- Inherent discontinuities: They relate to the solidification of the molten metal. Primary: They relate to the original condition and solidification of the metal or ingot. Secondary: It is related to the emptying and solidification of the article. Process discontinuities: They are related to various manufacturing processes such as: machining, forging, extruding, rolling, welding, heat treatments and coatings. Service discontinuities: Are those related to various service conditions, such as: Corrosion, fatigue and erosion stresses. Superficial discontinuities: They are seen with the naked eye, no matter their depth. Subsurface discontinuities: They are inside the material and do not reach the surface.
TYPES OF DISCONTINUITIES
POROSITY
They are gases trapped round or similar to it, they can appear on the surface or inside the metal
METAL INCLUSIONS
They are irregularly shaped and consist of impurities accidentally included in the molten metal.
CAVITY
They have an elongated shape, their interior is filled with air or some other gas. It is caused by shrinkage when the metal solidifies. It can extend deep into the center of the ingot.
LAMINATION
They are non-metallic inclusions, pores or cavities flattened during the rolling or sandwiching process in the steel. They are oriented in the direction of rolling.
CRACKS
They appear as tears in the material, they are caused by changes in the sections of the castings
WELDING
CRACKS
- BY CRATER
They are produced at the edge of the weld crater due to an inappropriate termination of the weld bead, it can also be caused by excessive shrinkage stresses due to the lack of stress relieved heat treatments, or to excess moisture in the electrode.
- BY EFFORT
They can be caused by a lack of stress relief heat treatments and can occur transverse to the weld bead or along its length.
POROSITIES
Caused by trapped gases (humidity), they are circular in shape, they can be caused by having the electrode high with respect to the weld bead puddle
SLAG INCLUSIONS
It may be due to slag trapping during the process due to the procedure used, or, in multiple strands, not having previously cleaned the previous strand of slag.
LACK OF PENETRATION
Due to poor welding technique, lack of current or poor penetration of the weld joint.
LACK OF FUSION
Due to a flaw in the welding technique, the electrode “points” more to one side than the other, or the electrode is not symmetrical with respect to its coating, causing distortion of the arc.
SOCAVATION
It occurs on the surface of the welded material and it is because due to the failure of the welder, excess current part of the base material is eaten.
REASONS WHY NON-DESTRUCTIVE TESTS ARE USED
- To save money by rejecting defective material during receipt inspection, before accepting and paying for the service To detect discontinuities that occur during the manufacturing process, before spending time and money continuing the process To improve manufacturing techniques by inspecting parts before and after the process Give safety to workers by periodically inspecting equipment and facilities to detect defects before they can cause failures Give reliability to the product To confirm the integrity of the parts during stoppages for preventive maintenance
SELECTION OF THE EDN METHOD
For the evaluation of a specific discontinuity, it must be taken into account that the NDT methods complement each other.
The selection of one method over another is based on variables such as:
- Type and origin of the discontinuity. Material manufacturing process. Accessibility of the article. € Desired acceptance level. Equipment available.
The limitations of the applicable END will vary with the applicable standard, the material and the service to which they will be used.
The first interchangeable parts manufacturing tests took place at the same time in Europe and the United States. These experiments were based on the use of cataloging gauges, with which parts could be classified into practically identical dimensions. The first true mass production system was created by the American inventor Eli Whitney, who in 1798 was awarded a government contract to produce 10,000 muskets made from interchangeable parts.
During the 19th century, a relatively high degree of precision was achieved in lathes, profiling machines, planing machines, polishing machines, saws, milling cutters, drilling machines, and drilling machines. The use of these machines spread to all industrialized countries. Larger and more precise machine tools appeared at the dawn of the 20th century. From 1920 these machines became specialized and between 1930 and 1950 more powerful and rigid machines were developed that took advantage of the new cutting materials developed at that time. These specialized machines made it possible to manufacture standardized products at a low cost, using unskilled labor. However, they lacked flexibility and could not be used for various products or for variations in manufacturing standards.To solve this problem, engineers have dedicated themselves over the past decades to designing highly versatile and precise machine tools, controlled by computers or computers, that allow products with complex shapes to be manufactured cheaply. These new machines are applied today in all fields.
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