What Makes a Stainless Steel Weld Sanitary?

In this Tech Info article, we are going to focus on welding and what makes a weld sanitary in today’s sanitary (hygienic) processing facilities. To start, let’s look at a very general definition and overview of welding. Welding is the joining process in which two (or more) parts are coalesced at their contacting surfaces by application of heat and/or pressure. Welding is achieved by two basic categories; fusion welding and solid state welding. Fusion welding is accomplished by melting the two parts to be joined, and in some cases, adding a filler metal to the joint. Examples of fusion welding processes include arc welding, resistance welding, and laser beam welding. Solid state welding involves the application of heat and/or pressure but no melting of base metals occurs and no filler metal is added. Some examples include forge welding, diffusion welding, and friction welding. The American Welding Society AWS D18 Committee was formed by the request of the 3A Sanitary Standards Committee for help in outlining welding standards for use in the manufacture and construction of dairy and food product processing plants. Within the AWS D18.1/D18 Specification is where it is defined that all welds for austenitic stainless steel tube and pipe are to be done by the gas tungsten arc welding (GTAW) process (also known as the tungsten inert gas, or TIG, process). This process uses a non-consumable tungsten electrode along with an inert gas (argon or helium) for arc shielding. The purpose of arc shielding is because at high temperatures, metals are chemically reactive to the oxygen, nitrogen, and hydrogen that is present in air, and thus without a shielding gas the mechanical properties of the joint are subject to oxidation. So the advantage of the GTAW welding process is that it produces high quality welds with little or no post-weld cleaning.

GTAW Welding

According to the AWS D18.1/D18 Specification, welds are to be fully penetrated to the ID to prevent the formation of crevices which could entrap product and lead to contamination. In order to determine acceptable oxidation levels, the AWS D18.2/D18 Specification provides a visual examination guide to aid in the inspection of color in the heat affected zone (HAZ) for welds in piping systems. The heat affected zone is the area of base metal that has experience temperatures below melting point, but high enough to cause changes in the properties and microstructure of the metal.

AWS Weld Discoloration & Acceptance Criteria Scale As the demands of the bioprocess industry for clean, smooth product contact surfaces increased, advances in process piping technology and equipment fabrication technology have followed. Orbital welding was developed as an automated process to address the risk of operator error in GTAW processes. Orbital welding offers the advantage of a computer controlled system that combines arc current, feed and speed to allow the ‘orbital weld head’ to travel around the tube in a steady manner in order to produce consistent and repeatable weld profiles. The 3A Standard was extensively used by the pharmaceutical food and dairy industries but with the emerging bioprocess industry, higher standards were needed for equipment design that would be both cleanable and sterilizable. The ASME published the first edition of the ASME Bioprocessing Equipment Standard in 1997 (BPE-97) to help address this need. Part MJ (Materials Joining) of the ASME BPE Standard requires that the weld criteria of ASME B31.3 – Process Piping be met for acceptable metallic materials. While ASME B31.3 prohibits weld discontinuities such as cracks, voids, porosity, lack-of fusion, and incomplete penetration, the ASME BPE Standard also provides visual examination acceptance criteria to determine the hygienic condition of the piping system. It should be noted that the BPE standard specifies orbital welding as the preferred joining technology for bioprocess tubing, and that manual welding may be performed with owner/user and contactor agreement.

Acceptable and Unacceptable Weld Profiles

Hopefully this Tech Info article has provided a good overview of the welding practices used in our industry and what criteria and standards define practices to be sanitary. If you have any questions about standard practices in sanitary industries, please contact one of our Inside Sales Representatives or email us at [email protected].