Why is the controlled sulfur content for Grade 316/316L specified in ASTM A270 S2 and is higher than that of ordinary?

Each published standard by ASTM International includes Supplementary Requirements, if necessary, that are not specifically covered in the main body of the standard. These supplementary requirements are optional and can be requested by the purchaser or referred to in other specifications. They are designated as S1, S2, and so on. The specific content of each supplement varies depending on the standard it is associated with.

While in ASTM A270(the latest version), there are three Supplementary Requirements: S1-Intergranular Corrosion Test; S2-Pharmaceutical Quality Tubing; S3-Chemical Cleaning(Passivation). In S2, it specifically prescribed When S31600(316) and S31603(316L) are ordered, sulfur content shall be restricted to the range of 0.005 to 0.017%. Normally, the sulfur content is lower than that. So why is higher? What are the reasons of the controlled sulfur content?

Because it is in order to improve the machinability of the material and is more conducive to orbital welding.

Machinability refers to the ease with which a material can be cut, shaped, and formed using machining processes such as drilling, milling, and turning. Because of the higher sulfur content, the material becomes more “free-machining.” This means that it can be more easily and efficiently machined, resulting in smoother and faster production processes. Higher sulfur content creates sulfide inclusions within the steel, which act as chip breakers during machining. These inclusions help to break the chips into smaller, more manageable sizes and reduce the tendency of the material to stick to cutting tools.

And orbital welding is commonly used in equipment manufacturing in the food and pharmaceutical industries. Orbital welding is a specialized welding technique that offers precise control and automation, making it ideal for industries that require high-quality, hygienic welds with minimal contamination risks.

In the food and pharmaceutical industries, strict sanitary requirements are essential to ensure product integrity and safety. Orbital welding provides several advantages in meeting these requirements. It produces consistent, high-quality welds with minimal heat input, reducing the potential for distortion or damage to the stainless steel tubes. The technique also minimizes the need for manual intervention, reducing the risk of human error and ensuring a more controlled and repeatable welding process.

In orbital welding, precise control and consistency are crucial for achieving high-quality welds. The controlled sulfur content in stainless steel helps minimize the risk of porosity formation during welding. When sulfur reacts with oxygen during the welding process, it can form sulfur dioxide gas, which can lead to the formation of porosity in the weld. Porosity can reduce the integrity and strength of the welded joint. By controlling the sulfur content within specified limits, the risk of porosity formation can be minimized, resulting in higher-quality welds. Minimizing sulfur content in stainless steel welded tubes for orbital welding helps ensure the integrity and performance of the welded joints.

By specifying a controlled sulfur content range of 0.005-0.017%, ASTM A270 S2 ensures that the risk of porosity formation in the weld is minimized. This range strikes a balance between achieving good weldability and maintaining the desired properties and performance of the stainless steel welded tubes.

Controlling the sulfur content within this specific range helps produce high-quality welds with proper integrity and strength, making the stainless steel welded tubes suitable for a wide range of applications in industries such as food, dairy, and pharmaceuticals, where cleanliness and corrosion resistance are crucial.