Technical Article | 316L Vs. Double-Melted 316L (VIM/VAR): What’s The Difference?

You must have seen these markings on product manuals within the fluid control industry: Body Material: 316L Stainless Steel Body Material: Double-Melted 316L (VIM/VAR)

Both bear the name 316L. Why is "double-melted" specified for some grades? Behind this label lies a massive gap in material purity. For ultra-high-purity applications such as semiconductor manufacturing, this difference directly determines whether a product is fit for service.

316L is a low-carbon molybdenum-bearing stainless steel, with chemical composition: Carbon ≤0.03%, Chromium 16~18%, Nickel 10~14%, Molybdenum 2~3%. The addition of molybdenum improves resistance to pitting and crevice corrosion, while the low carbon content mitigates intergranular corrosion risks.

Within fluid control, 316L serves as the baseline material. VIGOUR equips its VHPS High-Purity Series with standard 316L stainless steel. For non-corrosive or mildly corrosive gases with purity Grade 6.0 (99.9999%) and below, standard 316L delivers reliable performance.

However, standard 316L falls short when deployed for semiconductor Ultra-High Purity (UHP) gas delivery.

 AOD: Starting Point of Conventional Melting

The dominant industrial melting route for stainless steel is AOD (Argon Oxygen Decarburization). The AOD process offers balanced cost efficiency and produces specification-compliant 316L, yet it suffers one critical limitation: limited impurity control.

AOD-melted 316L contains higher levels of non-metallic inclusions (oxides, sulfides, etc.) and uneven grain structures. These micro-definitions exert negligible impact on general industrial applications, yet they become contamination sources in UHP gas environments. As gas flows through piping, inclusions may shed particulates or act as corrosion initiation sites.

 VIM: Vacuum Induction Melting – First Purification Step

VIM (Vacuum Induction Melting) uses electromagnetic induction to melt metals under vacuum. The vacuum atmosphere effectively reduces gaseous impurities including oxygen, nitrogen and hydrogen, while electromagnetic stirring homogenizes chemical composition.

VIM-treated 316L features drastically reduced non-metallic inclusions and tighter compositional tolerances. VIGOUR's VUPS UHP Series and VIGS Integrated Systems adopt VIM/VAR double-melted 316L - the mandatory material standard for entry into the semiconductor UHP sector.

 VAR: Vacuum Arc Remelting – Second Purification Step

VAR (Vacuum Arc Remelting) is the secondary melting process conducted after VIM. The VIM ingot acts as a consumable electrode, which is remelted and solidified inside a water-cooled copper crucible under vacuum.

Core benefits of VAR: • Further lowers inclusion content and boosts material cleanliness • Optimizes ingot solidification structure and reduces segregation • Enhances consistency and reliability of mechanical properties

316L refined via sequential VIM + VAR treatment is widely referred to as double-melted 316L. It far surpasses conventional AOD 316L in purity and constitutes the standard material for semiconductor UHP systems.

 Surface Finish: A Magnifier for Material Cleanness

Material purity directly governs surface treatment outcomes. Even with identical electropolishing (EP) processes, conventional 316L and double-melted 316L yield distinctly different surface roughness:

Standard 316L after EP: Ra approx. 10~25 μin. (0.25~0.63 μm)

Double-melted 316L after EP: Ra down to 5 μin. (0.13 μm)

Smoother surfaces generate fewer particulates under gas flow and minimize dead zones (micro-pits prone to trapping contaminants). In semiconductor fabrication, a single particulate can lead to wafer scrapping - this explains why UHP systems require double-melted material.

 VIGOUR Material Selection Guidelines

Quick reference to judge whether double-melted material is required:

✅ Semiconductor UHP gases (ultra-high-purity gases, corrosive specialty gases) → Mandatory: 316L VIM/VAR + EP finish

✅ Semiconductor bulk gases (N₂, Ar, He and other high-purity grades) → Recommended: 316L VIM/VAR, minimum BA finish

✅ Industrial gases & general high-purity gases (Grade 6.0 and below) → Standard 316L sufficient

✅ Food & pharmaceutical applications → 316L with clamp connections + surface passivation

Material forms the first line of safety and purity protection for fluid control systems. Selecting the correct material paves the way for successful project execution.

Feel free to contact VIGOUR technical team for one-on-one material selection support.~