Nickel based alloys- Applications, corrosion resistance and welding behavior
Nickel with its extensive characteristics and applications, has now become a root of the industrial processing primary minerals. It is combined on the vast level with other metals to produce alloys with specific characteristics for example ductility, extremely high temperature strength, corrosion resistance, magnetic attraction and low expansion. With these and other characteristics, nickel offers crucial advantages to manufacturers of the finished products in the several ways of final applications such as cars, jets, ships, electricity production, process apparatus, IT, construction and building, cutlery, coins, medical equipments and others.
Nickel renders with crucial benefits to produce alloys and to manufacturers of several finished products. It hence keeps a complex value-chain in creating output and wealth.
The resulted Nickel based superalloys are an outstanding class of structural materials for the elevated temperature applications in the specifically challenging conditions of turbine section of the aircraft engine. Regular enhancements in the characteristics of these materials have been feasible by close control of chemistry and microstructure and the introduction of advanced processing techniques. Additional enhancements are with the development and use of tools for alloy design and microstructure process evolution.
Ultimate applications of Nickel Alloys
Nickel containing alloys are used on the vast level in the products however there are several crucial applications of nickel where nickel based alloys considerably transform with the production procedure or the final product being produced. Additionally, in the several of these uses, this conversion cannot be obtained in any other way or solely by the considerable compromise with the product performance or modifying the production procedure. The products made in this way can be crucially based on nickel such as:
Turbochargers- Nickel is utilized in manufacturing housings and turbine wheels in turbo-chargers (to improve functionality, enhance fuel consumption and decrease emission of pollutants).
Jet Engines- the entire components of jet engine that operate in the very high temperature media are made from nickel based super alloys. Moreover, most of the steels utilized in the production of shafts also include nickel to enhance the performance at high temperature and reduce corrosion.
Industrial & marine gas turbines- Nickel based super alloys are significant for the competitive functionality of the gas turbine. They are used in manufacturing modern, complex gas turbines for power production, oil refining, petrochemicals and oil production, oil and gas refining. In the oil refinery industry, for instance nickel based catalysts, by their crucial role in reforming, hydrocracking and desulfurization, empower oil refiners to meet the customer needs and standards for controlled sulfur fuels, simultaneously enable to reduce the raw material costs by offering vast use of economical crude oils and by improving energy efficiency. Nickel based catalysts also increase operation output and hence control the product price by enhanced latest conversion and upgrading technologies utilized to increase output of the lighter transport fuels.
Process plant equipment- the industries related with food and beverages, oil and gas refining, petroleum refining, chemical and medicine are crucially based on nickel. The principal application of this metal is in alloys development such as stainless steels however, in particular conditions of elevated temperature or corrosive media, nickel alloys are used instead stainless steel.
Commercial market- the characteristics of stainless steels that are mostly recognized by the buyers of commercial catering equipment include corrosion resistance, easy fabrication, heat resistance and quick cleaning.
CD/DVD- Electroformed nickel is utilized in the formation of stampers that are utilized in manufacturing CDs and DVDs. Plated nickel is also utilized in the final steps of producing glass master from which the stampers are made.
Medical and dental equipments: Stainless steels products for example injection needles, pincers, drills and surgical apparatus, and specialized hospital furniture.
Melting and refining of high temperature steels and super alloys
The recent increase in count and capacity of special melting techniques and refining procedures is associated to regular development of high temperature alloy applications in the wrought and cast form as well as the metallurgical and technological strength of traditional melting and casting procedures have widely exhausted. The inception of the major melting procedures for high functional materials and the increasing demand for advanced melting and refining operations into unique individual performance for the highest use of every unit are outlined. Use of auxiliary refining apparatus after air melting is demanding for producers of specialty steels but has also application for nickel base alloys. More auxiliary refining technologies are stated in the light of their capability of several functions such as degassing, desulfurization, carbon deoxidation, alloying and decarburization.
The metallurgical process properties, basic properties and drawbacks of the different production and pilot melting techniques are compared with each other on base of their strength. The advanced wrought alloy production procedures offer versatility in choosing the most suitable combination of the specific melting procedures for the various alloys to obtain the superior quality material at the reasonable price.
High temperature steels and superalloys describe the class of materials that offer significant strength features at temperatures above 550oC or 1022oF. The super alloys widely specified as materials with outstanding oxidation resistance properties for applications at high stress levels at temperatures above 650oC or 1202of.
A perfect melting and casting process is the one that provides-
- Clean and reliable product with the minimum concentration of gases, contaminants and nonmetallic additions
- Outstanding control of chemistry and high and suitable recovery of composition elements
- Good control on efficient and quick refining reactions
- Contamination – free from container and atmosphere
- Versatility and adaptability for diverse alloys and materials
- Suitable control on solidification procedure
- Good reliability and productivity at low price
It is totally unfeasible to combine all of these benefits in single melting, refining and casting unit however efforts have been made. The key melting methods for special alloys are categorized into primary and secondary procedures. The primary melting methods are used to produce or synthesize the alloys from the pure metals or scrap material.
Secondary melting, refining and casting have different treatment objectives. Major emphasis is on solidification control to develop an ingot product of high structural integrity for after thermomechanical processing.
Electric Arc Melting
With the latest demand to duplexing operations, the electric furnace has been selected as basic melting unit and refining and alloying are conducted more specifically and efficiently in the separate auxiliary installations. The principal features of the electric arc furnace are:
- Wide flexibility in charge material conditions
- High temperature and quick operation control
- Hot, reactive slag for intensive metallurgical work
- Slag reactions adaptable to specific needs
- High productivity, efficiency and small melting cost
Many basic melting practices for complex high temperature alloys have been developed to tailor the working procedures to specific material needs.
Super alloys are an exceptional class of metallic materials with an outstanding combination of high temperature strength, hardness and cracking resistance in the corrosive or oxidizing media. These materials are extensively utilized in aircraft and power production turbines, rocket engines and other severe media such as nuclear power and chemical treatment units. Exhaustive alloy and processing activities in the past years have produced alloys that withstand average temperature of 1050oC with occasional excursions to increasing temperatures up to 1200oC that is around 90% of the melting point of a material.
Nickel based superalloys generally are consumed by 40 to 50% in the aircraft engines and are widely used in combustor and turbine engines where high temperatures are continued during the operation. The creep resistant turbine blades and vanes are generally produced by complex investment casting procedures that are important for introducing extended cooling systems and controlled grain structure. These components may comprise of equiaxed grains or columnar grains or single crystals, fully discarding the high angle grain boundaries. Because grain boundaries are areas for damage at the elevated temperatures, the blades in the beginning level of the turbines are produced from equiaxed alloys. The structural parts for example engine cases are also manufactured by investment casting procedures. The turbine disks are formed through wrought processing approaches that involve using cast ingots or consolidated super alloy powder performs. Outstanding combinations of strength, hardness and crack development resistance can be obtained in these materials by close control of microstructure through the various levels of wrought processing.
Super alloy processing starts with manufacturing large ingots that are then utilized for one of three principal processing treatments – remelting and investment casting, remelting after wrought processing and remelting to produce super alloy powder that is subsequently consolidated and subjected to wrought processing applications. The ingots are produced by vacuum induction melting (VIM) for example in a refractory crucible to consolidate elemental and/or revert materials to produce a base alloy. However some nickel based super alloys can be melted in air or slag conditions using electric arc furnaces, VIM melting of super alloys is extremely beneficial in discarding the low melting point trace contaminants.
Cast Super alloys
Investment casting is basically the casting procedure for fabrication of super alloy components in the composite shapes such as blades and vanes. The ceramic molds comprising of alumina, silica or zirconia are used in this procedure. The molds are formed by progressive formation of ceramic layers around a wax pattern of the cast material. The ceramic cores can be embedded in the wax to receive complex internal quenching structures. A thermal cycle eradicates the wax and remelted superalloy is poured in the mold in a preheated vacuum container to receive the desired shaped casting. The mold is removed as soon as the alloy is quenched to room temperature.
Casting may be equiaxed, columnar grained or single crystal. Equiaxed castings harden uniformly across their volume and single crystal castings are withdrawn from a warm zone in the furnace to a cold region at a controlled rate. In the entire casting procedures the final structure of the material is sensitive to thermal conditions that occur while hardening of castings.
Wrought alloys are generally produced by remelting of VIM ingots to produce a secondary ingot or powder for immediate deformation processing. The secondary melting process is essential for wrought alloys because the elevated temperature characteristics of Nickel based super alloys are extremely sensitive to microstructural changes, chemical non-uniformity and additions. With increase in ingot size, VIM melting usually results in macrosegregation or the production of big contractual cavities while solidification. The production of such solidification flaws is resulted by the large scale segregation linked with the dendritic solidifications at small thermal gradients.
Super alloys contribute a large part of construction materials in turbine engines due to their exclusive set of physical and mechanical characteristics. Optimization of the relevant set of mechanical characteristics is of great importance and is based on a large level of control, as the mechanical characteristics are a strong performance of microstructure. The mechanical structures of primary interest include tensile characteristics, creep, fatigue and cyclic crack development.
Nickel based super alloys have comparatively high yield and ultimate tensile strengths and yield strengths range in 900 to 1300 MPa and ultimate tensile strengths of 1200 to 1600 MPa at room temperature. The turbine disk alloys are normally made to have greater strength for flexibility at temperatures below 800oC in design to secure against burst of the turbine disk in the event of an engine overspeed. The tensile characteristics do not significantly loss as long as the temperatures are more than about 850oC. The slight increase in yield strength of alloys at medium temperatures is because of abnormal flow nature of the Ni3Al gamma prime phase.
Since the super alloys undergo large stress at the elevated temperatures, extensive resistance to time based creep deformation is important. It is extremely crucial for cast blade alloys however they will undergo temperatures about 1100oC whilst the disk alloys are generally limited to operation temperatures below 700oC. For a constant temperature and stress, dual phase super alloys have greater creep resistance as compared to their single phase counterparts. Considering the entire characteristics are leaded by the plastic deformation procedure, creep characteristics are sensitive to microstructure.
Regular advancement in the metallurgical and production techniques has spurred the development of nickel alloys and their extensive use in the chemical industry.
The nickel alloys introduce a combination of outstanding corrosion resistance, strength, hardness, metallurgical stability, fabricability and weldability. Most of nickel alloys offer excellent heat resistance making them fit for applications that need high temperature chemical resistance and strength.
Nickel alloys are costlier than stainless steels. Although the price evaluation corresponding to their service life and performance, the investment on nickel alloys would be lower. Owing to the outstanding corrosion resistance of nickel alloys, the initial cost can usually be recovered by prolong savings because of extended service life, lower maintenance and lesser downtimes.
The physical characteristics of nickel alloys are almost similar to austenitic stainless steel series 300. They exhibit thermal expansion equivalent to carbon steel but lower than SS 300 series.
However the thermal conductivity of pure nickel is better than carbon steel, many nickel alloys have noticeably smaller conductivities in few cases in fact smaller than austenitic steel family. Except pure nickel, the nickel based alloys for chemical processing applications are significantly stronger than SS 300 series. Nickel alloys also have excellent ductility and hardness.
Nickel alloys have completely austenitic microstructures. Almost all grades used in the chemical industry are solid solution strengthened. They receive their improved strength characteristics from the inclusion of suitable hardeners like molybdenum and tungsten instead from carbide formation. Similar to austenitic stainless steels, solid solution nickel alloys cannot be reinforced by heat processing only through cold processing.
Another extensive series of nickel based alloys are reinforced by precipitation hardening heat processing. They are broadly used in ultrahigh strength applications like those occurred in deep oil and gas production and extensive high pressure procedures.
Excluding the chosen parts in valves and rotating machines, precipitation hardened nickel alloys find limited application in the chemical units. This class of alloys called heat resistant superalloys are used in gas turbines, combustion chambers and aerospace applications.
Nickel alloys are much better than from traditional stainless steels and super-austenitic iron based alloys in preventing corrosion by a wide range of acids, alkalis and salts. An excellent feature of nickel alloys is outstanding resistance to aqueous solutions comprising of halide ions. In this regards, nickel alloys are far much better to austenitic stainless steels that are sensitive to corrosion by wet chlorides and fluorides.
The outstanding corrosion resistance nature of nickel alloys manifest not only counting the smaller metal loss, but also the potential to better withstand localized corrosion, strong pitting or crevice attack, intergranular corrosion and stress corrosion cracking. These kinds of localized corrosion are more than general thinning, account for the most of the corrosion based damages in the chemical industry.
Nickel alloys offer their corrosion resistance properties by lower reactivity of nickel as determined by its nobler oxidation potential in the EMF series. Like stainless steels, chromium containing nickel alloys have the ability to passivate. The supreme benefit of nickel over iron is its ability to accept wide fractions of alloying elements without producing brittle phases. The popular alloying elements for obtaining great corrosion resistance are chromium, molybdenum and copper.
Most of nickel alloy welding are done by shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW). The nickel alloy weldments are extremely ductile and their controlled thermal expansion properties decrease residual stresses and warpage. The postweld heat treatment is normally needed only for precipitation hardenable grades.
The welding processes for nickel alloys are specifically similar to those implemented on austenitic stainless steels. Although, because of wider indolence of nickel rich weld puddles and smaller penetration properties of nickel alloys, the formation of complete penetration welds may need modification of joint configurations and welding methods.
The combination of high ductility, small thermal expansion and potential to withstand dilutions by various metallic elements has made nickel rich welding consumables widely accepted for joining various metals. It includes welding of nickel based alloys to iron based alloys and also welding of stainless steels to carbon and alloy steels. Identically nickel alloys can be weld accumulated on carbon steel without the threat of fracture.
Chemical composition of popularly industrially used Nickel based super alloys
|Nickel 200||N02200||99.6 %||–||–||–||–||–||–|
|Monel 400||N04400||66.5 %||–||–||1 %||–||31.5 %||Mn 1|
|Inconel 600||N06600||75 %||15.5 %||–||8 %||–||–||–|
|Inconel 625||N06625||62 %||21.5 %||9 %||2.5 %||–||–||(Nb + Ta) 3.8|
|Inconel 690||N06690||61 %||29 %||–||9 %||–||–||–|
|Incoloy 825||N08825||42 %||21.5 %||3 %||29.5 %||–||2.3 %||Ti 1|
|Hastelloy G-3||N06985||44 %||22 %||7 %||19.5 %||1.5 %||2 %||Nb 2.1|
|Hastelloy G30||N06030||43 %||29.8 %||5 %||15 %||2.8 %||1.7 %||Nb + Ta 1|
|Hastelloy C276||N10276||57 %||15.5 %||16 %||5.5 %||3.8 %||–||–|
|Hastelloy C22||N06022||56 %||22 %||13 %||3 %||3 %||–||–|
|Hastelloy C2000||N10200||59 %||23 %||16 %||1.5 %||–||1.6 %||–|
|Inconel 622||N06022||58 %||20.5 %||14.2 %||2.3 %||3.2 %||–||–|
|Inconel 686||N06686||60 %||21 %||16 %||5 %||3.7 %||–||–|
|Alloy 59||N06059||60 %||23 %||15.8 %||1.5 %||–||1.6 %||–|
|Hastelloy B2||N10665||69 %||1.0 %||28 %||2 %||–||–||–|
|Hastelloy B3||N10675||68.5 %||1.5 %||28.5 %||1.5 %||3 %||–||–|
|Hastelloy B-4||N10675||68.5 %||1.5 %||28.5 %||1.5 %||3 %||–||–|
|Alloys||Ultimate tensile strength||Yield strength, ksi||Elongation, %|
|Nickel 200||55 ksi||15 ksi||40 %|
|Monel 400||70 ksi||28 ksi||35 %|
|Inconel 600||80 ksi||35 ksi||30 %|
|Inconel 625||110 ksi||55 ksi||30 %|
|Inconel 690||85 ksi||35 ksi||30 %|
|Incoloy 825||85 ksi||35 ksi||30 %|
|Hastelloy G-3||90 ksi||35 ksi||45 %|
|Hastelloy G30||85 ksi||35 ksi||30 %|
|Hastelloy C276||100 ksi||41 ksi||40 %|
|Hastelloy C22||100 ksi||45 ksi||45 %|
|Hastelloy C2000||100 ksi||45 ksi||45 %|
|Inconel 622||110 ksi||51 ksi||45 %|
|Alloy 59||110 ksi||51 ksi||40 %|
|Hastelloy B2||110 ksi||51 ksi||40 %|
|Hastelloy B3||110 ksi||51 ksi||40 %|
|Hastelloy B-4||110 ksi||51 ksi||40 %|