You can’t tell the difference between Titanium Grade 1 and Titanium Grade 2 just by looking at them. They have the same silver-grey shine, the same density (4.51 g/cm³), and the same resistance to corrosion in most places. However, mixing up these two “Commercially Pure” (CP) grades on an engineering drawing can cause a catastrophic failure. For example, using Grade 2 incorrectly can cause cracks during deep drawing processes, and using Grade 1 unnecessarily can cause structural yielding under pressure.
When engineers and procurement experts choose between Grade 1 (UNS R50250) and Grade 2 (UNS R50400), they are usually not thinking about “quality” in the usual way. Instead, it’s a strategic choice between the most ductility and moderate structural strength.
This guide goes beyond simple data sheets to help you make a real-world decision by breaking down the chemical, mechanical, and operational differences between grades so you can choose the right one for your needs.
The Critical Difference: Chemical Composition & Purity
While both alloys are classified as “unalloyed” titanium, Grade 2 is not simply a “lower quality” version of Grade 1. It is intentionally engineered with slightly higher levels of interstitial elements to achieve greater strength.
According to ASTM B265 (Standard Specification for Titanium and Titanium Alloy Strip, Sheet, and Plate), the compositional limits are defined as follows:
| Element | Grade 1 (UNS R50250) | Grade 2 (UNS R50400) | The Impact |
|---|---|---|---|
| Nitrogen (N) | Max 0.03% | Max 0.03% | Minor strength contribution. |
| Carbon (C) | Max 0.08% | Max 0.08% | Kept low to prevent carbide formation. |
| Hydrogen (H) | Max 0.015% | Max 0.015% | Strictly controlled to prevent hydrogen embrittlement. |
| Iron (Fe) | Max 0.20% | Max 0.30% | Iron adds strength but reduces corrosion resistance slightly. |
| Oxygen (O) | Max 0.18% | Max 0.25% | The primary strengthening agent. |
| Titanium (Ti) | Balance | Balance | Base matrix. |
The “Oxygen Knob”: Interstitial Strengthening Explained
The most significant differentiator lies in the oxygen content. The maximum allowable oxygen content for Grade 2 (0.25%) is 0.07 percentage points higher than for Grade 1 (0.18%).
While this difference seems negligible, in material science, oxygen acts as an interstitial alloying element. The small oxygen atoms fit into the spaces (interstices) between the larger titanium atoms in the hexagonal close-packed (HCP) crystal lattice. These interstitial atoms impede the movement of dislocations within the crystal structure.
- Grade 1 (Low Oxygen): Fewer barriers to dislocation movement, high ductility, and lower strength.
- Grade 2 (Higher Oxygen): More barriers “pin” the dislocations , Higher Strength, moderate ductility.
Therefore, Grade 2 obtains its “workhorse” status not by adding expensive metals like vanadium, but by carefully controlling the “impurity” of oxygen to boost tensile strength without sacrificing too much formability.
Technical Resource: For a deeper dive into standard specifications, refer to the ASTM B265 official documentation regarding strip, sheet, and plate requirements.
Mechanical Properties: Strength vs. Formability
When designing load-bearing equipment, the trade-off becomes numerical. Grade 2 generally offers a 30%–40% increase in yield strength over Grade 1.
Typical Mechanical Properties (At Room Temperature)
| Property | Grade 1 (Softest) | Grade 2 (Standard) |
|---|---|---|
| Tensile Strength (UTS) | 240 MPa (35 ksi) min | 345 MPa (50 ksi) min |
| Yield Strength (0.2% Offset) | 170 MPa (25 ksi) min | 275 MPa (40 ksi) min |
| Elongation | 24% min (often >30%) | 20% min |
| Hardness (Vickers) | ~120 HV | ~145 HV |
The ASME Design Perspective (Crucial for Pressure Vessels)
If you are designing heat exchangers or pressure vessels under the ASME Boiler and Pressure Vessel Code (BPVC), you don’t just look at yield strength; you look at maximum allowable stress.
This is where Grade 2 shines. Because its allowable stress values are significantly higher, engineers can specify thinner wall thicknesses for the same pressure rating.
- Scenario: A chemical reactor operating at 150°C.
- Grade 1: Requires thick walls to handle the pressure, increasing material weight and cost.
- Grade 2: Allows for thinner walls, reducing the total weight of titanium required.
Takeaway: Unless you need the extreme formability of Grade 1, Grade 2 is almost always the more economic choice for pressure containment due to its superior strength-to-weight ratio in code calculations.
Fabrication Guide: Working with the Material
The choice between Grade 1 and Grade 2 often comes down to how the part will be made. The shop floor reality can be quite different from the design office theory.
1. Cold Forming and Bending Radius
This is Grade 1’s territory. Because of its high elongation and lower oxygen content, Grade 1 possesses excellent “deep drawability.”
- Grade 1: Can often be bent to a radius of 1T to 1.5T (where T is material thickness) without cracking. It is the material of choice for plate-and-frame heat exchanger plates, which undergo severe deformation.
- Grade 2: Typically requires a generous bend radius of 2T to 2.5T. If you try to deep draw Grade 2 into a complex shape meant for Grade 1, you will likely experience “orange peeling” (surface roughening) or immediate cracking.
2. Machining: The “Gummy” Factor
Counter-intuitively, the “softer” Grade 1 can be more difficult to machine than Grade 2. Because Grade 1 is so ductile, it tends to be “gummy”. The material doesn’t chip away cleanly; instead, it smears and builds up on the cutting edge (Built-Up Edge or BUE).
- Machining Tip: When machining Grade 1, use sharp carbide tools, high positive rake angles, and copious coolant to prevent heat buildup and galling. Grade 2 behaves slightly more like stainless steel, offering better chip control.
3. Weldability
Both grades are excellent candidates for welding. Since they are single-phase alpha alloys, they do not suffer from the thermal cracking issues common in some steel alloys. However, gas shielding is non-negotiable. Molten titanium absorbs oxygen and nitrogen instantly from the air. Without proper argon backing (shielding), a Grade 1 weld will become brittle and crack—essentially turning into “Grade 100” due to oxygen contamination.
Critical Welding Practice: Achieving a high-integrity titanium weld requires more than just argon shielding gas. It demands:
-
Trailing Shields & Backing Gas: To protect the molten weld pool and the critical 400°C+ (750°F+) heat-affected zone from air contamination until it cools sufficiently.
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Meticulous Cleanliness: All surfaces, filler wires, and tools must be free of oil, grease, moisture, and fingerprints. Any organic residue will decompose in the arc, introducing hydrogen (causing porosity) and carbon (causing embrittlement).
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High-Purity Argon: The shielding gas must be of high purity (typically 99.998% or better) with low dew point to prevent moisture introduction.
Application Scenarios: When to Use Which?
Choose Grade 1 When:
- Deep Drawing is Required: Manufacturing plate heat exchangers, corrugated sheets, or complex architectural panels.
- Explosive Cladding: Bonding titanium to steel plates (the ductility helps absorb the explosive shock).
- Maximum Corrosion Resistance: In extremely marginal environments where the slightly lower iron content (0.20% vs 0.30%) might theoretically delay crevice corrosion, although this is rare.
Choose Grade 2 When:
- General Fabrication: Piping, flanges, fittings, and valves.
- Pressure Vessels: Tanks and reactors where ASME code compliance dictates wall thickness.
- Lining: Loose lining of steel tanks where the titanium is primarily a corrosion barrier, not a structural member.
- Availability Matters: You need standard sheet sizes delivered tomorrow.
Critical Warning: The “Grade 5” Trap
A common mistake made by junior engineers is looking at the strength of Grade 2 (345 MPa), finding it too low, and immediately jumping to Grade 5 (Ti-6Al-4V), which boasts nearly 900 MPa tensile strength.
Do not do this without validating your forming process.
Grade 5 is an alpha-beta alloy. It is incredibly strong but possesses minimal cold formability. Cold bending Grade 5 tubing or sheet is difficult, and attempting to do so will cause it to snap. If you need higher strength than Grade 2 but must retain cold formability, consider Grade 9 (Ti-3Al-2.5V) or simply increase the wall thickness of Grade 2.
Procurement Guide: Cost & Availability
The ‘Volume-Driven Pricing’ often confuses buyers. Logically, one might assume Grade 1 is pricier because it is ‘purer.’ While Grade 1 does command a premium due to tighter chemistry controls, the price difference is often driven by economies of scale.
Grade 2 is the industrial “workhorse.” It accounts for the vast majority of the CP Titanium market.
- Stock: Service centers stock tons of Grade 2 sheets and plates. Grade 1 is often a “special order” item or stocked in limited quantities.
- Price: Due to high volume turnover, Grade 2 is frequently cheaper per kilogram than Grade 1.
- Strategy: If your design allows for Grade 2, specify it. You will likely secure better pricing and shorter lead times. If you specify Grade 1 for a standard flange, you might pay a premium and wait weeks for production.
Summary: Quick Selection Matrix
| Decision Factor | Choose Grade 1 (UNS R50250) | Choose Grade 2 (UNS R50400) |
|---|---|---|
| Forming Process | Deep drawing, severe bending (>50% stretch). | Rolling, simple bending, standard fabrication. |
| Design Driver | Formability / Ductility. | Strength / ASME Pressure Rating. |
| Bend Radius | Tight bends (1T – 1.5T). | Generous bends (2T – 2.5T). |
| Availability | Moderate / Low (Specialty items). | High (The Industry Standard). |
| Corrosion | Excellent. | Excellent (Virtually identical). |
Frequently Asked Questions (FAQ)
Q: Is Grade 2 titanium magnetic?
A: No. Both Grade 1 and Grade 2 titanium are non-magnetic. This makes them ideal for medical imaging equipment (MRI) and sensitive electronics housing.
Q: Can I weld Grade 1 to Grade 2 titanium?
A: Yes, they are compatible. You can weld Grade 1 to Grade 2 using a matching filler wire (usually ERTi-1 or ERTi-2). The resulting weld zone will have mechanical properties intermediate between the two.
Q: Does Grade 2 titanium rust?
A: No. Titanium does not “rust” like iron. It forms a stable, passive oxide film that renders it immune to corrosion in seawater, wet chlorine, and most organic acids.
Q: Why is Grade 2 more popular than Grade 1?
A: It offers the “Goldilocks” balance: strong enough for structural use (unlike Gr1) but formable enough for fabrication (unlike Gr5), combined with the highest availability in the supply chain.