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

▶ With the increasing chromium additions, the boride changes from Fe 2B to (Fe,Cr) 2B-type boride. ▶ The matrix of Fe–3.5B alloy transforms to supersaturated α-(Fe,Cr) solid solution when high chromium concentration is added. ▶ The fracture toughness of boride increases with the increase of chromium addition. ▶ Secondary phase precipitates during the heat treatment of Fe–3.5B alloy with various chromium concentrations. 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 2B, 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 Fe 2B to (Fe,Cr) 2B when the chromium concentration in Fe–3.5B alloy exceeds 8 wt.%. Meanwhile, some primary M 2B-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 2B, which may be attributed to the change of spatial structure of boride.