Alloy 725, UNS N07725, Inconel® 725 - nickel alloy

Alloy 725, UNS N07725, Inconel® 725 - Nickel alloy according to ASTM B805, ISO 21457

Alloy 725, Inconel® 725 – description

Alloy 725 (N07725) is a precipitacion hardenable Ni-Cr-Fe-Mo alloy with niobium alloying addition. It can be seen as an age-hardenable variation of alloy 625, with comparable corrosion resistance, but higher strength.

Resistant to general corrosion, pitting, sulfide stress cracking, hydrogen embrittlement and stress-corrosion cracking in sour gas wells.

Applications

Hangers, landing nipples, side pocket mandrels, polished bore receptacles in sour gas service, high-strength fasteners in marine applications, polymer extrusion dies.

Physical and mechanical properties

Room temperature physical properties:

• Density: 8.31 g/cm³
• Melting temperature: 1271-1343 °C
• Magnetic permeability: <1.001

High-temperature Coefficient of thermal expansion, from 20°C to:

• 100°C: 13.0 μm/m⋅K
• 200°C: 13.1 μm/m⋅K
• 300°C: 13.4 μm/m⋅K
• 400°C: 13.7 μm/m⋅K
• 500°C: 14.1 μm/m⋅K
• 600°C: 14.4 μm/m⋅K

Thermal conductivity:

• 20°C: 10.631 W/m · K
• 100°C: 11.827 W/m · K
• 200°C: 13.544 W/m · K
• 300°C: 15.122 W/m · K
• 400°C: 16.843 W/m · K
• 500°C: 18.152 W/m · K
• 600°C: 20.037 W/m · K
• 700°C: 22.424 W/m · K
• 800°C: 23.062 W/m · K
• 900°C: 23.812 W/m · K
• 1000°C: 25.361 W/m · K
• 1100°C: 27.038 W/m · K
• 1200°C: 29.604 W/m · K

Electrical resistivity:

• 20°C: 1.144 μΩ·m
• 100°C: 1.158 μΩ·m
• 200°C: 1.179 μΩ·m
• 300°C: 1.206 μΩ·m
• 400°C: 1.226 μΩ·m
• 500°C: 1.251 μΩ·m
• 600°C: 1.265 μΩ·m
• 700°C: 1.273 μΩ·m
• 800°C: 1.302 μΩ·m

Specific heat capacity:

• 20°C: 430 J/kg⋅K
• 100°C: 447 J/kg⋅K
• 200°C: 468 J/kg⋅K
• 300°C: 589 J/kg⋅K
• 400°C: 508 J/kg⋅K
• 500°C: 519 J/kg⋅K
• 600°C: 552 J/kg⋅K
• 700°C: 604 J/kg⋅K
• 800°C: 609 J/kg⋅K
• 900°C: 618 J/kg⋅K
• 1000°C: 639 J/kg⋅K
• 1100°C: 654 J/kg⋅K
• 1200°C: 673 J/kg⋅K

Young's modulus:

• 20°C: 204 GPa
• 100°C: 200 GPa
• 200°C: 194 GPa
• 300°C: 188 GPa
• 400°C: 182 GPa
• 500°C: 177 GPa
• 600°C: 169 GPa
• 700°C: 160 GPa
• 800°C: 150 GPa

Shear modulus:

• 20°C: 78 GPa
• 100°C: 76 GPa
• 200°C: 74 GPa
• 300°C: 71 GPa
• 400°C: 69 GPa
• 500°C: 67 GPa
• 600°C: 63 GPa
• 700°C: 61 GPa
• 800°C: 56 GPa

Poisson's ratio:

• 20°C: 0.31
• 500°C: 0.32
• 600°C: 0.35
• 700°C: 0.32
• 800°C: 0.33

Mechanical properties of Inconel® 725 bars according to the manufacturer:

• Solution treated:
  • Yield strength: 427 MPa
  • Tensile strength: 855 MPa
  • Elongation: 57 %
  • Hardness: 5 Rc
• Age hardened:
  • Yield strength: 917 MPa
  • Tensile strength: 1241 MPa
  • Elongation: 30 %
  • Hardness: 36 Rc
  • Impact value: 92 J

Mechanical properties of Inconel® 725 tubes according to the manufacturer:

• Solution treated:
  • Yield strength: 334 MPa
  • Tensile strength: 783 MPa
  • Elongation: 60 %
  • Hardness: 5 Rc
• Age hardened:
  • Yield strength: 921 MPa
  • Tensile strength: 1268 MPa
  • Elongation: 27 %
  • Hardness: 39 Rc

High-temperature strength properties of Inconel® 725 alloy rods (16-165mm thick), age hardened to work in sour gas wells:

• 20°C:
  • Yield strength: 892 MPa
  • Tensile strength: 1254 MPa
  • Elongation: 32 %
  • Reduction of area: 48.4 %
• 93°C:
  • Yield strength: 868 MPa
  • Tensile strength: 1230 MPa
  • Elongation: 29.6 %
  • Reduction of area: 47.0 %
• 204°C:
  • Yield strength: 824 MPa
  • Tensile strength: 1170 MPa
  • Elongation: 30.7 %
  • Reduction of area: 52.4 %
• 316°C:
  • Yield strength: 782 MPa
  • Tensile strength: 1099 MPa
  • Elongation: 32.4 %
  • Reduction of area: 54.2 %
• 427°C:
  • Yield strength: 818 MPa
  • Tensile strength: 1106 MPa
  • Elongation: 29.6 %
  • Reduction of area: 49.6 %
• 538°C:
  • Yield strength: 778 MPa
  • Tensile strength: 1058 MPa
  • Elongation: 31.0 %
  • Reduction of area: 47.7 %

Corrosion behaviour

Alloy 725 is highly resistant sour gas well environments and other media containing carbon dioxide, chlorides and hydrogen sulfide. It is also highly resistant to brines, seawater and other corrosive media.

Mineral acids - very good resistance. Average corrosion rates for Inconel® 725 in various mineral acids (2 week test duration), as a function of heat treatment:

• Solution-treated (1038°C):
  • Hydrochloric acid (HCl) 3%°C, at 66°C: 0.01 mm/year
  • 5%°C, at 66°C: 0.01 mm/year
  • 10%°C, at 66°C: 2.67 mm/year
  • Sulfuric acid (H₂SO₄) 10%, boiling: 0.64 mm/year
  • Nitric acid (HNO₃) 10%, boiling: 0.01 mm/year
  • Phosphoric acid (H₃PO₄) 30%, boiling: 0.08 mm/year
  • 85%, boiling: 1.85 mm/year
• Annealed (1038°C) and age hardened (760°C for 6h, air cooling):
  • Hydrochloric acid (HCl) 3%°C, at 66°C: 0.01 mm/year
  • 5%°C, at 66°C: 0.01 mm/year
  • 10%°C, at 66°C: 6.81 mm/year
  • Sulfuric acid (H₂SO₄) 10%, boiling: 0.64 mm/year
  • Nitric acid (HNO₃) 10%, boiling: 0.01 mm/year
  • Phosphoric acid (H₃PO₄) 30%, boiling: 0.13 mm/year
  • 85%, boiling: 1.57 mm/year
• Annealed (1038°C) and age hardened (746°C for 8h, furnace cooling [56°C/h] down to 620°C, holding at 620°C for 8h, air cooling):
  • Hydrochloric acid (HCl) 3%°C, at 66°C: 0.01 mm/year
  • 5%°C, at 66°C: 0.01 mm/year
  • 10%°C, at 66°C: 6.35 mm/year
  • Sulfuric acid (H₂SO₄) 10%, boiling: 0.64 mm/year
  • Nitric acid (HNO₃) 10%, boiling: 0.01 mm/year
  • Phosphoric acid (H₃PO₄) 30%, boiling: 0.08 mm/year
  • 85%, boiling: 1.14 mm/year
• Annealed (1038°C) and age hardened (732°C for 8h, furnace cooling [56°C/h] down to 620°C, holding at 620°C for 8h, air cooling):
  • Hydrochloric acid (HCl) 3%°C, at 66°C: 0.01 mm/year
  • 5%°C, at 66°C: 0.01 mm/year
  • 10%°C, at 66°C: 5.54 mm/year
  • Sulfuric acid (H₂SO₄) 10%, boiling: 0.71 mm/year
  • Nitric acid (HNO₃) 10%, boiling: 0.01 mm/year
  • Phosphoric acid (H₃PO₄) 30%, boiling: 0.05 mm/year
  • 85%, boiling: 0.89 mm/year

Stress-corrosion cracking in a simulated sour well environment - very good resistance. C-ring autoclave tests of 14-day duration at 100% of yield strength in 25% NaCl + 0,5% acetic acid + 1g/l sulfur + 827 kPa H₂S:

• Yield strength: 811 MPa
  • 177-218°C - no cracks
  • 232°C - no cracks
  • 246°C - one of two specimens had cracks
  • 260°C - no cracks
• Yield strength: 887 MPa
  • 177-218°C - no cracks
  • 232°C - both specimens had cracks
• Yield strength: 916 MPa
  • 177-218°C - no cracks
  • 232°C - no cracks
  • 246°C - no cracks
  • 260°C - no cracks
• Yield strength: 917 MPa
  • 177-218°C - no cracks
  • 232°C - no cracks
  • 246°C - one of two specimens had cracks
  • 260°C - no cracks

Weldability

Very good weldability, preferably using GTAW or GMAW. Selection of welding consumables:

• ERNiCrMo-17 / SG-NiCr23Mo16Cu (Filler metal)

For best results, anneal alloy 725 prior to welding, then apply a solution anneal and age hardening after welding.

Heat treatment, working and machining

Hot working - alloy can be hot forged, rolled, upset, extruded and formed in the 900-1120°C temperature range. In the last operation, the metal should be worked uniformly with a gradually decreasing temperature. To avoid duplex grain structure, alloy 725 should be given uniform reductions. Final reductions of 20% minimum should be used for open-die work, and 10% minimum for closed-die work. Air cooling.

Solution heat-treatment involves holding at 1040°C and air cooling. Higher temperature of 1066°C can be chosen as post-welding heat treatment for optimal impact strength. Overall, however, 1040°C is preferable.

Solutioned parts should be Age hardened. There are multiple age hardening procedures, but for sour gas applications the recommended procedure is:

• Age hardening: 730°C; 8h
• Furnace cooling: 56°C/h
• Age hardening: 620°C 8h
• Air cooling

Machining - group D-2, meaning it's very hard to machine. Rough machine in solution annealed condition and then finish machine after aging. In rough machining allow for the size contraction up to about 0,07%, which takes place upon aging. Recommended turning parameters using coated carbide:

• Roughing in annealed condition:
  • Depth of cut: 6.35 mm
  • Feed: 0.25 mm/rev
  • Cutting speed: 244 m/min
• Finishing in aged condition:
  • Depth of cut: 1.02 mm
  • Feed: 0.13 mm/rev
  • Cutting speed: 35 m/min

Twist drilling with high-speed steel drills recommended parameters:

• Annealed condition:
  • Drill diameter: 9.5-11 mm
  • Feed: 0.10-0.18 mm/rev
  • Cutting speed: 3-4 m/min
• In aged condition:
  • Depth of cut: 13-17 mm
  • Feed: 0.15-0.25 mm/rev
  • Cutting speed: 2-3 m/min

We deliver this steel alloy as:

• Bars according to ASTM B 805 / ASME SB.805, ASME Code Case 2217
• Forgings according to SMC specification HA91, ASME Code Case 2217
• Materials for use in sour gas wells according to NACE International MR-01-75, API Standard 6ACRA

Replacements, equivalents and other designations:

Inconel® alloy 725, Inconel® 725, Alloy 725, BNi7725, N07725, Ni6200, Ni7725, NiCr21Mo8Nb3Ti, NiCr23Mo16Cu2, SNi7725, ASTM B 1007 (N07725) / ASME SB-1007 (N07725), ASTM B 637 (N07725) / ASME SB-637 (N07725), ASTM B 805 (N07725) / ASME SB-805 (N07725), ASTM B 872 (N07725) / ASME SB-872 (N07725), ASTM B 983 (N07725) / ASME SB-983 (N07725), N07725 API Standard 6ACRA-2015, Nickelvac W-722