A Comprehensive Guide to Nickel-based super alloy: Incoloy 20 (UNS N08020/DIN 2.4660)
What is Incoloy 20?
Table of Contents
- What is Incoloy 20?
- Chemical Composition Requirements
- Mechanical Properties of Alloy 20
- Physical Properties of Alloy 20
- Thermal Conductivity & Thermal Diffusivity of Alloy 20
- Coefficient of Thermal Conductivity of Alloy 20 (UNS N08020)
- Modulus of Elasticity of Alloy 20 at Different Temperatures
- PMI Test on Alloy 20 Butt Welding Fittings
- Product Forms and Standards
- Application areas of nickel-based alloy incoloy 20 (UNS N08020):
- Alloy 20 Used for Flue-Gas Scrubber
- Alloy 20 for Sulfuric Acid Storage Tank
- Alloy 20 Used for Hydrometallurgy Facilities
- Other Applications
- Alloy 20 Used for Alkylation Process
- ASTM B473 Alloy 20 Bar
- Alloy 20 (UNS N08020) Pipes & Tubes
- ASTM B463 Alloy 20 Plate, Sheet, Strip
- ASTM B366 Alloy 20 Fittings
- Alloy 20 (UNS N08020) Flanges
- Alloy 20 Weld Outlet Class 3000
- Alloy 20 Applicability and Max. Temperature Limits
- Allowable Stress of Alloy 20 at Different Temperatures
- Incoloy 20 (N08020, Alloy 20) nickel-based alloy phase structure:
- Incoloy 20 (N08020, Alloy 20) nickel-based alloy process performance and requirements:
- The Dissimilar Welding of Alloy 20
- Variety specifications and supply status of Nickel-based super alloy: Incoloy 20 (UNS N08020/DIN 2.4660)
- The effect of heat treatment on the organization and properties of UNS N08020 alloy pipe
- 1. Test materials and methods
- 2. Test results and discussion
- 3. Conclusion
Alloy 20, UNS N08020, is also known as Incoloy 020, Incoloy 20, 20Cb-3, CN7M(castings), or DIN 2.4660. It is a nickel-iron-chromium alloy with additions of copper and molybdenum. It has exceptional corrosion resistance to sulfuric acid, choloride stress-corrosion cracking, nitric acid, and phosphoric acid. Alloy 20 can be readily hot-formed or cold-formed to valves, pipe fittings, flanges, fasteners, pumps, tanks, as well as heat exchanger components. The hot forming temperature should be in the range of 1400-2150°F [760-1175°C]. Usually, the heat treatment of annealing should be conducted at the temperature range of 1800-1850°F [982-1010°C]. Alloy 20 is widely used for the production of gasoline, organic & inorganic chemicals, pharmaceutical processing, and food industry.
The Cr content is usually 19.0-21.0%, and the nickel content is 32.0-38.0%. Incoloy 20 alloy is a nickel-based alloy containing molybdenum and copper, which has good hot and cold working properties. Incoloy 20 is a corrosion-resistant alloy with many excellent properties. It has good resistance to oxidizing and moderately reducing corrosion. It has excellent resistance to stress corrosion cracking and good local corrosion resistance. It is satisfactory in many chemical process media. The corrosion resistance characteristics.
Alloy 20 weld neck flanges for an Indonesian client.
Chemical Composition Requirements
|The Chemical Composition of Alloy 20, %|
Mechanical Properties of Alloy 20
|Tensile Strength, min.||Yield Strength, min.||Elongation, min.||Young’s Modulus|
Physical Properties of Alloy 20
|Density||Specific Heat||Electrical Resistivity||Thermal Conductivity|
Tensile Strength of Alloy 20 at Different Temperatures
|Temp °F||-20 to 100||200||300||400||500||600||650||700||750||800|
- *TS1(tensile strength of): ASTM B462 UNS N08020 forgings, annealed; ASTM B463 Alloy 20 plate, annealed; ASTM B473 Alloy 20 bar, annealed; ASTM B729 Alloy 20 seamless pipe & tube, annealed; ASTM B464 UNS N08020 welded pipe, annealed; ASTM B468 UNS N08020 welded tube, annealed.
- *TS2(tensile strength of): ASTM B366 Alloy 20(WP20CB) seamless & welded fittings, annealed.
- *Temp. refers to metal temperature, not exceeding.
- *TS1 & TS2: ksi unit.
Yield Strength of Alloy 20 at Different Temperatures
|Temp. °F||-20 to 100||150||200||250||300||350||400||450|
- *YS1(yield strength of): ASTM B462 Alloy 20 forgings, annealed; ASTM B463 UNS N08020 plate, annealed; ASTM B473 UNS N08020 bar, annealed; ASTM B729 Alloy 20 smls. pipe & tube, annealed; ASTM B464 UNS N08020 wld pipe, annealed; ASTM B468 UNS N08020 wld. tube, annealed; unit: ksi.
- *YS2(yield strength of): ASTM B366 UNS N08020(WP20CB) smls.& wld. fittings, annealed; unit: ksi.
- *Temp.: the metal temperature, not exceeding.
Thermal Conductivity & Thermal Diffusivity of Alloy 20
Coefficient of Thermal Conductivity of Alloy 20 (UNS N08020)
- *TC: the coefficient of thermal conductivity, Btu/hr-ft-°F.
- *Temp.: metal temperature, not exceeding.
Coefficient of Thermal Diffusivity of Alloy 20 (UNS N08020)
- *TD: the coefficient of thermal diffusivity, ft2/hr.
- *Temp.: metal temperature, not exceeding.
All tabulated values are in accordance with ASME Boiler & Pressure Vessel Code Section II, Part D.
Modulus of Elasticity of Alloy 20 at Different Temperatures
|Modulus of Elasticity||30.0||29.3||28.8||28.0||27.4||27.0||26.6||26.2||25.8|
|Modulus of Elasticity||25.4||24.9||24.4||23.9||23.4||22.8||22.2||21.6||20.9|
- *The unit for modulus of elasticity is 106 psi; Temperature unit: °F.
- *All tabular values are in accordance with ASME BPVC Section II Part D Alloy 20 (UNS N08020).
- *Other physical properties of Alloy 20: poisson’s rate – 0.31; density – 0.291 lb/inch3.
PMI Test on Alloy 20 Butt Welding Fittings
PMI test on Alloy 20 Elbows
The elbow specification: 90° elbow, long radius(L/R), 4″ SCH40 butt welding(BW), ASME B16.9. Materials: ASTM B366 WP20CB (UNS N08020/ 20Cb3/ Alloy 20). The PMI test was conducted by using handheld spectrometer which read: Cr: 19.47%, Mn: 0.91%, Fe: 39.02%, Ni: 34.28%, Cu: 3.6%, Nb: 0.58%, Mo: 2.14%.
PMI test on Alloy 20 Stub Ends
The specification of the stub end: ASME B16.9 4″ long pattern. PMI was conducted by using spectrometer which read: Cr: 19.86%, Mn: 0.79%, Fe: 38.50%, Ni: 34.63%, Cu: 3.41%, Nb: 0.57%, Mo: 2.24%. The results conforms to ASME B366 WP20CB (Alloy 20).
Product Forms and Standards
|Rod, bar and wire||ASTM B473, B472, B462|
|Plate, sheet and strip||ASTM A240, A480, B463, B906|
|Seamless pipe and tube||ASTM B729, B829|
|Welded pipe||ASTM B464, B775|
|Welded tube||ASTM B468, B751|
|Welded fittings||ASTM B366|
|Forged flanges and forged fittings||ASTM B462, B472|
Application areas of nickel-based alloy incoloy 20 (UNS N08020):
- Heating pipes, containers, baskets and chains used in sulfuric acid pickling plants.
- Sea water cooling heat exchanger, marine product pipeline system, acid gas environment pipeline.
- Heat exchanger, evaporator, washing, dip tube, etc. in phosphoric acid production.
- Air heat exchanger in petroleum refining.
- Food engineering, chemical process.
- Flame retardant alloy for high pressure oxygen application.
- Heat exchange tubes
- Pipe fittings
Alloy 20 Used for Flue-Gas Scrubber
The flue-gas scrubber is widely used for flue gas desulfurization and denitration in residue fluid catalytic cracking unit (RFCCU) of oil refinery, coal-fired power plant, chemical fertilizer plant, and pharmaceutical plant. Considering the corrosive & abrasive environment, Alloy 20 materials shall be selected for the fabrication of accumulator pool, flue-gas distribution baffles, and flue-gas entrance joint. These materials may include ASTM B729 UNS N08020 seamless pipes, ASTM B463 Alloy 20 plates, ASTM B366 Alloy 20 fittings, ASTM B462 Alloy 20 flanges.
Alloy 20 for Sulfuric Acid Storage Tank
Generally, the most common material for sulfuric acid (with concentration ≥70%) storage tanks is carbon steel due to its low cost. SS 316 can be adopted to avoid iron contamination if required or at low concentrations(≤20%). However, the fittings, flanges, or valves at the feeding entrance and outlets may be made from Alloy 20 which has superior corrosion-resistance & pitting-resistance.
Alloy 20 Used for Hydrometallurgy Facilities
The wetted parts of hydrometallurgy facilities involving sulfuric acid shall be made of Alloy 20.
Alloy 20 Used for Alkylation Process
Recently, Yaang has been supplying a set of Alloy 20 piping materials to an oil-refinery project in Czech. This project involves piping construction of the alkylation process facilities. Due to its exceptional corrosion resistance to sulfuric acid, Alloy 20 (UNS N08020) is selected to displace a section of carbon steels in the original design.
|ASTM B729 Alloy 20 seamless pipes, 6″ SCH40, length=6m||16 pieces|
|ASTM B366 Alloy 20 fittings (WP20CB), 6″ elbows L/R 90° SCH40||6 pieces|
|Alloy 20 weld neck flange, 6“ SCH40, 150#||8 pieces|
|Alloy 20 lap joint stub ends, 4″ long pattern, ASME B16.9.||7 pieces|
- *All the products shall be annealed.
ASTM B473 Alloy 20 Bar
The Alloy 20 bars manufactured to ASTM B473 can be furnished in several types:
- (1) Hot-finished rounds, square, octagons, and hexagons: 0.25″(6.35 mm) and over in diameter or size;
- (2) Hot-finished flats: 0.25″-10″, inclusive, in width, 0.125″(3.175 mm) and over in thickness;
- (3) Cold-finished rounds, squares, octagons, hexagons, and shapes: over 0.5″ in diameter or size;
- (4) Cold-finished flats: 0.375″(9.525 mm) and over in width, 0.125″ and over in thickness.
Chemical Composition Requirements
The chemical requirements of ASTM B473 Alloy 20 bar are the same as those of ASTM B564 alloy 20 flanges. Corrosion test in accordance with ASTM A262 practice a shall be performed per heat.
Mechanical Properties of ASTM B473 Alloy 20 Bars
|Heat Treatment||Diameter||Tensile Strength, min.||Yield Strength, min.||Elongation, min.||Reduction of Area|
|Condition||mm||ksi||MPa||ksi||MPa||in 2″, %||min. %|
- *A: Alloy 20 hot-finished or cold-finished bars, annealed.
- *B: Annealed Alloy 20 bars, strain-hardened.
Alloy 20 (UNS N08020) Pipes & Tubes
|Product Form||Standard Specification|
|Alloy 20 Seamless Pipes & Tubes||ASTM B729 UNS N08020|
|Alloy 20 Welded Pipes||ASTM B464 UNS N08020|
|Alloy 20 Welded Tubes||ASTM B468 UNS N08020|
- *All pipes or tubes shall be furnished in stabilize-annealed condition.
Chemical & Mechanical Requirements of Alloy 20 Pipe & Tubes
The chemical & mechanical requirements of Alloy 20 (UNS N08020) is the same as those of alloy 20 flanges & Alloy 20 fittings.
Available Dimensions of Alloy 20 Pipe & Tubes
- For ASTM B729 Alloy 20 seamless pipes & tubes: 1/2″-16″, SCH10~SCH160.
- For ASTM B464 Alloy 20 welded pipes: 1/2″-6″, SCH 5S, SCH 10S, SCH 40S, SCH 80S (custom-design welded pipes up to 30″).
- For ASTM B468 Alloy 20 welded tubes: 1/2″-5″, wall thickness range: 0.38-12.7 mm.
- The pipe or tube can be furnished in either single random length(SRL), double random length(DRL), or specific cut length.
ASTM B463 Alloy 20 Plate, Sheet, Strip
ASTM B463 Alloy 20 plates, sheets, strips are widely used in the fabrication of pressure vessels, welded pipes or tubes, and other piping components. They can be furnished in various forms which are defined in below table.
|Spec.||Thickness, t [mm]||Width, w [mm]|
|Alloy 20 cold rolled plate||4.76≤t≤9.52||w>254.0|
|Alloy 20 hot rolled plate||t≥4.76||w>254.0|
|Alloy 20 plate||t≥4.76||w>254.0|
|Alloy 20 sheet||t＜4.76||w≥609.6|
|Alloy 20 strip||t＜4.76||w<609.6|
*The UNS designation is UNS N08020.
Chemical Requirements of ASTM B463 Alloy 20 (UNS N08020)
- *Values shall be based on product analysis.
Mechanical Properties of ASTM B463 Alloy 20 (UNS N08020)
|Tensile Strength, min.||Yield Strength, min.||Elongation in 2″||Hardness, max.|
|ksi||MPa||ksi||MPa||min. %||Brinell||Rockwell B|
- *The Alloy 20 plates shall be furnished in stabilize-annealed condition. The annealing temperature shall be 1800-1850°F [982-1010°C].
Available Specification in Stock
Thickness: 5mm-52mm; Width: 100mm-2000mm; Length: 100mm-6000mm.
ASTM B366 Alloy 20 Fittings
Yaang has committed experience of manufacturing a variety of ASTM B366 Alloy 20 fittings including 90° & 45° elbows, tees, reducers, 180° bends, caps, crosses, lap-joint stub ends, plugs, bushings, street elbows, etc. These fittings has the designation of UNS N08020 and can be used in many corrosive environments.
Product Forms & Dimension Standards of Alloy 20 Fittings
|Product Form||Dimension Standard|
|ASME B16.9||Alloy 20 butt welding pipe fittings|
|ASME B16.11||Alloy 20 forged fittings: threaded or socket welding.|
|MSS SP 43||Alloy 20 butt welding fittings for low pressure, corrosion resistant applications|
|MSS SP 95||Alloy 20 swaged nipples and bull plugs|
|MSS SP 97||Alloy 20 branch outlets: weld outlets, threaded outlet, socket outlets.|
- *The fittings can be either welded or seamless.
- *UNS N08020 fittings can be furnished in two classes: CR20CB: corrosion-resistant fittings; WP20CB: ASME pressure fittings.
Raw Materials for Alloy 20 Fittings
|Pipe or Tube||N08020||ASTM B464, B468, B729|
|Plate, Sheet, or Strip||N08020||ASTM B463|
|Bar or Forging||N08020||ASTM B472, B473, B462|
- *The chemical composition & mechanical properties shall conform to corresponding material standard specifications in the table.
Alloy 20 fittings shall be annealed at temperature 1700-1850°F [927-1010°C] and quenched at rapid air or water. See more technical specification for alloy 20.
Alloy 20 (UNS N08020) Flanges
Alloy 20 flanges can be readily made from ASTM B462 UNS N08020 forgings or ASTM B463 UNS N08020 plates. They can be furnished in a variety of flange types including weld neck flange, slip on flange, threaded flange, socket welding flange, blind flange, lap joint flange, and LWN flange, etc. There are 7 rating classes available: Class 150, Class 300, Class 400, Class 600, Class 900, Class 1500, Class 2500.
- *The tabulated values are abstracted from ASTM B462 Gr. UNS N08020.
|Alloy||Tensile Strength, min.||Yield Strength, min.||Elongation, min.||Reduction of Area, min.|
- *All values shall be measured at room temperature.
- *Alloy 20 flanges shall be supplied in stabilized-annealed condition.
Pressure-Temperature Ratings for Alloy 20(UNS N08020) Flanges
|Temp. °C||Class 150||Class 300||Class 400||Class 600||Class 900||Class 1500||Class 2500|
- *Working pressures by classes, unit: bar.
|Temp. °F||Class 150||Class 300||Class 400||Class 600||Class 900||Class 1500||Class 2500|
- *Working pressures by classes, unit: psig.
Alloy 20 Weld Outlet Class 3000
Alloy 20 Applicability and Max. Temperature Limits
Alloy 20 is endorsed by relative ASME Boiler & Pressure Vessel Codes (BPVC). A variety of product forms and conditions of Alloy 20 (UNS N08020) are listed in below table which are applicable for corresponding ASME BPVC specifications and temperature limits.
|ASME BPVC Applicability & Maximum Temperature Limits of Alloy 20|
- 1&2: ASTM B462 UNS N08020 forgings, annealed;
- 3&4: ASTM B463 UNS N08020 plates, annealed;
- 5&6:ASTM B473 UNS N08020 bars, annealed;
- 7&8: ASTM B729 Alloy 20 seamless pipe & tube, annealed;
- 9,10 & 11: ASTM B464 Alloy 20 welded pipe, annealed;
- 12, 13 & 14: ASTM B468 Alloy 20 welded tube, annealed;
- 15: ASTM B366 Alloy 20 seamless or welded fitting.
- I: ASME BPVC Section I Power Boilers;
- III: ASME BPVC Section III Nuclear Facility Components, Class 2 & 3;
- VIII-1: ASME BPVC Section VIII Division 1, Pressure Vessels;
- XII: ASME BPVC Section XII Transport Tanks.
- NP: not permitted.
Conclusion: Alloy 20 (UNS N08020) materials are not allowed to be used for the construction of power boilers. However, they can be used for the construction of Class 2 & 3 nuclear facility components and transport tanks with maximum temperature limits of 800°F and 650°F respectively.
Allowable Stress of Alloy 20 at Different Temperatures
|Temp. °F||-20 to 100||200||300||400||500||600||650||700||750||800|
- *Temp.: metal temperature, not exceeding.
- *MAS1, MAS2, MAS3, MAS4: maximum allowable stress of different product forms/conditions of Alloy 20.
- *Group MAS1: ASTM B462 UNS N08020 forgings, annealed; ASTM B463 UNS N08020 plate, annealed; ASTM B473 UNS N08020 bar, annealed; ASTM B729 UNS N08020 seamless pipe & tube, annealed; ASTM B464 UNS N08020 welded pipe, annealed; ASTM B468 UNS N08020 welded tube, annealed; ASTM B366 UNS N08020 seamless & welded fittings, annealed,
- *Group MAS2: ASTM B462 Alloy 20 forgings, annealed; ASTM B463 Alloy 20 plate, annealed; ASTM B473 Alloy 20 bar, annealed; ASTM B729 UNS Alloy 20 seamless pipe & tube, annealed.
- *Group MAS3: ASTM B464 UNS N08020 welded pipe, annealed; ASTM B468 UNS N08020 welded tube, annealed.
- *Group MAS4: ASTM B464 Alloy 20 welded pipe, annealed; ASTM B468 Alloy 20 welded tube, annealed.
Incoloy 20 (N08020, Alloy 20) nickel-based alloy phase structure:
Incoloy 20 (N08020, Alloy 20) nickel-based alloy is treated at 1120°C for 2h, only TiN nitrides and Cr7C3 type carbides are treated at 870°C by 1500 After long-term aging at ℃, the structure is still Cr7C3 and TiN, indicating that the structure of the alloy is stable.
Incoloy 20 (N08020, Alloy 20) nickel-based alloy process performance and requirements:
- 1. Incoloy 20 (N08020, Alloy 20) nickel-based alloy has good hot workability, and the heating temperature for ingot forging is 1110℃～1140℃. The average grain size of the alloy is closely related to the degree of deformation of the forging and the final forging temperature.
- 2. Incoloy 20 (N08020, Alloy 20) nickel-based alloys have good welding performance and can be connected by various methods such as arc welding, argon arc welding, resistance welding and brazing. Large or complex welding structural parts can be connected after melting. It should be annealed at 870°C for 1h to eliminate welding stress.
- 3. Incoloy 20 (N08020, Alloy 20) nickel-based alloys must be machined after heat treatment. Due to the work hardening of the material, it is advisable to use a lower cutting speed and heavy feed than processing low-alloy standard austenitic stainless steel. In order to drive into the underside of the hardened surface.
The Dissimilar Welding of Alloy 20
Alloy 20, also known as 20Cb3, is an iron-based austenitic nickel alloy. It has useful corrosion resistance to oxidizing and medium reducing environments. The alloy also exhibits useful resistance to stress-cracking corrosion and pitting corrosion. Alloy 20 is an ideal piping material for alkylation process in oil refinery. This article introduces the dissimilar welding process of ASTM B729 UNS N08020 (Alloy 20) seamless pipes in SINOPEC’s alkylation facilities in Selangor, Malaysia.
The issues during the welding of Alloy 20 seamless pipes include:
- High heat-susceptibility of Alloy 20 which may result in porosity during the welding.
- The tendency of hot cracking during the welding, as well as intergranular corrosion in welding area.
- Due to the high nickel content of Alloy 20 and its low flowability in molten status, incomplete fusion may occur.
- The formation of decarburized zone of pearlite and carburized zone of austenite may cause stress concentration.
- Residue stress may exist due to the different coefficient of linear thermal expansion between pearlite and austenite of the weld metal.
Actual Chemical Composition & Mechanical Properties
Materials specification: ASTM B729 Alloy 20 (UNS N08020) seamless pipes, 8″ SCH40, bevel ends, length = 6 meters, annealed, 15 pieces. Its actual chemical composition and mechanical properties are listed in below two tables.
- *All values are based on actual product analysis. The P Number of Alloy 20 is 45 according to ASME BPVC Section IX.
|Tensile Strength, MPa||Yield Strength, MPa||Elongation, %|
- *The heat number of all Alloy 20 seamless pipes: G8656H.
Welding Methods, Filler Metals, Welding Parameters for Alloy 20
|Welding Method||Polarity||Filler Metal||Current||Voltage||Welding Velocity|
- *Welding electrodes of ERNiCrMo-3 or ENiCrMo-3 are selected respectively as the filler metal for Alloy 20 dissimilar welding.
- *The root pass shall be welded by GTAW/ TIG method; the filler passes & cover passes shall be welded by SMAW method.
- *The heat input of each pass shall be < 20kJ/cm to prevent Alloy 20 from heat cracking. Interpass temperature shall be < 100°C.
Chemical Composition and Mechanical Properties of Filler Metal
- *The filler metals for Alloy 20 welding shall be selected in accordance with AWS A5.14(GTAW) or AWS A5.11(SMAW) respectively.
- *The UNS designation of AWS A5.14 ERNiCrMo-3 is N06625; the UNS designation of AWS A5.11 ENiCrMo-3 is W86112.
- *All tabulated values are actually measured.
|Classification||Tensile Strength, MPa||Yield Strength, MPa||Elongation, %|
*All values of the mechanical properties are actually measured.
Important Notice for Dissimilar Welding of Alloy 20
Before the dissimilar welding, the bevel ends of the Alloy 20 seamless pipe and SS 316 seamless pipe shall be carefully cleaned. Generally, preheating is not required. However, when the base metal temperature is lower than 15°C, induction heating shall be conducted to avoid air condensing. During the welding, there shall be no appreciable weaving motion.
Variety specifications and supply status of Nickel-based super alloy: Incoloy 20 (UNS N08020/DIN 2.4660)
Yaang Pipe Industry can produce various specifications of Incoloy20 seamless pipe, Incoloy20 steel plate, Incoloy20 round bar, Incoloy20 forgings, Incoloy20 flange, Incoloy20 pipe fittings, Incoloy20 welded pipe, Incoloy20 steel strip, Incoloy20 wire and supporting welding materials.
- Seamless pipe: solid solution + acid white, length can be set;
- Plate: solid solution, pickling, trimming;
- Welded pipe: solid solution acid white + RT% flaw detection;
- Forging: annealing + car polish; Bars are forged and rolled, surface polished or car polished;
- Strips are delivered after cold rolling, solid solution soft state, and deoxidized;
- Wire rods are finely ground in solid solution pickled disk or straight strips, solid solution straight strips Delivery in light state.
The effect of heat treatment on the organization and properties of UNS N08020 alloy pipe
Metallographic microscope, tensile testing machine, hardness tester scanning electron microscope, and other test methods to study the effect of different heat treatment process systems on the organization and properties of UNS N08020 alloy pipe. The results show that: hot extrusion of steel alloy pipe intermetallic phase precipitation, after 1200 ℃ high-temperature solid solution, intermetallic phase all dissolved; steel alloy pipe after large deformation of cold rolling and 1000 ℃ of the subsequent finished product heat treatment, the formation of the average grain size of 22 μm and no intermetallic phase precipitation of the best organization; and then after 3h stabilization treatment, through the precipitation of Nb (C, N), to obtain the best overall physical and chemical properties. Better comprehensive physical and chemical properties.
With the development of society, people’s demand for environmental protection is getting higher and higher; the traditional industrial fields are under increasingly severe pressure for environmental protection, especially in the chemistry and chemical industry, which has the most prominent impact on the environment. As the basic industry of the national economy, the chemical industry must strive to improve the efficiency of energy utilization and reduce the emission of pollutants, so it is necessary to improve the traditional process to improve efficiency. The primary issue for process improvement is material selection, for which many iron-nickel-based and nickel-based alloys are used. UNS N08020 alloy is widely used for its excellent resistance to acid and stress corrosion.
UNS N08020 (Chinese standard grade NS143) alloy, also known as No. 20 alloy, is a high-Cr, Ni, Mo, Nb single-phase austenitic alloy; with the increase of alloying elements, ordinary austenitic organization will produce carbides, nitrides, precipitation phases, such as a series of harmful phases, these precipitation phases of the mechanical properties of the material and the corrosion resistance of the impact of the larger. In other words, the stability of the UNS N08020 organization needs to be further improved. This paper focuses on the impact of the heat treatment process on the organization and properties of the UNS N08020 steel alloy pipe. It puts forward the production of the alloy steel alloy pipe specific heat treatment system.
1. Test materials and methods
1.1 Test materials
The test material is smelted by VIM (vacuum melting) + ESR (electroslag remelting), then forged into Φ260mm alloy pipe billet and extruded into Φ114mm×15mm seamless steel alloy pipe. The main chemical composition of the test material is shown in Table 1.
Table.1 Main chemical composition of the test material (mass fraction)
1.2 Test method
The test was completed by industrial production. Firstly, the hot extrusion process was used to produce gross alloy pipes. Then, the extruded gross alloy pipes were subjected to high-temperature solution treatment, cold rolling, heat treatment, and stabilization of the finished products.
DMI3000M metallurgical microscope was used to observe the metallurgical organization and grain size of the extruded state, high-temperature solid solution state, cold rolled + solid solution state, solid solution + stabilized state; Inspekt1000 electronic tensile tester, and ZHU3000 hardness tester were used to carry out tensile and hardness tests on the different states of the alloy pipe; Nova Nano SEM430 scanning The morphology and composition of the precipitated phases were analyzed by Nova Nano SEM430 scanning electron microscope (SEM) to determine the structure and type.
2. Test results and discussion
2.1 Organizational properties after hot extrusion
After the hot extrusion of the gross alloy pipe, the pipe is immediately cooled in water. Samples were taken along the radial direction and the organization and morphology of the alloy pipe. The sample size was Φ10mm×10mm, and the metallographic organization of Φ114mm×15mm extruded alloy pipe is shown in Figure 1. As shown in Fig. 1, the second phase on the outer surface has more content, which is in the form of strips across several grains, as shown in Fig. 1(a)-(b). The inner surface of the second phase content is less, and the metallurgical organization is purer, showing a typical austenitic organization and more twins. The twin crystals are mainly generated during the recovery and recrystallization process and grow with the grain growth.
SEM spectral analysis of the second phase shows the elemental distribution of high Cr, high Mo, and low Fe. Combined with the compositional characteristics of Fe-Ni and Ni-based alloys, it can be determined that the second phase should be a σ-phase organization. The high magnification morphology and energy spectrum of the precipitated phase are shown in Fig. 2.
The second phase of the Φ114mm×15mm extruded alloy pipe appears within 3mm from the outer surface. Based on the large difference in the content of the precipitated phase between the inner and outer surfaces, it can be judged that the extrusion process is reasonable. Extrusion is completed with a hot saw cut pressure balance of the process; the process time is about 1 minute. In the natural cooling of the alloy pipe exposed to the air, the outer surface and the air have a large temperature gradient, heat dissipation is faster, and the temperature will fall dramatically. In contrast, the inner surface of the alloy pipe cannot be quickly dissipated; the heat will be supplemented to the outer surface of the alloy pipe, resulting in the outer wall not being immediately cooled to room temperature; the temperature of the outer surface will be in the precipitation The temperature of the outer surface will stay in the precipitation zone, thus precipitating a large number of the second phase; In contrast, the inner surface cannot quickly dissipate heat, the temperature drop is small, not in the precipitation range, so there is no precipitation phase on the inner wall.
Figure.1 Metallographic organization of Φ114mm×15mm extruded alloy pipe
2.2 Microstructure after solid solution treatment
UNS N08020 extrusion alloy pipe for solution treatment, at 1200 ℃ insulation for 30min, UNS N08020 alloy pipe high temperature solid solution microstructure morphology shown in Figure 3. After solid solution treatment, UNS N08020 extrusion alloy pipe organization of precipitation phase all dissolved, the organization of a single austenitic organization; grain size uniformity is poor, and there is some abnormal grain growth. For UNS N08020 material, the alloy contains 0.5% of the Nb element, Nb as a grain refining element, with a strong Nb (C, N) precipitation tendency, hindering grain growth. For ordinary stainless steel containing Nb elements, 1200 ℃ solid solution temperature cannot make the grain abnormally large; for stainless steel containing Nb, the grain size is too coarse, which is inevitably related to the matrix organization and elements of the steel. Meng Fanmao et al.’s research proved this point: Nb solid solubility and Ni element content, with the increase of Ni element content, Nb solid solubility significantly increased.
Figure.2 High magnification morphology and energy spectrum of the analyzed phase
Figure.3 High temperature solid solution microstructure of UNS N08020 alloy pipe
2.3 Effect of solid solution treatment on grain size
After the solid solution treatment of the gross alloy pipe, there is no precipitated phase on the inner and outer surfaces of the alloy pipe. After Φ114mm×15mm→Φ88.9mm×11.13mm cold rolling, the material has accumulated huge deformation energy, and after re-solid solution, the recrystallization can be completed. Therefore, it is important to study the effect of heat treatment temperature on recrystallization.
The grain morphology after heat treatment at different temperatures for 20 min is shown in Fig. 4. The alloy was recrystallized at 1000 ℃, with no residual deformation zone, clean grain boundaries, no visible second phase, and an average grain size of about 22 μm. As the solid solution temperature increased to 1050 ℃, the grain size tended to grow to an average size of about 30 μm, and there was no significant change in the organization and morphology compared with that of the alloy at 1000 ℃. When the solid solution temperature increased to 1100 ℃, the grain size grew rapidly, with an average grain size of about 57 μm. When the solid solution temperature increased to 1150 ℃, the average grain size of about 73 μm is more compatible with the solid solution effect of barren alloy pipe. The average grain size of the barren alloy pipe after solid solution at 1200℃ is about 100μm.
Figure.4 Grain morphology of UNS N08020 alloy pipe after heat treatment at different temperatures
Figure 4 also shows the process of recrystallization and grain growth. The driving force for grain growth is the reduction of interfacial energy. The interfacial energy, grain boundary migration, and grain growth drive it. Grain size is small, and with more grain boundaries, interface free energy is higher, so under certain thermodynamic conditions, reducing the area of grain boundaries and spontaneously reducing the direction of interfacial energy is inevitable. Therefore, with the increase of solid solution heat treatment temperature, it is inevitable that small grains will merge, curved interface into a straight phenomenon.
The essence of grain boundary migration is the diffusion of atoms across the interface; this diffusion is the macro-statistical manifestation of the atomic microscopic random jump, with the common characteristics of the general thermal activation process. The temperature significantly affects the grain boundary migration rate; the Arrhenius formula can describe the grain growth process.
In the formula:
- D – the average grain size at a solid solution temperature, μm.
- D0 – Original grain size of austenite just after nucleation, μm.
- A – Factor.
- Q – apparent activation energy of grain growth, kJ/mol.
- R – gas constant; T—solid solution temperature, kJ/mol
- T – solid solution temperature, K.
Considering D02<<D2, Eq. (1) can be approximated as follows.
The logarithmic conversion of Eq. (2) leads to Eq. (3).
Figure 4 in the solid solution temperature is converted to K-1 form, the grain size to do logarithmic processing, grain size, and temperature data are shown in Table 2.
Table.2 Grain size and temperature data
Plotting ln D and 1/T relationship curve, UNS N08020 alloy average grain size and solid solution temperature of the relationship shown in Figure 5, the data has a linear characteristic, the slope is a constant so that you can estimate the activation energy of grain growth through the slope. The first and the last two points were chosen for the slope calculation, and the result was 1.46×104. Substituting into Eq. (3), the activation energy of diffusion for grain growth was 242kJ/mol.
The structure type of the crystal has a great influence on the diffusion coefficient. Fe has two kinds of dot matrix structure, body-centered cubic dot matrix α-Fe, and face-centered cubic dot matrix γ-Fe, and the diffusion activation energies of alloying elements in different dot matrices are different; the diffusion activation energies of Ni element in γ-Fe are 296.8kJ/mol, the diffusion activation energy of Cr element in γ-Fe is 286.8kJ/mol, and Mo element in γ-Fe is 2.46×104. The activation energy of the Mo element in grain growth is 242kJ/mol. The diffusion activation energy of Mo in γ-Fe is 239.8 kJ/mol, and the diffusion activation energies of Ni, Cr, and Mo are not much different from those of grain growth. For UNS N08020 alloy, the atomic radii of Cr-Ni-Mo-Fe elements in the crystal do not differ much, and all of them should occupy the node position of the crystal so the metal elements can only be replaced by a solid solution. Therefore, the process of grain recrystallization is completed by diffusion through the vacancy mechanism.
Figure.5 Average grain size of UNS N08020 alloy as a function of solid solution temperature
2.4 Effect of stabilization
The stabilization treatment is different from the solid solution treatment mechanism. Solid solution treatment avoids the precipitation of Cr23C6, usually high-temperature heat treatment followed by rapid cooling to form a supersaturated solid solution process. The process of stabilization is the process of destabilization of the supersaturated solid solution, and the formation of the stable phase; the phase transition process is spontaneous, the stabilization process at constant temperature and pressure, the Gibbs free energy will be reduced, and usually, this free energy will be converted into the driving force of the phase transition. In other words, the supersaturated solid solution formed by solid solution treatment is an unstable state, which, under certain thermodynamic conditions, promotes the precipitation of carbides and leads to the deterioration of the material’s corrosion resistance. Stabilization is mainly for the stainless steel containing Nb, Ti elements, usually in the Nb (C, N) precipitation interval for long-time insulation, generally more than 3h, to promote the precipitation of Nb (C, N) a large number of precipitations, thus hindering the formation of Cr23C6. On the Nb-containing austenitic stainless steel, GB 5310-2008 “high-pressure boilers with seamless steel alloy pipe” on its solid solution stabilization requirements.
They are combined with the characteristics of UNS N08020 steel grade, a long-time heat treatment at 1000 ℃ to achieve the effect of stabilization. Usually, the stabilization time is 3 hours. The matrix organization of Nb (C, N) will also be on the metal slip pinning, and in the process of recrystallization to hinder the growth of grains, play the role of grain refinement. At the same time, the strengthening of fine grains can also be reflected; a long stabilization period did not make the grain grow significantly. Still, the organization of the high magnification of the second phase of the precipitation of the obvious precipitation phase is a rounded particle, the size of less than 0.1 μm, and the size of the second phase is not as large as the size of the second phase. The size of the precipitated phase is less than 0.1 μm, and the SEM spectroscopy reveals that the main components of the second phase are compounds containing Nb elements.
Combined with the principle of Nb element precipitation, Nb (C, N) and Ni3Nb precipitation phases are usually called γ”. Considering that the size of Ni3Nb particles is very small, about 20 nm, which SEM cannot observe, it can be determined that the precipitation of fine particles of the second phase is Nb (C, N). A large amount of precipitation of Nb (C, N) can improve the intergranular corrosion resistance of the alloy. In contrast, the precipitation of fine particles in the second phase will impact the material’s mechanical properties.
The second phase always tends to form when the total surface energy and strain energy are minimized, so the shape of the precipitates results from the combined effect of the total strain energy and the total surface energy. Whether the precipitates are eutectic or non-eutectic, the spherical shape has the least resistance. The microscopic morphology and precipitation phases of UNS N08020 alloy after stabilization heat treatment are shown in Fig. 6, and the stabilized precipitates all appear spherical.
Figure.6 Microstructure and precipitation phases of UNS N08020 alloy after stabilized heat treatment
2.5 Effect of heat treatment on mechanical properties
The hardness and mechanical properties of UNS N08020 alloy pipes in different states were tested. The hardness test results of UNS N08020 alloy pipes in different states are shown in Figure 7. The Rockwell hardness of the hot extruded state steel alloy pipe reaches 74 HRB; on the one hand, the deformation of the extrusion process is large, and there are residual tissue stresses inside the grains; on the other hand, after a large thermal deformation, the grains re-completion of the recovery and recrystallization, the grains are finer, and there is a precipitation of diffuse second phase on the outer surface, which makes the organization of the existence of internal aberrant energy, resulting in higher hardness. Subsequently, after 1200 ℃ high-temperature heat treatment, the hardness is reduced to 68HRB. High-temperature heat treatment makes the precipitated phase back to dissolve while eliminating the hot extrusion process of the alloy pipe’s internal thermal stress and organizational stress so that the existence of internal grain distortion caused by dislocations vacancies and other crystalline defects is eliminated, resulting in a significant decrease in the hardness of the alloy pipe. After cold rolling + 1000 ℃ (20min) solution, the hardness of the alloy pipe is 73HRB, compared with the barren alloy pipe after solution, increased by 5HRB. Still, with the extension of the stabilization time, the hardness then produced a large increase. It shows that after a long stabilization time, the diffuse bulk precipitation of Nb (C, N) has a greater effect on the hardness.
Figure.7 Hardness test results of UNS N08020 alloy pipes in different states
Figure 8 shows the mechanical properties test results of UNS N08020 alloy pipes in different states. The yield strength of the hot extruded state is 300MPa, tensile strength is 600MPa, and elongation is 50%. After high-temperature heat treatment, mechanical property indicators showed a large degree of decline, yield strength directly down to 230MPa, tensile strength down to 550MPa, and elongation increased to 60%; it can be seen high-temperature heat treatment on the plasticity of the improvement is very obvious; after cold rolling + solution, due to the use of 1000 ℃ of the solution temperature, the average grain size of 22 μm or so, yield strength increased to 250MPa, tensile strength, elongation of 50%. After cold rolling + solid solution, the average grain size is about 22μm, the yield strength is increased to 250MPa, the tensile strength is increased to 560MPa, the elongation is decreased to 55%, the effect of fine grain strengthening is more obvious; and after 3h of stabilization, the strength value is increased to a certain extent, and the elongation is decreased. The dissolution process of the second phase caused the internal distortion of the organization, resulting in an increase in strength and a decrease in plasticity. This is in good agreement with the hardness index and metallographic appearance.
Figure.8 Mechanical properties of UNS N08020 alloy pipes in different states
In summary, the effects of different heat treatment systems on the properties of N08020 alloy are very obvious, and the effects of grain strengthening and precipitation strengthening on the microstructure cannot be ignored.
- (1) The σ phase must be precipitated during the extrusion process of the UNS N08020 seamless steel pipe. The precipitated phase’s content on the pipe’s outer surface is obviously more than that on the inner surface. The cooling process after extrusion is the cause of the precipitated phase.
- (2) After solution treatment at 1 200 °C for 30 min, the precipitated phase in the microstructure of UNS N08020 alloy can be completely dissolved, and the average grain size grows to about 100 μm.
- (3) The solution treatment temperature greatly influences the grain size of the UNS N08020 alloy. The average grain size increases from 22 μm to 73 μm with increasing temperature from 1000 °C to 1150 °C. The grain growth process conforms to the Arrhenius formula, and the activation energy of grain growth is close to the diffusion activation energy of alloying elements.
- (4) Different heat treatment processes significantly affect the hardness and mechanical properties of the UNS N08020 alloy. The stabilization process at 1000 °C for 3 h can make the matrix dissolve out of the dispersed Nb (C, N) second phase and improve the material’s hardness, mechanical properties, and corrosion resistance. This process is the best heat treatment process for UNS N08020 alloy.
Author: Nie Fei