High-strength, high-nitrogen stainless steel–copper composite

January 2002Ž.Materials Letters 522002154–158 r locate r matletHigh-strength,high-nitrogen stainless steel–copper compositewires for conductors in pulsed high-field magnetsW.Grunberger ),M.Heilmaier,L.Schultz ¨IFW Dresden,Institute for Metallic Materials,P.O.Box 270016,D-01171Dresden,GermanyReceived 10March 2001;accepted 16April 2001AbstractCopper r high-nitrogen austenitic stainless steel macrocomposites based on Cr–Ni steel and Cr–Mn steel,respectively,were fabricated by the ‘rod-in-tube’technology.A strength level of 1.2GPa was achieved at room temperature for both Ž.composites Cu fractions f 53vol.%.In composites with Cr–Mn steel,the necessary intermediate heat treatments during the drawing process lead to the formation of a low melting Cu–Mn alloy at the interface Cu r steel,which finally requires the use of a diffusion barrier.q 2002Elsevier Science B.V.All rights reserved.Keywords:Cu r steel macrocomposite;High-nitrogen steel;Drawing;Mechanical properties1.IntroductionThe main goal for conductor wires used in pulsed high-field magnets is a beneficial combination of high mechanical strength,high electrical conductiv-ity and sufficient ductility at ambient and cryogenic w x temperatures 1–3.In order to obtain the required strength level,the work hardening effect due to heavy cold deformation is used for the conductor materials under consideration.Besides microcompos-ite wires based on Cu–Nb or Cu–Ag,attractive candidates are Cu core r steel jacket macrocompos-ites.Currently,stainless steel is mainly applied for the steel jacket.The fabrication of these composites is carried out by utilizing the ‘rod-in-tube’technol-)Corresponding author.Tel.:q 49-351-4659-667;fax:q 49-351-4659-781.E-mail address:w.gruenberger@ifw-dresden.de Ž.W.Grunberger .¨ogy.Tensile strengths of composites with 50–60%volume fraction of Cu are reported to yield 0.8–1.0w x GPa at room temperature 4–6.In this work,an approach is presented for surpass-ing this strength level by using stainless steel vari-ants with a relatively high nitrogen content.It is well known that nitrogen plays a threefold beneficial role Ž.in austenitic stainless steel.Nitrogen i is a very Ž.effective austenite stabilizer,ii is a strong solid Ž.solution hardener,and iii improves the corrosion w x resistance in combination with molybdenum 7–9.The expected high strength level of high-nitrogen austenitic steels can be rationalized in terms of a superposition of four basic hardening mechanisms:work hardening,fine grain strengthening,solid solu-w x tion and precipitation hardening 7.This is already exemplified comparing 0.2%yield strength in the solution-annealed condition:while conventional aus-tenitic stainless steels hardly exceed 0.2GPa,ad-vanced austenitic steels containing up to 1wt.%N show 0.2%yield strength values of 0.6–0.7GPa.00167-577X r 02r $-see front matter q 2002Elsevier Science B.V.All rights reserved.Ž.PII:S 0167-577X 0100383-4()W.Grunberger et al.r Materials Letters522002154–158¨155Furthermore,they show a remarkably high cold working capacity,coupled with a beneficial absence of forming deformation-induced martensite.More-over,the increase of the work hardening rate with increasing nitrogen content was reported,and hence,w x 0.2%yield strengths above2GPa can be reached8.In the present study,the principle suitability of two commercial high-nitrogen steels on the base of Fe–Cr–Ni and Fe–Cr–Mn is demonstrated for the production of high-strength copper r steel compos-ites.Particular attention is given to the problem that recrystallization temperatures have to be maintained below the melting point of copper for the Fe–Cr–Ni steel.In the case of the Fe–Cr–Mn steel,the atten-tion focuses on the possible formation of a low melting Cu–Mn alloy at the interface Cu r steel.This can be expected since the melting temperature in the binary phase diagram Cu–Mn passes through a mini-mum at8718C for the composition Cu–37wt.% Mn.2.ExperimentalTubes of two austenitic stainless steels were usedŽ.Žin this study:i Nicrofer3033e designated as steel.3033in the following of KRUPP VDM,Werdohl,Ž.having the composition wt.%Fe,31Ni,33Cr,.0.6Cu,1.7Mo,0.39N with an outer diameter of BŽ.18mm and2-mm wall thickness,ii a B25=2Žmm tube of the steel P900VSG Energie-und Schmiedetechnik,Essen,provided by the Mannes-.mann Forschungsinstitut,Duisburg with the compo-Ž.sition wt.%Fe,18.8Mn,18.7Cr,0.34Ni,0.07C, 0.08Mo,0.77N.For adjusting the desired fractions of the conducting and the strengthening component of the composites to approximately50vol.%each,the tubes were drawn to different starting diameters prior to co-deformation with the Cu core.Steel3033was drawn to a diameter of15.5mm and filled with a copper bar of10.5-mm diameter for further co-defor-mation.Steel P900was drawn to a diameter of17.3 mm and then the solution was annealed at11208C for10min with subsequent water quenching.After cleaning,the composite was assembled with a copper rod of11.1-mm diameter.The drawing strain subse-quently used as the key parameter in the assessment of the resulting strength values is defined as h sŽ.ln A r A,where A and A are the initial and final 00cross-sectional areas,respectively.The necessary in-termediate heat treatments were performed in argon atmosphere followed by water quenching.The mi-crostructure was investigated by scanning electron Ž.microscopy SEM,Jeol JSM6400.The ultimate ten-Ž.sile strength UTS and the elongation to fracture Ž.A were measured at room temperature with wiretpieces of a total length of150mm and an original gauge length of25mm using an electromechanical Instron8562testing machine,with a cross-head speed of0.2mm r min.3.Results and discussionposites with the Fe–Ni–Cr–N steelw x According to the material data sheet10,the mechanical properties of steel3033in the solution-annealed condition are as follows:0.2%yield strength R)0.38GPa,ultimate tensile strength UTS) p0.20.72GPa,elongation to fracture A)40%.A solu-5tion heat treatment at11208C with subsequent waterw x quenching is recommended by the vendor10.Pre-studies with rods turned from rolled bulk material of the same steel revealed that solution heat treatments at1120and10508C for10min yield the same mechanical properties after heat treatment and subse-quent drawing.This is shown in Fig.1,depictingtheFig.1.UTS as a function of drawing strain of the high-nitrogenŽaustenitic stainless steel Nicrofer3033initial diameter B s6.7i .mm.()W.Grunberger et al.r Materials Letters 522002154–158¨156evolution of the UTS of rods with an initial diameterof 6.7mm as a function of drawing strain h .Essen-tially,no differences are visible for the two heat treatments leading to the same strength level of about 1.9GPa after a mere drawing strain of h s 1.8.Therefore,the alloy should be suitable for the co-de-formation with copper when heat-treated at tempera-Žtures lower than the melting point of copper 1083.8C .Based on these results,the co-drawing of the copper r steel 3033composite,starting from the outer diameter of 15.5mm,was interrupted at intermediate Ž.Ž.diameters of 11.8h s 0.84and 7.5mm h s 0.91for two heat treatments at 10508C for 10min with subsequent water quenching.For a Cu r steel 3033composite with a copper fraction of 52vol.%,Fig.2Ž.full squares exemplifies the development of the UTS as a function of drawing strain h .After h s 2.3,UTS of 1.2GPa at an elongation to a fracture of about 3%is achieved.The diagram also contains the Ž.results of the Cu r steel 900composite full triangles and of a macrocomposite with the conventional Ž.w x stainless steel 304L full circles after Dupouy 6.The observed work hardening rate of the Cu r steel 3033composite is found to be comparable to that of conventional stainless steel r copper composites.However,due to the higher initial strength level in alloy 3033,the final UTS exceeds those of the conventional Cu r stainless steel composites by roughly 0.2GPa after equivalent drawingstrain.Fig.2.UTS as a function of drawing strain of two Cu r high-Žnitrogen stainless steel composites last heat treatment at B s 7.5i .w x mm and of a Cu r steel 304L composite 6.Ž.Ž.Ž.Fig.3.a UTS and b elongation to fracture A of the Cu r steel t Ž3033composite after different final heat treatments drawing .strain h s 2.32.Hence,UTS value of more than 1GPa can certainly be obtained in macrocomposites containing more than 50vol.%copper.Fig.3shows that a final heat treatment even at modest temperatures lower than 6008C leads to both significant decrease of strength values and deteriora-tion of ductility.The elongation to fracture drops from 3%to values less than 1%.This effect is very likely attributed to the precipitation of nitrides which w x cause embrittlement 9.As a consequence,isother-mal annealing of the highly deformed composite is not an appropriate way for improving its formability.Thus,the adequate strength level for a crack-free transition from round to rectangular cross-section has to be determined yet.posites with the Fe–Cr–Mn–N steel According to the data sheet of VSG,the mechani-cal data of the comparable steel P900N Mo in the solution-annealed condition are:R s 0.6GPa,p0.2w x UTS s 0.95GPa,A s 55%11.The solution heat 5treatment is recommended to be performed at tem-peratures of 1100–11508C with subsequent water quenching.Obviously,this thermal treatment cannot be utilized for the Cu r steel P900macrocomposites.The P900series shows an extremely high work hardening rate.For nitrogen contents of about 0.7()W.Grunberger et al.r Materials Letters 522002154–158¨157Fig.4.SEM micrographs of the Cu r steel P900interface after a heat treatment at 10008C for 10min.wt.%,a strength level close to 2GPa after a defor-w x mation strain of only h s 0.9was reported 11.Co-drawing of the Cu r steel P900composite was Ž.interrupted at a diameter of 11.8mm h s 0.77for performing heat treatments for 10min in the temper-ature range of 950–10508C.At temperatures G 9758C,visible changes at the interface between the copper core and the steel jacket were observed due to the formation of a low-melting Cu–Mn solid solution.Incipient melting at the interface concerned both components.XEDS analysis after annealing at 9758C revealed Mn depletion and slight Cu enrich-ment in a width of about 10m m in the steel jacket.Corresponding enrichments of Mn and,to a lesser amount,of Fe,are found on the side of the copper core up to about the same depth.Fig.4a and b shows the melted zones after annealing at 10008C for10Fig.5.XEDS analysis of the interface region of a Cu r steel P900composite after a heat treatment at 10008C for 10min.min.The width of the melted zones already amounts to more than 100m m on both sides.This fact is supported in Fig.5by XEDS analysis results for the distribution of Fe,Cu and Mn at the interface.Since no diffusion barrier was used in the present study,the necessary annealing treatments were per-formed at 9758C for 10min at wire diameters of 11.8and 7.5mm.In spite of the low solution temperature,the formability of the composite was satisfactory.As already mentioned,Fig.2compares the development of the ultimate tensile strength of Žthe Cu r steel P900composite full triangles,copper .fraction s 53.5vol.%as a function of drawing strain Žwith those of the composites Cu r steel 3033full .Ž.squares and Cu r steel 304L full circles .The re-sults confirm the extremely high work hardening rate of the steel P900.The strength level of 1.2GPa is reached after a drawing strain of only h s 0.8,with an elongation to fracture between 2%and 3%.Again,as already observed in a similar way for the Cu r steel Ž.3033composite Fig.3,final heat treatments at temperatures F 5508C result in a steep drop of the Ž.UTS as well as of A not plotted here .t 4.ConclusionsCu r high-nitrogen stainless steel composites based on Cr–Ni steel and Cr–Mn steel,respectively,are able to achieve UTS levels of 1.2GPa at room temperature at comparably low levels of drawing Ž.strain h Cu fraction s 53vol.%.The composite with the Mn-rich high-nitrogen stainless steel reveals the formation of a low-melting CuMn alloy at the()W.Grunberger et al.r Materials Letters 522002154–158¨158interface Cu core r steel jacket during the necessaryheat treatments.In addition,its extremely high work hardening rate requires frequently repeated heat treatments during the cold deformation.As a conse-quence of these facts,the use of a diffusion barrier is inevitable.Therefore,future work will be directed towards the determination of the strength r ductility limits of Cu r high nitrogen Cr–Ni steel composites for the crack-free transition from round to rectangu-lar cross-sections of conductors.In addition,this variant promises higher fracture toughness values at cryogenic temperatures compared to those of the w x high-nitrogen Mn-rich steel 9.AcknowledgementsThe work was funded by Sachsisches Ministerium ¨Ž.fur Wissenschaft und Kunst SMWK and Bun-¨Ž.desministerium fur Bildung und Forschung BMBF .¨Referencesw x Ž.1 D.Dew-Hughes,Mater.Sci.Eng.,A 168199335.w x Ž.2J.T.Wood,J.D.Embury,M.Ashby,Acta Mater.4519971099.w x 3J.D.Embury,K.Han,Curr.Opin.Solid State Mater.Sci.3Ž.1998304.w x 4H.Jones,M.Van Cleemput,in:H.-J.Schneider-MuntauŽ.Ed.,High Magnetic Fields,Application,Generation,Mate-rials.World Scientific,NJ,USA,1997,p.499.w x 5H.Jones,M.Van Cleemput,A.L.Hickmann,D.T.Ryan,Ž.P.M.Saleh,Physica B 246–2471998337.w x 6 F.Dupouy,PhD Thesis,no.367,INSA,Toulouse,1995.w x Ž.7M.O.Speidel,in:G.Stein,H.Witulski Eds.,Proc.2nd Int.Conf.on High Nitrogen Steels,HNS90,Aachen.Stahleisen,Dusseldorf,1990,p.128.¨w x Ž.8G.Stein,J.Lueg,in:V.G.Gavriljuk,V.M.Nadutov Eds.,Proc.3rd Int.Conf.on High Nitrogen Steels,HNS93,Kiev.Institute for Metal Physics,Kiev,1993,p.31.w x Ž.9J.W.Simmons,Mater.Sci.Eng.,A 2071996159.w x w 10Data sheet:Nicrofer 3033alloy33,KRUPP VDM,Wer-–dohl,Germany,1998.w x 11Data sheet:Ni-free Austenitic Steel P 900N Mo,VSGEnergie-und Schmiedetechnik,Essen,Germany.。