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A Comprehensive Guide to Nickel-based super alloy: Incoloy 901 (UNS N09901)

What is Incoloy 901?

The Cr content in nickel-based alloy incoloy 901 is usually 11.0-14.0%, and the nickel content is 40.0-45.00%. Incoloy 901 alloy is a nickel-based alloy containing molybdenum and copper, with good hot and cold working properties. Incoloy901 is an austenitic age-hardening alloy based on Fe-43Ni-12Cr, adding titanium, aluminum and equal strengthening elements, and contains a small amount of boron and lower carbon. It is metastable γ”[Ni3(Ti, Al)] phase dispersion strengthening, a small amount of aluminum can inhibit the conversion of γ” to η-Ni3Ti phase.
Incoloy 901 alloy has high yield strength and endurance strength below 650℃, good oxidation resistance below 760℃, and stable organization after long-term use. This alloy is a relatively mature alloy developed in the early stage. It is widely used in the manufacture of rotating discs (turbine discs, compressor discs, journals, etc.), static structural parts, turbines for aviation and ground gas turbine engines operating below 650°C. Parts such as outer ring and fasteners.

Standard Inventory Specifications:

  • UNS N90901
  • AMS 5660
  • AMS 5661
  • B50A305B
  • Werkstoff 2.4662
  • DIN 1.4898
  • AISI 681, 68

Common Trade Names:

  • Incoloy 901 (USA)
  • Nimonic 901 (UK)
  • Z8NCDT42 (France)
  • 2.4662 (Germany)
  • GH901 (China)

Chemical composition (%) of nickel-based alloy incoloy 901 (UNS N09901)

Alloy 901 has good corrosion resistance to the atmospheres normally found in jet engine operations. It has slightly lower scaling resistance than such alloy as Types 309 and 310 stainless steel.

C≤ Si≤ Mn≤ P≤ S≤ Cr≥ Ni≥ Mo≥ Cu≤
0.02-0.06 0.40 0.50 0.020 0.008 11.0-14.0 40.0-45.0 5.0-6.5 0.20
other B≤ Al≤ Ti≤ Fe≤ Bi≤ Pb≤ Ag≤ Nb≤
0.01-0.02 0.30 2.80-3.10 margin 0.0001 0.001 0.0005

Typical Mechanical Properties of nickel-based alloy incoloy 901 (UNS N09901)

Tensile Properties

Solution Treated: 2000°F (1093°C), 2 hours, water quenched. Stabilized: 1425°F (774°C), 4 hours , air cooled. Aged: 1350°F (732°C), 24 hours, air cooled.


0.2% Offset
Yield Strength

Tensile Strength

% Elongation
in 2″

% Reduction
of Area















Stress Rupture Strength

Solution Treated: 2000°F (1093°C), 2 hours, water quenched. Stabilized: 1425°F (774°C), 4 hours , air cooled. Aged: 1350°F (732°C), 24 hours, air cooled.


Stress to Produce Rupture in



10 Hours

100 Hours

1000 Hours















Physical Properties nickel-based alloy incoloy 901 (UNS N09901)

  • Density, lb/cu in……………………………………………………………….. 0.294
  •           Mg/cu m……………………………………………………………….. 8.14
  • Melting Range, °F……………………………………………………… 2335-2455
  •                     °C……………………………………………………… 1280-1345
  • Specific heat, Btu/lb-°F………………………………………………………. 0.103
  •                      J/kg-°C…………………………………………………………… 431
  •  Permeability at 200 oersted (15.9 kA/m)………………………………..1.013
  •  Coeffiecent of Expansion,  68-212°F, 10(-6) in/in-°F………………. 7.50
  •                                              20-100°C, æm/m-°C……………………. 13.50
  •  Electrical Resistivity, ohm-cir mil/ft……………………………………….. 674
  •                                      æê-m………………………………………………….. 1.12

Heat treatment system of nickel-based alloy incoloy incoloy 901 (UNS N09901):

Solution Treatment

1975/2025°F (1080/1107°C), hold 2 hours at heat, and water quench.

Stabiliztion Treatment

1400/1475°F (760/802°C), hold 2 to 4 hrs, and air cool.

Precipitation Hardening Treatment

1300/1375°F (704/746°C), hold for 24 hours, and air cool.

Smelting and casting process of nickel-based alloy incoloy incoloy 901 (UNS N09901)

Alloy is produced by the dual process of vacuum induction and vacuum arc remelting, or vacuum induction and electroslag remelting process.

Workability of nickel-based alloy incoloy 901 (UNS N09901)

Hot Working

Alloy 901 is forged between 2050°F (1121°C) and 1850°F (1010°C), metal temperature. Light hot work may be continued down to 1600°F (871°C) but not below. Metal temperature should not exceed 2050°F (1121°C) during rapid working. Billets should be charged into a hot furnace and heated rapidly through the precipitation hardening range.


Alloy 901 can be welded by the inert-gas-arc method. It is difficult to weld. All welding should be done in the solution treated condition. Cold worked parts should be re-solution treated before welding. A re-solution treatment is recommended after welding before stabilizing and aging.

Application areas of nickel-based alloy incoloy 901 (UNS N09901)

This alloy is widely used abroad for rotating parts and fasteners on aero engines and ground gas turbines that work below 650°C, and has a long service life. It has also been used in aero engines in China, and it is a mature alloy that has passed the test of use. Alloy forging is advanced, if the process parameters are selected or operated improperly, its performance will show obvious directionality and may cause notch sensitivity, but as long as the process is strict, this phenomenon will not occur. The expansion coefficient of the alloy is close to ferritic heat-strength alloy steel, so that the two materials can be connected and there is no special requirement for the heat account.

  • 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 pipes
  • Pipe fittings
  • Flanges
  • Valves

Variety specifications and supply status of Nickel-based super alloy: Incoloy 901 (UNS N09901)

Variety classification:

Yaang Pipe Industry can produce various specifications of Incoloy 901 seamless pipe, Incoloy 901 steel plate, Incoloy 901 round bar, Incoloy 901 forgingsIncoloy 901 flangeIncoloy 901 pipe fittings, Incoloy 901 welded pipe , Incoloy 901 steel strip, Incoloy 901 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.

The effect of heat treatment on the organization and hardness of GH901 alloy

The microstructure observation and hardness test of GH901 high temperature alloy after solid solution treatment and aging treatment at different temperatures were carried out to analyze the influence of different factors on the hardness of the alloy. The results show that for the relationship between the grain size and hardness value of GH901 alloy, the H-P relationship is not applicable; the primary aging temperature has a greater effect on the hardness of the alloy.
GH901 alloy is a kind of Fe-Ni alloy with high Ni content. The alloy has high strength and oxidation resistance at high temperatures, and it has good comprehensive performance with stable organization and performance when used for a long time at 650°C. GH901 alloy has been widely used in the jet engines of the United Kingdom, the United States, and other countries. China’s alloy introduction mainly manufactures engine turbine disks, compressor disks, journals, and solid parts. With the localization of a certain engine, the demand for GH901 alloy will increase dramatically. GH901 alloy is mainly shaped through the forging process, solid solution, and aging treatment to obtain the required organization. The actual production found that the hardness of the alloy will often appear unqualified or have a lower limit value. Given this situation, this paper tries to find out the reasons for the low hardness value of the alloy. In this paper, after heat treatment of GH901 alloy at different temperatures, metallographic organization observation and hardness test were carried out to analyze the effect of different aging temperatures on the organization and hardness of GH901 alloy.

1. Test material and method

Selected into the factory re-examination of GH901 high-temperature alloy, production in recent years has been followed by a material standard heat treatment system: 1090 ℃ × 3h, WC; 775 ℃ × 3h, AC; 720 ℃ × 24h, AC. Test alloy solution treatment system for the 1090 ℃ × 3h, water-cooled; aging was carried out in 600, 700, 775, 800, and 900 ℃, aging time is 3h, aging time is 3h. The aging treatment was carried out at 600, 700, 775, 800, and 900 ℃, respectively, and the aging time was 3h. The specimens were quenched in water quickly after being taken out of the furnace to retain the high temperature organization of the alloy.
Firstly, remove the oxide skin on the surface of the specimen, and then use the Brinell hardness machine to determine the hardness value of the alloy after different aging treatments; the diameter of the indenter used is quasi 5mm, the load is 7.5kN, and the loading time is 12s. Metallurgical specimens were corroded with Nimonecle corrosion solution, and after corrosion, the specimens were observed and photographed under the metallurgical microscope, and the grains were graded.

2. Test results

2.1 Metallographic organization of the alloy after different heat treatment

Figure 1 shows the metallographic microstructure of GH901 alloy after different heat treatments. It can be seen that the grain boundaries of the alloy after solution treatment are straight and clean, as shown in Fig. 1(a). With the increase of aging temperature, the alloy grain size increases slightly, and its grain size is basically in the 1-2 level, or close to the 2 level, as shown in Figure 1(b)-(f). Figure 1 (g) for the GH901 alloy secondary aging after the completion of the metallographic organization of the alloy, compared with Figure 1 (d), can be found that after the second aging, the alloy grain size changes are not obvious.

2.2 Hardness of the alloy after different heat treatments

Table 1 shows the Brinell hardness values of GH901 alloy after different heat treatments. It can be seen that after different heat treatments, the hardness of the alloy varies greatly, of which the hardness of the alloy is the lowest when only solid solution treatment and the hardness of the alloy is higher when the solid solution temperature is lower.
20230916012552 46272 - A Comprehensive Guide to Nickel-based super alloy: Incoloy 901 (UNS N09901)
Figure.1 Microstructure of GH901 alloy under different heat treatment conditions
Table.1 Brinell hardness of GH901 alloy after different heat treatments.

Heat treatment schedule HB
1050 °C x 3h (WC) 152
1090 °C x 3h (WC) 143
1090 °C x 3h (WC) + 600 °C x 3 h (AC) 148
1090 °C x 3h (WC) + 700 °C x 3h (AC) 195
1090 °C x 3h (WC) + 775 °C x 3h (AC) 343
1090 °C x 3h (WC) + 800 °C x 3h (AC) 339
1090 °C x 3h (WC) + 900 C x 3h (AC) 226
1090 °C x 3 h (WC) + 775 °C x 3h (AC) + 720 °C x 24h (AC) 345

The relationship curve between the aging temperature and hardness of GH901 alloy is shown in Fig.2. It can be seen that the alloy is aged in the range of 600-775°C, and the hardness value increases gradually with the increase of aging temperature. At 775 ℃, the hardness value of the alloy reaches the maximum value of 343 HB. When the aging temperature exceeds 800 ℃, the alloy’s hardness decreases with the aging temperature increase. 900 ℃ aging hardness value is lower than the hardness of 800 ℃, which is only 226 HB. After the second aging, the hardness value of the alloy is slightly increased, which is 345 HB.

20230916012955 69631 - A Comprehensive Guide to Nickel-based super alloy: Incoloy 901 (UNS N09901)
Figure.2 Hardness change curve of GH901 alloy after aging at different temperatures

3. Analysis and discussion

3.1 The effect of grain size on alloy hardness

According to the single crystal and polycrystalline materials, dislocation plugging theory summarizes the material’s yield strength (or hardness) and grain size of the relationship, that is, the Hall-Peach (H-P) relationship formula:

σs0 + kd-1/2 (1)

The H-P equation can also describe the effect of grain size on hardness:

H=H0 + kd-1/2 (2)

In the formula:

  • H is the hardness value of the material, and H0 and k are constants.
  • H0 is the matrix hardness, mainly determined by the crystal structure and dislocation density.
  • k is a constant, which is different for different materials; d is the average grain diameter.
  • d is the average grain diameter.

Combined with the previous data can be seen that GH901 alloy grain size changes and hardness value of the law of change is not consistent; that is, with the aging temperature increases, grain size increases slightly, while the hardness value is the first increase and then decrease. GH901 alloy hardness value and the inverse of the square root of the grain size is not a linear relationship and does not meet the H-P equation. Therefore, caused by GH901 alloy hardness is low, not its grain size growth, but there are deeper reasons!

3.2 Analysis of other reasons

As we all know, in addition to fine grain strengthening, the second phase strengthening also greatly impacts the alloy’s strength; GH901 alloy is strengthened by adding Mo, Ti, and Al elements to strengthen the Ni-Fe-Cr based high temperature alloy. Studies have shown that the alloy is strengthened by the main intermetallic compound Ni3(Al, Ti)γ′ phase and trace amounts of (Ti, Mo)MC-type carbides. When the alloy is aged at 600°C, its hardness value is the same as that of the solid solution treatment only. In 700 ℃ aging, the hardness has a significant increase, which indicates that GH901 in the γ′ phase precipitation temperature should be between 600-700 ℃. As the aging temperature increases, the hardness value increases gradually, and the hardness of the alloy reaches a maximum value of 343 HB at 775°C. When the aging temperature exceeds 800°C, the hardness starts to decrease.
In early aging, hardness increase is not obvious; the reason should be that the temperature is lower when the nucleation and diffusion are more difficult, γ′ phase precipitation is less, and the degree of diffuse distribution is not very high. With the increase of aging temperature, the γ′ phase precipitates continuously, so the hardness of the alloy increases rapidly. When the aging temperature is 775-800℃, the hardness value does not change much, which is around 340HB, but it is higher than the hardness under other aging temperatures. It shows that under this aging condition, the number and size of γ′ phase precipitated in the alloy do not show obvious changes. It can also be inferred that the precipitation amount and critical size of γ′ phase have reached a better matching state. At the same time, it shows that the more suitable aging temperature of the alloy should be located in this temperature range. With the further increase of aging temperature, although the amount of γ′ phase precipitation increased, the size of γ′ phase also increased, and the degree of dispersion distribution further weakened. Hence, the hardness dropped sharply to about 226 HB at 900 ℃. It was obviously in the over-aging stage.
Compared with the first aging, after the second aging, the hardness value of the alloy slightly increased, but not obvious. The reason is that there are two sizes of γ′phase in the microstructure after two aging, and the large size γ′is precipitated during the primary aging process and grows slightly in the secondary aging. Small size γ′ is in the aging process of additional precipitation. In addition, the role of the second aging is mainly to adjust the size of the reinforcing phase to obtain the appropriate microstructure and adjust the uniformity of the organization to obtain the strength and plasticity of the best match. Thus, aging treatment plays a decisive role in the hardness of the alloy. To improve the hardness of the alloy, it is important to control the aging treatment at a good level.

4. Conclusion

  • (1) With the increase of aging temperature, the grain size of GH901 alloy increases slightly, and the grain size is maintained at about 1-2 levels.
  • (2) The hardness of the alloy changes considerably after heat treatment at different temperatures. Only solid solution treatment when the hardness of the alloy is the lowest; with the aging temperature increasing, the hardness value of the alloy first increases and then decreases. With the aging temperature of 775 ℃, the hardness of the alloy reached a maximum value of 343HB.
  • (3) For the GH901 alloy, aging treatment on the alloy grain size is not obvious, and the contribution to the hardness value, the H-P relationship, is not applicable. Adjustment of the aging treatment process plays an important role in the hardness of the alloy.

Author: Zhang Huan



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