Svetsaren 1/2007 - Esab

Disbonding of Austenitic Weld Overlays
in Hydroprocessing Applications
R. PASCHOLD, ESAB GMBH, SOLINGEN, GERMANY, L. KARLSSON, ESAB AB, GÖTEBORG, SWEDEN AND M. F. GITTOS, TWI, CAMBRIDGE, UK
Many large pressure vessels
operate at high temperatures and
at high hydrogen partial pressures.
These are typically fabricated from
low alloy Cr-Mo steel and internally
weld overlaid with austenitic
stainless steel. During shutdown,
hydrogen accumulates at the
interface between the cladding and
the parent material which
occasionally causes disbonding of
the stainless layer. This paper
discusses mechanisms, testing
and factors influencing the risk of
disbonding, focussing on welding
related aspects.
Large pressure vessels are used in hydrogen
containing environments, for example, in the
petroleum industry in hydrocracking,
hydrodesulphurisation and catalytic reforming
processes as well as in the chemical and coal
conversion industries. Many reactor vessels
operate at high temperatures and at high
hydrogen partial pressures, with 450°C and 15
MPa often being mentioned as typical values [1].
The vessels are generally fabricated from low
alloy, creep resistant steels [1, 2].
It is estimated that well over one thousand
hydroprocessing reactors have been fabricated
from the 2¼Cr-1Mo alloy, some few dozens from
the new generation vanadium modified 3Cr-1Mo
steel and a few from vanadium modified
2¼Cr-1Mo steel. Today, with hydrogen partial
pressures, in some applications, ranging as high
as 35 MPa, the new generation vanadium
modified steels exhibit service life improvements
and, in many cases cost advantages, in high
temperature and high pressure hydroprocessing
reactor applications [3].
Weld cladding overlays
All hydroprocessing reactors require internal
protection of the reactor vessel walls to resist
the high temperature corrosion effects of
sulphur in the process stream. This protection
is generally provided by stainless steel weld
overlays, typically a type 347 (18Cr 8Ni + Nb)
deposit. A stabilised 347 composition overlay
also prevents sensitisation during the final post
weld heat treatment (PWHT) cycle of the
reactor [2, 3].
to 8 or 10 % (or Ferrite Number: FN). Some
specifications also require disbonding tests to
be done by the reactor producer.
Strip cladding
Strip cladding by submerged arc welding (SAW)
(Fig. 1) or by electroslag welding (ESW) are the
preferred methods for cladding of larger areas
such as pressure vessels. Both methods offer a
high deposition rate, in terms of both kg/h and
area coverage (m 2 /h), combined with low
penetration and high deposit quality. However,
today, single layer electroslag strip cladding tends
to be more frequently used than double layer
procedures with submerged arc strip cladding.
Typical specifications for the cladding include a
chemical composition corresponding to AWS
EQ347 with a ferrite content in the range of 3
Figure 1. Strip cladding of vessel head by submerged
arc welding (SAW).
10 - Svetsaren no. 1 - 2007