Effect of chromium concentration on microstructure and properties of Fe-3.5B alloy

The cast low carbon Fe-3.5B alloys containing various chromium concentrations were prepared in a 10kg 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 Fe₂B, 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 α-Fe solid solution while the morphology of boride becomes dispersed due to the transformation of boride from simple Fe₂B to (Fe,Cr)₂B when the chromium concentration in Fe-3.5B alloy exceeds 8wt.%. Meanwhile, some primary M₂B-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 Fe₂B, which may be attributed to the change of spatial structure of boride.