A Comprehensive Guide to Nickel-based super alloy: Incoloy 800 (UNS N08800/W.Nr. 1.4876)

In the world of advanced materials, nickel-based super alloys have carved out a significant niche due to their exceptional properties and wide range of applications. One such remarkable alloy is Incoloy 800, renowned for its outstanding resistance to high temperatures, corrosion, and pressure. This comprehensive guide’ll delve into the fascinating world of Incoloy 800, exploring its composition, properties, applications, and benefits.

What is Incoloy 800?

Designated as UNS N08800 or DIN W.Nr. 1.4876, Incoloy 800 (also known as “Alloy 800”) is an iron-nickel-chromium alloy with good strength and excellent resistance to oxidation and carburization in high-temperature atmospheres. It also has excellent resistance to corrosion by many aqueous environments and resistance to chloride stress-corrosion cracking. Incoloy 800 alloy maintains a stable austenitic structure during prolonged exposure to high temperatures since it doesn’t form the embrittling sigma phase after long time exposure at 1200°F [649°C]. It’s mainly used for petrochemical process piping, heat exchangers, carburizing equipment, heating element sheathing, and nuclear steam generator tubing.

Incoloy 800 (UNS N08800) has a high chromium content and a sufficient nickel content, so it has a high resistance to high temperature corrosion, and it is widely used in industry. In chloride, low concentration NaOH aqueous solution and high temperature and high pressure water, it has excellent stress corrosion cracking resistance, so it is used to manufacture stress corrosion cracking resistance equipment. Corrosion resistance is better than general austenitic stainless steel, and better than Inconel 600 alloy; in the manufacture of corrosion-resistant stress cracking equipment, it is better than Inconel 600 and Monel 400 alloy.

ASTM B564 Incoloy 800 weld neck flange, 3″ SCH80 600#, ASME B16.5.

Chemical Composition Requirements of Incoloy 800

The Chemical Composition of Incoloy 800, %
Ni	30.0-35.0
Cr	19.0-23.0
Fe	≥39.5
Ti	0.15-0.60
Al	0.15-0.60
C	≤0.10
Mn	≤1.50
Si	≤1.00
S	≤0.015
Cu	≤0.75

Physical Properties of Incoloy 800

Density	Melting Range		Specific Heat		Electrical Resistivity
g/cm3	°C	°F	J/kg. k	Btu/lb.°F	µΩ·m
7.94	1357-1385	2475-2525	460	0.110	989

Temperature ℃	σb/ MPa	σ0.2/MPa	δ5/%
100	425	140	160
300	390	95	115
500	360	80	100
600	300	75	95

• Incoloy 800 (N08800) alloy melting temperature range: 1350-1400℃. 
• Incoloy 800 (N08800) alloy specific heat capacity: 455J/(kg·℃). 
• Incoloy 800 (N08800) alloy density: 8.0 g/cm³. 
• Incoloy 800 (N08800) alloy magnetism: none.

Mechanical Properties of Incoloy 800

Tensile Strength, min.		Yield Strength, min.		Elongation, min.	Hardness, min.
Mpa	ksi	Mpa	ksi	%	HB
600	87	295	43	44	138

Processing performance of nickel-based alloy incoloy 800 (N08800): 

Incoloy 800 (N08800) alloy hot processing:

• (1) The temperature range is 1200℃-950℃, and the cooling method is water quenching or rapid air cooling. 
• (2) In order to obtain the best performance and creep resistance, solution treatment should be carried out after thermal processing.
• (3) The material can be directly fed into the furnace that has been heated to 1200°C, and after holding for sufficient time, the material can be quickly discharged, and thermal processing can be carried out in the specified temperature range. When the material temperature drops below the thermal processing temperature, it needs to be reheated. 

Cold processing of Incoloy 800 (N08800) alloy:

• (1) The work hardening rate is greater than that of austenitic stainless steel, so the processing equipment needs to be selected. The cold-worked material should be in a solution heat-treated state, and intermediate annealing should be carried out when the amount of cold work is large. 
• (2) If the amount of cold working is greater than 10%, the workpiece needs to be treated with a second solution. 

Incoloy 800 (N08800) alloy welding process:

Suitable for welding with the same metal material or other metal metals 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. If manual arc welding is used, it is recommended to use (Ar+He+H₂+CO₂) as the shielding gas.

Product Forms and Standards of Incoloy 800

Product From	Standard
Rod and Bar	ASTM B408, EN 10095
Plate, Sheet & Strip	ASTM A240, A480, ASTM B409, B906
Seamless pipe and tube	ASTM B829, B407
Welded pipe	ASTM B514, B515, B751, B775
Welded pipe fitting	ASTM B366
Forgings	ASTM B564, DIN 17460

Application areas of nickel-based alloy incoloy 800 (N08800):

• Chemical industry
• Nuclear generators
• Nitric acid coolers
• Acetic anhydride cracking tubes
• Heat exchange equipment
• Heat exchange tubes
• Pipe fittings
• Flanges
• Valves

Varieties, specifications and supply status of Incoloy 800 (N08800) nickel-based alloys:

Variety classification:

Yaang Pipe Industry can produce Incoloy 800 seamless pipes, Incoloy 800 pipe fitting, Incoloy 800 round bar, Incoloy 800 forgings, Incoloy 800 flanges of various specifications , Incoloy 800 ring, Incoloy 800 welded pipe, Incoloy 800 steel strip, Incoloy 800 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.

Effect of heat treatment on microstructure and properties of Fe-Ni based alloy UNS N08800

The effects of heat treatment on the microstructure and properties of UNS N08800 alloy cold rolled sheet were studied using an optical microscope, scanning electron microscope, and mechanical properties test. The results show that the grains grow gradually with the increase of heat treatment temperature and holding time, and the grains grow rapidly at 1120 °C. With the increase of heat treatment temperature, the alloy’s tensile strength and yield strength at room temperature decrease gradually, and the plasticity increases gradually. The better heat treatment process of the UNS N08800 alloy cold plate is (1070 ± 10) °C, holding time 3-4 min.
UNS N08800 is an ultra-low carbon austenitic iron-nickel-based alloy with good oxidation, high strength, excellent pitting corrosion resistance, and stress corrosion resistance. It has been widely used in industrial and household high-end electric heating pipe industry, industrial heat treatment furnaces, and structural parts under high temperature conditions. During the heat treatment process of the alloy sheet, there will be precipitation and dissolution of the second phase, insufficient recrystallization recovery of the grain structure, and grain coarsening, which will affect the sheet’s corrosion resistance and mechanical properties. Therefore, to obtain the UNS N08800 alloy plate with excellent microstructure and properties, this paper mainly studies the influence of the heat treatment process on the microstructure and properties of the UNS N08800 plate and puts forward a better heat treatment process, which provides technical guidance for the formulation of actual heat treatment production process.

1. Test materials and methods

The test material is a 6mm UNS N08800 hot-rolled plate after solid solution treatment at 1130 °C/10 min, then cold rolled to a 2mm plate. The main chemical composition is shown in Table 1. The cold-rolled plate samples were heat-treated in a box-type resistance furnace at 1060-1120 °C for 1-6 min and then water-cooled. The samples after heat treatment were polished,
Table.1 Chemical composition of test materials %

w(C)	w(Si)	w(Mn)	w(P)	w(S)	w(Ni)	w(Cr)	w(Al)	w(Ti)	w(Cu)	w(Fe)
0.012	0.4	0.55	0.015	0.001	30.43	19.98	0.35	0.35	0.02	Bal.

2. Results and discussion

2.1 Hot rolled plate solution and cold rolled microstructure analysis

Fig.1 is the microstructure of the UNS N08800 alloy hot-rolled plate after solution treatment and cold rolling. It can be seen from Fig.1-1 that the microstructure of the hot rolled plate after solution treatment is single austenite, uniform structure, no precipitated phase, and the grain size is grade 5. Figure 1-2 shows that after 60% cold rolling deformation of the hot rolled plate, the grain is deformed along the rolling direction and is elongated. The cold rolling deformation is relatively uniform as a whole.

Fig.1 Microstructure of annealed and cold rolled UNS N08800 alloy hot rolled sheet

2.2 Effect of heat treatment on grain structure

Figure 2 and Figure 3 are the microstructure of the UNS N08800 cold rolled plate after heat treatment under different conditions. It can be seen from Fig.2 that when holding at 1060 °C for 1 min, the grain structure is mixed crystal, only part of the deformed structure is recrystallized, and the banded structure is obvious, which is due to the insufficient nucleation time of recrystallized crystal nucleus caused by too short time. When holding at 1060 °C for 2 min, the deformed microstructure has completed recrystallization, the grain structure is uniform, and the grain size is 8-9 grade; as the holding time is extended to 3 min and 4 min, the grains gradually grow, and the grain size is 7-8 grade. When the holding time is extended to 6 min, the growth phenomenon of large grains annexing small grains begins to appear, and the grain size is 7-5 grade.

Fig.2 Grain structure after holding at 1060 °C for different time
Figure 3 shows that under holding at 1060-1120 °C for 4 min, the alloy has completely recrystallized, and the structure is uniform. This is due to the deformation of the alloy matrix. There is a high enough distortion energy everywhere. Under suitable thermodynamic conditions, the energy conditions are created for the recrystallized crystal nucleus to form and grow quickly, and the distortion caused by cold processing is consumed to complete the recrystallization. With the increase of heat treatment temperature, the grain size gradually increases, and the grain grows at 1060-1090 °C. When the temperature increases to 1120 °C, the grains begin to coarsen. Due to the enhancement of thermal activation, the driving force of recrystallization increases, and the grains grow up after the recrystallization of the deformed structure.

Fig.3 Morphology of grain structure after heat treatment at different temperatures for 4 min

2.3 Effect of heat treatment temperature on mechanical properties

Fig.6 shows the scanning structure of the UNS N08800 cold plate, a typical austenite. No obvious precipitated phase is found in intragranular and intergranular, and the grain size is 7-8 grade.Table 2 is the mechanical properties test results of the UNS N08800 cold rolled plate after holding for 4 min. As the temperature increases, the tensile strength and yield strength of the alloy gradually decrease, and the elongation gradually increases. ASTM A240 standard stipulates room temperature mechanical properties: Rp0.2 ≥ 205MPa, Rm > 520MPa, A50 ≥ 30%. According to the test results, it can be concluded that the heat treatment process (1070 ± 10) °C for 3-4 min is a better heat treatment process for the UNS N08800 cold plate with a thickness of 2.0 mm.
Table.2 Test results of mechanical properties of UNS N08800 cold plate at room temperature after heat treatment

Temperature/℃	Yield strength, Rp0.2/MPa	Tensile strength, Rm/MPa	Elongation after fracture, 50/%
1060	223	545	44
1090	208	532	47
1120	187	510	49

The room temperature tensile test results of the UNS N08800 alloy 2.0mm cold plate are shown in Table 3, and the properties of the cold plate meet the requirements of ASTM A240 standard.
Table.3 Mechanical properties of UNS N08800 cold plate at room temperature

Sample	Yield strength, Rp0.2/MPa	Tensile strength, Rm/MPa	Elongation after fracture, 50/%
Taiyuan Iron and Steel Cold Plate	260	565	45
ASTM A240	M205	M520	M30

Fig.6 Microstructure of UNS N08800 cold plate finished product
The intergranular corrosion test of the N08800 cold plate was carried out according to the E method in GB/T 4334 standard after sensitization at 650 °C/2h. The macroscopic morphology of the crystal corrosion sample after flattening is shown in Fig.7.No intergranular corrosion crack was found on the sample’s surface, and the crystal corrosion was qualified.

Fig.7 Macroscopic morphology of crystal rot samples of UNS N08800 cold plate finished products
The stress corrosion resistance test of the UNS N08800 cold plate was carried out according to ASTM G36 standard. The test conditions were: w (MgCl2) = 42% solution, boiling, test for 200 h. After the test, the sample was checked, and there was no stress crack. It can be seen that the UNS N08800 cold plate has excellent stress corrosion resistance.

3. Conclusion

• 1) The heat treatment system greatly influences the grain size of UNS N08800 alloy. The grains gradually grow up in the range of 1060-1090 °C. At 1120 °C, the grain growth trend is obvious.
• 2) Heat treatment significantly affects the mechanical properties of UNS N08800 alloy. When the heat treatment temperature increases from 1060 °C to 1120 °C, the strength index of the alloy gradually decreases, and the plasticity gradually increases.
• 3) The optimum heat treatment process of UNS N08800 alloy cold plate is (1070 ± 10) °C, holding time 3-4 min. Under this process, the grain structure of the plate is uniform, the mechanical properties meet the standard requirements, and the corrosion resistance is excellent.

Author: Li Sha