“A form, Friction-induced martensite transformation of FeCrNi alloys in cryogenic 
environment 
An0G- 
ene ” Sigrid Binkowski, Wolfgang Hiibner, Olaf Berndes, Sigrid Benemann, Jiirgen Schwenzien, Werner 
eK 0 Oesterle 
0 ie Bundesanstalt fiir Materialforschung und — prüfung (BAM), Berlin 
Offa bn 
er Hd Introduction 
Timer Austenitic stainless steels are favoured alloys for cryogenic use because of the mechanical and 
Yandund. magnetical properties of the fcc austenite lattice. Also at very low temperatures these alloys retain 
high strength together with sufficient ductility. In applications of superconductivity stainless steels 
are used because of their non-magnetic behaviour. 
2a 104 In tribosystems of cryogenic equipment the surface of these alloys are subjected simultaneously to 
may - low service temperatures 
. plastic deformation 
vertu . reactive media in the case of hydrogen and oxygen environments. 
Thereby, the conditions exist for austenite/martensite transformations, and the material can lose its 
favourable properties. Martensite generation is followed by a decrease in ductility of stainless steels 
and can be the cause of materials deterioration. Especially, care must be taken in hydrogen- 
containing environments because it is well known that hydrogen-induced fracture starts at 
Ser Maer martensitic components. a ; 
nese The transformation also affects the applicability of stainless steels in superconductivity, because, in 
ih contrast to austenite martensite is ferromagnetic and its generation considerably deteriorates the 
desired magnetic properties of the materials. 
ren die This project aims to get stability data of the austenitic structure of FeCrNi alloys in cryogenic 
a tribosystems. This paper deals with metallographic explanations of the magnetic behaviour 
observed. 
Experimental 
The tribological tests were realized by ball-on-disc assemblies working in a bath cryostat or in a 
continuous flow cryostat [ 1]. In the bath cryostat the service temperature is determined by the 
cryogen used: 77 K in liquid nitrogen (LN), 4,2 K in liquid helium (LHe). In the continuous flow 
cryostat temperatures can be adjusted between 20 and 293 K. In this case the environment is a inert 
gas (He). 
The tests were started with a normal force of Fy = 5 N and sliding velocity of vs = 0,2 ms”. The 
sliding distance was normally 1000 m. 
For the experiments materials were chosen with different compositions (table 1): 1.4301, 1.4439, 
cungstechnik 1.4958, 1.4591 (German Materials Numbers). Depending on these compositions the alloys differ in 
ers Zeitschrift their nominal austenite stability as it is represented by Ms and My3o values resp. in the table. 
Material | Composition in % Martensite temperature * 
Fed Circuit In °C 
C Cr Ni N Mo Ms Mazo ] 
1.4301 <0,07 17,0-19,5 8,0-10,5 <0,11 - -411 -33 
ert 1.4439 0,03 16,5-18,5 12,5-14,5 0,12-0,22 4,0-4,5 - 673 - 152 
wmckaging and 1.4958 0,061 20,5 30,65 - - - 1553 - 197 
14591 0.007 32.85 30.95 0.39 1,67 - 2637 - 553 
Table 1: Composition and Mg and Mg; values (after | 2]) of the alloys chosen. * The values are not observable 
temperatures but only represent stability data of the austenite 
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