Mixed Microstructure Of Bainite Research Articles

Influence of different deposition strategies on the microstructure and mechanical properties of the laminated heterostructured material with ER130S-G HSS and 316 L SS fabricated by WAAM

Two groups of laminated heterostructured materials (LHM) thin walls with different deposition strategies and the corresponding single-material thin walls were successfully fabricated by wire and arc additive manufacturing (WAAM) technology. The deposition strategy for the LHM-1 thin walls was to change the deposited material every layer, and for the LHM-2 thin walls was to change the deposited material every two layers. The results indicate that changes in the deposition strategy could affect the volume fractions and distribution locations of different phases in LHM samples, thereby influencing the mechanical properties. Due to the dilution effect, when the two materials in the LHM samples were deposited on each other, the chemical composition of the two regions deviated. Two sets of LHM samples were prepared using the same volume fraction of 316 L stainless steel (SS) and ER130S-G high-strength steel (HSS), and the LHM-1 samples showed higher tensile strength and lower elongation. In HSS areas of the LHM-1 sample, every layer consisted of the martensite microstructure. In the HSS areas of the LHM-2 sample, one layer was composed of the martensite microstructure, while the adjacent layer was a mixed microstructure of bainite and ferrite. For LHM samples, the softer regions experiencing greater deformation during the tensile process. The lower Schmid factor of the FCC phase in the LHM-2 sample compared to the LHM-1 sample, makes it easier for the former to yield, thereby resulting in superior ductility of the LHM-2 sample. The results of the investigate reveal that the possibility of controlling the microstructure and properties by altering deposition strategies in WAAM technology.

Influence of microstructure on the fracture toughness of hot stamped boron steel

The automotive industry's desire for weight reduction while maintaining crashworthiness demands development of materials and material properties within the economic framework of consumers. The industrial process of hot stamping provides a technique to utilize steel in an efficient way. In hot stamping, microstructural characteristics of a steel blank are influenced by controlling the cooling rate. Hot stamping has become a prevalent method for lightweight solutions in car bodies without sacrificing passenger safety. The process of hot stamping applies sequential forming and quenching in a single production step. During the cooling of the blank, various microstructures can be formed depending on the cooling rate or holding temperature. Special tooling allows the application of different cooling rates within the same blank. Thus, the microstructure and mechanical properties can be influenced in designated areas of a blank.Fracture toughness properties of sheet metal are necessary to better understand fracture initiation and crack propagation during crash loading as well as improve crashworthiness predictions. This paper focus on fracture toughness of low-alloyed boron steel sheet common in the automotive industry. A heat treatment process is used to form different microstructures, predominately consisting of one single phase or mixed microstructures with two distinct phases. The fracture toughness of the present microstructures is evaluated using the Essential Work of Fracture methodology. Results are discussed in terms of the different microstructures obtained and the consequent part performance.Results show a strong connection between microstructure and fracture toughness. The bainitic grade shows favorable fracture toughness while a mixed microstructure of bainite and martensite shows a very brittle fracture behavior. A post heat treatment in the form of paint bake curing shows a negligible effect on fracture toughness of martensite.

Hydrogen embrittlement behavior of high strength rail steels: A comparison between pearlitic and bainitic microstructures

The present study was attempted to evaluate the hydrogen embrittlement (HE) behavior of two high strength rail steels with pearlitic and bainitic microstructures by using slow strain rate test (SSRT) with notched round bar specimens. The results show that the bainitic rail steel with a mixed microstructure of bainite, tempered martensite and M/A constituents exhibits superior mechanical properties in comparison with the pearlitic rail steel. The results of SSRT revealed that the bainitic rail steel is more susceptible to HE than the pearlitic rail steel, which is ascribed mainly to the higher strength level and the microstructural characteristics of the bainitic rail steel. The attempt of re-tempering treatment of the as-received bainitic rail steel exhibits no notable improvement in the resistance to HE at no expense of strength. It is thus suggested that further efforts concerning obtaining low susceptibility to HE besides obtaining excellent combination of strength and toughness should be conducted to guarantee the safety service of the bainitic steel rails.

Austenite grain growth and microstructure control in simulated heat affected zones of microalloyed HSLA steel

The roles of microalloying niobium, titanium and vanadium for controlling austenite grain growth, microstructure evolution and hardness were investigated at different simulated heat affected zones (HAZ) for high strength low alloy (HSLA) S690QL steel. High resolution FEG-SEM has been used to characterize fine bainitic ferrite, martensite and nanosized second phases at simulated coarse and fine grain HAZs. It was found that for Ti bearing steel (Ti/N ratio is 2) austenite grain had the slowest growth rate due to the presence of most stable TiN. The fine cuboidal particles promoted intragranular acicular ferrite (IGF) formation. Nb bearing steel exhibited relatively weaker grain growth retardation compared with titanium bearing steels and a mixed microstructure of bainite and martensite was present for all simulated HAZs. IGF existed at coarse grain HAZ of Ti+V bearing steel but it was totally replaced by bainite at fine grain HAZs. Hardness result was closely related to the morphology of bainitic ferrite, intragranular ferrite and second phases within ferrite. The microstructure and hardness results of different simulated HAZs were in good agreement with welded experimental results.

Structure and Properties of a Low-Carbon, Microalloyed, Ultra-High-Strength Steel

The current study concerns the development of a low-carbon, microalloyed ultra-high-strength steel on a pilot scale. The continuous cooling transformation has been evaluated, and a flat top “C” curve with a mixed microstructure of bainite and martensite has been obtained at a lower transformation temperature. The steel has been processed thermomechanically, followed by air cooling and water quenching. In addition, a variation in microstructure and mechanical properties at different finish rolling temperatures has been studied. Although a mixture of granular bainite and bainitic ferrite with interlath and intralath precipitation of NbC/NbC(N)/TiC(N) particles are the characteristic microstructural feature of air-cooled steel, the lath martensitic structure along the fine NbC/NbC(N)/TiC(N) precipitate is obtained in case of a water-quenched steel. The high-strength value obtained in the current steel is caused by the accumulated contribution of fine-grained, pancaked austenite, highly dislocated fine lath martensitic structure along with the presence of tiny precipitates of microalloy carbide/carbonitride.

Changes in chemical composition of carbides in 2·25Cr–1Mo power plant steel

It is found that the carbide chemistry changes at a slower rate in a mixed microstructure of bainite and allotriomorphic ferrite when compared with carbides in a fully bainitic microstructure. These results are expected intuitively and confirmed theoretically.MST/1896