1. Cast Titanium Alloy Properties
Cast titanium alloys provide the same corrosion resistance as wrought titanium and slightly different mechanical properties because of the cast microstructure. The strength is comparable to the wrought form, while ductility is lower because cast structures contain some porosity and coarse grains. For valve bodies, the material offers immunity to seawater, chlorides and many chemical media, eliminating the need for linings and coatings. The light weight of titanium also reduces the mass of large valves.
2. Casting Processes for Titanium
Titanium valves are cast by investment casting for complex, thin-walled bodies and by rammed graphite or graphite mold casting for larger, simpler parts. Investment casting gives a good surface finish and close dimensional accuracy, reducing the machining required. The melting and pouring are carried out in vacuum or inert gas because molten titanium reacts with air. The mold is designed to fill smoothly and to feed shrinkage with risers.
3. Grades for Valve Service
ASTM B367 covers titanium castings with unalloyed grades such as C-2 for corrosion service and alloy grades such as C-5, the cast Ti-6Al-4V, for higher strength. The grade is selected for the pressure rating and the fluid. For seawater and chloride service, unalloyed grades perform well, and palladium-alloyed cast grades are available where crevice conditions are severe. The casting specification defines the composition, mechanical properties and testing for each grade.
4. Testing of Cast Valve Components
Cast valve bodies are radiographed to ASTM E1320 to detect internal shrinkage and porosity, and penetrant tested to ASTM E165 for surface defects. Pressure-containing castings are hydrostatically tested to the valve rating, and the seat and flange faces are dimensionally verified. Where the specification requires, mechanical test coupons are cast with the batch and tested for tensile properties. The results are reported on the certificate with the chemical analysis.
5. Weld Repair and Machining
Castings that show acceptable defects are repaired by gas tungsten arc welding under inert gas with matching filler, using qualified procedures. The repaired areas are re-inspected by the same methods as the original defect. The cast body is then machined on the critical faces: the flange faces, the bore, the seat and the gland area. Machining of titanium requires the usual care with sharp tools and coolant, and the finished surfaces are verified.
6. Valve Applications and Advantages
Titanium cast valves are used in seawater systems, chemical plants, desalination, offshore platforms and food processing where corrosion resistance is essential. The advantages over lined steel valves are the absence of lining failure, lower maintenance, and the ability to handle aggressive fluids at higher temperatures. The service life of a titanium valve is typically many times that of a lined or stainless valve in the same duty, which offsets the higher initial cost.
Investment casting suits complex valve bodies with good accuracy.
ASTM B367 grades such as C-2 and C-5 cover valve service.
Radiography and hydrostatic testing verify the pressure integrity.
Frequently Asked Questions
Q: Why cast titanium valves instead of forged or fabricated ones?
Casting gives the complex internal geometry of a valve body in one piece, which is difficult and expensive to produce by forging or fabrication.
Q: What is the difference between cast and wrought titanium strength?
Cast titanium has comparable strength to wrought material but lower ductility because of the cast microstructure, and the design accounts for this.
Q: Can cast titanium valve bodies be repaired?
Yes, acceptable defects are repaired by qualified gas tungsten arc welding and the areas are re-inspected afterwards.
Q: What standards apply to titanium valve castings?
ASTM B367 covers the cast material, ASTM E1320 covers radiography and ASTM E165 covers penetrant testing.
Q: Are titanium valves expensive?
The initial cost is higher than steel, but the longer service life and lower maintenance in corrosive service reduce the life-cycle cost.
