Composition Design Of High-strength Ferritic Ageing Stainless Steels And Their Electrochemical And Mechanical Properties

A large portion of structural materials currently used in ocean exploitation are maternstitic and austenitic stainless steels. These materials, however, are not sufficently stable against pitting corrosion in sea water. While ferritic stainless steels meet the surface stablity requirement in such a corrosion enviroment, their low material strengths (σ0.2<300MPa in most cases) have limited their practical applications as various structural parts. High strength ferritic stainless steels having a good combination of material cost and properties are therefore very desirable. The present work aims at exploring new high strength ferritic stainless steels by composition design. The electrochemical and mechanical properties of alloys thus designed are also studied.The so-called atomic cluster plus glue atom model is able to show the energetically most probable short-range order topology and chemistry of a complex solid solution single phase alloy with the atomic cluster and by compoistion averaging. Here the atomic cluster is a pattern occurred in all examples of a given ensemble. For Fe-Cr bianry solid solution alloys, a basic atomic cluster formula,[Cr-Fe10Cr4]Cr, was determined taking both the phase diagram characteristics, enthalpy of mixing and the local structure topology of a BCC structure into consideration. Substitution alloying on the atomic scale can be viewed as carried out on specific atomic sites in the cluster formula, in particular, Ni、Cu and Al for the cluster center atom Cr; Mo、Ti、Nb and V, partially, for Cr. An atomic cluster formula,[(Ni16-m-nCumAln)-Fe160Cr64](Cr16-o-p-q-rMOoTipNbqVr), is eventually produced, which may be used to describe a stable complex solid solution structure at elevated temperatures. A series of such alloy compositions are emplicitly presented:1#[Cr-Fe12Cr2]Cr;2#[(Ni14Cu2)-(Fe160Cr64)](Cr7Mo6Ti2Nb1);3#[(Ni12Cu2Al2)-(Fe160Cr64)](Cr7Mo7Ti1Nb1);4#[(Ni12Cu3Al1)-(Fe160Cr64)](Cr7Mo6V1Ti1Nb1);5#(Ni14Cu2)-Fe160Cr64-(Cr8Mo5.5Ti2Nb0.5);6#(Ni12Cu4)-Fe16oCr64-(Cr8Mo5.5Ti2Nb0.5);7#(Ni14Cu2)-Fe160Cr64-(Cr8Mo5.5Ti1Nb0.5Al1);8#(Ni13Cu3)-Fe16oCr64-(Cr8Mo5V0.5Nb0.5Ti1Al1)。Alloy rods of6mm in diameter were made by means of copper mould suction casting. The as-cast rods were solid soluted at1030℃and1150℃for0.5h, respectively, followed by water quenching, and then ageing at555℃for3h. Phase identification by X-ray diffraction indicated that the1150℃solid soluted alloys had a single phase BCC structure, and secondary phases precipitated in the case of1030℃treatment. Optical metallurgical images of the aged alloys showed the dispersive distribution of secondary phase particles. In5#,6#7#and8#alloys, the paricles have a size smaller than0.5μm, and their precipitation are rather uniform and disperse.The potental/current density behaviors of these aged alloys are studied in simulated sea water at a constant potential sweeping rate. All these alloys exhibited a distinct passivation stage. From the anodic polarization curves, the self-corrosion potential (Ecorr), the self-corrosion current density(icorr) and the pitting potentals (Eb) are obtained: Eorr=-0.23～--0.12V(S.C.E.); icorr=0.987～4.41μA/mm2; Eb=1.45～1.5V(S.C.E.). The pitting corrosion resistance of these alloys were found to be significantly improved in comparison with the known00Cr26Ni6Mo4CulTi ferritic stainless steel.The microharness values of these aged alloys are ranged from Hv347to Hv436. Upon room temperature uniaxial tensile testing,2#alloy showed a yield strenth σ0.2=1017MPa, a tensile strenth σb=1287MPa and a plastic strain ε=7.7%. The reinforcement effect of the secodary phase particles are remarble. The new alloys are much more stronger than the known00Cr26Ni6Mo4CulTi(σ0.2=890MPa, ε=17.5%) alloy.The contents of specific alloying elements in these alloy are recalled:Cr:-25.0-27.0wt.%;Ni:-4.0-6.0wt.%; Mo:～3.0-6.0wt.%; Cu:～1wt.%; Ti:-0.3-0.7wt.%; Nb:-0.3-0.7wt.%; Al～0.2wt.%; V:-0.2wt.%. Comparing with the known ferritic stainless steels, the new alloys contain several minor alloying elements such as Nb、Al and V, wherease the contents of Mo are reduced. The lab success of making high strength ferritic stainless steels in a fairly wide composition range, along with the favorable heat treatment specifications, would imply a useful way for the compositional and property modifcations of complex enginerring solid solution alloys.