Page 56 - Handbook of Materials Failure Analysis

50 CHAPTER 3 Boiler tube failures: Some case studies

FIGURE 3.1
Typical cross section of large drum-type utility boiler, showing major water and stream-cooled
tube circuits [3].

Exposure of the tube steel to elevated temperature causes microstructural changes
of the steel constituents [4–6]. As for example, carbon steel will experience spher-
oidization in which iron carbide particles change from plate or rod shapes to more or
less spherical ones. Plain carbon and carbon molybdenum steel will experience
graphitization in which Fe 3 C particles transform into free iron and graphite mole-
cules. Austenite stainless steel will develop carbide precipitates and intermetallic
compounds. Microstructural changes occur even at design conditions with long-term
service. Their formations are accelerated when tube steel is operated above design
conditions.
Much of work has been carried out and still going on regarding boiler tube failure
mechanisms by renowned organizations and individuals. Some of the organizations
who are in the field are Electric Power Research Institute (EPRI, USA), Central Elec-
tricity Generating Board (CEGB, UK), ASME Boiler and Pressure Vessels, USA,
Combustion Engineering, USA, etc. Failure of boiler tubes is the prime reason of
boiler outage in most of the countries. Boiler tube failures are of immense concern
to utility companies and boiler manufacturers. There are many reports regarding fail-
ure modes and mechanisms. Damage mechanisms which mostly contribute the