High Performance Alloys – a short history
Almost 250 years ago in November 1751 nickel was discovered as an element. Monel*400 , a nickel copper alloy was one of the first high nickel alloys introduced to industry in 1905 . Today almost 100 years later it is still widely used and will continue to be used due to its outstanding corrosion resistance in many industrial applications.
Before the 1950’s the choice of alloys to combat corrosion was very limited. The latter half of the 20th Century saw a phenomenal growth in the development of new nickel base alloys for highly corrosive applications. This has primarily been due to the
excellent compatibility of nickel with other alloying elements.
Key factors in these developments have been improved melting technology and innovations in thermo-mechanical processes. In additions there has been a great advancement in the understanding of various roles of alloying elements such as chromium ,molybdenum, cobalt,copper, niobium etc .
The life of nickel alloys in the 20th Century
|Pre 1950’s||Stainless Steel 300 series Monel* 400 Inconel 600, Alloy B and C|
|1950’s||Alloy 20Cb Incoloy* 800 and 825 Alloy F and Alloy X|
|1960’s||Stainless steel 300 Low Carbon series Alloy 20 Cb3, 904L , 6mo Series duplex stainless steels Inconel 700 and 625 Alloy G and Alloy C276|
|1970’s||317LM , 254Smo Alloy 28 , Alloy c4 and B2|
|1980’s||Alloy G30 and 22|
|1990’s||Inconel* 686 , 690 and 693 Memory Shape alloy|
Todays engineers and designers have a wide spectrum of corrosion resistant alloys
to chose from when trying to prevent a corrosion problem developing or trying to solve an existing problem. As can be seen from the alloy groups table as one moves up from the 300 series stainless steels in group I to the materials in the higher groups the corrosion resistance in specific environments improves as the alloy content increases. This is evidence by the calculated PRE ( Pitting Resistance Equivalent) .
|Alloy Group||Generic Description||UNS no||PRE’s|
|Group I||Fe based 18/8 austenitic stainless steels||S30400,S31600,S32100 etc||24|
|Group II||High performance austenitic stainless steels||N08028,N08020 etc||29 – 38|
|Group III||Ni based general purpose alloys||N02200,N04400,N06600
|32 – 45|
|Group IV||6% molybdenum superaustinetic stainless steels||N08926,N08267, N08904 etc||37 – 48|
|Group V||Nickel based high performance alloys||N06686,N10276,N06022||65 – 74|
The drive to develop improved corrosion-resistant alloys has been lead by the need to increase production efficiency, leading to the introduction of higher temperatures, higher pressures and more aggressive catalysts in the petrochemical and chemical industries. Hence the need for better performance from the materials used in process equipment manufacture and the demand for shorter and fewer maintenece shutdowns. In addition the need for environmental protection measures has meant that these new alloys have to posses the necessary versatility to handle highly variable conditions.
Installing a high-alloy stainless steel and nickel alloy liner in a flue gas desulphurisation duct
In the past research revealed that the presence of chromium and molybdenum improved localised corrosion resistance but also that increased nickel and nitrogen enhanced resistance to stress corrosion cracking in chlorine environments. This lead to a class of alloys being developed which were a cost effective way of dealing with these problems in many applications. The 6Mo alloys are used extensively in pulp and paper the production of phosphoric acid , copper smelting , production and reclamation of sulphuric acid , marine and offshore applications , heat exchangers using seawater and brackish water as coolant and pickling baths.
Nickel Based Alloys
The combination of a high melting point, a face-centered cubic crystal structure, an adherent oxide, and good alloying ability has allowed nickel to form the basis of a wide range of heat- and creep-resistant alloys that are essential materials in the chemical and aerospace industries.
80% Ni- 20% Cr alloys have long been used as heating elements. Additional alloying elements such as cobalt, molybdenum and tungsten provide solid solution strength; aluminium and titanium additions give precipitation hardening; chromium improves corrosion resistance; small amounts of carbon, zirconium and boron are important for strength and ductility; oxide dispersions can provide additional strength; and single-crystal components can offer improved creep resistance.
With all these variables, the composition must be carefully balanced and processing tightly controlled. Remarkably, some of these materials can be stronger at their operating temperatures than mild steel at room temperature. Yet new materials continue to achieve still-higher operating temperatures –eg, intermetallics such as nickel aluminide.
Cast high nickel-base alloy blades and vanes in an industrial gas turbine
Nickel-based materials have a number of special properties that open up additional applications, Nickel-iron alloys have low expansion characteristics as a result of a balance between thermal expansion and magnetostrictive changes with temperature. Originally used in clock pendulums, these alloys are now widely employed as lead-frames in packaging electronic chips and in shadow-masks in television tubes. On a much larger scale, they provide one way of coping with the thermal expansion of storage and transportation tanks for liquid natural gas.
Liquid natural gas storage tank lined with low expansion 35% Ni-Fe alloy — Courtesy of Gaz-Transport
Equiatomic nickel-titanium shape memory alloys have gone from being mere curiosities to having real applications. Components are formed into shape at an elevated temperature. Deformation at the lower temperature causes a martensitic transformation — this can be reversed by reheating so that the components regain their original shape. The transformation temperatures are determined by composition and processing. Current applications include actuators, hydraulic connectors and spectacle frames. Superelastic alloys are closely related materials that can undergo large elastic strains without plastic deformation. Medical devices and mobile telephone aerials are two applications in which this property is exploited.
Nickel also plays a part in portable power provision. Nickel-cadmium rechargeable batteries containing nickel plates and nickel hydroxide have been in use for several years. More recently, we have seen the introduction of nickel metal-hydride batteries, which employ some nickel rare-earth alloys to absorb large amounts of hydrogen. These higher-performance rechargeable batteries have, in turn, led to improved performance from cordless power tools, portable computers and other mobile electronic equipment. The hydrogen storage alloys may find wider application if greater use is made of hydrogen as a fuel.
There are over 150 alloys available within the nickel alloys range . The table below show some applications for the some of the widely used alloys in the high nickel range .
Applications for Nickel Alloys
|Nickel Alloy||Properties||Some Applications|
|Monel* 400||Resistant to seawater and steam at high temperatures and to saline and caustic solution||Shipbuilding offshore and marine technology feedwater heaters in fossil fired power stations.|
|Incoloy* 800/H||Resistant to stress corrosion cracking and polythionic acid cracking .||Steam generators and feedwater heaters in nuclear power plants ammonia plants handling very high temperature fluids.|
|Incoloy* 825||Resistance to stress corrosion cracking in sulphuric and phosphoric acid Good resistance to in sodium and potassium hydroxide up to 190Deg C||Nuclear waste re-processing, chemical plant , sour gas oil and gas . pickling plants , concentrated caustic soda|
|Inconel* 600||Good resistance to high temperature oxidation , chloride-ion stress corrosion cracking and caustic corrosion.||Thermal processing, chemical and food processing, nuclear steam generators , moderator coolers|
|Inconel* 625||Excellent resistance to pitting crevice and stress corrosion cracking. Highly resistant to a wide range of organic and mineral acids. Good mechanical properties up to 450deg C||Flu Gas scrubbers, superphosphoric acids, nuclear waste processing,. Downhole equipment in sour gas production. Offshore and onshore piping systems|
|Inconel* 686||Corrosion resistant superalloy . resistant to oxidizing and reducing acids, mixed acids, and localised corrosion.||Flu Gas desulphurization systems , aggressive chemical and process environments. Nitric acid and hydrofluoric acid processing.|
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