Welding and valves officially became joined at the hip with the perfection of the arc-welding process in the late 1920s. Up until then, every component in a valve was made of solid, homogeneous material, and all the end connections were either threaded or flanged.
Successful arc welding fostered the development of butt-weld end and socket-weld end connections for pipes, valves and fittings. The arc-welding process also allowed valve parts that were difficult to cast or forge to be joined together, and one of the biggest innovations was the development of weld-applied hardfacings.
The most prevalent and effective joint in piping systems today is the welded connection. It is used to join sections of pipe together, join flanges and fittings to pipe, as well as join butt-weld-end and socket-weld-end valves to other piping components.
Several welding processes are used in welded piping systems, and many of those processes are used in the manufacture of valves.
Just what is welding anyway? Welding is the process of joining where two components are attached by melting the area where they join, while also adding a filler metal to the molten mix. Since the welding joint area needs to become molten, the temperatures reached in the welding process equal or exceed the melting point of the base metal.
Decades ago the only welding method available was the oxyacetylene process, which used an open-flame fueled by acetylene gas and supported by 100% oxygen. This was not an efficient method, and it was soon replaced by the arc-welding process. Still, applications where oxy/acetylene hardfacings were applied to valve components existed through the 1950s.
In arc welding, low-voltage and high-amperage electricity is directed through an electrode on one side of a circuit and through the piece to be welded on the other side. When the electrode gets close enough to the work piece, a very high-energy arc jumps from the electrode to the piece. This high-energy arc is powerful enough to melt both the electrode and the affected area of the work piece.
The most common “arc welding” process is the stick welding process, also called shielded metal arc welding (SMAW). However, others are more efficient, faster or produce a better weld. (See Table 1, page 39.)
One of the primary uses for welding in valve manufacturing is for applying hardfacing to seating surfaces. Decades ago, this laborious process was done with an acetylene gas torch and a long rod of filler material. Today, automated and semi-automated PAW and GTAW are popular because of their high deposition rates and excellent, repeatable weld quality.
An important consideration in hardfacing operations is preventing loss of desired properties of the applied “hard” metal (e.g., hardness, abrasion-resistance and corrosion-resistance) through a process called dilution. Dilution in hardfacing is the mixing of elements of the base metal with that of the filler metal, resulting in the hardfacing material’s chemical analysis being compromised enough to affect the desired properties of the hardfacing. To reduce dilution, welding procedures sometimes call for applying multiple layers of filler material, or in some cases, first applying an intermediate layer of a material metallurgically friendly to both the overlay and base material. These applications are oftentimes referred to as butter passes.
Because of the tremendous amount of heat applied during welding, great stress and strain can be put on both the weldment itself and the area adjacent to the weldment. That adjacent area is called the heat-affected-zone (HAZ). Many materials are stressed so greatly during the welding process that the mechanical properties of the weldment or HAZ are compromised. To restore the original desired properties to the components, a process called stress relief or post-weld heat-treatment (PWHT) is often required after welding. In the valve industry, this applies primarily to air-hardenable materials such as chrome/moly alloys and some martensitic stainless steels (400 series ss). Carbon steel components for use in hydrogen sulfide service also often require PWHT after welding.
The PWHT process consists of raising the temperature of the welded area to a specific amount that is much lower than the welding temperature and then holding that temperature for a prescribed amount of time to let the heat “soak” into the material. After the stress relief process, the desired properties have been restored to the components, and the stresses are relieved.
Like any other technical process, welding requires specific instructions to perform the process correctly. In the case of welding, these instructions are called the welding procedure. The correct term is “welding procedure specification” (WPS). A WPS is created by a welding engineer or other trained welding professional, and it is designed to meet specific piping construction codes or other standards. In the refining and power industries, for example, the primary code to be met is Section IX of the Boiler and Pressure Vessel Code.
For the WPS to become meaningful and valid, it must be qualified. This is done through a process called the “procedure qualification record” (PQR), which verifies that the WPS actually works as it should. All the directions and requirements in the WPS are closely followed so that the welder performs the weld exactly as stated in the WPS. In addition, all data and settings of the welding equipment are recorded on the PQR document. Following the actual PQR welding step, a “coupon” (sample weldment) is subjected to various mechanical and chemical tests to verify that the WPS creates a good, sound weld.
After the procedure is written and qualified, the individual welder that will perform a weld needs to prove that he or she can actually make a quality weld in accordance with that particular procedure. This third part of the welding procedure documentation trail is called the “welder performance qualification” (WPQ). After the weld is made, specific mechanical and or chemical tests are performed on the test sample to verify that the welder performed the procedure correctly, and the weld was sound.
To sum, the welding process consists of:
- A welding procedure (WPS) is written.
- The procedure is qualified via a PQR.
- An individual qualifies to weld that particular procedure via a WPQ.
WELDING VALVES INTO PIPING
The popularity of welded connections in piping systems means that valves must have the proper end connections for weld attachment. The two welded connection types are the butt-weld and the socket-weld. The socket-weld is used for piping that is 2-inch nominal pipe size (2” NPS) and smaller. The butt-weld is sometimes used for small-diameter pipe; but it is normally applied to sizes above 2” NPS.
The socket-weld is the easier weld to perform since the pipe is slipped into the close-fitting socket of the valve or fitting, and there is a ready-made pocket in which to lay down the welding material. It can be performed using the stick, MIG or TIG welding process. This weld is usually made with one or two passes, depending on the pipe size.
The butt-weld on the other hand, is more complex because there is no socket alignment, and the ends of both the pipe and the valve must have matching bevels as well as be in perfect alignment during the welding process. The initial welding pass in butt-weld joints is called the root pass. It requires skill and practice to perform correctly. The root pass is followed by subsequent passes until the weld is completed.
Any requirement for PWHT also must be followed. With small valves, extreme care must be taken to cause no damage to the critical valve components, which may be close to the weld area, thereby receiving unwanted high heat input. Extreme care must be taken when welding soft-seated ball valves into piping systems, for example.
WELDING IS VITAL
Welding will always have a place in valve manufacturing because of the impracticality of casting or forging some alloys or material combinations. Over the last 80 years or so, the welding industry and the valve industry have worked together to meet the challenge of creating quality valves and effective welded piping systems. They will continue to march side by side as operating pressures increase and the need for higher integrity, environmentally-friendly, joint-free piping continues to grow.