WELDING:
Welding is a fabrication process that joins materials, usually metals or thermoplastics, by melting the workpieces and adding a filler material to it. The workpieces and the filler material are melted to form a pool of molten material (the weld pool) that cools to become a strong joint. To weld metals, although heating is used but sometimes high pressure is also used to fuse the workpieces with filler material.
This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.
Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding may be performed in many different environments, including open air, under water and in outer space.
Welding is a potentially hazardous undertaking and precautions are required to avoid burns, electric shock, vision damage, inhalation of poisonous gases and fumes, and exposure to intense ultraviolet radiation.
Metallurgy of the Welding Process:
Most solids that are used engineering materials consist of crystalline solids in which the atoms or ions are arranged in a repetitive geometric pattern which is knows as a lattice structure. The only exception is materials that are made from glass which is a combination of a supercooled liquid and polymers which are aggregates of large organic molecules.
Crystalline solids cohesion is obtained by a metallic or chemical bond which is formed between the constituent atoms. Chemical bonds can be grouped into two types consisting of ionic and covalent. To form an ionic bond, either a valence or bonding electron separates from one atom and becomes attached to another atom to form oppositely charged ions. The bonding in the static position is when the ions occupy an equilibrium position where the resulting force between them are zero. When the ions are exerted in tension force, the inter-ionic spacing increases creating an electrostatic attractive force, while a repulsing force under compressive force between the atomic nuclei is dominant.
Covalent bonding is when the constituent atoms lose an electron(s) to form a cluster of ions, resulting in a electron cloud that is shared by the molecule as a whole. In both ionic and covalent boding the location of the ions and electrons are constrained relative to each other, thereby resulting in the bond being characteristically brittle.
Metallic bonding can be classified as a type of covalent bonding for which the constituent atoms of the same type and do not combine with one another to form a chemical bond. Atoms will lose an electron(s) forming an array of positive ions. These electrons are shared by the lattice which makes the electron cluster mobile, as the electrons are free to move as well as the ions. For this, it gives metals their relatively high thermal and electrical conductivity as well as being characteristically ductile.
Three of the most commonly used crystal lattice structures in metals are the body-centred cubic, face-centred cubic and close-packed hexagonal. Ferritic steel has a body-centred cubic structure and austenitic steel, non-ferrous metals like aluminium, copper and nickel have the face-centred cubic structure.
Ductility is an important factor in ensuring the integrity of structures by enabling them to sustain local stress concentrations without fracture. In addition, structures are required to be of an acceptable strength, which is related to a materials yield strength. In general, as the yield strength of a material increases, their is a corresponding reduction in fracture toughness.
Steel Weld Metallurgy
Welding is a fabrication process that joins materials, usually metals or thermoplastics, by melting the workpieces and adding a filler material to it. The workpieces and the filler material are melted to form a pool of molten material (the weld pool) that cools to become a strong joint. To weld metals, although heating is used but sometimes high pressure is also used to fuse the workpieces with filler material.
This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.
Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding may be performed in many different environments, including open air, under water and in outer space.
Welding is a potentially hazardous undertaking and precautions are required to avoid burns, electric shock, vision damage, inhalation of poisonous gases and fumes, and exposure to intense ultraviolet radiation.
Metallurgy of the Welding Process:
Most solids that are used engineering materials consist of crystalline solids in which the atoms or ions are arranged in a repetitive geometric pattern which is knows as a lattice structure. The only exception is materials that are made from glass which is a combination of a supercooled liquid and polymers which are aggregates of large organic molecules.
Crystalline solids cohesion is obtained by a metallic or chemical bond which is formed between the constituent atoms. Chemical bonds can be grouped into two types consisting of ionic and covalent. To form an ionic bond, either a valence or bonding electron separates from one atom and becomes attached to another atom to form oppositely charged ions. The bonding in the static position is when the ions occupy an equilibrium position where the resulting force between them are zero. When the ions are exerted in tension force, the inter-ionic spacing increases creating an electrostatic attractive force, while a repulsing force under compressive force between the atomic nuclei is dominant.
Covalent bonding is when the constituent atoms lose an electron(s) to form a cluster of ions, resulting in a electron cloud that is shared by the molecule as a whole. In both ionic and covalent boding the location of the ions and electrons are constrained relative to each other, thereby resulting in the bond being characteristically brittle.
Metallic bonding can be classified as a type of covalent bonding for which the constituent atoms of the same type and do not combine with one another to form a chemical bond. Atoms will lose an electron(s) forming an array of positive ions. These electrons are shared by the lattice which makes the electron cluster mobile, as the electrons are free to move as well as the ions. For this, it gives metals their relatively high thermal and electrical conductivity as well as being characteristically ductile.
Three of the most commonly used crystal lattice structures in metals are the body-centred cubic, face-centred cubic and close-packed hexagonal. Ferritic steel has a body-centred cubic structure and austenitic steel, non-ferrous metals like aluminium, copper and nickel have the face-centred cubic structure.
Ductility is an important factor in ensuring the integrity of structures by enabling them to sustain local stress concentrations without fracture. In addition, structures are required to be of an acceptable strength, which is related to a materials yield strength. In general, as the yield strength of a material increases, their is a corresponding reduction in fracture toughness.
Steel Weld Metallurgy
Carbon: Major element in steels, influences strength, toughness and ductility
Manganese: Secondary only to carbon for strength toughness and ductility, secondary de-oxidiser and also acts as a de-sulphuriser.
Silicon: Primary de-oxidizer
Molybdenum: Effects hardenability, and has high creep strength at high temperatures. Steels containing molybdenum are less susceptible to temper brittleness than other alloy steels.
Chromium: Widely used in stainless steels for corrosion resistance, increases hardness and strength but reduces ductility.
Nickel: Used in stainless steels, high resistance to corrosion from acids, increases strength and toughness
Classification of Steel
Steels are classified into groups as follows
- 1. Low Carbon Steel 0.01 – 0.3% Carbon
- 2. Medium Carbon Steel 0.3 – 0.6% Carbon
- 3. High Carbon Steel 0.6 – 1.4% Carbon
Plain carbon steels contain only iron & carbon as main alloying elements, traces of Mn, Si, Al, S & P may also be present.
ALLOY STEEL
Alloy steel is one that contains more than Iron & Carbon as main alloying elements
Alloy steels are divided into 2 groups
- Low Alloy Steels < 7% extra alloying elements
- High Alloy Steels > 7% extra alloying elements
Steel Weld Metallurgy
The grain structure of steel will influence its weldability, mechanical properties and in-service performance. The grain structure present in a material is influenced by:
- The type and number of elements present in the material
- The temperature reached during welding and or PWHT.
- The cooling rate after welding and or PWHT
Heat Affected Zone:
The parent material undergoes microstructure changes due to the influence of the welding process. This area, which lies between the fusion boundary and the unaffected parent material, is called the heat affected zone (HAZ). The extent of changes will be dependent upon the following..
- Material composition
- Cooling rate, fast cooling higher hardness
- Heat input, high heat inputs wider HAZ
- The HAZ can not be eliminated in a fusion weld
Heat Input Calculation:
Amps = 200 Volts = 32
Travel speed = 240 mm/min
Heat input = (Amps x volts)/(Travel speed mm/sec X 1000)
Heat input = (200 X 32 X 60)/(240 X 1000)
Heat input = 1.6 kJ/mm
Heat Input:
High heat input - slow cooling
- Low toughness
- Reduction in yield strength
Low heat input - fast cooling
- Increased hardness
- Hydrogen entrapment
- Lack of fusion
WELDABILITY
Weldability can be defined as the ability of a material to be welded by most of the common welding processes, and retain the properties for which it has been designed.
- A steel which can be welded without any real dangerous consequences is said to possess Good Weldability.
- A steel which can not be welded without any dangerous consequences occurring is said to possess Poor Weldability. Poor weldability normally generally results in the occurrence of some sort of cracking problem.
Weldability is a function of many inter-related
factors but these may be summarised as:
- Composition of parent material
- Joint design and size
- Process and technique
- Access
It is very difficult to asses weldability in absolute terms therefore it is normally assessed in relative terms.
There are many factors which affect weldabilty e.g. material type, welding parameters amps, volts travel speed, heat input.
Other factors affecting weldabilty are welding position and welding techniques.
Basically speaking weldabilty is the ease with which a material or materials can be welded to give an acceptable joint.
