Weldability
Encyclopedia
The weldability, also known as joinability, of a material refers to its ability to be welded
. Many metal
s and thermoplastic
s can be welded, but some are easier to weld than others. A material's weldability is used to determine the welding process and to compare the final weld quality to other materials.
Weldability is often hard to define quantitatively, so most standards define it qualitatively. For instance the International Organization for Standardization
(ISO) defines weldability in ISO standard 581-1980 as: "Metallic material is considered to be susceptible to welding to an established extent with given processes and for given purposes when welding provides metal integrity by a corresponding technological process for welded parts to meet technical requirements as to their own qualities as well as to their influence on a structure they form." Other welding organizations define it similarly.
there are three major failure mode
s by which weldability can be measured: hydrogen-induced cold cracking, lamellar tearing, and spot-weld peeling. The most prominent of these is hydrogen induced cold cracking.
, is inversely proportional to the hardenability
of the steel, which measures the ease of forming martensite
during heat treatment. The hardenability of steel depends on its chemical composition, with greater quantities of carbon and other alloy
ing elements resulting in a higher hardenability and thus a lower weldability. In order to be able to judge alloys made up of many distinct materials, a measure known as the equivalent carbon content
is used to compare the relative weldabilities of different alloys by comparing their properties to a plain carbon steel. The effect on weldability of elements like chromium
and vanadium
, while not as great as carbon
, is more significant than that of copper
and nickel
, for example. As the equivalent carbon content rises, the weldability of the alloy decreases.
High-strength low-alloy steels (HSLA) were developed especially for welding applications during the 1970s, and these generally easy to weld materials have good strength, making them ideal for many welding applications.
Stainless steel
s, because of their high chromium content, tend to behave differently with respect to weldability than other steels. Austenitic grades of stainless steels tend to be the most weldable, but they are especially susceptible to distortion due to their high coefficient of thermal expansion. Some alloys of this type are prone to cracking and reduced corrosion resistance as well. Hot cracking is possible if the amount of ferrite
in the weld is not controlled—to alleviate the problem, an electrode is used that deposits a weld metal containing a small amount of ferrite. Other types of stainless steels, such as ferritic and martensitic stainless steels, are not as easily welded, and must often be preheated and welded with special electrodes.
HSLA steel can be an issue. The equivalent carbon content
can be used as a parameter to evaluate the propensity for failure.
alloys varies significantly, depending on the chemical composition of the alloy used. Aluminium alloys are susceptible to hot cracking, and to combat the problem, welders increase the welding speed to lower the heat input. Preheating reduces the temperature gradient across the weld zone and thus helps reduce hot cracking, but it can reduce the mechanical properties of the base material and should not be used when the base material is restrained. The design of the joint can be changed as well, and a more compatible filler alloy can be selected to decrease the likelihood of hot cracking. Aluminium alloys should also be cleaned prior to welding, with the goal of removing all oxide
s, oil
s, and loose particles from the surface to be welded. This is especially important because of an aluminium weld's susceptibility to porosity due to hydrogen and dross
due to oxygen.
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
. Many metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s and thermoplastic
Thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...
s can be welded, but some are easier to weld than others. A material's weldability is used to determine the welding process and to compare the final weld quality to other materials.
Weldability is often hard to define quantitatively, so most standards define it qualitatively. For instance the International Organization for Standardization
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
(ISO) defines weldability in ISO standard 581-1980 as: "Metallic material is considered to be susceptible to welding to an established extent with given processes and for given purposes when welding provides metal integrity by a corresponding technological process for welded parts to meet technical requirements as to their own qualities as well as to their influence on a structure they form." Other welding organizations define it similarly.
Steels
For steelSteel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
there are three major failure mode
Failure mode
Failure causes are defects in design, process, quality, or part application, which are the underlying cause of a failure or which initiate a process which leads to failure. Where failure depends on the user of the product or process, then human error must be considered.-Component failure:A part...
s by which weldability can be measured: hydrogen-induced cold cracking, lamellar tearing, and spot-weld peeling. The most prominent of these is hydrogen induced cold cracking.
Hydrogen-induced cold cracking
The weldability of steel, with regard to hydrogen-induced cold crackingFracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
, is inversely proportional to the hardenability
Hardenability
The hardenability of a metal alloy is its capability to be hardened by heat treatment. It should not be confused with hardness, which is a measure of a sample's resistance to indentation or scratching. It is an important property for welding, since it is inversely proportional to weldability,...
of the steel, which measures the ease of forming martensite
Martensite
Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...
during heat treatment. The hardenability of steel depends on its chemical composition, with greater quantities of carbon and other alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
ing elements resulting in a higher hardenability and thus a lower weldability. In order to be able to judge alloys made up of many distinct materials, a measure known as the equivalent carbon content
Equivalent carbon content
The equivalent carbon content concept is used on ferrous materials, typically steel and cast iron, to determine various properties of the alloy when more than just carbon is used as an alloyant, which is typical...
is used to compare the relative weldabilities of different alloys by comparing their properties to a plain carbon steel. The effect on weldability of elements like chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
and vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
, while not as great as carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, is more significant than that of copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, for example. As the equivalent carbon content rises, the weldability of the alloy decreases.
High-strength low-alloy steels (HSLA) were developed especially for welding applications during the 1970s, and these generally easy to weld materials have good strength, making them ideal for many welding applications.
Stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
s, because of their high chromium content, tend to behave differently with respect to weldability than other steels. Austenitic grades of stainless steels tend to be the most weldable, but they are especially susceptible to distortion due to their high coefficient of thermal expansion. Some alloys of this type are prone to cracking and reduced corrosion resistance as well. Hot cracking is possible if the amount of ferrite
Ferrite (iron)
Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure. It is the component which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material...
in the weld is not controlled—to alleviate the problem, an electrode is used that deposits a weld metal containing a small amount of ferrite. Other types of stainless steels, such as ferritic and martensitic stainless steels, are not as easily welded, and must often be preheated and welded with special electrodes.
Lamellar tearing
Lamellar tearing is a type of failure mode that only occurs in rolled steel products that has been virtually eliminated with cleaner steels.Spot-weld peeling
The excessive hardenability that can occur when spot weldingSpot welding
Spot welding is a process in which contacting metal surfaces are joined by the heat obtained from resistance to electric current flow. Work-pieces are held together under pressure exerted by electrodes. Typically the sheets are in the thickness range...
HSLA steel can be an issue. The equivalent carbon content
Equivalent carbon content
The equivalent carbon content concept is used on ferrous materials, typically steel and cast iron, to determine various properties of the alloy when more than just carbon is used as an alloyant, which is typical...
can be used as a parameter to evaluate the propensity for failure.
Aluminium
The weldability of aluminiumAluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
alloys varies significantly, depending on the chemical composition of the alloy used. Aluminium alloys are susceptible to hot cracking, and to combat the problem, welders increase the welding speed to lower the heat input. Preheating reduces the temperature gradient across the weld zone and thus helps reduce hot cracking, but it can reduce the mechanical properties of the base material and should not be used when the base material is restrained. The design of the joint can be changed as well, and a more compatible filler alloy can be selected to decrease the likelihood of hot cracking. Aluminium alloys should also be cleaned prior to welding, with the goal of removing all oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s, oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....
s, and loose particles from the surface to be welded. This is especially important because of an aluminium weld's susceptibility to porosity due to hydrogen and dross
Dross
Dross is a mass of solid impurities floating on a molten metal. It appears usually on the melting of low-melting-point metals or alloys such as tin, lead, zinc or aluminium, or by oxidation of the metal. It can also consist of impurities such as paint leftovers...
due to oxygen.
Process factors
While weldability can be generally defined for various materials, some welding processes work better for a given material then others. Even within a certain process the quality of the weld may vary greatly depending on parameters, such as the electrode material, shielding gases, welding speed, and cooling rate.| Material | Arc welding | Oxy-acetylene welding | Electron beam welding | Resistance welding | Brazing | Soldering | Adhesive bonding |
|---|---|---|---|---|---|---|---|
| Cast iron | C | R | N | S | D | N | C |
| Carbon steel and low-alloy steel | R | R | C | R | R | D | C |
| Stainless steel | R | C | C | R | R | C | C |
| Aluminum and magnesium | C | C | C | C | C | S | R |
| Copper and copper alloys | C | C | C | C | R | R | C |
| Nickel and nickel alloys | R | C | C | R | R | C | C |
| Titanium | C | N | C | C | D | S | C |
| Lead and zinc | C | C | N | D | N | R | R |
| Thermoplastic† | N | N | N | N | N | N | C |
| Thermosets | N | N | N | N | N | N | C |
| Elastomers | N | N | N | N | N | N | R |
| Ceramics | N | S | C | N | N | N | R |
| Dissimilar metals | D | D | C | D | D/C | R | R |
| †Heated tool = R; Hot gas = R; Induction = C Key: C = Commonly performed; R = Recommended; D = Difficult; S = Seldom; N = Not used |
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