6 percent. These alloys are often used in welded structures, since their strength is not drastically reduced in the heat-affected zone.|
|6XXX||Mag-Si||By means heat-treatment the strength of these alloys is increased up to f0.2 @ 250 Nmm-2, with a quite good ductility, being et up to 12 percent. These alloys are corrosion resistant.||These are particularly suitable for extrusion, but also rolled sections as well as tubes can be produced. These alloys are used either in welded structures and in bolted or riveted connections|
|7XXX||Zinc||– AlZnMg alloys reach a remarkable strength, being the elastic limit f0.2 greater than 250 Nmm-2, with a quite good ductility (et @ 10 percent). They are also corrosion resistant.
– AlZnMgCu alloys are the highest resistance alloys after heat-treatment, reaching f0.2 @ 500 Nmm-2; conversely, they have low weldability and are not corrosion resistant,
|These alloys are generally used in structural applications, because they are particularly suitable in welded structures owing to their self-tempering behaviour, which allows to recover the initial strength in the heat-affected zone.
|This series is preferably used as material for packaging but, due to its advantages in fabrication, it finds more and more application in building industry for facades.|
Copper and copper-base alloys have specific properties which make them widely used. Their high electrical conductivity makes them widely used in the electrical industries and corrosion resistance of certain alloys makes them very useful in the process industries. Copper alloys are also widely used for friction or bearing applications.
Copper shares some of the characteristics of aluminum. Attention should be given to its properties that make the welding of copper and copper alloys different from the welding of carbon steels.
Copper alloys possess properties that require special attention when welding. These are:
- High thermal conductivity.
- High thermal expansion coefficient.
- Relatively low melting point.
- It is hot short, i.e., brittle at elevated temperatures.
- The molten metal is very fluid.
- It has high electrical conductivity.
- It owes much of its strength to cold working.
Magnesium – Base Alloys
Magnesium is the lightest structural metal. It is approximately two-thirds as heavy as aluminum and one-fourth as heavy as steel. Magnesium alloys containing small amounts of aluminum, manganese, zinc, zirconium, etc., have strengths equaling that of mild steels. They can be rolled into plate, shapes, and strip.
Magnesium can be cast, forged, fabricated, and machined. As a structural metal it is used in aircraft. It is used by the materials-moving industry for parts of machinery and for hand-power tools due to its strength to weight ratio.
Magnesium can be welded by many of the arc and resistance welding processes, as well as by the oxy-fuel gas welding process, and it can be brazed. Magnesium possesses properties that make welding it different than the welding of steels. Many of these are the same as for aluminum. These are:
- Magnesium oxide surface coating
- High thermal conductivity
- Relatively high thermal expansion coefficient
- Relatively low melting temperature
- The absence of color change as temperature approaches the melting point.
Nickel – Base Alloys
Nickel and the high-nickel alloys are commonly used when corrosion resistance is required. They are used in the chemical industry and the food industry. Nickel and nickel alloys are also widely used as filler metals for joining dissimilar materials and cast iron.
When welding, the nickel alloys can be treated much in the same manner as austenitic stainless steels with a few exceptions. These exceptions are:
- The nickel alloys will acquire a surface oxide coating which melts at a temperature approximately 538oC above the meting point of the base metal.
- The nickel alloys are susceptible to embrittlement at welding temperatures by lead, sulphur, phosphorus, and some low temperature metals and alloys.
- Weld penetration is less than expected with other metals.
FIG. – III STRENGTH – TOUGHNESS DIAGRAMS
TABLE – II CORROSION OF METALS
|Stainless steel (AISI 304), passive state||+0.09|
|Stainless steel (AISI 304), active state||-0.29|
TABLE. – III. PHYSICAL PROPERTIES OF ALUMINIUM
|Melting Point||655 Degree centigrade|
|Density||2.70 gm/ cm3|
|Thermal Expansion||23.5 x 10^-6/K|
|Modulus of Elasticity||69.5 GPa|
|Thermal Conductivity||201 W/m.K|
|Electrical Resistivity||0.033 x 10^-6 Ohm.m|
TABLE – IV. STEELS – COMPOSITION & STRENGTHS
|HY – 80||0.18||0.40||0.38||Ni-3.5Cr-1.8. Cu .25 .Mo-0.6||710||890|
|HSLA-80||0.06||0.70||0.40||Ni-3.5.Cr 0.90 Cu-1.30||710||800|
TABLE – V .IS : 2062 – Specification of Steel for General Structural Purposes
|IS : 2062 – Specification of Steel for General Structural Purposes|
|Grade||C% Max.||Mn% Max.||S% Max.||P% Max.||Si% Max.||C.E.% Max.|
|Grade||UTS(MPa) Min.||Y.S.(MPa) Min.||El.% Min. 5.65 Sqrt(So)||Bend
|B||410||250||240||230||23||2T & 3T *|
TABLE – VI COMPOSITION PROPERTIES OF MATERIAL : IRS : M44
|MANGANESE||0.8 – 1.5|
|CHROMIUM||10.8 – 12.5|
|U.T.S. : 50 KG / mm||ELONGATION : 25 %
TABLE – VII Properties of typical Micro-alloyed steels
|Trade name||% C||% Mn||% Si||% MA||YS MPa||UTS MPa|
|ASTM A633 Gr C||0.20||1.50||0.50||0.05 Nb||350 min||600 min|
|SAILMA 410||0.25||1.50||0.50||Nb+V+Ti =0.20||410 min||540 – 660|
|SAILMA 450||0.25||1.50||0.50||Nb+V+Ti =0.20||450 min||570 – 720|
|SAILMA 450HI||0.20||1.50||0.50||Nb+V+Ti =0.20||450 min||570 – 720 CVN =19.6J|
|TISTEN 60||0.20||1.80||0.50||0.20||440 min||590 min|
TABLE VIII. Composition Of Two Typical Duplex Steels
|Elements (%)||2205||UR52N + (Super Duplex)|
|C||0.03 Max||0.03 max|
|Cu||0.5 – 3.0|