Stainless steels are grouped into five metallurgical families: (1) Austenitic – the most common family, including 304 and 316, non-magnetic and non-hardenable by heat treatment; (2) Ferritic – magnetic, moderate corrosion resistance, including 409 and 430; (3) Martensitic – magnetic, hardenable, used for cutlery and tools, including 410, 416, and 440; (4) Duplex – a mix of austenitic and ferritic microstructure with high strength and good corrosion resistance; and (5) Precipitation Hardening (PH) – can be hardened by aging, including 17-4 PH and 15-5 PH.

The three most commonly referenced grades are 304 (the workhorse austenitic grade), 316 (the marine-grade austenitic with molybdenum), and 410 (a basic martensitic grade). Each covers a broad range of applications: 304 for general use, 316 for corrosive environments, and 410 for applications needing moderate hardness and wear resistance.

The five key characteristics of stainless steel are: (1) Corrosion resistance – provided by at least 10.5% chromium forming a passive oxide layer; (2) High strength-to-weight ratio; (3) Low maintenance and long service life; (4) Hygienic surface – easy to clean and resistant to bacteria; and (5) Aesthetic appearance – bright, attractive finish suitable for architectural applications.

Several ASTM standards govern stainless steel. ASTM A276 covers stainless steel bars and shapes. ASTM A240 applies to plate and sheet. ASTM A312 covers seamless and welded pipe. ASTM A484 covers general requirements for all stainless steel bar, billet, and forging stock. ASTM A193 applies to high-temperature fastener materials. Stainless Shapes can provide certified material test reports (CMTRs) per applicable ASTM standards.

Grade 304 is far more versatile and widely used than 409. Grade 304 is an austenitic alloy with much better corrosion resistance, formability, and weldability. Grade 409 is a ferritic grade commonly used in automotive exhaust systems and agricultural equipment—it is less expensive but significantly less corrosion-resistant than 304. For most industrial or food-processing applications, 304 is the preferred choice.

Grade 304 is the best all-around choice for general stainless steel use. It offers an excellent combination of corrosion resistance, formability, weldability, and cost. Where chloride environments are a factor, 316 is the appropriate upgrade. For applications requiring higher strength, 17-4 PH or Duplex grades should be considered.

ASTM A276 is the primary standard covering 304 stainless steel bar and shapes. For 304 stainless plate and sheet, ASTM A240 applies. The material must contain 18–20% chromium and 8–12% nickel (the basis for the common name '18-8 stainless'). Chemical composition and mechanical properties are verified via certified mill test reports available from Stainless Shapes.

304 and 316 have virtually identical hardness in the annealed condition—typically 80–95 HRB (Rockwell B scale). Neither grade can be hardened by heat treatment as they are austenitic. Cold working can increase the hardness and strength of both grades somewhat, but for applications requiring high hardness, a martensitic grade (410, 440C) or precipitation-hardening grade (17-4 PH) is required.

The most accurate field identification method is X-ray fluorescence (XRF) or optical emission spectroscopy (OES). In the absence of testing equipment, check the markings on bar ends or mill certifications. Visual inspection alone cannot reliably distinguish 304 from 316. Stainless Shapes provides certified mill test reports with every order to confirm the exact grade and heat number.