Metal Science and Heat Treatment
Vol. 38. Nos. 11 - 12, 1996
UDC 669.295:620.178.2
EFFECT OF TEMPERATURE ON THE SUSCEPTIBILITY OF VT6ch ALLOY TO HYDROGEN EMBRITTLEMENT B. A. Kolachev, t N. N. Kondrashova, t V. N. Skol'tsov, t and P. D. Drozdov t Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 28 - 32, December, 1996. Alloys of the VT6 type are used for parts operating in the temperature range from 450 - 500°C to cryogenic temperatures. At room temperature and below, alloys of this type can develop hydrogen embrittlement. This paper is devoted to the effect of hydrogen on the mechanical properties of the alloy for temperatures ranging from room to that of liquid helium.
chanical properties of annealed VT6ch alloy with different hydrogen concentrations. Most of the tests were conducted on hot-rolled rods 22 mm in diameter of alloy VT6ch with a composition including 5.9% AI, 3.9% V, 0.12% O, 0.01% N, 0.04% C, 0.07% Fe, 0.08% Si, and 0.008% H 2. The fracture toughness was determined on specimens cut from a plate 30 mm thick of VT6ch alloy with a composition including 6.3% AI, 4.3% V, 0.10% O, 0.01% N, 0.02% C, 0.1% Fe, 0,01% Si, and 0.008% H 2. The chemical composition of the semifinished products studied corresponded to the specifications for VT6kt and T i - 6 % A I - 4 % V (ELI) cryogenic alloys [6, 7]. The rod preforms were subjected to preliminary annealing at 880°C for I h in order to form a globular structure in the alloy and at 1150°C in order to obtain a lamellar [3-transformed structure. Differences in the hydrogen content were attained by charging the preforms in a medium of molecular hydrogen in a Sieverts apparatus. The preforms were first annealed in vacuum at 800°C for 2 h at a pressure of 6.65 10-3 Pa to remove hydrogen from the metal. This annealing did not cause noticeable grain growth. After vacuum annealing, the requisite amount of hydrogen was introduced into the preforms. In hydrogen charging, the preforms were subjected to a 2-h hold at 800°C in a medium of atomic hydrogen in order to attain a uniform distribution of hydrogen over the volume of the metal. After hydrogen-charging annealing, the specimens were cooled with the furnace. The correspondence of the hydrogen content in the preforms to the design value was checked by a spectral method. The mass fraction of hydrogen in the specimens studied corresponded to 0.003.0.0 I, 0.03, and 0. 1%. The microstructure of the rods and plate of alloy VT6ch as received consisted of worm-like grains of or-phase with ~3-1ayers. Vacuum annealing causes globularization of the a-phase, and introduction of hydrogen causes coagulation of
At room temperature hydrogen affects the mechanical properties of titanium alloys of the VT6 type subjected to tensile and impact tests very little [1]+ The impact strength of VT6 alloy of the standard composition begins to decrease substantially only when the mass fraction of hydrogen attains 0.2%. Alloys of the VT6 type are highly susceptible to slow fracture caused by hydrogen [1]. In tests by the Troyano method at room temperature for 100 days the disruptive stresses for annealed VT6 alloy are decreased considerably at a mass fraction of hydrogen above 0.03%. The susceptibility of VT6-type alloys to slow fracture depends substantially on the kind of heat treatment and the type of microstructure [2, 3]. The critical concentration of hydrogen at which the disruptive stresses begin to decrease in tests at room temperature is equal to 0.03, 0.05, and 0.015% t'or annealed, hardened, and aged VT6, respectively. The critical hydrogen concentration for annealed VT6ch alloy with a lamellar structure is much lower (0.01%) than that for an alloy with a globular structure (0.03%) [2, 3]. The effect of the temperature on slow fracture of titanium alloys caused by hydrogen was studied for one alloy (Ti - 4% AI - 4% Mn) in 1968 [ I ]. Greater attention has been devoted to the growth of preliminarily created cracks in slow fracture induced by hydrogen. It has been established in [4, 5] that for temperatures ranging from - 77 to + 20c'C hydrogen embrittlement can develop in tests for crack resistance at hydrogen concentrations lower than is envisaged by the corresponding specifications. The rate of growth of cracks induced by hydrogen in Ti - 6% AI - 4°/; V alloy is maximum at 0°C [3, 4]. The aim of the present work is to evaluate the susceptibility of VT6ch alloy to hydrogen embrittlement. For this purpose we studied the effect of the test temperature on the meK E+ Tsiolko~skii Moscow State Aircraft Engineermg Uni~,ersity, Moscow. Russia,
531 ¢~)26-0673 96 1112-0531$15 ~ ¢; 1997 Plenum Publishing Corporalion
532
B . A . Kolachev et al.
the c~-phase and increases somewhat the amount of the 13-phase. At a 0. I% concentration of H in the alloy we determined traces of hydrides. Tensile tests were conducted in an FP-100 machine with a mechanical drive that provides constant-velocity displacement of the mobile crosspiece. Low-temperature tests were conducted using a cryostat to cool the preforms to - 7 7 ° C by a mixture of "dry ice" and gasoline. Tests a t - 4 0 ° C were conducted in a mixture of ice and calcium chloride (CaCI 2 ___6H,O), tests at - 18°C were conducted in a mixture of ice and sodium chloride, and at 0°C in pure melting ice. In order to eliminate the effect of the medium the specimens were coated with a gun lubricant. The test temperature was maintained accurate to ~ 2°C. Tests at cryogenic temperatures were conducted in liquid nitrogen, hydrogen, and helium. Five specimens were tested at each temperature, and the mean value of the parameter studied was determined. The susceptibility of alloy VT6ch to slow fracture at low temperatures was determined by the method of Troyano [I] on round specimens with a V-shaped notch with a tip radius of 0. I mm. The mean value for three tests was determined. ',. Tests for fracture toughness were conducted by off-center stretching of standard fiat specimens 25 mm thick under conditions of plane deformation. The notch was oriented along the direction of rolling. In order to determine the nature of the fracture the appearance of ruptures was examined and cross sections of rupture specimens were analyzed macro- and microscopically. A
more detailed analysis of the ruptured zones was conducted with the help o f a Stereoscan microscope. Mechanical properties in stretching of rods 22 mm in diameter of alloy VT6ch with different microstructures and contents of hydrogen are presented in Table I. it should be noted that the table does not give values of the yield strength o02 or the contraction ~, which depend on the temperature and the hydrogen content like the ultimate rupture strength and the elongation, respectively. The strength properties and plasticity characteristics of rods with a globular structure are for the most part somewhat higher than for specimens with a lamellar structure. An exception is specimens with 0.003% H tested at temperatures ranging from - 196 to -269°C. The impact strength a t of specimens with the investigated structures is the same at + 20 and - 77°C. At lower temperatures a t of specimens with a lamellar structure is lower than that of specimens with a globular structure. At temperatures of + 20 and - 77°C hydrogen hardly affects the ultimate rupture strength or the yield strength of the alloy independently of the type of structure, although for a lamellar structure they are lower. A t - 196°C the rupture strength of specimens with a globular structure increases with increase in the hydrogen content, whereas in specimens with a lameilar structure it remains constant. At - 296°C o r and o t decrease with increase in the hydrogen content from 0.003 to 0.03%. At room temperature hydrogen in an amount of 0.1% does not decrease 8 and ~. Below this temperature the negative effect of hydrogen on the plastic characteristics manifests itself more strongly the lower the test temperature. The critical concentration of hydrogen Her above which the elongation and the contraction begin to decrease noticeably is presented below for different temperatures.
TABLE I
t,°C 20
- 77
-196
-253
H.%
a r , N/mm 2
ai,J/cm 2
0.003
890/810
18/9
100/100
0.01
900/825
20/9
80/80
0.03
900/840
18/9
81/79
0.1
910/850
18/11
75/75
0.003
1140/1060
16/8
48/48
0.01
1220/1070
14/6
40/39
003
1120/1020
10/5
32/33
0.1
1120/1020
10/5
29/29
0003
1325/1370
12/8
36/21
001
/1360
--/4
32/20
0,03
1400/1370
12/5
28/17
0~l
32/16
1460/1380
9/I
0003
--
--
0.01
.
.
.
.
.
0~03 (kl -.269
5,%
24/17 27/15
. . . . . .
25/13 25/12
0003
1650/1950
8/9
-
0,01
1720/1550
8/8
--.-
0,03
1560/1430
7/4
.........
OI
1690/1550
4/I
.......
Note. The numerators present the properties of specimens ~,ith a globular structure, the denominators present the ',alues tbr specimens with a lamellar structure.
t?C 20 - 77 - 196 - 269
........................ ........................ . . . . . . . . . . . . . . . . . . . . . . ........................
Hcr, % > 0.1/:>0.1 0.01/0,0 I 0.03/0.003 0.03/0.0 I
Note, The numerators present the critical concentration for specimens with a globular structure, the denominators present the values for specimens with a lamellar structure.
In tensile tests conducted at - 2 6 9 ° C the deformation is jumpwise and the number of jumps decreases with increase in the hydrogen content. In our tests the number of jumps for 0.003, 0.3, and 0.1% H was I1, 8, and I, respectively. Jumpwise deforrnation at the temperature of liquid helium is usually associated with adiabatic effects [8]. In this connection we can infer that hydrogen should change the thermophysical properties of titanium and its alloys. The impact strength of alloy VT6ch with both types of structure at room temperature decreases with increase in the hydrogen content from 0.003 to 0.01% and remains virtually constant with change in the hydrogen concentration from 0.01 to 0.1%. A decrease in the impact strength in the range of superlow hydrogen concentrations has been observed for some ct + 13titanium alloys [I]. Only when the amount ofhy-
Effect of Temperature on the Susceptibility of VT6ch Alloy to Hydrogen Embrittlement
KI, •
N/mmV2
% 2, N / m m 2
53.t,M~ o-,-Q. ~ . , ~ ~
1700
-
47,500 O
_,60o
0
~
~
,...1" O02
~
1500
t4o0 /c . m m L0 ~-
2.5 ~
" /c
2.O 0
' 0025
0~05
0075
H. %
Fig. I. Dependence o f the yield strength o 0 2 , the critical coefficient o f stress intensity KI, • and the permissible crack length /¢ at - 253°C in specimens o f a 30-ram-thick plate o f alloy VT6ch on the content o f hydrogen.
drogen was quite large ( - 0.2%) was a second, marked jump in the impact strength (to almost zero) observed [1]. It has been established in [9] that the critical coefficient of stress intensity K~ for T i - 6% A 1 - 4 % V alloy is also decreased at hydrogen concentrations ranging from 0.001 to 0.005% and then remains constant. Since at room temperature the impact strength is decreased at superlow concentrations of hydrogen, whereas at a conventional content of the latter it remains at a quite high level, this effect is not taken into account in evaluating the maximum permissible concentration of hydrogen. At low and cryogenic temperatures an increase in the hydrogen concentration causes a slight smooth decrease in the impact strength. A decrease in the test temperature has a stronger effect than increase in the hydrogen content. Figure 1 presents the dependence of the fracture toughness of a 30-mm-thick plate of alloy VT6ch at the temperature of liquid hydrogen. The thickness of the specimens permitted reliable values of the critical coefficient of stress intensity Kleto be obtained; the Pmax/PQ ratio did not exceed 1.1. The permissible crack length Ic was determined from the formula lc ~ 2.5 (Kic/Oo~2)2. It can be seen that the fracture toughness and the permissible crack length lc remain constant up to 0.01% H and then decrease considerably. A fractographic analysis of the tested specimens did not show signs of brittle fracture at all, even in specimens with 0. I% H after tests in liquid helium. The fracture surthce had a dimple structure without spalling. Since traces of titanium hydrides were observed only at a 0.1% hydrogen concentration, the decrease in the plasticity parameters of hydrogen-charged specimens should be connected with solution strengthening of the [:~-phase, Only the almost zero elongation (~ 1%) of the specimens with 0. I% H and a lamellar structure at the test temperatures of - 196 and - 269°C can be a consequence of hydride embrittlement. At temperatures ranging from - 1 9 6 to - 2 6 9 ° C hydrogen embrittlement of the second kind [ 1 - 5 ] , caused by transfer of hydrogen, does not develop in titanium alloys;
533
only hydrogen embrittlement of the first kind, caused by a source (dissolved hydrogen, hydrides) that is present in the initial hydrogen-charged metal before any stress is applied to it, can develop [10]. For this reason, the maximum permissible hydrogen concentration in an alloy to be used for structures (parts) operating in this temperature range should be established in tensile tests and tests for impact strength. For alloy VT6kt to be used at cryogenic temperatures the permissible hydrogen concentration is 0.006% [6]. It follows from the data presented above that at ( - 196°C) - (-269°C) hydrogen embrittlement does not develop in alloys with this concentration. Moreover, we have grounds to assume that the permissible hydrogen concentration can be increased to 0.01%. It should be noted that a standard for alloy T i - 6 % AI - 4 % V (ELI') establishes a maximum permissible hydrogen concentration equal to 0.1% [7]. At the same time, the results of our tests cannot be a basis for evaluating the maximum permissible hydrogen concentrations at ( - 170°C) - (+20°C) (or somewhat higher) because these conditions are favorable for hydrogen embrittlement of the second kind due to decomposition of hydrogen-supersaturated solid solutions and to diffusion of hydrogen under the effect of stresses [10]. In order to determine the possibility of development of hydrogen embrittlement of the second kind tests should be conducted for tension at low deformation rates, slow fracture, and the rate of crack growth. We studied the effect of the test temperature at different deformation rates for specimens of alloy VT6ch with a globular structure that contained 0.003 and 0.05% H. The plastic properties of specimet.s with 0.003% H depend but little on the deformation rate in the range from 1~3 x 10 -3 to 1.5 × 10 -3 sec ~ at ( - 77°C) - (+ 20°C). Under these test conditions all the specimens had W ---43%. On the other hand, the temperature dependence of the contraction of alloy VT6ch with 0.05% H has a minimum at about 0°C (Fig. 2). The depth of this minimum and the width of the diminished contraction values decrease with increase in the detbrrnation rate, and at ~: = 1.6 x 10 -2 s e c - I (24 mm/min) we did not observe signs of hydrogen embrittlement in the alloy. Such plasticity minima have been observed at ( - 50 °) - (+ 20°C) in titanium alloys VT3-1, VTI5, and Ti-140A (2% Fe, 2% Cr, 2% Mo, the remainder Ti) [1] and in various steels and nonferrous metals and alloys [10]. These plasticity minima are associated with transportation of hydrogen atoms by sliding dislocations in a certain range of temperatures and deformation rates [ 1, I 0]. A decrease in the plasticity with decrease in the deformation rate occurs in tensile tests of specimens of alloy VT6ch with 0.05% H (Fig. 3). In the initial specimens not charged with hydrogen (0.003%) the elongation does not change. Hydrogen-charged specimens are elongated considerably compared to the initial specimens at high detbrmation rates, which can be caused by the increase in the amount of [3-phase in titanium alloys that accompanies their hydrogen-charging or by the diminished sharpness of the notch in hydrogen
534
B.A. Kolachev et al.
%,
N/mm
O r , N/ram 2
2
k
1200
2
....
3,4
I10~
IO(K
I
I0
I00
t, h
3(3
20
Fig, 4. DeFendence of the disruptive stresses a on the time of their action r for alloy VT6ch with 0.05% H for different temperatures (the vertical arrows indicate that the specimens broke in the tests, the horizontal arrows indicate unfractured specimens): e ) + 20°C; O) 0°C; I"1) - 18oC: &) -50°C; ×) - 77°C,
"'---2 !
40
- 20
0
t, °C
Fig. 2. Temperature dependences ofer r and ~ of alloy VT6ch with 0.05% H in tensile tests with deformation rates(sec -I ): / ) 1.3 × 10-4; 2) I x 10-3; 3 ) 2 × 10-3; 4 ) 4 x 10 -3,
o r , Nlmm
2
1500 ,~.
,,
~
-~
i --,{ 1450' O " - ~ " ~
'~ ,O. . . .
1400 j
6,%
0
,/
c~,.,..f 5
9
13 c , ~ 1 0 I~ sec l
Fig. 3. Ultimate rupture strength and elongation of notched specimens of alloy VT6ch with O05°i) H as a function of'the teslmg rate at a temperature of: A) 50~'C O) 18°C: e){}°C
annealing. It should be noted that at 0°C a decrease in the deformation rate decreases cL, which is not obse~'ed at - 18 and - 50°C. At room temperature hydrogen-induced slow fracture of alloy VT6 manifests itself at > 0.03% H. For specimens with 0.05% H tested for 100h the disruptive stresses equal
1 0 0 0 N / m m 2 instead of 1 4 5 0 N / m m 2 in short-duration stretching [2, 3]. In this connection we chose a concentration of 0.05% H, which is known to exceed the critical value for standard VT6 alloy, to assess the effect of the temperature on the development of slow fracture. Not a single specimen o f alloy VT6ch with 0.003% H under a stress o = 0.90 r broke in ! 00 h at any temperature ranging from - 7 7 to + 20°C. On the other hand, specimens of VT6ch alloy with 0.05% H were susceptible to slow fracture (Fig. 4). At the same time, alloy VT6ch from the heat studied turned out to be less susceptible to slow fracture than alloy VT6 studied in [I - 3]. This difference seems to be connected with the lower content of oxygen in alloy VT6ch (0. I%) relative to alloy VT6 (0,015%), Slow fracture in alloy VT6ch develops most intensely at room temperature. Based on this fact, we can make the important practical conclusion that the maximum permissible hydrogen concentration in semifinished products (0.01%) and the safe concentration in structures (0.015%) established from tests for slow fracture at room temperature can be generalized to operating temperatures ranging from + 20 to - 7 7 ° C [1 - 3 ] . Slow fracture in alloy VT6ch can be associated with transportation of hydrogen atoms by sliding dislocations. Indeed, upward hydrogen diffusion plays a certain role in slow fracture at comparatively low stresses o = ( 0 . 5 - 0 , 8 ) o r, when the sliding of dislocations is unlikely. At high static stresses o = ( 0 . 8 5 - 0 . 9 5 ) o r the main contribution to slow fracture can be made by dislocation-transported hydrogen atoms. Tensile tests of notched specimens at different deformation rates indicate this possibility. As a result of transportation of hydrogen atoms to the tip of a crack, the concentration of hydrogen in the zone of triaxial stretching betbre it can be sufficient for segregation of hydrides and development of hydride embrittlement. Autocatalytic initiation and development of hydrides before a crack tip have been observed in ex
Effect of Temperature on the Susceptibility of VT6ch Alloy to Hydrogen Embrittlement
periments with alloy VT5L [2] and alloy Ti - 6% AI - 4% V
535
REFERENCES
[ll], CONCLUSIONS I. Tensile tests at standard deformation rates and impact tests o f alloy VT6ch show its low susceptibility to hydrogen embrittlement for temperatures ranging from + 20 to - 2 6 9 ° C . A considerable decrease in elongation caused by hydrogen is observed only at - 1 9 6 and - 2 6 9 ° C for rods with a lamellar structure that contain over 0.003% H. Purely brittle fracture on a microscopic scale has not been observed at any hydrogen concentration, even at the temperature o f liquid helium. 2. The elongation and contraction at all the temperatures and the impact strength at - 2 5 3 and - 196°C are considerably lower in alloy VT6ch with a lamellar structure than in this alloy with a globular structure at all hydrogen concentrations. 3. At - 2 5 3 ° C the fracture toughness o f alloy VT6ch does not depend on the hydrogen concentration up to 0.01% H and fMIs at higher concentrations. 4. In the range from - 5 0 to + 20°C the plasticity parameters in tensile tests exhibit a minimum, and its depth increases with decrease in the deformation rate. 5. Hydrogen-induced slow fracture o f alloy VT6ch develops most intensely at temperatures close to room. 6. A concentration o f 0.01% H should be considered the maximum permissible one in VT6ch alloy operating at low and cryogenic temperatures if the alloy possesses a globular structure.
I. B. A. Kolachev, V. A. Livanov, and A. A. Bukhanova, Mechanical Properties of Etanium and Its Alloys [in Russian], Metallurgiya, Moscow ( 1974), 2. B. A. Kolachev and A. V. Mal'kov, Physical Foundations Of Fracture of Etanium [in Russian], Metallurgiya, Moscow ( 1983L 3. B. A. Kolachev, "Hydrohen embrittlement of titanium and its alloys," in: Etanium. Physical Metallurgy and Technology. Proc. 3rd Int. Conf on Etanium [in Russian], Vol. I, VILS, Moscow (1977L 4. N. Paton, "Low temperature hydrogen embrittlement of titanium alloys," in: "iTtanium: Sci. and Tech., Proc. 5th Int. Conf on /7tanium, Munich, 1984, VoL 4, Oberursel (1985), pp. 2519 - 2526. 5, D. A. Meyn, "Temperature dependence of maintained loud cracking caused by residual hydrogen in Yi - 6AI -- 4V," in: /7tanium: Sci. and Tech., Proc. 5th Int. Con/? on /Ttanium, Munich, 1984, Vol. 4, Oberursel (1985), pp. 2565 - 2570. 6. A. G. Bratukhin, N. F. Anoshkin, V. N. Moiseev, et al., "'Use of titanium alloys tbr aircraft structures," ITtan, No. 1, 7 7 - 81 (1993). 7. "'Age hardenable titanium alloy," Alloy Digest, Sept., II - 12 (1992). 8. D. A. Wigley, Mechanical Properties o f Materials at Low Temperatures, Plenum, New York ( 1971 ). 9. K. K. Chen and D. E. Koin, "Dependence of the fracture toughness of forgings of Ti - 6AI - 4V alloy on the concentration of hydrogen and the cross-sectional area," in: Etanium. Physical Metallurgy and Technology, Proc. 3rd Int. Con./'. on /Ttanium [in Russian], Vol. 1, VILS, Moscow (1977). 10. B. A. Kolachev, Hydrogen Embrittlement q[" Metals [in Russian], Metallurgiya, Moscow (1985). 1I. H.Z. Xiao, S. Y. Gao, and X. Y. Wan, "In-situ TEM study of the strain-induced hydrides in T i - 6 A I - 4 V , " Z Electron. Microsc., 35 Suppl.(2), 1581 - 1582 (1986).