Mathematical forecasting composition of secondary carbides in the single-crystal superalloys

Purpose: Predicting the specifics of the distribution of alloying elements between secondary carbides, their topology, and morphology, as well as the composition for a single-crystal multicomponent system of the type Ni-11.5Cr-5Co-3.6Al-4.5Ti-7W-0.8Mo-0.06C using the calculated CALPHAD (passive experiment) versus scanning electron microscopy (active experiment). Design/methodology/approach: This work presents the results of studies of the distribution of chemical elements in the composition of carbides, depending on their content in the system. The studies were carried out using an electron microscope with computer analysis of images and chemical composition. Findings: It was found that the influence of alloying elements on the composition of carbides is complex and is described by complex dependencies that correlate well with the obtained experimental results. Research limitations/implications: An essential problem is the prediction of the structure and properties of superalloys without or with a minimum number of experiments. Practical implications: The obtained dependencies can be used both for designing new superalloys and for improving the compositions of industrial alloys. Originality/value: The value of this work is that the obtained dependences of the influence of alloying elements on the dissolution (precipitation) temperatures and the distribution of elements in secondary carbides in the superalloy of the Ni-11.5Cr-5Co-3.6Al-4.5Ti-7W-0.8Mo-0.06C system. It was found that changes in the course of the curves of temperature dependence on the element content closely correlate with thermodynamic processes occurring in the system, that is, the curves exhibit extrema accompanying the change in the stoichiometry of carbides or the precipitation of new phases.

Weldability and Damage Evaluations of Fresh-to-Aged Reformer Furnace Tubes

The microstructures and tensile properties of fresh and aged reformer furnace tubes and a fresh-to-aged welded joint were investigated to assess the weldability of fresh-to-aged reformer furnace tubes. Damage evaluation of the fresh-to-aged welded joint was also carried out using the modified Kachanov–Rabotnov model. The experimental results showed that M7C3 carbide transforms into M23C6 carbide and secondary carbides precipitate in the matrix after aging treatment. With continuous exposure, the interdendritic precipitates coalesced and coarsened and the number of secondary carbides reduced gradually. Microdefects were absent in the fresh-to-aged welded joint, and the tensile properties of the welded joint were close to the as-cast alloy, which confirms the weldability of fresh-to-aged furnace tubes. According to the results of the simulation, stress redistribution occurred during the creep process and the peak damage of the welded joint was located in the aged tube. The maximum damage of the fresh-to-aged welded joint reached 34.01% at 1.5 × 105 h.

Characteristics of the Evolution of Carbide Morphology in the Haynes® 230® Alloy as a Result of High Temperature Annealing

In this work, results of an investigation of the microstructure evolution in Haynes® 230® alloy are presented. The morphological and chemical compositions of the chosen microstructure’s constituents, such as the primary and secondary carbides, were analyzed based on tests in the temperature range 700–800 °C for 1000–3000 h. The prediction of phase evolution within the microstructure was proposed based on the analysis of mutual replacement of carbide-forming elements at the carbide/matrix interface. Based on the results, some complementary markers were considered to describe Haynes® 230® microstructure evolution. Qualitative markers, i.e., defined morphological features, were related to the shape and distribution of microstructure constituents. The study also used quantitative markers related to the local chemical compositions of carbide particles, determined as the ratio of the concentrations of carbide-forming elements Crc/Wc, Crc/CrM and Wc/WM. Microstructure maps created on the basis of these complementary markers for the successive annealing stages reflected the course of its morphological evolution.

Cyclic annealing versus continuous annealing of 20 wt.% chromium white cast iron

To destabilize as-cast microstructure of 20 wt.% chromium white iron, cyclic annealing involving repeated austenitization for short duration of 0.6 h at 900, 950, 1000, 1050 and 1100 °C followed by forced air cooling is conducted as an alternative to continuous annealing requiring austenitization for longer period of 4–6 h at the said temperatures followed by furnace cooling. Continuous austenitization destabilizes the austenite matrix through precipitation of secondary carbides and transforms the alloy depleted austenite to pearlite on furnace cooling, thereby reducing the as-cast hardness from HV 669 to HV466. In contrast, repeated austenitization not only destabilizes the austenite matrix through precipitation of secondary carbides followed by its transformation to martensite on forced air cooling, but also causes disintegration of longer eutectic carbides to shorter segments with subsequent increase in hardness to as high as HV 890. Notched impact toughness after both continuous and cyclic annealing remains uniformly at 12.0 J as compared to as-cast value of 6.0 J. Besides, an unexpected rise in abrasive wear resistance after cyclic annealing treatment makes the alloy superior than that obtained by continuous annealing treatment as practiced in industries.