Page 390 - Handbook of Materials Failure Analysis

388 CHAPTER 15 Welding-associated failures in power boilers

1 MATERIAL APPLICATIONS IN POWER BOILERS –
INTRODUCTION

The function of a boiler is to produce superheated steam by transferring the heat pro-
duced by the combustion of oil or natural gas to water and steam flowing through the
tubes that form the boiler. The super-heaters consist of banks of tubes suspended in
the combustion gas flow path in the upper parts of the boiler. The steam flows
from the super-heaters through the main steam header to the main turbine, where
it drives the turbine-generator. The service conditions involved in boilers, which
are mainly governed by alternating high temperatures and induced thermal stresses;
residual stresses in welds; as well as the harsh corrosive environments, promotes fail-
ure causes such as thermal fatigue; stress-corrosion cracking (SCC); corrosion; and
premature creep failures due to stress raisers at welding defects.
In high-temperature service (345-815 °C), and in the design of pressure vessels and
piping, engineers and designers are confronted with the problem of selecting materials
for a wide range of service conditions. The chromium molybdenum ferritic and austen-
iticstainlesssteels(SSs)aregenerallyusedfordesigntemperaturesabove425 °C[1].In
addition to service temperature, corrosion resistance, and fabricability, the following
conditions should beconsidered in high-temperature applications: allowable maximum
temperature and stress, type and value of load, expected life of the structure, and cost.
Austenitic SSs are generally used for design temperatures above 425 °C, the
ASME code, section VIII, division 1, shows the SS grades that are suitable for
the normal temperature range of usage (345-815 °C). Also some creep-resistant mar-
tensitic and ferritic SSs are used for high-temperature applications. The duplex stain-
less steels (DSSs) with both ferrite and austenite phases have gained increasing
interest in recent years in power generation applications in chloride-containing solu-
tions due to their better resistance to localized corrosion compared to single-phase
austenitic SSs [2–4]. Welded DSSs are used at temperatures up to 250 °C. The
increase in the thermal efficiency of fossil fuel-fired steam power plants, that can
be achieved by increasing the steam temperature and pressure, has provided the
incentive for the development of heat-resistant steels with excellent creep properties,
as well as superior oxidation and corrosion resistance properties [5]. In the last two
decades and to face up these requirements; several new Cr-Mo and 9-12% chromium
steels were developed ranging from P11 (1Cr-0.5Mo) to P122 (12Cr-1Mo) [6].
Table 15.1 summarizes the main types of steels used in pressure vessels with their
working conditions.
Table 15.1 The Main Types of Steels Used in Pressure Vessels
with Their Working Conditions
Steel Type Working Conditions
Ferritic stainless steels 430-510 °C
Austenitic stainless steels 595-815 °C
Duplex stainless steels Temperatures up to 250 °C, with
chloride-containing solutions
Creep-resistant martensitic steels 510-650 °C, with oxidation environments