The welding of duplex stainless steel S32750

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Compared with Austenitic stainless steel, super duplex stainless steel has more Cr and Mo contents, which is beneficial to form ferrite and improve the corrosion resistance of the steel. The addition of Ni, N, Cu and Cu can improve the corrosion resistance of steel to non-oxidizing medium. Super duplex stainless steel has good weldability without welding hot and cold cracks. Under the influence of welding heat cycle, Ferrite increases and the grain size enlarge, while too slow cooling will also lead to the precipitation of harmful phase, which may destroy the balance between Austenite and Ferrite, affect the mechanical properties and corrosion resistance of welded joints. Here this article will introduce the welding process of S32750 stainless steel.

 

Welding methods

Tungsten argon arc welding is characterized by energy concentration, a small amount of heat input, easy to control the welding quality. Reasonable control of the welding heat input, multi-layer welding, multi-channel and low deposition rate, the tungsten electrode argon arc welding and auxiliary to 99.99% pure argon gas protection weld molten pool implementation super duplex stainless steel welding, can get better welding quality and good mechanical properties and corrosion resistance.

 

Welding materials

According to the chemical composition and mechanical properties of the base material, ER2594 wire is an ideal choice. The weld metal is allowed to be called “super duplex stainless steel” when the PRENE(pitting resistance equivalent value) is greater than 40.

 

Welding parameter

Sample operation is specified in ASME B31.1andASME Ⅸ。

Firstly, take the base material sample S32750 pipe with the specification f114.3mm 6.02mm and open the V-shaped groove. The groove and weld bead is shown in the figure.

The welding material model is ER2594 with the specification f2.0mm. Note that too much current is easy to burn through, too little current is easy to cause incomplete fusion or incomplete welding. In the process of operation, the weld groove Angle can be appropriately increased to control the fusion ratio and adjust the metal composition of the weld.

Secondly, it is strictly prohibited to start the arc and test current on the surface of the base metal outside the groove to prevent arc damage to the base metal. The quality of the starting and ending arc should be guaranteed during welding. The same welding material and welding process as the root pass shall be used for welding positioning weld. The number of locating solder joints shall be 2, 3 or 4 points and shall be fixed on average. The thickness shall not exceed 2/3 of the pipe wall to ensure that the welding seam will not crack and remove defects during the formal welding process.

Welding shall be done in strict accordance with the relevant parameters selected. In order to make the welding can cover the actual construction of large-diameter, thick-wall pipe should be used as wide as possible range of parameters. Controlling interpass temperature less than 120 ℃, the welding heat input 1500 j/mm or less, on the premise of guarantee the quality of welding, as far as possible use small current, fast welding, small heat input and welding layer, bead for welding.

 

It is worth noting that the groove and the surface 50mm away from the groove shall be cleared before welding, and there shall be no water vapor, phosphating substances, carbon-containing materials (such as oil, paint, scale, rust, burr and halogen, etc.) and cracks, interlayer and other defects. Proper measures such as isolation and stacking should be taken to prevent the contamination of the super duplex stainless steel by iron elements. Super duplex stainless steel has good weldability and is not easy to produce hot cracks, been widely used in seawater and wastewater treatment equipment, papermaking, petrochemical equipment and other environments where require strict corrosion resistance.

What’s the effect of Co for carbide alloys?

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Cobalt (Co) is a magnetic and scarce metal element. Cobalt resources are widely but imbalanced in the world. The reserves of Congo (DRC), Australia and Cuba account for 70% of the total global reserves, of which Congo accounts for 47.89%. Cobalt resources are mostly associated with copper-cobalt ore, nickel-cobalt ore, arsenic-cobalt ore and pyrite deposits.

As one of the important strategic resources, Co plays an important role in many applications. Cobalt alloys or mixtures of metals, make up half the cobalt used each year. Currently, the largest consumption and application of cobalt are mainly lithium battery, of which the positive electrode material is lithium cobalt oxide. In addition, Cobalt is also used in the traditional areas of super heat-resistant alloys, tool steels, and type of magnetic materials. The cobalt compounds are mainly used as catalysts, desiccants, reagents, pigments and dyes, which is essential for industrial processing.

In cobalt-based alloys, the addition of Co increases strength and hardness especially the red hardness, permits higher quenching temperatures. It also intensifies the individual effects of other major elements in more complex steels. The consumption and application of carbide alloys are the Tungsten carbides. It is mainly used for cutting tools, molds, cobalt heads, nozzles, perforating tools and corrosion-resistant and wear-resisting parts, such as sealing rings, cylinder linings, ball-point pens, etc.

Cutter steel with a certain amount of cobalt has good wear resistance and cutting performance. Cobalt combines other metal carbide grains in the alloy composition to make the alloy more ductile and less sensitive to impact. This alloy is welded to the surface of the parts, which can increase the service life of the parts by 3-7 times. Sterlite carbide containing more than 50% cobalt will not lose its original hardness even if heated to 1000 ℃.

The cobalt-based alloy still shows good performanceunder high temperature above 1038 ℃, especially used for the production of high temperature engines and steam turbines, so it is widely used in aerospace and modern military field. The use of cobalt-based alloys containing 20-27% chromium in the structural materials of aero-turbine engines can achieve high oxidation resistance without any protective coating. A thermal-medium turbine engine in a nuclear reactor heating studio can operate continuously for more than one year without maintenance.

What’ s metal cladding plate?

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Metal cladding plate is composed of base steel (plain carbon steel) and composite layer (corrosion resistant metal) through the explosion and rolling process. The base material can be ASTM A 516 Gr70, ASTM A36, ASTM A283, ASTM A387 and other ordinary carbon steel and special steel. The cladding material can be ordinary stainless steel such as 304, 304L, 316L, S31603; Commercial pure titanium Gr1, Gr2, titanium alloy Gr.5, etc. Duplex stainless steel like 2205, 2507,904L; Nickel alloys such as Hastelloy C-276, C-22, Monel 400, Inconel600, Inconel 825, etc and other metal like Copper and Aluminum, etc. Metal cladding plate offers good process performance and can be hot pressing, cold bending, cutting, welding.

 

In theory, all kinds of malleable, corrosion-resistant and high-strength specialty metals can be used to make cladding materials. Cladding plates reduce the use of precious metals, their material and thickness can be produced by demands, combined the advantages of low cost and high performance. It has been widely used in petrochemical, coal chemical, fluorine chemical, fine chemical, acetic anhydride, PTA, chlor-alkali, salt, metallurgy, medicine, electric power and other fields. There are two main methods for industrial production of metal cladding plates: explosive cladding and hot rolling cladding.

 

Explosive cladding

Also known as explosive welding. superimposed the cladding layer on the base layer to make them keep a certain distance. The explosive instantly produces huge energy, which causes these two metals to collide at high speed to form plastic deformation, melting and welding together. Ideally, the shear strength per square millimeter can be up to 400 Mpa, which can meet almost any processing requirements.

The features of explosive cladding:

1. Cold processing, can produce a variety of metal cladding plate, such as titanium, copper, nickel, aluminum and a variety of non-ferrous metals.

2. Explosive cladding can produce metal with thickness up to hundreds millimeter for used in some large water conservancy project base and ultra-thick tube plate. It’t suitable for the production of composite steel plates with a total thickness less than 8 mm.

3. Less investment and can be small-scale production or mass production, also achieved a small number of pipe &plate or special-shape workpiece combination. The cladding layer can be spliced and polished before the explosion.

4. Due to the characteristics of explosives limited by weather and other technological conditions, explosive compound production efficiency is low. In addition, explosives can cause vibration, noise and smoke pollution, usually produces in the remote outdoor factory.

 

Hot rolling cladding

The hot rolling cladding is the process that the cladding material and the base material are overlapped and assembled into the slab to be rolled. In order to improve the bonding strength, a series of technical measures such as vacuum hot rolling or shielded gas rolling should be taken. The features of hot rolling cladding:

1. Using large plate mill and hot continuous rolling mill, so it offers high production efficiency and fast production speed.

2. More choice of product thickness, the stainless steel coating thickness of more than 0.5mm can be produced. Limited by the compression ratio of steel rolling, hot rolling production cannot produce a thickness of more than 50mm cladding steel plate, it is not convenient to produce round and other special shape clad plates.

3. Cladding plates of 6, 8 and 10 mm is ideal for this process. Under the condition of hot continuous rolling, the scale production can be realized and the length can be determined according to the needs to meet the needs of users.

4. Limited by technical conditions, the hot rolling process to directly produce titanium, copper, aluminum and other non-ferrous metal composite need to be improved.

 

Both explosive and rolling cladding are in accordance with GB/T-8165-2008 standard, by which main technical indicators are the same or higher than the Japanese standard JIS G3601-1990. They specified in different standards for pressure vessels. From the terms of cost accounting, the rolling cladding is calculated by ton while explosive cladding is calculated by explosive area. Some engineers concluded that from the actual production: with the limit of 20mm, the thick steel plate should be explosive composite while the thin steel plate should be rolled composite.

 

How to make titanium alloy easy to machine?

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As everyone knows, Titanium and its alloys are difficult to machine and process due to their high strength, low thermal conductivity and chemical reactivity with tool materials (at elevated temperatures), pose a hazard to the tool and significantly reduce the tool life. In addition, a relatively low Young’s modulus leads to spring-back and chatter leading to poor surface quality of the finished product. During turning and drilling, long continuous chips are produced; causing their entanglement with the cutting tool and making automated machining near impossible. In the last article, we explained what-makes-alloyed-titanium-grade5-so-difficult-to-machine in details.  Today I will discuss what should I do to makes Titanium easy to machine and supplied several tips about the  processing of most commonly used alloy Titanium grade 5 alloy ( 6Al-4V) :

 

  • Using carbide cutting tools. Tungsten-cobalt cemented carbides are characterized by high strength and good thermal conductivity and are not easy to react with titanium at high temperature. They are suitable for processing titanium alloys.
  • Select reasonable geometry parameters of tools. In order to reduce the cutting temperature and tool bonding, the front Angle of the tool can be appropriately reduced and the contact area between the chip and the front cutter surface can be increased to dissipate heat. At the same time, the rear angle of the cutter is increased to reduce the phenomenon of tool bonding and the precision of the machined surface is reduced due to the rebound of the machined surface and the friction contact between the machined surface and the machined surface. The tip should use arc transition to enhance tool strength. It is necessary to maintain the sharpness of the sharpener frequently to ensure that as little cutting heat as possible is generated during the processing.
  • Appropriate cutting parameters. Lower cutting speed – high cutting speed will lead to a sharp increase in cutting temperature; Moderate feed — large feed leads to high cutting temperature, while small feed leads to accelerated wear of the blade due to long cutting time in the hardened layer; Greater cutting depth — cutting beyond the hardened layer on the titanium alloy surface of the tip improves tool life.
  • Maintain a high cutting fluid flow and pressure. Sufficient continuous cooling of the machining area is required to reduce the cutting temperature.
  • Avoid machine tool vibration. Vibration can cause blade breakage and blade damage. Choose a larger cutting depth, but the titanium alloy machining rebound, the larger clamping force will aggravate the workpiece deformation, finishing can consider the use of jigs and other auxiliary tools to meet the stiffness requirements of the processing system.
  • The climbing milling methods by which milling is carried out. In titanium alloy processing, the milling cutter caused by reverse milling is much more damaged than the milling cutter caused by climbing milling.
  • Grinding with a green silicon carbide grinding wheel. The sticky chips will cause the blockage of grinding wheel and the surface burns of parts. Therefore, it is appropriate to use the green silicon carbide grinding wheel with sharp grinding particles, high hardness and good thermal conductivity. The grinding wheel size can be F36 ~ F80 that depending on the surface finish. The hardness of the grinding wheel should be soft so as to reduce the adhesion between grinding particles and grinding chips and the grinding heat. At the same time to ensure a small grinding and low speed, sufficient emulsion.
  • Drilling. Standard drill bits need to be polished during drilling to reduce burners and broken bits. Grinding method: increase the top Angle and decrease the front angle of cutting part, increase the back Angle of cutting part, double the number of taper of cylindrical edge. During processing, the cutting times should be increased and the drill bit should not stay in the hole, sufficient emulsion cooling, timely removal of chips and observe whether the drill bit becomes blunt.
  • Titanium alloy reaming needs to be calibrated. The width of the blade belt should be less than 0.15mm. Multiple reamer sets can be used for multiple reamers. The diameter of each reamer increase shall be less than 0.1mm. Reaming with this method can meet the requirement of a high finish. Handle cleaned titanium alloy parts should wear clean gloves, to avoid sodium chloride stress corrosion.
  • Tapping is the most difficult process in titanium alloy processing. Excessive torque causes rapid wear of the tap cutter teeth, and the rebound of the processed part can even break the tap in the hole. Ordinary tap processing should be according to the diameter size appropriate to reduce the number of the chip to increase the space, set aside on the calibration of tooth belt should be after 0.15 mm width of blade Angle increases to about 30 °, remove tooth back 1/2 ~ 1/3, calibration tooth number 3 after increases taper pouring. If you want to achieve better processing results, jumping wire is a good choice, which can effectively reduce the cutting tool and the workpiece contact area.

 

It is worth noting that: It is important to use non-combustible or non-combustible tools to transfer titanium chips and ensure that the cutting area has fire protection facilities. Trace cutting of titanium chips once a fire can be dry powder extinguishing agent or dry soil, dry sand extinguishing.

Anodic oxidation technology of titanium and its alloy

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Anodic oxidation is a commonly used surface protection treatment method for titanium and its alloys. Anodic oxidation of titanium refers to the method of generating oxygen on the anode in an electrochemical way and reacting with the surface of the anode titanium to form an oxidation film, also known as titanium electrode with metal oxide coating, it was first used in chlor-alkali production and has been widely used in chemical industry, environmental protection, water electrolysis, water treatment, electrometallurgy, electroplating, foil production, organic synthesis, electrodialysis, cathodic protection and other fields.

There are several ways to anodize titanium workpiece. At present, the anodic oxidation of titanium and titanium alloys is mainly carried out in acidic solution. The color, thickness and properties of the anodized film are different from the anodizing solution and process conditions. Place the titanium workpiece into the anodizing bath which can be regarded as an electrolyte solution. Opposite to a plating process, the current flows from the workpiece in the anodizing process. The following is the composition, content and process conditions of various anodic oxidation solutions:

 

  • Oxalic acid oxidation

Oxalate: 55 to 60 g/L

PH: 0.5 to 1.0

Temperature: 18 to 25 ℃

The area ratio of the anode and the cathode: 1:10

Anodic oxidation initial current density of 1.0-1.5 min, maintain the current density gradually will continue to oxidation voltage up to 100-120 – v then 30-50 min, current density automatically dropped to 1.0 0.3 A/d ㎡. Oxalic acid extremely oxygen change got oxygen of the film thickness is only 0.2-0.3 (including m, the pale gray, abrasion resistance and corrosion resistance. If the anodic oxidation under 8 ℃, and can improve the wear resistance of oxide film.

 

  • Pulsar oxidation

Sulfuric acid： 360-370 g/L

Phosphate： 16 and 32 g/L

Temperature： 0-13 ℃

The anode and the cathode area ratio： 2:1

Pulse time：0.15-0.30s

Pulse frequency：40-120 / min

Current density：30-7 A/d ㎡

The oxidation time ：10-20min

The solution at the beginning of the oxidation temperature was controlled in the lower, at 1.0 to 1.5 min flat steady current flow degree up to 7 a/d ㎡ keep 1.0 to 2.0 min. Allows for 1.0 2.0 A/d electrical flow dense degree of oxygen 40 ㎡ plus or minus 10 min. The thickness of oxide film is 2 um, brown has good wear resistance, corrosion resistance and prevent stickiness.

 

  • Thick film oxidation

Sulfuric acid：350-400 g/L

Hydrochloric acid：60-65 g/L

Temperature：40-50 ℃

Current density：2-4 A/d ㎡

At the rate of 0.3A/3dm, the current density was gradually increased to 2-4A /dm, and this current density was maintained to oxidize to the required oxide film thickness. The thickness of the oxide film obtained by this process can reach 20-40 m. With the increase of the thickness of the oxide film, the color of the oxide film changes from gray to gray-black. The oxide film has high hardness and good wear resistance. The oxidation film is porous and has good adsorption. If the colloidal graphite and dry film lubricant are immersed, the wear resistance can be further improved.

 

  • Colour anodic oxidation

There are many formulas for anodizing titanium and titanium alloys. The color of oxidant film was obviously affected by the change of process conditions.

Formula①:

chromic anhydride: 120-150g/L

Boric acid: 3 to 5 g/L

Temperature: 18 to 25 ℃

Formula②：

Phosphate: 50-200 g/L

Organic acids: 20-100 g/L

Temperature: 18 to 25 ℃

When formula① was used, the voltage was gradually increased from 5V to 50V within 15MIN, and the oxide film was first light brown, then blue-purple, and finally gold. If the voltage rises steadily to 50V within 1-2min and is maintained for 15MIN, the oxide film changes from light blue to golden yellow. When using formula② for oxidation, gradually increase the voltage from 5V to 9V within 20MIN, and oxidation films of different colors will appear in different voltage segments.

 

When titanium and titanium alloy anodic oxide film does not meet the requirements, you can also use mechanical processing or blowing sand or chemical method to remove the oxide film according to the size and accuracy of the parts, and then re-anodic oxidation.

What makes alloyed titanium grade5 so difficult to machine?

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Titanium alloy has the advantages of low density, high strength ratio (strength/density), good corrosion resistance, heat resistance, toughness and plasticity, etc. It is widely used in many fields such as aerospace, automobile, medicine, sports goods and electrolysis industry. However, its poor thermal conductivity, high hardness, low modulus of elasticity and other characteristics make titanium alloys become more difficult to process metal materials. Gr.5(Ti6AI-4V ) is the most commonly used titanium alloy grades. Today we will analyze why gr5 is difficult to machine.

 

  1. Low thermal conductivity

Grade 5 titanium alloy at 200 ℃ heat conductivity l = 16.8 W/m?℃, the coefficient of thermal conductivity is 0.036 ℃ / cm, is only a quarter of the steel, aluminum, 1/13 of 1/25 of the copper. Heat in the process of cutting of titanium alloy will not quickly be passed to the workpiece or taken away by chip, and agglomeration in cutting area, the temperature can be as high as 1 000 ℃ above, make the cutter blade quickly wear, split, produce more heat and the cutting area, further shorten the tool life. The high temperature in the cutting process destroys the surface integrity of the titanium alloy parts at the same time, which leads to the decrease of the geometrical precision of the parts and the machining hardening phenomenon which seriously reduces their fatigue strength.

  1. Low elastic modulus

The surface of the machined parts has great resilience, which leads to the increase of the contact area between the machined surface and the cutter surface, which not only affects the dimensional accuracy of the parts but also reduces the tool durability. The elasticity of titanium alloy may be beneficial to the performance of parts, but in the cutting process, the elastic deformation of the workpiece is an important cause of vibration. Cutting pressure causes the “elastic” workpiece to leave the tool and rebound, thus making the friction between the tool and the workpiece greater than the cutting effect. Friction process also produces heat, aggravating the poor thermal conductivity of titanium alloys. This problem is more serious when machining thin-walled or annular parts, which are easily deformed. It is not easy to process titanium alloy thin-walled parts to the expected dimensional accuracy. Because as the workpiece material is pushed by the tool, the local deformation of the thin wall has exceeded the elastic range and plastic deformation, cutting point material strength and hardness increased significantly. At this point, machining at the previously determined cutting speed becomes too high, further resulting in sharp tool wear.

  1. High hardness

When some titanium alloys with low hardness value are processed, the viscosity increases and the chip sticks to the cutting edge of the cutter to form a chip tumor, which affects the processing effect. The hardness of titanium alloy such as Grade5 titanium alloy machining is easy to make the tool edge collapse and abrasion. These characteristics result in a low metal removal rate of titanium alloy, which is only 1/4 of that of steel pieces while the processing time is much longer than that of steel pieces of the same size.

  1. Strong chemical affinity

Titanium can not only react with the main components in the air such as nitrogen, oxygen, carbon monoxide and other substances, forming TiC and TiN hardened layer on the surface of the alloy, but also react with the tool material under the high-temperature conditions generated by cutting, which reduces the tool durability.

  1. Poor safety performance in the cutting process

Titanium is a flammable metal. The high temperature and spark are extremely flammable and if ignited, are almost impossible to put out during the micro-cutting.

 

Compared with most other metal materials, titanium alloys have higher processing requirements and more restrictions. Satisfactory machining results for titanium alloys can also be obtained if the proper cutting tools are used and the machine tools and configurations are optimized to the best condition according to the machining requirements.

The commonly used materials of metal bellows

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Metallic bellows are wave-shaped tubes, also known as expansion or expansion joints. Metal bellows are elastic pipes that can be compressed or extended when pressure is applied to the outside of the pipe. When the pressure released and the material has not been stressed past its yield strength, the bellows will return to its original shape. The metal bellows are mainly used for non-concentric axial transmission with small bending radius, irregular turning, expansion or absorption of thermal deformation of pipes, etc., which is not convenient to connect pipes with pipes or connect pipes with equipment in the installation of fixed elbows.

Due to its excellent performance, it is widely used in automatic control and measuring instruments, vacuum technology, mechanical industry, electric power industry, transportation and atomic energy industry and other fields such as sensitive components, shock absorption components, non-concentric axial drive components, compensation components, sealing components, valve components and pipeline connections. Metal bellows should be selected according to the application and performance of different metal materials. The Common materials of metal bellows are carbon steel, copper, stainless steel, steel lined plastic, aluminum, titanium and so on. Today, lkalloy will discuss with you the common materials of metal bellows in details:

 

Copper alloy

Tin bronze C51900 ~ 0.1 X – 60 ℃ ~ + 100 ℃ metal bellows tube combined excellent flexibility and strength, has a good brazing and corrosion resistant performance. Hysteresis and elastic aftereffect are small and widely used as measuring elements.

Brass Bellows C24000 H – 60 ℃ ~ + 100 ℃ with low elasticity, large hysteresis and aftereffect, brazing sex is good.Can be used for non-corrosive media and precision is not high in the instrument as a measuring element.

Beryllium bronze Bellows C17510 P – 60 ℃ ~ + 150 ℃, nonmagnetic, have a very small hysteresis and elastic aftereffect, high flexibility, corrosion resistance, used for high precision measuring instrument.

 

Stainless steel

Bellows SUS321 G – 194 ℃ ~ + 400 ℃ has the very high bending fatigue strength and corrosion resistance, good welding performance, can be used as a measure of corrosive medium, sealing, connection and compensating element. Common brands are 304, 304L, 316, 316L, 317L, 310S, 321, etc. There are more special, high corrosion resistant of 6Mo special stainless steel (super stainless steel) such as 254SMO, 904L, N08367 or N08926. Duplex stainless steel offers high strength but poor elongation, it’s not recommended for small bellows, large bellows processing can be considered according to the technical conditions.

 

Nickel-based alloy

High corrosion and high-temperature conditions should take into account the resistance to chloride ions, sea water, acid, alkali corrosion and high temperature resistant alloy materials, such as high nickel alloy, nickel ferrochrome alloys such as Incoloy 800H, Incoloy 825, Inconel 625, Monel400 and alloy c-276.

 

Titanium

Generally, it is pure titanium Gr.1. It has high elongation while poor welding performance, which should be taken into consideration.

 

 

What are the effects of alloying elements on Nickel-based steel?

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The Nickel retains its austenitic cubic structure until it reaches the melting point, which provides freedom for ductile-brittle transition and greatly reduces manufacturing problems caused by the presence of other metals. Nickel is more inert than iron and more active than copper, so the order of corrosion resistance in reducing environment is Copper > Nickel > Iron. The addition of chromium on the basis of nickel provides the alloy with antioxidant properties, which can produce a wide range of alloys with the best resistance to both reducing and oxidizing environments.

For a certain nickel-based alloy, there are a variety of changes in the specific environment such as concentration, temperature, ventilation, liquid (gas) flow velocity, impurities, abrasion, cycle process conditions, etc., which will produce a variety of corrosion problems. These questions can be answered in nickel and other alloying elements.

Nickel-based alloys can hold more alloying elements in the solid solution state while maintaining good metallurgical stability compared with stainless steel and other Fe-based alloys. These factors allow the addition of a wide variety of alloying elements, making nickel-based alloys widely used in a wide variety of corrosive environments. The common elements in Nickel-based alloys are:

Ni

Nickel provides metallurgical stability, improves thermal stability and weldability, increases corrosion resistance to reducing acids and caustic soda, and in particular increases stress corrosion cracking resistance in chloride and caustic soda environments.

Cr

Chromium improves the properties of oxidation resistance, high-temperature oxidation resistance, vulcanization resistance, high pitting resistance and gap corrosion resistance.

Mo

Molybdenum improves the corrosion resistance of reducing acid, the resistance to pitting corrosion, gap corrosion and high-temperature strength under the environment of chloride aqueous solution.

Fe

Iron improves resistance to high temperature carburizing environment, reduces alloy cost and controls thermal expansion.

Cu

Copper improves corrosion resistance to reducing acids (especially sulfuric acid and hydrofluoric acid, which are used in airless applications) and salts. The addition of copper to nickel-chromium-molybdenum-ferroalloys helps to improve corrosion resistance to hydrofluoric acid, phosphoric acid and sulfuric acid.

Al

Aluminum improves high-temperature oxidation resistance and aging hardening.

Ti

The combination of titanium and carbon reduces the intergranular corrosion caused by chromium carbide precipitation during heat treatment and improves the aging strengthening.

Nb

The combination of Niobium and carbon reduces intercrystalline corrosion caused by chromium carbide precipitation during heat treatment and improves pitting corrosion resistance, clearance corrosion performance and high-temperature strength.

W

Tungsten improves the resistance to reducing acid and local corrosion as well as its strength and weldability.

N

Nitrogen improves metallurgical stability, pitting corrosion resistance, gap corrosion resistance and strength.

Co

Cobalt provides enhanced high temperature strength, carbonation resistance, and vulcanization resistance.

 

Many of these alloying elements can be combined with nickel over a wide range of compositions to form single-phase solid solutions, ensuring excellent corrosion resistance of the alloys under many corrosive conditions. The alloy also has good mechanical properties under complete annealing without worrying about harmful metallurgical changes during manufacturing or hot working. Many high Nickel alloys can be strengthened by solid solution hardening, carbide precipitation, precipitation (aging) hardening and dispersion hardening.

Which alloy material is most resistant to caustic soda?

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Caustic alkali generally refers to caustic soda and caustic potassium, namely sodium hydroxide and potassium hydroxide. Caustic embrittlement or caustic cracking occurs when the alloy material cracks under the action of tensile stress and corrosion medium in alkaline solution. In alkaline solutions, the concentration of hydrogen ions is usually low and the corrosion rate in chemical media usually decreases with the increase of PH value. At certain PH values, the corrosion rate of some metals hovers around the lowest value, while the PH value increases continuously as the corrosion degree increases.

Caustic soda has a wide range of applications. It can be used in papermaking, textile printing and dyeing, alumina, daily chemical, pharmaceutical, water treatment, steel and other industries. Caustic corrosion occurs when metals come into contact with caustic soda. Caustic corrosion can lead to pitting and other localized corrosion, as they tend to form cathode films that concentrate corrosion in susceptible anode areas. Austenitic stainless steels and other low Nickel materials may experience stress corrosion cracking or general corrosion in hot caustic soda.

Nickel and its alloys offer excellent alkali corrosion resistance for high temperature and concentration environments. Generally, the corrosion resistance of caustic soda usually increases with the increase of nickel content. In alkaline media, the most commonly used nickel alloys are Nickel 200/201, Monel 400, Nickel 600 and 625.

Plant tests in 23% caustic soda in tank receiving liquor from evaporator. average temperature,104℃
Material Corrosion rate，mpy(mm/a)
Nickel 200 0.16(0.004)
Monel alloy 400 0.20(0.005)
Inconel alloy 600 0.17(0.004)

Factory and laboratory test results, some of which are shown in the table below, show that nickel 200/201 and high nickel alloys are very satisfactory for the treatment of these materials

Laboratory corrosion tests in evaporation of caustic soda from 73-96 %, with and without chlorate
Materials Corrosion rate, mpy(mm/a)
Without chlorate with 0.30% Chlorate
Nickel 200 1.5(0.038) 260(6.60)
Inconel alloy 600 2.2(0.056) 380(9.65)

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Anti-corrosiont nickel based alloy Hastelloy C22 VS C276

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Hastelloy alloy has good resistance to pitting corrosion, crevice corrosion and stress corrosion cracking. It was well tolerated for a variety of corrosive medium in oxidation and reduction environments, even as high temperature up to 677 ℃; Hastelloy c-22 and c-276 are the only materials that can resist the corrosion of wet chlorine gas, hypochlorite and chlorine dioxide solution, and offering significant corrosion resistance to high concentration of chloride solution (such as iron chloride and copper chloride).

Hastelloy c-22 and c-276 have excellent welding performance and have been widely used in chemical and petrochemical fields, such as components in contact with chlorinated organics and catalytic systems. They are especially suitable for high temperature, mixed with impurities of inorganic acids and organic acids (such as formic acid and acetic acid), seawater corrosion environment.

The specification difference from different counties is as follows:

Chinese GB/T 15007-2008 NS3308 NS334
American ASTM B462-2006

ASTM B574-2006

ASTM B575-2006

 N06022 N10276
Special Metals INCONEL® alloy 22 INCONEL® alloy 276
Haynes International HASTELLOY® C-22® HASTELLOY® C-276®
Japanese JIS H 4551:2000 NW6022 NW 0276
Nippon Yakin NAS NW22 NAS NW276
German DIN 17744:2002 NiCr21Mo14W

2.4602

 NiMo16Cr15W

2.4819
VDM Metals VDM® Alloy 22

Nicrofer® 5621 hMoW

 VDM® Alloy 276

 

The chemical composition (%)difference is as follows:

Grade Ni C Mo Cr Fe W Co Mn V
C22 56 0.01 13 22 3 3 2.5 0.5 0.35
C276 57 0.01 16 16 5 4 2.5 1 0.35

We can learn that from the table above, there are few differences in the term of chemical analysis except the content of Mo and Cr. Alloy 22 has a higher Chromium and Molybdenum content than C276, demonstrating improved corrosion resistance in oxidizing aqueous media when high levels of chloride ions are present. In addition, the other differences are:

  1. Welding material：Varestraint hot-cracking tests performed on commercial alloys revealed a weldability ranking as follows: C-22 > C-276. Intermetallic secondary solidification constituents have been found associated with weld metal hot cracks in Alloys C-276 and C-22(gas-tungsten-arc welds). In Alloy C-276, this constituent is a combination of P and ώ phases, and in Alloy C-22, this constituent is composed of σ, P, and ώ C276 welding materials are wire ernicrmo-4 and electrode enicrmo-4; C22 welding material is wire ERnicrmo-10, electrode Enicrmo-10.
  2. C22 offers better and more comprehensive corrosion resistance compared with common nickel-chromium-molybdenum alloys such as C-276, C-4 and 625. C22 can be considered as an upgraded version of C276.
  3. Hastelloy c-22 alloy has a density of 8.7g/cm3, less than that of C276 alloy (8.9). However, C22 is more expensive because the manufacturing cost of C22 is higher than that of C276.

Each alloy has been manufactured to provide a unique set of benefits, wherever it is applied. Both commonly used corrosion resistant alloy. Hastelloy C22 and C276Hastelloy C22 and C276 generally, replace each other in most cases. C276 is more versatile and the C22 offers better performance.