USE OF A NONMAGNETIC STAINLESS STEEL

Abstract: The invention advises use of a stainless steel alloy as construction material in superconducting magnetic components, said alloy containing in weight percent 0.05-0.25 % C, 0.1-1.5 %, Si, 3.5-7.5 % Mn, 17-21 % Cr, 6-10 % Ni, 0.10-0.50 % N, the remainder being Fe and normal impurities.

Description: Use of a nonmagnetic stainless steel The present invention relates to the use of a non-magnetic high strength stainless steel for the manufacture of super conducting magnet components such as magnet collars used in particle accelerator apparatuses. The rapid development of research within various advanced physical laboratories has created an increased demand for more sophisticated materials with combinations of properties not previously considered or easily achievable such as, for example the combination of high mechanical strength and a non-magnetic structure for materials to be used in applications where the material is required to be magnetically inert also at low temperatures. Among high strength steels, the so-called non-stable austenitic spring steels, SS2331 with a typical nominal analysis of 17 Cr, 7 Ni, 0.8 Si, 1.2 Mn, 0.1 C and 0.03 N are in a special position because of their combination of high strength and good corrosion properties. Thanks to a systematic development work it has now been found that it is possible, by a carefully selected composition, to achieve, by cold working, a specific deformation hardening effect while preserving a non-magnetic structure. In addition, it has been found possible without affecting the magnetic properties, to provide precipitation hardening of the alloy such that a very high strength combined with low magnetic permeability and good thermal contraction values is achieved at very low temperatures. The optimized composition (in weight-%) of the alloy of the present invention in its broadest aspect is as follows: C: 0.05-0.25 Si: 0.1-1.5 Mn: 3.5-7.5 Cr: 17-21 Ni: 6-10 N: 0.10-0.50 The remainder being Fe and normal impurities. Cr content should be high in order to achieve good corrosion resistance. The alloy can, to advantage, be annealed and precipitate high chromium containing nitrides. In order to reduce the tendency for excessive local reduction of Cr-content with the non- stabilization of the austenite phase and reduction in corrosion resistance the Cr content should exceed 16 %. Since Cr is a ferrite stabilizing element, the presence of very high Cr contents with lead to the presence of ferromagnetic ferrite. The Cr content should therefore be less than 21 %, preferably less than 19 %. Ni is a very efficient austenite stabilizing element. Ni also increases austenite stability against deformation into martensite. In order to achieve a sufficiently stable non¬ magnetic structure the Ni-content should exceed 6 % and preferably exceed 7 %. In order to achieve high strength after cold working the Ni-content should not exceed 10 %. Mn has beside an austenite stabilizing effect the important -ability of providing solubility of nitrogen, both in melted and solid phases. The Mn-content should therefore exceed 3.5 %. High amounts of Mn, however, reduce the corrosion resistance in chloride containing environments and should therefore not exceed 7.5 %. The amounts of the various components of the alloy should be selected such that the nickel equivalent calculated as Ni-equiv = Ni + 30 C + 0.5 Mn + 25 N, and the chromium equivalent calculated as Cr-equiv = Cr + Mo + 1.5 Si both amount to values in the range 16-22, preferably 18-20. The invention will in the following be disclosed by way of results from research carried out whereby further details about mechanical properties and magnetic properties will be disclosed. Example Production of the testing materials included melting in a high-frequency induction furnace and casting to ingots at about 1600°C. These ingots were heated to about 1200°C and hot worked by forging the material into bars. The materials were then subjected to hot rolling into strips which thereafter were quench annealed and clean pickled. The quench anneal was carried out at about 1080°C and quenching occurred in water. The strips obtained after quench annealing were then cold rolled to various amounts of reduction after which test samples were taken out for various tests. In order to avoid variations in temperature and their possible impact on magnetic properties the samples were cooled to room temperature after each cold rolling step. Table 1 : Chemical analysis, in weight-%, of testing material. * alloys of the invention ** comparison samples Steel No. C Si Mn Cr Ni Mo Al N 869* 0.11 0.69 4.29 18.52 - - - 0.27 880* 0.052 0.89 3.82 20.25 10.01 - - 0.29 866** 0.11 0.83 1.49 18.79 9.47 - - 0.20 AISI** 0.034 0.59 1.35 18.56 9.50 - - 0.17 304 AISI** 0.042 0.42 1.72 18.44 11.54 - - 0.036 305 P, S < 0.030 weight-% is valid for all alloys above. The strength of the alloys when subjected to uniaxial tensile testing as function of cold working degree appears from Table 2, where R- 0.05 and R_ 0.2 correspond to the load that gives 0.05 % and 0.2 % remaining elongation, and where R.., corresponds with the maximum load value in the load-elongation diagram and where A10 corresponds with ultimate elongation. Table 2. Yield point, tensile strength and elongation of testing materials. * alloys of the invention ** comparison samples Steel No. Condition R-.0.05 R p O.2. Rm A10 MPa MPa MPa % 869* 35 % reduction 792 1062 1203 9 50 1007 1311 1464 6 75 1082 1434 1638 4 880* 35 836 1086 1208 7 50 1025 1288 1410 5 75 985 1343 ' 1566 4 866** 35 796 1036 1151 8 50 986 1239 1366 5 75 997 1356 1558 4 AISI** 35 683 912 1080 9 304 50 841 1127 1301 6 75 910 1300 1526 5 AISI** 35 555 701 791 15 305 50 841 1042 1139 6 75 868 1177 1338 5 Table 2 shows that with alloys of the invention very high strength levels can be obtained at cold working. AISI 305 appears to show a substantially slower work hardening due to its low contents of dissolved alloy elements, i.e. nitrogen and carbon, combined with rather high nickel content. For a material according to this invention there is the requirement that this material, whilst exhibiting high strength, also has as low magnetic permeability as possible, i.e. close to 1. Table 3 shows the magnetic permeability depending upon field strength for the various alloys after 75 % cold reduction and annealing at 450°C/2 h. Table 3. Permeability values of test alloys. Underlined values indicate maximal measured permeability. The value at the bottom indicates tensile strength in corresponding condition. * alloys of the invention ** comparison samples Field strength Steel No. Oersted 869* 880* 866 ** AISI AISI 304** 305** 25 1.0350 50 1.0389 1.0099 1.0346 1.5231 1.0593 100 1.0372 1.0118 1.0248 1.8930 1.0666 150 1.0359 1.0115 1.0413 2.1056 1.0688 200 1.0350 1.0110 1.0505 2.2136 1.0729 300 1.0329 1.0099 1.0640 2.2258 1.0803 400 1.0322 1.0089 1.0754 2.1506 1.0855 500 1.0321 1.0081 1.0843 2.0601 1.0884 700 - 1.0071 1.0917 1.0859 1000 - 1.0882 Rm MPa 1840 1740 1720 1644 1380 Table 3 shows that with alloys of this invention it is possible, by coldworking and precipitation hardening, to achieve high strength exceeding 1700 or even 1800 MPa combined with very low values of the magnetic permeability < 1.05. The reference alloys with compositions outside the scope of this invention and the reference steels AISI 304 and AISI 305 appear to be too unstable in austenite, or appear to have an insufficient degree of work hardening. As appears from the results in Table 4 it is impossible with alloys of this invention, by cold working and precipitation hardening, to achieve a strength exceeding 1700 MPa combined with very low values of the magnetic permeability of < 1.05. The reference steels AISI 304 and AISI 305 appear to be too unstable in austenite, and alloys 866 and AISI 304 appear to be magnetic at high strength or appear to have an insufficient degree of work-hardening. As a further result of such material having low values of magnetic permeability it was found that such material also possesses a desirable degree of thermal contraction value at low temperatures. Conducted measurements have shown that integrated thermal contraction for a temperature range 77K-300K is about 0.25 %. Further, for the material in the annealed or slightly cold rolled conditions (tensile strength ~ 1000 N/mm 2 ) the relative magnetic permeability coefficient has been measured to a value below 1.005 for temperatures down to 4.2 K or even 1.8 K. Measurements have been carried out on a material with following anah sis with amounts given in weight-%: c si Mn Cr Ni N 0.11 0.8 6.0 18.5 7.2 0.-25 the remainder being Fe and normal impurities. Table 4. Condition Temp. K R-, 0.2 Rm Annealed 293 475 850 N/mm 2 77 1090 1620 Cold rolled 293 1375 1630 77 1820 2385

Claims: Claims 1. Use of a non-magnetic, stainless steel alloy having high strength, consisting essentially of, in percent by weight: C: 0.05-0.25 Si: 0.1-1.5 Mn: 3.5-7.5 Cr: 17-21 Ni: 6-10 N: 0.10-0.50 the remainder being Fe and normal impurities, as material for the manufacture of superconducting magnet components to satisfy demands on low magnetic permeability and good thermal contraction values at low temperatures. 2. Use of a non-magnetic stainless steel as defined in claim 1, characterized in that the alloy contains 17-19 % Cr and 7-10 % Ni. 3. Use of a non-magnetic stainless steel as defined in claim 1 or 2, characterized in that the contents of the alloying elements are so adjusted that the following conditions are fulfilled: Cr-equiv = Cr + Mo + 1.15 Si = 16-22. Ni-equiv = Ni + 30 C + 0.5 Mn + 25 N = 16-22. 4. Use of a non-magnetic stainless steel as defined in any of the claims 1-3, characterized in that the contents of the alloying elements are so adjusted that the following conditions are fulfilled: Cr-equiv = Cr + Mo + 1.5 Si + 0.5 Nb = 18-20. Ni-equiv = Ni + 30 C + 0.5 Mn + 25 N = 18-20.

Inventors: HOLMBERG HAAKAN, HOLMBERG, HAAKAN
Applicants: SANDVIK AB
Application year: 1995
Main classification: C22C38
IPC codes: C21D8, C22C38