A Comprehensive Guide to Nickel-based Super Alloy: Inconel 625 (UNS N06625/W.Nr. 2.4856)

What is inconel 625?

Inconel 625 is a kind of nickel-based super alloy with density of 8.4 g/cm³, melting point of 1290-1350 ℃, excellent resistance to inorganic acid corrosion, and excellent corrosion resistance to various corrosive media in oxidation and reduction environment.
625 alloy shows excellent corrosion resistance in many media. It has excellent resistance to pitting corrosion, crevice corrosion, intergranular corrosion and erosion in chloride medium.

Designated as UNS N06625 or W.Nr. 2.4856, Inconel 625, also known as alloy 625, is a nickel-chromium-molybdenum alloy with an addition of niobium that act with the molybdenum to stiffen the alloy’s matrix and thereby provide high strength without a strengthening heat treatment. The alloy resists a wide range of severely corrosive environments and is especially resistant to pitting and crevice corrosion and it has excellent fatigue strength and stress-corrosion cracking resistance to chloride ions. Inconel 625 can be used in chemical processing, aerospace and marine piping, pollution control equipment, and nuclear reactors. Usually it can be used at the temperature up to 980°C[1800°F] in oxidation service.

Inconel 625 seamless pipe, ASTM B444 UNS N06625, 19mmx3.5mmx5620mm.

Chemical Composition

Element	Content, %
Nickel	≥58.0
Chromium	20.0-23.0
Iron	≤5.0
Molybdenum	8.0-10.0
Niobium(plus Tantalum)	3.15-4.15
Carbon	≤0.10
Manganese	≤0.50
Silicon	≤0.50
Phosphorus	≤0.015
Sulfur	≤0.015
Aluminum	≤0.40
Titanium	≤0.40
Cobalt	≤1.0

Physical Properties

Density		Melting Range		Specific Heat	Permeability at 200 Oersted	Curie Temperature
lb/cu in	gram/cc	°F	°C	Btu/lb°F (J/kg°C) 70°F(21°C)	15.9 kA/m	°F	°C
0.305	8.44	2350-2460	1290-1350	0.098(410)	1.0006	<-320	-196

Product Forms and Related Standards of Inconel 625

Products Form	Standard
Rod and bar	ASTM B446
Plate, sheet and strip	ASTM B443, B906
Seamless pipe and tube	ASTM B444, B829
Welded pipe	ASTM B705, B775
Welded tube	ASTM B704. B751
Pipe fitting	ASTM B366
Billet and bar for reforging	ASTM B472
Forging	ASTM B564

Characteristic

• 1. It has excellent corrosion resistance to various corrosive media in oxidation and reduction environment.
• 2. Excellent resistance to pitting corrosion and crevice corrosion, and no stress corrosion cracking due to chloride.
• 3. Excellent corrosion resistance to inorganic acids, such as nitric acid, phosphoric acid, sulfuric acid, hydrochloric acid and mixed acid of sulfuric acid and hydrochloric acid.
• 4. Excellent corrosion resistance of various inorganic acid mixed solution.
• 5. When the temperature is up to 40 ℃, it has good corrosion resistance in various concentrations of hydrochloric acid solution.
• 6. Good processability and weldability, no post weld cracking sensitivity.
• 7. Manufacture certification of pressure vessel with wall temperature between – 196 ℃ and 450 ℃.
• 8. Certified by NACE standard (mr-01-75) of American Society of corrosion engineers, meeting the highest standard grade VII for sour gas service.

Metallographic structure of Inconel 625

625 is a face centered cubic lattice structure. After holding at 650 ℃ for a long time, carbon particles and unstable quaternary phase will be precipitated and transformed into stable Ni3 (Nb, Ti) rhombic lattice phase. After solution strengthening, Mo and Nb in Ni Cr matrix will improve the mechanical properties, but the plasticity will be reduced.

Corrosion resistance of Inconel 625

625 alloy shows excellent corrosion resistance in many media. It has excellent resistance to pitting corrosion, crevice corrosion, intergranular corrosion and erosion in chloride medium. It has good resistance to inorganic acid corrosion, such as nitric acid, phosphoric acid, sulfuric acid, hydrochloric acid and so on. It also has the corrosion resistance of alkali and organic acid in oxidation and reduction environment. Effective resistance to chloride reduction stress corrosion cracking. It has high corrosion resistance to seawater and salt solution, as well as high temperature. There is no sensitivity during welding. It is resistant to carbonization and oxidation in static or cyclic environment, and resistant to gas corrosion containing chlorine.

Medium concentration (%)	15	50	60	70	80
Corrosion rate（mm/a）	0.188	0.432	0.711	1.626	2.286

 

Hydrochloric acid medium: (temperature is 66 ℃)

The application scope of Inconel 625:

The low carbon alloy 625 after softening and annealing is widely used in chemical process industry. Its good corrosion resistance and high strength make it a thin structural component. 625 alloy can be used in the case of contact with sea water and high mechanical stress.

Typical applications:

• 1. Components of organic chemical processes containing chlorides, especially where acid chloride catalysts are used.
• 2. Digesters and bleachers for pulp and paper industry.
• 3. Absorber, reheater, flue gas inlet baffle, fan (wet), agitator, guide plate and flue in flue gas desulfurization system.
• 4. It is used for manufacturing equipment and components used in sour gas environment.
• 5. Acetic acid and acetic anhydride reaction generator.
• 6. Sulfuric acid condenser.

Smelting of Inconel 625

Inconel625 without or less aluminum and titanium is usually smelted by electric arc furnace or non vacuum induction furnace. If Inconel625 with high content of aluminum and titanium is smelted in the atmosphere, the element burning loss is not easy to control and the gas and inclusions enter more, so vacuum smelting should be adopted. In order to further reduce the content of inclusions, improve the distribution of inclusions and the crystal structure of ingots, the duplex process of smelting and secondary remelting can be adopted. The main means of smelting are electric arc furnace, vacuum induction furnace and non vacuum induction furnace; the main means of remelting are vacuum consumable furnace and electroslag furnace.
Solid solution strengthened alloy and alloy ingot with low aluminum and titanium content (the total amount of aluminum and titanium is less than 4.5%) can be opened by forging; the alloy with high aluminum and titanium content generally needs extrusion or rolling, and then hot rolled, and some products need further cold rolling or cold drawing. The alloy ingot or cake with larger diameter shall be forged by hydraulic press or fast forging hydraulic press.
Precision casting is widely used for the alloy with high alloying degree and low deformation, such as turbine blade and guide blade. In order to reduce or eliminate grain boundaries perpendicular to the stress axis and to reduce or eliminate porosity in cast alloys, directional crystallization technology has been developed in recent years. In this process, the grains grow along one crystal direction to obtain parallel columnar grains without transverse grain boundaries. The primary technological conditions for directional crystallization are to establish and maintain a large axial temperature gradient and good axial heat dissipation conditions between liquidus and solidus. In addition, in order to eliminate all grain boundaries, it is necessary to study the manufacturing process of single crystal blades.
The powder metallurgy process is mainly used to produce the precipitation strengthened and oxide dispersion strengthened Inconel 625. This process can make Inconel 625, which can not be deformed, obtain plasticity or even superplasticity.
The properties of Inconel625 are closely related to the microstructure of the alloy, and the microstructure is controlled by the metal heat treatment. Inconel625 generally needs heat treatment. Precipitation strengthened alloys are usually solution treated and aged. The solution strengthening alloy is only treated by solution treatment. Some alloys have to go through one or two intermediate treatments before aging treatment. The purpose of solution treatment is to dissolve the second phase into the alloy matrix, so that the strengthening phases such as γ and carbide (Co based alloy) can be uniformly precipitated during aging treatment, and the second is to obtain appropriate grain size to ensure high temperature creep and creep rupture properties.
The solution treatment temperature is generally 1040 – 1220 ℃. The widely used alloys are mostly treated at 1050 – 1100 ℃ before aging treatment. The main function of intermediate treatment is to precipitate carbides and γ films at grain boundaries to improve the state of grain boundaries. At the same time, some alloys also precipitate some larger γ phase to form a reasonable combination with the fine γ phase precipitated during aging treatment. The purpose of aging treatment is to make the supersaturated solid solution precipitate γ phase or carbide (cobalt based alloy) uniformly to improve the high temperature strength. The aging temperature is generally 700-1000 ℃.

Thermal Expansion Coefficients for Alloy 625 at Elevated Temperatures

The thermal expansion coefficients for Inconel alloy 625 (UNS N06625) includes:

• Coefficient A – the instantaneous coefficient of thermal expansion x 10^-6(inch/inch/°F);

• Coefficient B – the mean coefficient of thermal expansion x 10^-6(inch/inch/°F) in going from 70°F to indicated temperature; Coefficient C – the linear thermal expansion (inch/100 ft) in going from 70°F to indicated temperature. Their values at elevated temperatures are listed in below three tables.

Thermal Expansion Coefficients for Alloy 625 (70°F – 500°F)

Temp.	Coefficient A	Coefficient B	Coefficient C
70	6.7	6.7	0
100	6.9	6.8	0.2
150	7.2	7.0	0.7
200	7.4	7.1	1.1
250	7.4	7.2	1.6
300	7.5	7.2	2.0
350	7.5	7.3	2.4
400	7.5	7.3	2.9
450	7.5	7.3	3.3
500	7.5	7.4	3.8

*Temp.: the indicated temperature, °F.

Thermal Expansion Coefficients for Alloy 625 (550°F – 1000°F)

Temp.	Coefficient A	Coefficient B	Coefficient C
550	7.6	7.4	4.2
600	7.7	7.4	4.7
650	7.8	7.4	5.2
700	8.0	7.5	5.6
750	8.2	7.5	6.1
800	8.4	7.6	6.6
850	8.7	7.6	7.1
900	8.9	7.7	7.7
950	9.1	7.8	8.2
1000	9.4	7.9	8.8

*Temp.: the indicated temperature, °F.

Thermal Expansion Coefficients for Alloy 625 (1050°F – 1500°F)

Temp.	Coefficient A	Coefficient B	Coefficient C
1050	9.6	7.9	9.3
1100	9.8	8.0	9.9
1150	9.9	8.1	10.5
1200	10.1	8.2	11.1
1250	10.2	8.3	11.7
1300	10.4	8.4	12.3
1350	10.5	8.4	13.0
1400	10.7	8.5	13.6
1450	10.9	8.6	14.2
1500	11.3	8.7	14.9

*Temp.: the indicated temperature, °F.

Thermal Conductivity and Thermal Diffusivity for Alloy 625

TC is the thermal conductivity with an unit of Btu/hr-ft-°F, and TD is the thermal diffusivity with and unit of ft²/hr. The nominal coefficients of TC and TD for nickel alloy 625 (UNS N06625) at elevated temperatures are listed in below two tables. Note that ±10% uncertainty may exist due to compositional variations and analytic data deviation.

 Coefficients of TC & TD for Alloy 625 @ (70°F-750°F)

Temp.	TC	TD
70	5.7	0.110
100	5.8	0.113
150	6.0	0.116
200	6.3	0.119
250	6.5	0.121
300	6.7	0.124
350	7.0	0.126
400	7.2	0.128
450	7.5	0.130
500	7.7	0.132
550	7.9	0.134
600	8.2	0.136
650	8.4	0.138
700	8.7	0.140
750	8.9	0.142

*Temp.: indicated temperature, °F.

 Coefficients of TC & TD for Alloy 625 @ (800°F-1550°F)

Temp.	TC	TD
800	9.1	0.144
850	9.4	0.146
900	9.6	0.148
950	9.8	0.150
1000	10.1	0.152
1050	10.3	0.154
1100	10.5	0.156
1150	10.8	0.158
1200	11.0	0.159
1250	11.3	0.161
1300	11.5	0.163
1350	11.8	0.165
1400	12.0	0.167
1450	12.3	0.169
1500	12.6	0.172
1550	–	0.175

*Temp.: indicated temperature, °F

Welding process and analysis of Inconel 625

Weldability analysis of nickel alloy

Nickel alloy has unidirectional structure, and macro cracks and micro cracks will be produced during welding. Therefore, the most common defect in Inconel 625 welding is hot crack, which is mainly caused by the mixing of sulfur, lead, phosphorus or low melting point metal, resulting in serious embrittlement at high temperature. Especially, the melting point of sulfur and phosphorus eutectic is much lower than that of ferronickel, and the liquid film of eutectic with low melting point remains in the grain boundary area when the weld is crystallized. In addition, the large heat input makes the weld joint overheat and produces coarse grains. Some eutectic crystals with low melting point are concentrated on the boundary of coarse columnar grains, which are of low strength and brittleness, and easy to form hot cracks under the action of welding stress.
Another defect that needs to be avoided in welding is blowhole. The molten pool of nickel alloy is thick and the fluidity is poor. When the welding is rapidly cooled, it is easy to produce blowholes. The solubility of oxygen, hydrogen, nitrogen, carbon dioxide and carbon monoxide in the molten liquid nickel base alloy is very high, but the solubility is greatly reduced in the solid state. When the nickel base alloy welding process changes from high temperature to cold, the gas in the deposited metal will change The solubility in the solution also decreases. The free gas in liquid nickel with poor fluidity can not overflow completely and form pores before the weld solidifies. Moreover, poor fluidity is easy to produce slag inclusion defects.
Crystallization crack is a kind of welding hot crack, which generally occurs in the temperature range of solid-liquid phase line, that is, the later stage of weld crystallization, so it is named after it. The sensitivity of Inconel625 welding to crystal crack is very high, mainly because the trace elements sulfur, phosphorus and carbon will gather to the grain boundary in the process of crystallization. Carbon promotes the segregation of sulfur and phosphorus. However, there is a large amount of nickel in the weld, which is easy to form nickel sulfide at the grain boundary position. The melting point of the mixed product of nickel and nickel sulfide is only about 630 ℃, which belongs to low melting eutectic and weakens the grain Because of the interaction between them, the weld will crack along the grain boundary under the action of tensile stress.
Various measures can be taken to eliminate the crystal cracks in Inconel625 welding: firstly, cleaning before welding is very important, grinding with grinding wheel or wiping with acetone to remove oxide skin, oil and other pollutants on the surface, so as to avoid sulfur, phosphorus and other impurities from mixing into the molten pool; secondly, formulate reasonable welding process parameters, and adopt small welding current and welding heat input; finally, the anti cracking performance can be better Good welding material.

Cold crack

Compared with crystal crack, Inconel625 has little tendency to weld cold crack, which is attributed to the fact that it does not induce cold crack during or after welding: hydrogen content in Inconel625 is very rare, and strengthening gas protection can effectively reduce hydrogen diffusion in deposited metal; Inconel625 has less carbon content and less hardening tendency, which successfully inhibits martensitic transformation after welding and avoids brittleness Moreover, Inconel625 has a certain plasticity, and when it is cooled to room temperature or lower temperature after welding, its plasticity can resist the shrinkage stress caused by the decrease of temperature and avoid the generation of cold crack.

Stoma

Nickel based alloy Inconel625 is very sensitive to the pores formed by hydrogen and oxygen. The solubility of oxygen in nickel decreases with the decrease of temperature. When the temperature is reduced from 1720 ℃ to 1470 ℃, the solubility of oxygen can be reduced from 1.18% to 0.06%. The oxygen precipitated rapidly combines with nickel to form nickel oxide. However, near the fusion zone, hydrogen in liquid nickel can reduce nickel in nickel oxide and form pores.
The reduction of porosity needs to be realized by strict pre welding cleaning and gas protection.

Welding method

Considering the characteristics of field construction, the method of TIG backing and electrode arc welding filling cover is adopted. During argon arc welding, the back must be filled with argon to prevent the alloy elements from being oxidized.
3. Selection of welding materials
The selection of welding materials is based on the chemical composition, mechanical properties, service conditions and welding conditions. For nickel alloy welding, the same welding materials as the base metal alloy system should be selected.
Considering the instability of welding materials quality of some domestic manufacturers, it is recommended to use imported welding materials or welding materials of Shanghai electric power and Sichuan Atlantic brands.

Welding material drying

Before use, the welding rod should be dried according to the product manual. The dried welding rod shall be stored in a constant temperature box of about 100-150 ℃. When the welder receives the welding rod, it shall be picked up with qualified heat preservation cylinder. If the receiving time is more than 4 h, it shall be re baked, but the re baking times shall not exceed 2 times. Baking temperature of welding rod.

Construction environment

During the construction of high-temperature pipeline, when the welding environment has the following conditions, effective protection measures (such as tarpaulin, heating, etc.) must be adopted before welding, otherwise welding is prohibited.

• (1) The ambient temperature is lower than 0 ℃;
• (2) For manual welding, the wind speed is greater than 8m / S; for tungsten argon arc welding, the wind speed is greater than 2m / s;
• (3) The relative humidity of air is more than 90%;
• (4) Rain and snow weather.

Construction preparation

Welder requirements

According to the requirements of relevant regulations, welders welding nickel base materials shall be tested according to the contents, methods and results required in code for construction and acceptance of industrial pipeline welding engineering of field equipment (GB 50236-1998), and the organization, supervision, certification and management of welders with certificates shall be conducted according to the examination and management rules for welders of boiler and pressure vessel and pressure pipeline.
The welder welding nickel base materials must be tested according to the qualified welding process. Only after passing the examination and obtaining the corresponding qualification certificate can they undertake the corresponding welding engineering construction.

Welding process card

When welding on site, the welding process card must be formulated according to the qualified welding procedure qualification to guide the welding construction.

Welding seam assembly

(1) Form and size of butt weld groove.
(2) Alignment misalignment requirements
When the pipeline or weldment is assembled, the inner wall shall be flush, the inner wall misalignment shall not exceed 0.5mm, and the outer wall misalignment shall not exceed 1.0mm.
(3) Location welding size
The location welding of welded junction adopts the form of root positioning weld. The length, thickness and spacing of the welding seam of the positioning welding shall be able to ensure that the weld does not crack in the welding process.
If TIG welding with solid core wire is used for positioning welding, the back side of the weld shall be protected with nitrogen; the length of the weld should be 10-15mm, and not more than 2 / 3 of the wall thickness.

Welding process

Cleaning before welding

Lead, sulfur, phosphorus and some low melting point elements can increase the tendency of nickel alloy welding crack. Therefore, these impurities must be completely removed before welding. Before construction, the oil, paint, rust, scale, burr and other foreign matters within the range of 50 mm on both sides of the groove surface shall be removed with angle grinder, and there shall be no cracks, interlayer and other defects.

Welding process requirements

• (1) Welding penetration and fusion must be ensured to ensure weld quality. In order to reduce hot cracks, on the premise of ensuring full penetration, small linear energy, short arc and no swing or small swing operation method shall be adopted as far as possible.
• (2) In multi-layer welding, low current multi-layer multi pass welding is adopted. The inter layer temperature should be controlled below 100 ℃, the inter layer inspection should be careful and the cleaning should be thorough, and each layer should be staggered with each other.
• (3) During argon arc welding, the back must be filled with argon for protection, and local argon filling can be adopted for protection measures (water solubility is pasted on both sides of the groove). At the beginning of argon filling, the flow rate should be increased properly, and welding can be carried out after the air in the pipe is exhausted. During welding, the argon flow rate is gradually reduced to avoid the phenomenon that the argon flow rate is too large and the pressure in the pipe is too high, which may cause the back of the weld to be concave or the root is not fully penetrated.
• (4) During welding, the temperature between layers must be strictly controlled. Infrared thermometer is used for measurement. When the temperature is lower than 100 ℃, the next layer can be welded.
• (5) Each layer of weld bead must be cleaned before the next layer can be welded; after welding, the slag and spatter on the weld surface shall be cleaned in time. When the corner grinder is used to clean up the defects during welding, the hot cracks caused by excessive local heat must be prevented.
• (6) Due to the small requirement of linear energy, the welding flesh is thin after the TIG welding is used. In order to ensure the welding quality, penetrant inspection shall be carried out to see whether there are cracks. After the welding is qualified, the TIG welding shall be continued. After three layers of TIG welding are used, the penetration inspection shall be carried out. After the welding is qualified, the welding seam surface can be cleaned For foreign matters, penetrant inspection shall be conducted.
• (7) During welding, the quality of arc striking and arc stopping shall be ensured, and the arc pit shall be filled during arc extinguishing. When nickel alloy is welded, crater cracks are easy to occur at the arc stopping position. After arc extinguishing, the arc crater shall be carefully inspected, and the micro cracks shall be removed by grinding.

Quality inspection of welded joint

• (1) The appearance quality requirements shall meet the relevant requirements of GB 50236-1998 “code for construction and acceptance of industrial pipeline welding engineering for field equipment” and SH / T 3523-1999 “welding specification for chromium nickel austenitic steel, iron nickel alloy and nickel alloy pipeline in petrochemical industry”.
• (2) 100% radiographic inspection for all butt welds.
• (3) After the backing welding, TIG welding and all welding of Inconel625, 100% penetrant testing must be carried out, and the next process can be carried out after all qualified.
• (4) Non destructive testing standard: JB / T4730-2005 nondestructive testing of pressure equipment.

Stress relief heat treatment

After one layer of surfacing welding, stress relief heat treatment is needed. The whole heating is 620 ℃ and the furnace is kept for 2 hours before air cooling. 80% – 90% of welding residual stress can be eliminated by this method, and no heat treatment is needed after all welding.

Inconel 625 Weld Overlay on Carbon/ Alloy Steel Flanges

In mordern oil & gas industry, the crude oil or natural gas are usually extracted from deep layers below the ground. This requies large volume of various piping materials including valves, pumps, flanges, pipes, and pipe fittings. The fluid contains rich content of hydrogen sulfide (H₂S), carbon dioxide (CO₂), and chloride ion (Cl^–) which are extremely corrosive and may attack the contact metal surface easily. Pitting corrosion and crevice corrosion are commonly seen under the circumstances.

Pipe flanges, as the main connection components in crude oil or natural gas piping system, are generally furnished in carbon steel or alloy steel materials with corrosion-resistance alloy (CRA) weld overlay on its surface in contact with the fluid. The Inconel Alloy 625 is a prevalent corrosion-resistance alloy that can be used in the weld overlay process. By depositing a thick layer of Inconel 625 (UNS N06625) alloy onto the carbon steel or alloy steel substrate, we impart outstanding resistance to crevice & pittings corrosion, stress corrosion cracking, and oxidation, especially in severe service, while maintaining a relatively low cost. The weld overlay area may cover flange-bore internal surface, raised face, and RTJ face which are in contact with the fluid.

The CRA Inconel 625 weld overlay on carbon/ alloy steel flanges can be done by submerged arc welding (SAW), shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW/TIG), plasma arc welding, laser welding, and friction welding. Due to the relatively high dilution rate, corrosion resistance of the deposition may decrease in SAW. The plasma arc welding, laser welding and friction welding require more complex and costly welding equipment. The GTAW/ TIG is the most widely used optimum technique in Inconel 625 weld overly. It provides relatively low cost, stable arc during welding, low-dilution and excellent-quality welding deposition. The filler metal of GTAW shall be ERNiCrMo-3 in accordance with AWS A5.14 or ASME II SFA-5.14. They are furnished in the product form of bare welding rod or welding wire in either straight length, coiled, or spooled style.

A weld neck flange, RTJ 600LB SCH80, ASME B16.5; Inconel 625 weld overlay on the sealing RTJ face and internal bore surface.

Typical sizes of Inconel 625 (ERNiCrMo-3) welding wires available include: Φ0.8mm, Φ0.9mm, Φ1.0mm, Φ1.14mm, Φ1.2mm, Φ1.6mm, Φ2.4mm, Φ3.2mm. Minimum order quantity (MOQ): 1 kg.

Yaang’s inconel 625 products:

Inconel alloy 625 Plates, Sheets & Strips

Generally, Inconel alloy 625 plate, sheet and strip shall be manufactured in accordance with ASTM B443/ ASME SB-443. Designated as UNS N06625, the material has a nominal chemical composition of 60Ni-22Cr-9Mo-3.5Cb, which indicates that it is a nickel-chromium-molybdenum-columbium alloy. The Inconel 625 alloy plate, sheet and strip can be furnished in two grades of different heat-treated conditions: Grade 1 (annealed) – Material is normally employed in service temperatures up to 1100°F (593°C); Grade 2 (solution annealed) – Material is normally employed in service temperatures above 1100°F (593°C) when resistance to creep and rupture is required.

Conventionally, the Inconel alloy 625 plate, sheet, and strip can be defined or clarified roughly based on its thickness: plates – with thickness ≥ 0.188″ (4.8 mm), hot-rolled or cold-rolled, especially that cold-rolled plates are available in thickness from 0.188″ (4.8 mm) to 3/8″ (9.5 mm); sheets – with thickness from 0.018″(0.46 mm) to 0.188″ (4.8 mm), available in both hot-rolled and cold rolled conditions; strip – with thickness from 0.005″ (0.13 mm) to 0.018″ (0.46 mm), available only in cold-rolled condition. The alloy 625 plates are generally furnished with sheared or cut edges while sheets and strips are usually furnished with sheared or slit edges. The cutting methods include machined, abrasive cut, powder cut, inert arc cut, plasma-torch cut, etc.

Inconel Alloy 625 Flanges

Inconel alloy 625 flanges manufactured to ASME B16.5 fall into the Group 3.8 materials of the standard specification. They can be supplied in 7 classes: 150, 300, 400, 600, 900, 1500, 2500. Flanges with hub, such as weld neck, slip on, threaded, socket weld, and LWN, are made in accordance with ASTM B564 Gr.N06625; flanges without hub, such as blind or flat, can be either made from ASTM B564 Gr.N06625 forgings or ASTM B443 Gr.N06625 plates.

The Inconel alloy 625 flange shall always be furnished in annealed condition. Since alloy 625 in the annealed condition is subject to severe loss of impact strength at room temperatures after exposure in the range of 538°C to 760°C， it shall not be used over 645°C. The typical pressure-temperature ratings for Inconel alloy 625 pipe flanges are listed in below tables.

Pressure-Temperature Rating for Alloy 625 flanges Class 150 & 300

Temp.	Working Pressures by Classes
	Class 150	Class 300
-29 to 38	20.0	51.7
50	19.5	51.7
100	17.7	51.5
150	15.8	50.3
200	13.8	48.3
250	12.1	46.3
300	10.2	42.9
325	9.3	41.4
350	8.4	40.3
375	7.4	38.9
400	6.5	36.5
425	5.5	35.2
450	4.6	33.7
475	3.7	31.7
500	2.8	28.2
538	1.4	25.2
550	. . .	25.0
575	. . .	24.0
600	. . .	21.6
625	. . .	18.3
650	. . .	14.1
675	. . .	11.5
700	. . .	8.8

*Temp.: °C; Working pressure: bar.

Pressure-Temperature Rating for Alloy 625 flanges Class 600 & 900

Temp.	Working Pressures by Classes
	Class 600	Class 900
-29 to 38	103.4	155.1
50	103.4	155.1
100	103.0	154.6
150	100.3	150.6
200	96.7	145.0
250	92.7	139.0
300	85.7	128.6
325	82.6	124.0
350	80.4	120.7
375	77.6	116.5
400	73.3	109.8
425	70.0	105.1
450	67.7	101.4
475	63.4	95.1
500	56.5	84.7
538	50.0	75.2
550	49.8	74.8
575	47.9	71.8
600	42.9	64.2
625	36.6	54.9
650	28.1	42.2
675	23.0	34.6
700	17.5	26.3

*Temp.:℃; Working pressure: bar.

Pressure-Temperature Rating for Alloy 625 flanges Class 1500 & 2500

Temp.	Working Pressures by Classes
	Class 1500	Class 2500
-29 to 38	258.6	430.9
50	258.6	430.9
100	257.6	429.4
150	250.8	418.2
200	241.7	402.8
250	231.8	386.2
300	214.4	357.1
325	206.6	344.3
350	201.1	335.3
375	194.1	323.2
400	183.1	304.9
425	175.1	291.6
450	169.0	281.8
475	158.2	263.9
500	140.9	235.0
538	125.5	208.9
550	124.9	208.0
575	119.7	199.5
600	107.0	178.5
625	91.2	152.0
650	70.4	117.3
675	57.6	96.0
700	43.8	73.0

*Temp.℃; Working pressure: bar.

ASTM B444 UNS N06625 (Alloy 625) Tubes

The alloy 625 tubes, which are widely used for superheater or heat exchanger tubing, are generally manufactured as per ASTM B444 (ASME SB-444) UNS N06625. It is a typical nickel-chromium-molybdenum-columbium alloy, which is commercially known as a proprietary brand of “Inconel alloy 625”. The cold-worked alloy 625 seamless tubes may be furnished in two grades of different heat-treated conditions:

• Grade 1 (annealed) – tubes are normally employed in service temperatures up to 1100°F (593°C);
• Grade 2 (solution annealed) – material is normally employed in service temperatures above 1100°F (593°C) when resistance to creep and rupture is required.

Generally, unless otherwise specified, Grade 1 will be supplied.

The cold-drawn alloy 625 tubes can be furnished with ground bright surfaces or pickled matte surfaces depending on the requirements of clients. Especially, small-diameter and light-wall tubes of UNS N06625 are widely used for heat exchanger or instrumentation tubing. The “small-diameter” refers to outside diameters 1-1/4″ (31.8 mm) and under, while “light-wall” refers to a tube with specified wall thickness 3% or less of the specified outside diameter.

Tests and Inspection

• Hydrostatic Test: each alloy 625 tube (seamless & cold-worked) with an outside diameter of 3 mm and larger and with wall thickness of 0.38 mm and larger shall be tested in accordance with ASTM B829. The allowable fiber stresses for UNS N06625 Grade 1 and Grade 2 are 30 ksi [207 MPa] and 25 ksi [172 Mpa], respectively.
• Dimensional inspection and visual examination: outside diameter, wall thickness, inside diameter, length, straightness, plain ends, finish & surface appearance.
• Tension testing (ASTM E8) and hardness test (ASTM E18).
• One or more non-destructive tests (NDT) may be specified: PMI, ultrasonic examination (ASTM E213), eddy current examination (ASTM E426/ E571).

Weight of Alloy 625 Tubes

The weight of an alloy 625 tube can be calculated using the equation: W = ( D – t ) * t * 0.0246615 * F * L. W: weight of the tube, kg; D: outside diameter of the tube, mm; t: wall thickness of the tube, mm; F: correction factor, 1.075; L: length of the tube, meter.

Maximum Allowable Stress, Tensile Strength, Yield Strength @ Different Temperatures

Alloy N06625 tubes in the annealed condition are subject to severe loss of impact strength at room temperatures after exposure in the range of 1000°F to 1400°F. When used for applications designed to ASME Boiler and Pressure Vessel Codes, the alloy 625 tubes, made to ASTM B444 UNS N06625, shall be furnished in solution annealed condition. They can be applied to temperatures up to 1600°F. The values of maximum allowable stress, tensile strength and yield strength at different elevated temperatures are listed in below tables.

*Temp.	*MAS	*Temp.	*MAS
-20-100	26.7	1000	19.6
200	24.6	1050	19.5
300	23.4	1100	19.4
400	22.4	1150	19.3
500	21.7	1200	19.3
600	21.0	1250	15.0
650	20.8	1300	11.6
700	20.5	1350	8.5
750	20.3	1400	6.7
800	20.1	1450	4.9
850	20.0	1500	3.8
900	19.8	1550	2.6
950	19.7	1600	1.9

• Temp.: metal temperature, °F, not exceeding;
• MAS: maximum allowable stress for ASTM B444 UNS N06625 Grade 2 tubes, ksi.

*Temp.	*T.S	*Temp.	*T.S
-20-100	100.0	700	97.3
200	100.0	750	97.1
300	100.0	800	96.9
400	100.0	850	96.7
500	99.7	900	96.2
600	98.1	950	95.5
650	97.6	1000	94.5

• Temp.: metal temperature, °F, not exceeding;
• T.S: tensile strength for ASTM B444 UNS N06625 Grade 2 tubes, ksi.

*Temp.	*Y.S	*Temp.	*Y.S
-20-100	40.0	600	31.5
150	38.0	650	31.2
200	36.9	700	30.8
250	36.0	750	30.5
300	35.1	800	30.2
350	34.3	850	30.0
400	33.7	900	29.7
450	33.0	950	29.6
500	32.5	1000	29.4
550	32.0	–	–

• Temp.: metal temperature, °F, not exceeding;
• Y.S: yield strength for ASTM B444 UNS N06625 Grade 2 tubes, ksi.

Inconel 625 90° & 45° 5D Bends

The Alloy 625 (UNS N06625) 5D pipe bends 90° & 45° SCH 80 manufactured to ASME B16.9. These are supplied to a chemical project in Poland.

Inconel 625 Corrugated Bellows

Inconel 625 corrugated bellows for the manufacture of a bellows expansion joint. This bellows expansion joint will be used for a catalytic cracking unit in an Indonesian oil refinery. It is designed to absorb and compensate the displacement of pipeline at the flue gas turbine.

Inconel 625 corrugated bellows, made of ASTM B443 Gr. UNS N06625 plates. Working temperature: 650°C, flow media: flue gas & catalysts.

The heat treatment process of Inconel 625 alloy

Inconel 625 alloy is a solid solution strengthening corrosion resistant deformation superalloy, widely used in petrochemical, shipbuilding, and nuclear power industries. The alloy has a high content of Cr, Mo, and Nb, and the solid solution strengthening effect is strong, so it is difficult to form by hot working. The initial microstructure is important for thermal processing, mainly controlled by heat treatment. Therefore, it is of great significance to study the heat treatment process of the alloy to study its hot working performance.

1. Test materials and methods

The experimental alloys were prepared by vacuum induction melting (VIM), and the chemical composition is shown in Table 1. 
Table.1 Chemical composition (mass fraction) (%) of Inconel 625 alloy)

Ni	Cr	Mo	Nb	Fe	C	Si	Al	Ti	Mn	S
61	21.5	9	3.6	2	0.05	0.2	0.2	0.2	0.2	0.001

The ingot is processed into profiles by homogenization, billet forging, rolling, etc. Samples were taken from this profile, and a series of heat treatment process tests were carried out. The specific process is shown in Table 2.
Table.2 Heat treatment process of Inconel 625 alloy

Test number	Solid solution temperature/℃	Heat treatment time/min	Cooling method
1	950	60	Air cooling
2	950	60	Water quenching
3	975	60	Air cooling
4	975	60	Water quenching
5	990	60	Air cooling
6	990	90	Air cooling
7	990	120	Air cooling
8	1000	60	Air cooling
9	1000	60	Water quenching
10	1030	60	Air cooling
11	1030	60	Water quenching

2. Test results

(1) Grain size because most of the sample grains are not uniform, the grain size is only evaluated for recrystallized small grains, as shown in Table 3, which can also reflect whether the degree of recrystallization is sufficient.

Table.3 Grain size of Inconel 625 alloy after different heat treatment processes

Sample number	1	2	3	4	5	6	7	8	9	10	11
Grain size (grade)	8	9	7.5	8	7	6.5	6	6.5	7	6	7

The thermal processing of Inconel 625 is generally carried out above 950 °C, which has reached the recrystallization temperature. The degree of recrystallization relates to temperature, holding time, and cooling method. It can be seen from Table 3 that when the heat treatment time and cooling method are the same, the higher the temperature, the larger the grain size after recrystallization; when the temperature and cooling method is the same, the longer the heat treatment time, the coarser the recrystallized grains; when the temperature and heat treatment time are the same, the grains of water quenching are slightly finer than those of air cooling because the recrystallized grains of air cooling samples have sufficient time to grow.
The metallographic structure of Inconel 625 under different heat treatment processes is shown in Fig.1-Fig.3.
After 950 °C × 60 min treatment, there are both recrystallized grains and original grains in the small deformation zone and large deformation zone of Inconel 625 alloy, as shown in Fig.1. However, the original grain size in the small deformation zone is larger and more than that in the large deformation zone due to insufficient recrystallization. The grain size distribution in the large deformation zone is more uniform, and almost no particularly large grains are observed in the large deformation zone of the water-quenched sample. And the grain size is slightly smaller than that of the air-cooled sample. At this temperature, the recrystallization is incomplete.

Fig.1 Microstructure of Inconel 625 alloy after 950 °C × 60 min heat treatment
Compared with the 950 °C heat treatment, the grains grow slightly at 975 °C, and the recrystallization in the large deformation zone is still incomplete. At 990 °C, the recrystallization range is further increased, and the grain size is uniform. At 990 °C, the grains grow slightly with the extension of time.
When the temperature rises to 1000 °C, the microstructure of the recrystallization zone is more uniform than that of the heat treatment below 1000 °C, but there are also more twins, as shown in Fig.2.

Fig.2 Microstructure of Inconel 625 alloy after 1000 °C × 60 min heat treatment
After heat treatment at 1030 °C, the grains of the samples grow further, and some larger grains appear, as shown in Figure 3. Because the temperature is higher, the grain is significantly larger than the lower temperature, indicating that the grain growth is serious at this temperature. The large grains are irregular blocks, while the small grains are equiaxed. Twins are often found between large grains in the water-quenched microstructure. These belong to annealing twins, which are generally formed at the triple junction of grains and surrounded by recrystallized grains that continue to grow.

Fig.3 Metallographic photographs of Inconel 625 alloy after deformation and heat treatment at 1030 °C

3. Results analysis

In these groups of heat treatment samples, from the area ratio of the recrystallization zone and the non-recrystallization zone, the area of the non-recrystallization zone gradually decreases with the increase of temperature because the increase of temperature enhances the driving force of recrystallization, and the recrystallization process is more and more sufficient. The recrystallization is not sufficient below 975 °C. When the temperature reaches above 990 °C, the recrystallization is completed, the grains after recrystallization are more uniform, and the grain size increases by 0.5 grade for each 30 min extension of the treatment time. As the temperature increases, the grains gradually increase. The water-quenched samples are slightly smaller at different treatment temperatures than the air-cooled grains. A large number of annealing twins are formed at 1000 °C.
Recrystallization is a process of eli minating deformation and internal stress by for ming new fine equiaxed grains. The driving force of recrystallization is the remaining strain energy after recovery. Due to the uneven deformation of the sample itself, the strain energy is different everywhere, so the progress of recrystallization is also different. The more complete the recrystallization is, the more strain energy is released, the less residual stress in the matrix is, and the more favorable it is for future processing. In the same sample, the recrystallization of the large deformation zone is the first. When the recrystallization of the large deformation zone is sufficient, the recrystallization of the small deformation zone has yet to be fully carried out. Suppose it is not advisable to increase the temperature or prolong the time unthinkingly. In that case, it should be carefully considered because the recrystallization of the small deformation zone is further carried out. At the same time, the recrystallized grains in the large deformation zone will continue to grow, which may not be conducive to subsequent processing. Therefore, heat treatment should make the alloy structure more balanced and uniform. 
As far as this alloy is concerned, metallographic observation shows that 990 °C × 120 min and 1000 °C × 60 min are better heat treatment processes before processing. The grain size is relatively small, the size distribution is more uniform, and there is no large grain.

4. Conclusion

After the above test and analysis, the following conclusions are drawn:

• (1) Under the combined influence of deformation and heat treatment temperature of as-rolled 625 alloys, the grain size increases slowly with the temperature increase from 950 °C to 1030 °C, and the grain size decreases from grade 8 and 9 to grade 6 and 7. At the same temperature, the grains of water quenching are slightly finer than those of air cooling.
• (2) The recrystallization is insufficient below 975 °C, and the recrystallization is complete when the temperature reaches above 990 °C. 990 °C × 120 min and 1000 °C × 60 min is a better heat treatment process before processing.

Author: Yu Qiujing