The carburization and nitriding of molybdenum and TZM

Int. J. of Refractory Metals & Hard Materials 12 (1993-1994) 179-186

O 1994 ElsevierScienceLimited Printed in GreatBritain.All rightsreserved 0263-4368/94/S7.00

ELSEVIER

The Carburization and Nitriding of Molybdenum and TZM Hans-Peter Martinz & Klaus Prandini Metallwerk Plansee GmbH, A-6600 Reutte, Austria

Abstract: Molybdenum is finding wide application as constructive material in

high temperature furnaces, which are operated under high vacuum or inert/ protective gas conditions. Direct contact of Mo with graphite often is inevitable. It is known that high bulk-concentrations of carbon in Mo lead to an embrittlement of the metal. In the paper to be presented, the kinetics of the Mo-carburization in the temperature range of 1250-1550"C were derived and correlated with the decreasing ductility of the material. Enhanced concentrations of nitrogen in Mo and TZM also embrittle the materials. Less known and therefore the topic of this study is the velocity of the nitriding of Mo and TZM in various nitrogen-containing protective atmospheres. The resulting changes of the mechanical and oxidation properties are treated subsequently.

molybdenum

also not very well so they have been examined here at the same temperatures as above. Consequently, DBTTs were determined with molybdenum and TZM specimens which were exposed to various nitriding conditions and were compared with literature d a t a . 13,20,24-26 Finally, with the nitrided samples, oxidation tests at 640 and 900°C were performed. Thermodynamical calculations helped to understand the results.

INTRODUCTION

are

k n o w n 4'6-8'19'21-23 and

From the literature it is not clear at which temperature molybdenum and carbon start to react) -8 The reaction product seems to be M02C in m o s t cases, 9-11 but the growth rate has not been quantified ~2 up to now for the temperature range of 1250-15500C. Furthermore, it is not well established -- and is therefore of interest -- how surface carburization with carbon affects the DBTT of deformed molybdenum in comparison with results from carbon-doped molybdenum in the literature) 3-15 These questions are important for applications of molybdenum in high temperature furnaces where direct contact with carbon may occur. Likewise, there are some contradictory statements in the literature concerning the nitriding of molybdenum with molecular nitrogen: 5,16-~8 the reaction is said to start betwen 1100 and 1500°C at nitrogen pressures between 1 and 300 bar. Using pure nitrogen and forming gases (N 2 + HE) this question was answered for molybdenum and TZM in the temperature range of 800-1000°C. The nitriding characteristics of ammonia with

EXPERIMENTAL PROCEDURES

Carburizations were performed in a high vacuum (pressure 10 -3 mbar), the specimens (30 x 5 x 1 mm 3) being fully immersed in carbon-filled alumina crucibles. After 1 and 10 h respectively, at 1250-1550°C the samples were removed and evaluated concerning weight change, total carbon, oxygen and nitrogen content, structure (XRD) and thickness of the carbide (from cross-sections), structure of the metal (from cross-sections), hardness (HV) and DBTT (from bending tests). 179

Hans-Peter Martinz, Klaus Prandini

180

Gas-nitriding was run in a tubular furnace under dynamic conditions (pressure=l bar, flux = 0.75 litres/min) at 800-1200°C for 20-50 h. Afterwards the samples (again 30 × 5 × 1 mm 3) were evaluated in the same way as in case of the carburization experiments. Oxidation tests were performed in a chamber furnace using air as oxidizing agent.

RESULTS AND DISCUSSION Carburization of molybdenum The carburization experiments led to recrystallization of the deformed molybdenum (Table 1) and to carbide formation (a-M02C; Table 2) in all cases, facts which had been expected from the literature 27 and thermodynamical calculations (Fig. 1).28,29Owing to residual gases in the vacuum chamber, the oxygen level in the samples increased significantly whereas the nitrogen content stayed rather stable because of the low

solubility of nitrogen in molybdenum (Fig. 2). In particular, enhanced oxygen concentrations may have an influence on the mechanical properties of the carburized molybdenum. However, naturally the main compositional change was owing to carbon uptake. Figure 3 shows the dependence of the total carbon content on the temperature and duration of the carburization. The curves are roughly parabolic so that parabolic rate constants kp could be evaluated and subjected to an Arrhenius plot. The resulting temperature dependence is kp--A .exp( - Ea/ (R. T)) with A = 9"7 x 1022 (ppm2/h) and activation energy E a = 534.1 kJ/mol. The specific mass gain during carburization is presented in Fig. 4 leading to the empirical law

(g2/(cm4.h)).

kp-- 3"0 × 1 0 4 . e x p ( - 332"9/(R. T))

From metallographic cross-sections, finally, the thickness of the carbides versus the duration and

2 log K (Bqulllbrlum constant)

Table 1. Structural changes of deformed molybdenum and TZM during carburization and nitriding

Treatment

Structure

Mo+C, 1250°C, 1 h Mo+C, 12500C, 10h Mo + C, 14000C, I h Mo+C, 14000C, 10 h Mo+C, 1550°C, I h Mo+C, 1550°C, 10 h

Recrystallized Recrystallized Recrystallized Recrystallized Recrystallized Recrystallized

Mo + FG70/30 or NH3, 800°C, 20 h Mo + NH3, 800"C, 50 h Mo + NH 3, 1000°C, 22 h Mo + NH 3, 1000°C, 50 h Mo + FG70/30, 10000C, 50 h Mo + FG70/30 or NH3, 1200°C, 20 h Mo + FG70/30 or NH3, 1200°C, 50 h

Deformed Deformed Deformed Partly recrystallized Recrystallized RecrystaUized RecrystaUized

TZM + FG70/30 or NH3, 800-1200°C, 50 h

Deformed

FG70/30, Forming gas 70/30.

2Mo* C

"~*

Mo2C

2Mo * C

'*'--

Mo=C

0

b .l I 1500

Fig. I.

140

I

I

I

1600

1700 Temperature IX]

1800

1900

Equilibrium constant K for the reaction 2Mo + C-- Mo2C as a function of temperature.

Total Oxygen and Nitrogen content [ppm]

140

Molybdenum/Carbon /

ls6o'c

120

-120

I00

100

-80

Table 2. Near surface phases after carburization detected byXRD

6O

"60 I

Treatment of Mo C/HV/1250*C/1 h C/HV/1250°C/10 h C/HV/14000C/1 h C/HV/1400"C/10 h

C/HV/15500C/1 h

C/HV/1550°C/10 h

i4o

Detected phases

Mo, ct-Mo2C Mo, a-MoEC Mo, a-Mo:C (Mo), a-MozC

(Mo), ct-Mo2C ct-Mo2C

Nitrogen

zo 0

Fig. 2.

1

2

3 4 $ 6 7 Duration of Carburization [h]

8

9

2O 1560"C 1400*C

0 10

It60"C

Oxygen and nitrogen uptake of molybdenum during carburizafion.

Carburization and nitriding of molybdenum and TZM 25000

TotniOJrbon c o n ~ n t l p p m ]

3SO

Molybdenum/Carbon . .....-"""

Mol

//~55°°c

ZSO

15000

ZOO 150

..--

tO000

..~..-/

/ / ///~"

+ 1400°C 50

f i

2

Fig. 3.

,4oo.

tO0

...-

5000

0,014

T h l c k n e u of Carblde [tam]

300

1550°C

20000

181

6 8 Duration of Carburlzatlon [h]

i 10

1250°C

1250°C

12

Carbon uptake of molybdenum during carburization.

0

2

4

6

8

10

12

Duration of Carburization [hi

Fig. 5.

Growth of the carbide during carburization.

Specific mass change [S*cn/2 ]

Nitriding of molybdenum and TZM 0,012 0,01 0,000 0,006 0,004 0.002

2

4

6

S

I0

12

Duration of Carburizatlon [h]

Fig. 4.

Specific mass gain during carburization.

temperature of the carburization is plotted in Fig. 5: k~ = 7.4 × 1013.exp( - 343"3/(R. T ))(/~m2/h). Figure 6 shows the cross-sections of untreated and carburized specimens with bright carbide phases (a-Mo2C; for hardness see Table 3) of uneven thickness distribution. A comparison of the data from Figs 3-5 gives an agreement within about + 45% which is not so bad because of uncertainties mainly in the evaluation of carbide thickness. The DBTT (Fig. 7) of the specimens is only slightly increased at 1250°C, whereas owing to enhanced carbon uptake (and oxygen absorption) at 1400 and 1550°C the embrittlement is very distinct. The literature ~3 would suggest an even more detrimental influence of C and O of comparable total concentrations. The explanation of this difference is that real bulk contents ~3 are more effective then surface-enrichments mainly of carbon as in this work.

In agreement with the literature27 molybdenum recrystallized above about 1000°C under the conditions given, whereas TZM remained on the deformation structure without exception (Table 1 ). Nitrogen (N2) and forming gases (N 2 + 20 or 30 vol.% H2) did not react to nitride layers in any c a s e 13 (Table 4 and Fig. 16); in some cases they were not even protective against oxidation by residual air (N2-1000°C, forming gas 80/ 20-1200°C, Table 4). This behaviour can be explained by thermodynamical calculations (Fig. 8; data from Ref. 28 and partly extrapolated (Mo2N)), which show that under the conditions given, Mo2N formation is very unlikely at N 2 pressures below a few hundred bar. For ammonia equilbrium-calculations (Fig. 9) nitride formation can be predicted only at about 800°C, but dynamic experimental conditions may be a good deal away from full equilbrium. Figure 10 shows that ammonia and forming gas 70/30 in the temperature range of 800-1200°C are really protective against oxygen uptake into Mo and TZM. The absorption of nitrogen from forming gas 70/30 is only remarkable for TZM, whereas Mo resists even up to 1200°C (Fig. 11). This may be explained by the reactivity of titanium with nitrogen to stable TiN of 1400 ppm maximum concentration (from 0.5 wt% Ti in TZM). With ammonia the nitrogen uptake is much higher and of comparable order of magnitude for Mo and TZM. Nevertheless, TZM again shows a stronger affinity to nitrogen (Fig. 12).

182

Hans-Peter Martinz, Klaus Prandini

[

untreated I c, 1250"c, lh I C, 1250"C, 10hi C, 1400"C, lh [ C, 1400"C, t0hl C, 1550"C, lh I C, 1550"C, 10h I Fig. 6.

all x 100 Light micrographs of cross-sections of carburized Mo.

Table 3. Hardness of the phases formed during carburization and nitriding Phase

4

Hardness

a-Mo2C y-Mo2N

s log P~llmrl

1382 + 33 (HV 0.2) 1760 + 23 (HV 0"5)

4Mo + N 2 ' ~

2Mo2N

3

2 4Mo * N 2 '*--'- 2MozN

1

DBTT [ ' C ]

D • PN~ from Nitrogen .... G-i&-eT80120

and Forming lind 7 0 / 3 0

200

Molybdenum/Carbon

0

155o*c

IS0

8

D

i I

-I I000

100

1100

Fig. 8.

50

1400°C

I

i

1200 1300 Temperature [E]

L

1400

1300

Formation or decomposition of Mo2N as a function of nitrogen pressure and temperature.

0 P".~t

120o0c

j: /

61°8 ( P . . , )

-50 13

-I00 0

r

t

1

2

Fig. 7.

i

i

i

i

i

3 4 $ 6 7 Duration of Carburir~tloa [h]

p

i

8

9

_

10

q2

+m.

4

DBTT of carburized molybdenum.

4Mo+ 2NH e m 2 M o a N *

3H s

'

3

Table 4. Phases detected by XRD on surfaces of molybdenum exposed to various nitrogen-containing gases Treatment of Mo

1000°C/23.5 h/N z 1000°C/23.5 h/forming gas 80/20 1000°C/17.0 h/forming gas 70/30 1200°C/20.0 h/forming gas 80•20

[]

~-~, from NH 3

Detected phases

Mo, MoO 2 Mo Mo Mo, MoOz, MoO 3

T h e t e m p e r a t u r e of 800°C seems to be a lower limit for N-absorption; whereas at 1200°C after 20 h T Z M begins to lose nitrogen again by s o m e d e s o r p t i o n (see also Fig. 18). A s s u m p t i o n of parabolic rate laws and A r r h e n i u s plots lead to the

i

1000

1100

I

i

1200 1300 Temperature [K]

I

1400

1300

Fig. 9. Formation or decomposition of Mo2N as a function of the ratio of hydrogen and nitrogen pressures.

following formulas 1200°C ( < 20 h):

valid

between

800

and

Mo: kp = 1.11 x 1019. e x p ( - 346.8/(R. T )) T Z M : kp = 2.36 x 1022.exp( - 398.3/(R. T ))

Carburization and nitridingof molybdenum and TZM Plots of the specific mass gain of Mo and TZM versus the duration of the nitriding under various conditions (Figs 13 and 14) confirm the above results and can be expressed by: Mo: k'p=0.0052.exp(- 124.7/(R.T)) 1 TZM: k'p -- 123.9 .exp( - 215-3/(R. T ))

l

183

The thickness of the nitrides (y-Mo2N in all cases), which are formed on Mo and TZM in ammonia, can be seen in Fig 15-18. At 1200°C and > 20 h, x-MoEN partly decomposed forming an outer layer of metal (Figs 16 and 18), which was not taken into account in the diagrams of Figs 15 and 17. The Arrhenius eva-

for 800-1200°C ( < 20 h) Total)Ozygon content [ppm]

250

0,006

Specific mat. change [8*cm"=]

0,005 0,004

200

,ybdenum

0,003 0,002

150

0,001 Ammonia, Forming Gas

/

I O0 1

0

800 - i200°C

~

~

~

:

:

......... ~:.....~..:~ ............. ..............................

i

~

::::::::::::::::::::::::::::::: ..........

-0,001 50

0

t

MO

40

50

.... ,~ .............

~

0

20 Durotion of N i t r i d i n g [h]

40

50

0,006 TZM

+

NH3 1000°C " *+ NH3 800°C lO00°C o..-

FGTOI30

PG70130

/

I

800°C

Fig. 13. Specific mass gain of Mo during nitriding.

Total Nitropn content [ppm]

Forming Gas 70•30

NH3 1200°C

FGT0130 1200°C x

Fig. 10. Oxygen uptake during nitriding. 600 -

20

Duration of Nitriding [h]

1200°C]

Specific mass change [g*cm2]

0,005 0,004

500

_TZM

0,003 400

0,002 0,00l

300

..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0 ZOO

-0,001

I ..............

::::::::::::::::::::::::::::::::::::::::::: :::--

,

I

I

I

L

I

L

t

L

5

10

15

20

25

30

35

40

45

50

¼ I

Duration of Nitridin 0 [h]

I00 ~

TZM

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

800

°C "

-0

Mo 800"1200°C 20 Duration of N i t r i d i n l [h]

0

Fig. 11.

16000

40

50

Ammonia

160 Mo 1200°C

+

NH3 1000°C ' - ~ NH3 800°C

FG 1200°C

×

]FG IO00°C

"~

FG 800°C

I

Fig. 14. Specific mass gain of TZM during nitriding.

Nitrogen uptake during nitriding with forming gas 70/30.

TotalNit~sen content[ppm]

NH3 1200°C

Thickness of Nitride [pm]

~oJybdenum/Ammonia

140

t:

14000 120 12000 100 10000 80 8000 60 6000 40 4000 20.

":J •

2000 0 0

0

20

Nitrogen uptake during nitriding with NH 3.

4~6

800°C

i

J

20

40

50

Duration of Nitridin8 [h]

40 TZM 800°C

Duration of Nitridin$ [h]

Fig. 12.

................

Fig. 15.

Growth of Mo2N during nitriding of Mo with ammonia.

184

Hans-Peter Martinz, Klaus Prandini

all x 500 Fig. 16.

200

T h i c k n e a of Nitride

Light micrographs of cross-sections of nitrided Mo-samples.

[Urn]

luation gave the following formulas: o

is0

1'

TZM/Ammonia

Mo: kp= 3.24 x 101°.exp(- 220.3/(R. T)) 1 TZM: kp = 2.76 x 1011.exp( - 239.6/(R. T))

~

100

.,.,.

...................

for 800-1200°C

"

" ..........-+ 800Oc

0

0

......

~,

20

Duration of Nitridiag

Fig. 17.

u

.........

[hi

40

60

Growth of MoaN during nitriding of TZM with ammonia.

Except at 800"C (lower limit of nitriding) the data from Figs 12-18 agree within _+17%. The change of DBTTs in forming gas 70/30 and ammonia at 800, 1000 and 1200°C with time is shown in Figs 19-21. In agreement with the literature, 13'2°'24'25 in all cases Mo and TZM become more brittle, and in most cases (except TZM 800°C) ammonia is more detrimental than forming gas and TZM is much more sensitive than

i.lntl~ated r NH3'] 800°C' 2I NH3'50h 8°°°e' 0 [ NI'I3' h 20h 1000~C' I NI'x3~50h 'e°01

Fig. 18.

1

N~I3t11200*Cp 2 0 0 " CI 'N2~103'h50h

x 500 x 200 x200 Light micrographs of cross-sections of nitrided TZM-samples.

Carburization and nitriding of molybdenum and TZM

of cracks (see Fig. 16) within the nitrides and the fact that nitrogen (like carbon in carbides) forms gaseous (and therefore non-protective) oxides. At 800 K the equilibrium constants for the reactions Mo2C + 402 = 2 MoO3 + CO2 and Mo2N + 402 = 2 M o O 3 -t-NO 2 are 10 l°° and 1064, respectively, which means a high probability of oxidation (data from Ref. 28).

, D B T T [*C!

250 "

, 200 FG 70•30, 800PC ~' 200 .+ ...............................................................

200

.,'*

TZM

• 150 .. : ~ j ~ " - ~

150 -

NHa' 800o C

100 -

, 100

50

o ~ - 20

NH~ 8000C

~

-so

~ (-90

-100

-0

,

500

...... ~2-................................................... i~ ...... [ -65 F G 7 0 / 3 0 , 8 0 0 C .-65 , , , , , , lS 20 "25 30 35 40 45 50 IDuration of N i t r t d i n g [h]

DBTT of Mo, TZM after nitriding at 8000C.

D B T T [*C]

) 450

NHa, 10000C

,4o~

400 t

TZM 300 t

_250 / " F G 70/30, 1000°C

2O0 I00 ....

0

(..

20

-100

5

Fig. 20.

500

10

15 20 25 30 3S Duration of N l t r l d i n | [h]

40

45

SO

DBTT of Mo, TZM after nitriding at 1000*C.

DBTT [*C] NH o 1200°C

> 400

> 400

400

/ +,/'

TZM 300

Molybdenum is carburized by carbon in high vacuum at temperatures between 1250 and 1550°C, forming layers of a-Mo2C obeying parabolic rate laws. The ductility of Mo decreases with enhanced carbon uptake but not so much as it would have been had the carbon been absorbed equally in the bulk of the material. Nitride layers (~-Mo2 N) on molybdenum and TZM are formed at 800-1200°C only in ammonia-atmosphere, whereas nitrogen and forming gases (N2/H2) lead to some nitrogen absorption into the bulk, especially with TZM. The nitride formation roughly obeys a parabolic rate law. It is very slow to non-existing at 800°C and is combined with some parallel decomposition of Mo2N at 1200°C. TZM is strongly embrittled by forming gas and ammonia over the whole temperature range, whereas molybdenum is affected decisively by ammonia above 1000°C. Neither the carbide nor nitride layers improve the hot air oxidation resistance of molybdenum above 640°C.

" ......................................... . .........;35iiFG70/30t1200C

) 350

REFERENCES

200 ~ 0

"/

NH~ 1200°C

> 10(

lO0 0 FG 70/30, 12000C

..............

-100

SUMMARY

,

lO

Fig. 19.

¢-40

-

MO

..........

r 5

185

.gD

Fig. 21.

i

T

5

l0

i

i

i

r

J

15 20 25 $0 35' Duration of N l t r l d l n g [h]

J

i

40

45

$0

DBTT of Mo, TZM afer nitriding at 1200"C.

Mo. This can be explained by the higher amount of nitrogen absorbed in the case of TZM and ammonia. The resistance of molybdenum against hot air oxidation at 640 and 900°C is not improved by nitriding. This can be explained by the existence

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186

Hans-Peter Martinz, Klaus Prandini

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21. Climax Molybdenum Co., New York, 1960, pp. 1-110, 53. 22. Agte, C. & Vacek, J., Wolfram und Molydbiin. Akad.Verlag, Berlin, 1959, pp. 214-34, 218. 23. Lakhtin, Yu. M. & Kogan, Ya. D., Met. Term. Obrab. Metallov., 1 (1968) 24; Metal Sci. Heat Treatm. (1968) 24. 24. Kogan, Ya. D., Lakhtin, Yu. M. & Shaskov, D. P., Met. Term. Obrab. Metallov., 9 (1968) 20; Sci. Heat Treatm. (1968) 6922. 25. Lakhtin, Yu. M. & Kogan, Ya. D., lzv. Akad. Nauk. SSSR, Metally, 5 (1971) 145; Russ. Metallurgy, 5 (1971) 109. 26. Skuratov, L. P., Dovbnya, V. K. & Kalashnik, M. V., F/z. Khim. Mekh. Mater., 4 (1984) 118-19. 27. Metallwerk Plansee, Molybd/in, 530 DEF 10,85, p. 18. 28. Barin, I. & Knacke, O., Thermochemical Properties o] Inorganic Substances. Springer Verlag, Berlin, 1973. 29. Fromm, E. & Gebhardt, E. (ed.), Gase undKohlenstoffin Metallen. Springer Verlag, Berlin, 1976.