Effect of alloying element
- 1. Gandhinagar Institute of Technology Submitted by: Faculty Guide: Pavan Narkhede Prof. Amit Patel (130120119111) Academic Year (2015-16) A Case Study On EFFECT OF ALLOYING ELEMENT LIKE NICKEL, CHROMIUM AND TUNGSTON Under a subject of Design of machine Element B.E. III, Semester – V_B2 (Mechanical Engineering)
- 2. 1)Nickel : Nickel is a silver-white transition metal with a relative atomic mass (12 C=12) of 58.69, an atomic number of 28, a melting point of 1,453°Cand a density of 8.902 kg/dm3 . It is in group VIII on the periodic table. It has a face- centred-cubic (f.c.c.) crystalstructure. Nickel is ferromagnetic up to 353°C, its Curie point. Effect of nickel on stainless steel In stainless steels, nickel has no direct influence on the passive layer but exerts a beneficial effect, particularly in sulphuric acid environments. Thanks to nickel austenitic stainless steels, i.e. Fe-Cr-Ni (Mo) alloys, exhibit a wide range of mechanical properties that are unparalleled by any other alloy system today. For instance, these alloys exhibit excellent ductility and toughness, even at high strength levels and these properties are retained up to cryogenic temperatures. Nickel promotes the resistance to corrosion of the nickel-based alloys as compared with the iron-based alloys under conditions where the passive layersmay beabsent, or may be destroyedlocally or uniformly.For example, pitting corrosion tends to progress less rapidly in high-nickel alloys. In Fe-Ni alloys, their original features can be explained by two major phenomena. A one is the abnormally low expansion in compositions closeto INVAR (36% Ni) due to a large spontaneous volume magnetrostriction and
- 3. exceptionally high electrical permeabilities resulting fromthe disappearance of various anisotropies in the vicinity of 80% Ni. Nickel forms the base of high temperature super alloys becauseof its ability to develop an adherent oxide and precipitation hardening phases based on Ni3Al. Nickel is a moderate strengthener, and consequently large amounts can be added to low-alloy steel beforestrength increases to an undesirablelevel. In low-alloy steel, nickel appears to have a greater overall, beneficial effect on toughness transition temperature than any other substitutional alloying element. NICKEL IN MARTENSITIC STAINLESS STEEL Effect on Microstructure Nickel is one element that increases the amount of Cr that can be added and still form austenite at high temperatures, necessary to get martensite formation when quenched. Effect on Corrosion Properties Most standard martensitic SS have relatively low Cr content, 11.5-13.5%, and thus have relatively low general corrosion resistance compared to austenitic grades with higher Cr content. Nickel increasesthecorrosionresistanceof the martensitic gradesto both general corrosion and localized corrosion. The higher Cr S43100 has the highest corrosion resistance of any of the standard martensitic SS.
- 4. All the martensitic SS havetheir bestcorrosionresistancein the hardened and tempered condition; corrosion resistance is much poorer in the annealed condition. NICKEL IN MARTENSITIC STAINLESS STEEL Martensitic-ferritic-austenitic grades (triplex) 1.4418 grade is typically 65% martensite, 30%austeniteand 5% ferrite in the tempered condition. It is a weldable martensitic SS with corrosion resistance, good strength and good ductility. Major use in small to medium- sized water turbines (Francis, Kaplan), also used in Pulp & Paper industry. Super-Martensitic grades Super-martensitic grades were developed specifically for high pressure, generally sweet gas applications for offshore use. There are grades with 2.5-6.5% nickel, some containing Mo, some without. They are produced as seamless or welded pipe, but they must be welded on an offshore pipe-laying platform. A short Post Weld Heat Treatment is usually performed (e.g. a few minutes at 600 o C). NICKEL IN PH GRADE STAINLESS STEEL
- 5. Role of Nickel in PH Grades All PH grades contain nickel, which is needed to obtain austenite to martensite transformation. Nickel gives higher corrosion resistance (general corrosion,localizedcorrosion,stresscorrosioncracking). Nickel gives improved ductility and notch toughness. Exampleof Nickelascritical raw materialin technologiesfor Mitigatingthe Climate Change and Low Carbon Economies.
- 6. 2)Chromium : Chromium is a silver-grey transition metal with a relative atomic mass 12 (C=12) of 51.996, an atomic number of 24, and a melting point of 1,875°C and a density of 7.190 kg/dm3 . It is in group VI of the periodic table. Chromium has a body-centred-cubic (B.Sc.) crystal structure. The Effect of Chromium in Stainless Steels The properties that distinguish stainless steels i.e. Fe-Cr-(Mo) alloys and Fe-Cr-Ni-(Mo) alloys from other corrosion-resistant materials depend essentially on chromium. Thehigh degreeof reactivity of chromiumis the basisfor the effectivenessof chromiumas an alloying element in stainless steels. The resistanceof these metallic alloys to the chemical effects of corrosive agents is determined by their ability to protect themselves through the formation of an adherent, insoluble film of reaction products that shields the metal substrate from uniform and localised attack. Theprotective filmcalled passivelayerorpassivefilm. Itis a veryfine layer on the surface, of the order of 1.0 to 2.0 nm, which reduces the corrosion rate to negligible levels and has a structure similar to chromite.
- 7. For passivation to occur and remain stable, the Fe-Cr alloy must have a minimum chromium content of about 11% by weight, above which passivity can occur and below which it is impossible. The corrosionresistanceofFe-Cralloys tends to improveasthe chromium content is increased, and definite changes happen at about 11% Cr, and again around 17% Cr. USES OF CHROMIUM : Chrome Metal Chrome metal consists of nearly 100% chromium. Impurities include Fe, Al, and Si. There are two ways of producing chromium metal, the aluminothermy process chrome oxide and the electrolytic process using ferrochromium or chromic acid. Chromium metal standard grades range from 99% to 99.4%. Higher grades areavailable as 99.6% Cr and degassed quality with 99.8% Cr and low gas. Chrome metal is mainly used in the production of specialty alloys, nickel and cobalt -based alloys (super alloys) where low iron is required. Due to their unique high temperature and corrosion resistanceproperties, these high performancealloys are used in the mostcritical environments, such as aeronautic, oil & gas production, land based turbines, petrochemical and chemical processing. In addition, chromium metal powder is used in the production of welding electrodes and cored wires, aluminium briquettes and master alloys. SPECIAL STEELS Chrome brings amazing properties to the metals with which it is alloyed. Add it to carbon steel in sufficient quantities and the steel miraculously becomes “stainless” – in other words, corrosion resistant, mechanically strong, and heat resistant, hard wearing, shiny and glamorous. Stainless steel, which accounts for some 66% of the use of chrometoday, is found
- 8. everywhere in modern life, from nuclear reactors to exhaust pipes, architecture, kitchenware and a host of other applications. The corrosion resistance and shiny appearance of stainless steel come from an extremely thin, continuous chromium-oxide film which spontaneously forms on the surface of the steel, in the presence of air. This film renders the surface inert to chemical reaction, thus protecting steel fromcorrosiveattack. Should the surfacebe damaged or scratched, this “passive” layer instantly re-forms. Thanks to chrome, stainless steel literally self-heals. In fact, chrome is the one and only magic ingredient making stainless steel “stainless”, whatever the grade. Speciality steels producedfor applications suchas tools, injection moulds, camshafts, dies, bearings and mill rollers also derive the high mechanical strength, hardness and heat-resistance required from their chrome content. Certain exceptionally demanding applications require an alloy known as chrome metal, which, is almost pure chrome (99%). Chrome metal provides the solidity and resistance to wear and high temperatures required for critical applications in the aircraft, gas, petrochemical and nuclear sectors. Chrome is also used in alloyed cast irons, to bestow hardness and resistance to abrasion and impact. These alloys are used for applications such as pumps, valves, pipes, rolls and wear plates. OTHER USES OF CHROME Chromium and health Chromium is a metallic element which exists primarily in the mineral, chromite. This mineral is the main constituent of chromeoreand the only commercial source of chromium. After processing, chromium occurs in several forms, also called oxidation states... Chromium in cement World-wide, there is an annual production of some 1.5 billion tonnes of cement - the concrete of our modern global infrastructure, fromroads to
- 9. houses, from dams to water treatment systems, from schools to hospitals... Chromium and leather Making leather from the skins of animals, reptiles and fish is one of society'soldestindustries. Mostleather is madefromthe hides of animals reared for their meat and hence tanning converts what would otherwise be waste products from the food industry into valuable retail products... 3)TUNGSTON : Tungsten is known chiefly for its strong tendency to form extremely hard and stable_carbides. This property, in fact, is the basis for the most impo rtant use of tungsten insteels: as a constituent of high speed and other t ool steels. About 30% of all tungsten_produced is used in this fashion. Ca rbide cutting tools account for one-half of the totaltungsten market; allo y steels, superalloys, lamp filaments and miscellaneous applications make up the remainder. The economic history of tungsten has been one of the mosterratic of all steel alloying_constituents. Prices have risen and fallen abruptly in irregu lar cycles throughoutthe_years. Adequate supply has been an occasiona l problem, despite massivegovernment_and private stockpileaccumulat ion. Ithas been estimated that China holds over 50% of world tungsten reser ves, followedby Canada and the U.S. Bolivia, Peru, Thailand and Korea al so have important_reserves. TheChinese and, more recently, the Russia ns have taken over. Effect of Tungsten Alloy Tungsten is a refractory metal with a high melting point and a very high density. Itcan be used in a pure form but it becomes more useful as an engineering material when alloyed with small quantities of other el ements to form a group of products sometimes_referred to as Tungs
- 10. ten Heavy Metal Alloys (WHAs). These alloys usually contain 90-97% tun gsten and initial forming requires a_process of pressing and sintering. Wrought shapes can then be produced however near-final-shapesinteri ng is more common. Most of the major applications for tungsten alloys are based on its very high density where it is used to control or distribute weight i n_some way. Tungsten is up to 65% denser than lead and 130% denser than steel. Radiation shielding is a second common application area. Tungsten alloys generally have high strength and good creep resistance however at low temperatures ductility is poor making it unsuitable for some applications. Corrosion resistance and magnetic properties canal so be factors in alloy selection. Tungsten can be used in a granular form to provide balance weight when used as simple ballast. However, more advanced applications form an actual component out of a suitable tungsten alloy making it a functioning part of a final assembly. Examples of this are found in aerospace(wing balances),defence(missilefin balance) and motorsport. Other application areas include computer disk balances,
- 11. ordnance (kinetic energy_penetration), marine balancing components a nd gyroscope components.