Translated Title

Translated Abstract

Hadfield　steel　cannot　show　good　wear　resistant　properties　under　low　impact　energy　or　lighter　load　conditions.　In　this　thesis,　a　nanostructured　surface　layer　of　a　Hadfield　steel　was　synthesized　by　shot　peening.　The　shot　peening　is　expected　to　improve　mechanical　and　wear　resistance　properties　of　Hadfield　steel.　Shot　peening　approach,　of　course,　has　a　great　potential　in　industrial　applications.　It　is　also　a　flexible　approach　that　makes　it　possible　to　meet　specific　structure/property　requirements　during　services.　Based　above　considerations,　Nanocrystallization　mechanism　of　surface　on　Hadfield　steel,　thermodynamics　and　kinetics　of　nanocrystilline　formation　in　surface　of　Hadfield　steel　and,　effect　of　surface　work　hardening　on　wear　behavior　of　Hadfield　steel　were　discussed　in　the　present　thesis.The　grain　size　about　3-8　nm　was　formed　in　the　sample　surface　after　60　min　shot　peening.　The　treated　subsurface　can　be　subdivided　into　three　zones　along　depth:　nanostructured　regime　(0-20　m)　micro-sized　regime　(20-100　m)　and　matrix　with　plastic　deformation　evidences　(100-400　m).　Nanocrystalline　can　also　be　found　even　though　very　short　peening　duration　was　done,　such　as,　2　min　and　30　min　peening　duration.　The　average　grain　size　in　the　surface　was　about　40　nm　and　30　nm　for　2　min　and　30　min　shot　peening,　respectively.　The　evolution　of　nanocrystalline　can　be　described　as　follows:　Dislocation　tangles　are　gradually　developed　into　dislocation　cells　by　dislocation　density　multiplication.　Dislocation　annihilation　and　rearrangement　in　the　austenite　coarse　grains　can　also　occur　in　the　same　time.　Mechanical　twin　is　induced　with　strain　and　strain　rate　increasing.　Single　system　deformation　twins　are　gradually　developed　to　multisystem　twins.　With　twins　increasing　and　twin　intersection　enhancing,　grains　are　further　refined.　Finally,　nano-scale　grains　forms　in　the　equiaxed　shape　and　random　crystallographic　orientations　which　is　dependent　upon　increasing　dislocation　and　twin　density.　Grain　sizes　are　continuously　decreased　with　increasing　the　shot　peening　duration.It　has　been　shown　that　Hadfield　steel　surface　can　be　obviously　strengthened　by　shot　peening.　The　hardness　is　increased　rapidly　with　the　depth　decreasing.　Surface　work　hardening　is　attributed　to　grains　refinement,　dislocation　and　twin.　It　does　not　result　from　phase　transformation.　Surface　hardness　is　also　increased　with　increasing　the　shot　peening　duration.In　the　present　thesis,　a　thermodynamic　model　was　proposed　based　on　Pringogine’s　nonequilibrium　thermodynamics.　Kirchhoff‘s　theory　and　Miedema’s　thermodynamic　theory　can　be　used　to　describe　that　strain　induced　the　formation　of　nanocrystalline　in　Hadfield　steel.　The　difference　of　Gibbs　free　energy　of　coarse　grain-nanocrystalline　and　coarse　grain-amorphous　transformation　was　calculated　by　this　model.　The　thermodynamical　calculation　results　suggested　that　Gibbs　free　energies　to　form　nanocrystalline　and　amorphous　are　positive,　which　means　that　there　was　not　energy　potential　for　strain　induced　nanocrystallization.　The　material　surface　gets　the　external　mechanical　energy　to　increase　the　internal　free　energy　of　the　system　during　shot　peening.　Nanocrystalline　and　amorphous　transformation　finally　can　occur　if　the　internal　energy　is　increased　to　responded　critical　values.Base　on　thermodynamics,　a　kinetic　model　was　proposed　based　on　the　relation　between　the　hardness　and　grain　size　in　subsurface.　The　transformed　volume　faction　of　kinetic　was　expressed　as　the　function　of　hardness.In　two-body　abrasive　wear,　after　surface　nanocrystallization　surface　hardness　was　increased.　When　hard　particles　were　used　as　abrasives,　such　as,　emery,　the　wear　resistance　can　be　not　improved.　However,　when　soft　particles　were　used,　such　as,　glass　particles,　wear　resistance　can　be　improved　for　2-30　min　shot　peening.　Especially,　the　wear　resistance　of　treated　surface　can　be　increased　by　70%　for　2　min　peening　duration.　Under　the　conditions　of　impact　wear,　harder　abrasive　particles,　such　as,　quartz　sands　were　used　using　0.5　J　impact　load,　wear　resistance　can　be　improved　to　some　extent　after　2-30　min　shot　peening.　On　the　contrary,　shot　peening　is　harmful　to　wear　resistance　of　Hadfield　steel　in　3.0　J.　The　longer　the　shot　peening　duration　is,　the　worse　the　wear　resistance　is.　In　loose　abrasive　wear,　the　wear　resistance　after　2-30　min　shot　peening　can　be　improved　to　some　extent　when　soft　abrasives　were　used,　such　as,　glass　particles.　In　the　similar　situation,　the　wear　resistance　of　treated　surface　can　be　increased　by　72%　for　30　min　peening　duration.　Therefore,　the　wear　resistance　of　Hadfield　steel　can　be　improved　in　light　impact　load,　soft　particles　abrasive　wear　after　suitable　shot　peening　duration.

Translated Keyword

[Hadfield SteelNanocrystallizationShot peeningThermodynamicsAbrasive wear]

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