Application of nickel base alloy Inconel 690 in coal to methanol process pipeline
In the coal to methanol production unit, the pipeline from the heating furnace to the reformer is used to transport the mixture of gas and steam under high temperature and high pressure. The material for manufacturing this pipeline is required to have lasting strength, oxidation resistance and vulcanization resistance under high temperature and high pressure. The mixture of gas and steam is a flammable and explosive medium, which is very dangerous. It is required that the pipeline conveying this medium must be safe. The nickel base alloy inconel 690 is an ideal material for manufacturing this kind of nickel base alloy pipe. It has been running for many years without accidents, which shows that the application of inconel 690 alloy in coal to methanol process pipeline is feasible.
China coal energy and a Coal Chemical Co., Ltd. built a set of methanol production unit with an annual output of 250000 tons. Pipes, pipe fittings, flanges and valves of methanol partial oxidation unit are made of special materials Inconel 690 or Inconel 600. The partial oxidation unit adopts steam catalytic partial oxidation technology to convert the hydrocarbon gas in the purified gas into hydrogen, carbon monoxide and carbon dioxide. The special material Inconel 690 is used for the pipeline from heating furnace to reformer. The medium is the mixture of purified gas and steam at 550 ℃ and pressure of 2.1MPa.
The pipes are rolled from plates and imported from ThyssenKrupp VDM (Germany), with the commercial brand of nicrofer6030, i.e. Inconel 690. The plate standard is asmesb168, which is hot-rolled plate in solution treated state.
Based on the chemical composition of Inconel 600 (nominal composition 76Ni-15Cr-8Fe-0.08C), the chromium content is increased to 30% and the carbon content is reduced to less than 0.05%, which is inconel 690 (nominal composition 58Ni-29Cr-9Fe-0.02C) alloy. Inconel 690 has better stress corrosion cracking resistance than Inconel 600 alloy when used as heat exchange tube of steam generator of nuclear island PWR, which is a typical application and characteristic of inconel 690 alloy. Inconel 690 alloy is also used for nitric acid and sulfur-containing gas at high temperature.
Inconel 690 alloy has excellent corrosion resistance in many alkaline or acidic solutions. It is especially suitable for treating nitric acid or mixed acid of nitric acid and hydrofluoric acid. The most prominent corrosion characteristic of Inconel 690 alloy is stress corrosion cracking resistance. It has stronger stress corrosion cracking resistance than Inconel 600, Incoloy 800 (nominal composition 33Ni-42Fe-21Cr-0.05C), s31603 stainless steel in high temperature and high pressure pure water, chloride containing solution and sodium hydroxide solution. The alloy has good resistance to oxidation and vulcanization at high temperature and high chromium content, which makes it especially suitable for sulfur etching at high temperature. It has high strength in a wide temperature range. High metallurgical thermal stability. Easy to cold and hot processing, forming, etc.
Inconel 690 alloy has a density of 8.2g/cm3 and a melting point of 1345 ℃ – 1375 ℃ .
Inconel 690 is a Ni Cr Fe alloy, and its chemical composition is shown in Table 1. The outstanding feature of composition is high chromium content and very low carbon content. Nickel can reduce the stress corrosion cracking sensitivity of Inconel 690 alloy in alkaline solution. The action principle of chromium on the corrosion resistance of alloy is the same as that of stainless steel. The higher the chromium content, the better the oxidation resistance, medium corrosion resistance, high temperature oxidation resistance and vulcanization of the alloy. Increasing chromium content can reduce the sensitivity of Inconel 690 alloy to stress corrosion cracking in high temperature water containing chloride ion and oxygen. When the carbon content of Inconel 690 alloy is less than 0.03%, it has small tendency of medium temperature sensitization and stronger ability of intergranular corrosion resistance.
Table.1 chemical composition (wt%) of Inconel 690 plate (12mm thick)
|ASME SB168 standard value ||≥58.0||27.0-31.0||7.0-11.0||≤0.50||≤0.05||≤0.05||≤0.05||≤0.015|
Table.2 mechanical properties of Inconel 690 plate (12mm thick) at room temperature
|Project||Tensile strength RM (MPA)||Yield strength Rp0.2 (MPA)||Elongation A5 (%)|
|ASME SB168 standard value ||≥586||≥240||≥30|
Inconel 690 alloy has medium yield strength of 240mpa-345mpa and high elongation of 55% – 35% in solid solution state, which makes the alloy easy to cold and hot forming.
The mechanical properties of Inconel 690 are similar to those of ordinary austenitic stainless steel s30408, and have a good combination of strength, plasticity and toughness. Cold forming has work hardening, and the hardening rate is higher than Inconel 600 and lower than austenitic stainless steel s30408 . Inconel 690 is a solid solution alloy, which can not be strengthened by heat treatment, but can be strengthened by cold deformation.
Inconel 690 is both a corrosion-resistant alloy and a heat-resistant alloy. The mechanical properties of the supplied plates are shown in Table 2. See Table 3  and table 4  for mechanical properties at high temperature. At 550 ℃ when the pipeline is running, the tensile strength reaches 470mpa, with medium strength, and the creep strength is not lower than 100MPa.
Table.3 Mechanical Properties of Inconel 690 at different temperatures for typical short time
|Temperature (℃)||Tensile strength RM (MPA)||Yield strength Rp0.2 (MPA)||Elongation A5 (%)|
Table.4 typical creep properties of Inconel 690 in solid solution state
|Temperature (℃)||Creep strength (MPA)|
Alloys have excellent corrosion resistance in many alkaline or acidic solutions and high temperature gas environment. The high chromium content makes Inconel 690 alloy especially suitable for oxidizing acid corrosion and sulfur corrosion at high temperature.
Inconel 690 alloy has good corrosion resistance to oxidizing acids due to its high chromium content. It is especially suitable for treating nitric acid or mixed acid of nitric acid and hydrofluoric acid. Inconel 690 alloy shows excellent corrosion resistance in the mixed acid of nitric acid and hydrofluoric acid. Table 5  shows the corrosion rate data of solution treated thin plate samples in mixed acid with three concentrations and ratios obtained by the laboratory, and the test temperature is 60 ℃.
Table.5 corrosion rate of Inconel 690 alloy in mixed nitric acid and hydrofluoric acid
|Solution ratio||Corrosion rate|
|10% nitric acid + 3% hydrofluoric acid||6||0.15|
|15% nitric acid + 3% hydrofluoric acid||10||0.25|
|20% nitric acid + 2% hydrofluoric acid||8||0.15|
*The above is the average value of corrosion rate of double samples * corrosion rate unit: 1mpy (milliperyear, British wire/year) = 0.0254mm/a (millimeterperannum, mm/year)
Inconel 690 alloy has high corrosion resistance to phosphoric acid at room temperature and medium temperature. The test shows that the corrosion rate of the alloy in phosphoric acid with a concentration of 85% from room temperature to 80 ℃ does not exceed 0.03 mm/year. However, at boiling temperature, the alloy can only resist low concentration phosphoric acid. For example, when the concentration is 20%, the corrosion rate is about 30mpy (0.8mm/year); At higher concentrations, the corrosion rate will exceed 100mpy (2.5mm/year).
Inconel 690 alloy has good corrosion resistance to all concentrations of sulfuric acid at room temperature.
Stress corrosion cracking
The most prominent corrosion characteristic of Inconel 690 alloy is stress corrosion cracking resistance.
Inconel 690 alloy has good resistance to stress corrosion cracking in many environments including chloride solution, high-temperature water, polysulfuric acid and medium concentration sodium hydroxide solution.
The U-shaped bending specimen of Inconel 690 alloy did not crack after 30 days in boiling 45% magnesium chloride solution.
Inconel 690 alloy has been tested for stress corrosion cracking in high temperature water encountered in steam generators of nuclear power plants. The results show that the alloy has high cracking resistance in chloride containing water, oxygen containing water under crevice conditions and deoxidized water (oxygen content of 0.02 ppm or less).
In the continuous polysulfuric acid solution, no matter the solid solution treated sample or the sample sensitized at 315 ℃ for 1000 hours, the U-shaped bending sample of Inconel 690 alloy did not crack after 720 hours, while the sensitized 304 stainless steel cracked within 1 hour.
Table 6  shows the stress corrosion cracking test results of Inconel 690 alloy U-bend specimen in sodium hydroxide solution. The test is carried out in the autoclave. There is air in the upper part of the autoclave, that is, the sodium hydroxide solution is non deoxidized, and the test temperatures are 260 ℃ and 320 ℃ respectively. The test results show that under the condition of high temperature and pressure, Inconel 690 alloy will produce stress corrosion cracking in 30% sodium hydroxide solution; Under atmospheric pressure, Inconel 690 alloy has the ability to resist stress corrosion cracking in boiling sodium hydroxide solution with concentration up to 50%.
When the concentration of sodium hydroxide solution reaches more than 30% at 260 ℃ and 320 ℃, Inconel 690 alloy will undergo serious uniform corrosion.
Table.6 stress corrosion cracking test data in sodium hydroxide solution
|Concentration (%)||Temperature (℃)||Test result|
|10||320||No cracking after 100 hours|
|20||260||No cracking after 1000 hours|
|20||320||No cracking after 1000 hours|
|30||260||Cracking after 1000 hours|
|40||260||Cracking after 1000 hours|
|40||320||Cracking after 100 hours|
|50||260||Cracking after 100 hours|
|50||150*||No cracking after 1000 hours|
|60||160*||Cracking after 1000 hours|
*Is the boiling temperature of sodium hydroxide solution at this concentration under atmospheric pressure
In deoxidized 1% sodium hydroxide solution at 316 ℃, the U-shaped bending sample does not crack after 9400 hours.
Metallurgical thermal stability
When the nickel base alloy with chromium as the main alloy element is subjected to welding thermal cycle and incorrect heat treatment, chromium carbides will precipitate from the weld metal, heat affected zone or base metal, resulting in “chromium deficiency” at the grain boundary. This phenomenon is called sensitization. The sensitization is related to the carbon content of the alloy. The higher the carbon content, the greater the sensitization tendency. Sensitization will bring two hazards: one is to embrittle the alloy and reduce the impact toughness; Second, the sensitized alloy will accelerate intergranular corrosion in some water-soluble media. The sensitization temperature of nickel base alloy is not very high, mostly between 550 ℃ – 900 ℃, so it is called “medium temperature sensitization”. The sensitization temperature of Inconel 690 is 550 ℃ – 850 ℃, and the optimum sensitization temperature is 750 ℃ .
Inconel 690 is an austenitic solid solution alloy with metallurgical thermal stability. No brittle metal phase, such as sigma phase, was found after exposure to critical medium temperature (560 ℃ – 760 ℃) for 12000 hours or more（ σ Phase). However, due to the low solubility of carbon in solid solution, chromium carbides generally appear in the gold phase. The precipitation amount of carbides changes with the carbon content in the alloy and the length of high-temperature exposure time. Table 7  shows that there is no significant change in room temperature tensile strength and impact toughness of Inconel 690 alloy exposed to high temperature for a long time. This is due to the low carbon content and strong anti sensitization ability of Inconel 690 alloy.
Table.7 Effect of medium temperature sensitization on mechanical properties of Inconel 690 at room temperature
|Sensitization temperature (℃)||Sensitization time (H)||Yield strength Rp0.2 (MPA)||Tensile strength RM (MPA)||Elongation A5 (%)||Impact toughness AK (J)|
*Inconel 690 was softened and annealed before sensitization.
Inconel 690 alloy has good oxidation resistance and vulcanization resistance in high temperature sulfur-containing gas environment.
Figure 1  shows the ability of Inconel 690 alloy to resist periodic oxidation at 1095 ℃ (heating for 15 minutes and cooling for 5 minutes). It can be seen that the oxidation resistance of Inconel 690 alloy is stronger than that of Incoloy 825 and Inconel 600.
Fig.1 high temperature oxidation resistance curve of inconel 690 alloy
Table 8  shows the corrosion rate of Inconel 690 alloy in oxidizing and reducing sulfide atmosphere. It can be seen that the vulcanization resistance of Inconel 690 alloy is stronger than Inconel 600.
Table.8 corrosion rate of Inconel 690 in 727 ℃ vulcanization atmosphere
*The sample is a round bar sample, and the corrosion rate comes from the weight loss of the sample with completely removed oxide scale. The test duration is 96 hours.
Metal pulverization 
When the mixed gas composed of CO, H2 and H2O is transported in the pipeline and the temperature is 400 ℃ – 800 ℃, a so-called “metal dusting” phenomenon may become a potential problem of serious corrosion of the pipeline. This erosion often has disastrous consequences.
Metal pulverization is the result of high temperature carburization. Carbon penetrates into the alloy atomic lattice through the damaged oxide film on the alloy surface to form a supersaturated solid solution of carbon. For nickel based alloys, this supersaturated solid solution will be directly decomposed into metal particles and graphite, leaving pits on the metal surface after falling off.
The oxide film plays an important role in protecting against metal powder etching. The oxide film is required to be firm and dense and have the ability of automatic repair. Chromium is the main element to form protective oxidation film. Increasing the chromium content of the alloy can significantly improve the protection of the surface film. Therefore, increasing the chromium content is conducive to the ability of the alloy to resist metal pulverization.
Fig.2 metal pulverization resistance curve of inconel 690 alloy
Inconel690 alloy has good resistance to metal pulverization. Figure 2 shows the exposure time and maximum etching depth of inconel 690 alloy in 80% CO + 20% H2 mixture at 621 ℃ compared with the other two heat-resistant alloys inconel 600 and Incoloy 800. The chromium content (30%) of inconel 690 alloy is higher than inconel 600 (Cr15.5%) and Incoloy 800 (Cr21%), and the resistance to metal pulverization is also much stronger than inconel 600 and Incoloy 800.
The softening annealing temperature of Inconel 690 is 1020 ℃ – 1070 ℃ , and the solution treatment temperature is 1080 ℃ – 1150 ℃ . The strength of the alloy after softening annealing is higher.
The three conditions for deciding whether to carry out solution treatment are:
- 1) the size of cold deformation;
- 2) Whether it has been formed by hot working;
- 3) Whether harmful phase precipitates after welding.
According to the data , softening or solution treatment shall be carried out when the cold deformation is greater than 10%. The formula  for calculating the deformation of plate drum is as follows:
- ε— Deformation
- R – inner radius of cylinder
- S – wall thickness
According to this formula, when the inner radius of the cylinder is equal to 3 times the wall thickness, the deformation is about 14%. about φ 508mm × The deformation of 12mm pipeline is 2.42%; φ 406mm × The deformation of 9mm is 2.26%. The deformation of these two specifications of pipes is far less than 10%. Therefore, softening annealing or solution treatment after cold deformation is not required.
The temperature range of hot working of inconel 690 alloy  is that the maximum hot working (forging or hot rolling) temperature is 1230 ℃; The forging or bending deformation temperature with large deformation is 1040 ℃ – 1230 ℃; Forging with small deformation 870 ℃ – 1040 ℃; Forging or bending deformation is not allowed at 540 ℃ – 870 ℃ (in this temperature range, the change (decrease) of tensile strength and yield strength is not obvious, the elongation does not reach the stable maximum value, and this temperature range coincides with the sensitization temperature). Incorrect thermal processes are taboo for nickel base alloys, such as heating within the sensitization temperature range. In order to obtain good mechanical properties and the best corrosion resistance, inconel 690 alloy should be solution treated after hot deformation.
The straight pipe section of the pipeline is formed by cold rolling, and the elbow is formed by cold stamping (assembly welding in two segments). Therefore, solid solution treatment is not required.
Harmful phase precipitation
The metallographic pictures obtained by German Zeiss metallographic microscope axiovert200mat, FIG. 3-fig. 6 show that chromium carbide (M23C6)  precipitates in the weld and heat affected zone, and the base metal is fine equiaxed crystal.
Fig.3 metallographic diagram of weld center
Fig.4 metallographic diagram of fusion line
Fig.5 metallographic diagram of heat affected zone
Fig.6 metallographic diagram of base metal
Although solution treatment can eliminate the precipitation of chromium carbide after welding, it is not necessary for the working condition (non-water-soluble electrolyte) of this pipeline application.
- 1) The room temperature mechanical properties of the welding procedure qualification are obtained by Changchun Kexin electro-hydraulic servo universal testing machine waw-600 (the tensile sample is plastic broken in the weld, and the average value is 652mpa; The bending samples are intact (no cracking) and meet the requirements;
- 2) The medium high-temperature gas contained in the pipeline is not electrolyte aqueous solution and does not have the tendency of intergranular corrosion;
- 3) There are risks in solution treatment, such as serious deformation of pipeline.
- Based on the above considerations, the solution heat treatment after pipeline welding is not carried out.
Inconel 690 alloy is superior to inconel 600, Incoloy 800 and austenitic stainless steel in many aspects such as high temperature strength, metallurgical thermal stability, oxidation resistance, vulcanization resistance and high temperature carburization (metal pulverization). It is an ideal corrosion-resistant and heat-resistant material for dealing with high-temperature sulfur-containing carbon gas in coal gasification unit.
Solution treatment after welding is good, but the necessity, risk, feasibility and economy of solution treatment should be considered.
Inconel 690 alloy pipeline has been in safe operation for many years without quality accidents.
Authors: Xing Zhuo, Guo Hairong
Source: China Inconel 690 Pipelines Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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