Why Nickel-Chromium-Molybdenum (NiCrMo) alloys are preferred over stainless steels?

The major development of chemical products and processes demand materials with enhanced mechanical characteristics, metallurgical stability and supreme corrosion resistance in severe conditions. Over the time, highly corrosion resistant alloys made of nickel, chromium and molybdenum have been proven to prevent corrosion in vigorous environments in the various industries such as chemical, pollution control, marine, oil and gas units.

Hastelloy alloy C-276 and Hastelloy C-22 offer excellent resistance to general and localized corrosion. The service of the material in preventing acid corrosion, mixed acid and localized corrosion in acid and marine media. These alloys have supreme metallurgical stability and can be easily formed and welded.

Plant operators are widely using Inconel-Weld filler metal as filler metal for a great range of nickel alloy based compositions. The versatility of the welded product and high alloying concentration has improved the use of the material in extremely vigorous environments.

Widely corrosion resistant materials like Inconel alloy 625 and Hastelloy C276 wire offer supreme corrosion resistance for use in chemical, power production, pollution control, marine and major applications. The reason for the use of these alloys is the development of new chemicals and improved processes. The latest challenge has been met by Hastelloy C-2000 that is increasingly used to prevent corrosion and safeguard product purity in media, that was not feasible for stainless steels or other materials. These materials can offer several benefits such as enhanced life cycle cost service, enhanced reliability, nominal maintenance and decreased downtime costs.

Significant inclusions of nickel, chromium, molybdenum and other elements are required for several applications that offer supreme corrosion resistance. Chromium offers resistance to oxidizing media while molybdenum enhances resistance to reducing media. Chromium and molybdenum prevent localized attack. Inclusion of tungsten also increases to localized corrosion. However nickel prevents attack in caustic and mild reducing media, its main advantage in alloys consisting of chromium and molybdenum is to retain stable austenitic single-phase structure. It is significant in offering adequate corrosion resistance in an alloy to make it economically manufactured and fabricated.

Inconel 625, Hastelloy C276, Hastelloy C22 and Hastelloy C2000 offer enhanced corrosion resistance with better flexibility and versatility in preventing attack in diverse conditions. The Nickel – chromium – molybdenum alloys are used in corrodents and fluctuating environments including oxidizing and reducing media. While comparing alloys for a specific service environment it is often recommended to test the alloy in the service media or in a lab test that most closely approximates it.

Localized corrosion attack that is most often  occurring failure mechanisms of stainless steels and Ni-Cr-Mo alloys, it is less predictable as compare to general corrosion.  

The high alloyed materials such as Hastelloy C22 and Hastelloy C2000 are adequately alloyed to prevent pitting and crevice corrosion to the test temperature of 85oC. The highly aggressive oxidizing chloride conditions differentiate the influence of chemical composition on localized corrosion resistance of the advanced nickel alloys. At 125oC, Hastelloy grades C22, C276 and C2000 receive some corrosion however these alloys are fully resistant to stress corrosion cracking in waters including chlorides.

Stress-rupture strengths of super alloys

People specifically in industries dealing with superalloys, usually need valuable information to do their jobs more efficiently. Designers need stronger and corrosion resistant materials for elevated temperature applications. The stainless steels developed in the 20^th century acted as a beginning for the meeting of high temperature industrial requirements. They had limited characteristics. The industries demanded the superalloys.

Before Second World War, the gas turbine became an agent for alloy invention. However patents for Nichrome alloys were released in 1920s, the industry of superalloys boomed with the introduction of cobalt alloy that was used in dentistry for high temperature strength demands of aircraft engineers. The nickel-chromium alloys such as Inconel grades, toaster materials Nichrome were introduced.

What can be done to Superalloys?

Superalloys are nickel, iron-nickel and cobalt based alloys that are used at temperatures over 1000oF or 540oC. Popular alloy Inconel 718 wire is an example of enhancement of stainless steel technology that is a wrought form. Cobalt-base and nickel base super alloys are wrought or cast for the specific applications. Although many alloys have been developed over the years, only a few are widely used. Alloys use is a basis of industry such as gas turbines, steam turbines and more. Suitable compositions of super alloys can be forged, rolled to sheet or developed in different shapes. The fabricated structures can be developed by welding or brazing, however various highly alloyed compositions comprising a large magnitude of hardening phase are hard to weld.

High temperature strength of Metals

At general temperature limits, the strengths of many metals are measured as short term characteristics for example yield strength or ultimate strength. Although, with increase in temperature limits, specifically to temperatures of up to 50% of the melting point, strengths can be reckoned with time. Therefore some load is subjected to a metal of less than a breakable value at room temperature, however at the elevated temperature, the metal will extend with the passage of time.

This time based extension is known as creep and if it happens for a long time, it will cause fracture. So the creep strength of a metal or rupture strength or both are essential to describe the mechanical behavior of a metal like yield and ultimate strengths. Similarly fatigue potential will be decreased. Therefore, to completely validate the potential of a metallic material, based on application temperature and load, it is essential to offer yield and ultimate strengths, creep strengths, stress rupture strengths and suitable fatigue strengths.

Physical characteristics like coefficient of thermal expansion and density are also considerable.

Basic Metallurgy

Iron, nickel and cobalt are normally face centered cubic structures. The upper service temperature limit for super alloys is not limited by the occurrence of allotropic phase transformation reactions in fact it is based on incipient melting points of alloys and dissolutions of strengthening phases. 

Incipient melting is melting that occurs in a part of alloy that when solidified, hence melts at a lower temperature than that at which it might otherwise melt. The whole alloys have a melting limit, hence melting is not at a certain temperature in fact there is no non-equilibrium segregation of alloy elements. Superalloys are reinforced not just by the basic nature of the FCC matrix and its composition however also by the availability of special strengthening phases often precipitates. Processing of a Superalloy can also increase strength, however that strength may not withstand at high temperatures. 

Cleaning processes used in Gas-Solid filtration systems

Gas-solid filtration applications follow the process designs- continuous process filters and trap filters. These designs are perfect for sintered porous metal elements and they function in an equivalent manner- gas travels through the metal mesh where particles are hold and captured on it. The basic difference between them is the frequency and particle elimination methods and element reproduction.  The decision for the type of filter used is based on the individual process factors and basically the solids loading.

The trap filters are used on clean streams for protection of downstream process and system. This type of filters does not need in-situ cleaning and solid elimination needs disassembly. The filter components are usually cleaned through chemical methods. The cleaning frequency varies with solid load and feed gas.

A regular process tungsten wire mesh filter is perfect for heavily particulate streams or in processes involving the use of dangerous materials. The cleaning interval is based on solid loading. Usually the cleaning durations vary from 1-2 minutes to a few hours. The blowback cycle can be initiated manually or automatically.

The applications involving gas stream processing need on-line blowback cleaning, the sintered mesh filters are perfect for automated process control and in-situ cleaning.

The particulate loading on the filter cartridges is similar for both filter designs. During the process, gas-solid combination moves towards outside the sintered metal filter components that have captured the solids. The processed gas travels along the filter wall and is released from the unit.

After reaching the specific differential pressure or cycle time, the supply is stopped at this time the backflow cycle starts. The system is separated and gas enters. Reverse flowing through the chamber and elements release the cake from the filter.

The sintered mesh featured system offers high throughput with nominal backpulse gas needs. Sintered metal filters are backpulsed sequentially even when the system remains active. When the specific differential pressure or cycle time limit reaches, the mesh elements are backpulsed to eliminate the cake. During the service, a massive high pressure gas enters the nozzle along the valves. The blowback gas releases from the nozzles and enters into the venturis entraining the gas from the chamber. The resulting gas flow develops a strong backpulse on the filter components that uplifts the filter cake. The cake drops on the discharge hopper then discarded. The reverse pulse of blowback gas usually remains for a few seconds. A part of the filter elements are pulse cleaned at one moment while the rest remains active to continue the filtration process hence ensuring the regular flow of the process gas stream.

The choice of suitable media and the equipment service conditions for liquid-gas filtration units is often confirmed through prolong testing to receive performance efficiency and required operation rates. The recovery pressure drop after blowback can be confirmed to offer long service. Suitable service media are essential for prolong operation life. Suitable particle loading and pulse blowback cleaning are essential choosing the ideal functional conditions. The mesh filters use diverse filtration technologies such as wide level cross flow filtration, in-situ back pulse and in-line gas filtration. 

Why Porous Sintered metal mesh media is fit for filter cartridges

The suitable choice of filter media with optimum pore size, strength and corrosion resistance allows prolong filter service with good efficiency particle retention. The sintered porous metal media meets the criteria and offers good particle separation efficiency while meeting the tight emission regulations for modern industrial processes. The production of specifically designed and engineered sintered porous metal media with consistent porous matrix, precise bubble point dimensions, close thickness tolerances and permeability uniformity ensures consistent filtration performance, suitable blowback cleaning and prolong service life.

The characteristics of metal filters, developed from different metal alloys, permit their service in the wide conditions such as high temperature, large pressure and corrosive media. Service temperature can increase up to 1000oC on the basis of choice of application metal. The basic advantages of sintered metal filters are- good strength and cracking hardness, large pressure and temperature potential, extensive resistance to thermal shock, corrosion resistance, easily cleanability, high welded assembly and prolong service.

Sintered mesh media have shown good particle efficiency, consistent filtration performance, suitable backwash and on-stream efficient performance. These elements can offer particle capture efficiency about 99.9% by using surface or depth techniques. In addition of efficient filtration consideration, the factors also include corrosion resistance, mechanical strength at application temperature, cake elimination and longer service life. These factors are essential for the successful and economical services. The performance of these filter media is based on the particulate holding capacity and the related pressure drop. This accumulating cake can be regularly eradicated during the blowback cycle. This suitability of the blowback cycle and filter pressure drop recovery is crucial function of characteristics of the accumulated particles in the cake and filter elements.

The sintered mesh is considered as partial-permanent media with the entire welded structure. The benefit of metallic filters is that can be welded to the components to receive durable sealed joints. The filter elements can withstand pressure spikes without media migration. The inherent hardness of the filter offers regular and back pulsed service for long duration. For the high temperature applications creep-fatigue interactions and elevated temperature corrosion processes are taken into account. The filters with partial-permanent media are economical like units offer minimal downtime, automatic operation with minor contribution of operation and mild maintenance.

Sintered mesh porous metal provides service at temperature limits varying from 400 – 950oC on the basis of alloy material and atmospheric media. Sintered metal filter elements can withstand differential pressures above 210 bar. The sintered metal is a permanent media that can pair with the whole welded structure if needed and can work with pressure spikes without movement of media.

Permanent structure of sintered metal mesh

Sintered mesh media features permanent structure that enables cleaning of filter cartridges in the diverse ways without any migration of media. An optimum choice of alloy for corrosion resistance and media grade for particle elimination ensures liquid purity or gas during separation. In-situ cleaning in process filters by liquid or gas backwash. The chemical cleaning with suitable materials or ultrasonic cleaning in the detergent solution will prevent insoluble contaminants from inside the filter media. Sintered metal mesh is made from the diverse corrosion resistant alloys such as Monel 400, Inconel 625, Nickel 200, Stainless steel 304/304l, Inconel 600 and Titanium.