No. 37

1

Gas Metal Arc Narrow-Gap Welding of Pressure Vessels

Made from the Nickel Alloy 2.4663

IllXA0055847

by K. Iversen and A. Palussek

1. Introduction

The highly heat-resistant nickel alloy 2.4663 is used

for the construction of test components of the nuclear

heat reactor PUP as structural material. This material

is given preference for strength reasons in particular

in the range of peak temperatures of 950 °C at pressures

up to 40 bar.

Since no construction and operation experience is yet

available with primary components for the process heat

reactor, test components shall be developed, manu-

factured, and tested. These works are sponsered by the

German Minister of Virtschaft, Mittelstand und Verkehr

of the state of Nordrhein-Westfalen. With the

helium intermediate heat exchanger, two 10 MW types

come under consideration, these being the helical tube

and the straight tube versions. The hot gas collector

component part places the highest demands on the wel-

ding and testing technology. Workpieces of 1000 mm

diameter and wall thicknesses of 42 to 100 mm are to

be forged from material 2.4663, to be joined together,

to be nondestructively tested and to be tested in a

largescale test plant under operating conditions.

2. Design and materials of a hot gas collector test model

Before the construction of the two 10 MW heat ex-

changers, a hot gas collector model shortened in the

longitudinal direction with original wall thicknesses

and diameters was manufactured amongst other things

for the development of the manufacturing technology,

in order to gather in good time experience for the

construction of the components.

Figure 1 shows the design of the hot gas collector

model. Three welds are to be made in the wall thickness

range between 35 - 70 mm. The nickel alloy 2.4663 was

selected as base material and filler wire of the same

composition 1.2 mm diameter was chosen.

Table 1 shows the chemical analyses of base metal, filler

wire and welded joint.

All figures had been within the standards. The burn-out

of the elements aluminum and titanium amounted to 0,19 %•

Therefore limitations of titanium and aluminum to £1,5 %

from the point of embrittlement of the deposited metal

is not necessary according to the latest experiences.

Maximum values of Ti + Al ~ 1#9 % (Al max 1.3 % and

Ti max 0#6 %) were permitted.

3. Welding and testing the hot gas collector model

3.1 Selection of the suitable welding process for the

manufacture_of_the_circular_welds

The manufacture of pressure vessels for the reactor con-

struction necessitates the use of proven reliable welding

procedures. However, with the selection of the welding

procedure neither tested electrodes nor powder for sub-

merged arc have been available for the material 2.4663*

Experience had been gained only for the manual arc wel-

ding processes TIG and GMA each with argon as shielding

gas.

Other welding procedures with high deposition rates like

electro-slag and submerged-arc process could not be used

by means of their hot cracking sensivity caused by the

high heat input. On the other hand efficieny must be an

extremely important point of view if new components are

to be developed. Therefore the decision was made to chose

narrow gap welding with an inert gas.

With expensive base metals and filler wire as well as

high labour costs, the gap width and the side wall angle

effect decisively the quantity of the weld metal to be

filled in, the melting rate and the manufacturing costs.

This applies in particular with larger wall thicknesses,

Fig. 2. According to experience made so far with the nickel

alloy 2.4663, however, the intermediate layer temperature

should be limited to 150 °C, the weld pool size should be

small and overheating of the weld puddle should be

avoided. This led to the selection of a GKA process with

a melting rate of maximum 3 - 5 kg/h. Therefore orLJ-y

single wire processes came into consideration.

For reasons including experience available with pressure

vessels in Japan, the Babcock-Hitachi oscillation flap

process was chosen /~1_7. Fig. 3 and 4 shows the method

of process operation C^.J\ f^bj*

3.2 Qualification 2£_i^£_£§§_S®5ai_§£c_Ba£r!2HzS§2 E£°£eSS

Unfortunately no experience was available for the pro-

duction of circular welds on forged shells of the nickel

alloy 2.4663 with the above-mentioned narrow-gap process.

The same applied for stainless Cr-Ni steels too. There-

fore suitability for welding had to be proven and the

first parameters had to be found with plate samples of

s » 60 mm by fundamental research at the ISF in Aachen/~4^7-

Here it was demonstrated that neither the commercially

used gas mixture of 82 % Ar/18 % COp nor pure argon came

into consideration because of the high arc length and the

too small side wall penetration, Fig. 5« Only the use of

pure helium (99*995 °/°) in combination with the pulse

technique led to a short arc and to a low risk of burn-

back as well as to reduced spattering in the 9 - 11 mm

wide gap. The side wall penetration could be increased

decisively from approx. 0.5 nua to>1 mm and produced

sound welds, Fig. 6.

The low density of the helium had a disadvantageous effect

in the narrow gap because of amongst other things the

formation of very adhesive titanium, aluminum and chromium

oxides on the layer surfaces which could only be removed

by grinding.

At the welding equipment itself, the wire feed system

above all had to be adapted to the significantly stiffer

nickel wire (2.-4-663). The tests in the ISF in Aachen were

concluded successfully after the above-mentioned improve-

ments C^l' The plate samples had to be provided with

an angle of 18° out of the flat for the start of welding

by means of shrinkage.

The continuation of the tests at Interatom took place

initially on circular seams 1000 mm o.d. x 125 mm wall

thickness of 2.4-663. For this the ISF welding data were

taken over and the welding system was completed appro-

priately, Fig. 7. However, because of the impeded

shrinking, heat cracking occurred in the center of the

weld. Only by reducing the layer height, the heat

absorption, the weld pool overheating and the weld pool

size welds could be done successfully without cracks.

Before the optimum data were achieved, single pores and

lack of fusion were found by means of radiographic and

ultrasonic testing. The first circular weld was followed

by the german official TuY process inspection with the

same dimensions.

The root path had been welded manually from the inside

while fillet and the final runs had been welded from the

outside automatically by the narrow gap G-MA process.

For the manual weld Nicrofer S 5520 of 2,4- mm 0 was used

while the automatic GMA process consumed Nicrofer S 5520

wire of 1,2 mm 0 diameter.

All welds had been done in the horizontal position while

the workpiece was turned and the torch was stable. The

opening of the weld preparation was app. 2° to avoid the

squeezing of weld head which was just 7 mm in width by means

of transverse shrinkage, Fig. 8. The opening of the root

srf

gap on the surface was app. 17,8 mm before welding and

lowered down according picture 8 to app. 14 mm before

welding the last path, Fig. 9. The narrow gap welding

with the Babcock-Hitachi-Process is capable of one path

per layer for a gap of 9 - 15 mm and two pathes per layer

till 18 mm.

During this inspection, influences on the welding results

such as not grinding the bead surfaces, welding over of

start and stop points, working out and filling in local

repair places in seam depth and seam width were also tried.

It was shown that welding over ungrinded layers led to

cracks and lack of fusion. All filling layers and the

repair simulations met the required values for BS

according to DIN 8563> Part J. For tbis, however, all

parameters had to be matched exactly to one another and

kept within a close tolerance band. The computer-assisted

welding data monitoring system was here of outstanding

benefit.

After completion of the test weld (81 layers) it was

x-rayed by a linear accelerator. An evaluation of the

films did not lead to a conclusion about weld quality.

It must be stated that good results can be obtained

til 80 mm wall thickness only. The evaluation of the

ultrasonic testing, manually as well as automatically,

Fig. 10, did not show any defect outside tolerance

band. The dye check did not show any defects too.

According to the fixed plan of specimen the forged ring

had to be split up. Six cross sections had to be

examined by TtJV while another 39 had been studied by

INTERATOM. Macro sections did not show any defects

without some minor pores, Fig. 11. Micro sections

(v » 50 or 100:1) did show layers according to speci-

fication but with the exception of some lower area,

where the layers had not been grinded.

In that ungrinded and unsound area lack of fusion as

well as porosity had to "be stated. Most of these defects

had been detected already during x-ray and ultrasonic

testing. The figures of the other testing procedures

are given in table 2.

- £ound_tensile S2ecimen_according_to_DIN_50125

The requirements for the base metal values of Em 2"

700 N/mm2 and Ep 0,2 - 300 N/mm2 were met extremely

close because the lowest value was 302 N/mm . On the

other hand the values of the deposited metal (4-23 N/mm )

as well as the welded Joint (486 N/mm ) exceeded the

specification by far.

The requirements for the yield point had been exceeded

with Ep 0,2 = 328 N/mm but the figures for the ulti-

mate tensile strength did not meet the requirements.

The rupture had been secured in the base metal beyond

the joint. Therefore the weld had met the requirements.

former - 0 : 3 x wall thickness

The following specimen had been taken:

" out of a repaired section bend test specimen

had been taken two for root bend testing and

another two for final layer bend test

* four bend test specimen (two of each for root bend

test and final layer bend test) had been taken from

all around the weld. All specimen did show 180

bend angle without any cracks.

7

notched tar_impact_test_according to_DIN 50115

The specimen had been taken from a place of repair

and from other areas all around the joint. The

requirements of - 40 J was met by all specimen. The

lowest value was shown with 71 J at the weld edge

while the highest was shown with 151 J in the deposited

metal. It can be emphasized that narrow-gap-welds do

show much better results than there is the demand

according to HP 2/1.

With these results the process qualification for the

narrow .gap welding with the single wire snake-wave

process (VP 70) was successfully finished.

According to Fig. 1 three circular seams of different

wall thicknesses (35? 42 and 70 mm) had to be produced.

Narrow-gap welding took place with the welding data proven

in the above-mentioned process inspection. About JO layers

were required for the 70 mm wall thickness, this meant an

average layer height of 2.*? mm. The heat input was 14 kJ/cm,

the welding speed 30 cm/min and the melting rate app.3»8 kg/h.

Radiographic inspections were carried out after about

4-0 mm of weld metal and after laying the final layer

(70 mm). The results of the radiation testing and the

results of the manual as well as computerassisted

mechanized ultrasonic testing, Fig. 9 did show good

harmony.

With the exception of some single pores neither lack

of fusion nor cracks had been detected. After finishing

of the welds (T) and (2) the workpiece had to be oval

shaped followed by welding (seam (^) ) to the bottom,

figure 12.

Fig. 13 shows the figures of transverse and longitudinal

shrinkage depending on wall thickness.

8

4. Summary

The GMA narrow-gap welding process with helium as

shielding gas showed circular welds free of defects up

to s = 126 mm with the nickel alloy 2.4663. The commer-

cially available pulse power source had to be modified at

the shielding gas supply and the wire feed. The selection

of the welding parameters had particular significance. It

was shown that with shrinking-impeded welding of the

circular welds, an overheated weld pool must absolutely

be avoided and that a small weld pool size must be main-

tained. The nickel alloy 2.4663 which contains aluminum

and titanium necessitates grinding of the single layer

surfaces.

However, it remains for the future to show with further

applications if a technical breakthrough of the narrow-

gap welding process with the manufacture of thick-walled

pressure vessels of stainless Cr-Ni steels and nickel

alloys can been made.

5. References

/""1J7 S.Swada, K.Hori, M.Kawahara, M.Takao, I.Asano;

Application of Narrow-Gap Welding Process;

AVS 60th Annual Meeting, 5 April 1979

</"~2 7 C.Ferling; Gas metal arc narrow-gap welding,

a new technology for joining thick plates;

study project of the ISP Aachen, Jan. 1981

/"~3 7 K. Iversen; Review of the process of narrow-gap

welding; Colloquium narrow-gap welding

SLV Duisburg, June 1982

/~4_7 P. Eichhorn, P.Groger; Gas metal arc narrow-gap

welding tests on austenitic chrome-nickel steels

and nickel based alloys; DVS report Vol. 75, 1982

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Properties of Base Metal, Filler Metal andWelded Joints (Narrow Gap )

weld (1

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1350 1020

\ \ \ \ _.\i \ \ \ \ \ 1

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i>zzzzzzz^

Fig. Hot gas collector

T3

6

Y=[b-S+2sZ^H)x length

20 40 50 30 100 120 140 160 *80 200 220 thickness [mm]

Fig: 2 Seam volumes as a function of wall thickness and

aoerature anale

We !<Jinq d)tec ft o n

Fig.3 Process with oscillating flap - a) Flap plate

b) Feed rollers

c) Contact nozzle

d) Upper gas protection

e) Welding gap

f) Wire electrode

g) Water cooling

h) Front gas protection nozzle

i) Rear gas protection nozzle

input

vD wire feed speed

8 wire deflection

fp oscillation frequency

output

X wave length

Y oscillation

Wire. Def6rsn/r?3

•Jff

Fig. 5 Penetration depending upon type of shielding gasBase metal: X 5 CrNi 18 9

Filler Metal: X 5 CrNi 18 9

The first 6 layers were welded with argon, thefollowing layers with helium

I

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j j j

1 Helium

I

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Fig. 6 Influence of the shielding gas on the arc lengthand side wall penetration

Fig. "$- G'tA narrow gap welding equipment

81 pathesMIG -Narrow Gap

mTIG ( 2 p a t h e s )

Wsld preparation for MIG-Narrow-Gap

Groove widthon vesselsurface

[mm]

50 60 70 80 125Welded thickness [mm]

Shrinkage of MIG-(Narrow- Gap)-Welding

Fig 1o Fully mechanized ultra-sonic testing of the hot gascollector with computer evaluation

Fig. 44 Narrow gap weld (wall thickness 12^ nun)

Fig 72, H o t <3as collector with 3 circular welds

Transverseshrinkage

Longitudinalshrinkage

125 [mm]wall thickness

a Longitudinal and transverseshrinkage depending onwall thickness