### 1. What is the difference between 304 and 202 steel?
The main difference lies in their composition and resulting corrosion resistance. **304 stainless steel** is part of the 300 series and uses nickel and chromium as its primary alloying elements. This gives it excellent corrosion resistance. **202 stainless steel** is part of the 200 series, where a significant amount of the expensive nickel is replaced with manganese and nitrogen. This makes 202 less expensive but also significantly less resistant to corrosion, especially in harsh environments like coastal areas or where exposed to chemicals. 304 is generally considered a higher-quality, more durable material than 202.
### 2. Is 304 stronger than 316?
No, 304 is not stronger than 316 in terms of tensile or yield strength. Their mechanical strengths are actually very similar. The key difference is in their corrosion resistance. **316 stainless steel** contains molybdenum, an element that 304 lacks. This molybdenum dramatically increases 316's resistance to chlorides and acids, making it the "marine-grade" stainless ideal for harsh environments like coastal settings or chemical processing. So, while not "stronger" in a mechanical sense, 316 is vastly superior in its ability to resist corrosion and pitting.
### 3. Which steel is best, 302 or 304?
For the vast majority of applications, **304 is considered better and is the more commonly used grade**. The two grades are very similar, but 304 has a slightly lower carbon content and a slightly higher nickel content than 302. This small difference makes 304 much less prone to "sensitization" (carbide precipitation at grain boundaries) during welding, which can lead to premature corrosion. Therefore, 304 is preferred for any application that requires welding. 302 is slightly stronger in its cold-worked state but is largely considered obsolete for new designs in favor of the more weldable 304.
### 4. How do I know if my SS is 304 or 316?
It is very difficult to tell them apart by sight or touch as they look identical. The most reliable methods require testing:
* **Chemical Test (Most Accurate):** Use a **Molybdenum (Mo) test kit**. A drop of testing solution is applied to the surface. If the steel contains molybdenum (indicating 316), the drop will turn a specific color (often brown or gold). If it contains no molybdenum (indicating 304), it will show a different reaction (often red or no color change).
* **Spark Test (For Experienced Personnel):** When grinded, 316 typically produces sparks that are shorter, straighter, and less "bursting" than the sparks from 304. This method requires significant experience.
* **XRF Analyzer (Professional Tool):** This is an expensive handheld instrument that can precisely identify the alloy composition within seconds. It is used by scrap metal dealers and quality control professionals.
### 5. What are the four types of stainless steel?
The four primary types of stainless steel are categorized by their crystalline microstructure:
1. **Austenitic (e.g., 200 and 300 series like 304, 316):** This is the most common type. It is non-magnetic, highly weldable, and offers excellent corrosion resistance and formability. It is hardened by cold working, not by heat treatment.
2. **Ferritic (e.g., 430, 409):** These are magnetic steels with good corrosion resistance (though less than austenitic) and good resistance to stress cracking. They have a lower nickel content, making them more economical. They cannot be hardened by heat treatment.
3. **Martensitic (e.g., 410, 420):** These are magnetic and can be hardened by heat treatment (like carbon steel), making them very hard and strong. However, this comes at the cost of lower toughness and much lower corrosion resistance compared to austenitic grades. Often used for cutlery, blades, and mechanical parts.
4. **Duplex (e.g., 2205):** This is a modern composite structure that is a mix of austenitic and ferritic grains. It offers very high strength (about double that of standard austenitic grades) and excellent resistance to stress corrosion cracking. It is commonly used in demanding offshore, chemical, and oil and gas applications.
