WELDABILITY OF MATERIALS

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WELDABILITY OF MATERIALS

• K. A. Yushchenko, V. V. Derlomenko

E.O.Paton Electric Welding Institute Kyiv,
Ukraine
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• Introduction
• Analysis of modern views on weldability
• Weldability of different materials
• Scheme of changes in properties, degradation,
  energy criteria
• New definition of weldability
• Conclusions

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Analysis of weldability

• The term "welding" is gradually more and more
  replaced by the term "joining" of materials.
  Sometimes these two terms are used together. It
  is likely that these are not just changes in
  terminology. They are of a conceptual character,
  implying a more general understanding of the
  process of making permanent joints in any
  materials, where the "weld" can be formed by
  omitting the stage of melting of materials, and
  have no clearly defined sizes and attributes.
• No generally accepted definition of
  "weldability" or "joinability" of materials
  exists as yet

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What is weldability

• Different materials react to changes in the
  joining and adjacent zones in a different way,
  depending upon their thermal-physical properties,
  completeness of structural transformations
  induced by the thermal energy input and a number
  of other factors characteristic of given welding
  conditions. The entire set of the changes
  determines the ability of a material to form a
  permanent joint with a guaranteed level of
  performance. In the scientific literature, this
  ability is called "weldability"

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Analysis of weldability standards

• ISO 581-1980 "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"
• DIN 8528 "Weldability of parts of a metallic
  material takes place if joining of materials is
  achieved by using a certain welding method and by
  keeping to a certain technology. Herewith the
  welds should comply with the corresponding
  requirements for properties and influence on a
  structure a part of which they are".
• TWI, Great Britain "A perfect weldability is the
  ability of material to be welded by any method
  and using no special measures to produce a joint
  with properties allowing a complete utilisation
  of the material".
• Bratislava Institute of Welding, Slovakia
  "Weldability is the ability of a material that
  enables welded joints with required properties to
  be produced by welding under certain
  technological conditions".

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Analysis of weldability standards (continue)

• GOST 2601-84, USSR "Weldability is the property
  of metal or a combination of metals to form a
  welded joint by an established technology to meet
  requirements stipulated by design and service
  conditions of a product".
• DSTU 3761.1-98, Ukraine "Metallic material is
  considered to be weldable to an established
  extent by a given method and for a given purpose
  if a given welding procedure provides the
  integrity of metal that guarantees compliance
  with the requirements imposed on welded joints
  both in terms of their properties and in terms of
  their influence on a structure a part of which
  they are."
• AWS "The capacity of a material to be welded
  under fabrication conditions imposed into a
  specific, suitably designed structure and to
  perform satisfactory in the intended service

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Analysis of the "weldability" term

• In the absolute majority of cases "weldability"
  is evaluated qualitatively and subjectively, on
  the "yes" or "no" principle
• The term "weldability" of materials in five of
  the above six cases is treated as a philosophical
  concept, namely "the ability", "takes place",
  "considered to be susceptible to welding to an
  established extent" etc, thus implying a
  subjective evaluation method
• Almost all of the above definitions of
  "weldability" note that it is necessary to use "a
  corresponding technological process"
• Recommendations to allow for the influence of a
  technology on weldability are of a conditional,
  i.e. "philosophic", character

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Analysis of available approaches to evaluation of
"weldability" show that

• in the absolute majority of cases, "weldability"
  is evaluated qualitatively and subjectively, on
  the "yes" or "no" principle (i.e. the material is
  weldable or unweldable)
• the term "weldability" of materials is treated as
  a philosophical concept, namely "the ability",
  "takes place", "considered to be susceptible to
  welding to an established extent" etc, thus
  implying a subjective evaluation method
• it is stated in some cases that "weldability is
  the property (meaning the ability) of metal to
  form a welded joint". This definition indicates
  neither what this property is nor how it can be
  measured. Therefore, this approach is also a
  subjective evaluation

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Weldability or joinability

• What is "weldability? Is it a property of a
  material or some philosophic term used in many
  documents and defined as the ability to form a
  permanent joint?

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• Characterisation of "weldability"
  (joinability) should allow for the following
  factors
• comprehensive data on a material joined (chemical
  and structural composition, physical-mechanical
  properties)
• effect of the suggested technology on the
  material welded (joined) (structural and
  thermally stressed state, formation of defects,
  properties)
• effect of environment on the welded (joined)
  material during its operation as a product (time
  variations in structure and properties caused by
  the presence of a welded joint in the product)

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Principals of materials joining

• Physical
• Chemical

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Joining (welding) of materials in different of
the aggregate state
solid
liquid
vapour-gas
The fourth aggregate state of high-charged
particles (i.e. ions of the materials) can be
deposited.
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• With different technological processes the
  heat and force effects on the weld and HAZ
  influence structural, physical-chemical,
  mechanical and other functional properties of
  materials.

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Examples of degradation of properties
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Appearance of Kh23N18 steel test specimens with
solidification cracks in the weld metal (a) and
HAZ (b) (x5)
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Effect of arc heat input q/v on impact toughness
of Shnadt specimens, aR (notch radius 0.025 mm
(1), 0.5 mm (2) and 8 (3)), and Mesnager
specimens. an (4), made from 16 mm thick plates
of steel 45 with a notch in the heat-affected
zone
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Changes in properties - degradation

• There are many other examples, where in the
  process of a technological effect on a material a
  change in its properties, i.e. degradation, takes
  place in the joining zone. It should be
  highlighted that degradation in terms of
  weldability is a negative change in properties of
  a material joined, compared with its state before
  welding, caused by the effect of a technology
  and/or service conditions. It changes the
  estimate of weldability and makes the material
  transform from weldable (joinable) to hard to
  weld (hard to join), which is seen in the weld
  and HAZ metals because of

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Changes in properties degradation (continue)

• thermal and force effects on the weld and HAZ.
  Different technological processes have different
  effects on structural, physical-chemical,
  mechanical and other functional properties of
  materials. The thermal and force effects change
  also the value and character of occurrence and
  fixation of effective and residual stresses and
  strains in the material joined and in the joint
  as a whole
• the technological process and associated thermal
  and force effects on a material and its
  stress-strain state determine the level of
  degradation of the material. The level of
  degradation in different welding (joining) zones
  is evaluated by critical permissible requirements
  to the values of functional properties and
  quality, both in manufacture and operation of
  structures

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Scheme of changes in properties - degradation
Distance from centre of WM
Schematic of a possible level of degradation of
weld and HAZ metals 1 well-weldable 2
having limited weldability 3 unweldable
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• Degradation in terms of weldability is a
  negative change in properties of a material
  joined, compared with its state before welding,
  caused by the effect of a technology and/or
  service conditions

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How to calculate degradation

• Degradation of properties is a qualitative
  and quantitative criterion of weldability of
  materials.
• Technological process and an accompanying heat
  and force impact on a material, as well as a
  stress-strain state, determine the level of
  degradation of the material. The level of
  degradation of the material in different zones of
  a joint is evaluated relative to the initial
  material or critical requirements for permissible
  values of functional properties and quality both
  during the process of fabrication of a structure
  and during its operation.

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Energy criteria

• Any change of functional properties with
  respect to the initial equilibrium state of a
  system as a whole can be estimated on the basis
  of the energy criterion, i.e. specific energy
  necessary for such a changes will be possible.

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New definition of weldability of materials

• "Weldability (joinability) is a physical
  property of a material to form a permanent joint
  of the required quality and level of
  physical-mechanical and functional properties
  exhibited both during the process of its
  production and during operation of a product.
• "Weldability (joinability) is a time- dependent
  reaction of a material to the joining technology
  and operation conditions"

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• Generalised indicators of weldability can and
  should be established as a specification
  characteristics for a given material. In analogy
  with such properties as yield stress, tensile
  strength, etc., they should be used and allowed
  for both in design of new structural materials
  and in their operation.