Hydrogen Damage of Nickel-base Heat Resistant Alloys*

Hydrogen Damage

UDC

of Nickel-base

669.245'1'26'28'25'27 : 669.245'1'26'25 : 620.193.55 : 669.018.85

Heat Resistant Alloys*

By Masayoshi HASEGAWA** and Motoaki OSAWA***

Synopsis

The damage of nickel-base heat resistant alloys exposed to hydrogen at high temperature and pressure was investigated. Tested materials were mainly Inconel 600, Hastelloy X and nickel. Inconel 600 solution-treated exhibited embrittlement due to the occluded hydrogen. Hastelloy x solution-treated did not exhibit embrittlement. Nickel remained the irreversible damage even after hydrogen was completely eliminated. Stress concentration enhanced the susceptibility to embrittlement for Inconel 600. Hastelloy X which did not show embrittlement in smooth specimen was also embrittled in notched specimen in which stress was concentrated. Degree of embrittlement depended on nickel content, and alloys with higher nickel content exhibited an irreversible damage. The susceptibility to hydrogen embrittlement of Inconel 600 and Hastelloy X increased when they were aged and carbides precipitated at grain boundaries. This effect could be explained by hydrogen trapping.

I. Introduction

As a result of the development of petrochemical,

high temperature gas and energy plants dealing with hydrogen containing gases, their structural materials have been required to resist hydrogen as well as to

have high strength at elevated temperatures. Nickel-base heat resistant alloys are considered to be de-

sirable materials and many studies have been carried out from the point of view of high temperature strength. A few investigations about the effect of

hydrogen have been reported, but those researchers were principally performed on Ni-Fe alloys cathodi-

cally hydrogen charged,l-3) and the investigations on commercial alloys were very few. For commercial alloys, hydrogen penetration4-$~ and embrittlement

under exposure to hydrogen at room temperature9-iii

were mainly investigated. This paper deals with hydrogen damage of commercial alloys, such as In-

conel 600 and Hastelloy X, exposed to hydrogen at

high temperature and pressure, and hydrogen damage of nickel which is the main element of these alloys.

II. Experimental Procedure

The materials used were Inconel 600, Hastelloy

x, pure nickel, and Ni-Cr alloys for comparison. Their chemical compositions are shown in Table 1. These materials were machined into smooth speci-

mens, 8 mm in gauge length and 2 mm in thickness, and notched specimens, 2 mm in thickness and 2.5 to 11 mm in width at notches, as shown in Fig. 1. Inconel 600 and Hastelloy X were heated at 1 473 K for 1 h, followed by water cooling. After the heat treatment, some specimens were heated at 873 K to 1 173 K for up to 100 h. Pure nickel and Ni-Cr alloys were annealed at 1 473 K for 1 h. After

polishing, test specimens were exposed to 1 30 MPa hydrogen in an autoclave at 723 K for 20 h. To estimate the reversibility of hydrogen damage, dehydrogenation was applied to some specimens at 473 K to 723 K in vacuum. Tensile test was carried out at a cross head speed of 0.5 mm/min at room temperature. Hydrogen content was determined by means of an extraction apparatus joined to a gas-chromatograph. Tensile fracture surface was ex-amined in a scanning electron microscope and the determination of the precipitated carbides was done by means of X-ray diffraction method. All the hydrogen-free specimens were heated in the same condition as for the hydrogenation (723 K x 20 h).

III. Results and Discussion

1. Hydrogen Damage of Nickel-base Heat Resistant Alloys

The changes of the elongation and ultimate tensile strength were examined on Inconel 600, Hastelloy X and pure nickel, which were solution-treated and exposed to hydrogen at the high temperature and

pressures. These results are shown in Fig. 2 and the previous result of hydrogenated SUS304 steel is also plotted in this figure for comparison. Hydrogen-free reference specimens were heated at the same tempera-ture and for the same time as these for the hydrogena-tion to eliminate the effect of the thermal history. No change of mechanical properties with heating was observed in all the materials. As shown in Fig. 2, substantial decreases of the elongation and ultimate tensile strength were observed on Inconel 600 and

pure nickel under exposure to hydrogen at the high temperature and pressure, but only the small changes

Table 1. Chemical composition of tested alloys. (Wt%)

(26) Transactions ISIJ, Vol. 21, 1981

were observed in the hydrogenated Hastelloy X. From the change of elongation, the hydrogen em-brittlement susceptibility of Inconel 600 and Hastelloy x were relatively small compared with that of aus-tenitic stainless steel.

2. Hydrogen Damage of Inconel 600 1. Effect of Austenitic Matrix

As shown in previous section, Inconel 600 solution-treated is susceptible to hydrogen damage. As hy-drogen damage is generally concerned with the dif-fusion of hydrogen in alloy, the diffusion behavior of hydrogen was examined. Inconel 600 exposed to 30 MPa hydrogen at 723 K for 20 h contained about 40 cm3 H2/ 100 g. The diffusion coefficient of hydrogen was calculated from evolution curves by heating at 473 K to 673 K in vacuum. Figure 3 shows the result with some other reference data.7,13,14) The activation energy of diffusion of hydrogen in Inconel 600 is 10.4 kcal/mol.

Hydrogen embrittlement characteristics of hydro-

genated Inconel 600 was investigated. Figure 4 shows the effect of the dissolved hydrogen content on hydrogen embrittlement obtained by varying hy-drogen pressure. When hydrogen content exceeds 15 cm3 H2/ 100 g, the degradation of elongation and ultimate tensile strength is observed with increasing hydrogen content. Compared with the fact that austenitic stainless steel, SUS304 steel, containing about 10 cm3 H2/ 100 g of hydrogen exhibits a decrease of the elongation, the hydrogen embrittle-ment susceptibility of Inconel 600 is smaller than that of SUS304 steel.

Figure 5 shows the effect of strain rate on hydrogen embrittlement of Inconel 600 containing about 40 cm3 H2/100 g of hydrogen. Hydrogen embrittlement was revealed in low strain rate region and increasing of strain rate decreased the hydrogen embrittlement susceptibility. This tendency which was also ob-served in ferritic steels15) and austenitic stainless

Fig. 1. Geometry of tensile specimens.

Fig. 2. Change of tensile properties of nickel-alloys,

and SUS304 steel with hydrogenation.

nickel

Fig. 3. Temperature

hydrogen in

dependence

Inconel 600.

of diffusion coefficient of

Fig. 4. Relation between tensile

and hydrogen content.

properties of Inconel 600

Transactions ISIJ, Vol. 21, 1981 (27)

steels,16~ stems from the diffusion process of dissolved hydrogen toward crack initiation site.

In this way, it became clear that Inconel 600 was embrittled by exposure to hydrogen at the high tem-

perature and pressure, so the reversibility of this embrittlement was investigated. Figure 6 shows the change of the elongation of hydrogenated Inconel 600 with dehydrogenation time at 473 K to 734 K. The elongation recovers completely to the value of hydrogen-free states at all temperatures examined, though the rate depended on the temperature. The relation between 50% recovery time and reciprocal of absolute temperature as shown in Fig. 7 shows the Arehenius' one. This activation energy, 10.9 kcal/ mol, is the same as the one of diffusion of hydrogen in Inconel 600, 10.4 kcal/mol. From this fact, the recovery process of elongation by dehydrogenation seems to be controlled by the diffusion process of the dissolved hydrogen.

The above results were derived from the smooth specimens. On the other hand, a notched specimen has " notch effect ",17) so the effect of stress concentra-tion was investigated using notched specimens. The stress concentration factor was varied by changing the notch width and thickness. The hydrogen em-brittlement index from the change of the elongation by hydrogenation are defined as follows;

Elongation of X 100%) hydrogen-free specimen

As shown in Fig. 8, the hydrogen embrittlement susceptibility increased with increasing stress con-centration factor. 2. Effect of Aging Treatment

Inconel 600 precipitates chromium carbides when heated at medium and elevated temperatures. This

precipitated carbide seems to serve as a trapping site of hydrogen and increases the hydrogen embrittlement susceptibility, as in the investigation on austenitic stainless steels carried out by one of the authors.8~ Therefore, the relation between hydrogen embrittle-ment and the morphology and the distribution of pre-cipitated carbides was investigated. Figure 9 shows the change of the elongation of Inconel 600 aged at 973 K with hydrogenation. Elongation of hydrogen-free specimens is about 55 to 62% only slightly dependent on the aging time, and, on the other hand, that of hydrogenated ones decreases with increasing aging time, from 55 to 18% when aged for 24 h. This change of the elongation seems to be

Elongation of hydrogen-free

specimen-(Elongation of

hydrogenated specimen

Fig. 5. Effect of strain rate on tensile properties of Inconel

600 with and without hydrogenation.

Fig. 6. Recovery of elongation of

600 by dehydrogenation.

hydrogenated Inconel

Fig. 7. Arrhenius' plot of the relation 50% recovery of elongation and

Inconel 600.

between time

temperature

of

for

Fig. 8. Effect of stress concentration on

brittlement, calculated from loss in

Inconel 600.

hydrogen em-

elongation, of

Research Article


related to precipitated carbide. That is, hydrogen embrittlement susceptibility increases when carbides

precipitate at grain boundaries with aging treatment. The change of the elongation with aging tempera-

ture is shown in Fig. 10. The elongations of hydro-

genated specimens were smaller than those of hydro-

gen-free specimens, especially when aged at 873 K to 1 073 K. As the decrease of the elongation by aging treatment is considered to be related to

precipitated carbides, the change of the distribution and the morphology of the precipitated carbides by

aging treatment were investigated. Photograph 1 shows the change of microstructures by aging treat-

ment. In solution-treated condition, Inconel 600 shows a single phase of austenite, by aging at 973 K

carbides precipitate continuously at grain boundaries, and by aging at 1 073 K to 1 173 K precipitated

carbides coarsen at grain boundaries. Hydrogen

embrittlement susceptibility is correlated with such

precipitation behavior and is the greatest when

carbides precipitate finely and continuously at grain

boundaries. The precipitated carbides were iden-tified to be Cr23C6 above 1 033 K, and Cr7C3 below

1 033 K.

Photograph 2 shows the tensile fracture surfaces of Inconel 600 as solution-treated and aged at

1 023 K for 24 h after solution treatment. Hydro-

gen-free specimens show dimple pattern, while hy-drogenated ones show intergranular fracture. By hydrogenation, the solution-treated specimen reveals

partly dimple pattern, but the aged specimen shows completely intergranular fracture. From these facts,

the enhancement of hydrogen embrittlement of the aged specimen is related to carbides precipitated

at grain boundaries. Since this behavior of the

precipitated carbides on hydrogen embrittlement sus-ceptibility is considered to be based on the hydrogen

trapping on the carbide interface, the activation energy of diffusion of hydrogen was calculated on Inconel 600 aged at 973 K from the evolution curve

Fig. 9. Change of elongation of Inconel 600 aged at 973 K

with and without hydrogenation.

Fig. 10. Effect of aging

of Inconel 600

temperature (24 h) on elongation with and without hydrogenation.

Photo. 1. Microstructures of Inconel 600

(d) aged at 1 073 K for 24 h,(a)

(e)

as solution-treated, (b) aged aged at 1173K for 24h.

at 873 K for 24 h, (c) aged at 973 K for 24 h,

Research Article


of hydrogen content, like that of solution-treated specimens, and it was 15 kcal/mol. This value is larger than that of the solution-treated specimen, 10.4 kcal/mol; the difference may be caused by hydrogen trapping on precipitated carbides and thereby the delay of hydrogen diffusion.

Consequently, the enhancement of hydrogen em-brittlement susceptibility by aging is considered to be due to hydrogen trapped on precipitated carbides. This phenomenon, at the same time, is predicted to occur when the solution-treated alloy is used practical-ly for long term.

3. Hydrogen Damage of Hastelloy X 1. Effect of Austenitic Matrix

As shown in Fig. 2, the smooth specimens of Hastel-loy X do not show hydrogen embrittlement. In this section, the notch effect on hydrogen embrittlement was investigated on Hastelloy X. Figure 11 shows that the hydrogen embrittlement index increases with increasing stress concentration factor. 2. Effect of Aging Treatment

In the same way as described on Inconel 600, the relation between the hydrogen embrittlement sus-ceptibility and carbides precipitated by aging treat-ment was investigated. Figure 12 shows the change of the elongation and ultimate tensile strength with aging time at 1 173 K. The elongation and ultimate tensile strength of hydrogenated and hydro-

gen-free specimens decrease with increasing aging time, especially the degradation of hydrogenated specimen is larger than that of hydrogen-free ones. Aging for 2 h produced the elongation decrement of 20% and the ultimate tensile strength decrement of 10 kg/mm2. This tendency seems to be associated with the precipitated carbides. Figure 13 shows the change of the elongation and ultimate tensile strength with aging temperature. For the specimens aged at 873 K and below the degradation of hydrogenated

Hastelloy X was not observed, but for the specimens aged at 973 K and above the elongation of both

hydrogenated and hydrogen-free specimens decrease

with aging temperature, especially the decrease in hydrogenated ones is remarkable. This trend, as

Photo. 2. Tensile fracture surfaces of Inconel 600 as solution-treated and aged at 973 K for 24 h after heat treatment, (a) solution-treated and hydrogen free, (b) solution-treated and hydrogenated, (c) aged and hydrogen free, (d) aged and hy-drogenated.

Fig. 11. Effect of stress concentration on

brittlement, calculated from loss

of Hastelloy X.

hydrogen em-

in elongation,

Fig. 12. Change of tensile properties of Hastelloy X aged

at 1 173 K with and without hydrogenation.


well as in Inconel 600, seems to be connected with

precipitated carbides. Aging at below 873 K is not accompanied by the precipitation of carbides. Aging at 973 K and above makes precipitation of carbides in matrix and continuously at grain boundaries, and thereby hydrogen embrittlement susceptibility in-creases because of large trapping on the interface be-tween the precipitated carbides and matrix. For the specimens aged at 1 073 K and above, the precipitated carbides are inclined to coarsen and thereby hydrogen embrittlement susceptibility decreases.

4. Hydrogen Damage of Nickel

As the elongation of pure nickel decreased from exposure to hydrogen at the high temperature and pressure (see Fig. 2), the reversibility of the embrittle-ment was then examined. Figure 14 shows the change of the elongation and hydrogen content of

pure nickel with dehydrogenation at 723 K. The elongation corresponds to the residual hydrogen content, increasing with decreasing hydrogen content. However, the elongation does not recover to the value of hydrogen-free ones, which shows that nickel re-mains irreversible embrittlement. From the ob-servation of the hydrogenated specimen surface, as shown in Photo. 3, cracks are formed along grain boundaries; this is considered to cause irreversible embrittlement. It was reported that pure nickel forms hydride by hydrogen catholic charging,2,19~ but in this case, esposure to hydrogen at the high temperature and pressure, any trace of hydride was not detected.

5. Hydrogen Damage Susceptibility of Nickel-base Alloys

As described above, it became clear that Inconel 600, Hastelloy X and pure nickel have hydrogen embrittlement susceptibility. From these results, the authors discuss the difference of hydrogen embrittle-ment susceptibility of these alloys in reference to nickel content. That is, from special attention of nickel and chromium which are main elements of

Inconel 600 and Hastelloy X, four Ni-Cr alloys of different nickel contents were melted and examined to measure their hydrogen embrittlement susceptibili-ty. Hydrogen embrittlement susceptibility is defined as hydrogen embrittlement index divided by dis-solved hydrogen content. The hydrogen damage susceptibility is defined as follows :

Elongation of` Elongation of hydrogen-free - dehydrogenated

specimen specimen x 100 (% ) Elongation of Dissolved

hydrogen-free X hydrogen specimen content

Figure 15 shows the result for smooth specimens. Hy-drogen embrittlement index decreases with decreasing nickel content and the embrittlement is not seen below nickel content of 60%. The hydrogen damage index also decreases with decreasing nickel content and the damage does not appear when nickel content is below 80%. These tendency coincides with the results of Inconel 600 and Hastelloy X. Hydrogen embrittlement susceptibility of other nickel-base heat resistant alloys may not always follow the results of Ni-Cr alloys, but may be estimated qualitatively from their nickel contents. Low nickel iron-base alloys (up to 25 % nickel content) such as austenitic stainless steels, however, show the embrittlement.12~

Fig. 13. Effect of aging temperature (24 h) on elongation of Hastelloy X with and without hydrogenation.

Fig. 14. Change of elongation and

nickel with dehydrogenation.

hydrogen content of

Photo. 3. Cracks observed in free (not etched).

hydrogenated nickel under stress


When carbides precipitate finely and continuously at grain boundaries by aging, hydrogen embrittle-ment susceptibility increases. It was shown by one of the authors that austenitic stainless steels show the same phenomenon and it was caused by hydrogen trapped at the interface between carbide and matrix.18) The elongation was completely recovered to the value of hydrogen-free specimen by dehydrogenation.

As described above, it became clear that hydrogen embrittlement susceptibility (1) depends on nickel content of matrix, and (2) increases by carbide pre-cipitation. These data were from specimens hydrogen-ated for only short times. However, from the facts that austenitic stainless steels form internal micro-defects by hydrogen charging20} and was observed to have the irreversible damage in a plant test,21~ nickel-base heat resistant alloys also have the pos-sibility to show the irreversible damage in long term service.

Iv. Conclusions

The following conclusions are offered from the investigation on hydrogen damage of nickel-base heat resistant alloys, exposed to hydrogen at the high temperature and pressure.

(1) Inconel 600 solution-treated exhibited em-brittlement due to the occluded hydrogen.

(2) Hastelloy X solution-treated did not exhibit embrittlement.

(3) Pure nickel gave the irreversible damage. (4) Stress concentration enlarged the susceptibili-

ty to embrittlement. Hastelloy X which did not show embrittlement in smooth specimen was also embrittled when stress concentrated.

(5) Hydrogen embrittlement susceptibility de-pended on nickel content, increasing with increasing nickel content. Hydrogen damage susceptibility showed the same tendency.

(6) Hydrogen embrittlement susceptibility of In-conel 600 and Hastelloy X increased when they were aged and carbides precipitated finely and continuously at grain boundaries, and this was caused by hydrogen trapped on the precipitated carbides. This pheno-menon can be applied to the case that solution-treated material is used as structural material for a long time.

From these facts, when nickel-base heat resistant alloys are used as construction materials for chemical plant, it is desirable to use a low nickel alloy, to avoid local stress concentration, and to take care of the

precipitated carbides at grain boundaries by aging.Fig. 15. Effect of nickel content on hydrogen embrittlement

and damage of nickel, Ni-Cr alloys, Inconel 600

and Hastelloy X, exposed 30MPa hydrogen at

723 K for 20 h.

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Documents

Hydrogen Damage of Nickel-base Heat Resistant Alloys*