Ni superalloys: Turbine disks and Processing

Question 1

Give 7 requirements for turbine disks

Answer 1

1. High tensile strength to prevent yield and fracture in over-stressed conditions
2. resistance to fatigue crack nucleation
3. slow crack nucleation rates
4. high fracture toughness
5. adequate creep resistance for operation at high temperatures (973K)
6. high ductility to allow relaxation at stress concentrations
7. oxidation/hot corrosion resistance

Question 2

How do operating temperature of disks relate to that of turbine blades

Answer 2

operating temperature of disks are much lower than that of blades (873K)
stresses are much higher though

Question 3

How is design carried out for turbine disks

Answer 3

with relation to the yield characteristics of the material

this is because creep is not a significant problem

Question 4

Why is it necessary for reliable non-destructive testing to be carried out

Answer 4

because fracture toughness of disks are very low, so catastrophic failure could occur if there are any unexpected defects in material

Question 5

Describe the important parameters when designing a turbine disks

Answer 5

1. Statistical distribution of inclusions
2. Fatigue crack law

this means large components will have inferior low cycle fatigue to that of small components as more likely to have larger inclusions thus

Question 6

How is forging carried out for superalloys with low volume fraction of γ’ phase

Answer 6

• at very high temperatures where the alloy is relatively weak and ductile
• usually at temperatures where γ’ is dissolved in solution
• this means about 200C between γ’ solvus and alloy melting temperature

Question 7

How where high volume fractions of the γ’ phase superalloys processed

Answer 7

• this was carried out by direct casting as complex shapes and with internal cooling channels

Question 8

How does directionally solidified materials compare with conventional cast alloys

Answer 8

Directionally solidified materials how much higher ductilities which contributes to improved creep rupture and thermal fatigue resistance

Question 9

What is the major benefit with respect to the thermal fatigue resistance

Answer 9

it results from the crystallographic texture produced in the process

Question 10

Equation for cyclic stress in a turbine blade

Answer 10

Δσ = αEΔT

α - coefficient of linear expansion
E - Youngs Modulus

Therefore stress range varies with E which is determined by the crystal orientation

Question 11

When can single crystal superalloys be used

Answer 11

only when their integrity in complex castings can be assured

Question 12

Why are investment casting, directional solidification and single crystal processes inappropriate for turbine disks

Answer 12

• this is because turbine disks operate at very high stresses but reasonably low temperatures which mean fatigue rather than creep will restrict their service life
• to improve fatigue, small grain sizes are more appropriate

Question 13

What processes have been advanced for the manufacture of turbine disks

Answer 13

• electron beam cold hearth refining
• plasma melting
• electroslag refining