Page 79 - Handbook of Materials Failure Analysis

1 Introduction 73

A new generation of reformer tube alloy began to be used in the 1980s, charac-
terized by the presence of Nb, in the range 0.8-1.2%, as carbide-forming element
stronger than Cr; moreover, the addition of cobalt and tungsten enhances the effects
of both solution and precipitation hardening, improving creep resistance [14].
As pointed out in literature, a fine dispersion of carbide particles can be stabilized
by modifying the HP grade composition adding stabilizing elements, such as the
aforementioned niobium, titanium up to 0.8% [15], or yttrium about 0.3% [16].
The outcome of this microalloying is a strong refinement of microprecipitates and
improved carbides stability [14].
Starting from the early studies of Wen-Tai and Honeycombe [17], new austenitic
alloys were developed adding, in addition to niobium, microalloying elements from
groups III and IV of the periodic table (Ti, Ta, and Zr) and also rare earth elements
(Cs, Nd, Pr, and Hf). These elements usually do not exceed a content of 0.5%,
because it was experimented that when taken individually their effect on creep resis-
tance may be contained, whereas taken jointly they show a synergic effect at lower
concentrations and for given proportions. For example, the best combined effect of
the joint addition of Nb and Ti is reached with a proportional share of their atomic
contents equal to Ti/(Ti+Nb) 0.5 [14].

1.2 PREDICTION OF CREEP LIFE BY MEANS OF THE
LARSON-MILLER PARAMETER

Material selection for reformer tubes needs a complete understanding of the risk of
probable failures caused by cracks within tubes during long-time service, as well as
during the thermal cycles associated to the planned start-up and shut-down stages of
plants.
Since the Second World War, materials researches in the field of heat-resistant
steels and alloys were addressed to investigate creep properties, performing tests, and
collecting data by public institutes and within industry.
Table 4.1 summarizes chemical composition and relative strength of heat-
resistant alloys developed during the last 50 years.

Table 4.1 Heat-Resistant Alloys Compositions Developed During
the Last 50 Years
Chemical Composition (wt%)
Other Relative
Decade Cr Ni C Si Nb elem. Fe Strength [14]
1960s 25 20 0.2-0.6 1-2 bal. 1.0
1970s 25 24 0.2-0.6 1-2 bal. 1.4
1980s 23 22 0.2-0.6 1-2 1 Ti bal. 1.7
1990s to 25 35 0.2-0.6 1-2 1 Ti, Zr, W, bal. 2.2
day and Cs