Additive manufacturing technologies, also known as 3D printing, have demonstrated the potential to fabricate complex geometrical components, but the resulting microstructures and mechanical properties of these materials are not well understood due to unique and complex thermal cycles observed during processing. The electron beam melting (EBM) process is unique because the powder bed temperature can be elevated and maintained at temperatures over 1000°C for the duration of the process. This results in three specific stages of microstructural phase evolution: (a) rapid cool down from the melting temperature to the process temperature, (b) extended hold at the process temperature, and (c) slow cool down to the room temperature. In this work, the mechanisms for reported microstructural differences in EBM are rationalized for Inconel 718 based on measured thermal cycles, preliminary thermal modeling, and computational thermodynamics models. The relationship between processing parameters, solidification microstructure, interdendritic segregation, and phase precipitation (d, c9, and c0) are discussed.
Flash processing of an AISI8620 steel sheet, which involves rapid heating and cooling with an overall process duration of <10 s, produced a steel microstructure with a high tensile strength and good ductility similar to that of advanced high strength steels. Flash processed steel [ultimate tensile strength (UTS): 1694 MPa, elongation: 7·1], showed at least 7 higher UTS and 30 greater elongation than published results on martensitic advanced high strength steel (UTS: 1585 MPa, elongation: 5·1). The underlying microstructure was characterised with optical, scanning electron, transmission electron microscopy as well as hardness mapping. A complex distribution of bainitic and martensite microstructures with carbides was observed. A mechanism for the above microstructure evolution is proposed.
In recent years, several incidents of cracking and failures have been observed in Stellite (Stellite is a registered trademark of the Deloro-Stellite Corporation) hardfacing used in valves of modern high temperature combined cycle gas fired power plants. These hardfacing layers are applied as an overlay onto a steel substrate, such as CrMo steel (i.e. Grade 22, WC9) or creep strength enhanced ferritic steel (i.e. Grade 91, C12A). Cracking has been observed in valve components at the Stellite/steel interface and in the weld dilution zone formed between the steel and clad. Ultimately, disbonding or delamination of the weld hardfacing from the valve body occurs and has resulted in collateral damage to components in the plant (such as to the turbine) or valve failure. In this study, the microstructure formed near the Stellite/steel interface is investigated. Based on thermodynamic modelling, microstructure formed at these regions is hypothesised and a simple methodology is proposed to predict the occurrence of these failures.
API Specification 5CT [1] and NACE MR0175 [2] and TM0177 [3] provide guidelines for the use of carbon steels with desired characteristics in oxygen-free environment containing H2S and CO2. TMK IPSCO has developed a range of suitable grades to meet such service requirements; this paper describes in detail the necessary processing tools that have been developed and implemented to achieve the desired pipe properties and consequently ensure their performance in the well. It is well known that low alloy carbon steels of higher strengths are susceptible to SSC (Sulphide Stress Cracking) in such sour service conditions. As the strength (and hardness) of steel increases, controlling impurities within the steel matrix becomes critical in the development of pipes for such applications. Development of technologies to reduce the inclusion content and ensure "clean" steel for the production of these grades at our steel production facility in Koppel is presented. Additionally, the use of rigorous heat treatment practices aimed at controlling the micro-structure and mechanical properties of the steel, which are achieved by monitoring and controlling critical parameters such as the furnace temperatures, are discussed. To meet the stringent quality requirements for these grades, improve process efficiency and reduce operational error, the use of automatic hardness testing machines have been implemented to batch process the hardness tests on such grades. Furthermore, a novel use of automated microhardness machine to predict the disposition of test samples to failure during SSC testing is presented. Investigations of steel specimens have shown that application of "clean" steel practices, like the implementation of TruStir® technology has resulted in a marked and consistent enhancement in the cleanliness of such sour service steel grades. Heat treatment studies point to enhancements in hardness control and distribution resulting from heat treatment practices involving double quench and tempering. Such heat treatment routes are believed to result in a better control of prior austenite grain size and prevent formation of localized "hard" spots in the material, which could become sites susceptible to SSC. Recent developments of the control of furnace operating parameters at our Baytown Heat Treating facility and resulting improvements in the performance of our sour service grades are discussed. Production of these sour service grades require stringent quality checks as mandated by API specification 5CT and ISO specification 15156. Apart from extensive hardness testing, these grades require prolonged corrosion testing. Aside from standardized testing methods for these grade, our internal microhardness testing of these grades have revealed a relation between the frequency of microhardness distribution and the corrosion performance of these grades. Based on these results, a quick methodology of predicting the corrosion performance of a material is being explored.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.