The Effect of Processing Conditions and Cooling Rate on the Microstructure and Properties of API X-70 and API X-100 Steels

Abstract

The final microstructure and subsequent properties of pipeline steel have been shown to be highly dependent of both the controlled rolling conditions and accelerated cooling conditions. This research utilized a novel physical deformation simulator to evaluate the evolution of microstructure in an X-70 and an X-100 grade pipeline steel during finish hot deformation and cooling. A series of plane strain compression (PSC) tests have been designed to simulate the processes of plate rolling and strip rolling, with the influence of finishing temperature and cooling rate being of particular interest. Single and multi-pass reductions at a constant strain rate of 70 s −1 , together with constant delay times were used in this work. A total of three different deformation temperatures, namely, 950 C, 900 C, and 850 C were used for the present experiments, in which multipass isothermal deformation took place. Additionally, multipass deformation took place under non-isothermal conditions (i.e., descending temperature processing). Microstructures were characterized optically, and micro-hardness were obtained from the as-deformed PSC specimens. Analyses from the microstructural characterization together with the results from mechanical testing will reveal the optimal processing-microstructure-properties window for this important engineering alloy.

On the Post-Printing Heat Treatment of a Wire Arc Additively Manufactured ER70S Part

“…It should be mentioned that similar to the scenario of the normalizing heat treatment, the hardening heat treatment also resulted in the formation of a homogeneous microstructure with a uniform grain size throughout the sample from the bottom to the top of the WAAM-fabricated wall. The formation of the acicular ferrite and bainite phases by quenching of the sample at higher cooling rates from the austenite stability region has also been reported in other low-carbon low-alloy steels, such as API X70 and X80 [26,27,35]. For the initial austenitizing step, the sample was heated up to 900 • C, where γ is the only stable phase since the Ac 3 temperature of the alloy was calculated to be at~883 • C, using a reported empirical equation that predicts austenite formation temperatures, i.e., Ac 1 and Ac 3 , for the low-alloy steels with less than 0.6 wt.% C [32].…”

“…It is well established that the presence of acicular ferrite and bainite constituents in the microstructure of steels can promote the mechanical properties of the component. This is primarily resulted from the finer structure of both phases, a more uniform distribution of carbide and higher dislocation density and internal stresses in the bainite phase, contributing to a higher hardness/strength and ductility in the alloy [26][27][28]. However, it should be noted that since the volume fraction of acicular ferrite and bainite constituents are negligible as compared to the dominant ferritic and pearlitic microstructure of the alloy, the presence of acicular ferrite and bainite cannot have a significant contribution to the mechanical properties of the WAAM-ER70S sample.…”

Effect of convection and cooling on microstructure of mid-chilled solidified casting

“…These parameters were obtained from differential cooling curves calculated using thermal analysis. 11,12 There was only a small difference between the temperature of T EU (eutectic undercooling) and T ER (eutectic recalescence) at TC0 z , TC0, and TC0 2 as a result of the rapid absorption of latent heat by the chilled mould. However, at TC1 z , TC2 z , TC1 2 , and TC2 2 , the T ER was a few degrees higher than T EU , indicating a more obvious recalescence respectively.…”

“…The absence of convection in zone A resulted in a greater concentration, which segregated to form Mo rich carbides in interdendrite regions, whereas the presence of convection in zone B mitigated the problem of element segregation. [9][10][11] For the casting process, an optimal set-up of the chilling block not only removes hot spots in heavy section castings but also reduces solidification time and induces convection, thereby reducing the degree of segregation and enhancing the mechanical properties of the castings.…”

“…The measuring system comprised an A/D card and a PC. The thermal analysis techniques [11][12][13] were used to analyse the cooling curves to employ first derivative curve to predict and control the microstructure in the castings.…”