Understanding of welding technology is critical for all workers in the welding industry. In a preview of its wielding course, the American Society of Mechanical Engineers posited that the process of welding affects welded materials and structures. Therefore, understanding of the electric circuits used to generate welding arcs, properties of materials, and metallurgical and dimensional aspects of welding processes is imperative. This essay examines some of the techniques used to strengthen metal, forms of heat treatments used by welders, and the properties that make steel a versatile material.
Techniques Used to Strengthen Metals
Metals have the capacity to deform plastically is dependent on the ability of its dislocations to move. How easily the metal plastically deforms determines its hardness and strength. The mechanical strength can be improved by reducing the dislocation movement. Metal with easy dislocation movement tends to be soft and easy to deform. There are four main mechanisms through which metals are strengthened:
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1. Grain Size Reduction
Slips in metals are prevented by grain boundaries that serve as barriers. The strength of these barriers increases with misorientation. More barriers to slip can be introduced through smaller grain size. The relationship between grain size and barriers to slip in metals is defined by the Hall Petch Relation equation that indicates the presence of an inverse square root between yield strength and grain size.
2. Solid Solution Alloying
Solid solution alloying can be used to introduce impurities on the metal. Impurity atoms have the potential to distort the lattice generating stress in the process. The resultant stress can yield a barrier to dislocation. This technique of metal strengthening can use small substitutional impurity or large substitutional impurity to generate a local shear at specific points that opposes dislocation in a particular direction.
3. Strain Hardening – Cold Working
The technique is used to deform metals at room temperature. The cross-section area of metal can be changed using common forming techniques such as forging, rolling, drawing, and extrusion. Cold Work operates on the principle of dislocations entangling in the process. As a result motion of the dislocations is impaired making the metal stronger. Cold work also increases the dislocation density, increasing the yield strength of the material, making the metal stronger. The technique also increases the ductility and tensile strength of the metal. For instance, rolling changes metal grains from isotropic to anisotropic by changing orientation from spherical, equiaxed and random to directional.
4. Annealing
Annealing is a technique that involves the use of heat. Metal samples are heated to above the recrystallization temperature and then cooled down. The objective is to improve the properties of cold work such as ductility and hardness. Annealing involves three important steps: recovery, recrystallization, and grain growth. Recovery releases some of the energy stored in the internal strain through dislocation motion leading to a reduction in the number of dislocations. Recrystallization helps in the formation of a new set of strain-free and uniaxial grains with low dislocation densities. Grain growth at high temperatures causes reduction in total boundary area and total energy-yielding a stronger metal.
Heat Treatments Used in Welding
Heat treatments in welding are necessary to optimize the mechanical properties of the metal. However, the techniques can result to quality problems due to challenges in the regulation of temperatures and timing during the process. The common techniques used include:
Annealing
The method involves heating of the alloy to a specific temperature and then cooling to form a redefined microstructure. Processes such as normalizing can be used to achieve uniformity in grain size throughout the metal; stress relieving, which removes the stresses in the internal structure of the metal.
Aging
The technique is used for precipitation hardening metals. When such metals are quenched, it allows elements remain in the solution resulting to an overall soft metal. Aging the metals in solution involves diffusion of alloying elements through the microstructure forming intermetallic particles that nucleate and fall out of the solution acting as a reinforcing phase.
Quenching
The technique is used to cool the metal rapidly often with the objective of producing martensite transformation. The metal must be heated above its upper critical temperature and then cooled for the method to work.
Tempering
Some untampered metals such steel are very hard but also be very brittle to be useful in many applications. Tempering eliminates the problem and involves heating the alloy below the lower critical temperature to give it some toughness. Tempering colors that result from the process can be used to judge the strength of the metal with hard metals tempered in light to dark straw and springs to blue.
Selective Heat Treatment
The technique is crucial in altering properties of only a select portion of the metal. Sections of a metal can be cooled at different rates, quickly heat and quenched, or by tempering. Selective heat treatment techniques commonly used are differential hardening, flame hardening, and induction hardening.
Why Steel is a Versatile Material
It is important to understand that there is no one material called steel. Steel is a name for a large family of iron alloys containing carbon and other different elements. Steel versatility stems from the ability of its internal structure and composition to be adjusted to come up with desired properties. For this reason, steel can be used to produce beams for bridges, columns, and skyscraper, paper clips, or even razor blades. The answer about the versatility of steel depends on the area of application, but generally, the properties that give it that characteristic include:
High tensile and high specific strength
High toughness
High ductility
Affordability and availability in mass
Ability of its strength to be varied
