STAINLESS STEEL

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In metallurgy, stainless steel is defined as a steel alloy with a minimum of 11%chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as ordinary steel (it stains less), but it is not stain-proof. It is also called corrosion-resistant steel or CRES when the alloy type and grade are not detailed, particularly in the aviation industry. There are different grades and surface finishes of stainless steel to suit the environment to which the material will be subjected in its lifetime. Stainless steel differs from carbon steel by the amount of chromium present. Carbon steel rusts when exposed to air and moisture. This iron oxide film is active and accelerates corrosion by forming more iron oxide. Stainless steels have sufficient amounts of chromium present so that a passive film of chromium oxide forms which prevents further surface corrosion and blocks corrosion from spreading into the metal's internal structure. There are different types of stainless steels: when nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures. For greater hardness and strength, carbon is added. When subjected to adequate heat treatment, these steels are used as razor blades, cutlery, tools, etc. Significant quantities of manganese have been used in many stainless steel compositions. Manganese preserves an austenitic structure in the steel as does nickel, but at a lower cost.

 

Stainless steels are also classified by their crystalline structure:

 

§Austenitic, or 300 series, stainless steels comprise over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy. A typical composition of 18% chromium and 10% nickel, commonly known as 18/10 stainless. Similarly, 18/0 and 18/8 are also available. Super austenitic stainless steels, such as alloyAL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion due to high molybdenum content (>6%) and nitrogen additions, and the higher nickel content ensures better resistance to stress-corrosion cracking versus the 300 series. The higher alloy content of super austenitic steels makes them more expensive. Other steels can offer similar performance at lower cost and are preferred in certain applications. The low carbon versions of the Austenitic Stainless Steel, for example 316L or 304L, are used to avoid corrosion problem caused by welding. The "L" means that the carbon content of the Stainless Steel is below 0.03%, this will reduce the sensitization effect, precipitation of Chromium Carbides at grain boundaries, due to the high temperature produced by welding operation.

§Ferritic stainless steels are highly corrosion-resistant, but less durable than austenitic grades. They contain between 10.5% and 27% chromium and very little nickel, if any, but some types can contain lead. Most compositions include molybdenum; some, aluminum or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni. These alloys can be degraded by the presence of σ chromium, a intermetallic phase which can precipitate upon welding.

§Martensitic stainless steels are not as corrosion-resistant as the other two classes but are extremely strong and tough, as well as highly machineable, and can be hardened by heat treatment. Martensitic stainless steel contains chromium (1214%), molybdenum (0.2-1%), nickel (0-<2%), and carbon (about 0.1-1%) (giving it more hardness but making the material a bit more brittle). It is quenched and magnetic. Precipitation-hardening martensitic stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than the other martensitic grades. The most common, 17-4 PH, uses about 17% chromium and 4% nickel. There is a rising trend in defense budgets to opt for an ultra-high-strength stainless steel when possible in new projects, as it is estimated that 2% of the US GDP is spent dealing with corrosion. The Lockheed-Martin Joint Strike Fighter is the first aircraft to use a precipitation-hardenable stainless steel—Custom 465—in its airframe.

 

 

 

 

§Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim being to produce a 50/50 mix, although in commercial alloys, the mix may be 40/60 respectively. Duplex steels have improved strength over austenitic stainless steels and also improved resistance to localized corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are characterized by high chromium (19–28%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. The most used Duplex Stainless Steel are the 2205 (22% Chromium, 5% Nickel) and 2507 (25% Chromium, 7% Nickel); the 2507 is also known as "Super Duplex" due to its higher corrosion resistance.

Stainless Steel Grades

 

 

• 100 Series- austenitic chromium-nickel-manganese alloys

   

• Type 101— austenitic that is hardenable through cold working for furniture

   

• Type 102— austenitic general purpose stainless steel working for furniture

   

• 200 Series—austenitic chromium-nickel-manganese alloys

   

• Type 201— austenitic that is hardenable through cold working

   

• Type 202— austenitic general purpose stainless steel

   

• 300 Series— austenitic chromium-nickel alloys

   

• Type 301— highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304.

   

• Type 302—same corrosion resistance as 304, with slightly higher strength due to additional carbon.

   

• Type 303—free machining version of 304 via addition of sulfur and phosphorus. Also referred to as "A1" in accordance with ISO 3506.

   

• Type 304—the most common grade; the classic 18/8 stainless steel. Also referred to as "A2" in accordance with ISO 3506.

   

• Type 304L— same as the 304 grade but contains less carbon to increase weldability. Is slightly weaker than 304.

   

• Type 304LN—same as 304L, but also nitrogen is added to obtain a much higher yield and tensile strength than 304L.

   

• Type 308—used as the filler metal when welding 304

   

• Type 309—better temperature resistance than 304, also sometimes used as filler metal when welding dissimilar steels, along with Inconel.

   

• Type 316—the second most common grade (after 304); for food and surgical stainless steel uses; alloy addition of molybdenum prevents specific forms      of  corrosion. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304. 316 is often used for building nuclear reprocessing plants.

   

• Type 316L— extra low carbon grade of 316, generally used in stainless steel watches and marine applications due to its high resistance to corrosion. Also referred to as "A4" in accordance with ISO 3506.

   

• Type 316Ti—includes titanium for heat resistance, therefore it is used in flexible chimney liners.

   

• Type 321—similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of niobium for desensitization during welding.

   

• 400 Series—ferritic and martensitic chromium alloys

   

• Type 405— ferritic for welding applications

   

• Type 408—heat-resistant; poor corrosion resistance; 11% chromium, 8% nickel.

   

• Type 409—cheapest type; used for automobile exhausts; ferritic (iron/chromium only).

   

• Type 410—martensitic (high-strength iron/chromium). Wear-resistant, but less corrosion-resistant.

   

• Type 416— 416 SS is easy to machine due to additional sulfur

   

• Type 420—Cutlery Grade martensitic. Excellent polishability.

   

• Type 430—decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced temperature and corrosion resistance.

   

• Type 439—ferritic grade, a higher grade version of 409 used for catalytic converter exhaust sections. Increased chromium for improved high temperature corrosion/oxidation resistance.

   

• Type 440—a higher grade of cutlery steel, with more carbon, allowing for much better edge retention when properly heat-treated. It can be hardened to approximately Rockwell 58 hardness, making it one of the hardest stainless steels. Due to its toughness and relatively low cost, most display-only and replica swords or knives are made of 440 stainless. Also known as razor blade steel. Available in four grades: 440A, 440B, 440C, and the uncommon 440F (free machinable). 440A, having the least amount of carbon in it, is the most stain-resistant; 440C, having the most, is the strongest and is usually considered more desirable in knife making than 440A, except for diving or other salt-water applications.

   

• Type 446—For elevated temperature service

   

• 500 Series—heat-resisting chromium alloys

   

• 600 Series—martensitic precipitation hardening alloys

   

• 601 through 604: Martensitic low-alloy steels.

   

• 610 through 613: Martensitic secondary hardening steels.

   

• 614 through 619: Martensitic chromium steels.

   

• 630 through 635: Semi austenitic and martensitic precipitation-hardening stainless steels.

   

• Type 630 is most common PH stainless, better known as 17-4 PH; 17% chromium, 4% nickel.

   

• 650 through 653: Austenitic steels strengthened by hot/cold work.

   

• 660 through 665: Austenitic super alloys; all grades except alloy 661 are strengthened by second-phase precipitation.

   

• Type 2205— the most widely used duplex (ferritic/austenitic) stainless steel grade. It has both excellent corrosion resistance and high strength.