A Study Of The Processing And Mechanisms Of Grain Refinement By The Addition Of Refiners For Superalloy K4169

The main disadvantages of conventional investment cast superalloys are micro- structural coarseness and non-uniformity of grain structures, resulting in lower yield strength and shorter low-cycle fatigue life of castings. Since 1 980s, the integral fine- grained cast techniques have been extensively progressed which leads to improved uniformity in properties and precipitate morphology. One of the effective methods to obtain fine-grained products of superalloys is based on the control of foundry variables, whereas micro-porosities are typically present in interdendritic areas due to lower melt pouring temperature. Therefore, Hot Isostatic Processing (HIP) is required, which increases the production cost. In addition, agitation of the melt during solidification through which grains can be refined, will cause the similar structural problem. Moreover, a specially designed casting furnace and more complicated post-treatment for the castings are needed, resulting in higher production cost. Therefore, an alternative method of adding refiner to the melt, namely chemical method is considered. Whereas, the selection of refiner is still based on a trial and error method. In this thesis, the effects of the addition of refiners on grain size and microstructures, together with the grain refinement mechanisms, and the mechanisms of transition from dendrite to granulation after grain refinement are investigated. The main conclusions can be drawn as follows: (1) Suitable refiners for superalloy K4 169 were selected and prepared, and they are two bi-component and two tn-component intermetallic compounds. Trace addition of them to the melt did not bring in inclusion and change the phase constitution of the alloy. (2) By the addition of mixed refiner and at a conventional pouring temperature of 1 4000C, the average equiaxed grains (r) could be refined to the order of ASTM 1.7, the proportion of equiaxed grains at traverse cross-section (p) could be improved from 54% to 96% with the melt homogeneous treatment. Furthermore, r could be refined to 0.1158mm, i.e. to ASTM 3.2, and p has been increased to 99% without the melt homogeneous treatment. While at a pouring temperature of 1 4200C and with the melt homogeneous treatment, r could be refined to the order of ASTM M10.5, and p has been increased from 53% to 99%. (3) The remarkably refined turbine blade having uniform structures and no casting defects after the melt superheating treatment, was obtained by the addition of mixed refiner while at a pouring temperature of 14000C and with the melt superheating treatment. III ABSTRACT (4) The addition of single or mixed refiners could lead to the reduction in grain size. The grain refinement mechanisms are that the particles of refiner can act as substrata of the y matrix so as to reduce nucleation work and promote nucleation. In addition, the refining capacity of the mixed refiner is superior to the single one, which could be attributed to the enlarged effective nucleation temperature range, owing to the result of cooperation of two refiners. (5) Effects of processing parameters on grain structures were studied. The grain size in castings with melt homogeneous treatment is coarser than that in castings without such treatment, ascribed to the effectiveness of MC carbides and other refractory particles in promoting nucleation to be weakened. Low pouring temperature can...