Key Engineering Materials Vol. 658

Abstract: Nickel based super alloy MoNiCr was developed as the material with high corrosion and creep resistance against aggressive molten fluoride salt environment. This medium is used by Fluoride salt cooled High temperature Reactor (FHR) and Molten Salt Reactor (MSR) systems. The elements Mo, Cr and Fe are most important alloying elements accompanied by very low amount of Al + Ti. Necessity to prepare hot formed semi products such as sheets, bars, wires and tubes was the motivation for this new experimental program execution searching for suitable hot forming conditions. In the case of MoNiCr components forming, recrystallization is a crucial process allowing successful forming. Special attention was paid to this process after finding out, that this process is running very slowly or even not running at all, if inappropriate conditions are met. Proper temperature and strain rate range were determined and furthermore the influence of cold deformation on recrystallization progress was determined. It was found that cold deformation before hot forming as well as after hot forming can be successfully implemented into technological chain. Almost fully recrystallized microstructure can be achieved using synergy of both cold and hot deformation.

Abstract: The present research work has an aim to modify microstructure and oxidation behavior of Hastelloy X, a solid solution nickel base alloy, by both aluminium and titanium additions by mean of arc melting process. The Hastelloy X was added both Al and Ti (50:50) for 2% 4% and 6% by weight and casted by vacuum arc melting furnace. Then all received specimens were performed heat treatment, which consist of solutioning treatment at 1175°C for 4 hours and aging temperatures for 760°C, 800°C and 845°C for 24 hours. From the obtained results, it was found that the amount of both Al and Ti additions as well as precipitation aging temperature provided significant effect on both final microstructure and oxidation behaviors at 900°C and 1000°C. Widmanstatten type of microstructure was found in many case. Intermetallic phase formation of molybdenum and chromium was also found in all cases by element mapping. This phase should be γ’-phase. Both aluminium and titanium additions could not provide beneficial effect on oxidation resistance tests at temperature of 900°C and 1000°C. However, with 4%wt. of both aluminium and titanium addition, it resulted in slightly increasing of oxidation resistance at temperature of 1000°C

Abstract: Nickel-base superalloys have been used as high temperature materials in land-base gas turbine application. When subjected to long term, high temperature service, large crack propagation was observed. Typical refurbishment method of these turbines is carried out by using TIG welding followed by post-weld standard heat treatment. However, new crack initiation is found in the heat-affected zone after TIG welding. Pre-weld heat treatment has been discovered to improves final γ + γ’ microstructure. This study focuses on the effect of pre-weld heat treatment temperature on final γ + γ’ microstructure. Seven different conditions of pre-weld heat treatment temperature were investigated. Scanning electron microscopy studies were carried out after pre-weld and post-weld heat treatments to compare the γ + γ’ microstructure and capture microcracks. The best pre-weld heat treatment temperature produces uniform distribution of finely dispersed γ’ precipitates in the γ matrix without post-weld crack.

Abstract: The objectives of this research are to search for the most appropriate heat treatment condition for rejuvenating microstructure of cast nickel base superalloy, grade IN-738, turbine blades after using to prolong its life time service again. The turbine blades that had been used for long term service under load and high temperatures resulted in small gamma prime particles connecting to each other and thus forming into larger particles. This effect generally reduces creep resistance and increases the failure. In this research, 5 IN-738 superalloy samples were reheat-treated under simulation of 2 working conditions. First, they were heated at 900oC. At every 400 hours from the beginning of heat treatment until time reaching 1600 hours, these samples were collected and examined the microstructures, size and area fraction of gamma prime particles. Another heating program, they received an over thermal exposure heating at 1125oC for 1 hour after long-term heated at 900oC after every 400 hours-heating. Then the results were analyzed from working conditions. It was found that the sample passed solutioning at 1125oC for 4 hours and aging at 845oC for 24 hours with the over thermal exposure showed the most stable phase stability with γ’ phase increasing after long-term simulated working conditions .

Abstract: In this research, 5 superalloys Udimet-500 samples were reheat-treated under simulation of 2 working conditions. First, they were heated at 900oC for 400 hours of each heating cycle from the beginning until accumulated heating time reached 1600 hours, these samples were collected and examined the microstructures for size and shape of gamma prime particles. Another heating condition, they had an additional over thermal exposure heating at 1125oC for 1 hour after heating at 900oC of each 400 hours-heating cycle. Then the results were analyzed from 2 working conditions. It was found that the area fraction, average area of gamma prime particle values of nickel base superalloy turbine blade, grade Udimet-500 happened in the same directions in first heating program. However, with the over thermal exposure heating at 1125oC for 1 hour after every heating at 900oC for 400 hours-heating, the obtained results show rates of the area fraction, average area of gamma prime particle values had changed more slowly because the temperature 1125oC at the end of each 900C heating could solutioning some coarse gamma prime phase back to the matrix in some degree. Therefore, the rate of ripening of gamma particle size was slower due to that the microstructural rejuvenation was always performed after each heating cycle and also allowed the reprecipitation of uniform finer gamma prime particles providing longer service time than those of first heating condition.

Abstract: Cobalt based alloys have been widely used in orthopedic implants. These alloys are an allotropic metal, which commonly exhibits two crystal structures, namely, FCC and HCP lattice. In this work, developed microstructure and hardness of a Co-Cr-Mo alloy after isothermal aging treatment were investigated. The applied aging procedure included soaking at the temperature of 850°C for five different holding times of 1, 3, 6, 9 and 12 h with subsequent water quenching. Microstructure examination, X-ray diffraction analysis and micro-hardness test were carried out for both as-received and heat-treated cobalt based alloys. The results showed that the FCC to HCP phase transformation occurred during the isothermal aging. It was observed that phase fraction of the identified HCP phase increased with longer aging time. Microstructure of the samples aged for 12 h showed very fine lamellae morphologies similar to a pearlitic structure with different orientations within each FCC grain. Apparently, these occurred lamellae structures could be well correlated with the formation of the HCP martensite. Additionally, it was found that in the Co-Cr-Mo alloy sigma phase precipitated early at the grain boundaries and further grew along these boundaries by increasing aging time. The hardness value of the examined alloy slightly increased with larger HCP phase fraction. The increased aging time certainly led to higher amount of the HCP martensite and consequently increased hardness and possible wear resistance properties.

Abstract: In-Situ direct observation was used to observe the liquation in an Inconel 617. High-Temperature Confocal Laser Scanning Microscopy (HT-CLSM) was used to observe the phenomena. Experiment was perform at 1330 °C with heating rate 5 °C/s. Results showed that the liquidation started at 1319.2 °C (268.08 seconds) in which liquid formed around precipitate. At 1330.5 °C (272.05 seconds) the liquid film wet and penetrated along grain boundaries. Then after 277.05 seconds, sufficient liquid film formed and moved freely on the surface of the specimen across grain boundaries. After cooling to room temperature, the test specimen was analyzed for local chemical composition gradient of precipitates and liquid film. Precipitate morphology changed from polygonal shape to round shape using SEM and EPMA. The results from this observation could be used to explain liquation mechanism occurred in Inconel 617 during heating.

Abstract: Accelerated atmospheric corrosion behavior of carbon steel pipes subjected to cyclic salt spray test was performed according to ISO 14993 – Corrosion of metals and alloys – Accelerated testing involving cyclic exposure to salt mist, “dry” and “wet” conditions [1]. In order to investigate the effect of degree of exposure to environment of inner surface of the pipe on corrosion behavior of inner surface of the specimens, degree of completeness of weldment, 0%, 50%, 80%, and 100%, of steel cover plate is varied. Exposure times in this study are 168, 336, and 504 hours which can be correlated to 10, 20, and 30 years exposed to C3 corrosivity category according to the definition of ISO 9223 – Corrosion of metals and alloys – Corrosivity of atmospheres – Classification, determination and estimation [2,3]. After testing, visual inspection on outer and inner surfaces of the specimens were performed. In addition, average corrosion rate of the specimens were performed according to ISO 8407 – Corrosion of metals and alloys – Removal of corrosion products from corrosion test specimens [4]. The results show that as-received steel pipe specimens exhibited great degree of corrosion attack on the inner surface of the specimens while slight amount of corrosion was observed from the welded pipe specimens.

Abstract: In this work, 3D ductile fracture locus was determined for the advanced high strength (AHS) steel sheet grade DP780 using a hybrid approach between experiment and FE simulation. Tensile tests of different sample geometries were performed for the investigated dual phase steel, by which varying stress triaxiality (η) and lode angle (θ) values developed in the material during loading were introduced. During the tests, the direct current potential drop (DCPD) method and digital image correlation (DIC) technique were applied for identifying crack initiation on the micro-scale and fracture of the specimens due to local plastic deformation. Obtained force and displacement curves were correlated with the electric potential curves. Then, the moments of crack onset were determined for various states of stress. In parallel, the most critical areas of deformed samples before fracture were observed by the DIC method. Subsequently, FE simulations of the tensile tests were carried out and calculated local stresses and strains were gathered. The stress triaxialities, equivalent plastic strains and lode angles were evaluated for the corresponding detected areas. These threshold variables obtained from different specimens were plotted as the 3D failure locus for defining crack initiation and fracture occurrence in the DP steel.