A Comprehensive Guide to Nickel-based super alloy: Incoloy 825 (UNS N08825/W.Nr. 2.4858)

What is Incoloy 825?

Designated as UNS N08825 or DIN W.Nr. 2.4858, Incoloy 825 (also known as “Alloy 825”) is an iron-nickel-chromium alloy with additions of molybdenum, cooper and titanium. The molybdenum addition improves its resistance to pitting corrosion in aqueous corrosion application while copper content bestows resistance to sulfuric acid. Titanium is added for stabilization. The Alloy 825 has excellent resistance to both reducing and oxidizing acids, to stress-corrosion cracking, and to localized attack such as pitting and crevice corrosion. It is especially resistant to sulfuric and phosphoric acids. Incoloy 825 alloy is mainly used for chemical processing, petrochemical piping, pollution-control equipment, oil and gas well piping, nuclear fuel reprocessing, acid production, and pickling equipment.

ASTM B423 UNS N08825, Incoloy 825 seamless pipes.

Incoloy 825 (N08825, 2.4858) The alloy is an all-austenitic nickel-iron-chromium alloy stabilized by titanium, with copper and molybdenum added. 

Characteristics of Incoloy 825 (UNS N08825/W.Nr. 2.4858):

• (1) Good stress corrosion cracking resistance.
• (2) Good resistance to pitting and crevice corrosion.
• (3) Good oxidation resistance and non-oxidizing thermal acid performance.
• (4) Good mechanical properties at room temperature and high temperatures up to 550℃.
• (5) With the certification of the pressure vessel manufacturing temperature up to 450 ℃.

Chemical Composition Requirements

The Chemical Composition of Incoloy 825, %
Nickel	38.0-46.0
Iron	≥22.0
Chromium	19.5-23.5
Molybdenum	2.5-3.5
Copper	1.5-3.0
Titanium	0.6-1.2
Carbon	≤0.05
Manganese	≤1.00
Sulfur	≤0.030
Silicon	≤0.50
Aluminum	≤0.20

The role of each element in 825 alloy

• Ni: The role of Ni is to improve the resistance of the alloy to chloride stress corrosion cracking.
• The role of chromium: Cr is to provide the ability of the antioxidant environment.
• Titanium: adding 0.6 % -1.2 % Ti plays a stabilizing role.
• Molybdenum: 3 % Mo is added to improve the ability to resist pitting corrosion in water corrosion applications.
• Copper: Adding 1.5 % -3.0 % Cu gives the alloy the ability to resist sulfuric acid corrosion.

Mechanical Properties of Incoloy 825

Incoloy 825 weld neck flanges 600# SCH80, manufactured to ASTM B564.

Tensile Strength, min.		Yield Strength, min.		Elongation, min.	Elastic Modulus
Mpa	ksi	Mpa	ksi	%	Gpa	106psi
690	100	310	45	45	206	29.8

The properties listed in the following table are applicable to the specified specifications of Incoloy 825 alloy after softening annealing (stabilizing annealing). The special properties of non-standard size materials can be provided according to the requirements of specific applications. 

Mechanical properties at room temperature (minimum)

Product	Specification	Yield strength  RP0.2 N/mm2	Yield strength  RP1.0 N/mm2	Tensile strength  Rm N/mm2	Elongation  A50%
Sheet and strip/ cold rolled  plate/ hot rolled	0.5-6.4	240	265	585	30
	5-100
Cold  processing and  hot processing of bars	1.6-64
	25-100
	>100-240	220	250	550	35
Cold  processing and  hot processing of pipes	64-240	180	–	530	30
	5-100	240	265	585	30
Condensation and  heat exchanger	16-76

High temperature mechanical properties (minimum value)

Product	Yield strength RP0.2 N/mm2					Yield strength RP1.0 N/mm2
Temperature ℃	100	200	300	400	450	100	200	300	400	450
Plate, strip, tube	205	180	170	160	155	235	205	195	185	180
Baton	190	165	155	145	140	220	190	180	170	165

Incoloy 825 ISO V-notch impact test

Average room temperature:

• Axial direction>=150J/cm²
• Radial direction>=100J/cm²

Time-temperature-sensitization curve. 

Incoloy 825 (N08825, 2.4858) alloy conditional stress value

The high conditional stress value that reaches 90% of the yield strength can be used in applications that allow a little larger deformation. The permanent stresses caused by these stresses can cause dimensional changes, so it is not recommended for flange and gasket connections. 

Incoloy 825 (N08825, 2.4858) alloy metallographic structure

Incoloy 825 alloy has a stable face-centered cubic structure. The chemical composition and proper heat treatment ensure that the corrosion resistance is not impaired by sensitization. 

Incoloy 825 (N08825, 2.4858) alloy corrosion resistance

Incoloy 825 is a general engineering alloy, which has resistance to acid and alkali metal corrosion in both oxidation and reduction environments. The high nickel content makes the alloy have effective resistance to stress corrosion cracking. It has good corrosion resistance in various media, such as sulfuric acid, phosphoric acid, nitric acid and organic acids, and alkali metals such as sodium hydroxide, potassium hydroxide and hydrochloric acid solutions. The high comprehensive performance of Incoloy 825 is manifested in nuclear combustion dissolvers with various corrosive media, such as sulfuric acid, nitric acid and sodium hydroxide, which are all processed in the same equipment.

Physical Properties of Incoloy 825

Density	Melting Range		Specific Heat		Electrical Resistivity
g/cm3	°C	°F	J/kg.k	Btu/lb. °F	µΩ·m
8.14	1370-1400	2500-2550	440	0.105	1130

• (1). Density: ρ=8.1g/cm³
• (2). Melting temperature range: 1370-1400℃ 

Temperature ℃	Specific heat capacity J/Kg.K	Thermal conductivity W/mK	Resistivity μΩcm	Elastic modulus KN/mm2	Coefficient of thermal expansion 10-6/K
20	440	10.8	112	195
93
100	462	12.4	114	190	14.1
200	488	14.1	118	185	14.9
204
300	514	15.6	120	179	15.2
316
400	540	16.9	124	174	15.6
427
500	565	18.3	126	168	15.8
538
600	590	19.6	126	161	16.0
649
700	615	21.0	127	154	16.7
760
800	655	23.2	128	142	17.2
871
900	680	25.7	129	130	17.6
982
1000	710	28.1	130	119	17.9

Product Forms and Standards of Incoloy 825

Product form	Standard
Rod and Bar	ASTM B425, DIN17752
Plate, Sheet & Strip	ASTM B906, B424
Seamless Pipe & Tube	ASTM B423, B829
Welded pipes	ASTM B705, B775
Welded tubes	ASTM B704, B751
Welded pipe fittings	ASTM A366
Forgings	ASTM B564, DIN17754

Application areas of nickel-based alloy incoloy 825 (UNS N08825/W.Nr. 2.4858)

The Incoloy 825 alloy can be used in many corrosive environments, including the treatment and storage of reducing acids such as sulphuric acid and phosphoric acid solutions. It can also be used in a variety of oxidizing environments, including nitric acid solutions, nitrites and oxidizing salts. Incoloy 825 also has excellent seawater corrosion resistance. 
(1) Chemical treatment: 
Heat exchange tubes, pipe fittings, evaporators, flanges, pumps, pipes, valves, containers, expansion bellows, tankers, scrubbers, condensers, tanks and container liners; processing and transportation of sulfuric acid, phosphoric acid, hydrochloric acid, nitric acid, nitrite and oxide salts.
(2) Pulp and paper:
Wet scrubber lining in sodium sulfate and sodium bisulfite solution system.
(3) Pickling treatment:
Acid tank heater and hook clamp.
(4) Mineral processing:
Plate heat exchanger in hydrometallurgy and copper refining.
(5) Pollution control:
Feeding bottom and disk in electrostatic precipitator; pipes and baffles, flue gas reheater, waste heat circulation exchanger, chimney lining and other wet scrubber components; residue incinerator, wet scrubber; marine inert gas scrubber; radioactive waste dryer.

Incoloy 825 Used for Air-Cooled Heat Exchangers

In an oil refinery’s ethylene project of Indonesia, air-cooled heat exchangers are required for the gas oil hydrotreating process. The product of hydrogen cracking process is separated high-pressure hot gas which consists of hydrogen sulfide(H₂S), ammonia(NH₃), nitrogen(N₂), etc. Considering the harsh conditions, Incoloy 825 (UNS N08825) materials are chosen for the manufacture of the air-cooled heat exchanger.

The air-cooled heat exchanger for an ethylene project in Indonesia.

The design pressure and design temperature for tube-side pass are 15.2 MPa and 250°C respectively with flow media of hot gas(H₂S+NH₃+N₂ ,etc). The design pressure and design temperature for shell-side pass are 0.1 MPa and 35°C respectively with flow media of air. The air-cooler dimensions are 10.5m (length) x 3m (width).

The plug-type air cooled heat exchanger made of Incoloy 825.

The air cooled heat exchanger mainly consists of headers, plug-type tube bundles and joint flanges. The header is made of Incoloy 825 plates (ASTM B424 UNS N08825) in solution annealed condition. The tube bundles are made of Incoloy 825 seamless tubes manufactured to ASTM B163 Gr. N08825 also in solution annealed condition. The joint flanges shall be manufactured to ASTM B564 Gr. N08825 forgings. Especially, the tube bundles are furnished with finned tubes. The extruded aluminum fins are firmly seated on the Incoloy 825 base tubes.

Incoloy 825 Quench Ring and Dip Tube for Gasifiers

Generally, a gasifier of texaco coal gasification process consists of the upper combustion chamber and the lower cooling zone. The pressurized coal-water slurry is injected into the combustion chamber together with oxygen or air through a injection burner. A useful synthetic gas will be produced at a temperature of approx. 1400°C. Along with the hot gas, large amount of hot ash slags are generated. All the hot products shall be discharged to the cooling zone through a quench ring and dip tube structure. After been through the cooling zone, the temperature of the hot gas is reduced to about 300°C. And it is then directed to other facilities for further processing.

A quench ring fixed with the dip tube for Texaco coal-water slurry gasifier.

Both the quench ring and the dip tube are working in extremely harsh conditions: sudden temperature drop from 1400°C to 300°C; high pressures which may be as high as 25 MPa; severe corrosion and erosion caused by the hot gas & slag mixtures. Incoloy 825 is chosen for the fabrication of these components.

Function of The Incoloy 825 Quench Ring

Incoloy 825 gasifier quench rings.

The main body of the Incoloy 825 quench ring is made of ASTM B424 UNS N08825 plates with cooling water inlet made of ASTM B564 UNS N08825 forgings. The quench ring can be furnished in either integral or segmented pattern. It is in close contact with the combustion chamber. The holding coolant in the quench ring flows down the internal surface of the adjacent dip tube and forms a film of water over the latter.

Function of The Incoloy 825 Dip Tube

A dip tube made of Incoloy 825 plates manufactured to ASTM B704 Gr. N08825.

The dip tube is located below the quench ring. It is wetted by the coolant held by the quench ring. The lower serrated edge of the dip tube shall be dipped into the coolant. Hot gas and particulated slag will be passed through it and get cooled in the coolant. The dip tube shall be made from Incoloy 825 plates manufactured to ASTM B704 Gr. N08825.

Incoloy 825 Tubes Used for Distillation Column Reboiler in AlF3 Production

The distillation column reboiler is widely utilized for the production of dry aluminum fluoride (AlF₃). The hydrogen fluoride (HF) is passed through the distillation column and get rectified. Then it is re-vaporized in the reboiler for further processing. Due to the corrosive and hazardous properties, hydrogen fluoride(fluid/gas) shall be properly handled. Generally, the high-temperature corrosion resistant material Incoloy 825 is chosen for the fabrication of tube pass of the reboiler.

The distillation-column reboiler with Incoloy 825 tube pass for a AlF3 producer in UAE.

We have supplied Incoloy 825 heat-exchanger tubes of 2″ x 2.6mm x 3m (manufactured to ASTM B163 UNS N08825) for the fabrication of a distillation-column reboiler’s tube pass.

• Design temperature: 110°C
• Design pressure: 0.8 MPa
• Flow media: HF

The facilities belong to a reputable aluminum fluoride producer, Gulf Fluor, located in Abu Dhabi, UAE.

Pressure-Temperature Ratings for Incoloy 825 Flanges

Incoloy 825 flanges manufactured to ASME B16.5 can only be made from annealed material and its maximum applicable temperature is 538°C [1000°F]. It can be furnished in a variety of types including welding neck, slip on, blind, threaded, lap joint, long weld neck, and socket welding. The flange can be either made from ASTM B564 UNS N08825 forging or ASTM B424 Gr. N08825 plate (only for blind flange and plate flange) with nominal composition of 42Ni-21.5Cr-3Mo-2.3Cu.

ASME B16.5 welding neck flange, RTJ end, 3″ SCH80 600#. Material: Incoloy 825

Pressure-Temperature Ratings for Incoloy 825: Metric Unit

*The table lists working pressures by classes at different temperatures, in bar unit.

Pressure-Temperature Ratings for Incoloy 825: US Customary Unit

*The table lists working pressures by classes at different temperatures, in psig unit.

The Manufacture of Incoloy 825 Welding Neck Flanges

Incoloy 825 welding neck flanges are widely used in sour crude oil services. The primary manufacturing processes include: electric arc furnace melting → ingot casting → cogging → descaling → cutting → preheating → loose tooling forging → heat treatment → machining → inspection.

There are some difficulties in the manufacturing of Incoloy 825 flanges:

• (1). The material has very high resistance to deformation: even at 980°C, its flow stress can be as high as 240 MPa.

• (2). In order to prevent the intergranular corrosion and improve its resistance to hydrogen sulfide stress corrosion, fined grain is required. However, phase transformation is not available when Incoloy 825 is heated or cooled. Fined grain can only be achieved by re-crystallization or plastic deformation.

• (3). The deformation on each direction shall be 15% larger than the critical values to avoid coarse grain.

Processing and heat treatment of Incoloy 825 (UNS N08825/W.Nr. 2.4858)

Incoloy 825 alloy is easy to process in general industrial processes. 

Incoloy 825 (N08825, 2.4858) alloy preheating

Temperature control is very important to ensure that the corrosion resistance of the alloy is not weakened by sensitization. The surface of the workpiece must be cleaned before and during the heating process to keep the surface clean. If the heating environment contains sulfur, phosphorus, lead or other low melting point metals, Nicrofer 6023/6023H alloy will become brittle. Impurities come from marking paint, chalk, lubricating oil, water, fuel, etc. The sulfur content of fuel should be low. For example, the impurity content of liquefied gas and natural gas should be less than 0.1%, the sulfur content of city gas should be less than 0.25g/m³, and the sulfur content of petroleum gas should be less than 0.5%. 
The heat treatment is best carried out in a vacuum resistance furnace or an inert gas protective atmosphere, because it can control the temperature accurately and is not contaminated by impurities. If the impurity content of the gas is low, a gas heating furnace can also be considered, so that a neutral or weak oxidizing atmosphere can be obtained. It should be avoided that the composition of the furnace gas fluctuates in the oxidation and reducibility, and the combustion flame cannot directly burn the workpiece. 

Incoloy 825 (N08825, 2.4858) alloy thermal processing

The suitable thermal processing temperature of Incoloy 825 alloy is 1150-900℃, and the cooling method can be water quenching or rapid air cooling. During hot processing, the workpiece can be directly fed into the heated furnace. After the furnace is warm, the holding time of the material is 60 minutes per 100mm thickness. After the heat preservation reaches the specified time, it will be released immediately and processed within the specified temperature range. If the temperature of the metal drops below the minimum processing temperature, it should be heated again. It should be annealed in time after hot working to ensure the best corrosion resistance and suitable crystal structure. 

Cold processing of Incoloy 825 (UNS N08825/W.Nr. 2.4858)

Cold working should be carried out after solution treatment. The work hardening rate of Incoloy 825 is close to that of austenitic stainless steel, so the processing equipment should be adjusted accordingly. When the amount of cold work is large, there should be an intermediate annealing process. When the final cold deformation is greater than 15%, final stabilization is required. 

Heat treatment of Incoloy 825 (UNS N08825/W.Nr. 2.4858)

The temperature range of softening annealing or stabilization treatment is 920-980℃, and the most suitable is 940±10℃. For workpieces with a thickness greater than 1.5mm, it is recommended to use water quenching or rapid air cooling to obtain the maximum creep resistance. In any heat treatment process, we must pay attention to the aforementioned matters about keeping clean. 

Incoloy 825 (N08825, 2.4858) alloy grinding and pickling

The oxide near the weld of Incoloy 825 workpiece is more difficult to remove than stainless steel, and it needs to be polished with a fine abrasive belt or grinding wheel. To maintain the luster of the metal, be very careful when polishing it. Before pickling in a mixed acid of nitric acid and hydrofluoric acid, sandpaper should be used to remove oxides or salt bath pretreatment. Pay special attention to the pickling time. 

Machining of Incoloy 825 (UNS N08825/W.Nr. 2.4858)

Machining of incoloy825 needs to be carried out after solution treatment. The work hardening of the material must be considered. Unlike austenitic stainless steel, it needs to adopt a low surface cutting speed and heavy weight. Infeed amount. 

Incoloy 825 (N08825, 2.4858) alloy welding

When welding nickel-based alloys, the following procedures should be followed

• (1). Work site: The work site should be separated or far enough from the carbon steel processing area Keep the distance as clean as possible, with partitions and avoid ventilation between the two areas.
• (2). Work clothes and auxiliary supplies: clean fine-grained leather gloves and clean work clothes should be worn.
• (3). Tools and machinery and equipment: there should be special tools for nickel-based alloy and nickel-chromium steel. The wire brush should be made of stainless steel. The machinery and equipment such as shears, punches, rolling mills, etc. should be covered with felt, cardboard or plastic. The paper prevents iron-carbon metal from falling on the surface of the machine and sticking to the processed materials, causing corrosion. Remove oxide scale, oil stains and various marking marks, and use acetone to clean the base metal and filler alloys (such as welding rods) in the welding area. Note that trichloroethylene TRI, perchloroethylene PER and tetrachloride TETRA cannot be used.

Incoloy 825 edge preparation

It is best to use machining, such as turning, milling, planing, or plasma cutting. If the latter is used, the cutting edge (welding surface) must be ground clean and smooth, allowing fine grinding without overheating. The base material on both sides of the weld should be polished to expose the bright metal. 

Incoloy 825 groove angle

Compared with carbon steel, the physical properties of nickel-based alloys and special stainless steels are mainly low thermal conductivity and high expansion coefficient. These characteristics must be considered when welding groove preparation, including Widen the bottom gap (1-3mm). At the same time, due to the viscosity of the molten metal, a larger groove angle (60-70°) should be used in butt welding to offset the shrinkage of the material. 

Incoloy 825 arc starting

The arc cannot be started on the surface of the workpiece, and the arc should be started on the welding surface to prevent the arc starting point from causing corrosion. 

Incoloy 825 welding process

• (1). Incoloy 825 is suitable for welding with the same material or other metal materials using any traditional welding process, such as tungsten electrode inert gas arc welding, plasma arc welding, manual sub-arc welding, metal inert gas arc welding, MIG welding, pulse arc welding is the first choice. When using manual arc welding, it is recommended to use (Ar+He+H₂+CO₂) shielding gas mixed with multiple components.
• (2). The welding of Incoloy 825 must be carried out in the annealed state, and a stainless steel wire brush should be used to clean up stains, dust and various marks. When welding the root of the weld, in order to obtain the best quality of the root weld, the operation must be very careful (argon 99.99), so that the weld does not produce oxides after the root is welded. The color generated in the welding heat-affected zone should be brushed off with a stainless steel brush when the weld area is not cooled. 
• (3). Incoloy 825 filler metal: welding rod: ENiCrMo-3, welding wire: ERNiCrMo-3.

Incoloy 825 welding parameters and influence (heat input)

Welding operation should be carried out under the low heat input specified in the heat input table, using stacked beads For welding seam technology, the temperature between layers does not exceed 120°C, and welding specifications must be followed. The heat input Q is calculated according to the following formula: 

• U = arc voltage, volt 
• I = welding current, ampere 
• V = welding speed, cm/min. 

Incoloy 825 post-welding treatment (pickling, brushing to remove oxides and heat treatment)

Immediately after welding, use a stainless steel wire brush to remove oxides, that is to say, brush before the metal has no welding color, so that you can get Ideal surface quality without pickling. If there are no special requirements or regulations, pickling is usually the last process in welding. Please refer to the section on descaling and pickling.No heat treatment is required before and after welding. 
Heat input per unit length (guide value)

Welding process	Heat input per unit length  kJ/cm	Welding process	Heat input per unit length  kJ/cm
GTAW, manual/fully mechanized	≤ 10	MIG/MAG, manual/fully mechanized	≤ 11
Hot wire GTAW	≤ 6	Manual arc welding (SMAW)	≤ 7
Plasma arc	≤ 10

Pitting Test on Incoloy 825 Materials

ASTM G48 Method A: Ferric Chloride Pitting Test

In order to study the pitting resistance of Incoloy 825 in high chloride-ion environments, pitting test shall be conducted in accordance with ASTM G48 Method A. The specimen is a piece of Incoloy 825 plate of 30 mm x 10 mm x 3 mm manufactured to ASTM B424 UNS N08825. It is immersed into 6% FeCl₃ solution at 50(±1)°C. Maintain the test solution temperature for 72 hours. Then, remove the specimen , rinse with water and scrub with a nylon bristle brush under running water to remove corrosion products, dip in acetone or methanol, and air-dry. Weigh the specimen to the accuracy of 0.0001g. Analyze corrosion products at the corrosion pit by using SEM and X-ray EDS methods respectively.

Actual Chemical Composition of Incoloy 825 Specimen

C	S	Si	Mn	P	Ni	Cr	Mo	Nb	Ti	Cu
0.0062	0.0009	0.31	0.47	0.014	43.34	22.78	3.28	0.18	0.94	2.04

*All values are by weight percentage.

The SEM & EDS Results

The main elements remaining at the corrosion pit are Fe, Mo, Ti and O while the content of base element nickel is relatively low. It is concluded that oxidizing ions react with Cr & Ni in the corrosive solution. The product of the reaction is dissolved in the solution.

Thermal Expansion for Incoloy 825

Instantaneous Coefficient of Thermal Expansion for Incoloy 825

Temp. °F	70	100	150	200	250	300	350	400
*TA	7.5	7.6	7.8	8.0	8.1	8.2	8.2	8.3
Temp. °F	450	500	550	600	650	700	750	800
*TA	8.3	8.5	8.7	8.9	9.2	9.2	8.9	7.8

*TA: instantaneous coefficient of thermal expansion. Unit:10^-6 inch/inch/°F.

Mean Coefficient of Thermal Expansion for Incoloy 825

Temp. °F	70	100	150	200	250	300	350	400
*TB	7.5	7.5	7.6	7.7	7.8	7.9	7.9	8.0
Temp. °F	450	500	550	600	650	700	750	800
*TB	8.0	8.1	8.1	8.2	8.3	8.3	8.4	8.4

*TB: mean coefficient of thermal expansion. Unit: 10^-6 inch/inch/°F. Going from 70°F to indicated temperature.

Linear Thermal Expansion for Incoloy 825

Temp. °F	70	100	150	200	250	300	350	400
*TC	0	0.3	0.7	1.2	1.7	2.2	2.7	3.2
Temp. °F	450	500	550	600	650	700	750	800
*TC	3.7	4.2	4.7	5.2	5.8	6.3	6.9	7.4

*TC: linear thermal expansion for Incoloy 825. Unit: inch/100 ft. Going from 70°F to indicated temperature.

Design Stress Intensity of ASTM B163 UNS N08825 Tubes

The design stress intensity of ASTM B163 Gr. N08825 seamless tubes are listed in below table:

Temp. °F	-20~100	150	200	250	300	350	400	450
A*	23.3	23.3	23.3	23.3	23.3	23.3	23.3	23.3
Temp. °F	500	550	600	650	700	750	800	850
A	23.3	23.3	23.3	23.3	23.3	23.2	23.0	–

*Nominal composition: 42Ni–21.5Cr–5Mo–2.3Cu.
*A: ASTM B163 Gr. N08825 seamless tubes, annealed, P-No.: 45.

Allowable Stress & Applicable Temperature of ASTM B163 UNS N08825

Seamless tubes manufactured to ASTM B163 UNS N08825 (Incoloy Alloy 825) are primarily used for condenser and heat-exchanger service. They are applicable for piping systems designed according to ASME BPVC Section III Class 3 or ASME BPVC Section VIII Division 1 with maximum applicable temperature 800°F or 1000°F, respectively. They are also applicable for transport tanks of ASME BPVC Section XII with maximum applicable temperature 650°F.

Max. Allowable Stress Values at Different Temperatures

*Temp. °F	-20~100	200	300	400	500	600	650
*A	23.3	21.4	20.3	19.4	18.5	17.8	17.5
*B	23.3	23.3	23.3	23.3	23.3	23.3	23.3
*Temp. °F	700	750	800	850	900	950	1000
*A	17.3	17.2	17.0	17.0	16.9	16.8	16.5
*B	23.3	23.2	23.0	22.9	22.8	22.6	22.3

*Temp.: metal temperature, not exceeding.
*A: Standard ASTM B163 UNS N08825 seamless tubes, annealed.
*B: ASTM B163 UNS N08825 seamless tubes, annealed; slightly greater deformation caused by stresses is acceptable.

PMI Test on Incoloy 825 Bars

Recently Yaang has procured two pieces of Incoloy 825 bars for further processing. The bars are manufactured to ASTM B425 Gr. UNS N08825, hot rolled, annealed and roughly ground. PMI test was conducted to verify its chemical composition. The specification of the bar: 200 mm(diameter) x 3.2 meter(length).

The PMI test result(actual chemical composition) is listed in below table:

Ti	Cr	Mn	Fe	Ni	Cu	Mo
0.92	20.56	0.52	32.56	40.42	2.25	2.78

*All values are in % unit and are read from the direct-reading spectrometer.

Variety specifications and supply status of Nickel-based super alloy: Incoloy 825 (UNS N08825/W.Nr. 2.4858)

Variety classification

Yaang Pipe Industry can produce various specifications of Incoloy 825 seamless pipe, Incoloy 825 steel plate, Incoloy 825 round bar, Incoloy 825 forgings, Incoloy 825 flange, Incoloy 825 pipe fittings, Incoloy 825 welded pipe, Incoloy 825 steel strip, Incoloy 825 wire and supporting welding materials. 

Delivery status

• 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.

Microstructure and properties of Incoloy 825 alloy after solid solution at different temperatures

The microstructure, mechanical properties, and corrosion resistance of Incoloy 825 alloy after solid solution at different temperatures were studied by scanning electron microscope, transmission electron microscope, brinell hardness tester, and tensile tester. The results show that when the solid solution temperature is 980-1050 ℃, the grain size of the alloy does not change significantly; when the solid solution temperature is higher than 1050 ℃, the grain size increases at a faster rate; with the increase of solid solution temperature, the hardness and tensile strength of the alloy decreases gradually, and the elongation increases; the grain boundary precipitation phase mainly consists of chromium-rich, molybdenum-rich M₂₃C₆ carbides and intermetallic compounds containing chromium, nickel, iron and molybdenum, and the grain boundary precipitation phase mainly consist of chromium and molybdenum rich M₂₃C₆ carbides and chromium, nickel, iron, and molybdenum-rich intermetallic compounds. For intermetallic compounds containing chromium, nickel, iron, and molybdenum, the number of grain boundary precipitation phases with the solid solution temperature shows a trend of increasing and then decreasing, the solid solution temperature of 1015 ℃ when the grain boundary precipitation phase is the most, at this time the alloy’s resistance to intergranular corrosion is the worst performance.

0. Introduction

NiFeCr alloy has good resistance to stress corrosion cracking, crevice corrosion and pitting corrosion resistance, oxidation resistance, and reducing thermo-acidic properties. It is mainly used in offshore engineering piping systems, petroleum processing heat exchangers, heating tubes in pickling equipment, etc. Incoloy 825 alloy is a titanium-stabilized austenitic NiFeCr alloy with high hardness and strength and has good corrosion resistance in high temperature environments. High hardness and strength, by adding molybdenum, copper, and other alloying elements, can be used for stainless steel is difficult to withstand the more complex and harsh corrosion environment. When the alloy composition is certain, the corrosion resistance and mechanical properties of the alloy mainly depend on its microstructure, composition, and distribution of precipitated phases, etc., and different heat treatment processes on the microstructure of the alloy, grain size, composition, distribution, number, and size of precipitated phases will have a greater impact, so it is necessary to study the alloy’s heat treatment process. Usually, the hot rolling temperature of Incoloy 825 alloy is 900-1150 ℃, and the cooling method is water cooling or rapid air cooling. The alloy needs to be solution-treated between 1150-1250 ℃ to obtain better resistance to pitting and intergranular corrosion cracking. Currently, the research on Incoloy 825 alloy mainly focuses on intergranular corrosion, precipitation phase analysis, etc. Still, there is less in-depth research on the alloy’s mechanical properties and corrosion resistance after solid solution treatment at different temperatures and the relationship between the precipitation phase and microstructure and corrosion resistance. Therefore, the authors of the microstructure, precipitation phase, corrosion resistance, and mechanical properties of Incoloy 825 after solid solution treatment at different temperatures are of great significance for optimizing the heat treatment process and studying alloying elements’ strengthening mechanism.

1. Sample preparation and test methods

1.1 Sample preparation

Test materials used for homemade Incoloy 825 alloy, through the vacuum induction furnace melting, billet thickness of 120mm, open rolling temperature 1100-1180 ℃, the final rolling temperature of not less than 900 ℃, hot rolled into a 12mm thick plate, and then after hot pickling to remove the surface oxide skin, its chemical composition as shown in Table 1. Take 5 groups of samples for solution treatment, sample size of 100mm × 12mm × 12mm, solid solution temperature were 980 ℃, 1015 ℃, 1050 ℃, 1100 ℃, 1200 ℃, insulation 20 min after quenching.
Table.1 Chemical composition of Incoloy 825 alloy (mass fraction)

C	0.023
Si	0.064
Mn	0.4
p	0.002
S	0.0038
Ni	42.8
Cr	20.8
Mo	2.99
Ti	0.98
Al	0.1
Fe	29.43
Cu	2.15

1.2 Test method

According to GB/T 6394-2002 “metal average grain size assessment method”, the use of Zeiss Axio Imager Z1m-type optical microscope on the grain size of heat-treated specimens for rating; the use of Zeiss EVO-18 scanning electron microscope on the specimen microstructure and the distribution of precipitated phases for observation, corrosive agent for the copper sulfate – hydrochloric acid aqueous solution (copper sulfate 2g, hydrochloric acid 10mL, distilled water 10mL), corrosive agent for the copper sulfate – hydrochloric acid solution (copper sulfate 2g, hydrochloric acid, distilled water 10mL), corrosive agent for the water solution (copper sulfate 2g, hydrochloric acid, distilled water 10mL). The area fraction of precipitates on the sample surface was statistically analyzed by INCA Feature, an inclusions analysis system equipped with a Zeiss ΣIGMA field emission scanning electron microscope; the precipitated phases in the samples were separated from the matrix by carbon extraction compounding and analyzed with a JEM-2100F field emission transmission electron microscope and an Oxford Instruments INCA Energy350 energy spectrometer. The precipitated phase’s morphology, structure, and composition were characterized by a JEM-2100F field emission transmission electron microscope and Oxford Instruments INCA350 energy spectrometer.
By GB/T 228.1-2010 and GB/T 231.1-2009, room temperature tensile and Brinell hardness tests were performed on a Model 5585H tensile tester and an Instron CLB3 Brinell hardness tester, and the tensile specimen was a Φ8-mm round bar, and the average value of the three data of each was taken as the result of the test; by the method A of ASTM G28-2015, 600 mL of the sample with a mass fraction of 50 % was prepared. By method A of ASTM G28-2015, 600mL of boiling sulfuric acid-ferric sulfate solution with a 50% mass fraction was prepared, and the specimens, after solid solution treatment at different temperatures, were weighed and immersed into the boiling solution and then kept in the boiling solution for 120 h. The corrosion rate was calculated by washing, drying, and weighing the specimens.

2. Test results and discussion

2.1 The effect of solid solution temperature on microstructure

As can be seen from Figure 1, Incoloy 825 alloy is a single austenitic organization; after 980 ℃, 1015 ℃, 1050 ℃, 1100 ℃, and 1200 ℃ solid solution treatment, the grain size level of 9.5, 9.5, 9.0, 3.5 and 1.5 levels, respectively. This shows that with the solid solution temperature between 980 ℃-1050 ℃, grain size change is not obvious; when the solid solution temperature of 1100 ℃, the austenite grain grows sharply; when the solid solution temperature of 1200 ℃, the grain continues to grow. This is because as the solid solution temperature increases, the dislocation density decreases, the migration speed of the grain boundary is accelerated, and the growth rate of the grain is accelerated.

Figure.1 Microstructure of Incoloy 825 alloy after solid solution at different temperatures

2.2 Effect of solid solution temperature on mechanical properties

Figure 2(a) shows that the Brinell hardness of the alloy and the average grain size have a good correspondence; with the solid solution temperature increases, Brinell hardness decreases; when the solid solution temperature of 980-1050 ℃, the hardness value of the decline is small when the solid solution temperature is higher than 1050 ℃, Brinell hardness decreases sharply, which is consistent with the change rule of the average grain size and organization. Figure 2(b) shows with the solid solution temperature increases, the elongation after break gradually increases, and the tensile strength of the trend of change and grain size and Brinell hardness change rule is the same, which indicates that the alloy’s hardness, strength and plasticity of the alloy and the alloy grain size there is a certain degree of correlation. When the solid solution temperature exceeds 1050 ℃, the austenite grain grows rapidly, the number of grain boundaries is reduced sharply, the bonding force between the grains is weakened, and thus, the tensile strength decreases rapidly. Still, at this time, the solid solution temperature is higher, the number of dislocations within the grain is low, and the internal stress is small. Hence, the elongation after break reflects the plasticity increases rapidly.

Figure.2 Mechanical properties and average grain size of Incoloy 825 alloy after solid solution at different temperatures: (a) Brinell hardness and average grain size and (b) tensile strength and elongation after fracture

2.3 Effect of solid solution temperature on corrosion resistance properties

As can be seen in Fig. 3(a), when the solid solution temperature is 980-1200 ℃, the corrosion rate of the alloy and the content of the precipitated phase (area fraction) show a trend of increasing and then decreasing with the increase of the solid solution temperature. The two patterns of change are the same. Figures 3 (b) and (c) show solid solution temperature in the range of 980-1015 ℃, the organization of a large number of grain boundary precipitation phase, the alloy grain size is the same, which indicates that the precipitation phase of the grain boundaries of the pinning effect of the inhibition of the growth of grains, at the same time, in the temperature interval of the emergence of a large number of precipitation phase led to the intergranular corrosion rate increases. When the solid solution temperature increases to 1050 ℃, the grain grows slightly, as shown in Fig. 3 (d), which is mainly due to the precipitated phase on the grain boundary beginning to dissolve, the number of precipitated phases has been reduced, resulting in the pinning effect on the grain boundary is weakened. When the solid solution temperature of 1100 ℃, the grain size increases significantly, as shown in Figure 3 (e); no obvious precipitates appear at the grain boundary, corresponding to the intergranular corrosion rate decreases. When the solid solution temperature of 1200 ℃, as shown in Figure 3 (f), is compared with the solid solution temperature of 1100 ℃, the grain boundary precipitates are the same, and the grain size increases. Still, the intergranular corrosion rate does not change significantly, indicating that the grain size of the intergranular corrosion rate does not significantly impact.

Figure.3 Precipitated phase content, intergranular corrosion rate, and secondary electron image of Incoloy 825 alloy after solid solution at different temperatures (b-f)

2.4 Micro-morphology of precipitated phase

As can be seen from Figure 4: 980 ℃ solution treatment after the test alloy precipitation phase is mainly face-centered cubic structure M₂₃C₆ carbide, M to chromium, molybdenum-based elements, as well as chromium, nickel, iron, molybdenum, and other intermetallic compounds; these two precipitation phases are chromium-rich, and the precipitation phase along the grain boundaries of the distribution. Due to the chromium to the grain boundaries of the diffusion rate being lower than the speed of its aggregation to the crystal, resulting in the formation of a chromium-poor zone around the grain boundaries, the grain boundaries are easy to be corroded, which can be a good explanation for the precipitation phase increased in Figure 3 (a) and the corresponding corrosion rate of the grain boundaries increased phenomenon.

Figure.4 TEM bright field image and element mapping distribution of two precipitated phases in Incoloy 825 alloy after solid solution treatment at 980 ℃: (a) carbide bright field image; (b) carbide diffraction patterns and element mapping scanning result; (c) intermetallic phase distribution in Incoloy 825 alloy after solid solution treatment at 980 ℃. Result; (c) intermetallic compound bright field image; (d) intermetallic compound element mapping scanning result

3. Conclusion

• (1) With the increase of solid solution temperature, the grain size of Incoloy 825 alloy shows an increasing trend; when the temperature is between 980 ℃ and 1050 ℃, the grain size increases insignificantly; when the temperature is higher than 1050 ℃, the grain size increases at a faster rate; the hardness and tensile strength of Incoloy 825 alloy decreases, and the elongation increases continuously.
• (2) Incoloy 825 alloy after solution treatment, the grain boundary precipitation phase mainly comprises chromium-rich, molybdenum carbides and intermetallic compounds containing chromium, nickel, iron, and molybdenum.
• (3) With the increase of solid solution temperature, the number of grain boundary precipitated phases of Incoloy 825 alloy increases and then decreases; the intergranular corrosion rate shows a tendency to increase and then decrease, and the solid solution temperature of 1015 ℃ has the most precipitated phases at the grain boundaries, and the corrosion rate is the largest.

Author: Hong Huimin