Within any domestic or civil system it is possible to find pipes that have orthogonal axes to each other or with an angle that does not make them parallel such as, for example, at 45 °. For the realization of the connection, then, some fundamental components are chosen that allow the junction: the elbows. In particular, the
austenitic stainless steel elbows (in 304 / 304L and 316 / 316L) have a series of mechanical and resistance characteristics:
- Mechanical resistance: allows to avoid breakages in operation;
- Good ductility: useful in the component forming process;
- Resistance in temperature: allows to face severe working conditions that could lead to the premature failure of the structures;
- Corrosion resistance: a central aspect for devices that are in contact with particularly aggressive fluids and environments.
Within the Intertubi catalog there are numerous types of elbows that differ in some fundamental characteristics:
- Reference standard: the two main ones are the American standard ASME B16.9 and the European standard EN-10253;
- Characteristic dimensions: among which it is appropriate to remember the radius of curvature, the thickness and the diameter of the duct;
- The angle: that is, the inclination between the axes of the two pipes placed in connection.
As regards the first distinction, based on the reference standards, in the American case there are 45 ° elbows without welding, with a diameter from ½ " to 24" and a radius of curvature equal to 1.5 times the diameter, and 90 °, from 1" to 24" with a radius of curvature equal to the diameter.
For the European standard, however, the elbows are welded with an angle of 45 °, from 13.7 mm to 609.6 mm with a radius of curvature equal to 1.5 times the diameter, and at 90 °, from 21.3 mm to 114.3 mm with a radius of curvature equal to 2.5 times the diameter.
The fundamental distinction is the presence of the welding that derives from the different production method. Welding is a widely used joining technology for metallic materials with many benefits but also some limitations due to the microstructural change, the possible presence of defects and, in general, the quality of the product obtained.
The forging process of stainless steel elbows
In order to be able to create stainless steel elbows that comply with the regulations and quality requirements requested by customers, Intertubi uses, in addition to materials with excellent characteristics and properties, controlled production processes following the best procedures deriving from both study and experience in the field.
In particular, the forging process assumes considerable importance. This operation allows the transformation of stainless steel by bringing it to high temperatures and plastically deforming it to obtain a semi-finished product. The correct choice of the parameters to be set for the process is fundamental, in addition to the continuous maintenance of the systems, both to avoid subsequent damage during operation and not to produce an excessive number of rejects which represents damage from an economic point of view.
Among these are:
- Temperature of the starting steel;
- Form of the starting material;
- Type of material and chemical composition;
- Presence and intensity of tensions;
- Type of forging operation carried out;
- Type of lubricant used in the process.
Based on the foregoing, it is easy to understand how the study and analysis of the mutual influence of these parameters is an essential requirement. In addition to this, even the hammers or matrices used must be constantly maintained to prevent problems such as thermal fatigue and other types of failure from altering the quality of the product.
At the end of this operation, however, the stainless steel elbows must be finished and further processed before they can be put on the market.
How to choose the most suitable stainless steel elbow?
The strategy to follow for choosing the correct stainless steel elbow for the specific application is not unique. In fact, it depends not only on the design characteristics of the system in question, which means the geometry, the arrangement, the inclination of the pipes and the dimensions, but also on the surrounding conditions, such as the environment and the stresses present, which represent additional but certainly not secondary characteristics. The experience and quality of Intertubi allows us to meet the needs of the customer by supporting him in the most suitable choice for the specific case of study, avoiding inconvenience upstream of the project and reducing costs, maintenance and related times.
Welding of stainless steel elbows
Welding stainless steels is not as simple as for other typical materials of the hydraulic world. In fact, stainless steel usually has a characteristic oxide layer (suitable, for example, for protection against the phenomenon of corrosion) which tends to make the process difficult and requires some precautions:
- Preparation for caulking: it is important to remove the oxide layer in correspondence with the welding area by means of mechanical abrasion;
- Use of appropriate filler material: this addition improves the characteristics of the area, avoiding resistance problems that can arise in correspondence with the welded or heat-altered area.
- Use of austenitic stainless steels: they have better weldability than other stainless steels, reducing the upstream problem.
- For the latter reason, austenitic stainless steels such as those previously mentioned in the appropriate paragraph are chosen for this type of application.
- Certainly following these indications represents a good starting point but it is not sufficient to achieve a work of art welding. In fact, the welder must be adequately trained and must know the importance of each characteristic parameter of welding such as, for example, the depth and the speed of the pass. If the welding is not carried out correctly, it will bring with it numerous problems from a resistance and structural point of view which can act as areas of weakness for the system once the stainless steel elbows have been put into operation.
The characteristic dimensions of the elbows and their design
Given the regulations, the types and methods of connection, it is necessary to shift attention to the design phase of the stainless steel elbows and to the characteristic dimensions that act as a cornerstone both for the choice and for the realization.
In particular:
- external diameter: whether in inches or mm, it represents the main characteristic for the choice as it must be made on the basis of the system in which it will operate.
- thickness: it is essential for the structural resistance of the system as it guarantees certain resistance characteristics. For this reason it must be made uniform along the entire section.
- distance between the entrance section and the axis of the other entrance: it is an important dimensional characteristic and valid only for 90° elbows;
- distance between the entry section and the center of the elbow: it is an important dimensional characteristic and valid only for 45° elbows;
- elbow weight: it may be necessary to estimate the loads and inertias during operation.
The role of the designer, while adhering to the stringent rules highlighted above, is to understand the main possible "failure modes" and anticipate them in the design phase, preventing them from occurring. Furthermore, it must be able to provide support to the customer regarding the specific application of the stainless steel elbows, highlighting the advantages and criticalities in order to carry out a complete service that allows the interconnection between the manufacturer and the customer, improving the final yield of the system.
The history of the use of stainless steel
As already introduced in the previous paragraphs, stainless steel has exceptional characteristics for the creation of elbows such as, for example, the combination of resistance and anti-corrosion properties that are essential in water systems due to the presence of stresses and degradation phenomena of considerable entity.
The first traces of the use of this material in the war field date back to antiquity and have been confirmed by the presence of manuscripts that describe stainless steel. However, the year that is conventionally recognized as the zero point in the use of this formidable material is 1872, when some scientists added chromium to steel and noted that this addition had a beneficial effect on the formation of rust. Despite this, as with many inventions in history, large-scale use occurred only with the advent of World War I and the first large-scale industrialization process dates back to 1913 in the United States. The explanation of these characteristics, however, will only occur with the study of the phenomenon of passivation that will be carried out in the first decades of the twentieth century.
Since then, this material has increasingly entered our lives and is used to create many components, whether they are for the hydraulic sector, such as the elbows we are interested in, or for other application sectors that have to deal with environments that tend to rapidly degrade the material.
