5.6.2   Titanium

 

Because of its high strength/weight ratio and excellent corrosion resistance, titanium has been considered as an alternative to 12% Cr steel for low pressure turbine blading. Since the density of titanium is about 60% of that of steel, a titanium blade nearly 40% longer than a steel blade can be substituted, assuming comparable stress levels in the blade root.

The corrosion resistance of titanium makes the material highly attractive for blading subject to pitting, stress corrosion and corrosion fatigue. Laboratory tests have also shown that certain titanium alloys are superior to 12% chrome stainless steel in erosion resistance but not as good as Stellite 6, which is commonly used as an erosion shield on steel LP blading.

The yield strength of a typical titanium alloy in the annealed condition is approximately 50% better than standard 12% chromium stainless steel material, and even slightly superior to the Super 12% Cr steels: the ductility is somewhat less. The fatigue strength of titanium alloy is, however, generally higher than the 12% Cr steels.

Titanium alloy is not as readily forgeable as 12% Cr materials. The titanium alloy has to be forged at a lower temperature which means that more energy is required to forge a certain amount. Also, since all titanium alloys are very active chemically at high temperatures, surface contamination is another problem encountered when forging.

Machining is possible, if slow speeds are used, but a propensity for embrittlement by contamination and lack of heat penetration make welding difficult. Inert gas welding is the normal solution but this is time consuming and costly.

The main problems of titanium are high cost and poor vibration characteristics. The cost of titanium alloy is several times that of standard 12% Cr blade material and a row of titanium blades normally costs two to three times that of a steel row, all factors considered.

The material damping of titanium is very poor, varying from 10% to 40% of that of 12% Cr blade material. Thus there is a potential for higher vibratory stresses when operating the titanium blade at or near resonance. Also Young's Modulus of titanium is low and, since the flexibility is relatively high, natural frequencies are depressed and harmonics are more crowded in the running range.

In view of the lower cost and improved properties of the Super 12% Cr steels, it is unlikely that titanium alloys will supersede these steels for LP moving blading on UK steam turbines in the near future. However, because of its high strength/weight ratio, titanium is used extensively for lacing wire and for coverbanding and shrouding.

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