Optimization of mechanical alloying and spark-plasma sintering regimes to obtain ferrite–martensitic ODS steel

The results of structure investigation, distribution uniformity of dispersed particles of Y2O3, porosity and density of the ferritic/martensitic reactor steel EP-450 (0.12C–13Cr–2Mo–Nb–V–B, wt%) produced by spark-plasma sintering (SPS) are presented. More than 140 samples were produced using different combinations of mechanical alloying (time, speed of attritor rotation) and SPS parameters (temperature, speed of reaching preset temperature, pressure and time of exposure under pressure, concentration of strengthening particles). It is determined that the absence of strengthening Y2O3 nano-particles in local volumes of sintered specimens is connected with the imperfection of mechanical alloying, namely, the formation of agglomerates of matrix steel powder containing no oxide nano-particles. It has been determined that the time of mechanical alloying should not exceed 30 h to provide minimum powder agglomeration, uniform distribution of Y2O3 particles in the powder mixture and minimum porosity of sintered samples. Spark-plasma sintering should be performed at the lowest possible temperature. As a result it was found that samples with 99% theoretical density can be obtained using the following optimized SPS-parameters: sintering temperature is 1098 ÷ 1163 K; speed for reaching the preset temperature is > 573 K/min; load is 70 ÷ 80 MPa; time of exposure under pressure – either without isothermal exposure, or exposure during ≥ 3 min; optimum quantity of Y2O3 is 0.2 ÷ 0.5 wt%.