Page 10 - Handbook of Materials Failure Analysis

2 CHAPTER 1 Progressive failures of components

technical/economic activity offering significant potential in problem-solving viable
growth.
Metal components exhibit lower service life when operated at elevated temper-
atures. Depending on the particular environmental and loading conditions, the most
common causes of elevated-temperature failures can be classified as creep-
originated, environmentally induced fractures, high-temperature fatigue, and ther-
mal fatigue [2]. Creep is realized through the nucleation and growth of transgranular
or intergranular voids depended on the applied operating conditions, that is, stress
and temperature [3]. Creep is a progressive time-dependent deformation which leads
to final rupture with potential catastrophic consequence and implications to health,
safety, and environment. Remaining life-assessment techniques are a valuable asset
in failure prevention and cost minimization of high-temperature working equipment.
Methods using the Larson-Miller parameter (LMP), expressed in Equation 1.1, life
fraction rules, and damage accumulation conditions are very popular in remaining
life prediction [4,5], along with finite-element analysis (FEA) modeling presented
in the relevant literature [6]

LMP ¼ T 23 + logtÞ (1.1)
ð
where T is the operating temperature (K) and t is the service time (h).
Larson-Miller creep rupture curves present the stress variation as a function of
LMP. Knowledge of stress and temperature conditions could therefore lead to the
determination of the estimated creep life with at a certain statistical confidence level.
Life fraction is used for the estimation of remaining life of structural elements
exposed at high temperatures, according to Equation 1.2:

t op t test
+ ¼ 1 (1.2)
T op T test
where t op is the operating time, T op is the total operating time at service conditions,
t test is the time for rupture at the test under accelerating conditions, and T test is the
time for rupture of fresh component under accelerating conditions.
The commonly referred eight forms of corrosion were addressed in the classical
corrosion literature [7]. The issue of stress corrosion cracking (SCC) of stainless
steels is a critical research field due to the extensive use of such materials in special
applications and the associated risks (unexpected failure without warning, severe
impact to human safety and environment). It is a dominant failure mechanism for
metallic components in unit operations industries that embrace many types of phys-
ical and chemical processes, under elevated temperatures, high pressures, and chem-
ically reactive environments. SCC of a stainless steel spiral heat exchanger in a
distillation column has been attributed to the action of chloride ions in the cooling
water, compromising the integrity of the protective oxide layer, producing corrosion
pits and facilitating crack formation [8]. SCC was also attributed to the detrimental
effect of chloride ions as reported in the case of failure of an AISI 304 styrene