The　cast　low　carbon　Fe-3.5B　alloys　containing　various　chromium　concentrations　were　prepared　in　a　10　kg　medium　frequency　induction　furnace　and　the　effects　of　chromium　concentration　on　microstructure　and　properties　of　Fe-3.5B　alloys　have　been　examined　by　means　of　optical　microscope　(OM),　scanning　electron　microscope　(SEM),　back-scattered　electron　microscope　(BSE),　electron　probe　microanalyzer　(EPMA),　energy　dispersive　spectrum　(EDS),　X-ray　diffraction　(XRD),　transmission　electron　microscopy　(TEM)　and　Vickers　hardness.　As　a　result,　the　as-cast　structures　of　Fe-3.5B-XCr　(X　=　0,　2,　5,　8,　12,　18,　mass　fraction)　alloys　are　mainly　composed　of　dendrite　ferrite.　martensite,　pearlite　and　boride.　The　boride　in　the　alloy　without　chromium　addition　comprises　the　eutectic　Fe(2)B,　which　is　continuous　netlike　or　fish-bone　structure　distributed　over　the　metallic　matrix.　With　the　increase　of　chromium　concentration　in　Fe-3.5B　alloy,　matrix　structure　turns　into　the　supersaturated　alpha-Fe　solid　solution　while　the　morphology　of　boride　becomes　dispersed　due　to　the　transformation　of　boride　from　simple　Fe(2)B　to　(Fe,Cr)(2)B　when　the　chromium　concentration　in　Fe-3.5B　alloy　exceeds　8　wt.%.　Meanwhile,　some　primary　M(2)　B-type　borides　may　precipitate　under　this　condition.　The　bulk　hardness　of　the　as-cast　alloy　ranges　from　41.8　to　46.8　HRC.　However,　the　bulk　hardness　of　the　heat　treated　alloy　rises　first　and　falls　later　mainly　because　of　the　morphology　variation　of　structure.　Fracture　toughness　of　boride　is　improved　gradually　owing　to　the　entrance　of　chromium　into　Fe(2)B,　which　may　be　attributed　to　the　change　of　spatial　structure　of　boride.　(C)　2010　Elsevier　B.V.　All　rights　reserved.