56 METALLURGY AND CORROSION CONTROL IN OIL AND GAS PRODUCTION
Martensitic Stainless Steels Martensitic stainless
steels are used in upstream production and pipelines
more than any other class of CRAs.
58 – 61
There are two
reasons for this: the martensitic structure produces
strong, tough alloys and, compared with other stainless
steels, the martensitic alloys have the lowest alloying
content and are thus less expensive. They can also be
heat - treated to approximately the same strengths as
carbon steel tubular goods.
Martensite is produced when high - temperature aus-
tenite is oil - quenched to martensite transformation
temperatures, between 250 ° C and 350 ° C (440 ° F and
660 ° F) depending on alloy composition. The alloys are
then tempered to improve toughness. API casing,
tubing, and line pipe specifi cations identify two differ-
ent chromium contents for iron - chromium alloys — 9%
and 13% chrome. The 9% chrome has insuffi cient alloy-
ing to be a true stainless steel and is seldom specifi ed.
The use of 13% chrome alloys has increased in recent
years due to increased production in more aggressive
environments and concerns that the cost of corrosion
monitoring, corrosion inhibitors, repairs and, most
importantly, lost production, outweigh the added cost
of CRAs. As one example, the materials cost of a carbon
steel subsea pipeline, including both metal acquisition
and welding, is approximately 25% of the total cost.
Changing to CRAs doubles the materials cost, but it
only increases the total initial cost of the pipeline by
approximately 25%. Many operators consider this addi-
tional initial cost justifi ed.
Martensitic stainless steel tubular goods are usually
specifi ed using API specifi cations, while other industrial
specifi cations based on UNS or other international stan-
dards are common for other applications.
58
Table 4.5
shows some of the most commonly used martensitic
stainless steels used in oil and gas production.
The limited alloying content of martensitic stainless
steels means that while they are more corrosion resis-
tant than carbon steels, they cannot withstand aggres-
sive environments. They are mostly used in applications
where CO
2
corrosion is a problem and H
2
S, if present,
is at relatively low concentrations. NACE RP0175/
ISO15156 places limits on the hardness that these alloys
can have in H
2
S service depending on alloy type and
application.
12
The amount of chromium that can be added to mar-
tensitic stainless steels is limited because austenite,
which is necessary for heat treatment to produce mar-
tensite, does not form at chromium contents greater
than 12 – 17% Cr. The upper limits are determined by
other alloying additions, and the highest commercially
available martensitic stainless steel routinely approved
for H
2
S service, UNS S42500, has 15% chrome.
12
Iron -
chrome alloys with higher chrome contents have ferritic
Iron - Nickel Alloys
These alloys are more corrosion resistant than carbon
steels, but their relative ductility at low temperatures is
the more important reason for their use in oilfi eld
applications.
Iron - nickel alloys are used for above - ground cryo-
genic storage tanks for LNG, because these alloys have
suffi cient ductility at LNG temperatures ( − 162 ° C
[ − 260 ° F]). The addition of nickel to iron results in alloys
where the transition of high - temperature austenite to
α - ferrite is retarded. Alloys having 5 – 10% nickel will
have a mixture of high - iron α - ferrite, which is subject
to embrittlement at low temperatures, and austenite,
which is ductile at the temperatures found in LNG
storage and similar applications. The dual - phased struc-
ture still has ferrite, but the resulting alloy is ductile
enough for static structures like storage tanks.
LNG storage tank walls are typically constructed
from welded iron - nickel alloys (9% nickel alloys are
most commonly used),
51 – 54
with piping and similar
attachments made from austenitic stainless steel, which
is more expensive, but has better resistance to thermal
fatigue. Welds on these LNG tanks use nickel - based
alloys.
55,56
Corrosion is not a problem at cryogenic tem-
peratures, so galvanic coupling between nickel steel and
stainless steel is not a problem.
Austenitic stainless steel and aluminum alloys are
also ductile at LNG temperatures, and they are some-
times used for building smaller storage tanks, but large
containment vessels are usually welded from iron - 9
nickel because of expense considerations. This tech-
nology has been in use since the 1940s and is
well - established worldwide.
Stainless Steels
Stainless steels are usually defi ned as alloys having a
minimum of 11% chromium in addition to other alloy-
ing additions.
57
The names attached to the various
classes of stainless steels usually derive from the pre-
dominant crystal structure that determines their
mechanical properties. Stainless steels can have a wide
variety of mechanical properties, but the reason for
using them is for corrosion resistance, and this will be
emphasized in the following discussions.
The commonly recognized classes of stainless
steels are:
•
Martensitic stainless steels
•
Ferritic stainless steels
•
Austenitic stainless steels
•
Duplex stainless steels
•
Precipitation - hardening stainless steels
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