Experience with Cast Material for SteamReformer Furnaces

The relationship between chemical composition andmechanical properties is discussed, as well as how they affect the weldability and weld-

ing procedures after aging of20Cr-32Ni-Nb cast alloy s in actual plants.

T. Shibasaki, T. Mohri, and K. TakemwraChiyoda Corporation, Kanagawaku Yokohama 221, Japan

Outline

Since 1968 Chiyoda has selected 20Cr-32Ni-Nb

material for outlet manifolds. To date more than

120 sets of outlet manifolds have been put into

service on 27 reformers with no reported problems.

In response to user's requests, we have performed

metal lurgical investigations on the used outlet manifolds

in order to evaluate creep damage and to confirm

their safety in operation. The results of the investi-

gations on used 20Cr-32Ni-Nb material were pre-

sented at the 1986 Boston Symposium, where it was

reported that the Si and Nb contents had a negative

effect on the ductility of used outlet manifolds*.

This paper is concerned with the history of outlet

manifold material and findings by our investiga-

tions on the relation of chemical composition to

mechanical properties and the weldability and welding

procedure for use after the aging of 20Cr-32Ni-Nb

cast alloys in actual plants.

History of Outlet Manifold Material

Alloy 800 material was firstly applied for a outlet

manifold for a NH3 Reformer constructed by

Chiyoda in 1965. The wall thickness of the outlet

manifold was thicker since the creep rupture strength

of Alloy 800 was low at higher design temperature

service.

HU40 material containing higher carbon content

of around 0.4 % was applied for the manifold as the

second stage material to improve the creep rupture

strength higher than the Alloy 800 material as

shown in Table 1 and the wall thickness was

reduced.

166

The creep rupture damages were occurred on the

HU40 manifold mainly at the thicker wall parts of

the reducer due to thermal stress.

HK40 material was applied as substitute material of

the HU40 material for the manifold from 1969 to

increase the creep rupture strength as shown in

Figure 1 and expect more thinner wall thickness for

the manifold. It was basic design concept that the

creep damage would be solved by lowering the

thermal stress since the wall thickness could be

reduced.

However, the creep damages were also observed at

the same portion as the HU40 manifold although the

wall thickness was reduced to thinner. The destruc-

tive examination was performed for the damaged

parts and it was found that the HK40 material was

aged after using and its creep rupture ductility was

lowered as shown in Table 1 due to precipitation of

secondary carbides in the matrix of the metal and the

thermal stress could not be reduced to lower level.

As the results of material studies, 20Cr-32Ni-Nb

material was selected as design standard material

for the manifold in 1973 and applied during these

two decades. The ductility of the material after

aging was improved keeping creep rupture strength

same level as HK40 material as shown in Figure 2

and 3, by lowering carbon content to around 0.1 %

and adding Nb based on Alloy 800 material.

The destructive examination of the used 20Cr-

32Ni-Nb manifolds showed that no creep damage

was observed on the manifolds since the thermal

stress could be reduced and relaxed to lower level

due to above creep properties after aging.

Mechanica! Properties of 20Cr-32Ni-Nb After

Use

Used 20Cr-32Ni-Nb materials taken from four sets

of outlet manifolds and three bottom reducers of

catalyst tubes were investigated. Table 2 gives the

list of materials investigated and Figure 4 shows

the location the samples were taken from. Table 3

shows the chemical analysis results for each sample.

There was a fairly wide distribution in the content

of C, Si and Nb among the samples.

Tensile tests at room temperature and at 800°C

were performed. The tensile elongations at room

temperature are plotted against the contents of C,

Si, Mn and Nb as shown in Figures 5 and 6. Since

the effect of an aging period of more than one year

is not that significant, the data are plotted neglect-

ing the aging period.

At room temperature, the tensile strength and elon-

gations tend to decrease as the content of C, Si and

Nb increased, and tend to increase as the content of

Mn increased. The elongations at 800°C are apt to

decrease as the content of Si and Nb increased. It

was found that, after use, Si and Nb had a negative

effect on the tensile property of 20Cr-32Ni-Nb

material at both room and high temperatures.

Creep rupture tests on used 20Cr-32Ni-Nb materi-

als with different chemical compositions on Sample

L2, T2, L3 and T3 were also examined. These

samples were taken from different parts of outlet

manifolds which were used at approximately 820°C

in plant B for 8 years and plant C for 10 years. The

main differences in the composition of the four

samples were:

167

Sample L2 : higher content of C, Si and Nb

(0.12% C, 0.69% Si, 1.1% Nb )

Sample T2 : lower content of C, Si and Nb

(0.07% C, 0.35% Si, 0.8% Nb )

Sample L3 : higher content of C, Nb and lower Si

(0.12% C, 0.43% Si, 1.17% Nb )

Sample T3 : higher content of C, Nb and lower Si

(0.12% C, 0.47% Si, 1.22% Nb )

The results are shown in Figure 7.

Although there was no significant difference in the

rupture strength at 800°C for sample T2, sample T2

showed a higher rupture strength than sample L2 at

900°C. The rupture strengths of sample T3 and L3

are between sample L2 and sample T2. It is note-

worthy that sample T2 had a higher rupture strength

even though its carbon content was lower than that

of sample L2.

Figure 8 shows the creep rupture elongations of the

samples. The creep ductilities were comparable for

the four samples.

It was also found that the tensile property and creep

rupture strength of used 20Cr-32Ni-Nb were supe-

rior in material with a low Nb and a low Si.

Weldabilitv of Used 20Cr-32Ni-Nb Material

Three types of weldability tests were conducted on

used 20Cr-32Ni-Nb materials. One was performed

to find the effect of temperature during the opera-

tion. The second was performed to observe the

effect of the chemical compositions of alloys. The

third was performed to select the optimum filler

metal. The test samples were the same as those for

the mechanical tests ( T2, L2, T3, L3 and L4 ).

Effect of Operating Temperature

A bead-on-plate-test was performed on the outer

surface of a Tee-piece and reducer of an outlet

manifold ( sample T3 ) where the metal skin's

temperature changed from 800°C to 250°C as

shown in Figure 9. The result of a liquid pen-

etrant examination of the bead-on-plate-test is

shown in Photograph 1. Indications of cracks

due to welding were observed in the regions

higher operating temperatures, that is the Tee-

piece and upper portion of the reducer of an

outlet manifold where the temperature may be

over 600°C. No crack indications were found on

the lower outer surface of the reducer, where the

skin temperature was less than 600°C.

Effect of Chemical Composition

The test conditions are shown in Figure 10.

There were no cracks on sample T2 ( lower C, Si

and Nb ) but cracks were found adjacent to the

fusion line of samples L2, L3 and L4 welds.

Photograph 2 shows the microstructure of the

bead-on-plate-test for sample L3. These results

were explained by the difference in tensile ductilities

between the samples.

Difference of Filler Metai

The bead-on-plate-test for sample L4 was per-

formed using two kinds of filler metal. One is the

same as the base metal ( 20Cr-32Ni-Nb material

168

). The other is a high Nickel alloy metal ( Inconel

82 ). Differences were demonstrated in the re-

sults of liquid penetrant examinations of the

bead-on-plate-tests as shown in Photograph 3.

There are many more cracks in the bead-on-

plate-test using the same metal filler rather than

the high Nickel alloy metal.

Microstructures of Used 20Cr*32Ni-Nb Mate-

rial

To investigate the reasons for different mechanical

properties after use of materials with different Nb

and/or Si contents of these alloys, microscopic

investigations wereperibmiedhaMtion,thecharacterization

of the precipitates in each of the materials was done

by means of a SEM-EDX ( Scanning Electron

Microscope and Energy Dispersive X-ray Spectros-

copy ).

The samples used for these investigations were

same as those of the creep rupture tests .

Photograph 4 shows the secondary electron image

of sample L3 and the results of EDX analysis for the

precipitates.lt seems that the coalescence rate of the

primary precipitates in the high Si, Nb material was

faster than that in the low Si, Nb material. SEM-

EDX analyses for the interdendritic regions of each

of the samples were also performed. Only one type

of precipitate was found in the portion which was

operated at below 250°C ( cold-end ) by SEM and

it was considered to be Nb-carbide ( NbC ) as shown

by EDX analysis.

region on Samples T2 and L2, both of which had

been used at high temperatures. In Sample T2, they

were found to be Cr-carbide and Nb-carbide. As

for Sample L2, they were found to be Cr-carbide

and Ni-Nb-Si phase.

As mentioned above, only Nb-carbides ( NbC )

were found in materials of as-cast condition ( cold-

end ). However, two types of the precipitate were

found in the materials which had been used at high

temperatures. They were Cr-carbide ( M23C6 )

and Nb-carbide ( NbC ) in the low Si material.

But, in materials of high Si, Nb, Cr-carbide (

M23C6 ) and Ni-Nb-Si phase were found and NbC

was not found.

We consider that the inferior tensile and creep

rupture properties of high Si, Nb of 20Cr-32Ni-Nb

material are attributable to this phase.

Preferable Composition for 20Cr-32Ni-Nb

material

Since headers and fittings have relatively compli-

cated shapes and are used at various operating

temperature distributions on the metal, they are

liable to be damage by thermal stress. In order to

prevent failure due to thermal stress, it is important

to have materials with high ductility and relaxation

properties. The 20Cr-32Ni-Nb material has been

used for headers and fittings because of its superior

performance in the above mentioned categories, as

well as its high creep rupture strength.

Twotypesofprecipitatewerefoundintheinterdendritic It was proven that there was a wide distribution of

ductilities after use due to differences in the chemi-

169

cal composition of the alloy. And materials having

higher ductility were found to have superior in

weldability after use.

The effect of each element on after use tensile

ductility was evaluated and summarized using the

parameter shown in Figure 11.

The tensile ductilities after use decreased as the

parameter increased. The ductilities decrease to an

unacceptable level when the parameter is over 9. It

was also found that materials with a lower content

of Si and Nb had higher ductility after use as well as

higher creep rupture strength. The parameter ( P ) is

defined by the following equation;

P = 7xC + 5 x S i - 3 x M n + 8 x N b

C, Si, Mn, Nb : wt %

Welding Procedure of 20Cr-32Ni-Nb material

After Long Period Using

The preferred chemical composition for the 20Cr-

32Ni-Nb cast material described above would have

greater ductility after lengthy using at high tempera-

tures. If the chemical composition was not prefer-

able, it was possible to avoid crack formation by

means of changing the filler metal from the same

material as the base metal to the high Nickel alloy

metal ( Inconel 82 ). When the metal temperature is

kept below 600°C, welding can be performed since

embrittlement will not be caused by precipitation of

the Ni-Nb-Si phase.

Summary

(1) It was found that the tensile and creep rupture

properties of used 20Cr-32Ni-Nb material were

affected by variations in the elements of the chemi-

cal composition of the alloys.

(2) It was also found that Si, Nb and C had a

negative effect and Mn had a positive effect on the

tensile ductility after use. The effects of each ele-

ment on the tensile properties after use were sum-

marized using the parameter shown.

(3) The tensile ductilities after use at high tempera-

ture decreased as the parameter increased, espe-

cially when the parameter exceeded 9. The param-

eter ( P ) is defined by the following equation;

xC + 5 x S i - 3 x M n

C, Si, Mn, Nb : wt %

(4) The results of the SEM-EDX analyses revealed

that a Ni-Nb-Si rich phase was found at the site of

primary carbides in the high Si content and high Nb

content materials of 20Cr-32Ni-Nb. The inferior

creep rupture strength and ductility of the materials

were attributed to the precipitation of the Ni-Nb-Si

rich phase.

(5) When metal skin temperature is kept below

600°C, the embrittlement caused by the precipita-

tion of the Ni-Nb-Si phase will not occur and there

will be no problems in welding the 20Cr-32Ni-Nb

material after lengthy use.

170

(6) If the chemical composition was not preferable,

it was possible to avoid crack formation by means of

changing the filler metal from base metal material to

high Nickel alloy metal ( Inconel 82 ).

LITERATURE CITED

1. T. Shibasaki, K. Takemura, T. Kawai, and T.Mohri, "Experiences of Niobium-ContainingAlloys for Steam Reformers", Ammonia PlantSafety, 27 (1987) 56

T. Shibasaki

T. Mohri

K. Takemura

Figure 1. Change of manifold number for eachmaterial.

Stress

(kgf/mm1)

\

\

VvS

Alloy 800H

20Cr-32NI-Nb

Temperature (t)

Figure 2. Comparison of creep-rupture-strength in10,000 hours for Alloy 800H, HK40and 20Cr-32Ni-Nb.

Elongation

uc

6

0

•*:F

i

iö

(

» • >

3

<n

'ik

E

1 • ,» ^

(Ifir

0

0

o

fa

fir

t

N-Nb

'

°

4o

i

10 100 1000 10000 100000

Rupture Time {Hour )

Figure 3, Comparison of creep-rupture-elongationat 870°C for HK40 and 20Cr-32Ni-Nbcast materials.

171

30

Notation : T

Sampls Coupon

Outlet Manifold

Bottom Reducer of Catalyst Tube

D

Catalyst Tube

Figure 4. Location of test samples of used20Cr-32Ni-Nb materials.

T 20

I S

V

v° R

0.0 0.1 0.2

C(wt%)

30

T 20SÄ

1

10X

o«—i-

8

0.4 0.6 0.8 1.0

Si(wt%)

0.3

<x1.4

Figure 5. Effect of carbon and silicon on the roomtemperature tensile elongation of used20Cr-32Ni-Nb material.

172

30

— 20

.S

lS* 10

0.2 0.4 0.6 0.8

Mn(wt%)

1.0 1.2

1003_••Srit

lii^

10

,' :»_.|, T.»-

'

U..Lrf

13

F

JÈa J

v»>

N |

\

higher.

100

oi

: . *"**»„

• KL

\

:l

^^* -^

r i c

^i

1000

^

-

-B?--"K -v ,,

\

>L

t

\towerS

x-

4&

2S

fa '@ :

W £

i o eoot;« 900t

l

10000 100000

Time (hour)

0.8 1.0

Nb(wt%)

1.2 1.4

Figure 6. Effect of manganese and niobium on theroom temperature tensile elongation ofused 20Cr-32Ni-Nb material.

Figure 7. Comparison of the creep-rupture-strengths between used 20Cr-32Ni-Nbmaterials of different silicon and niobiumcontents.

1

\

3O

s"\

T.C

j13

s\

,

\1 l

'

! Xf13

\T,

^

\

1

\

t l1C

n

\

1

\N

1

L3o

L

IL3

\

<»

T2 C

^ v

» - 2

\

0 L2

•L

\S

s

o eoot• 900t

100 1000 10000

Rupture Him (hour)

Figure 8. Comparison of the creep-rupture-elongation between used 20Cr-32Ni-Nbmaterials of different silicon and niobiumcontents.

173

7

_) 200 300 400 500 600 700 BOO 900

Temperature c

Figure 9. Temperature distribution of manifoldreducer.

Groove WeldingJO mm

V. \

• 1 AA TVMVI100 mmBead on Plate Welding

10 mm

10mm

100 mm

Filler Metal

Filler Wire Size

Voltage

Amperage

Welding Speed

Number of Pass

(1) the same metal of 20Cr-32Ni-Nb material

(2) high Nickel alloy : Inconel 82

3.2 mm

13 to 15V

90 to 110A

80 to 90mm/min

(1) groove welding : 5 passes

(2) bead on plate welding : 1 pass

\

\

6 8 10 12 14 16 18

Parameter ( + 5xSi%-3xMn%

Figure 11. Effect of each element on the elongationat room temperature tensile test.

Figure 10. Weldability test conditions on used20Cr-32Ni-Nb materials (Sample L3). Photograph 1. Result of liquid penetrant

examination for the bead-on-plate-test to sample T3 (tee andreducer).

174

Precipitate 1

Photograph 2. Microstructure of cracks appearedafter welding on the used 20Cr-32Ni-Nb material with highersilicon and niobium contents(sample L3).

O fA-t

f>, v-i'<ri.Y . Y -,

<r»*S-,, ,,.,,<, „y»5«!r,«L,,„,, , „^ „» |« » ^"« « >'*ƒ**&] V

S^Œ^ fi!n**>™***™*7*>t™it*'fT<vi B

l Precipitate 2

R — Pr-ecipi- tate l ( S . E l . i . : G r - a y )

e.oo 4.00•Pr-ec i p i tat e J

6.0O 8.00

(S .E . I . : WH i

Photograph 3. Results of liquid penetrant Photograph 4. Results of SEM-EDX analysis of theexamination of the bead-on-plate- precipitates found in higher silicontest for sample L4. and niobium content 20Cr-32Ni-Nb(a) Filler metal is the same material material (sample L3).of the base metal; (b) filler metal is (a) Secondary electron imhigh nickel alloy material (b) EDX cfaart(Inconel 82).

175

Table 1. Summary of Material Properties forSteam Reformer Furnace's Manifold

Chemical Composition

0 %Ml %

Cr %Nb%

Creep Rupture Strength

in 100,000 hours at 845t: (kgl/mm )

Elongation at Room Temperature (%}

Before Aging

Mer Aging at 8SOt (or 5.000 hours

Stress Relaxation Property

Alloy BOOH

0.132

200.0

1.2

3045

Good

HU40

0.4

38

18

0.0

2.0

151

Poor

HK40

0.4

20

250.0

2.4

15

3

Poor

20Cr-32Ni-Nb

0.132

20

1.0

2.2

25

20

Good

»I: (kgr/mnrfj) «2.

Table 2. List of 20Cr-32Ni-Nb Materials forInvestigations

Source

Malarial

Manifold

Outlet

Manifold

Bottom

Reducer of

Catalyst Tube

Plant

A

B

C

D

E

F

G

Notation

of

Sample

L1.T1

12, T2

L3.T3

L4

R1

R2

R3

Operating Conditions

Temperature

(t)

820

830

820

830

830

820

840

Pressure

(MPa)

1.1

2.2

3.2

2.8

22

1.3

2.4

Period

(year)

2

8

10

15

3

6

9

Table 3. Chemical Composition of 20Cr-32Ni-NbMaterial Samples

SMMról

Manifold

Outlet

Manflold

Bottom

Reducer of

Catalyst Tube

No.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Chemical Compositions ( wt% )

C

0.11

0.12

0.12

0.12

0.13

0.07

0.07

0.12

0.12

0.14

0.12

0.12

0.09

0.08

0.23

0.11

Si

0.54

0.75

0.75

0.78

0.61

0.35

0.35

0.69

0.43

0.43

0.47

0.58

0.66

0.73

1.20

0.57

Mn

0.88

1.01

1.01

1.05

1.12

1.08

1.08

1.15

0.39

0.38

0.44

0.41

0.79

1.11

0.65

0.51

P

0.013

0.012

0.012

0.011

0.015

0.016

0.016

0.016

0.009

0.009

0.009

0.010

0.002

0.013

0.019

0.012

S

0.008

0.010

0.010

0.007

0.007

0.009

0.009

0.005

0.012

0.013

0.010

0.012

0.010

0.012

0.018

0.012

NI

32.58

31.91

31.91

31.56

34.22

33.35

33.35

32.35

34.53

34.74

34.45

34.60

3154

31.87

31.23

32.04

Cr

21.97

21.87

21.87

22.13

21.29

20.44

20.44

21.26

20.52

2057

20.16

2055

20.01

21.45

19.08

19.39

Nb

0.73

0.82

0.82

0.76

1.08

050

0.80

1.10

1.17

1.20

.1-22

1.12

0.87

0.55

1.32

0.81

Plant

A

A

A

A

B

B

B

B

C

C

C

C

D

E

F

G

176