Microstructure

0 5 10 15 20 25 30 35 400

5

10

15

20

25

30

35

Austenite

+ martensiteAustenite

Martensite

+ -ferritemartensite

Ausatenite + martensite + -ferrite

-ferrite

Ausatenite + -ferrite

Ni e

quiv

alen

t (%

)

Cr equivalent (%)

Creq = (Cr) + 2(Si) + 1.5(Mo) + 5(V) + 5.5(Al) + 1.75(Nb) + 1.5(Ti) + 0.75(W)= 21.37Nieq = (Ni) + (Co) + 0.5(Mn) + 0.3(Cu) + 25(N) + 30(C)= 33.514

4. FAILURE ANALYSIS OF COOLING FAN GEARBOX

IntroductionGearbox driving a cooling tower fan had led to an unplanned shutdown

VISUAL OBSERVATIONS

Two gears and two pinions submitted for analysis

Pitting, spalling and fatigue fractures seen on Gear 1 – beach marks can be clearly observed on the fracture faces Teeth Damage

MICRO-EXAMINATION

Tempered martensitic microstructureThe cracks formed an angle of about 45º with the surface. Note the corrosion pits on the tooth surface

Hardness examination

Shows the hardness profile at the tooth surface reached 645 hv compared to 421 hv at the middle of the cross section

(a–c) Pitting and spalling on the pinion teeth. Beach marks can be observed at the fracture surfaces. (d) Fatigue crack at the bottom of

the spalled region.

ANALYSIS OF FAILURE CAUSES

Pitting taking place on Gear 2 teeth. Note the pitting corrosion on the teeth cross section

• Conclusions• It is quite clear that gears failed by contact

fatigue accelerated by high alternating loads. • The increase in the gearbox start and down

frequency played a major role in the failure. • corrosion was not thought to have been a

major contributing factor as no cracks were found to have originated from the pits