Damage Investigation On Welded Tubes
Damage Investigation On Welded Tubes
Damage Investigation On Welded Tubes
In this work the creep damage of radiant tubes of a reforming furnace has been investigated. The considered furnace
contains a battery of tubes constructed by butt welding three spun cast pieces, made of ASTM 608 HP-Nb alloy.
They are designed to operate at temperatures of about 900°C, pressures of about 30 bars and times of the order of
100000 h. Tubes were inspected during the plant stops scheduled every two years, in order to identify and replace
the damaged ones with the aim to ensure conditions of safe operation in the furnace. They were selected though a
criterion based on measures of the internal diameter deformation performed in situ by Laser Optic Tube Inspection
System (LOTIS). For a verification of this method, optical and scanning electron microscopy observation, Vickers
microharndess and creep tests have been carried out on samples taken from tubes put out of service.
E. Guglielmino, A. Sili
Dipartimento di Ingegneria Elettronica, Chimica e
Ingegneria Industriale – Università di Messina
a) b)
R. Pino
Raffineria di Milazzo S.C.p.A Fig. 1 – Assembly of a tubes row (a) and detail of a
single tube (b). The arrows indicate the gas flows
C. Servetto
entering and leaving the tube.
IIS Service Srl, Genova
a) b)
Fig. 2 – Examples of radiant tube damage: a) creep expansion of a tube section; b) large longitudinal crack
across a butt welded joint.
Therefore good mechanical properties and corrosion resis- mal cycles associated to the furnace start-up and shut-
tance at high temperature are requested for radiant tube down. So adequate means of investigation giving reliable
alloys [3]. These materials have seen significant improve- information about tubes damage are required.
ments during the last fifty years: in the 60s and 70s the al- Nowadays reforming furnaces rely on the Laser Optic Tube
loy 25Cr-20Ni-Fe-0.4C, designated as HK-40, was the most Inspection System (LOTIS), a non-destructive control tech-
utilized, while in the following decades the HP-40 alloy nique based on creep deformation measures performed
(25Cr-35Ni-Fe-0.4C-1.5Si) has been widely considered [4]. in situ by driving a laser probe inside tubes [17]. It is very
The high concentrations of Cr and Ni give great mechanical promising for the development of a criterion for decom-
strength and corrosion resistance at service temperature, missioning tubes that would be no longer safe until the
the presence of Si improves carburization resistance. The next scheduled stop [18]. So furnaces have to be regularly
mechanical reinforcement of the HP-40 alloy is obtained inspected during each stop, in order to identify the dam-
by a dispersion of carbides particles with high hardness [5] aged components and replace them. The radiant tubes
whose stability is decisive for creep behavior [6]. Moreover, are visually observed with the aim of identifying macro-
because it has been tested that microalloying elements are scopic damage; creep deformations are detected through
able to stabilize a fine dispersion of carbides, starting from LOTIS measures in order to select the tubes to put out of
the 90s HP-40 alloys have been developed with addition service for safety reasons.
of Nb (about 1%) [7], Ti (up to about 0.8%) [8] and Y (about In previous works [18, 19] laboratory tests were carried
0.3% [9] to improve their creep behavior. out to correlate mechanical properties and microstructur-
Tubes life is shortened by creep damage, being character- al changes in tubes decommissioned after long time ser-
ized by progressive microstructural changes [10], as car- vice. In this paper the experimental investigations have
bides transformations, microcracks nucleation and forma- been extended to the butt welds that join the tube pieces.
tion of voids, typical of the final creep stage that precedes Samples for creep tests and microstructural observations
fracture [11, 12]. were cut from various sections also including welds.
Unfortunately, during the furnace scheduled stop, tubes
have to be decommissioned to cut metallographic sam-
ples and perform microscopic observations and me- MATERIALS AND METHODS
chanical test. However, because each single tube is very
expensive and its damage conditions are not necessar- As shown in the working drawing (Fig. 3), radiant tubes are
ily representative of the other tubes, many experimental constructed in three butt welded pieces (W1 welds); the
works have been addressed to non destructive methods, first one (located in the upper vertical position inside the
such as eddy current and ultrasonic measurements [13, furnace) is welded to the upper flange (W2), the third one
14, 15]. Moreover, being these investigations affected by is welded to the reducer (W1), in turn welded to the outlet
many uncertainties, studies on the relationship between tube (W3); other welds concern the gas inlet nozzle (W4)
microstructural degradation and mechanical properties and the catalyst support grid (W5).
are largely quoted in literature [2, 11, 16]. In this respect it Radiant tubes have an internal diameter of 101.6 mm, a
is worth noting that the residual life evaluation depends on nominal thickness of 10.5 mm and an overall length of
actual service temperatures (measured by optical pyrom- 12.8 m. The design conditions are pressure of 32.7 bar
eters with an error of +/- 20 °C), mechanical properties and temperature of 950°C, while the operating tempera-
values after long time service, actual stress state and ther- ture is 900°C.
Fig. 3 – Working drawing: a) upper flange, b) radiant tube constructed in three pieces, c) reducer, d) outlet tube,
e) catalyst support grid, f) gas inlet nozzle.
The three pieces of the radiant tube (b), the connection Alloy C Si Mn Cr Ni Nb Ti Fe
of the gas outlet (c) and the catalyst support grid (e) are
made of the ASTM 608 HP-Nb alloy, a HP alloy modified by A 608 HP-Nb 0.45 1.5 1 25 35 1.5 add. bal.
Nb micro additions; the upper flange (a) and the gas inlet Table 1 – Composition given by the manufacturer of
nozzle (f) are made of the ASTM A182 Type F22 steel; the the alloy.
gas outlet tube (d) is made of the ASTM B407 UNS NO8811
Incoloy.
The composition of the ASTM 608 HP-Nb alloy is given Root pass Subsequent Final pass
in table 1. The microstructure of the spun cast pieces is passes
characterized by radial austenitic dendrites, that are well Current DC DC DC
delineated by carbide particles precipitated in the inter-
Electrode polarity negative negative negative
dendritic spaces.
Welds are performed by means of GTAW, utilizing pure Ar Intensity (A) 140-170 100-150 100-130
as shielding gas. Fig. 4 shows the edge preparation for Voltage (V) 14-16 14-16 14-15
W1, W2 and W3 butt welds. Welding parameters are given Welding speed 4-7 6-9 5-8
in table 2. (cm/min)
a) Tungsten electrode Type WS2 Ø 2,4 mm
Filler material UTP A2535Nb (25Cr-35Ni-1.2Nb)
Ø 2,4 mm rod
Table 2 – Process parameters of the W1 welds.
Following a criterion developed in previous work [18, 19], between 920 and 980°C. In order to achieve acceptable
the tubes where deformation reached values greater than rupture times (between 200 and 1000 hours), the test
ε = 1.5% were put out of service and utilized to take the stress was assumed in the range 22-30 MPa, greater than
experimental samples. the hoop stress at the design pressure (σ = 17.5 MPa).
The decommissioned tubes were visually investigated for a For a useful comparison, in fig. 5 the creep test results
first inspection of their damage conditions. are plotted on a Larson Miller diagram (σ vs. LMP), being
Creep tests were performed on samples cut longitudinally σ test stress and LMP = T (C + log t) / 1000, with T (K)
from tube put out of service after about 100000 hours test temperature, t (h) rupture time and C=22.9 a constant
and, for comparison, from samples taken from a tube in characteristic of the considered material [10].
the as cast condition. Samples with welded joint were Moreover both average and minimum curves characteris-
also machined. tics of the A 608 HP-Nb alloy are given in the same dia-
For microhardness test and microscopic observations gram as reported in the manufacturer catalogue [20].
samples were metallographically prepared with the usual Creep tests results can be summarized as follows:
techniques and etched by a solution containing 15% of - the experimental point of the as supplied material (blue
glycerol (100% concentration), 45% of HNO3 (65% concen- triangle) is on the minimum curve of the manufacturer
tration) and 40% of HCl (37% concentration). catalogue,
Considering that the results of LOTIS measurements - results of samples taken from undeformed zones (yellow
showed inhomogeneous deformations along the tubes rhombus) are similar to the as supplied material,
axis, experimental surveys were carried out on samples - results of samples taken from deformed zones (blue cir-
cut longitudinally both from sections with high deforma- cle) are well below those of the undeformed material,
tion (1.5-2.0%) and from undeformed sections, in order to - samples cut transversely to the welded joint (red circle)
put in comparison their microstructure and creep strength. behave in agreement with the previous ones.
Samples of welded joints were also cut. The experimental diagram σ vs. LMP shows that speci-
Metallographic samples were observed optically and by mens with welds have behavior similar to the base mate-
scanning electron microscopy (SEM) with energy disper- rial with the same diameter deformation. However they
sive X-ray spectroscopy (EDS). differ in creep ductility: rupture elongation in creep test
resulted around 3-5% for samples with welds, while it was
20-30% for samples of base material.
RESULTS AND DISCUSSIONS The residual life of decommissioned tubes can be ob-
tained thanks to the curve extrapolated from experimen-
LOTIS measurements tal data: entering with the hoop stress σ = 17.5 MPa, the
The internal diameter creep expansion along each tube was corresponding LMP value, equal to about 32.6, allows to
accurately recorded through LOTIS measures, performed calculate the rupture time at a given temperature. For
in situ with the aim of selecting the tubes to put out of ser- example, with temperatures of 900, 920 and 940°C, the
vice when ε > 1.5%. Results can be summarized as 78% of corresponding rupture times are respectively 10, 3.2 and 1
tubes with diameter deformations less than 1.5%, while in year. Then rupture time after long time service is strongly
the remaining 22% were recorded deformations exceeding dependent on temperature changes that are in the range
the established limit [18]. of measurement errors typical of pyrometer systems used
Moreover LOTIS measurements showed that diameter de- in reforming furnaces.
formation increases from the upper flange, where in gen-
eral it is negligible, up to the lowest zone of the tube. In Optical microscopy and Vickers microhardness test
the decommissioned tubes diameter deformation values After long time service, the A 608 HP-Nb alloy is character-
of 1.5-2.0% were recorded near the catalyst grid. ized by microstructure modifications that can be related to
the degree of diameter expansion.
Visual inspections The radiant tube walls undergo creep caused by circumfer-
The tubes put out of service after 114000 hours show ential stress and temperature and each zone has specific
highly oxidized surfaces along all their length and zones metallurgical features.
located in the lower part which have undergone creep Some considerations can be drawn with reference to the
deformations detectable also by visual inspection. These optical micrographs shown in figure 6 at the respective
zones are mainly near the catalyst grid, where the diam- stages of a typical creep curve.
eter expansion is clearly observable (fig. 2a). - The as cast microstructure is characterized by austenitic
dendrites, well outlined by a network of coarse carbides
Creep tests precipitated in the interdendritic spaces.
Results of creep test refer to samples in the as cast condi- - The microstructure of samples cut from undeformed
tion and samples cut longitudinally from both undeformed zones of a decommissioned tube is nearly similar to the
and deformed (ε=1.5-2.0%) zones of a decommissioned one of as cast alloy, so it can be referred to the first
tube. Results of samples with welds are also considered creep stage.
(cross-weld test). The test temperature (T) was in the range - The microstructure of samples cut from deformed zone
REFERENCES