Welding is typically the best way to join two or more pieces of metal to create a solid junction that is durable, low-maintenance, and keeps the system clean. While welding stainless steel, there are some things to remember as not all welds are created equal.
What Type of Weld?
The majority of welding techniques can be summarized as using heat to fuse two metallic components together. Other welding techniques do not even entail high temperatures, such as impact or compression; however, heating is required to join metal for use in a high purity system.
Unlike soldering, which requires heating a soft metal to its melting point and using it to fuse two pieces together, welding involves heating the ends or surfaces of the metal pieces and joining them to the point they turn into a thick liquid and fuse together.
A metal filler rod or wire similarly melted and utilized as a filler in the joint region is also included in most melt-based processes. One significant exception to welding with filler rods or wires is orbital welding, which is explored below.
Even though various welding procedures use similar technologies, their precision and quality vary substantially. The undesirable materials that some welding processes leave on the surface of the welded component, like slag, make them wholly unsuitable for high-purity service. The most popular type of welding does not adhere to purity standards.
Shielded Metal Arc Welding (SMAW), sometimes known as “stick welding,” is utilized in shipbuilding, vehicle repair, and other industries. The consumable rod in SMAW is covered in a flux that vaporizes as the rod melts. The slag and the shielding provided by the generated gas serve to insulate the weld from ambient pollution.
Image Credit: Astro Pak Corporation
While SMAW does not produce welds with a high degree of purity, two additional arc welding techniques can. Gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) have several similarities. These processes are referred to as metal inert gas (MIG) welding and tungsten inert gas (TIG) welding, respectively.
Regarding industrial applications using high purity stainless steel systems, TIG is the method of choice and hence is the most popular procedure.
Image Credit: Astro Pak Corporation
Image Credit: Astro Pak Corporation
What is TIG?
As the name suggests, TIG protects the well pool from absorbing oxygen and nitrogen from the atmosphere by using an inert gas. The most widely used gas is argon, but helium can also be used. In some applications, a combination of the two noble gases is used when welding stainless steel. A powerful electric current is passed through a tungsten welding tip that does not melt.
Instead, a metal filler rod is used. When welding stainless steel, the rod is usually made of an alloy with a higher percentage of chromium and nickel than the metal being welded. This ensures that the weld will not fail or be a source of contamination.
Despite being more challenging, TIG welding offers greater control over elements like heat, travel speed, and metal feed. Compared to the MIG method, it uses less heat, reducing the resulting Heat Affected Zone (HAZ).
TIG welding can be done using either the standard approach or the orbital fusion method. The former is the typical manual technique. In an automated procedure called orbital fusion welding, two ends of sanitary tubing are joined by a tungsten electrode moving circumferentially around them while moving at a constant speed. A filler rod is not necessary for this technique.
What Comes After Welding?
The passive layer of chromium oxide that accumulates on the surface is what gives stainless steel its chemical resistance. When the proportion of chromium to iron atoms at the surface is greater than one, corrosion resistance increases.
Chromium to iron should be used in a 2-to-1 ratio; however, the surface composition varies, and the ratio could be substantially lower when it is melted and resolidified during the welding process. It thus cannot create a robust passive layer and is therefore susceptible to corrosion.
The heat-affected zone (HAZ) around the weld bead is also damaged, and the level of chromium is reduced. This immediate area around a weld must be treated so that it is as pure as well as the surrounding, unaffected stainless steel.
In most circumstances, a smooth surface cannot be achieved without first removing the physical weld bead of the elevated metal line. Weld bead mechanical sanding is often prohibited, especially in orbital welds, code welding, or high-pressure applications.
Since orbital welding is carried out through an automated process rather than manually, it often results in smaller weld beads.
It is also necessary to treat the sections that have been heat-tinted, and the metal might occasionally require pickling. Using acid, a thin layer of the metal surface must be removed.
Whole components can be pickled in a bath, or a gel technique can be used to spot-treat problem regions if only the weld has to be pickled. This discoloration could be eliminated using the electropolishing procedure based on the degree of damage.
Finally, the area needs to be passivated. The filler rod’s higher alloy content does not automatically make the weld more corrosion-resistant. Although it may have a higher chromium concentration, the surrounding heat-damaged metal must have surface iron removed.
A do-it-yourself passivating gel is, at most, a short-term fix. Operators will still need to guarantee the passivation’s effectiveness and ensure that the weld satisfies the requirements set by the industry for the entire surface.
It is only possible to declare that the weld area satisfies the strict purity criteria and that the project is complete once it has been expertly cleaned and passivated, and the required documentation has been provided.
This information has been sourced, reviewed, and adapted from materials provided by Astro Pak Corporation.
For more information on this source, please visit Astro Pak Corporation.