The construction of installations starts first from a good design of the same but above all from the choice of components. Let's talk today about carbon steel fittings which must guarantee some primary objectives:
The tightness and safety
The quality of the materials
The versatility of the figures
To achieve this, the starting point is an adequate construction technique of the fittings to be chosen in order to have the desired final shape while maintaining the necessary mechanical characteristics. This guarantees the necessary performance, together with, of course, the choice of the most suitable material for the operating conditions, in terms of pressure and temperature. Flexibility depends on an adequate choice of available figures, all in any case regulated and certified according to the reference standards.
The construction technique of carbon steel fittings
The starting point for the production of carbon steel fittings are welded steel pipes or sheets of the desired thickness which, depending on the particular to be made, are treated differently.
1. HYDROFORMING: a process used in particular in the production of normal or reduced tees, which is the forming of the lateral branch of the pipe through the action of a liquid under pressure, generally oil.
2. DRAWING: a process performed on sheet metal through the action of a press, usually made cold, which serves to give the fittings greater yield strength.
3. FORGING: hot forging is very similar to deep drawing but makes the final product more ductile than a deep drawn product, usually these two processing techniques are used for open products such as caps.
4. HOT DEFORMATION: after having brought the tube to the most suitable temperature for the process, the material is deformed by means of shaped punches to create details such as normal or wide curvature elbows. This is one of the most established processes ever. In fact, the experience and know-how developed over the years have made it possible to achieve the production of components in which the consistency of the thickness is an important plus, we remind you that for example in elbows the cut made to achieve angles of less than 90 ° still leads a circular head section of constant thickness.
If we talk about carbon steel fittings for butt welding, there are two reference standards worldwide, which must be respected in order to face both the market that refers to Anglo-Saxon measures and that which refers to metric measures.
- UNI EN 10253-2: 2008 Butt weld fittings for pipes - Part 2: Non alloyed steels and ferritic alloyed steels with specific control requirements, which is the transposition by Italy of the corresponding European standard which indicates the supply conditions of the butt weld fittings used for the transport of liquids and gases at both low and high temperatures.
- ASME B16.9 Butt weld fittings in carbon steel, also in this case defines the dimensions and tolerances for the construction of fittings, but also their marking and test conditions. Both high and low temperature alloy carbon steels can be used.
For carbon steels for the construction of welded fittings, the choice is very wide depending on the use for which they are intended. Mainly, the new ISO standards divide materials into two categories, on the one hand the steels designated according to their mechanical characteristics and on the other those designated according to their chemical composition.
In the construction of fittings, the first group of materials uses those with the suffix P which indicates materials suitable for use under pressure, usually with yield strengths starting from 235 Mpa at room temperature. In the second group, on the other hand, we find the steels classified by their chemical composition usually with a significant content of Molybdenum which improves the heat resistance of the material.
Unalloyed carbon steel P235NL or P265NL is used for fittings to be used at extremely low temperatures; if the temperatures are above the room temperature up to particularly high conditions, the choice can be between non-alloy steels such as P235GH or alloy steels with molybdenum content and other chemical compounds such as 13CrMo4-5 or X10CrMoVNb9.
As regards the Anglo-Saxon legislation, the classification is different but conceptually similar.
The figures available
The reference legislation does not leave much room for creativity in the realization of the fittings, a strong point is the availability of prompt delivery in the warehouses of the figures necessary for the installation of the systems.
The figures that we can find are similar for the two main reference standards and are: A. ELBOWS: the purpose of which is to change the direction of the fluid, exist both in the normal version and with a wider radius of curvature, usually equal to 5 times the diameter, able to reduce the pressure drops where the compactness of the system is not a requirement essential.
C. EQUAL AND REDUCER TEE: the purpose of this fitting is the creation of lateral branches derived from the main system. In particular situations the lateral branch of the tee can be extended by welding a piece of pipe which facilitates the subsequent connection operations.
D. CAPS: which can be used in the termination of system branches, the particular shape allows to reduce the end stresses by distributing them in an optimal way on the weld.
Why choose carbon steel as a construction material?
Carbon steel is a type of steel that has low percentages of elements other than iron or carbon. In particular, all the elements are below the limit reported within the BS-EN 10020 standard. Therefore, the fundamental element to guarantee the properties is the carbon itself.
This material is widely used from the construction point of view thanks to its properties, above all of workability, which make it suitable for many applications. In addition, based on the carbon content it is divided into:
- Low carbon steel (<0.3%): characterized by good weldability and softness;
- Medium carbon steel (between 0.3% and 0.6%): it is usually heat treated with a hardening treatment and is more resistant than low carbon steel. On the other hand, however, it is more difficult to work, weld and cut requiring specific processes;
- Steel with a high carbon content (> 0.6%): it is a steel which after heat treatment becomes extremely hard and brittle, but which is very difficult to cut, form and weld and for this reason it is used for making tools.
Furthermore, the quality is determined by the percentage of sulfur and phosphorus present within the alloy:
- High quality steel: if their content is <0.03%;
- Superior quality steel: if their content is <0.07%;
- Steel of ordinary quality: if their content is <0.1%.
The differences between carbon steel and stainless steel in the world of fittings
Steels are high-performance materials that guarantee durability and resistance of the fittings over time, avoiding frequent replacements that can lead to inconvenience or expense from an economic point of view.
Carbon steels, as seen, have important characteristics for this application category but they are not the only ones that can be used. In fact, part of the market is occupied by stainless steel fittings. The differences can be summarized in:
- From a chemical point of view: the presence of chromium and nickel is at decidedly higher levels for stainless steels compared to carbon steels which are composed of carbon and alloy elements in small percentages;
- From the point of view of mechanical properties: carbon steel is harder and has a higher resistance limit;
- From the point of view of corrosion resistance: carbon steel has lower corrosion resistance;
- From an economic point of view: stainless steel is generally more expensive;
- From an aesthetic point of view: stainless steels are preferred, reducing the need for painting.
Both steels therefore have strengths and characteristics that can be improved, but it emerges that it is essential to study the systems in advance and understand what are the major criticalities that will be faced during operation.
How to check the weldability of carbon steel?
As already verified in the previous paragraphs, carbon steels for the production of fittings can have different chemical compositions which influence the mechanical and technological properties of the components.
As far as weldability is concerned, there is a remarkably widespread empirical parameter called "Carbon Equivalent". It can be calculated with the following formula from the International Welding Institute.
CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
where the symbol of each element represents the percentage quantity present in the alloy. This is one of the many calculation formulas that can be found in manuals. It appears clear that not all elements have the same weight within the formula but there are more important elements (among all carbon but also manganese), while other elements are secondary (such as nickel and copper). Additionally, it is important to note that carbon equivalent is not the carbon content, which instead represents the % carbon content in the alloy. Which values are to be considered good and which are deleterious?
- CE <= 0,35: weldability is excellent and there are no problems;
- 0,35 <= CE <= 0,45: the weldability is very good and the necessary precautions are few;
- 0,45 <= CE <= 0,50: weldability is fair and care must be taken when carrying out the operations;
- CE >= 0,50: weldability is bad and difficult to make.