Atom Probe Tomography Study On Copper Precipitation Strengthening And Reverse Austenite Toughening In HSLA Ferritic Steel

The copper precipitation and the formation of reverse austenite in Cu-andNi-containing high-strength low-alloy steelsare comprehensively studied by atomprobe tomography (APT) companying with optical microscopy (OM), scanningelectronic microscopy (SEM) and transmission electronic microscopy (TEM). Threedifferent heat treatment procedures including solution pluscontrolled cooling (SCC),solution and quenching plus conventional tempering (SQCT), and solution andquenching plus interstitial tempering plus conventional tempering (SQIT) aredesigned. The nature of Cu precipitation,and the effects of heat treatment on themicrostructural evolution and the final mechanical strength are discussed.During continuously cooling of austenite, the solute elements of C, Ni, Mn andCu etc.are prone to diffuse into the untransformed austeniteand change the austenitedecomposition kinetics, leading to the formation of a variety of structural componentsof ferrite, bainite, martensite and/or retained austenite in the ambient microstructure.Besides, the Cu precipitation also greatly correlates with the austenite decomposition.The actual volume fractions of the final phases are as function of the cooling rates.The Cu phases are mostly formed by interphase precipitation in the polygonalferrite.But the Cu precipitation can also occur in polygonal ferrite after theaustenite-ferrite transformation is finished.The micro-constituents such as dislocations,carbides (cementite) and migrating austenite/ferrite heterophase interfaces duringaustenite decompositionhave an effect on the nucleation, growth and coarsening ofcopper precipitates. APT studies indicate that not all Cu precipitates are formed on thedislocations with P segregation, because the austenite/ferrite heterophase interfacesare more likely to induce the Cu nucleation compared to dislocation. The carbide/matrix interfaces are the preferential sites for Cu nucleation, and the formation ofcarbides will contribute to the growth and coarsening of Cu precipitates. Theinterphase precipitation of Cu phases is significantly impacted by migration rate ofaustenite/ferrite interfaces and the actual cooling rate. The structural andcompositional evolution is the same as that for the isothermal aging. Thesolution-quench microstructure of the Cu-Ni steels mainly consists of lathmartensite with Cu in solution. Duringtempering, the micro-hardness evolution wellreflects the microstructuralevolutionresulting from the softening of martensite, Cuprecipitation strengthening and reverse austenite toughening. The number density ofCu precipitates decrease whereas the sizes increase with increasing of temperingintensity. At the same time, the morphology evolve form sphere to ellipse. At theinitial nucleation stage, the Cu precipitates contains considerable high Fe content aswell as a small amount of Ni and Mn, which are prone to segregate at the matrix/Cuprecipitate interface at the later growth and coarsening stages.By additional interstitial tempering in QT procedure, a microstructure of lath-likemartensite with dispersed reverse austenite at lath boundary and prior austenite grainboundary is obtained.The mechanical and thermal stability of the reverseaustenitewhich is in direct proportion to the content of alloying elements especially Niplays an important role in toughening at low temperature. Copper can diffuse into andstabilize the reverse austenite and precipitate out companying with the partialdecomposition of reverse austenite during tempering. Copper still in solution duringinterstitial temperingis the contributor for the strengthening Cu precipitates formed insubsequent tempering.There are great difference in the final microstructure and thereby mechanicalproperties of the Cu-Ni steels. The disadvantages of SCC and SQCT processed steelsare lower temperature toughness and yield/tensile ratio, respectively, which can beoptimized by SQIT procedure. As to the shearable BCC coherent Cu precipitates, thestrength increments by the effects of chemical hardening, the coherency strain and themodulus strengthening are3-6,86and139MPa, respectively. For the incoherent,impenetrable FCC Cu precipitates, the strength increments by classical Orowanmechanism is18MPa. The critical transition size of Cu precipitates with differentstrengthening mechanism is calculated to be about2.9nm.