Stainless steel is an alloy of Iron with a minimum of 10.5% Chromium (typically between 10-20%). Chromium produces a thin layer of oxide on the surface of the steel known as the ‘passive layer’. This prevents any further corrosion of the surface.
Stainless Steel is valued due to its high corrosion resistance. Stainless steel is about 200 times more resistant to corrosion than mild steel.
Austenitic Stainless Steels.
Most frequently used types of stainless steels.
High chromium and Nickel content compared to other steel alloys, higher resistance to corrosion.
Non-magnetic + Most weld-able + not heat-treatable.
Can be largely divided into three groups: common chromium-nickel (300 series), manganese-chromium-nickel-nitrogen (200 series) and specialty alloys.
Use in food processing equipment, kitchen utensils and medical equipment.
Grade 304 – Chromium 18-20%, Nickel 5-10.5%. include manganese, silicon
Grade 316 – similar to Grade 304 but 316 containing a significant amount of molybdenum; typically 2 to 3 percent, hence increased corrosion resistance.
Ferritic Stainless Steels
Contain trace amounts of nickel, 12-17% chromium, less than 0.1% carbon, along with other alloying elements, such as molybdenum, aluminium or titanium.
Magnetic. Non heat treatable. Can be hardened through cold working.
Good ductility and formability, but high-temperature strengths are relatively poor.
Less expensive because of reduced nickel content.
Grade 430 – Fe, <0.12% C, 16-18% Cr, <0.75% Ni, <1.0% Mn, <1.0% Si, <0.040% P, <0.030% S
usually provided in bar form to be used in automatic screw machines.
Grade 434 – added molybdenum to increase its corrosion
Martensitic Stainless Steels
Contain 11-17% chromium, less than 0.4% nickel and up to 1.2% carbon
Less Corrosion resistance, but high tensile strength & high hardness.
Martensitic stainless steels, such as types 403, 410, 410NiMo and 420 are magnetic and heat-treatable. These Stainless steels are used in knives, cutting tools, as well as dental and surgical equipment.
Grade 420: carbon <0.15%, Chromium 12.0-14.0%, Manganese <1.0%, Silicon <1.0% …
It offers good ductility in its annealed state and excellent corrosion resistance properties when the metal is polished, surface grounded or hardened. This grade has the highest hardness – 50HRC
Duplex (or austenitic-ferritic): Primarily used in chemical plants and piping applications. Duplex stainless steels typically contain approximately 22-25% chromium and 5% nickel with molybdenum and nitrogen. Duplexes have higher yield strength and greater stress corrosion cracking resistance to chloride than austenitic stainless steels.
Both the low nickel content and the high strength (enabling thinner sections to be used) give significant cost benefits. They are therefore used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc and in the petrochemical industry in the form of pipelines and pressure vessels.
Standard Duplex ( PREN Range 28-38) typically Grade EN 1.4462 (also called 2205)
Super-duplex (PREN Range 38-45) Typically grade EN 1.4410 up to so-called Hyper duplex grades (PREN > 45)
Lean Duplex grades (PREN Range 22-27), typically grade EN 1.4362
Precipitation Hardening: This is a chromium-nickel stainless that also contains alloying additions such as aluminium, copper or titanium. These alloys allow the stainless to be hardened by a solution and aging heat treatment. They can be either austenitic or martensitic in the aged condition.
Grade 630 / 17-4 PH. The name comes from the additions 17% Chromium and 4% Nickel. It also contains 4% Copper and 0.3% Niobium.
Cost: 316 Cr+Ni+Moly (Austenitic) > 304 Cr+Ni (Austenitic) > 430 Cr 12-17 (Ferritic) > 420 Cr 12-14 (Martensitic)