High strength Material for turbine engine components

There are essential differences in the structure and service conditions of industrial and aero-gas turbines that prevent the direct use of standard alloys in the design of industrial engines. Different alloys are used. Blades for industrial gas turbines are often made from super alloys containing high chromium concentration that have been evaluated for service in vigorous conditions whilst those for aero gas engines are made from low carbon high strength alloys.

Blades made for industrial gas turbines are significantly larger than those for aero-engines. It results into significant differences in the process factors during directional solidification that could influence the structural and properties of the casting. In specific, slow heat flow through larger castings will influence the cooling rate and the larger mass of molten metal could cause issues in the containment by current moulding materials.

Industries demand longer lives from the aero-gas turbines that requires better corrosion resistance. Practical engine trials that are significant in the evaluation of aero engines, are impractical for industrial gas turbines, consequently suitable component design by using materials required for service in the required environment.

The industrial gas turbines are made from alloy that is directionally solidified by using lab and commercial observation. The operation of industrial gas turbines with specific emphasis on long term properties and on both the anisotropy and heterogeneity of the castings.

A major part of the turbine design is the selection or development of alloys suitable for application in the turbine. As long as candidate alloys are chosen, basic tests are performed on the alloys that are already selected such as nickel base super alloys Inconel 617. These alloys have very high strength and oxidation resistance at atmospheric pressure.

Initial oxidation tests are performed on the test materials to ensure that they can perform properly and can benefit the boiler research. Higher heating value also signifies the performance of alloys. Ferritic stainless steels comprising of 9 to 12% are presently used at steam temperatures up to 600oC. Most studies of the upper temperature limit are up to 650oC with high temperature strength as a limiting factor.

Austenitic stainless steels keep their strength at higher temperature as compare to ferritic alloys. Although vigorous thermal fatigue issues prevented their regular use at the original design temperatures and pressures. As thermal fatigue becomes more a problem in thicker component parts, austenitic alloys are still useful in the specific thinner components. They are used in boiler sections of advanced power stations.

Advancements in the application materials have contributed in a major way in the development of gas turbine engines with higher power ratings and efficiency levels. Enhancements in the design of gas turbine engines over the years have been possible due to the development f materials with improved performance. Gas turbines are widely used in aircraft engines and land based applications for power production.

The major use of Inconel wire made from alloy 617 is in the improvement of high temperature creep rupture strength without affecting oxidation and corrosion resistance. It is commonly used in manufacturing combustion system components for higher creep rupture strength.

Insight of Woven Mesh screens used in industrial services

Woven wire mesh is processed in various ways to develop the filter screen. Wire cloth and gauze are commonly use words to refer meshes made from fine wire grades. Bolting cloth refers to lightweight forms of square mesh clothes made from the finest wires.

A wide range of mesh screens are made by weaving wires separately. Two major categories of mesh screens are plan weave and zero aperture filter clothes.

In a woven mesh screen, each warp or weft wire bends where it passes over or beneath the wire of other type. Wire crimping occurs during the weaving process for fine wires, however the crimp has to be imposed on the wire above a specific thickness, before it is fed into the position. However this includes a stage to the weaving process, it has a significant advantage of retaining the crossing wire in a place.

Woven wire mesh screens are widely used for filtration for more than a century. They are made in diverse range of materials and specification. The mesh can be woven from virtually a metal with sufficient ductility to be drawn into wire form, specifically nickel-chrome steels, Monel mesh screens and other nickel base alloys. Nickel and nickel alloys are used for high temperature services. These alloys include Inconel, Hastelloy and Incoloy.

The least useful size of wire used is based on the alloy from which it is made, the mesh strength, the service temperature and corrosion level and abrasion probably occur during the service. So, the finer wire diameters in aluminium, copper etc are not usually used for other than ordinary applications. Stainless steel mesh are made from wires under 15 micro-m.

The plain dutch weave is a zero aperture weave. It is a plain weave with larger diameter wires similar to warp and straight, while the weft wires are crimped at each pass. The mesh developed from this weave extends from 340 micro-m to 15 micro-m in aperture size. The holes are small, and not straight through the mesh. The cloth is durable and compact with high strength. Two differences of this type of mesh occur, first that includes two warp wires rather one and that is usually selected for holes below 14 micro-m or when higher strength is needed. Other type includes finer warp and weft wires leading to better flow rates and higher tolerance.

In a reverse plain dutch weave, everything followed is similar, except the factor that thicker wire treats as weft. This type of mesh is substantially stronger and is even the most durable weave to produce filter mesh for commercial filtration applications. It features good flow and high dirt retaining capacity, for which it is widely used on the industrial scale.

With similar combination of warp and weft wires, two basic types of twilled dutch weave are developed. Using heavy warp wires in this weave allows the production of the finest grades of woven wire cloth, while also offering the benefit of smooth surface on both sides of the mesh. Dutch weave has less flow resistance and also rough surfaces on both sides.

Recommended super alloys for control on corrosion

It is well known that the major saving is great by using available and economic practices to enhance corrosion prevention and control. The designer should consider the initial material cost and also include maintenance cost, service life, downtime cost and replacement cost. This type of analysis helps better understand the cost-effectiveness of corrosion resistant materials.

Studies have stated that the overall cost of corrosion is surprising. The total cost of corrosion in U.S. is about $276 billion a year. About 25 to 305 of cost of corrosion is preventable and can controlled by using corrosion resistant materials and application of best anticorrosion technology from the design through maintenance. As becoming commercially available material, Nickel has become a crucial material in preventing corrosion. It is a major element in the coating and claddings applied to steels, copper-nickel and nickel-copper alloys as well as high performance nickel alloys. Nickel is not just a corrosion resistant element, it also has high tolerance for alloying that makes it possible to develop various high performance, special purpose alloys.

Nickel-Chromium alloys can be considered as the base for other alloys. Chromium offers resistance to oxidizing conditions and high temperature strength. It helps alloys resist stress corrosion cracking and corrosion in nitric acid, steam, and oxidizing gases. Alloys with high chromium contnent resist melting sulphates and vandates found in the fuel ash. Resistance to high temperature oxidation is also enhanced by alloying with aluminium in conjunction with high chromium. Nickel is used as a corrosion resistant material in food processing and in high temperature caustic and gaseous chlorine or chloride conditions.

Alloys offering significant performance in aqueous reducing acids, alloys for oxidizing chloride conditions and seawater- Inconel 625 and Hastelloy C276. Cobalt and other alloying element inclusions create materials for use in jet engines that combine high temperature strength with resistance to gaseous oxidation and sulfidation.

Other technologically important materials are high iron alloys that are made to conserve nickel and are usually considered as moderate in performance and cost between nickel alloys and stainless steels. Incoloy 800 is a general purpose alloy that has good high temperature strength and resistance to seam and oxidizing or carburizing gases. Addition of molybdenum and chromium similar to in Incoloy 825 and Hastelloy G enhances resistance to reducing acids and localized corrosion in chlorides.

Other essential group of alloys is nickel-copper alloys. Containing higher nickel, Monel alloys are the best for use in corrosive chemical conditions such as with hydrofluoric acid and vigorous marine conditions. Alloys with higher copper content such as copper-nickel alloys are commonly used in marine conditions for providing high fouling resistance.

Nickel base alloys offer outstanding resistance to corrosion in nitridingconditions and in chlorine or chloride gases. Corrosion in the chloride conditions at the high temperatures speeds up with the formation and volatization of chloride layers and high nickel alloys offer suitable performance because nickel develops least volatile chlorides. On the other hand, in sulfidiing conditions, high nickel alloys without chromium content can be corroded due to the development of low melting temperature Ni-Ni3Si2 eutectic.  Here Nickel-Chromium alloys are fit to use.

In demand grades of Nickel-Chromium-Iron alloys

Inconel 600

Inconel 600 offers outstanding mechanical properties and high strength and supreme workability.  It adequately performs up to 1200oF. Inconel 600 resists corrosion in fresh waters, including the highly corrosive natural waters comprising of free carbon dioxide, iron compounds and dissolved air. It remains free from stress corrosion cracking in fact in boiling magnesium chloride.

Alloy 600 offers supreme resistance to sulfuric acid in the oxidizing conditions than Nickel 200 or Monel 400. Inclusion of oxidizing salts to sulfuric acid passivates Inconel 600 that makes it fit for use with acid mine waters where Monel  400 cannot be used.

Inconel 600 is not susceptible to stress corrosion cracking in the chloride salts and has supreme resistance to the nonoxidizing halides. It offers outstanding resistance to dry halogens at high temperatures and is used successfully in the chlorination plants up to 1000oF. This alloy replaces Nickel 201 in specifically high temperature applications where sulfur offers enhanced resistance.

Incoloy 800

It is basically used for oxidation resistance and strength at high temperatures. Alloy 800 is used in high temperature equipments because it does not develop the embrittling delta phase after long exposures up to 1600oF. It offers high creep and rupture strengths that add in its functionality in the numerous applications.

At moderate temperatures the general corrosion resistance of Incoloy 800 is similar to other austenitic nickel-iron-chromium alloys. With increase in temperature, alloy 800 attains supreme corrosion resistance whilst other austenitic steel grades fail to perform.

Incoloy 800 offers supreme resistance to nitric acid up to 70% concentrations. Its Inconel bars resist corrosion in diverseoxidizing salts however except halide salts. It offers good resistance to organic acids like formic, acetic and propionic acids. Incoloy 800 is specifically fit for use in hot corrosive gases.

Incoloy 825

Alloy 825 is an enhanced version of Incoloy 800 containing higher nickel concentration that makes it resistant to chloride ion stress corrosion cracking. Inclusions of molybdenum and copper resists pitting in the reducing conditions for example sulfuric or phosphoric acid. Incoloy 825 prevents corrosion in pure sulfuric acid solutions up to 40% concentrations at boiling points. However it has limited applications in hydrochloric or hydrofluoric acids.

Chromium concentration in Incoloy 825 resists corrosion in diverse oxidizing conditions like nitrates, nitric acid solutions and oxidizing salts. It resists corrosion by stress cracking in neutral chloride conditions. For localized corrosion of stainless steel 300 series, Incoloy 825 is the best replacement. It also prevents corrosion in seawater.

Incoloy 800H

Incoloy 800H is a controlled carbon grade of alloy 800. Carbon concentration is maintained between 0.05% to 0.1% to offer superior high temperature creep and stress rupture properties. It is also used in diverse high temperature applications in the refining and heat processing plants.

Corrosion issues in Pressurized water reactor steam generators

Traditional domestic commercial nuclear power plants were made with austenitic stainless steel generators for their outstanding corrosion resistance, weldability and other properties. A pressurized water reactor steam generator started in 1967, is made of Inconel 600 due to its outstanding resistance to chloride stress corrosion cracking. Although since its use, wastage and intergranular  cracking of alloy 600 has occurred in plants involving the use of sodium phosphate or volatile water treatment.

Intergranular corrosion has been attributed to the presence of free caustic, there have also been cases of intergranular stress corrosion cracking on the tube used in the surface in relatively pure water. Cracking of highly stressed Inconel resulted into steam generator tube failures that was due to corrosion.

In the pressurized water reactor steam generators, the heat released from the primary coolant is used to develop steam that passes through the turbine generators to the condenser where the waste heat is removed. The steam generator tubes made from Inconel 600 and tube shells made from carbon steel, an Inconel 600 cladding to which the tubes are welded. To limit the tubes vibration, a series of carbon steel supports are used for the straight parts of the tubes and for the u tube design, a series of retstraints as antivibration bars are used for the curved part of the tubes.

Both design features and environmental factors have had a determined effect on the integrity of steam generator tubing. Contaminants in the secondary water that are allowed to focus due to their thermal and hydraulic factors that result in regions of low flow, have resulted into degradation of steam generator. The main causes of contaminants are corrosion of materials in the turbine, condenser and feedwater pipes and inleakage of the tertiary coolant in the condenser.

Secondary water chemistry:

Secondary water chemistry focuses on reducing the corrosion and prevent scale development on the high-heat flux surface of the tubing by the contaminants. Types of water chemistries like in PWR secondary water systems- all volatile treatment is increased and the oxygen scavenged by volatile additives like hydrazine or morpholine, a phosphate treatment, in which sodium phosphates are added to the coolant to raise the pH and react with scale developing contaminants to develop harmless soft phosphate precipitates, combined with inclusions of morpholine or sodium sulphite to scavenge oxygen that uses full-flow condensate demineralization to eradicate caustic forming contaminants and scale forming solids from the steam generator.

Phosphate Composition and wastage cracking

Sodium phosphate treatment is commonly used for steam generators that eliminate precipitated or suspended solids by blowdown. Different steam generators with Inconel 600 tubes recieved stress corrosion cracking. The cracking was featured to free caustic that can be developed when sodium ratio exceeds the preferred limit of 2.6.`They accumulate as sludge on the tube sheet and tube supports at the main part of the tube bundle where limited water supply and high heat flux occurs.

With the use of high corrosion resistant nickel alloys, the problems of corrosion are significantly minimized that ensure the considerably longer life of service equipments.

Corrosion resistance of Nickel alloys in dilute and anhydrous hydrofluoric acid

Anhydrous hydrogen fluoride and hydrofluoric acid are major industrial process materials. Hydrofluoric acid is highly corrosive and extremely reactive, it is used in large magnitudes for pickling of stainless steels as well as other metals. Additionally, it is used in acid treatment of wells and glass etching. Other applications of hydrofluoric acid include production of aluminium fluoride and synthetic cryolite, fluorinated organics like aerosol propellants and plastics.

Anhydrous hydrogen fluoride is the basis of fluorocarbon industry that mainly includes coolants, fire extinguishing elements, ultrasonic cleaning fluids and fluorocarbon plastics.

Hydrofluoric acid in aqueous and anhydrous form is extremely hazardous that it severely affects eyes, lungs and mucous membranes.

Anhydrous hydrogen fluoride is made by the reaction of sulfuric acid and calcium fluoride. Commonly used materials for hydrofluoric acid service are carbon and alloy steels, stainless steels, aluminium, copper and nickel and nickel alloys.

Nickel and its alloys

Nickel 200 is less resistant than Monel 400 to aqueous  hydrofluoric acid. Oxygen has higher accelerating effect on corrosion of Nickel 200. In aqueous hydrofluoric acid, use of Nickel 200 is limited to air-free systems below 80oC. However there are cases of stress corrosion cracking of Nickel 200 in aqueous hydrofluoric acid, they seem to be associated to contaminants like cupric fluoride. Nickel 200 is highly resistant to hot anhydrous fluoride however it may be embrittled by sulfur compound contaminants.

Monel 400: Alloy 400 is widely used in hydrofluoric acid alkylation units and in the production of hydrofluoric acid. It has supreme resistance to liquid hydrofluoric acid over the whole concentration range in the absence of oxygen to minimum 150oC.

Monel 400 resists stress corrosion cracking when subjected to wet vapors of hydrofluoric acid in the presence of oxygen. Intergranular cracking occurs. In one test, alloy 400 received transgranular stress corrosion cracking in the vapour phase of dilute hydrofluoric acid solutions at temperatures up to 95oC. The cracking sensitivity was not based on the presence of oxygen and no cracking was noticed in the liquid phase.

The cracking mechanism was unknown until it was noticed that aqueous hydrofluoric acid solutions comprising significant concentrations of cupric chloride would result in fast cracking of stressed Monel 400. Nominally resistant nickel-copper composition corresponds to that Monel 400.

Stress corrosion cracking is often limited to the vapour phase rather than liquid is enrichment of a thin layer of aqueous hydrofluoric acid in the vapour with copper fluoride corrosion products. The availability of oxygen speeds up corrosion and develops CuF2 from CuF. The much greater dilution of corrosion products prevents reaching a critical concentration of CuF2 in the liquid phase.

Inconel 600: Alloy 600 resists corrosion in dilute aqueous hydrofluoric acid at ambient temperatures and anhydrous hydrogen fluoride. It is used in valves and other systems replacing alloy 400 to prevent the feasible stress corrosion cracking. It is commonly used for hot hydrofluoric acid vapors, preventing chemical resistance and providing good metallurgical stability.

High performance Nickel alloys: Hastelloy alloys like grade C276, B2 and G grades as well as Inconel 625 offer supreme resistance to aqueous and anhydrous hydrofluoric acid and to high temperature hydrofluoric acid vapors.  

Behavioural approaches of Nickel alloys to NaOH solutions

Corrosion resistance of pure nickel-chromium alloys is determined to find their usefulness in the different temperature-concentration ranges with respect to general corrosion rate and stress corrosion cracking susceptibility.

In boiling 10% NaoH at 103oC, different alloys such as Monel 400, Inconel 600, Inconel 625, Hastelloy C276, Hastelloy C4, C22 and C2000 are tested to determine their low corrosion rates irrespective of nickel concentration. In a solution of boiling 50% NaOH at 14oC and 70% NaoH boiling at temperature of 182oC, the tested alloys show nominal corrosion rates. Inconel 600 and Monel 400 are the commonly used materials to handle hot concentrated caustic. They also offer supreme corrosion resistance at all concentration as anticipated.

In boiling 50% NaOH an anticipated advantageous effect of increasing nickel concentration is observed. It is found that critical nickel concentration for corrosion resistance in this solution is almost half. Although 20 to 30% nickel concentrations also offer a wide enhancement as compare to stainless steel 304.

Nickel alloys containing iron and chromium such Inconel 600 is distinguished by its excellent resistance to uniform surface corrosion and to stress corrosion cracking in NaOH solutions. During testing Inconel 600 in 10% NaOH solutions at temperature of 325oC and pressure of 275 MPa, where there is no sign of cracking after 100 hours.

With the systematic evaluation of the damage to components made from Nickel and Inconel 600, the sensitivity to stress corrosion cracking of the both alloys is observed for the given temperature and concentration limit. Application of nickel is larger than Inconel 600.

Carbon concentration and heat processing have a remarkable effect on the stress corrosion cracking behaviour of Inconel 600. Outcomes of stress corrosion cracking tests following the slow strain rates in 10% NaOH solutions at 288oC are observed. Alloy 600 pipes containing carbon are tested.

The percent part of intergranular fracture on the fracture surface of the failed samples was accounted as a measure of the sensitivity to stress corrosion cracking . For the samples in the as-delivered state, the intergranular fracture part that is the sensitivity to stress corrosion cracking, reduced with increase in carbon concentration to reach a minimum 0.05%C. Resistance to stress corrosion cracking significantly increased by after heat processing and carbon concentration had eventually no influence.

Inconel 600 with modified carbon concentration and heat processing and test temperature did not have a considerable effect on the stress corrosion cracking behaviour.

For most of the tested alloys, the U bend samples are the more critical test condition as compared to the C ring samples. The ferritic and martenisitic stainless steels showed significant resistance to stress corrosion cracking, although the excellent general corrosion and the risk to embrittlement restricts their user under specific conditions. For the austenitic alloys, more highly stressed U bend samples showed that the resistance to stress corrosion cracking increased with increase in nickel concentration. The alloys with low nickel concentrations could not perform in the concentrated NaOH solutions.