FERROMAGNETIC TRACES IN AUSTENITIC STAINLESS STEELS AT 4.2 K

Transcription

1 FERROMAGNETIC TRACES IN AUSTENITIC STAINLESS STEELS AT 4.2 K K. Pieterman, A. Ketting, J. Geerse To cite this version: K. Pieterman, A. Ketting, J. Geerse. FERROMAGNETIC TRACES IN AUSTENITIC STAIN- LESS STEELS AT 4.2 K. Journal de Physique Colloques, 1984, 45 (C1), pp.c1-625-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1984 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C1, suppl6ment au no 1, Tome 45, janvier 1984 page Cl-625 FERROMAGNETIC TRACES IN AUSTENITIC STAINLESS STEELS AT 4,2 K X. Pieterman, A. Ketting and J. Ch. Geerse Dept. for Applied Physics, Delft University of TechnoZogy, Lorentzweg 1, 2628 CJ Delft, The netherlands Resume - Dans cet article, sont decrites des mesures magnetiques qui se rapportent m r s aciers inoxydables de la serie ALSI ,2 K. I1 a 6t6 examine jusqu'ii quel point le comportement magnetique des different$ types, 6tai t influence par soudage ou par frequents r@froidissements, jusqu'ii 4,2 K. De plus, l'influence de differentes additions sur la stabilite de la structure austenitique a et6 consideree. Des preparations examinees, une grande partie (303, 304, 310, 316L, 316Ti, 321) semblait montrer une certaine forme de comportement ferro-magnetique, tandis que seulement peu d'autres (X6CrNi 1811,316LN) restaient compl6tement paramagnetiques 1 4,2 K. Abstract - This paper describes magnetisation experiments on austeni tic stainless steels of AISI 300 series at 4.2 K. The influence of welding and cyclic cooling on their magnetic behaviour was studied, as was the influence of trace additions for stabilizing the properties of stainless steel. A large number of samples has been studied, most of them showed ferro-magnetic traces at 4.2 K (303, 304, 310, 316L, 316Ti, 321). Only a few remain fully paramagnetic (X6CrNi 1811, 316LN) at 4.2 K. INTRODUCTION The stability of austenitic stainless steels has been studied extensively in the pastl '2 '3 '4'5'. It is well known that most of them are unstable below room temperature for the transition into the martensitic phase. Because the austenitic phase is paramagnetic and the martensitic phase is ferromagnetic, the magnetic character of the steel will change. Whether such a transition occurs depends very critically on the chemical composition of the steel, so different heats of the same type of stainless steet may show unlike magnetic behaviourl. When using stainless steels in the neighbourhood of magnetic fields it is important to know their magnetic behaviour, especially when high homogeneities are required. In that respect it is also necessary to know the influence of welding on the magnetic character of the stainless steel. In this paper maqnetisation measurements are reported on welded and unwelded samples of the types AISI 303, 304, 304N, 310, 310S, 316, 3161, 316LN, 316Ti and 321 at 4.2 K. PREPARATION OF THE SAMPLES The samples were machined out of bars received from different manufacturers into cylinders of 10 mm diameter and 10 mm long. For each type of stainless steel also two cylinders of 10 by 5 mm were machined and bounded together by fusion-welding. Welded and unwelded samples were machined out of the same bar. Chemical analysis of each specimen was carried out; the results are listed in table l. Out of the 32 samples used in this study 22 were annealed at 1080~~ for 90 minutes and quenched in water. The magnetisation of 3 samples was measured before and after heat treatment. The amount of ferromagnetism was reduced, but only one of them (316) was fully paramagnetic after heat treatment, indicating a pure austenitic structure (see table 2). Article published online by EDP Sciences and available at

3 C1-626 JOURNAL DE PHYSIQUE Table 1. Chemical analysis of the measured stainless steel samples. (Compositions in wt %) Alloyname C Cr Ni Mn Mo Ti Si Cu S (a) (b) N ' S (a) (b) (c) L LN Ti X6CrNi (a) (b) Table 2. Magnetic susceptibility and saturation magnetisation of different specimens after first cooling at 4.2 K A1 1 oy name ~ondi ti onx xo X, hat (ka/m) 303 AWQ (a) AM (b) AM (cj 304 N S 316 (a) 316 (6) 316 (cj 316 (cj 316 L 316 LN 316 Ti 323 (a) 321 (a) 321 (b) "AM = As machined AWQ = annealed at 1080~~ for 90 minutes, then water quenched EXPERIMENTAL PROCEDURE Measurements concerning the magnetic susceptibility x am = ( )T. have been made at 4.2 K in magnetic fields up to 1 T. The specimens were moungd ~nside a sample holder that fits in an ac magnetometer surroundedby a superconducting magnet. The sample holder could be easily taken out of the cryostat for quick changing the

4 sample. Cool-down time for the samples was typically 2 minutes, they were held at liquid helium temperature for about 15 minutes to sweep the magnetic field to 1 T and back and measure the magnetic susceptibility. Afterwards the samples were allowed to warm up to room temperature in air. Cyclic cooling effects were studied on 13 samples by cooling them down once every day during a period of 14 days. Holding time in liquid helium was 15 minutes each time. RESULTS Non-linearity of the magnetisation curves indicates ferromagnetic traces in the stainless steel. In the case of the unwelded samples this is caused by the formation of a' martensite. The saturation magnetisation Msat of the ferromagnetic part indicates the amount of a' martensite. In the case of the welded samples, ferromagnetism is caused by martensite formation on cooling and welding by 6 -ferrite and carbide formation induced by the welding process. Ferromagnetism induced by welding only can be found by substracting the measured Msat values (Msatw - Msatu in table 3). Table 3. Magnetic susceptibility and saturation magnetisation of different specimens after welding at 4.2 K. A1 loy name Condition xo X, Msat Msatw- Msatu ~tabil i tyx (ka/m) (ka/m) on welding (a) AM (b) AM (c) AM N A WQ AWQ S AM (a) AM (b) AWQ (c) L AWQ LN Ti A WQ X6CrNi 1811 AWQ (a) AWQ (b) A WQ = poor ++ = reasonable +++ = good ++++ = excel lent FERROMAGNETIC TRACES AT FIRST COOLING The results of the magnetisation of the unwelded samples on first cooling to 4.2 K are listed in table 2; x0 is the magnetic susceptibility at zero applied field, X, at maximum field where the ferromagnetic part is saturated and Msat is the saturation magnetisation. Out of 16 measured specimens only 6 showed fully paramagnetic behaviour on first cooling. Important is that all of the untraced 316 type specimens remain fully austenitic, where only one of the 304 samples showed no ferromagnetism. Also the 316LN specimen and the special composite X6CrNil811 remained austenitic. The 304N type shows a slightly non-linear behaviour, although it is reported to be fully paramagnetic in the The results indicate

5 ' JOURNAL DE PHYSIQUE that in the case of the 304 type the occurring of y - a' transformation is more critically dependent of the chemical composition than in the case of the 316 type of stainless steel. Comparing the chemical analyses with the magnetic behaviour of the 304 specimens one may conclude that a low carbon together with a high chromium content enhance martensite formation. Also the influence of the austenite stabilizing Mn addition is important as is seen in the case of sample 304(c) with a high value for MSat and a low Mn content. FERROMAGNETIC TRACES INDUCED BY WELDING Table 3 shows the results of the measurements of the welded samples. The chemical composition is the same as for the unwelded samples. After welding the number of specimens that remained fully paramagnetic decreased to four (316(b), 316(c), X6CrNi 1811 and 316LN). The first three showed a slight decrease in magnetic susceptibility (resp. 5, 4 and 2%) and only the 316 LN type of stainless steel showed no difference whatsoever. A large increase of the ferromagnetic part was found for the samples 303 and 304 whereas sample 304(c) showed a large decrease of the ferromagnetic part. This indicates again the rather unstable character of these types of stainless steel and the large dependence of the magnetic character on thermal history and chemical composition. Only the 304N type may be regarded as fairly stable with and without welding. With one exception (303) all of the annealed samples showed decreasing magnetisation after welding. Only in one case (304(c)) of the "as-machined" samples, indicated in table 3 by AM ("as-machined"), showed decreasing magnetisation. Table 3 also gives the stability on welding of the different types of stainless steel with respect to the changes in magnetisation. The Mo stabi 1 ised types 316 have excel lent properties in this respect except for sample 316(a) which may give better results when annealed. The low carbon steel 316L is stable on welding but it showed ferromagnetism on first cooling. Surprisingly the X6CrNi 1811 sample without Mo addition showed very good results. It remains fully paramagnetic and has excellent stability. The so called weldable steels with Ti addition of the types321 and 316Ti showed less stability than the 316 and 316LN types. Even after annealing they showed ferromagnetic traces and must be regarded as unstable as was stated in ref. 7. Fig. 1 shows some typically magnetisation curves for welded and unwelded specimens. FERROMAGNETIC TRACES ON CYCLIC COOLING Cyclic cooling effects have been studied on 13 different specimens: 304(d), 304N, 310, 310S, 316(a), 316(b), 316(c), 316L, 316LN, 316Ti, X6CrNi 1811, 321(a), 321(b). Of the measured specimens four showed increasing ferromagnetism on cyclic cooling namely: 304(d), 304N, 310 and 321(b). The results are shown in figure 2. The three types of stainless steel specimens showed different behaviour on cyclic cooling. Both 304 types showed no effect on the first four cooldown runs to 4.2 K. After that a sharp rise in M at was detected. 304 was reported unstable1, but cyclic cooling effects on 304N sampfes are not published as far as we know. In ref. 2, 4, 6 and 8 304N was reported stable. Again we must conclude that the magnetic character of these steels is unpredictable because of the wide commercial composition limits. Sample 310 shows a slight increase of ferromagnetism from the first cooling cycle onwards which remains the same after 10 cycles. The effect is rather small, only a 2% increase of Msat after 10 cycles. In ref. 2 it was reported stable. Sample 321(b) shows a large increase of Msat between the first and the second cooldown and a slight further increase in the next 9 cooldown runs. This effect was also reported in ref. 2. The other specimen of this type of steel (321(a)) showed no effects on cyclic cooling as was reported in ref. 1. The difference between these two samples could be explained by looking at the chemical composition: sample 321(a) contains twice as much of the austenite stabilising Mn as sample 321(b). This was exactly the case with the samples of Larbalestier and King2 (Mn0.77, unstable) and of Reed and Mikesell' (Mn2.0, stable).

6 Fig. 1 - Magnetisation curves of different stainless steel samples. Straight line - before welding, dotted line - after welding.

7 Cl-630 JOURNAL DE PHYSIQUE Fig. 2 - Magnetisation of different specimens as function of the number of cooling cycles N between room temperature and 4.2 K. Table 4. Summary of results A1 1 oy name ferromagnetic traces on first cooling on welding on cycls cooling 303 x 304 (a) + x 304 (b) - f 304 (c) + x 304N t + 310s (a) (b) (c) L LN Ti X6CrNi (a) (b) = detected - = not detected x = nbt tested

8 CONCLUSIONS Table 4 summarizes the results of the magnetisation measurements on 16 different types of stainless steel. Four samples remained paramagnetic under all circumstances: 316(b), 316(c), 316LN and X6CrNi These types may be regarded as stable austenitic stainless steels. Sample 316(a) showed some ferromagnetism on welding probably due to the low Mo content. In the case of the 303, 304 and 321 samples we can conclude that one cannot be sure about their magnetic character in advance. The knowledge of the chemical composition could be helpful to predict whether ferromagnetic traces will occur, but in general when magnetic behaviour is critical one should not use these types of stainless steel unless exact magnetisation measurements has been done. In the case of the 310 specimens absolute magnetisation is fairly large. There is a possibility of a ferromagnetic transitio: in the austenitic phase caused by the large amount of Ni in these types of steel. That leaves only the 316LN type of steel to be perfectly stable (no evidence of the contrary is known in the literature) and the type X6CrNi 1811, of which we are not aware of any results published before. ACKNOWLEDGEMENT The authors wish to thank Prof. H. Postma for his advice and encouragement. They are also grateful to Mr. G.J. Maanenschijn and Mr. P.F. Colijn of the Metallurgical Department for analyzing the samples and helpful discussions. The technical assistance of Mr. W. den Braver at the preparation of the experimental set-up is gratefully acknowledged. REFERENCES 1. REED R.P. and MIKESELL R.P., Adv. Cryog. Eng. 4 (1960) LARBALESTIER D.C. and KING H.W., Cryogenics 13-(1972) WARNES L.A.A. and KING H.W., Cryogenics 16 (m76) COLLINGS E.W., Adv. Cryog. Eng. 26 ( WARNES L.A.A. and KING H.W., CryGenics 21 (1982) COLLINGS E.W. and HART S.C., Cryogenics 2 (197 9) KING H.W.. CIM Bulletin July GEYARI C.; Vacuum 26 (1976) 587.