Why Titanium Exhaust Tubing Is Specified
Exhaust systems fail from the outside in and the inside out at the same time. Road salt and spray attack the outer surface in winter, while acidic condensate and high gas temperature attack the inner surface. Titanium addresses both because its passive titanium dioxide film is stable in chloride solution and in the mildly acidic condensate found in exhausts, so the tube can be used without paint or plating. The weight benefit is equally important: titanium has a density of 4.51 g/cm3 against about 7.85 g/cm3 for steel, so a titanium tube weighs roughly 57 percent of the steel tube it replaces.
The material for this application is supplied to ASTM B338, which covers seamless and welded titanium tube, in Grade 1 with UNS number R50250, equivalent to TA1 in the GB system. Grade 1 is the softest pure grade, which makes it easy to bend, flare and weld, and that formability is the reason it dominates decorative and moderate temperature exhaust sections.
Chemical Requirements
| Element | Maximum, percent by mass |
|---|---|
| Iron | 0.20 |
| Carbon | 0.08 |
| Nitrogen | 0.03 |
| Hydrogen | 0.015 |
| Oxygen | 0.18 |
| Other elements, each | 0.10 |
| Other elements, total | 0.40 |
Mechanical Requirements and Available Sizes
| Property, annealed | Minimum |
|---|---|
| Tensile strength | 240 MPa |
| Yield strength, 0.2 percent offset | 170 MPa |
| Elongation | 24 percent |
| Parameter | Value |
|---|---|
| Outside diameter | 38, 45, 55, 60, 63, 70, 76, 80 and 89 mm |
| Wall thickness | 1 mm as standard, other walls on request |
| Tube type | Seamless or welded |
| Surface | Bright, polished or pickled |
| Length | Straight lengths or coiled, cut to drawing where required |
Service Temperature Boundaries
Grade 1 is the lowest strength pure grade, and its yield strength of 170 MPa falls away quickly above roughly 300 to 400 °C. In practice that means Grade 1 is well suited to tailpipes, mid pipes, heat shields, tips and decorative sections, where the gas has already cooled, and it is not the right material for a header or collector bolted directly to the cylinder head, where sustained high temperature and thermal cycling demand higher strength. Those positions take Grade 2, which retains more strength at temperature, or a titanium alloy. The design should always be checked against the actual measured gas temperature rather than a nominal expectation, because exhaust gas temperature varies widely with load and tuning.
Bending, Welding and Fabrication Practice
Bending: Grade 1 tube is bent by mandrel or rotary draw bending. A mandrel is essential for 1 mm wall tube to prevent collapse, and the drawing should state a minimum bend radius, typically 1.5 to 3 times the outside diameter, together with the allowable wall thinning on the outside of the bend.
Welding: tungsten inert gas welding with matching ERTi-1 filler, argon shielding on the weld pool, the trailing area and the inside of the tube until the metal cools below about 450 °C. Purge the tube bore before welding, because the inner surface oxidises just as readily as the outside.
Weld appearance: accept silver or light straw; reject blue, purple and grey, which show oxygen and nitrogen absorption and a hard, brittle joint.
Cutting and forming: use dedicated tooling and avoid iron contamination from steel brushes, files or grinding wheels, which embeds particles that become corrosion initiation sites.
Joining to other metals: where titanium meets steel or stainless steel, use an insulated or bimetallic transition joint because a direct galvanic couple in wet service will accelerate corrosion of the less noble metal.
Frequently Asked Questions
Q: How much lighter is titanium exhaust tubing than steel?
A: Titanium has a density of about 4.51 g/cm3 against 7.85 g/cm3 for steel, so a titanium tube of the same dimensions weighs roughly 57 percent of the steel equivalent.
Q: Can Grade 1 tube be used for exhaust headers?
A: Generally no. Grade 1 loses strength rapidly above about 300 to 400 °C, so headers and collectors that see sustained high temperature take Grade 2 or a titanium alloy instead.
Q: What wall thickness is normal for titanium exhaust tube?
A: 1 mm wall is the common standard for the 38 mm to 89 mm outside diameter range, because the material weight saving and its corrosion resistance mean extra thickness is seldom needed.
Q: Why is a mandrel needed when bending thin wall titanium tube?
A: The 1 mm wall has limited resistance to ovalisation, so an internal mandrel supports the bore through the bend and prevents collapse and excessive wall thinning.
Q: What does weld colour tell the fabricator?
A: Silver or light straw means correct shielding; blue, purple or grey means oxygen and nitrogen were absorbed above 450 °C, leaving a hard and brittle weld that should be cut out and remade.
