1. **Which one is better, SUS 304 or 316?**
* Neither is universally "better." SUS 316 is generally superior for corrosion resistance, especially against chlorides, acids, and marine environments, due to its added molybdenum (2-3%). SUS 304 offers excellent corrosion resistance for many general applications and is more cost-effective. Choose 304 for everyday environments (kitchens, appliances, architecture). Choose 316 for harsher environments (coastal, chemical, marine, de-icing salts).
2. **What is the difference between 304 and 309 stainless steel?**
* The key differences are composition and purpose. SUS 304 (18% Cr, 8% Ni) is a versatile, general-purpose austenitic stainless steel. SUS 309 (23% Cr, 13% Ni) has significantly higher chromium and nickel content, making it much more resistant to oxidation (scaling) and corrosion at high temperatures (up to ~2000°F / 1093°C). 309 is primarily used for high-heat applications, while 304 is used for general corrosion resistance at lower temperatures.
3. **How can I tell if my stainless steel is 304 or 202?**
* Distinguishing 304 from 202 visually is difficult. Key methods:
* **Magnet Test:** Both are generally non-magnetic when annealed, but 202 may show slightly stronger magnetic response after forming/cutting than 304. This is unreliable.
* **Chemical Test Kits:** The most accessible method. Use a drop test kit specifically designed to detect Manganese (Mn). SUS 202 has high Manganese (~7.5-10%) and low Nickel (~4-6%), while SUS 304 has lower Manganese (max 2%) and higher Nickel (~8-10.5%). A strong color change indicates high Mn (likely 202).
* **Spark Test (Requires Skill):** 304 produces fewer, shorter, less-branched orange-yellow sparks than 202, which produces brighter, longer, more branched white sparks due to its high Mn. Requires experience and comparison.
* **Laboratory Analysis:** XRF (X-ray Fluorescence) or OES (Optical Emission Spectroscopy) provides definitive, accurate composition. This is the only sure way.
4. **What is 309 stainless steel used for?**
* SUS 309 is primarily used for high-temperature applications due to its excellent resistance to oxidation (scaling) and carburization. Common uses include furnace parts (e.g., heat exchanger tubes, burner parts, muffles, retorts, radiant tubes), high-temperature conveyor systems, kiln linings, combustion chambers, pyrolysis hardware, and as weld filler for joining dissimilar metals or overlaying carbon steel for high-temperature service.
5. **Can I weld 304 with 309?**
* Yes, you can weld SUS 304 base metal using SUS 309 filler metal. This is a common practice, especially when:
* Welding 304 to itself or other grades for high-temperature service (309 filler provides better heat resistance).
* Welding 304 to carbon steel or low-alloy steel (309 filler acts as a buffer layer, reducing dilution and cracking risk).
* Welding 304 in corrosive environments where slightly higher alloy content is beneficial.
* The weld will have a different composition (higher Cr/Ni) than the 304 base metal, but it generally creates a sound joint. Ensure proper welding procedures are followed.
