- Email: [email protected]
Austenitic stainless steels at cryogenic temperatures 1—Structural stability and magnetic properties
The structure and magnetic properties of some 15 austenitic stainless steels were examined after cyclic cooling treatments and low temperature deformation. Magnetic measurements at room temperature, 77 K, and 4.2 K and subsequent metalIographic examination suggest that many of the AISI 300 stainless steels such as 301, 302, 303, 304, 304L, 305, 316L, 321, and 347 must be considered potentially unstable with respect to the formation of the ferromagnetic ~' martensite phase on repeated cooling to low temperatures. This structural instability was increased significantly after a sensitizing treatment in the weldable steels 304L, 321, and 347 leading to the formation of up to 11.2% a' martensite, part
of which formed isothermally. Low temperature deformation is even more potent in promoting the transformation, at least 50%0~' martensite being induced by deformation at 4 . 2 K in the otherwise stable alloys such as 309 and the 0.2% N versions of 304L and 316L. The high alloy steels 310 and Kromarc 55 remain fully austenitic even after deformation to rupture at 4.2 K. The temperature dependence of the magnetic susceptibility of the latter alloys and Incoloy 800 indicates that their low temperature structural stability is associated with magnetic transitions which occur within the austenite phase.
Austenitic stainless steels at cryogenic temperatures 1--Structural stability and magnetic properties D. C. Larbalestier and H. W. King The austenitic stainless steels are widely used as constructional materials at cryogenic temperatures because of their high strength and excellent toughness even at temperatures down to 4.2 K. Their fabrication properties do not present any special grounds for concern, while their rustless properties are a considerable advantage in conditions where condensation of water vapour is frequent. A further important property of these steels is their low thermal conductivity which makes it possible in many instances to combine the load-bearing and thermal insulation functions in the same structure.
and Reed 3 also reported stable as well as unstable samples of both 304 and 304L in a survey of the mechanical properties of these steels at 4.2 K. Further, the previous finding of the present authors, 4 that a superconducting solenoid former made from the British equalent to AISI 321 transformed to ~5% a' on cooling to 4.2 K, showed that other grades besides 303 and 304 can become unstable at low temperatures. This latter work also pointed to other problems which can arise when such steels are subjected to changing magnetic fields with their consequent eddy currents.
Many of the machines and instrumentation designed for low temperature operation utilize magnetic fields and it is thus an additional important property of the AISI 300 stainless steels that the austenite phase, 7, of the 18 Cr[ 10 Ni grades is non-ferromagnetic. These steels are, however, metastable at or not far below room temperature and can thus be transformed to a ferromagnetic martensite, a', either by cooling or by the application of cold work.
The problem of the low temperature decomposition of austenite may be further aggravated in certain circumstances, since in addition to being ferromagnetic the a ' martensite phase has a higher specific volume then austenite, so that its formation is accompanied by a volume expansion. To find the clamping bolts of a liquid hydrogen bubble chamber expand instead of contract on cooling, can provide considerable cause for anxiety!
The possibility of the martensitic transformation occuring solely by cooling to low temperatures, was studied for a wide range of the AISI 300 stainless steels by Reed and Mikesell. 1 These authors found that, of the grades 302, 303,304,308,310,316,321, and 347, only samples of 303 and 304 showed any transformation from 7 to a ' after repeated cycling between room temperature (RT) and 77 or 20 K. Their study has so far remained the only comprehensive survey of the influence of cryogenic cooling on the martensitic transformation in the AISI 300 stainless steels even though subsequent isolated results have often been contradictory. Gonser et al, 2 for example measured the N~el temperature of a sample of 304 as 38 K, thus implying that no transformation to a ' had occurred, at least down to that temperature. Guntner
With these factors in mind, a comprehensive study was initiated on the effect of both cyclic cooling and deformation at 4.2 K on the low temperature stability and magnetic properties of the 300 series of stainless steels. The present paper gives general details of the different types of magnetic behaviour exhibited by these steels between RT and 4.2 K and the effect of mechanical and heat treatments on the transformation to a'. Methods for predicting the stability of a given steel will be discussed in a second paper 5 while a third 6 will be concerned with the relationship between magnetic structure and the relative stability of the 3' and a ' phases.
This work was carried out at the Department of Metallurgy, Imperial College, London, UK. DCL is now with Battelle Research Centre, Geneva, Switzerland and HWK with the Department of Engineering and Engineering Physics, Dalhousie University, Halifax, Nova Scotia, Canada. Received 28 July 1972.
160
Materials and experimental methods The alloys examined in this study were kindly made available by Messrs Firth Vickers Ltd, Samuel Fox Ltd (Special Steels Division of British Steel Corporation), and the Rutherford High Energy Laboratory. The chemical analyses of these alloys are listed in Table 1. The majority of the steels belong to the AISI 300 series of austenitic F e - C r - N i
C R Y O G E N I C S . M A R C H 1973
Table 1. Chemical analysis of stainless steels (All compositions are in wt %)
Alloy
C
Cr
Ni
Si
Mn
S
P
17.98 17.92 22.44 23.95 16.77 17.22 17.18 19.18
10.53 10.47 14.25 21.09 12.43 10.18 9.65 31.36
0.77 .49 0.54 0.75 0.38 0.60 0.72 0.56
1.37 1.6 1.23 1.65 1.49 0.77 0.70 0.76
0.08 0.02 0.01 0.03 0.03 0.01" 0.02 0.004
0.02 0.01 0.02 0.007 0.02 0.02 0.02 0.02
10.44 11.55 14.17 21.00 13.5 12.56 20
0.40 0.40 0.46 0.62 0.77 1.52 0.4
0.70 .1.30 1.17 1.57 0.27 0.41 9
0.013 0.008 0.003 0.014 0.016 -
0.020 0.022 0.020 0.028 0.033 -
Others
Cyclic cooling specimens 304L 304N 309* 310 316LN 321 347 Incoloy 800
0.031 0.061 0.120 0.075 0.025 0.050 0.071 0.014
-
0.181~* 0.18N*; 2.40Mo 0.41Ti 0.68Nb
Low temperature deformation specimens 304L* 304LN* 309* 310* 316L* 316LN* Kromarc 55t
0.020 0.011 0.120 0.060 0.028 0.027 0.04
18.63 17.30 22.32 24.64 18.1 17.5 16
0.2N 3.0Mo 0.12N, 2.8Mo 0.2N, 2Mo
*Cast analysis tTypical analysis Remaining data refer to specimen analyses
stainless steels; those with the non-standard N suffix denote high nitrogen content (a nominal 0.2 wt%) while the L suffix refers to the standard low carbon steels (0.03 wt% max). Incoloy 800 and Kromarc 55 complement the AISI steels, since they have a significantly higher substitutional alloy content; Incoloy 800 is an F e - C r - N i steel with 31% Ni, while Kromarc 55 is a special F e - C r - N i casting alloy containing 9% Mn. The steels used for cyclic cooling experiments were all supplied in the form of plate or bar stock, in the nominally fully-softened condition. These were machined into rods 4 mm in diameter and cut into adjacent 30 mm and 3 mm lengths for magnetization and metallographic specimens, respectively. Certain samples were pickled for half an hour in 30% HNO 3 at 50°C to dissolve any iron possibly picked up during machining, but subsequent magnetic measurements revealed this to be an unnecessary precaution.
adjacent to a fusion weld. Typical microstructures of the steels after the different heat treatments are shown in Fig.l, for steel 304L. The twinned matrix is the 7 austenite phase, while the precipates are carbides, phosphides, and sulphides. The figure also show~ the additional
a
In order to duplicate in the laboratory some of the conditions encountered in the general manufacture of cryogenic components, specimens were given one or more of the following prior treatments. (a) As-machined. Specimens left in the as-machined condition.
f
• °
•
(b) Fully-softened. Heated at 1 075°C for ½ hour and water quenched. (c) Sensitized. Heated for 1 week at 700°C and air cooled. Both magnetic and metallographic specimens were sealed together in evacuated silica capsules for these heat treatments. The as-machined condition reflects the condition of most stainless steels used in cryogenic engineering. Some small components may be fully-softened after manufacture to remove stresses or phase transformations due to machining or fabrication by welding. The sensitizing treatment reproduces in an extreme form the precipitation of carbides which occurs in the heat affected zone
CRYOGENICS. MARCH 1973
b
Fig.1 Microstructure of steel 304L -- Fully softened; b - sensitized at 700°C for 1 week
161
precipitation of carbides at twin and grain boundaries after the sensitizing treatment. The austenite grain size of the as-machined and the sensitized specimen of this steel is ASTM No 5, a value typical for most of the alloys studied, though Incoloy 800 and Steel 347 had grain sizes of 7 and 8 respectively. The fully-softening treatment in general caused the grain size to increase (a reduction of one or two units on the ASTM scale). Since a difference of one unit in grain size is known to raise the M s temperature by only 4 K in these steels, 7 the effect of grain size was not introduced as a variable in the present investigation. The alloys used for the low temperature deformation study are listed in the lower section of Table 1. These were received in the form of standard tensile test-pieces, supplied by the Bubble Chamber Group of the Rutherford High Energy Laboratory at the completion of their low temperature mechanical testing programme. Preparation of the test-pieces was by machining from original stock, except for the sample of Kromarc 55, which was cut from a section of a liquid hydrogen bubble chamber. Disc shaped magnetization specimens 2 - 3 mm thick were cut across the section of the test-pieces, both in the shoulder region and across the gauge length. In samples which had been deformed to rupture, the magnetization specimens were cut from the gauge length region adjacent to the fracture. Two types of martensite can form on cooling stainless steels below a critical temperature, M s, or by.deforming them below a critical temperature, M d. The martensite cz' is body centred cubic. This phase is closely related to the ferrite phase, cz, in pure iron and is also ferromagnetic at room temperature and below. The hexagonal closepacked martensite, e, on the other hand has magnetic properties indistinguishable from the FCC austenite phase and remains naramagnetic. These two types of martensite can ooviously be distinguished by x-ray diffraction, 8 but this method of detection and measurement is destructive to the specimen. Metallographic examination can also be used, since differential etches have been developed to distinguish the two phases. 8 This technique however, also tends to be destructive to a specimen, unless confined to the examination of the ends of the rod shaped specimens which may not be representative of the bulk. Hence, since the prime interest of the present study was concerned with the occurrence of ferromagnetism in stainless steels, magnetic methods were adopted to determine the presence and amount of the ~' martensite after various cooling or deformation treatments. The magnetization measurements in isolation are not necessarily unambiguous, however, since small quantities of the high temperature form of ferrite,/i, can also be found in the austenitic stainless steels. In the present samples, however, magnetization measurements made before exposure to cryogenic temperatures showed that 8-ferrite, if present, was always less than 0.1%. Magnetization measurements at low temperatures were made with a dc integration magnetometer while continuously sweeping the field. This instrument had an ultimate sensitivity of-+0.5 G, which allowed the detection of less than 0.1% ofcz' martensite. The apparatus was fitted in a liquid helium dewar in a 3 kOe water cooled copper solenoid and magnetization loops in increasing and decreasing field were recorded at RT, 77 K, and 4.2 K.
162
Since this field was ot)viously insufficient to saturate the specimens and thus allow a direct conversion from magnetization measurements to martensite contents, additional measurements were made at room temperature only, in a Sucksmith type force balance using the 3 mm length metallographic specimens and the discs cut from the tensile test-pieces. Since any ~' martensite formed at low temperature remains stable on warming up to room temperatures, 8 the magnetization values at a field of 14 kOe were converted to a ' martensite contents using the formula of Curtis and Sherwin, 9 who showed that the saturation moment of ferrite in stainless steels of these compositions can be expressed by the weighted mean of their Fe, Cr, and Ni contents, where the individual atoms are assigned moments of 2.22, 0.22, and 0.60 Bohr magnetons, respectively. The saturation magnetization of these alloys is thus clearly less than that of pure iron, but is still considerable, as shown by the figure o f ~ 1 6 000 G, derived for 100% ~' in the 18 Cr/10 Ni alloys. Magnetization measurements in the copper solenoid were taken at fields of 1.9 kOe and 2.3 kOe. These were converted to a ' martensite contents by observing the approach to saturation in the Sucksmith balance which showed that the specimens were 60% and 75% saturated at these respective fields. Using these values, the relative accuracy of the a ' martensite determinations was estimated to be -+5%.
Results Effect of cooling In initial experiments, samples of the steels in various conditions were examined in the magnetometer at 4.2 K. The measurements gave different types of magnetization curve as illustrated in Fig.2. The samples of steels 304L, 304N, 309, and 316LN gave plots which were linear with field within the limits of error, suggesting that the samples were paramagnetic or antiferromagnetic. This means that no a ' martensite forms in these steels on first cooling to 4.2 K. Non-linear magnetization curves, indicative of ferromagnetic behaviour, were observed for steels 321, 347, and 310, while a pronounced hysteresis was observed in the sample of Incoloy 800. The magnitude of the ferromagnetic magnetization in Incoloy 800 indicated that in this very high nickel alloy the FCC 3' phase may itself become ferromagnetic, by passing through a Curie Point, instead of transforming to ~' martensite. This possibility must also be considered for the other high nickel alloys such as 310. The two sources of ferromagnetic behaviour can easily be distinguished, however, since the formation of the ~' martensite is an irreversible process at these temperatures in these alloys. The magnetization measurements were thus repeated on all the alloys at RT and on a subsequent cool down to 77 K. These results revealed that the steels followed one of three basic patterns on cooling to cryogenic temperatures. (a) All samples of 304L, 304N, 309, and 316LN gave reversible, small, non-ferromagnetic magnetization curves, which were linear with field up to at least 3 kOe, at all temperatures studied. These steels thus remain fully austenitic. (b) Steels 321 and 347 showed weak ferromagnetism on the initial cool down to 4.2 K. This ferromagnetism
CRYOGENICS. MARCH 1973
600
2000
321
500
16OO O O
400
s"
E
,.J O N
.o
1200
8,
300
O
8'
800 771~
200 400 I00
~.. ...---"-"r"-"
I
a
304 L
J
I
l
2
3
4
I C
Fig.2
Field, kOe
2
3
4
Field, kOe
Typical magnetization loops
a -- 304L and 321 at 4.2 K; b - 310 at RT, 77, and 4.2 K; c -- Incoloy 800 at RT, 77, and 4.2 K
350 4.2K
was retained on returning to RT and increased in magnitude when the steels were subsequently measured at 77 K and again at 4.2 K. These specimens thus transformed to a ' martensite, the amount of which increased with each subsequent cooling to low temperature.
300
(c) The magnetization in steels 310 and Incoloy 800 was found to be variable with temperature but regardless of the order or number of cooling treatments was reproducible at any given temperature, as illustrated in Fig.2.
250
In order to investigate this behaviour more fully, the ac susceptibility of these alloys was followed over the temperature range 2 0 - 4 K for 310 and from 9 0 - 3 0 K for Incoloy 800. A sharp, reproducible increase in susceptibility was obtained at 7 and 74 K, respectively and this increase was associated with a paramagnetic-ferromagnetic transition within the austenite phase. While this appears to be a genuine Curie point for Incoloy 800, the absence of any hysteresis (<1 G) in the magnetization curve of steel 310 at 4.2 K suggests that a local clustering or a form of superparamagnetism probably occurs in this alloy. Measurements made in fields greater than 100 kOe, which shed more light on this transition will be discussed in more detail in a subsequent paper. 6
200
5
N C O
150
I00
50 77K
298 K O
I
Field, k0e
b CRYOGENICS.
2
MARCH
1973
3
4
No martensitic transformation was observed in any sample of the group (c) steels when they were continuously examined at temperatures down to 77 K, at a magnification of x330, using the low temperature microscope of King and Pollock. 10 Metallographic examination at RT of all the remaining alloys after the cooling treatment to 4.2 K, showed that martensitic structures were only present in the alloys listed in group (b) above. The martensitic structures observed in alloys 321 and 347 after many cooling cycles to 77 K are illustrated in Fig.3. These are
163
sirr.!lar to those reported previously by Lagneborg 11 and by Reed. 8 The martensite in steel 347 is plate-like, but it is difficult to resolve the detail between the plates. Similar plates, but more widely separated were also observed initially in steel 321 and after repeated cycling the area between the plates increasingly transformed. This area etched darkly with a preferential stain etch developed by Reed ~ for identifying a ' martensite. The structures are thus similar to the morphology reported by Lagneborg, 11 where e martensite first appears as parallel pairs of plates in preferred orientations in the 7 phase, while the a ' martensite forms between these plates. Almost all the plates were formed during the first or second quench and on repeated quenching subsequent transformation was predominantly ct', giving the plates a 'filled-iri' appearance as in Fig.3b. It is interesting to note in passing that many of the original plates of martensite appear to have nucleated from precipitate particles and hence the presence or absence, of carbide phases may have at least a secondary, if not a primary, effect on the M s temperatures of these alloys. It was also observed during the course of the above experiments that a certain amount of ~t' martensite forms isothermally in these steels. This was most evident in a sample of steel 347, when an isothermal increase in magnetization from 461 to 478 G occurred over a period of 8 minutes, while the magnetization loop was being
~f
''L
#'
e
•
%',
•
ql~,
•
•
m
w
e.
50~um a
ql
zcrjcu~
'
Fig•3 Microstructure of fully-softened specimens of steels after cyclic cooling treatments to 77 K and 4•2 K a -- steel 347; b -- steel 321
164
To determine whether steels become unstable on continued cycling and if the amount of martensite formed during cooling approaches a constant value for a given steel, a composite quench was devised to maximize the amount of athermal and isothermal martensite formed in a given cool down. For 77 K treatments, the specimens were plunged into liquid nitrogen, allowed to soak at 77 K overnight in a dewar flask, which was then emptied and stoppered so that the warm-up to RT took place over a period of 6 - 8 hours. A similar treatment for cycles to 4.2 K could not be applied, because of the consequent cost in terms of liquid helium, and hence the specimens were held at 4.2 K for about 5 minutes and then allowed to warm to RT in air. Steels 309, 310, and Incoloy 800 showed no change in magnetic properties after repeated cycles between RT and 77 K or RT and 4.2 K. The previous thermal history of the sample thus had no effect on the magnetization of these alloys, either ferromagnetic or otherwise, which was found to depend solely on the applied field at all temperatures investigated. Also, no trace of martensite was found in any of these specimens and hence it was concluded that, at least in the absence of an applied stress, the 7 phase in all these three alloys remains stable against a martenitic decomposition at temperatures down to 4.2 K.
The nitrogen containing steels 304N and 316LN show no change in magnetic properties as a result of either the prior heat treatment or the subsequent cyclic cooling and, since the limit of a ' detection lies in the vicinity of 0.05%, these steels must be considered perfectly stable• The as-machined and fully-softened samples of steel 304L also showed no effect due to the cyclic cooling, but the sensitized sample of this steel became unstable during the cyclic treatment, so that 1.4% a ' martensite was formed.
v'
~
The effect of cyclic cooling
The effect of prior specimen condition and cyclic cooling treatments on the amount of t~' martensite formed in steels 304L, 304N, 321, and 347 is summarized in Table 2. It is evident that, once a steel is susceptible to low temperature martensitic decomposition, the amount of martensite formed after a given cooling treatment depends on the prior heat treatment given to the specimen•
.olB
b
plotted at 77 K (at H = 2.3 kOe this represents an increase in a ' from 3.7 to 3.9%). On returning to RT and again cooling to 77 K, the magnetization of the sample increased to 570 G (4.6% a ' ) and after 4 hours at 77 K it further increased to 600 G (4.9% t~'). Thus, although direct cooling from RT to 77 K is more potent in increasing the amount of ct' transformation, the isothermal effect at 77 K is still quite significant and must be taken into account when determining the amount of martensite formed at a given temperature.
Both the prior heat treatment and the cyclic cooling influence the stability of steels 321 and 347. After the initial cool down, the amount of martensite is much the same in the as-machined and fully-softened specimens of these steels, but both differ appreciably from the amount of martensite present in the sensitized specimens• After the cyclic treatments, the amount of martensite is increased in all the specimens, but the effect is most enhanced in the sensitized samples, particularly in steel 347 where some
CRYOGENICS. MARCH
1973
11.2% martensite was detected after this treatment. It is also of interest to note that when the as-machined specimen contains more c~' than the fully-softened specimen, then less martensite is observed in this specimen on cyclic cooling. Hence, it appears that a limiting amount of martensite is able to form according to the chemistry of the alloy and the physical mechanism of its formation does not significantly influence the total amount. The rate of approach to a limiting amount of martensite following many cooling cycles was studied by measuring the magnetization of the as-machined metallographic samples in the Sucksmith balance each time they were returned to RT following a composite quench to 77 K. To facilitate these measurements, a small hole was drilled in the centre of each specimen and this deformation was sufficient to induce the formation of ~0.5% t~' in steel 321 and ~1% ~' in steel 347, indicating the relative instability of these two steels. The amount of martensite formed after the cooling cycles, shown graphically in Fig.4, confirms the previous results for the as-machined samples of steels 304L, 304N, and 316LN, which remain unaffected by the cyclic cooling treatments. The findings for the other steels show that while more martensite forms initially in steel 321 the cumulative effect of cyclic cooling is greater in steel 347. Although both steels show a fall off in the amount of a ' formed during each successive cycle, a limiting value of martensite was not achieved, since even after 23 cooling cycles significant changes in magnetization were still detected. This result is similar to that reported by Reed and Mikesell 1 for a sample of 304, which still showed evidence of martensitic decomposition after 3 months of daily cycling between RT to 77 K. Table 2. T r a n s f o r m a t i o n
The low temperature permeability of the martensite containing steels 304L, 321, and 347 at fields of 100 Oe and 1 000 Oe was derived from the full magnetization curves of these steels at 4.2 K. The results are plotted against the amount of a' martensite in Fig.5 and show a variation from 1.35 to 5.15 at 100 Oe and fall to values below 2 at 1 000 Oe. These values, though small, are quite significant, particularly in changing magnetic field situations. It is also of interest to note that the permeability varies
Iooo
347
-6
-4 0
600
_
"u ~o
2 400
-2
0
u~ 2 0 0 304L, 304N, 316LN ....
I 2
f--.f.__
II
--
15
$--
O
23
Cooling cycles to 77K Fig.4 Sucksmith magnetization measurements on as-machined specimens plotted as a function of the number of cooling cycles between RT and 77 K
to c~' martensite after cyclic cooling
O
a ' martensite
Alloy
Condition
Initial cooling*
304L
As-machined Fully-softened Sensitized
nd nd nd
nd nd 1.4%
304N
As-machined Fully-softened Sensitized
nd nd nd
nd nd nd
316LN
As-machined Fully-softened Sensitized
nd nd nd
nd nd nd
321
As-machined As-machined Fully-softened Sensitized
1.7% 2.6% 2.0% 1.1%
3.4% 2.9% 3.7% 3.1%
As-machined As-machined Fully-softened Sensitized
0.3% 1.3% 0.4% 3.4%
3.9% 3.7% 1.3% 11.2%
347
Cyclic coolingt
O =L
O
4
C R Y O G E N I C S . M A R C H 1973
12
2
O o "I
1.5
I
"1 cycle to 4.2 K or 77 K t l 0 further cycles to 77 K and 5 to 4.2 K nd Not detected. Limit of detection 0.05% cz' NB Steels 309, 310, Incoloy 800, and Kromarc 55 showed no transformation to ~x' martensite after any of the above treatments
8 O/o='
f
I 4
i 8
i 12
O/o c~' Fig.5 Plots of permeability (~) versus c~' martensite content at 4.2 K for steels 304L, 321, and 347 at fields of 100 Oe and I 000 Oe
165
Table 3. Magnetic susceptibility of the non-martensitic steels at different temperatures Magnetic susceptibility, X Alloy
Condition
RT
304L 304N
Fully-softened Fully-softened Sensitized Fully-softened Sensitized Fully-softened Sensitized Fully-softened Sensitized Fully-softened Sensitized As-machined
1.86 1.99 2.66 1.66 2.06 1.79 1.86 2.05 2.78 4.25 4.25
309 310 316LN Incoloy 800 Kromarc 55
4.2 K
77 K x x x x x x x x x x x
4.91 4.15 4.53 4.97 5.31 6.63 9.95 5.50 5.70 F F -
10 -4 10 .4 10 .4 10 -4 10 .4 10 .4 10 .4 10 .4 10 .4 10 .4 10 -4
X x x x x x k x x
10 .4 10 .4 10 .4 10 -4 10 .4 10 .4 10 -4 10 .4 10 .4
7 J 6 x 10 .4 5.68 X 10 -4 5.70 X 10 -4 18.7 X 10 .4 16.6 X 10 .4 F F 9.02 x 10 .4 9.02 x 10 -4 F F 3.1 x 10 .4
F Ferromagnetic at this temperature
linearly with the amount of a ' martensite, which implies that there is little magnetostatic interaction between neighbouring martensite plates. This is in accord with the observations of Henkel, 12 that such interactions do not become important until the a ' content exceeds 15%. Beyond this amount, it would be expected that the low field permeability should be significantly greater than that given by the straight line approximation. The magnetic properties of the non-martensitic steels are given in Table 3 in terms of the volume susceptibility x(=M/H). At low temperatures X approaches or exceeds 10-3 which is a sufficiently strong moment to give rise to appreciable magnetic forces in high fields. When the results are plotted as I/x versus temperature as in Fig.6, negative intercepts on the temperature axis are obtained for steels 304L, 304N, 309, and 316LN. This behaviour is that expected of a Curie-Weiss anti-ferromagnet and is in agreement with previously reported N~el temperatures in the region of 40 K for steels 304 2,13,14 and 347.15 In the absence of any other data, the much larger 4.2 K susceptibility of steel 309 suggests that if this alloy passes through a N~el temperature it must be close to 4.2 K. To check this possibility its magnetization versus temperature was measured over the range 4 - 2 0 K and found to peak at 11.5 K. The sample was therefore concluded to be anti-ferromagnetic below this temperature, as discussed in detail in a later paper. 6
Effects of deformation
6
L
/
~7
309 • 304 L ' s304N 316 LN
Fully softened condition 4 ? O x X --. 1
3
"S /#•
/
/x,, /
s'
,
at 4.2 K
As the Rutherford Laboratory programme, on the low temperature mechanical properties of the stainless steels, was conducted independently of the present investigation, it was not possible to use samples of steels from exactly the same heats for both studies. Nevertheless, Table 1 shows that several grades of steel were common to the two series of tests, thus enabling a close comparison to be made between the structural stability of the steels in the presence and absence of stress at 4.2 K. The a ' martensite contents derived from saturation magnetization measurements of specimens cut from tensile test-pieces and examined in the Sucksmith balance, are collected in Table 4. Deforma-
166
tion at low temperature is clearly much more potent in inducing the martensitic transformation than single or repeated cooling. The transformation in steel 304L goes to 83%, while even the high nitrogen versions of 304 and 316, which were stable against cyclic cooling (Table 2),
O '
I00
200
300
Temperature K Fig.6 The reciprocal of the susceptibility (1/×) versus temperature for steels 304L, 304N, 309, and 316LN in the fully softened condition
C R Y O G E N I C S . MARCH 1973
now transform at least 50%. Considerable transformation was also observed in steel 309, which again was stable when subjected to the previous cyclic cooling treatment. The degree of transformation is also clearly related to the amount of strain, being only 0.1% for 304L at 0.5% strain. In the high nitrogen steels and in 309, whereas considerable transformation occurs in the region near the fracture, no transformation is observed in the less strained shoulder region of the specimens. Only steel 310 and Kromarc 55 show no evidence of or" martensite after deformation, even in the region of the fracture on specimens deformed to rupture. This interesting result on Kromarc 55 prompted a measurement of its magnetic properties at 4.2 K. The susceptibility was found to be 3.1 x 10 -4 which lies somewhat lower than the values obtained for the other alloys (Table 3). The precise magnetic state of Kromarc 55 is still uncertain, but it appears that the substantial manganese content may encourage antiferromagnetic behaviour. Discussion
When the present findings on the low temperature stability of the austenitic stainless steels in the absence of stress are compared with the results of previous investigators, many direct conflicts become evident. In the case of steels 304 and 304L, some samples have been reported to be stable, 2,3 while others are found to transform. 1,3 The present sample of 304L remained stable under normal conditions, but became unstable after a sensitizing treatment. Similar discrepancies can also be noted for other grades, with Meiklejohn 15 and Reed and Mikesell 1 reporting both 321 and 347 to be stable, while the present samples of these alloys, and a previously examined sample equivalent to 321,4 were found to be definitely unstable. It is thus evident that, depending on their exact chemical composition or previous heat treatment, these steels can have M s temperatures either above or below 4.2 K. Table 4. Transformation to e" martensite after deformation at 4.2 K
(~' martensite Gauge length
Shoulder region
0.5% strain Fractured; 51.% elong
0.1% 83%
0.1% no test
304N
Fractured; 60.7% elong
65%
nil
309
0.5% strain Fractured
nil 50%
nil nil
310
Fractured; 44.0% elong
nil
nil
316L
Fractured
46%
0.5%
316N
Fractured; 69.0% elong
52%
nil
Kromarc 55
0.5% strain Fractured; 17.3% elong
nil nil
nil nil
Alloy
Deformation
304L
CRYOGENICS. MARCH 1973
The general effect of different solutes in lowering the M s temperature of F e - C r - N i alloys has been evaluated previously. 7,16 The interstitial solutes C and N have the greatest effect, causingMs to be lowered by about 17 K per 0.01 wt % solute. The equivalent figures for the common substitutional solutes are approximately 0.6 K (Ni), 0.4 K (Cr), 0.3 K (Mn), and 0.3 K (Si) per 0.01 wt % solute. Since the AISI specifications allow a variation of at least 2 wt % in the amount of Cr and Ni, and only quote maximum carbon contents, the M s temperature of a cast on the lean side of a specification may thus be as much as 300 K above that for a steel with the maximum permitted alloy content. The possibility of such wide variations in M s temperature means that many of AISI 300 grades must be considered as borderline with respect to austenite stability at low temperatures and hence any particular batch of such a grade is best regarded with suspicion until demonstrated to be stable. The grades concerned are 301, 302,303,304, 304L, 321, and 347. Although no evidence of martensite has been reported in the slightly higher alloy grades 305 and 316(L), these steels should probably also be regarded as borderline, until the compositional dependance of their low temperature stability is examined in more detail. The single and multiple cooling experiments have shown that there is comparatively little difference between the low temperature stability of the as-machined and fullysoftened specimens of various steels. The small amount of surface deformation associated with machining was thus substantially without effect on the austenite stability, as indeed was the small difference in grain size between the two sets of specimens. Precipitation similar to that shown in Fig.1 for 304L was observed in all the sensitized steels, but 304L was evidently the only one to be converted from stable to unstable by this heat treatment. The amount of martensite was not significantly affected by sensitizing the unstable steel 321, but the treatment greatly increased the amount of martensite formed in 347. The enhancement of instability by sensitizing is of particular significance in these steels since they are specially formulated to resist weld decay; that is, a loss of corrosion resistance caused by the depeletion of Cr from solid solution due to precipitation of Cr23C 6 in the heat affected zone adjacent to a fusion weld. In steel 304L, the precipitation of Cr23C 6 is minimized by specifying a low carbon content (0.03 wt % max), while steels 321 and 347 (0.08% C max) contain additions of Ti and Nb, respectively, which cause the preferential precipitation of TiC or NbC. Since carbon has such a marked effect on the M s temperature, however, the precipitation of these carbides in amounts insufficient to lower the corrosion resistance of the steel, may still remove enough C or Cr to tip the balance of its low temperature phase stability in favour of ~' martensite, as was found in the sensitized samples of 304L and 347. The lack of an increase in the martensite formed on cooling the sensitized sample of 321, may be associated with its exceptionally high Ti content of 8 x wt % C, compared to the specified 5 x wt % C min. The niobium content of steel 347 was in fact slightly under specification, being 9.6 x wt % C instead of 10 x wt % C min, which would allow additional carbon to be removed from solution during sensitizing. Hence, in view of the precuations taken to reduce their soluble carbon contents, it is clear that the weldable quality steels are in general
167
likely to be less stable on exposure to cryogenic temperature than their standard equivalents. In practice however, welding will often give serious magnetic problems of its own, since the preferred welding electrodes are generally those which deposit 5-10% ~-ferrite, to inhibit weld tearing. The 8-ferrite phase is of course ferromagnetic, like a', and fabrication and welding practice should thus be closely controlled when non-magnetic properties are required. The presence of 0.2 wt % nitrogen in the Hi-proof versions of 304N and 316N greatly increases their low temperature stability in comparison with their standard counterparts. This stability can be attributed directly to the pronounced effect of nitrogen on the M s of these steels. The addition gives the bonus of an increase of some 1.50% in yield strength with no loss of toughness at low temperatures. 17 The presence of nitrogen in solution also stabilizes the 5l phase at high temperatures, so that these steels are also protected against the formation of fi-ferrite, the presence of which is not uncommon in the standard grades. Unfortunately, due to the precipitation of nitrides or carbonitrides of Ti and Nb, nitrogen is of limited use in stabilizing the austenite phase in the weldable steels 321 and 347. The high alloy steels 309, 310, Incoloy 800, and Kromarc 55 also remained stably austenitic on cyclic cooling to 4.2 K. The relative stability of these alloys with respect to the lower alloy or high nitrogen grades is thus best considered by their resistance to transformation when subjected to plastic deformation at low temperatures. Steel 304L, being distinctly borderline in stability, transformed to a ' martensite even after a strain of 0.5% at 4.2 K. Extensive deformation at this temperature, however, was found to induce considerable amounts of tranformation not only in the high nitrogen steels, but also in 316L and the moderately high alloy steel 309. This leaves only 310 and Kromarc 55 as alloys which remain fully austenitic under all the experimental conditions. The stability of 310 with respect to low temperature deformation confirms the findings of previous workers.3,18
168
Evidently there is some critical composition beyond which the alloy content of the steel is sufficient to stabilize the austenite phase under all conditions. The interesting question of what relationship exists between such compositions and the magnetic properties of austenite remains to be explored in later papers in this series. The authors wish to thank Prof J. G. Ball for his advice and encouragement and Messrs Firth Vickers Ltd, Samuel Fox Ltd, and the International Nickel Co Ltd for samples of various stainless steels. They are also grateful to Mr P. Clee of the Rutherford High Energy Laboratory for providing additional alloys and for his stimulating cooperation with the deformation work at low temperatures. This work wJs supported financially by the Science Research Council and the UK Atomic Energy Research Establishment.
References 1 2
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Reed, R. P., Mikesell, R. P. Advances in Cryogenic Engineering 4 (1960) 84 Gonser, U., Meechan, C. J., Muir, A. H., Wiedersich, H. JApplPhys 34 (1963) 2373 Guntner, C. J., Reed, R. P. Trans ASM 55 (1962) 399 Larbalestier,D. C., King, H. W. Cryogenics 10 (1970) 410 Larbalestier,D. C., King, H. W. (forthcoming) Larbalestier,D. C., King, H. W. (forthcoming) Eichelman,G. H., Hull, F.C. TransASM45(1953) 77 Reed, R. P. Acta Met 10 (1962) 865 Curtis,C. J., Sherwin, F. C. BritJAppl Phys 12 (1961) 334 King,H. W., Pollock, .I.T.A. Cryogenics 7 (1967) 209 Lagneborg,R. Acta Met 12 (1964) 823 Henkel,O. Phys Stat Sol 2 (1962) 78 Mikesell,R. P., Reed, R. P. NBS JRes 70C (1966) 207 Kondorsky, E. 1., Sedov, V. L. JAppl Phys 31 (1960) 331 Meiklejohn,W. A. JAppl Phys 32 (1961) 2745 Monkman,F. C., Cuff, F. G., Grant, N. J. Metal Progress 71 (1957) 94 Sanderson,G. P., Liewellyn, D. T. JISI 207 (1969) 1129 Watson,J. F., Christian, J. L. ASTMSpec Tech Publn 287 (1960) 170
C R Y O G E N I C S . M A R C H 1973
of which formed isothermally. Low temperature deformation is even more potent in promoting the transformation, at least 50%0~' martensite being induced by deformation at 4 . 2 K in the otherwise stable alloys such as 309 and the 0.2% N versions of 304L and 316L. The high alloy steels 310 and Kromarc 55 remain fully austenitic even after deformation to rupture at 4.2 K. The temperature dependence of the magnetic susceptibility of the latter alloys and Incoloy 800 indicates that their low temperature structural stability is associated with magnetic transitions which occur within the austenite phase.
Austenitic stainless steels at cryogenic temperatures 1--Structural stability and magnetic properties D. C. Larbalestier and H. W. King The austenitic stainless steels are widely used as constructional materials at cryogenic temperatures because of their high strength and excellent toughness even at temperatures down to 4.2 K. Their fabrication properties do not present any special grounds for concern, while their rustless properties are a considerable advantage in conditions where condensation of water vapour is frequent. A further important property of these steels is their low thermal conductivity which makes it possible in many instances to combine the load-bearing and thermal insulation functions in the same structure.
and Reed 3 also reported stable as well as unstable samples of both 304 and 304L in a survey of the mechanical properties of these steels at 4.2 K. Further, the previous finding of the present authors, 4 that a superconducting solenoid former made from the British equalent to AISI 321 transformed to ~5% a' on cooling to 4.2 K, showed that other grades besides 303 and 304 can become unstable at low temperatures. This latter work also pointed to other problems which can arise when such steels are subjected to changing magnetic fields with their consequent eddy currents.
Many of the machines and instrumentation designed for low temperature operation utilize magnetic fields and it is thus an additional important property of the AISI 300 stainless steels that the austenite phase, 7, of the 18 Cr[ 10 Ni grades is non-ferromagnetic. These steels are, however, metastable at or not far below room temperature and can thus be transformed to a ferromagnetic martensite, a', either by cooling or by the application of cold work.
The problem of the low temperature decomposition of austenite may be further aggravated in certain circumstances, since in addition to being ferromagnetic the a ' martensite phase has a higher specific volume then austenite, so that its formation is accompanied by a volume expansion. To find the clamping bolts of a liquid hydrogen bubble chamber expand instead of contract on cooling, can provide considerable cause for anxiety!
The possibility of the martensitic transformation occuring solely by cooling to low temperatures, was studied for a wide range of the AISI 300 stainless steels by Reed and Mikesell. 1 These authors found that, of the grades 302, 303,304,308,310,316,321, and 347, only samples of 303 and 304 showed any transformation from 7 to a ' after repeated cycling between room temperature (RT) and 77 or 20 K. Their study has so far remained the only comprehensive survey of the influence of cryogenic cooling on the martensitic transformation in the AISI 300 stainless steels even though subsequent isolated results have often been contradictory. Gonser et al, 2 for example measured the N~el temperature of a sample of 304 as 38 K, thus implying that no transformation to a ' had occurred, at least down to that temperature. Guntner
With these factors in mind, a comprehensive study was initiated on the effect of both cyclic cooling and deformation at 4.2 K on the low temperature stability and magnetic properties of the 300 series of stainless steels. The present paper gives general details of the different types of magnetic behaviour exhibited by these steels between RT and 4.2 K and the effect of mechanical and heat treatments on the transformation to a'. Methods for predicting the stability of a given steel will be discussed in a second paper 5 while a third 6 will be concerned with the relationship between magnetic structure and the relative stability of the 3' and a ' phases.
This work was carried out at the Department of Metallurgy, Imperial College, London, UK. DCL is now with Battelle Research Centre, Geneva, Switzerland and HWK with the Department of Engineering and Engineering Physics, Dalhousie University, Halifax, Nova Scotia, Canada. Received 28 July 1972.
160
Materials and experimental methods The alloys examined in this study were kindly made available by Messrs Firth Vickers Ltd, Samuel Fox Ltd (Special Steels Division of British Steel Corporation), and the Rutherford High Energy Laboratory. The chemical analyses of these alloys are listed in Table 1. The majority of the steels belong to the AISI 300 series of austenitic F e - C r - N i
C R Y O G E N I C S . M A R C H 1973
Table 1. Chemical analysis of stainless steels (All compositions are in wt %)
Alloy
C
Cr
Ni
Si
Mn
S
P
17.98 17.92 22.44 23.95 16.77 17.22 17.18 19.18
10.53 10.47 14.25 21.09 12.43 10.18 9.65 31.36
0.77 .49 0.54 0.75 0.38 0.60 0.72 0.56
1.37 1.6 1.23 1.65 1.49 0.77 0.70 0.76
0.08 0.02 0.01 0.03 0.03 0.01" 0.02 0.004
0.02 0.01 0.02 0.007 0.02 0.02 0.02 0.02
10.44 11.55 14.17 21.00 13.5 12.56 20
0.40 0.40 0.46 0.62 0.77 1.52 0.4
0.70 .1.30 1.17 1.57 0.27 0.41 9
0.013 0.008 0.003 0.014 0.016 -
0.020 0.022 0.020 0.028 0.033 -
Others
Cyclic cooling specimens 304L 304N 309* 310 316LN 321 347 Incoloy 800
0.031 0.061 0.120 0.075 0.025 0.050 0.071 0.014
-
0.181~* 0.18N*; 2.40Mo 0.41Ti 0.68Nb
Low temperature deformation specimens 304L* 304LN* 309* 310* 316L* 316LN* Kromarc 55t
0.020 0.011 0.120 0.060 0.028 0.027 0.04
18.63 17.30 22.32 24.64 18.1 17.5 16
0.2N 3.0Mo 0.12N, 2.8Mo 0.2N, 2Mo
*Cast analysis tTypical analysis Remaining data refer to specimen analyses
stainless steels; those with the non-standard N suffix denote high nitrogen content (a nominal 0.2 wt%) while the L suffix refers to the standard low carbon steels (0.03 wt% max). Incoloy 800 and Kromarc 55 complement the AISI steels, since they have a significantly higher substitutional alloy content; Incoloy 800 is an F e - C r - N i steel with 31% Ni, while Kromarc 55 is a special F e - C r - N i casting alloy containing 9% Mn. The steels used for cyclic cooling experiments were all supplied in the form of plate or bar stock, in the nominally fully-softened condition. These were machined into rods 4 mm in diameter and cut into adjacent 30 mm and 3 mm lengths for magnetization and metallographic specimens, respectively. Certain samples were pickled for half an hour in 30% HNO 3 at 50°C to dissolve any iron possibly picked up during machining, but subsequent magnetic measurements revealed this to be an unnecessary precaution.
adjacent to a fusion weld. Typical microstructures of the steels after the different heat treatments are shown in Fig.l, for steel 304L. The twinned matrix is the 7 austenite phase, while the precipates are carbides, phosphides, and sulphides. The figure also show~ the additional
a
In order to duplicate in the laboratory some of the conditions encountered in the general manufacture of cryogenic components, specimens were given one or more of the following prior treatments. (a) As-machined. Specimens left in the as-machined condition.
f
• °
•
(b) Fully-softened. Heated at 1 075°C for ½ hour and water quenched. (c) Sensitized. Heated for 1 week at 700°C and air cooled. Both magnetic and metallographic specimens were sealed together in evacuated silica capsules for these heat treatments. The as-machined condition reflects the condition of most stainless steels used in cryogenic engineering. Some small components may be fully-softened after manufacture to remove stresses or phase transformations due to machining or fabrication by welding. The sensitizing treatment reproduces in an extreme form the precipitation of carbides which occurs in the heat affected zone
CRYOGENICS. MARCH 1973
b
Fig.1 Microstructure of steel 304L -- Fully softened; b - sensitized at 700°C for 1 week
161
precipitation of carbides at twin and grain boundaries after the sensitizing treatment. The austenite grain size of the as-machined and the sensitized specimen of this steel is ASTM No 5, a value typical for most of the alloys studied, though Incoloy 800 and Steel 347 had grain sizes of 7 and 8 respectively. The fully-softening treatment in general caused the grain size to increase (a reduction of one or two units on the ASTM scale). Since a difference of one unit in grain size is known to raise the M s temperature by only 4 K in these steels, 7 the effect of grain size was not introduced as a variable in the present investigation. The alloys used for the low temperature deformation study are listed in the lower section of Table 1. These were received in the form of standard tensile test-pieces, supplied by the Bubble Chamber Group of the Rutherford High Energy Laboratory at the completion of their low temperature mechanical testing programme. Preparation of the test-pieces was by machining from original stock, except for the sample of Kromarc 55, which was cut from a section of a liquid hydrogen bubble chamber. Disc shaped magnetization specimens 2 - 3 mm thick were cut across the section of the test-pieces, both in the shoulder region and across the gauge length. In samples which had been deformed to rupture, the magnetization specimens were cut from the gauge length region adjacent to the fracture. Two types of martensite can form on cooling stainless steels below a critical temperature, M s, or by.deforming them below a critical temperature, M d. The martensite cz' is body centred cubic. This phase is closely related to the ferrite phase, cz, in pure iron and is also ferromagnetic at room temperature and below. The hexagonal closepacked martensite, e, on the other hand has magnetic properties indistinguishable from the FCC austenite phase and remains naramagnetic. These two types of martensite can ooviously be distinguished by x-ray diffraction, 8 but this method of detection and measurement is destructive to the specimen. Metallographic examination can also be used, since differential etches have been developed to distinguish the two phases. 8 This technique however, also tends to be destructive to a specimen, unless confined to the examination of the ends of the rod shaped specimens which may not be representative of the bulk. Hence, since the prime interest of the present study was concerned with the occurrence of ferromagnetism in stainless steels, magnetic methods were adopted to determine the presence and amount of the ~' martensite after various cooling or deformation treatments. The magnetization measurements in isolation are not necessarily unambiguous, however, since small quantities of the high temperature form of ferrite,/i, can also be found in the austenitic stainless steels. In the present samples, however, magnetization measurements made before exposure to cryogenic temperatures showed that 8-ferrite, if present, was always less than 0.1%. Magnetization measurements at low temperatures were made with a dc integration magnetometer while continuously sweeping the field. This instrument had an ultimate sensitivity of-+0.5 G, which allowed the detection of less than 0.1% ofcz' martensite. The apparatus was fitted in a liquid helium dewar in a 3 kOe water cooled copper solenoid and magnetization loops in increasing and decreasing field were recorded at RT, 77 K, and 4.2 K.
162
Since this field was ot)viously insufficient to saturate the specimens and thus allow a direct conversion from magnetization measurements to martensite contents, additional measurements were made at room temperature only, in a Sucksmith type force balance using the 3 mm length metallographic specimens and the discs cut from the tensile test-pieces. Since any ~' martensite formed at low temperature remains stable on warming up to room temperatures, 8 the magnetization values at a field of 14 kOe were converted to a ' martensite contents using the formula of Curtis and Sherwin, 9 who showed that the saturation moment of ferrite in stainless steels of these compositions can be expressed by the weighted mean of their Fe, Cr, and Ni contents, where the individual atoms are assigned moments of 2.22, 0.22, and 0.60 Bohr magnetons, respectively. The saturation magnetization of these alloys is thus clearly less than that of pure iron, but is still considerable, as shown by the figure o f ~ 1 6 000 G, derived for 100% ~' in the 18 Cr/10 Ni alloys. Magnetization measurements in the copper solenoid were taken at fields of 1.9 kOe and 2.3 kOe. These were converted to a ' martensite contents by observing the approach to saturation in the Sucksmith balance which showed that the specimens were 60% and 75% saturated at these respective fields. Using these values, the relative accuracy of the a ' martensite determinations was estimated to be -+5%.
Results Effect of cooling In initial experiments, samples of the steels in various conditions were examined in the magnetometer at 4.2 K. The measurements gave different types of magnetization curve as illustrated in Fig.2. The samples of steels 304L, 304N, 309, and 316LN gave plots which were linear with field within the limits of error, suggesting that the samples were paramagnetic or antiferromagnetic. This means that no a ' martensite forms in these steels on first cooling to 4.2 K. Non-linear magnetization curves, indicative of ferromagnetic behaviour, were observed for steels 321, 347, and 310, while a pronounced hysteresis was observed in the sample of Incoloy 800. The magnitude of the ferromagnetic magnetization in Incoloy 800 indicated that in this very high nickel alloy the FCC 3' phase may itself become ferromagnetic, by passing through a Curie Point, instead of transforming to ~' martensite. This possibility must also be considered for the other high nickel alloys such as 310. The two sources of ferromagnetic behaviour can easily be distinguished, however, since the formation of the ~' martensite is an irreversible process at these temperatures in these alloys. The magnetization measurements were thus repeated on all the alloys at RT and on a subsequent cool down to 77 K. These results revealed that the steels followed one of three basic patterns on cooling to cryogenic temperatures. (a) All samples of 304L, 304N, 309, and 316LN gave reversible, small, non-ferromagnetic magnetization curves, which were linear with field up to at least 3 kOe, at all temperatures studied. These steels thus remain fully austenitic. (b) Steels 321 and 347 showed weak ferromagnetism on the initial cool down to 4.2 K. This ferromagnetism
CRYOGENICS. MARCH 1973
600
2000
321
500
16OO O O
400
s"
E
,.J O N
.o
1200
8,
300
O
8'
800 771~
200 400 I00
~.. ...---"-"r"-"
I
a
304 L
J
I
l
2
3
4
I C
Fig.2
Field, kOe
2
3
4
Field, kOe
Typical magnetization loops
a -- 304L and 321 at 4.2 K; b - 310 at RT, 77, and 4.2 K; c -- Incoloy 800 at RT, 77, and 4.2 K
350 4.2K
was retained on returning to RT and increased in magnitude when the steels were subsequently measured at 77 K and again at 4.2 K. These specimens thus transformed to a ' martensite, the amount of which increased with each subsequent cooling to low temperature.
300
(c) The magnetization in steels 310 and Incoloy 800 was found to be variable with temperature but regardless of the order or number of cooling treatments was reproducible at any given temperature, as illustrated in Fig.2.
250
In order to investigate this behaviour more fully, the ac susceptibility of these alloys was followed over the temperature range 2 0 - 4 K for 310 and from 9 0 - 3 0 K for Incoloy 800. A sharp, reproducible increase in susceptibility was obtained at 7 and 74 K, respectively and this increase was associated with a paramagnetic-ferromagnetic transition within the austenite phase. While this appears to be a genuine Curie point for Incoloy 800, the absence of any hysteresis (<1 G) in the magnetization curve of steel 310 at 4.2 K suggests that a local clustering or a form of superparamagnetism probably occurs in this alloy. Measurements made in fields greater than 100 kOe, which shed more light on this transition will be discussed in more detail in a subsequent paper. 6
200
5
N C O
150
I00
50 77K
298 K O
I
Field, k0e
b CRYOGENICS.
2
MARCH
1973
3
4
No martensitic transformation was observed in any sample of the group (c) steels when they were continuously examined at temperatures down to 77 K, at a magnification of x330, using the low temperature microscope of King and Pollock. 10 Metallographic examination at RT of all the remaining alloys after the cooling treatment to 4.2 K, showed that martensitic structures were only present in the alloys listed in group (b) above. The martensitic structures observed in alloys 321 and 347 after many cooling cycles to 77 K are illustrated in Fig.3. These are
163
sirr.!lar to those reported previously by Lagneborg 11 and by Reed. 8 The martensite in steel 347 is plate-like, but it is difficult to resolve the detail between the plates. Similar plates, but more widely separated were also observed initially in steel 321 and after repeated cycling the area between the plates increasingly transformed. This area etched darkly with a preferential stain etch developed by Reed ~ for identifying a ' martensite. The structures are thus similar to the morphology reported by Lagneborg, 11 where e martensite first appears as parallel pairs of plates in preferred orientations in the 7 phase, while the a ' martensite forms between these plates. Almost all the plates were formed during the first or second quench and on repeated quenching subsequent transformation was predominantly ct', giving the plates a 'filled-iri' appearance as in Fig.3b. It is interesting to note in passing that many of the original plates of martensite appear to have nucleated from precipitate particles and hence the presence or absence, of carbide phases may have at least a secondary, if not a primary, effect on the M s temperatures of these alloys. It was also observed during the course of the above experiments that a certain amount of ~t' martensite forms isothermally in these steels. This was most evident in a sample of steel 347, when an isothermal increase in magnetization from 461 to 478 G occurred over a period of 8 minutes, while the magnetization loop was being
~f
''L
#'
e
•
%',
•
ql~,
•
•
m
w
e.
50~um a
ql
zcrjcu~
'
Fig•3 Microstructure of fully-softened specimens of steels after cyclic cooling treatments to 77 K and 4•2 K a -- steel 347; b -- steel 321
164
To determine whether steels become unstable on continued cycling and if the amount of martensite formed during cooling approaches a constant value for a given steel, a composite quench was devised to maximize the amount of athermal and isothermal martensite formed in a given cool down. For 77 K treatments, the specimens were plunged into liquid nitrogen, allowed to soak at 77 K overnight in a dewar flask, which was then emptied and stoppered so that the warm-up to RT took place over a period of 6 - 8 hours. A similar treatment for cycles to 4.2 K could not be applied, because of the consequent cost in terms of liquid helium, and hence the specimens were held at 4.2 K for about 5 minutes and then allowed to warm to RT in air. Steels 309, 310, and Incoloy 800 showed no change in magnetic properties after repeated cycles between RT and 77 K or RT and 4.2 K. The previous thermal history of the sample thus had no effect on the magnetization of these alloys, either ferromagnetic or otherwise, which was found to depend solely on the applied field at all temperatures investigated. Also, no trace of martensite was found in any of these specimens and hence it was concluded that, at least in the absence of an applied stress, the 7 phase in all these three alloys remains stable against a martenitic decomposition at temperatures down to 4.2 K.
The nitrogen containing steels 304N and 316LN show no change in magnetic properties as a result of either the prior heat treatment or the subsequent cyclic cooling and, since the limit of a ' detection lies in the vicinity of 0.05%, these steels must be considered perfectly stable• The as-machined and fully-softened samples of steel 304L also showed no effect due to the cyclic cooling, but the sensitized sample of this steel became unstable during the cyclic treatment, so that 1.4% a ' martensite was formed.
v'
~
The effect of cyclic cooling
The effect of prior specimen condition and cyclic cooling treatments on the amount of t~' martensite formed in steels 304L, 304N, 321, and 347 is summarized in Table 2. It is evident that, once a steel is susceptible to low temperature martensitic decomposition, the amount of martensite formed after a given cooling treatment depends on the prior heat treatment given to the specimen•
.olB
b
plotted at 77 K (at H = 2.3 kOe this represents an increase in a ' from 3.7 to 3.9%). On returning to RT and again cooling to 77 K, the magnetization of the sample increased to 570 G (4.6% a ' ) and after 4 hours at 77 K it further increased to 600 G (4.9% t~'). Thus, although direct cooling from RT to 77 K is more potent in increasing the amount of ct' transformation, the isothermal effect at 77 K is still quite significant and must be taken into account when determining the amount of martensite formed at a given temperature.
Both the prior heat treatment and the cyclic cooling influence the stability of steels 321 and 347. After the initial cool down, the amount of martensite is much the same in the as-machined and fully-softened specimens of these steels, but both differ appreciably from the amount of martensite present in the sensitized specimens• After the cyclic treatments, the amount of martensite is increased in all the specimens, but the effect is most enhanced in the sensitized samples, particularly in steel 347 where some
CRYOGENICS. MARCH
1973
11.2% martensite was detected after this treatment. It is also of interest to note that when the as-machined specimen contains more c~' than the fully-softened specimen, then less martensite is observed in this specimen on cyclic cooling. Hence, it appears that a limiting amount of martensite is able to form according to the chemistry of the alloy and the physical mechanism of its formation does not significantly influence the total amount. The rate of approach to a limiting amount of martensite following many cooling cycles was studied by measuring the magnetization of the as-machined metallographic samples in the Sucksmith balance each time they were returned to RT following a composite quench to 77 K. To facilitate these measurements, a small hole was drilled in the centre of each specimen and this deformation was sufficient to induce the formation of ~0.5% t~' in steel 321 and ~1% ~' in steel 347, indicating the relative instability of these two steels. The amount of martensite formed after the cooling cycles, shown graphically in Fig.4, confirms the previous results for the as-machined samples of steels 304L, 304N, and 316LN, which remain unaffected by the cyclic cooling treatments. The findings for the other steels show that while more martensite forms initially in steel 321 the cumulative effect of cyclic cooling is greater in steel 347. Although both steels show a fall off in the amount of a ' formed during each successive cycle, a limiting value of martensite was not achieved, since even after 23 cooling cycles significant changes in magnetization were still detected. This result is similar to that reported by Reed and Mikesell 1 for a sample of 304, which still showed evidence of martensitic decomposition after 3 months of daily cycling between RT to 77 K. Table 2. T r a n s f o r m a t i o n
The low temperature permeability of the martensite containing steels 304L, 321, and 347 at fields of 100 Oe and 1 000 Oe was derived from the full magnetization curves of these steels at 4.2 K. The results are plotted against the amount of a' martensite in Fig.5 and show a variation from 1.35 to 5.15 at 100 Oe and fall to values below 2 at 1 000 Oe. These values, though small, are quite significant, particularly in changing magnetic field situations. It is also of interest to note that the permeability varies
Iooo
347
-6
-4 0
600
_
"u ~o
2 400
-2
0
u~ 2 0 0 304L, 304N, 316LN ....
I 2
f--.f.__
II
--
15
$--
O
23
Cooling cycles to 77K Fig.4 Sucksmith magnetization measurements on as-machined specimens plotted as a function of the number of cooling cycles between RT and 77 K
to c~' martensite after cyclic cooling
O
a ' martensite
Alloy
Condition
Initial cooling*
304L
As-machined Fully-softened Sensitized
nd nd nd
nd nd 1.4%
304N
As-machined Fully-softened Sensitized
nd nd nd
nd nd nd
316LN
As-machined Fully-softened Sensitized
nd nd nd
nd nd nd
321
As-machined As-machined Fully-softened Sensitized
1.7% 2.6% 2.0% 1.1%
3.4% 2.9% 3.7% 3.1%
As-machined As-machined Fully-softened Sensitized
0.3% 1.3% 0.4% 3.4%
3.9% 3.7% 1.3% 11.2%
347
Cyclic coolingt
O =L
O
4
C R Y O G E N I C S . M A R C H 1973
12
2
O o "I
1.5
I
"1 cycle to 4.2 K or 77 K t l 0 further cycles to 77 K and 5 to 4.2 K nd Not detected. Limit of detection 0.05% cz' NB Steels 309, 310, Incoloy 800, and Kromarc 55 showed no transformation to ~x' martensite after any of the above treatments
8 O/o='
f
I 4
i 8
i 12
O/o c~' Fig.5 Plots of permeability (~) versus c~' martensite content at 4.2 K for steels 304L, 321, and 347 at fields of 100 Oe and I 000 Oe
165
Table 3. Magnetic susceptibility of the non-martensitic steels at different temperatures Magnetic susceptibility, X Alloy
Condition
RT
304L 304N
Fully-softened Fully-softened Sensitized Fully-softened Sensitized Fully-softened Sensitized Fully-softened Sensitized Fully-softened Sensitized As-machined
1.86 1.99 2.66 1.66 2.06 1.79 1.86 2.05 2.78 4.25 4.25
309 310 316LN Incoloy 800 Kromarc 55
4.2 K
77 K x x x x x x x x x x x
4.91 4.15 4.53 4.97 5.31 6.63 9.95 5.50 5.70 F F -
10 -4 10 .4 10 .4 10 -4 10 .4 10 .4 10 .4 10 .4 10 .4 10 .4 10 -4
X x x x x x k x x
10 .4 10 .4 10 .4 10 -4 10 .4 10 .4 10 -4 10 .4 10 .4
7 J 6 x 10 .4 5.68 X 10 -4 5.70 X 10 -4 18.7 X 10 .4 16.6 X 10 .4 F F 9.02 x 10 .4 9.02 x 10 -4 F F 3.1 x 10 .4
F Ferromagnetic at this temperature
linearly with the amount of a ' martensite, which implies that there is little magnetostatic interaction between neighbouring martensite plates. This is in accord with the observations of Henkel, 12 that such interactions do not become important until the a ' content exceeds 15%. Beyond this amount, it would be expected that the low field permeability should be significantly greater than that given by the straight line approximation. The magnetic properties of the non-martensitic steels are given in Table 3 in terms of the volume susceptibility x(=M/H). At low temperatures X approaches or exceeds 10-3 which is a sufficiently strong moment to give rise to appreciable magnetic forces in high fields. When the results are plotted as I/x versus temperature as in Fig.6, negative intercepts on the temperature axis are obtained for steels 304L, 304N, 309, and 316LN. This behaviour is that expected of a Curie-Weiss anti-ferromagnet and is in agreement with previously reported N~el temperatures in the region of 40 K for steels 304 2,13,14 and 347.15 In the absence of any other data, the much larger 4.2 K susceptibility of steel 309 suggests that if this alloy passes through a N~el temperature it must be close to 4.2 K. To check this possibility its magnetization versus temperature was measured over the range 4 - 2 0 K and found to peak at 11.5 K. The sample was therefore concluded to be anti-ferromagnetic below this temperature, as discussed in detail in a later paper. 6
Effects of deformation
6
L
/
~7
309 • 304 L ' s304N 316 LN
Fully softened condition 4 ? O x X --. 1
3
"S /#•
/
/x,, /
s'
,
at 4.2 K
As the Rutherford Laboratory programme, on the low temperature mechanical properties of the stainless steels, was conducted independently of the present investigation, it was not possible to use samples of steels from exactly the same heats for both studies. Nevertheless, Table 1 shows that several grades of steel were common to the two series of tests, thus enabling a close comparison to be made between the structural stability of the steels in the presence and absence of stress at 4.2 K. The a ' martensite contents derived from saturation magnetization measurements of specimens cut from tensile test-pieces and examined in the Sucksmith balance, are collected in Table 4. Deforma-
166
tion at low temperature is clearly much more potent in inducing the martensitic transformation than single or repeated cooling. The transformation in steel 304L goes to 83%, while even the high nitrogen versions of 304 and 316, which were stable against cyclic cooling (Table 2),
O '
I00
200
300
Temperature K Fig.6 The reciprocal of the susceptibility (1/×) versus temperature for steels 304L, 304N, 309, and 316LN in the fully softened condition
C R Y O G E N I C S . MARCH 1973
now transform at least 50%. Considerable transformation was also observed in steel 309, which again was stable when subjected to the previous cyclic cooling treatment. The degree of transformation is also clearly related to the amount of strain, being only 0.1% for 304L at 0.5% strain. In the high nitrogen steels and in 309, whereas considerable transformation occurs in the region near the fracture, no transformation is observed in the less strained shoulder region of the specimens. Only steel 310 and Kromarc 55 show no evidence of or" martensite after deformation, even in the region of the fracture on specimens deformed to rupture. This interesting result on Kromarc 55 prompted a measurement of its magnetic properties at 4.2 K. The susceptibility was found to be 3.1 x 10 -4 which lies somewhat lower than the values obtained for the other alloys (Table 3). The precise magnetic state of Kromarc 55 is still uncertain, but it appears that the substantial manganese content may encourage antiferromagnetic behaviour. Discussion
When the present findings on the low temperature stability of the austenitic stainless steels in the absence of stress are compared with the results of previous investigators, many direct conflicts become evident. In the case of steels 304 and 304L, some samples have been reported to be stable, 2,3 while others are found to transform. 1,3 The present sample of 304L remained stable under normal conditions, but became unstable after a sensitizing treatment. Similar discrepancies can also be noted for other grades, with Meiklejohn 15 and Reed and Mikesell 1 reporting both 321 and 347 to be stable, while the present samples of these alloys, and a previously examined sample equivalent to 321,4 were found to be definitely unstable. It is thus evident that, depending on their exact chemical composition or previous heat treatment, these steels can have M s temperatures either above or below 4.2 K. Table 4. Transformation to e" martensite after deformation at 4.2 K
(~' martensite Gauge length
Shoulder region
0.5% strain Fractured; 51.% elong
0.1% 83%
0.1% no test
304N
Fractured; 60.7% elong
65%
nil
309
0.5% strain Fractured
nil 50%
nil nil
310
Fractured; 44.0% elong
nil
nil
316L
Fractured
46%
0.5%
316N
Fractured; 69.0% elong
52%
nil
Kromarc 55
0.5% strain Fractured; 17.3% elong
nil nil
nil nil
Alloy
Deformation
304L
CRYOGENICS. MARCH 1973
The general effect of different solutes in lowering the M s temperature of F e - C r - N i alloys has been evaluated previously. 7,16 The interstitial solutes C and N have the greatest effect, causingMs to be lowered by about 17 K per 0.01 wt % solute. The equivalent figures for the common substitutional solutes are approximately 0.6 K (Ni), 0.4 K (Cr), 0.3 K (Mn), and 0.3 K (Si) per 0.01 wt % solute. Since the AISI specifications allow a variation of at least 2 wt % in the amount of Cr and Ni, and only quote maximum carbon contents, the M s temperature of a cast on the lean side of a specification may thus be as much as 300 K above that for a steel with the maximum permitted alloy content. The possibility of such wide variations in M s temperature means that many of AISI 300 grades must be considered as borderline with respect to austenite stability at low temperatures and hence any particular batch of such a grade is best regarded with suspicion until demonstrated to be stable. The grades concerned are 301, 302,303,304, 304L, 321, and 347. Although no evidence of martensite has been reported in the slightly higher alloy grades 305 and 316(L), these steels should probably also be regarded as borderline, until the compositional dependance of their low temperature stability is examined in more detail. The single and multiple cooling experiments have shown that there is comparatively little difference between the low temperature stability of the as-machined and fullysoftened specimens of various steels. The small amount of surface deformation associated with machining was thus substantially without effect on the austenite stability, as indeed was the small difference in grain size between the two sets of specimens. Precipitation similar to that shown in Fig.1 for 304L was observed in all the sensitized steels, but 304L was evidently the only one to be converted from stable to unstable by this heat treatment. The amount of martensite was not significantly affected by sensitizing the unstable steel 321, but the treatment greatly increased the amount of martensite formed in 347. The enhancement of instability by sensitizing is of particular significance in these steels since they are specially formulated to resist weld decay; that is, a loss of corrosion resistance caused by the depeletion of Cr from solid solution due to precipitation of Cr23C 6 in the heat affected zone adjacent to a fusion weld. In steel 304L, the precipitation of Cr23C 6 is minimized by specifying a low carbon content (0.03 wt % max), while steels 321 and 347 (0.08% C max) contain additions of Ti and Nb, respectively, which cause the preferential precipitation of TiC or NbC. Since carbon has such a marked effect on the M s temperature, however, the precipitation of these carbides in amounts insufficient to lower the corrosion resistance of the steel, may still remove enough C or Cr to tip the balance of its low temperature phase stability in favour of ~' martensite, as was found in the sensitized samples of 304L and 347. The lack of an increase in the martensite formed on cooling the sensitized sample of 321, may be associated with its exceptionally high Ti content of 8 x wt % C, compared to the specified 5 x wt % C min. The niobium content of steel 347 was in fact slightly under specification, being 9.6 x wt % C instead of 10 x wt % C min, which would allow additional carbon to be removed from solution during sensitizing. Hence, in view of the precuations taken to reduce their soluble carbon contents, it is clear that the weldable quality steels are in general
167
likely to be less stable on exposure to cryogenic temperature than their standard equivalents. In practice however, welding will often give serious magnetic problems of its own, since the preferred welding electrodes are generally those which deposit 5-10% ~-ferrite, to inhibit weld tearing. The 8-ferrite phase is of course ferromagnetic, like a', and fabrication and welding practice should thus be closely controlled when non-magnetic properties are required. The presence of 0.2 wt % nitrogen in the Hi-proof versions of 304N and 316N greatly increases their low temperature stability in comparison with their standard counterparts. This stability can be attributed directly to the pronounced effect of nitrogen on the M s of these steels. The addition gives the bonus of an increase of some 1.50% in yield strength with no loss of toughness at low temperatures. 17 The presence of nitrogen in solution also stabilizes the 5l phase at high temperatures, so that these steels are also protected against the formation of fi-ferrite, the presence of which is not uncommon in the standard grades. Unfortunately, due to the precipitation of nitrides or carbonitrides of Ti and Nb, nitrogen is of limited use in stabilizing the austenite phase in the weldable steels 321 and 347. The high alloy steels 309, 310, Incoloy 800, and Kromarc 55 also remained stably austenitic on cyclic cooling to 4.2 K. The relative stability of these alloys with respect to the lower alloy or high nitrogen grades is thus best considered by their resistance to transformation when subjected to plastic deformation at low temperatures. Steel 304L, being distinctly borderline in stability, transformed to a ' martensite even after a strain of 0.5% at 4.2 K. Extensive deformation at this temperature, however, was found to induce considerable amounts of tranformation not only in the high nitrogen steels, but also in 316L and the moderately high alloy steel 309. This leaves only 310 and Kromarc 55 as alloys which remain fully austenitic under all the experimental conditions. The stability of 310 with respect to low temperature deformation confirms the findings of previous workers.3,18
168
Evidently there is some critical composition beyond which the alloy content of the steel is sufficient to stabilize the austenite phase under all conditions. The interesting question of what relationship exists between such compositions and the magnetic properties of austenite remains to be explored in later papers in this series. The authors wish to thank Prof J. G. Ball for his advice and encouragement and Messrs Firth Vickers Ltd, Samuel Fox Ltd, and the International Nickel Co Ltd for samples of various stainless steels. They are also grateful to Mr P. Clee of the Rutherford High Energy Laboratory for providing additional alloys and for his stimulating cooperation with the deformation work at low temperatures. This work wJs supported financially by the Science Research Council and the UK Atomic Energy Research Establishment.
References 1 2
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Reed, R. P., Mikesell, R. P. Advances in Cryogenic Engineering 4 (1960) 84 Gonser, U., Meechan, C. J., Muir, A. H., Wiedersich, H. JApplPhys 34 (1963) 2373 Guntner, C. J., Reed, R. P. Trans ASM 55 (1962) 399 Larbalestier,D. C., King, H. W. Cryogenics 10 (1970) 410 Larbalestier,D. C., King, H. W. (forthcoming) Larbalestier,D. C., King, H. W. (forthcoming) Eichelman,G. H., Hull, F.C. TransASM45(1953) 77 Reed, R. P. Acta Met 10 (1962) 865 Curtis,C. J., Sherwin, F. C. BritJAppl Phys 12 (1961) 334 King,H. W., Pollock, .I.T.A. Cryogenics 7 (1967) 209 Lagneborg,R. Acta Met 12 (1964) 823 Henkel,O. Phys Stat Sol 2 (1962) 78 Mikesell,R. P., Reed, R. P. NBS JRes 70C (1966) 207 Kondorsky, E. 1., Sedov, V. L. JAppl Phys 31 (1960) 331 Meiklejohn,W. A. JAppl Phys 32 (1961) 2745 Monkman,F. C., Cuff, F. G., Grant, N. J. Metal Progress 71 (1957) 94 Sanderson,G. P., Liewellyn, D. T. JISI 207 (1969) 1129 Watson,J. F., Christian, J. L. ASTMSpec Tech Publn 287 (1960) 170
C R Y O G E N I C S . M A R C H 1973