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.

Research on Carbide Characteristics and Their Influence on the Properties of Welding Joints for 2.25Cr1Mo0.25V Steel

The carbide characteristics of 2.25Cr1Mo0.25V steel have an extremely important influence on the mechanical properties of welding joints. In addition, hydrogen resistance behavior is crucial for steel applied in hydrogenation reactors. The carbide morphology was observed by scanning electron microscopy (SEM) and the carbide microstructure was characterized by transmission electron microscopy (TEM). Tensile and impact tests were carried out and the influence of carbides on properties was studied. A hydrogen diffusion test was carried out, and the hydrogen brittleness resistance of welding metal and base metal was studied by tensile testing of hydrogenated samples to evaluate the influence of hydrogen on the mechanical properties. The research results show that the strength of the welding metal was slightly higher and the Charpy impact value was significantly lower compared to the base metal. The hydrogen embrittlement resistance of the welding metal was stronger than that of the base metal. The presence of more carbides and inclusions was the main cause of the decreased impact property and hydrogen brittleness resistance of the welding metal. These conclusions have certain reference value for designing and manufacturing hydrogenation reactors.

Influence of Carbide Morphology on the Deformation and Fracture Mechanisms of Spheroidized 14CrMoR Steel

The influence of carbide morphology on the deformation and fracture mechanisms of as-received and complete spheroidization 14Cr1MoR steel was investigated using an in situ scanning electron microscope (SEM) under tension testing. During spheroidization damage, the carbide morphology changed from the original lamellar cementite present in pearlite to granular M23C6 carbide, which was concentrated along the ferrite grain boundaries. The yield strength and tensile strength of 14Cr1MoR steel decreased with the increasing degree of spheroidization damage. In situ SEM observations revealed that the deformation and crack initiation started from the ferrite matrix in both as-received and completely spheroidization-damaged 14Cr1MoR steel samples. However, the extension of slip bands and crack propagation behavior of both samples were different during the in situ tensile process, which could be ascribed to the difference in carbide morphology. In the as-received 14Cr1MoR steel sample, hard and brittle lamellar pearlite resulted in high-strength ferrite/ pearlite boundaries, which inhibited the movement of slip bands. With further deformation, the concentration of stress at the crack tip resulted in the emergence and propagation of cracks along the ferrite/pearlite boundaries. In the case of the completely spheroidized 14Cr1MoR steel sample, slip bands bypassed the grain boundary carbide and continuously expanded into the neighboring ferrite grain. In addition, micro-voids and fractures of grain boundary carbides were observed due to the large stress concentration at the front of crack tip. Then, the micro-voids connected with the main crack to complete the crack propagation behavior. The morphological changes of carbides deteriorated the mechanical properties and altered the fracture behavior of 14Cr1MoR steel. It is worth noting that the fracture surface morphology of 14Cr1MoR steel changed from a combination of lamellar fracture and dimpled morphology to a completely dimples-dominated morphology after spheroidization.

Effect of Grain Boundary Formation in Microstructural Changes in IN 738 Service Exposed Turbine Blade

Microstructural changes on a Nickel based superalloy service exposed gas turbine blade after 52,000 hours at 720°C temperature was investigated. The study examines the grain boundary growth sizes; the changes with respect to gamma prime (γi) precipitate and phase distributions that occurred during operation. In order to observe the changes some different portions of the turbine blade undergone scanning electron microscope (SEM) to determine the grain size measurement and the EDX was used to evaluate the elemental analysis. The grain growths were discrete in formation along the grain boundary (GB) and little difference in the size of (γi) precipitates existed in different locations. The gradual transition of grain boundary-carbide morphology from the original fine discrete to fine semi-continuous, continuous film-like and finally coarse is observed in the specimens. The changes mostly evolves around two distinct phases which is from the solid to the equilibrium (sigma coarse phase) and then back to the solid (hardening gamma prime precipitate) phase. The grain size of (γi) precipitate varies throughout the surface of the turbine blade and there is a significant difference between the leading and trailing edge. The microstructural changes on the serviced blade can be interpreted to indicate that no obvious microstructural degradation and phase deformation occurred during exposure time.

Pearlitic Lamellae Spheroidisation During Austenitization and Subsequent Temperature Hold

Typical processing routes for bearing steels include a soft annealing stage. The purpose of this procedure is to obtain a microstructure containing globular carbides in ferritic matrix. A newly developed process called ASR (Accelerated Spheroidisation and Refinement) cuts the carbide spheroidisation times several fold, producing considerably finer globular carbides than conventional soft annealing. Finer microstructure also leads to more homogeneous and finer structure after final hardening process. The present paper explores process of the accelerated spheroidisation (ASR) in steel 100CrMnSi6-4 with initial pearlitic structure. Cementite lamellae morphology was observed in different stages of austenitization. The heat treatment was performed using induction heating in quenching dilatometer. There was analysed influence of austenitization temperature and austenitization time on spheroidisation. Hardness and carbide morphology was observed. Deep etching was used to reveal evolution of cementitic lamellae fragmentation. It is favourable process especially in induction treatment of small parts.