Friction stir welding was carried out under different heat input and cooling rates to produce lap joints between high strength martensitic steel sheets. The microstructure of the welds was characterized, and microhardness was evaluated. Joint efficiency was determined by lap shear test. Variation in processing conditions governed total heat input, peak temperature and cooling rate during friction stir welding. Weld nugget microstructure depended principally on cooling rate. The slowest cooling rate promoted ferrite–pearlite and the fastest cooling rate resulted in martensite formation in the weld nugget. The weakest region of all the joints was the heat affected zone, which consists of ferrite with small quantities of pearlite. Fracture during shear testing occurred along the heat affected zone of welded joints. The width and grain size of ferrite in heat affected zone controlled the joint efficiency.

Friction stir lap welded advanced high strength steels: Microstructure and mechanical properties

Effect of gradient temperature rolling (GTR) and cooling on microstructure and properties of E40-grade heavy plate

A comparative study has been carried out to assess the effect of rolling and annealing processes on ridging generation of ferritic stainless steels (FSSs) 430 and 430LR after tensile deformation. The results show that FSS 430LR has better ridging resistance owing to the refinement of microstructure and crystallographic texture optimisation. A 30% reduction of ridging height can be achieved using FSS 430LR compared to FSS 430 after tension. Optimal reduction during cold rolling benefits the microstructural refinement and surface quality improvement of FSSs 430 and 430LR. Both theoretical calculations and experimental results indicate that the {1 1 2} and the {0 0 1} components are responsible for ridging in the FSSs after tension. Through preventing the formation of coarse bands and grains with the {0 0 1} component inside the FSSs, the ridging resistance of both FSSs 430 and 430LR can be improved.

https://ro.uow.edu.au/cgi/viewcontent.cgi?article=3895&context=eispapers1

Quantification of texture-induced ridging in ferritic stainless steels 430 and 430LR during tensile deformation

Weld metal mechanical properties and weldability of materials are closely related to the microstructure of the weld metal. A significant amount of research has been studied to improve microstructure of weldments such as weld pool stirring by using magnetic arc oscillation and arc pulsation. In this work, the effect of ultrasonic vibration was used to modify weld metal solidification to improve microstructure of the weld metal. Microstructure and mechanical properties of carbon steel weld metal (ER70S-G filler metal) were studied. Filler metal was melted by using Gas Tungsten Arc Welding (GTAW) in a water-cooled copper mold. Ultrasonic vibration with a frequency of 20 kHz was applied during solidification of the weld metal. Microstructure and mechanical properties of weld metal were compared with those of conventional weld metal (no ultrasonic vibration assistance). Scanning Electron Microscopy (SEM) was also used to determine microstructure and phases at high magnification. The results showed that ultrasonic vibration applied during solidification promoted grain refinement in the weld metal. Mechanical properties of weld metal were improved significantly, microstructure analysis correlated well with the mechanical test results.

The Effect of Ultrasonic Vibration on Properties of Weld Metal

In this work, Yb4Hf3O12 ceramic, a typical rare-earth hafnate, was exposed to water vapor with Al(OH)3 impurities at 1400 ℃, and microstructural evolution and corrosion mechanisms were investigated. The long-term corrosion can be divided into initial hydration and gasification of Hf4+ and steady-state corrosion after the formation of Al5Yb3O12. In the initial stage, Hf4+ and the surrounding O2- tend to be hydrated and then volatilized in the form of Hf(OH)4, causing a metastable state of Yb4Hf3O12 on the surface. Besides, the preferential corrosion of small grains, grain boundary, and specific crystal plane on the surface and sintered and densified grains result in varied corrosion behavior from the early linear law to the later parabolic law. In the steady-state corrosion stage, due to the consumption of Hf4+, more vacancies promote bonding and diffusion of Al3+ to form corrosion product (Al5Yb3O12), and during this stage, regrowth and shedding of the corrosion layer reach a dynamic balance. 

Corrosion behavior of ytterbium hafnate exposed to water-vapor with Al(OH)3 impurities

Abstract Thermal barrier coatings (TBCs) can effectively protect the alloy substrate of hot components in aeroengines or land-based gas turbines by the thermal insulation and corrosion/erosion resistance of the ceramic top coat. However, the continuous pursuit of a higher operating temperature leads to degradation, delamination, and premature failure of the top coat. Both new ceramic materials and new coating structures must be developed to meet the demand for future advanced TBC systems. In this paper, the latest progress of some new ceramic materials is first reviewed. Then, a comprehensive spalling mechanism of the ceramic top coat is summarized to understand the dependence of lifetime on various factors such as oxidation scale growth, ceramic sintering, erosion, and calcium-magnesium-aluminium-silicate (CMAS) molten salt corrosion. Finally, new structural design methods for high-performance TBCs are discussed from the perspectives of lamellar, columnar, and nanostructure inclusions. The latest developments of ceramic top coat will be presented in terms of material selection, structural design, and failure mechanism, and the comprehensive guidance will be provided for the development of next-generation advanced TBCs with higher temperature resistance, better thermal insulation, and longer lifetime. 

Progress in ceramic materials and structure design toward advanced thermal barrier coatings

Considering the specialities of the steel plate production, the TMCP study has been carried out with Gleeble 2000 tester to explore the possibility of fine grained ferrite in the low carbon steel plates with the chemical composition of C 0. 13–0. 18, Si 0. 12–0. 18, Mn 0. 50–0. 65, P 0. 010–0. 025, and S 0. 005–0. 028. The plates with thickness of 8. 7 mm in which the ferrite grain size is smaller than 8 μm have been produced by special deformation process in the laboratory. Furthermore, the trial production of special plain carbon steel plates of 16–25 mm in thickness and 2 000–2 800 mm in width with fine grained ferrite has been successfully carried out in the Shougang Steel Plate Rolling Plant. The ferrite grain size is 5. 5–7 μm in the surface layers and 9. 5–15 μm in the central layer respectively. The yield strength is 320–360 MPa, tensile strength is 440–520 MPa and the elongation is 25%–34%. It is very important for the rolling plants to improve the low carbon steel plates' mechanical properties. The results show that the ferrite grains in the surface layer can be refined effectively by the appropriate rolling process, and the strength can be also increased.

Wei-xu Zhang

Papers

Progress in ceramic materials and structure design toward advanced thermal barrier coatings

Surface Ferrite Grain Refinement and Mechanical Properties of Low Carbon Steel Plates