Metallographic Methods for Revealing the Multiphase Microstructure of TRIP-Assisted Steels - some applications for steels

Hydrogen-Induced Cracking (HIC) is a primary failure mechanism of pipeline-welded joints in the absence of external loading in the oil and gas exploration industries. Three different cooling rates after austenitization were used to simulate in the laboratory different regions of the heat-affected zone (HAZ) formed when welding an API X80 pipeline steel specially designed to enhance the HIC resistance. The samples were characterized with regard to microstructure and crystallography as well as HIC resistance. The HIC resistance test used NACE TM0284-2011 methodology. The microstructure and its homogeneity varied as a function of cooling rates. Samples containing inclusions and segregation zone from the segregation bands of specimens showed reduced HIC resistance, while specimens containing only acicular ferrite and granular bainite coupled with the absence of segregation zone showed significant improvement in HIC resistance. The best HIC resistance results came from samples presenting fine acicular ferrite consisting of fine interlocking plates, with divergent crystallographic orientations, preventing the formation of localized strain distribution inside the grain and at grain boundaries. It was also found that a large proportion of medium-angle boundaries prevent microcrack initiation and the transgranular mode of crack propagation.

Effects of different cooling rates on the microstructure, crystallographic features, and hydrogen induced cracking of API X80 pipeline steel

X70 (steel A) and X80 (steel B) pipeline steels were fabricated by ultra fast cooling (UFC). UFC processing improves not only ultimate tensile strength (UTS), yield strength (YS), yield ratio (YS/UTS), and total elongation of both steels, but also their Charpy absorbed energy (A
 K) as well. The microstructures of both steels were all composed of quasi polygonal, acicular ferrite (AF), and granular bainite. MA islands (the mixtures of brittle martensite and residual austenite) are more finely dispersed in steel B, and the amount of AF in steel B is much more than that in steel A. The strength of steel B is higher than that of steel A. This is mainly attributed to the effect of the ferrite grain refinement which is resulted from UFC processing. The finely dispersed MA islands not only provide dispersion strengthening, but also reduce loss of impact properties to pipeline steels. UFC produces low-temperature transformation microstructures containing larger amounts of AFs. The presence of AF is a crucial factor in achieving desired mechanical properties for both steels. It is suggested that the toughness of the experimental steel increases with increasing the amount of AF.

Effects of Ultra Fast Cooling on Microstructure and Mechanical Properties of Pipeline Steels

Triply periodic minimal surfaces (TPMS) allow the production of structural components with high mechanical strength and energy absorption capacity and significantly reduce material consumption during manufacturing. The fabrication provided by laser powder bed fusion expands the manufacture of TPMS by allowing the obtention of high-performance components. Thus, this study investigated the compression responses of as-built and aged (480 °C for 3 h) maraging steel 300 primitive scaffolds under compressive loading in different directions (0° and 90°). Compressive tests and failure analysis showed that the maraging steel specimens subjected to aging treatment had approximately 56% improved yield and 20% collapse strength, in comparison to the as-built specimens, due to the austenite reversion and intermetallic precipitates within the martensitic structure. Also, a non-linear correlation of energy absorption capacity with strain was observed during the compression of the TPMS. It was also well evidenced that mechanical performance and energy absorption capacity depends on post heat treatment and loading direction, which can contribute to an improved design strategy of TPMS devices produced by laser powder bed fusion.

Compression and energy absorption of maraging steel primitive scaffolds produced by powder bed fusion

A base low Si, high-Al transformation-induced plasticity (TRIP) steel and one with 0.03Nb and 0.02Ti (wt%) additions were subjected to thermo-mechanical processing (TMP) and galvanising simulations. The microstructure and mechanical properties were analysed using a combination of optical and electron microscopy, X-ray diffraction and tensile testing and the results compared with those from intercritically annealed–galvanised steels. The addition of Nb and Ti results in microstructure refinement and an increase in the amount of the retained austenite after TMP which in turn, leads to increases in the tensile strength (~750 MPa) and the total elongation (TE) (~29 %). A deterioration in the volume fraction of retained austenite and the mechanical properties was noted in both steels after the additional galvanising simulation. For the base steel, all TMP and galvanised samples presented with continuous yielding during tensile testing. The Nb–Ti steel exhibited discontinuous yielding and extended Luders banding when TMP was followed by a longer coiling time. Both steels returned discontinuous yielding after the intercritical annealing–galvanising treatment. The discontinuous yielding behaviour was associated with the much finer ferrite grain size in the intercritically annealed steels and the ageing processes that take place during galvanising.

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The effect of processing parameters on the microstructure and mechanical properties of low-Si transformation-induced plasticity steels

In the present paper, effects of hot deformation and subsequent austempering on the mechanical properties of high silicon and low silicon transformation induced plasticity (TRIP) steel were investigated. The results have shown that polygonal ferrite, granular bainite and a large amount of stabilised retained austenite can be obtained by thermomechanical processing (TMP). Mechanical properties increase with increasing finish rolling reduction and decreasing finish rolling temperature because of the stabilisation of retained austenite. The tensile strength and the total elongation of both steels reach the maximum (791, 794 MPa and 36, 37% respectively). Isothermal holding for 20 min at 400°C after hot deformation is the optimal process. Low silicon TRIP steel shows a larger uniform elongation owing to a stable n value, and it exhibits better mechanical properties than conventional high silicon TRIP steel.

Effects of hot deformation and subsequent austempering on mechanical properties of high silicon and low silicon TRIP steel

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Effects of hot deformation and subsequent austempering on mechanical properties of high silicon and low silicon TRIP steel