Selecting a woven wire screen for utmost separation service

In the screening application, for the best quality product and supreme efficiency needs precautions screen choice. This article shows how to select the right woven wire screen for your applications.

A screen or separator or called sifter mechanically separates dry free flowing materials by particle size by moving the material with respect to screen. Every screen is circular or rectangular and is connected to a frame in an assembly named as screen deck. The screener can be utilized in the various applications named as chemicals, pharmaceuticals, food products, minerals, pigments, eradicate fines or grade material. In these applications, selecting the right screen is the crucial factor in obtaining the best separation service.

Many screeners utilize woven wire screen with square openings that are described by mesh count, wire diameter, opening size and open area %. According to US systems the screens are described by mesh count and wire diameter. The mesh count describes the count of wires per linear inch. Specifying a wire mesh in countries by using the metric system is slightly more logical as the screen is mentioned by its opening size and is wire diameter or open area%.

Woven wire Inconel mesh screen is developed in high magnitudes by using the combinations of mesh count and

wire diameter. For use in dry bulk material, you need to choose single or three types of screen cloth- market grade, mill grade and tensile bolting cloth.

Many woven wire screens are constructed from stainless steel. Steel types 304 and 316 are commonly used for screening granules and powders.

Select opening size

The opening size is the crucial factor to choose when choosing a screen as it has the major impact on the screen’s separation quality. However before you select the opening size, you should know the specifications of final product that depend on scalping, fine removal or grading.

Stainless steel woven wire is the common screen material that is used for separation of dry bulk solids. In few applications, the screens used are not woven wire mesh but perforated plate containing round perforations instead square holes, it is stronger and sturdier than woven wire mesh. The perforated plate also offers smooth surface to the flowing material that helps separating extended particles from granules or spherical, evenly shaped particles. The round holes offer a more precise opening as the circular holes do not have square openings’s bigger diagonal dimension. Although the plate has a small open area% that decreases capacity and makes the plate sensitive to blinding. A perforated plate screen is commonly used in eradicating streamers and strands from the plastic pellets.

The screeners consist of multiple screen decks to separate material into different discharge streams. Every stream has its specific particle size distribution and generally single or more of discharge streams consist of final product that often has specific limiting magnitude of bigger and fine particles permitted in it. For grading, it is essential to select the correct opening size for a replacement screen because any variation in the opening size can affect the screened products. 

Benefits of Perforated Metal sheets

This article offers description of the vast potential of the services and applications of perforated metals that various perforated metals are used throughout the world in the different styles and materials. A part on the technical factors to be considered while choosing the perforated metal product is also stated that includes global standards approved by IPA and EUROPERF.

In any case while you are looking for a perforated metal sheet, Heanjia Super-Metals can offer the solutions that meet your requirements. By using the advanced equipments and with extremely trained professionals, the company delivers the wide range of perforated products with the full technical support to enable our customers in designing made to order perforated metal components.

The exclusive potential that the industry of perforated metals provides the designers and engineers is extensive versatility to all types of industries with the sole limit of imaging the customer requirements. The need of perforated metals can be in the manufacturing industry, furniture, agriculture, electronics, automotive industry, mining, sugar production, distillation and various other applications.

Perforated metal sheet has several technical advantages over other mesh materials such as woven wire mesh, welded mesh and expanded metal etc. The service of perforated metals is superior to these materials when considering the attributes like ventilation, filtration, sorting and choosing minerals, grains, and sound absorption, radiation security and more. Another advantage of perforated metal over other products is its versatility in permitting different combinations of open areas and solid areas in the single material sheet.

Characteristics of perforated metal sheets

Perforated metal is supreme in the different applications that demand holes. It offers specific control on the open areas that control the flow of sound, air, gases, liquid and solid particles. The characteristics of perforated sheets are:

a.       Uniform hole size and distance

b.      Flat and clean surface

c.       Strong, corrosion resistant and non-extensible

d.      Different hole patterns- round, square, slots, hexagonal, decorative designs

e.      Air, gas, liquid and sound flow supervision

f.        Radiation prevention

g.       Filtration and assortment

h.      Aesthetic look

i.         Structural sturdiness

j.        Economical

k.       Personalized attention to design and production of perforated metal sheet

l.         Fast and effective us

m.    Uniform structuring

n.      Outstanding floatation and ventilation characteristics

o.      Rust resistance

Uses of perforated sheet

Sound control, filtration of liquids, gases and solids, EMI and RFI radiation control, assortment and selection of minerals, grain, architectural elements, security grills for moving components, aeration for warm and moist regions, visibility of enclosed regions, drying grains, bread, brick and ceramics.

Farming – Silo ventilation, sifters, tumblers, grain separators

Automotive industry –Air and oil filters, radiator grilles, mufflers, exhaust pipes

Electronics – Decorative grills, lamp screens, radios and radar equipments

Food Processing-  Shredders, coffee bean toasters, tea separators, fruit dryers, presses,

Aeration- Air conditioners, ventilation fan ducts, return air grilles,

Acoustics-Wall and ceiling panels, sound control equipments

The perforated metal can be developed by using CNC,  sectional, turret and others. The pressure is developed by the presses is use to develop holes such as square, rounds, slots, hexagonal, rectangular, and various decorative patterns. 

Super alloy grades for tanks and rivet nuts

Elliptical Head Pressure vessels as filter tanks

Vast level elliptical head pressure vessels made from stainless steels are develop and designed for service at 100 psig a 230of following ASME code section 8 division 1 for treating as carbon filter tanks. The tank consists of different size nozzles, lifting nugs, pipe legs etc.

The materials used for constructing the tanks are stainless steel, Inconel, Hastelloy alloy, Monel and others.

Assembly with Rivet Nuts

Automakers are widely using hydroformed metal tubing in the automobile structures. As compare to stamped and welded metal components, hydroformed parts lightweight, control price and have greater stiffness to weight ratio.

Until the demand increases to connect other components to the structure with threaded fasteners. Tapping threads in the tube many not be probable because the material may not be sufficiently thick or durable. Self-clinching or weld nuts are not an option, as installation needs access to the reverse side of the metal.

Also called as the blind threaded inserts, rivet nuts offer strong fastening threads in thin panels. The fasteners were originally made decades ago.

Cage nuts are a possibility, but they require a square hole and are difficult to install. The rivet nuts are used in the different types of things. A rivet nut is a single piece internally threaded and counterbored tubular rivet is installed during the service completely from the single panel side. Similar to a traditional blind rivet, the rivet nut is developed on the blind side. The back side flange is big enough to prevent being drawn out in fact under conditions of eccentric load. As rivet nuts can be configured without accessing the panel sides, the fasteners are perfect for connecting components to housings, tubes or extrusions. The fasteners can be configured into metals, plastics and ceramics.

Rivet nut fasteners are constructed from steel, Monel and austenitic grade stainless steel 310/310S. The commonly used material is plated steel however you might specify stainless steel if you aim at corrosion. Stainless steel rivet nuts are usually used in solar panel structures and other outdoor systems.

A single fastener size can usually accommodate a different grip range. Rivet nuts are introduced with a different type of head styles. A wide front-side flange offers a wide load bearing surface. It reinforces the hole and avoids push-through. It is feasible to implement a sealant below the flange for weatherproof applications. A thick flange can treat as a space and offer additional push-out strength. Countersunk and low-profile heads ensure flush or near flush configuration. Wedges or knurls below the head are made to bite into the mating material and avoid the fastener from turning in the hole.

The wedge head is vast for soft materials. Although rivet nuts are annealed, so they are very soft. The wedges are not going to be perfect on the steel components.

Rivet nuts are also used in the different body styles. The standard rivet nut is cylindrical with a smooth surface however variations include splined, square, and hexagonal bodies. Many changes are all made to perform single thing- keep the fastener from turning in the hole, specifically in softer materials like aluminium. If they are not set simply perfect, round river nuts can roll in the hole at large torque levels, with a hex shaped fastener, there will not be a problem. 

Performance of Hastelloy X in the high temperature carburizing media of methane gas

Wrought Nickel alloy Hastelloy X tube was subjected to Argon-Methane mixture at 800oC and 1000oC to understand the carburization mechanism of alloy utilized for fuel injection nozzles of micro-gas turbine combustors. Three types of different internal carbides, (Cr,Mo)3C2, (Cr3Mo)7C3 and (Cr,Mo)23C6 were noticed in this order from the surface and the partial deformation to the external surface of the sample tube seemed similar to the metal dusting. The internal carburization mechanism on the inner and external components of tube were followed. The carbon permeability in Hastelloy X was received and was nominally lower than that of Nickel- 20%Chromium.

Hastelloy X is a key component for gas – turbine components like combustors and fuel injection nozzles. It offers supreme oxidation resistance at the elevated temperature oxidizing media by developing a security layer of chromium oxide. Although this alloy is rapidly subjected to low oxygen potential, high carbon containing media, specifically in combustion media with methane and propane gases that are commonly used. The oxide layer is anticipated to become unstable in these media and may damage to offer security.

Carburization analyses of iron and nickel based chromium alloys have been widely shown. Iron-Chromium-Nickel alloys in C3H6/H2 conditions at 900 – 1100oC and the development of partial outer Cr23C6 and Cr27C3 was noticed. Normally commercial alloys comprise of different alloying elements and various concentration of iron and nickel. This difference in alloy chemistries makes it very tough to state the corrosion nature of various alloys, for instance, Hastelloy X in real service media.

Although Nickel based super alloy Hastelloy X is widely utilized for combustor components, carburization analyses on alloy X are limited. Hastelloy X utilized solid carbon for their carburization analyses. To understand the alloy’s attack utilized for fuel injection nozzles in micro-gas turbines, carburization performance of alloy X is tested in gas combination of argon and methane at 800oC and 1000oC.

Experiment setup

Carburization specimens with length of 20mm were taken from a wrought Hastelloy X tube with internal and external diameters of 9.1 and 10.7mm. Plate shape specimens with 1.5mm thick were utilized for few corrosion analyses to recognize the products.

Argon travelled from the bottom of alumina tube, by the middle of the tubular sample, then in the reaction quartz tube. The carburization test was performed up to 800oC and 1000oC. The reaction tube was flushed with argon gas many times before every corrosion analysis. The furnace temperature was increased at a rate of 10oC per minute to the carburization temperature of 800 or 1000oC with a argon stream at a speed of 200Cm3/min. At the test temperature, Ar gas was replaced by methane -10% mixture with argon at a speed of 150 Cm3/min. Subsequent the test, the sample was furnace quenched in the carburization gas stream with a flow speed of 50 cm3/ min.

At 800oC, no internal carburization was noticed for initial 25 hours of the exposure, however it was noticed in few regions after 100 hours in the internal side of the tube. At 1000oC, the internally carburized layers were noticed to form after 60 minutes and depth of every layer increased with time. The growth rate of the internal and external regions carburization was different during smaller reaction periods.

The elements like iron, molybdenum and silicon may decrease the carbon permeability in grade X as these elements decrease carbon diffusivity.

The external tube surface was affected noticeably and graphite deposition was observed around the affected regions. Graphite can be precipitated on the reaction surface, can develop and include nickel particles and these corpuscles increase the reaction rate resulting into metal dusting.

Hastelloy X in argon-methane condition at 1000oC after a lengthy exposure received metal dusting after exposure for 100 hours. Metal dusting was one of the major causes of extreme corrosion of micro gas turbine parts created from metal dusting on alloy X.

Outline

Scratches from the surface grinding can still be noticed after 25 hour of carburization at 800oC and the surface was shielded with the needle like reaction product after 100 hour. At 1000oC, the internal and external surfaces were shielded by a fine grained reaction product after one hour of carburization and fine grained product became coarser after prolong carburization.

Carburization of Hastelloy X at 800oC – 1000oC in argon-10% methane gas was conducted. The outcomes may be stated as:

Internal carburization was noticed at these temperatures in the given gas mixture. Longer incubation periods were noticed at 800oC. Triple layered carburization regions with M3C2, M7C3 and M23C6 were developed in this range from the surface.

The development of the internal carburization regions developed on the internal and external part of the tube followed parabolic mechanism. Carbon permeability in alloy  X was slightly smaller than Ni-20Cr grade.

Metal dusting was noticed below graphite accumulation and may be one of the reasons of extreme corrosion of micro gas turbine parts. 

Inconel 617 alloy – Effect of heat processing on mechanical characteristics

Inconel 617, a high temperature nickel based alloy is a fit for use as a construction material for 700oC power plants as it offers superior creep strength and adequate fabrication characteristics. This alloy has been tested in various programs for use in the USC boilers. Depending on the received experience, the alloy is customized to fit the special application of USC boilers.

In this post Inconel 617 is evaluated for service in the elevated temperature gas cooled reactors (HTGR). Methods were created for the developed of sound welds and tests were conducted on base metal and metal welds. Samples of alloy were used for aging to 20,000 hours to determine the heat stability. Short term tensile tests were performed that have showed that aging widely decreased strain at fracture at ambient and high temperatures. The impact energy at ambient temperature was terribly decreased by aging. Creep tests describes that cracking is noticed at 593 – 704oC after 1 -2% strain and higher strains were observed at the elevated temperatures. The creep properties were same in air and reactor helium conditions.

Preface

Inconel alloy 617 was developed for service at the high temperatures. It is a solid solution alloy that features high strength at the elevated points. In the elevated temperature gas cooled reactor service, structural alloys are subjected to a gaseous media comprising of helium with nominal magnitudes of hydrogen, methane, carbon monoxide, carbon dioxide, nitrogen and water.

Various test specimens were included in this program. Inconel 617’s three heats were observed in addition of its weld metal heat. The base of alloy 617 received general coarse grains. A sample was aged at 593oC, and its grain size was not changed. This heat processing has the maximum grain size among the three heat processing of the metal.

The tests were conducted in stainless steel 304l retorts and in aluminium oxide. The specimens aged in the inert media at 538oC, 704oC and 871oC were gathered in the metal chambers. The samples were aged for 20,000 hours in HTGR helium in the steel retorts. Aging is continued up to 704oC.

Test conditions

The test gas is contained in pressurized cylinders and supplied to various chambers maintained at 83 kPa. The pipes and valves are organized to allow the parallel supply to all test chambers.

Outcomes

The tensile characteristics of the heated samples were varied significantly. The strength of sample heated from 600 – 750oC show an erratic nature. The cracking strain and reduction percent in area are unlike particularly above 600oC when the ductility increases with raising temperature whilst it decreases in other samples with increasing temperature. The yield stress increased by 20% by aging in inert condition at 538oC to 704oC and reinforcing level on the base of slight aging time was analyzed. Aging in HTGR in helium at 593 – 704oC raised the yield stress by 70%.

The aging time creates a wide effect on samples heated at 593oC with nominal strengthening for 10,000 hours aging whilst aging for 20,000 hours increased strength by 70%. The aging time has no noticeable effect at 704oC. Aging at 871oC for 10,000 hours in reactor containing helium gas showed 30% increased yield strength/

Aging significantly alters the ultimate tensile strength by 10%. Aging for 10,000 hours at 593oC in the reactors didn’t show any effect whilst aging for 20,000 hours increased the ultimate tensile strength above 20%. In aging at 871oC, the eventual tensile strength reduced for aging time above 2500 hours. The strength reductions were higher for samples aged in HTGR containing helium tan for samples aged in an inert media. After 20,000 hour aging in HTGR-helium, the ultimate tensile strength was decreased by above 30%.

Aging at 538oC created erratic influence however the elongation values lied in 53 – 69%, that was very large. The cracking length reduced with increasing aging time and temperature. In an inert media, the least cracking strain was 20% for a specimen aged at 20,000 hours at 871oC. Aging in reactor always caused smaller fracture strain than the contextual aging processing in an inert media. The least value observed after aging in the reactor was 6% subsequent 20,000 hours at 871oC.

Aged samples were also observed at the aging temperature and the outcomes have similar behaviour for specimens observed at 25oC. Aging in the limit about 500-700oC improved the yield and tensile strengths and the enhancement was higher in the reactor aging condition as compare to in the inert. Aging in both conditions at 871oC had nominal influence on the mechanical strength of alloy. Aging in inert media decreased the fracture elongation nominally at 704oC and create unnoticeable effect at other aging limits. Aging in reactor containing helium gas widely decreased the cracking length at all the given temperatures.

Inconel 617 welds

The eventual tensile strength of the weld metal was nominally higher than the base metal at the same temperature limit. The cracking extension of the weld metal was about half of the base metal the whole temperature limits. The area reduction for weld was larger at 25oC and smaller at the high temperatures as compare to the base metal. Although the weld was highly ductile under the whole test media.

Creep tests

Creep tests were performed on Inconel 617 base metal. It was found that the surrounding condition has no overall influence on the nominal creep rate. At 760oC to 871oC the cracking strains were very high however the effect of surrounding on the fracture strain was not evident.

Discontinued creeping
Alloy 617’s specimens were subjected to a creep load for long time length and short term tensile test at the ambient temperature. The initial test included alloy’s sample that was creep tested up to 871oC for 26,117 hours and received strain about 0.3%. The specimen was widely carburized and was discovered to comprise of 0.233% carbon. The yield stress of the creep specimen is larger and the ultimate tensile stress and elongation smaller. Carbon concentration of the creep sample is larger than sample aged in the reactor condition. It resulted into reduction of the ultimate tensile stress and fracture elongation. 

Super alloys for corrosion prevention in aggressive application media

The alloys containing high carbon content are referred as super alloys that are named as Incoloys, A-286, Inconels, Hastelloys etc. The super alloys offer good high temperature strength and oxidation resistance. The super alloys are based on nickel are widely used materials, as they offer superior services than FeNiCr alloys and less costlier than cobalt based alloys.

For comparison objectives, use of high temperature strength in heat resistant alloys is recommended. Although for design purposes creep or stress rupture data should be used. A design engineer should often find if the component is bounded by crack or extent of deformation. Generally the alloys offering superior stress rupture characteristics offer the excellent creep strengths.

The alloys referred above are wrought and mechanical alloyed types. Mechanically alloyed materials consist of fine dispersion of oxide particles and developed by powder metallurgy methods. Many of powder metallurgy alloys have been substituted to forged alloys and utilized as turbine discs.

However the use of powder metallurgy methods have been implemented to these alloys, development has been basically limited to warm isotatic pressing processes and warm compaction followed by extrusion procedures.

Several of wrought alloys are also fit for investment casting procedures. Moreover, nickel and cobalt base alloys have been made for service as cast materials.

Applications of Superalloys

Incoloy 800	Catalytic cracking tubes, reformer tubes, aqueous attack applications, sulphuric and phosphoric acid conditions, heat exchangers, industrial furnaces, steam producers
Inconel 617	Gas turbines, petrochemical treatment, heat processing unit, nitric acid development
Inconel 718 and X-750	Gas turbines, rocket motors, spacecraft and pumps

Nickel base alloys contribute by 65% in the aerospace engineering. These are widely used in rocket engines offering excellent corrosion resistance, elevated temperature oxidation resistance, maintaining significant characteristics over the large temperature range and in several cases, offer exclusive set of physical characteristics. These alloys are categorized into three groups depending on their applications:

1.       Nickel base alloys utilized because of their outstanding corrosion prevention potential for example Nickel 200, Monel grades, alloy 600, alloy 625 and electroformed nickel. The corrosion resistant nickel is not based on the metastable oxide layer for security and acting as firm electronegative element, it is not sensitive towards galvanic attack when interacts with other metallic materials. Monel alloys offer supreme corrosion resistance, possess significant magnetic characteristics at cryogenic limits and offer supreme resistant to inflammation in oxygen. Inconel grades consisting of nickel, chromium and iron prevent oxidation at temperatures about 1800oF.

2.       Nickel containing super alloys that possess supreme strength at the high temperatures. Inconel 718, the leader of this group offer supreme strength up to 1300oF, supreme cryogenic ductility and excellent welding potential. Fine grained material must be mentioned for components that require to be electron beam welded.

3.       Special purpose materials such as Nichrome, Incoloys and Invar.

The whole of these nickel based superalloys prevent attack and stress corrosion and oxygen computability however are sensitive to conditions containing hydrogen due to embrtillemen at temperatures about -200oF. Hydrogen embrittlement in nickel based alloys can be avoided by discarding plastic strains or by offering a security shield for example electroplating with corrosion resistant alloy.

Special austenitic stainless steels

There are many commercial proprietary heat resistant materials that are a member of austenitic stainless steel group considering nickel and chromium concentrations however with inclusion of silicon offers good resistance to oxidation and other high temperature corrosion attack. For example Incoloy 800H that offers service up to 1093 to 1150oC.

FeCrAl grades

Aluminum is a strong alloying element that enhances resistance to oxidation and other types of corrosion at the elevated point. The alloy needs about 4% aluminium to develop a regular alumina scale. The alumina layer offers outstanding security from the corrosive attack of oxidation. When the alloy is heated up to 1200oC or above, a layer of Cr2O3 is formed that grows gradually and develops volatile CrO3, becomes non-secured. Alumina layer offers supreme protection from oxidation. Due to very small growth rates at low and moderate temperatures, alumina scale offers low security at such limits. So high temperature alloys are made to develop alumina scale for extremely high temperature services also consist of sufficient chromium content to develop chromium oxide layer for moderate temperatures.

Few commercial electrical resistance heating materials are constructed from FeCrAl alloys like heating elements that depend on development of alumina layer for service up to 1400oC. For instance these alloys are made in wire, strip, rod and mesh forms. As these wrought alloy forms are basically ferrite materials, they attain small creep rupture strengths when the temperature limit goes above 650oC or 1200oF and is not feasible for high temperature structural materials. So the heating elements made from such alloys need to be adequately supported to prevent creep deformation for example sagging. The heating wires are used in flame spray or arc to develop an oxidation resistant coating or in weld overlay cladding by using gas metal arc welding process. A powder metallurgy process was utilized to develop a  supreme heating element FeCrAl Cr25Al5 that have excellent creep rupture strengths.

Several more FeCrAl grades are made for use as resistance heating elements such as foil to different temperature limits for 2 minute as long as it failed. The failure occurs when the foil was oxidation penetrated. The use of rare earth elements such as cerium is essential for improvement in alumina scale. A nominal studies have been performed on the suitability of cerium on adhesion of alumina layer. Many more analyses are performed on the influences of yttrium, zirconium and other reactive elements. In the FeCrAl alloy, the rare earth element such as yttrium is included to enhance adhesion of the alumina layer developed on the FeCrAl alloys hence enhancing the oxidation resistance of the alloy. A FeCrAl alloy is reinforced by oxide-dispersion strengthening mechanism to significantly enhance its high temperature strengths by mechanical alloying.

Iron-Nickel-Chromium Alloys

With increase in nickel concentration in the FeNiCr system from austenitic stainless steel grades to iron base alloys, the materials attain more stability such as metallurgical structure and good resistance to creep deformation. Normally these alloys offer superior oxidation prevention. For example wrought Incoloy 800H/800HT that resist corrosion in the prolong oxidizing media.

Ni-Cr/Co-Cr Super alloys

In various Nickel-Chromium alloys, the composition elements for example the solid solution reinforcing elements like molybdenum and tungsten, and precipitation reinforcing elements for example aluminium, titanium and niobium are included in to the alloys to offer reinforcement at the high temperatures. Most of these alloys are preferred as super alloys that involve oxide dispersion strengthened alloys.

Similar to FeCrAl alloys, aluminium acts as a composition element in the Nickel-Chromium alloys to enhance the oxidation resistance. However it usually needs least 4% in the Ni-Cr matrix to develop alumina scale, the inclusion of aluminium enhances oxidation resistance of alloy.

Inconel 601 contains just 1.3% aluminium and offers supreme oxidation resistance. However alloy 601 contains 1.4% aluminium that improves its oxidation resistance, the adherent oxide layers developed on this metal are usually enriched of chromium. But at high temperatures above 1100oC, these oxides become sensitive to failure, receiving scaling, deformation and spalling.

The oxidation resistance can be increased by modifying the concentration of chromium, aluminium or silicon, meanwhile many alloys are developed to offer sustained high temperature strengths by alloying with other elements. A big count of super alloys are developed to meet the challenging needs of gas turbine engines for critical service media including high stress and elevated temperatures. To meet the demands of high stresses at moderate temperatures, a group of wrought super alloys is reinforced by precipitation strengthening with Ni3X precipitates along with solid solution strengthening by using molybdenum or tungsten. These alloys include Inconel 718 and X750. Few applications are gas turbines components such as compressors, diffusers, turbine disks, cases, heat shields, exhaust units, thrust reversers and turbine shroud rings. Many alloys of this category are utilized in the heat processed conditions to get the benefit of precipitation strengthening. Many heat processing methods are followed at the moderate temperature limit. So the applications of these alloys are referred to be in the moderate temperature limits to avoid overaging of the reinforced precipitates. The oxidation of these alloys at moderate limits does not show a major problem in their service.

The alloys containing none or nominal chromium level for example Hastelloy B can only perform in the reducing media. Stainless steel type 304 and 316 offer good corrosion resistance in the oxidizing media. Austenitic stainless steel grade for example type 304 and SS 321 contain borderline limit of chromium content, these are susceptible to chromium concentration for heavy composition and surface composition in the material. When the surface penetration of chromium happens in stainless steel product when excessive chromium concentration is the bay of the specification, cracking oxidation occurs possibly so causing terrible oxidation corrosion.

Most of the oxidation attack is noticed in the form of weight change over the time or temperature. Although, it is possible to use the weigh change information to assess the service life of the component because of oxidation attack. The oxidation analysis is significant for engineering purposes that includes metal loss and depth of internal oxidation corrosion. The overall depth of the oxidation corrosion is responsible for loss in load bearing property of the material.

Nickel and cobalt base alloys comprising of molybdenum or tungsten or both also cannot withstand oxidation at the excessively high temperatures. The samples of nickel base alloys that are used at 1200oC were Hastelloy X and Inconel 625. Few of the nickel base alloys comprising of molybdenum or tungsten or both were not attacked up to 1200oC or 2200oF for example Inconel 617 so it is estimated that nickel base alloys comprising of molybdenum and tungsten can be used for high temperature reinforcing to prevent oxidation highly elevated points by modifying contents of other elements.

In the nickel base alloys comprising of high concentrations of molybdenum and tungsten or both, it is trusted that increasing chromium is certainly the most significant decision to prevent quick oxidation. Titanium is found to be very effective in the development of oxide layer. 

Effect of heat processing on the welded Inconel 625’s corrosion resistance

Inconel 625 is an outstanding heat resistant alloy with good mechanical characteristics at the high temperatures and supreme corrosion resistance. These characteristics make it significant for use as a structural material in steam engines, nuclear plants and aircraft engines. It offers supreme welding properties and as a result it is used for weld overlay in the carbon steel pipes. Therefore it can be used instead high corrosion resistant steels for example duplex stainless steel. It also develops synergistic effect by weld overlay with carbon steel materials to supplement the yield strength of Inconel alloy and small corrosion resistance of carbon steel so it can be utilized as structural materials in the severe media like for example in crude oil. Moreover the heat expansion coefficients of the to metals are identical that decreases the chances of cracks due to thermal stress under the high temperature media. Although the Inconel alloy comprises of nickel and chromium, that may develop carbides and secondary phases, on the base of specific temperature and exposure time. Such carbides and secondary phases have a significant role in affecting the corrosion resistance and physical characteristics of alloy and causing the crack development. 

With the passage of time different carbides and secondary phases are developed that precipitate in the temperature limits from 600oC to 950oC. The development of these carbides can be a contributing aspect to the reduction of corrosion resistance. Intergranular regions of carbides are thermodynamically more inconsistent and highly active than other types of intergranulars. Intergranular regions increase owing to development of carbides. Additionally the carbides have higher chromium content than that present in the base metal. So when carbides are developed in the intergranular regions, the chromium content reduces around them resulting in to areas with low chromium content along the intergranualr regions. In this mechanism, chromium lacking areas are more prone to intergranular attack than other regions, so causing corrosion, is named as sensitization.

Electroslag welding method is implemented with alloy EQNiCrMo-3 utilized as a filler metal.

Aging

Aging heat processing was performed to find the intergranular resistance at 500A to 620A samples. Chromium carbides were developed through age heat processing at constant temperature for 100 hours in a vertical furnace at 850oC.

Corrosion tests

A single and double loop electrochemical reactivation test was performed as an electrochemical method to estimate the corrosion resistance. This test shows more significant results than with other chemicals. The sensitivity of intergranular attack on Inconel meals is assessed. The outcomes are assessed from non-uniform attack in the intergranular region and the outcomes should be evaluated by the current ratio. On the other hand, mistakes due to surface conditions are nominal and the test values can be easily received and compared with the single loop analysis. In high concentration of sulphuric acid, intergranular and other types of corrosion increases whilst in the low content, the corrosion is not noticed. So depending on the test condition factors, the tests are performed on nickel based alloy.

In the chemical analysis to evaluate the intergranular attack on stainless steel or  super alloy Inconel 625 plate, the tests included sulphate- sulphuric acid and nitric acid were performed. The ferric sulphate sulphuric acid test in the ASTM G28 method was conducted to assess the corrosion sensitivity of super alloys.

In the test of ferric sulphate sulphuric acid, H2SO4 acid solution comprising of 400 ml water and 236ml sulphuric acid where 25g dissolved Fe2(SO4)3 was utilized. It is warmed on a hot plate, the alloy sample was plunged for around 120 hours and the material loss was evaluated. To avoid evaporation of the solution beyond 120 hours, the boiling stone was kept in the solution and the vapour was condensed through flowing water condenser. The test results were received by substituting the noticed material loss of the samples.

In the another chemical test, the nitric acid test was performed that was aimed on evaluating the austenitic stainless steel and therefore was organized to fit the nickel based alloys in this analysis. Nickel super alloys have higher PREN than austenitic steel grades; the plunging period was increased up to 120 hours. In this test nitric acid is taken in 65% content.

No corrosion sign was found by intergranular attack on Inconel 625 due to its small carbon concentration and sufficiently high niobium magnitude. With small carbon concentration, chromium carbides, the main reason of chromium lacking regions, were not precipitated during aging heat processing, and with niobium effect, the niobium carbides were accumulated, hence preventing the development of chromium carbide. So the alloy was stabilized by precipitation along the grain. It is found that increased weight loss occurs with high heat supply to the sample. The weight loss in material is a crucial factor that shows the speed of corrosion as it results into the material degradation in the component.

Various corrosion rates are based on the type of attack. To determine this, microstructures of the corrosive surfaces were noticed after these chemical tests. The samples initially experienced corrosion at their intergranular regions. Initially thin and lengthy corrosion shape was seen however later the corrosion area increased and then at the maximum value of current the corrosion area becomes round. Alike outcomes were observed in the both tests.

Generally, the bigger weld heat supply, high dilution occurs in the weld metal and the base metal. With increase in weld heat supply, it results into melting a part of the main metal sample, therefore the metal’s atoms are diluted in the weld metal. It causes to increased diluted iron content in the weld metal hence decreasing the breaking potential. This iron dilution mechanism is noticed in the melted part and can also be seen in the fusion line zones by energy dispersive X ray spectroscopy. Increased niobium and molybdenum contents resulted into cracking. Therefore with increase in heat supply, the dilution effect is enhanced hence increasing iron concentration in the fusion line. As a result, large magnitude of iron was coagulated, initially in the dendritic regions, through weld solidification, niobium and molybdenum were emitted into the interdendritic regions. This method created microcrakcing in the dendritic and interdendritic regions, hence showing a variation in the corrosion resistance offered in these areas, and an unlike corrosion shape.

Outline

The samples were not found to be sensitive towards intergranular attack. Irrespective of nitrogen aging heat processing, precipitation of chromium carbide didn’t occur and niobium carbide stabilized the sample.

In the ferric sulfate sulphuric and nitric acid tests, heat supply improved with increase in material loss, hence corrosion rate is accelerated.