Inconel 625 is a nickel-chromium-molybdenum-niobium alloy. Grade 1 and Grade 2 differ mainly in annealing temperature, service temperature range, microstructure and applicable working scenarios. The distinction originates from different thermal processing schedules, which adjust grain size and mechanical properties for targeted industrial service conditions.
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Core Difference Between Inconel 625 Grade 1 and Grade 2
The primary difference between ASTM B163 Inconel 625 Grade 1 and Grade 2 lies in their heat treatment (annealing) temperatures and intended service temperatures, which affect their mechanical performance and creep resistance at high heat.
1. Annealing Temperature
- Grade 1 adopts high-temperature full solution annealing at 1095–1150℃. This high temperature dissolves most niobium intermetallic phases and achieves full recrystallization.
- Grade 2 uses lower-temperature stress-relief / partial recrystallization annealing at 980–1040℃. It avoids excessive grain growth, preserving finer grain structure without full solution of alloying elements.
2. Service Temperature & Actual Usage
- Grade 1 is designed for sustained high-temperature environments. It maintains structural stability under long-term exposure up to 980℃, suitable for thermal equipment that bears continuous high-temperature creep load.
- Grade 2 targets moderate-temperature corrosive environments below 815℃. It does not require ultra-high creep resistance, prioritizing room and medium temperature mechanical strength for pressure-bearing corrosion equipment.
3. Microstructure
- Grade1: Coarse equiaxed grains after full recrystallization. Nb and Mo fully dissolve into nickel matrix. No brittle δ phase formed, which improves high-temperature creep performance.
- Grade2: Fine partially-recrystallized grains. Tiny NbC carbides remain inside grain boundaries. Fine grains strengthen matrix at moderate temperature, but creep performance degrades above 815℃.
4. Performance
- Grade1: Superior high-temperature creep rupture strength and thermal stability. Its tensile strength at room temperature is relatively low.
- Grade2: Higher yield and tensile strength at room & medium temperature, better fatigue resistance under 815℃. Creep resistance drops rapidly once service temperature exceeds 815℃.
Comparison Table
| Item | Inconel 625 Grade 1 | Inconel 625 Grade 2 |
|---|---|---|
| Annealing Temperature | 1095–1150 ℃, rapid cooling after annealing | 980–1040 ℃, air cooling after annealing |
| Service Temperature | Up to 980 ℃ for continuous service; short-term peak 1040 ℃ | Up to 815 ℃ for continuous service; short-term peak 870 ℃ |
| Microstructure | Fully recrystallized coarse equiaxed grains; dissolved niobium-rich precipitates (no δ phase) | Partially recrystallized fine grains; limited fine NbC precipitates, no massive δ phase |
| Core Performance | High creep rupture strength at elevated temperature; lower room-temperature tensile strength; excellent high-temperature fatigue resistance | Higher room-temperature tensile & yield strength; better low-cycle fatigue at moderate temperature; slightly lower creep resistance |
| Actual Usage Scenarios | Gas turbine combustors, furnace components, high-temperature piping exposed to long-term thermal load | Pressure vessels, chemical process equipment, seawater corrosion parts working under 815℃ |
Inconel 625 GRADE1 vs GRADE2 ：Chemical Composition (wt%)
| Element | ASTM B163 Inconel 625 Grade 1 | ASTM B163 Inconel 625 Grade 2 | Standard Limit |
|---|---|---|---|
| Ni | ≥58.0 | ≥58.0 | ≥58.0 |
| Cr | 20.0 ~ 23.0 | 20.0 ~ 23.0 | 20.0 ~ 23.0 |
| Mo | 8.0 ~ 10.0 | 8.0 ~ 10.0 | 8.0 ~ 10.0 |
| Nb+Ta | 3.15 ~ 4.15 | 3.15 ~ 4.15 | 3.15 ~ 4.15 |
| Fe | ≤5.0 | ≤5.0 | ≤5.0 |
| C | ≤0.10 | ≤0.10 | ≤0.10 |
| Si | ≤0.50 | ≤0.50 | ≤0.50 |
| Mn | ≤0.50 | ≤0.50 | ≤0.50 |
| S | ≤0.015 | ≤0.015 | ≤0.015 |
| P | ≤0.015 | ≤0.015 | ≤0.015 |
Note: Grade 1 and Grade 2 have identical chemical composition. Grade differentiation comes from heat treatment and grain size control instead of alloy chemistry.
GRADE1 vs GRADE2 ：Room-Temperature Mechanical Properties (ASTM B163)
| Property | ASTM B163 Grade 1 | ASTM B163 Grade 2 |
|---|---|---|
| Heat Treatment | Annealed | Solution Annealed, Controlled Grain Size |
| Minimum Tensile Strength | ≥827 MPa (120 ksi) | ≥827 MPa (120 ksi) |
| Minimum Yield Strength (0.2% offset) | ≥415 MPa (60 ksi) | ≥240 MPa (35 ksi) |
| Minimum Elongation (2 in / 50 mm gauge length) | ≥30 % | ≥40 % |
| Grain Size | No mandatory grain size requirement | Controlled fine and uniform grain structure |
| Cold Formability | Moderate; Not recommended for tight radius U-bend | Excellent, designed for U-bend heat exchanger tube |
| Primary Application | Straight heat exchanger tubes (no bending required) | U-bend tubes, high-temperature cyclic service heat exchangers |
Note: Grade 2 has lower minimum yield strength and higher elongation. This provides superior ductility to prevent microcracks during U-bend cold forming and delivers better thermal fatigue resistance under repeated hot-cold cycling for heat exchanger operation.
In-Depth Analysis of INCONEL 625 Grade 1 and Grade 2 Application Characteristics
ASTM B163 Inconel 625 Grade 1
Grade 1 is the conventional annealed grade of B163 625 heat exchanger tube. It features higher room-temperature yield strength and stable basic corrosion resistance, with more cost-effective pricing. After standard annealing treatment, the tube meets the basic mechanical and anti-corrosion requirements of industrial heat exchange equipment.
Applicable Scenarios: Pure straight tube heat exchangers, static heat exchange systems without cold bending processing, medium and low-temperature working conditions below 593℃, and general chemical corrosion heat exchange equipment with low cycle frequency.
ASTM B163 Inconel 625 Grade 2
Grade 2 is a high-precision solution-annealed grade exclusively optimized for heat exchanger forming working conditions. Through strict solution heat treatment and grain size homogenization control, the tube material obtains lower yield strength and greatly improved elongation and toughness. The internal structure is uniform and stable, eliminating residual stress generated during tube rolling.
Core Advantages: Grade 2 completely solves the cracking pain point of 625 tubes during tight-radius U-bending. It maintains structural integrity after cold forming without residual micro-defects. Meanwhile, the optimized grain structure provides outstanding high-temperature creep resistance and thermal fatigue resistance, adapting to 24-hour continuous operation and frequent temperature impact of high-end heat exchangers.
Applicable Scenarios: U-tube heat exchangers, shell-and-tube condenser core tubes, finned heat exchanger tubes, high-temperature heat exchange systems above 600℃, marine desalination heat exchange equipment, and ASME pressure vessel grade projects requiring strict NDT inspection.
How to Quickly Select Grade 1 or Grade 2? Clear Selection Guide
To help customers quickly confirm the correct grade and avoid over-design or under-standard procurement, we summarize the most practical selection rules：
Choose ASTM B163 625 Grade 1 if:
- Your project only needs straight tubes without any cold bending or secondary forming
- Working temperature is stable below 593℃ without frequent hot-cold alternating cycles
- General anti-corrosion heat exchange requirements, no strict fatigue life standards
- Project budget is limited and cost-effective procurement is prioritized
Choose ASTM B163 625 Grade 2 if:
- Tube requires U-bend, fin rolling, or other cold forming processing
- Working temperature exceeds 600℃, requiring high-temperature creep resistance
- Equipment runs continuously with frequent hot and cold alternating impact
- Project belongs to ASME pressure vessel, marine, petrochemical high-standard engineering
- On-site requires 100% NDT eddy current and ultrasonic inspection with zero defect standard
Wall Thickness Reference Range for ASTM B163 Inconel 625 Tubing
| Nominal Wall Thickness (mm) | Wall Thickness Tolerance Range (mm) |
|---|---|
| 1.0 ~ 3.0 | -0.10 ~ +0.30 |
| 3.0 ~ 6.0 | -0.15 ~ +0.45 |
| 6.0 ~ 10.0 | -0.20 ~ +0.70 |
| 11.1 (Project Reference) | -0.25 ~ +0.94 |
| 10.0 ~ 15.0 | -0.25 ~ +0.95 |
| 15.0 ~ 20.0 | -0.30 ~ +1.10 |
Dimension Reference & Tolerance Table (Length / Width)
| Nominal Dimension | Nominal Size (mm) | Tolerance Reference Range (mm) |
|---|---|---|
| Width | 1829 | -2.0 ~ +3.0 |
| Length | 6096 | -3.0 ~ +5.0 |
Real Project Reference Case
We have successfully supplied over 25 tons of ASTM B163 Inconel 625 Grade1 annealed and pickled seamless tubes, with a nominal wall thickness of 11.1 mm and finished dimensions of 1829 mm × 6096 mm. The agreed wall thickness tolerance was -0.25 / +0.94 mm.
We can produce both Grade1 and Grade2 Inconel 625 heat exchanger tubing in various wall thickness and length combinations. Send your required OD, wall thickness, length and grade requirement for a formal quotation.
| Material Description | Full English Description |
|---|---|
| INCONEL 625, GRADE 1, ANNEALED AND PICKLED | ASTM B163 Inconel 625 Grade 1, Annealed and Pickled seamless tube |
Our Supply Capacity & Supporting Service
We supply full-specification ASTM B163 Inconel 625 Grade 1 and Grade 2 heat exchanger seamless tubes, fully matching international engineering standards, supporting customized processing and strict third-party inspection:
- Complete specifications: straight tubes, U-bend tubes, finned tubes, precision capillary tubes
- Strict classification supply: Grade 1 and Grade 2 are produced and stored separately to avoid mixing
- Full inspection reports: MTR certificate, PMI report, UT/ NDT flaw detection report, grain size test report
- Custom service: fixed-length cutting, precision bending, inner and outer wall polishing, anti-oxidation packaging
Contact us for the latest export price quote for 625 Alloy
FAQ:
What is the ASTM standard for Inconel 625?
The ASTM standard for Inconel 625 depends on the specific product form, with the general designation universally tracked under UNS N06625.
Is Inconel 625 a stainless steel?
No, Inconel 625 is not a stainless steel; it is a nickel-based superalloy.
Can Inconel 625 rust?
No, Inconel 625 does not rust because it does not contain free iron to form traditional red iron oxide, though it can experience specialized chemical corrosion in extreme conditions.
