A Comprehensive Guide to Nickel-based super alloy: Alloy 31 (UNS N08031)

What is alloy 31?

Alloy 31 ( UNS N08031) nickel base alloy is a kind of iron nickel chromium molybdenum alloy containing nitrogen. Its properties are between super austenitic stainless steel and existing nickel base alloy. Alloy 31 (N08031) nickel base alloy is suitable for chemical and petrochemical industry, environmental engineering, oil and gas production and other industrial fields.

Designated as UNS N08031 or DIN W.Nr. 1.4562, Alloy 31 is an iron-nickel-chromium-molybdenum alloy with nitrogen and copper additions which has hybridized properties of austenitic super stainless steels and regular nickel alloys. Alloy 31 has exceptional corrosion resistance to alkaline & halide-containing media, sulfuric acid, phosphoric acid, chlorine dioxide media, nitric acid as well as reducing & oxidizing medias. It is widely used in offshore & marine industries, chemical & petrochemical processing, pharmaceutical piping, oil and gas extraction, and ore digestion plants. Especially, alloy 31 can be used for the fabrication of pressure vessels which may endure a wide temperature-range from cryogenic level to elevated temperatures(-196°C to 550 °C).

(UNS N08031) Alloy 31 weld neck flanges

Standards of Alloy 31 (UNS N08031)

Chemical Composition of Alloy 31 (UNS N08031)

Element	Min	Max
C	–	0.0015
Si	–	0.3
Mn	–	2
P	–	0.02
S	–	0.01
Cr	26	28
Cu	1	1.4
Fe	Balance
Mo	6	7
N	0.15	0.25
Ni	30	32

Mechanical Properties of Alloy 31 (UNS N08031)

Tensile Strength, min.		Yield Strength, min.		Elongation, min.	Hardness, min
Mpa	ksi	Mpa	ksi	%	HB
650	95	277	41	40	220

Physical Properties of Alloy 31 (UNS N08031)

Density		Melting Range		Permeability	Electrical Resistivity
g/cm3	lb/in3	°C	°F	@20°C	µΩ·m
8.05	0.290	1330-1370	2426-2498	1.001	104

Product Forms and Standards

Product Form	Standard
Plate, sheet and strip	ASTM A240, A480
Seamless pipe and tube	ASTM A213
Welded tube	ASTM A249
Forgings	ASTM A182

ASTM B366 UNS N08031 90° elbows L/R 18″ SCH160

Filler metal (for welding with 1.4562)

• Welding Rod Nicrofer S 5923 – FM 59 (Material Designation 2.4607)
• UNS N06059
• Covered electrode 2.4609
• UNS W86059
• AWS A5.11: ENiCrMo-13
• DIN EN ISO 14172: E Ni 6059 (NiCr23Mo16)

Characteristics of Alloy 31 (UNS N08031)

Temperature Range	Material for wet corrosion
Density	8.1 g/cm³
Elongation at break of Alloy 31	≥ 40 %
ISO-V notch impact toughness	≥ 185 J/cm² at room temperature ≥ 140 J/cm² at -196°C

Corrosion Properties of Alloy 31 (UNS N08031)

One of the most important properties for an alloy exposed to ehloride-eontaining solutions, sueh as seawater or flue-gas eondensate, is its resistanee to pitting and ereviee attaek. The pitting and ereviee eorrosion resistanee of an alloy in a ehloride environment ean be eorrelated to its eomposition by using the Pitting Resistanee Equivalent (PREN) equation, whieh is eommonly defined as PREN = % Cr + 3.3% Mo + 30% N, where ehromium, molybdenum and nitrogen are in weight %. For 31 alloy, the PREN is typieally about 53. The table below shows the PREN values and eritieal pitting temperatures of several alloys eompared to 31 alloy.

PREN values and Critical Pitting Temperatures (per ASTM G48 A)

Alloy	PREN	CPT
Alloy 316	24	59°F(15°C)
Alloy 317	30	77°F(25°C)
Alloy 904L	36	113°F(45°C)
AL-6XN	45	167°F(75°C)
Alloy 31	53	185°F(85°C)

The table below lists the corrosion rates of 31 alloy from immersion tests conducted according to ASTM G31 and ASTM A262 B. Duplicate samples were exposed for the times shown and an average corrosion rate was determined.

Solution	Time	Mils per year	mm/a
Boili ng 10% HSO4	Five 48-hr Periods	28.8	0.732
Boili ng 1% HCl	Five 48-hr Periods	9.6	0.244
Boili ng 65% HN03	Five 48-hr Periods	10.8	0.274
ASTM A262 B	120 hours	14.3	0.364

Formability of Alloy 31 (UNS N08031)

31 alloy can be formed like other austenitic stainless steels. This alloy is somewhat stronger than common
stainless steels like 304 and 316, so more force will be required when forming ATI 31 alloy.

Weldability of Alloy 31 (UNS N08031)

31 alloy can be welded using most conventional welding processes, including GTAW (TIG), GMAW (MIG) and plasma welding. A filler metal that has a matching composition to 31 alloy is commercially available. However, the use of a more highly alloyed austenitic filler metal, such as Alloy 59, should be considered if it is critical to achieve high corrosion resistance in the welds. Overmatched fillers are often used in applications such as fabricated scrubber components.

Heat Treatment of Alloy 31 (UNS N08031)

The 31 alloy should be solution annealed between 2100 and 2150°F (1149 and 1177°C) and quickly cooled.

Applications of Alloy 31 (UNS N08031)

Alloy31 is an iron-nickel-chromium-molybdenum alloy with nitrogen addition. The alloy fills the gap between special alloyed austenitic stainless steels and nickel alloys. Alloy 31 has been proven especially in the chemical and petrochemical industry, in ore extractors, environmental and marine engineering as well as in oil and gas production.

• Piping system, water purification system, marine engineering and hydraulic piping perfusion system.
• The pipeline, joint, air systems in the acidic gas production.
• Evaporator, the heat exchanger, filter, mixer, etc. in the phosphate production.
• Power plants condensation and pipe system in the use of sewage water to cool water.
• The use of acidic organic catalyst chlorinated derivatives production.
• The production of cellulose pulp,Polished bar in the orrosive oil wells.
• Hose system in the oceanngineering,Flue gas desulphurization system components.
• Sulfuric acid condensation and separation system,Crystalline salt concentration and the evaporator.
• Transport of corrosive chemicals containers,Reverse osmosis desalination plan.
• Heat exchange tubes
• Pipe fittings
• Flanges
• Valves

Development of UNS N08031 seamless steel tube for chemical heat exchangers

The research and development process of UNS N08031 seamless steel tube for chemical heat exchanger is introduced. The UNS N08031 seamless steel tube is successfully developed by extruding UNS N08031 barren pipe by hot extrusion process and deforming it by multi-stage cold rolling with the specification of Φ50.8 mm × 4 mm. The analysis results show that the grain distribution of UNS N08031 seamless steel tube is uniform, with an average grain size of 6, with excellent mechanical properties and processing performance, and its chemical composition, metallurgical organization, and mechanical properties meet the requirements of ASME SB622-2019 specifications.
The new Hastelloy UNS N08031 is a nitrogen-containing Fe-Ni-Cr-Mo alloy (U.S. brand UNS N08031) developed by ThyssenKrupp VDM in Germany specifically to resist corrosion of halide media, and its performance lies between super austenitic stainless steels and existing nickel-based alloys. The alloy is a face-centered cubic lattice structure with a high degree of alloying, Ni content of 30.0%-32.0%, Cr content of 26.0%-28.0%, Mo content of 6.0%-7.0%, and 0.15%-0.25% of N. The high Ni-Cr composition increases the alloy’s corrosion, oxidation resistance, and volatility. The high Mo design not only refines the grain size but also significantly increases the high temperature strength of the steel. Adding N stabilizes the austenite phase and reduces the tendency of intermetallic precipitation. The design of this chemical composition will UNS N08031 critical pitting temperature increased by 85 ℃, but also consider the high temperature oxidation resistance, corrosion resistance, and high temperature creep resistance properties. High Cr, Mo design, it is easy to make the material in the process of precipitation of a large number of Mo-rich phases, resulting in the alloy in the hot working of deformation and cold working molding process with greater difficulty. Currently used for chlorine dioxide and other highly corrosive media heat exchanger tube is mainly manufactured in Germany, the United States, Japan, and other countries; the domestic research and manufacturing reports on the UNS N08031 seamless steel tube is almost none; therefore, meet the needs of China’s high-end heat exchanger equipment for the localization of the need to vigorously carry out the development of seamless steel tube UNS N08031 alloy.

1. Product technical requirements

Yaang developed UNS N08031 alloy heat exchanger tubes for use in the chemical industry with chlorine dioxide as the medium in 2019 in response to the use requirements of well-known domestic manufacturers. The ASME SB622-2019 specification for UNS N08031 alloy tubes is shown in Table 1-2 for the chemical composition and mechanical property requirements.
Table.1 Chemical composition (mass fraction) requirements for UNS N08031 alloy tube %

C	Si	Mn	P	S	Ni	Cr	Mo
≤0.015	≤0.3	≤2.0	≤0.020	≤0.010	30.0-32.0	26.0-28.0	6.0-7.0

Table.2 UNS N08031 alloy tube mechanical properties requirements

Tensile strength Rp0.2/MPa	Yield strength Rm/MPa	Elongation A/%	Hardness HRB
≥ 650	≥ 276	≥ 40	≤ 95

2. UNS N08031 alloy steelmaking process

UNS N08031 in a Ni content of 30.0%-32.0%, Cr content of 26.0%-28.0%, and Mo content of 6.0%-7.0%; such a high content of alloying elements in steelmaking puts forward high requirements; Mo is a positive segregation of the element, if the smelting process is not appropriate to further affect the alloy’s processing performance and performance. The double-vacuum smelting method of vacuum induction melting (VIM) + vacuum self-consumption arc melting (VAR) was adopted to get the billet with uniform composition, organization, and high purity. The ingot was homogenized and then opened and forged into a round bar billet of Φ225 mm, and the chemical composition of the billet complied with the requirements in Table 1. Because of the billet’s high purity requirements, the inclusions must be controlled within the standard range.

3. UNS N08031 alloy extrusion process

UNS N08031 alloy pipe extrusion process route is mainly: billet processing and cleaning → ring heating furnace preheating → primary induction heating → lubrication → reaming → secondary induction heating → lubrication → extrusion → water → inspection. Extrusion of barren pipe specifications for Φ135 mm × 13 mm, extrusion using glass powder lubricant, including internal lubrication glass powder lubricant for imports of lubricants HNK-2, external lubrication glass powder lubricant 844-7, glass mat glass powder lubricant HDK-5, preheating temperature of 900-980 ° C, the preheating time of 2.5 h. Reaming temperature of 1160-1200 ° C, the reaming and reaming Coefficient of 1.23, reaming speed of 140-220 mm / s, extrusion temperature of 1180-1230 ℃, extrusion speed of 100-200 mm / s, extrusion pressure of 30-36MN, mold preheating temperature range of 300-400 ℃.
The advantage of extrusion pipe making is that the billet is subjected to three-way compressive stress during the extrusion process. This does not necessarily result in cracks, even if the material’s low thermoplasticity. Due to the high Mo content in UNS N08031, the solid solution strengthening effect is significant, thus resulting in a difficult hot deformation session of the billet. As the extrusion deformation usually leads to extrusion of the head of the barren tube, the middle and the end of the grain size differences, if the organization of the variability is too large, it will lead to the subsequent cold rolling deformation of the non-uniformity, and ultimately lead to the pipe rolling cracks. Therefore, it is necessary to reasonably adjust the extrusion process as far as possible to reduce the organization of the extruded pipe differences. Therefore, for the extrusion characteristics of UNS N08031 alloy, the extrusion ratio is designed to 7.68, and the secondary induction furnace temperature to 1180-1230 ℃, large extrusion ratio, high temperature, and short time operation is fully considered after the billet extrusion as far as possible to make the grain uniformity. UNS N08031 alloy extruded tube head, middle, and tail of the organization and morphology as shown in Figure 1. It can be seen that the pipe in the extrusion process has occurred a sufficient dynamic recrystallization and grain growth, presenting a uniform size distribution of equiaxial crystals, pipe head, middle and tail of the grain degree level difference of about 1 level, the organization is uniform, the longitudinal organization of the billet and precipitation phases is shown in Figure 2. It can be seen that there are more precipitation phases in the matrix that are banded along the forging direction. Combined with the equilibrium phase diagram and energy spectrum analysis, it can be seen that it is a Mo-rich σ phase.

Fig.1 Organizational morphology of different parts of UNS N08031 hot extruded tube

4. UNS N08031 alloy cold rolling process

UNS N08031 alloy tube cold rolling process route is mainly as follows: extrusion tube grinding → cold rolling → de-oiled → solution → straightening → flat head → flaw detection → inspection. Because of the heat exchanger pipe specification Φ50.8 mm × 4 mm, the specification of the barren pipe Φ135 mm × 13 mm, designed to be formed after three passes of cold rolling, the average distribution of deformation of each pass, and control the deformation of each pass at less than 50%, cold rolling in the Pilger two-roll rolling mill LG110, LG60 for processing. An intermediate solid solution at 1150-1200°C is required after cold deformation to enable the deformed organization to undergo static recrystallization and grain growth to eliminate work-hardening and facilitate the next pass of cold rolling deformation.

Fig.2 Longitudinal organization and precipitation phases of UNS N08031 billet
Through three passes of cold rolling, the finished pipe of Φ50.8 mm × 4 mm specification was finally rolled successfully. The organization and morphology of the cold rolled state are shown in Fig. 3, which shows that the isometric crystals of UNS N08031 alloy are elongated along the rolling direction after cold rolling. The billet has more σ phases along the rolling direction with a strip-like distribution. The existence of this precipitation phase affects the processing performance of the alloy, which in turn affects the performance of the use of the alloy. Therefore, whether hot or cold working, should try to reduce the precipitated phase σ in the alloy to improve the thermoplasticity and cold workability of the alloy to improve the deformation of a single pass to achieve a higher rate of material and economic benefits.

Fig.3 Microstructure of the alloy after cold rolling

5. Heat treatment of UNS N08031 alloy

UNS N08031 alloy shows excellent corrosion resistance and mechanical properties at high temperatures, mainly due to its high degree of alloying, especially up to 6.0%-7.0% Mo content. UNS N08031 alloy tubes are very susceptible to generating a large number of Mo-rich σ-phase during the manufacturing process; therefore, to ensure that the alloy has good machinability, it is necessary to take a reasonable heat treatment system so that the alloy will be dissolved back to the precipitated phase in the alloy. To ensure that the alloy has good machinability, it is necessary to adopt a reasonable heat treatment system so that the alloy precipitation phase dissolves or reduces the amount of precipitation as much as possible. Therefore, to develop a reasonable solution process for intermediate and finished products, it is necessary to analyze the phase precipitation pattern of UNS N08031 alloy.
The thermodynamic equilibrium calculation of the precipitation phase of UNS N08031 alloy was carried out using JMatPro software, and the equilibrium phase diagram of UNS N08031 alloy is shown in Figure 4. The precipitation phase diagram shows that the σ phase is the main intermetallic precipitation phase of the alloy during solidification, and the precipitation temperature is 450-1120°C. The precipitation phase of the alloy is the Mo-rich σ phase and the content of precipitation increases with the decrease of temperature. The maximum precipitation content is 0.28% at 550-700℃. According to thermodynamic equilibrium calculations, the solid solution temperature needs to be greater than 1120°C to ensure that the σ phase starts to dissolve back.

Figure.4 Equilibrium phase diagram of UNS N08031 alloy
Through the relevant solid solution test, it is found that the microstructure of UNS N08031 alloy under different solid solution temperatures is shown in Fig. 5. 1120 ℃ solid solution, there is still more σ phase in the matrix, 1150 ℃ solid solution, there are still a small portion of the σ phase in the alloy is not dissolved sufficiently. The precipitation phase in the alloy is completely dissolved when the solid solution is performed at 1180 ℃. Finished tube by 1180 ℃ after solid solution, the organization has undergone sufficient static recrystallization and grain growth for uniform isometric crystals, the average grain size of 6, grain boundaries, and the second phase precipitation within the grain is completely soluble.
At the same time, on the finished tube by ASTME8/E8M-2016a “Tensile Test Methods for Metallic Materials” for room temperature tensile, the finished product performance is shown in Table 3. Table 3 shows the performance of the UNS N08031. Each batch is stable, the alloy’s average tensile strength of up to 783.6 MPa, yield strength of up to 379.4 MPa, elongation rate of 54.9%, and the average hardness value of 83.31 HRB; all the properties are in full compliance with the ASME SB622-2019 specification.

Figure.5 Microstructure of UNS N08031 alloy at different solid solution temperatures
Table.3 Properties of UNS N08031 finished products.

Sample Number	Tensile Strength Rp0.2 /MPa	Yield Strength Rm/MPa	Elongation A/%	Hardness HRB
1	778	377	51	84.2
2	788	389	56	83
3	791	389	54	84.5
4	789	391	55	82.5
5	780	371	56	83.2
6	781	382	54	83.5
7	790	368	56	82.8
8	782	381	56	82.9
9	780	384	55	83.1
10	777	362	56	83.4
Average Value	783.6	379.4	54.9	83.31
Norm Value①	≥650	≥276	≥40	≤95

Note: ① ASME SB622-2019 specification.

6. Conclusion

The UNS N08031 alloy melted by VIM+VAR double vacuum method has high purity, accurate control of chemical composition, and meets the compositional requirements; the alloy seamless heat exchanger tube UNS N08031 is developed by hot extrusion + multiple-pass cold rolling process, with an average grain size of grade 6, good mechanical properties and processing performance, and meets the requirements of the relevant standards and specifications such as ASME SB622 -2019 and other related standards and specifications.
Author: Wang Lianhua