Gateway Arch in St Louis – 304 series SS F-35 Joint Strike Fighter (JSF) Lightning II, built by Lockheed Martin – airframe 17-7 PH – 600 series SS Prepared By: Mr. Prashant S. Kshirsagar (Sr.Manager-QA dept.)

 Definition of Stainless Steel  History of Stainless steel  Properties of Stainless steel  Different types of Stainless steel  Different grades of Stainless steel  Application of Stainless steel

 Stainless steel is an alloy of Iron with a minimum of 10.5% Chromium.  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. Increasing the amount of Chromium gives an increased resistance to corrosion. Stainless steel also contains varying amounts of Carbon, Silicon and Manganese. Other elements such as Nickel and Molybdenum may be added to impart other useful properties such as enhanced formability and increased corrosion resistance.

 There is a widely held view that stainless steel was discovered in 1913 by Sheffield metallurgist Harry Brearley. He was experimenting with different types of steel for weapons and noticed that a 13% Chromium steel had not corroded after several months.

Electrical Resistivity Electrical resistance is higher than that for plain-carbon steels Thermal Conductivity Stainless Steel has lower thermal conductivity than Carbon steel

Coefficient of Thermal Expansion Stainless steel is having greater coefficient of thermal expansion than plain-carbon steels High Strength Exhibit high strength at room and elevated temperatures Melting Temperature Plain-carbon: °C Martensitic: °C Ferritic: °C Austenitic: °C

Austenitic: This SS is having Austenitic structure (i.e. FCC) at room temp. They contain 16-25% chromium, and nitrogen. They are hardened only by cold working. Good toughness, formability, easily weldable, high corrosion resistance. Nonmagnetic except after excess cold working or welding due to martensitic formation. Can be successfully used from cryogenic temperatures to the red-hot temperatures of furnaces and jet engines. They are susceptible to SCC. App.: Automotive, Architecture, Food and beverage equip., Industrial equip.

 Martensitic: They have a structure i.e. BCT. Due to addition of ‘C’ (0.15-1%), they can be hardened and strengthened by heat treatment. When steel is heated it transform from ferrite to austenite & on slow cooling it transforms back to ferrite. However, with fast cooling through quenching in water or in oil the carbon atoms become trapped in a somewhat distorted atomic matrix and structure becomes i.e. BCT. The main alloying element is ‘Cr’ i.e % approx. App.: Steam turbine blades, valves body and seats, bolts and screws, springs, knives, surgical instruments, and chemical engg. equipments.

 Ferritic: This SS is having Ferritic structure (i.e. BCC) at room temp. It containing % ‘Cr’ & C: 0.20max. They are having good corrosion resistance, magnetic and hardenable only by cold working. They are less expensive than Austenitic SS. Limitations: i) poor toughness at sub zero temp ii) Poor weldability due to embrittlement.  App.: Automotive, building construction, Cladding, Urban furniture, commercial food equipment, industrial application

Duplex SS: This SS is having austenitic (50%) + ferritic (50%) microstructure. This SS is containing high Cr: %, Ni: 1.4-7%, Mo: 0.3-4% (e.g.2205; 2507). Combining many of the best features of both austenitic (i.e. application in cryogenic temp. to red-hot temp.) and ferritic types. It is magnetic, non-hardnable by heat treatment, has high TS than austenitic type, weldability similar to the austenitic stainless steel. App.: Heat exchangers, pressure vessels, Chemical-petrochemical ind., Oil-Gas ind., Nuclear Power plant

 Precipitation hardening: This SS are not defined by their microstructure, but rather by strengthening mechanism (e.g.17-4pH (martensite) or 17-7pH (austenitic)). These grades may have austenitic, semi-austenitic or martensitic microstructures and can be hardened by aging at elevated temp. i.e. 480deg.C to 620deg.C. The strengthening effect is due the formation of intermetallic precipitates from elements such as copper or aluminum. Have the highest strength and to be used for specialized application where high strength together with good corrosion resistance is required. App.: Aerospace, defense, offshore oil & gas industries, missile components, motor shafts, valve stems, gears

 Although stainless steel is much more resistant to corrosion than ordinary carbon or alloy steels, in some circumstances it can corrode. It is 'stain-less' not 'stain-impossible'. In normal atmospheric or water based environments, stainless steel will not corrode.  In more aggressive conditions, the basic types of stainless steel may corrode  1) Pitting corrosion  2) Crevice corrosion  3) General/Uniform corrosion  4) Stress corrosion cracking  5) Intergranular corrosion  6) Galvanic corrosion

 Stainless steels of various kinds are used in thousands of applications. Some of the main applications are as follows: Domestic – cutlery, sinks, saucepans, washing machine drums, microwave oven liners, razor blades Architectural/Civil Engineering – cladding, handrails, door and window fittings, street furniture, structural sections, reinforcement bar, lighting columns, lintels, masonry supports Transport – exhaust systems, car trim/grilles, road tankers, ship containers, ships chemical tankers, refuse vehicles Chemical/Pharmaceutical – pressure vessels, process piping. Oil and Gas – platform accommodation, cable trays, subsea pipelines. Medical – Surgical instruments, surgical implants, MRI scanners. Food and Drink – Catering equipment, brewing, distilling, food processing. Water – Water and sewage treatment, water tubing, hot water tanks. General – springs, fasteners (bolts, nuts and washers), wire.