In the infiltration casting process and heat treatment method, gray cast iron and niobium wires are compounded. The reinforcement phase NbC particles are synthesized in situ between niobium wires and graphite phases in the gray cast iron using heat treatment. The microstructure and micro-hardness of the compound region are observed and measured through X-ray diffraction, scanning electron microscopy, and micro-hardness testing. The results indicate that at 1172 A degrees C for 25 min, the in situ reaction area is diffusely extended eight times larger than the original diameter of 0.7 mm. The NbC particles synthesized in situ are tiny square- or rectangle-like particles that are evenly distributed in the gray cast iron. The highest micro-hardness value of the composite region is 1988 HV0.05, and the average micro-hardness value of the composite region after 20 min heat treatment is 953 HV0.05 or about five times larger than that of the matrix. After heat treatment at 1172 A degrees C for 25 min, the average micro-hardness value of the composite was about 458 HV0.05, and the average macro-hardness was 49.2 HRC.