Carbon welding nipples: materials, construction and function

Carbon steel welding nipples represent important components within the world of hydraulic fittings. Compared to the carbon couplings, which have a female thread on the internal surface along the entire axis of the component, the carbon welding nipples have a male thread only on one end, while the other surface is free and is usually intended for connection by welding.

The characteristic dimensions of the carbon welding nipples

Carbon welding nipples are very simple components which, however, must correctly comply with the dimensions and tolerances to ensure effective coupling with the system on which they are installed. The characteristic dimensions are:
- Φ1: is the nominal external diameter of the welding nipple. This dimension can vary from 17.2 to 114.3 mm.

- A: is the length of extension of the threaded section. It can vary from 14 to 32 mm depending on the model.

- L: is the length of the component and is always equal to 100 mm. It is immediate to notice how the ratio between A and L increases as the thread length increases.

- SP/THK: is the thickness of the component in the non-threaded area. It can vary between 2.20 and 4 mm.

The choice of carbon steel for the welding nipples

Carbon steel is a very common type of steel that is generally opposed to stainless steels which are characterized by alloying elements such as chromium and nickel. Carbon is a fundamental element in steels but the variation of the percentage is important to determine the mechanical, physical and chemical characteristics of the alloy.
The main differences between carbon steels and stainless steels can be summarized in the greater flexibility of the former compared to the latter as well as a better heat distribution (property suitable for welding, which however also depends on the carbon content in the alloy), even if the resistance to corrosion is less. Furthermore, the gloss of stainless steels is better and makes them suitable also for decorative applications while carbon steels have a higher degree of opacity. Based on these properties, it is easy to understand how an element such as the carbon welding nipple, which must be welded and does not constitute a decorative element, is made of carbon steel: in fact, it must also be emphasized that the price is certainly lower, especially due to of the alloy elements present.

The process of making carbon welding nipples

The production of carbon welding nipples follows a simple production process, but which requires precision to avoid subsequent problems during coupling and assembly. The starting semi-finished product is a portion of tube that is suitably sectioned and processed by machine tools: in particular, the thread is made by removing chips for a length corresponding to dimension A.

The connection of carbon couplings: possibilities and differences

In the previous paragraphs, much attention has been paid to the marking, the choice of material and the production method of the carbon steel couplings. All these steps are essential to obtain a quality product that complies with current regulations and allows to improve the efficiency of the systems in which it is installed.
Another fundamental aspect, however, concerns the implementation of these components. In fact, the connection of the couplings to the pipes that make up the reference system is central. This can be done according to two macro-methodologies:
- Through threaded fittings (GAS and NPT, for example);
- By welding (butt or pocket).
There are important differences between these types of connections:
- Reversibility of the connection: screwing onto a thread is a reversible process that allows you to get the coupling separated from the system again while the welding is permanent;
- Maintainability: the choice of threads is more suitable where it is believed that the aggressive environment or the stresses to which the couplings are subjected are such as to quickly damage the component and require quick and immediate replacements;
- Resistance: welding normally guarantees better resistance values and does not present problems, such as unscrewing, which can further reduce the properties of the fitting.
It is clear, however, that the personnel involved in the installation must be specialized and must be properly trained in order to make the most of the potential of the connection.


The types of threads

There are differences between GAS and NPT threads that may be of particular interest in the field of carbon steel couplings:
- The GAS thread can be either cylindrical or conical while the NPT thread is conical only;
- The NPT thread generally has a better seal than the GAS thread;
- The GAS thread has a thread angle of 55 ° against the 60 ° of the NPT thread: as the angle increases, the thread pitch increases.
- The GAS thread is more widespread in Europe and is the most used in the world in the field of hydraulic fittings, while the NPT thread is preferred in the American system.

The types of welding

As mentioned above, there are two main types of welding: butt and pocket. The main difference between the two is that the pocket weld is a fillet weld while the butt weld is frontal and generally allows for better resistance values. Despite this, it is certainly a less economical method and for this reason it is preferred only for pipes, and therefore couplings, with large diameters, while pocket welding is preferred on the smaller dimensions. In any case, it is essential that they are made in a workmanlike manner so as not to make the welded joint represent a weak point for the structure.