Research has shown that the acicular ferrite microstructure in steel weld metal, which provides an optimum combination of strength and toughness, is indeed intragranularly nucleated bainite. It is possible to maximize the content of acicular ferrite by increasing the intragranular nucleation sites while maintaining a critical weld metal cooling rate and the steel hardenability. This paper highlights recent research related to nucleation and growth of acicular ferrite during decomposition of austenite.

The mechanism of acicular ferrite in weld deposits

The correlation of microstructural characteristics and toughness of the simulated coarse grained heat affected zone (CGHAZ) of low carbon bainitic steel was investigated in this study. The toughness of simulated specimens was examined by using an instrumented Charpy impact tester after the simulation welding test was conducted with different cooling times. Microstructure observation and crystallographic feature analysis were conducted by means of optical microscope and scanning electron microscope equipped with electron back scattered diffraction (EBSD) system, respectively. The main microstructure of simulated specimen changes from lath martensite to coarse bainite with the increase in cooling time. The deterioration of its toughness occurs when the cooling time ranges from 10 to 50 s compared with base metal toughness, and the toughness becomes even worse when the cooling time increases to 90 s or more. The MA (martensite–austenite) constituent is primary responsible for the low toughness of simulated CGHAZ with high values of cooling time because the large MA constituent reduces the crack initiation energy significantly. For crack propagation energy, the small effective grain size of lath martensite plays an important role in improving the crack propagation energy. By contrast, high misorientation packet boundary in coarse bainite seems to have few contributions to the improvement of the toughness because cleavage fracture micromechanism of coarse bainite is mainly controlled by crack initiation.

Microstructural characteristics and toughness of the simulated coarse grained heat affected zone of high strength low carbon bainitic steel

This study aims at correlating microstructure and Charpy impact properties in high-toughness API X70 and X80 line-pipe steels. Three kinds of steels were fabricated by varying alloying elements and hot rolling conditions, and their microstructures and Charpy impact properties were investigated. In addition, their effective grain sizes were characterized by the electron back-scatter diffraction (EBSD) analysis. The Charpy impact test results indicated that the steels rolled in the single phase region had the higher upper shelf energy (USE) than the steel rolled in the two phase region because their microstructures were composed of acicular ferrites. In the X80 steel rolled in the single phase region, the decreased energy transition temperature (ETT) could be explained by the decrease in the overall effective grain size due to the presence of acicular ferrite having smaller effective grain size. Thus, it had excellent mechanical properties in yield and tensile strengths, absorbed energy, and transition temperature, except in ductility.

Correlation of microstructure and charpy impact properties in API X70 and X80 line-pipe steels

The correlation of microstructure and Charpy V-notch (CVN) impact properties of a high-toughness API X70 pipeline steel was investigated in this study. Six kinds of steel were fabricated by varying the hot-rolling conditions, and their microstructures, effective grain sizes, and CVN impact properties were analyzed. The CVN impact test results indicated that the steels rolled in the single-phase region had higher upper-shelf energies (USEs) and lower energy-transition temperatures (ETTs) than the steels rolled in the two-phase region because their microstructures were composed of acicular ferrite (AF) and fine polygonal ferrite (PF). The decreased ETT in the steels rolled in the single-phase region could be explained by the decrease in the overall effective grain size due to the presence of AF having a smaller effective grain size. On the other hand, the absorbed energy of the steels rolled in the two-phase region was considerably lower because a large amount of dislocations were generated inside PFs during rolling. It was further decreased when coarse martensite or cementite was formed during the cooling process.

Effective grain size and charpy impact properties of high-toughness X70 pipeline steels

In the present study, comparison studies on mechanical properties were made on a commercial X70 grade polygonal ferrite (PF) dominated pipeline steel and a laboratory developed X90 grade acicular ferrite (AF) dominated pipeline steel obtained by optimum thermo-mechanical controlled processing (TMCP). Charpy impact test results indicated that the upper shelfenergy (USE) of the AF pipeline steel was a little bit higher, but its energy transition temperature (ETT) was extremely low, about -162 degrees C, much lower than that of the PF pipeline steel of about -121 degrees C. It was analyzed that higher strength and better toughness of the AF pipeline steel came from its finer grain size and higher density of dislocations and subboundaries, which could be also further proved from the electron backscatter diffraction (EBSD) analysis for its finer effective grain size (EGS) and higher content of low angle grain boundaries (LAGBs), smaller cleavage fracture unit measured from the fracture surface around fracture origin fractured at -196 degrees C, and its more bent crack propagation path in the fracture. (C) 2008 Elsevier B.V. All rights reserved.

Study of high strength pipeline steels with different microstructures

Acicular ferrite formation, promoted by the intragranular nucleation of ferrite plates, is well known to be beneficial for achieving a good combination of mechanical properties. However, the set of microstructures that can be obtained during the subsequent development of the transformation from the primary plates generated at particles can be quite complex and depends on a certain number of variables: steel composition, temperature range, prior austenite grain size, and particle density. In the present work, acicular ferrite microstructures have been produced by isothermal treatments in three different steels with different active particle types and densities. The morphology of the obtained intragranular microstructures has been found to depend on the steel composition, the prior austenite grain size, and the density of particles able to promote intragranular nucleation. Electron backscattered diffraction (EBSD) techniques have been used to define the microstructural unit controlling toughness in these types of microstructures.

Analysis of different acicular ferrite microstructures in low-carbon steels by electron backscattered diffraction. Study of their toughness behavior

The degree of ferrite grain refinement that can be reached in low-carbon microalloyed steels by thermomechanical processing is limited, to a certain extent, by the grain coarsening which can take place behind the transformation front. The coarsening of ferrite grains is the result of two different mechanisms: elimination of ferrite grains produced by normal grain growth after full impingement on the austenite grain boundary plane and/or coalescence between different ferrite grains with close orientation formed from the same crystallographic variant. The lack of experimental data to support either process is due to the experimental difficulties encountered when analyzing the phenomenon. Some transmission electron microscope (TEM) observations reveal that the ferrite grains formed along a prior grain boundary in deformed austenite are separated by a mixture of low and high angle grain boundaries upon impingement. In the present work, the electron backscattered diffraction (EBSD) technique has been applied to investigate the microstructural evolution during transformation, with special emphasis placed on the α-α grain boundary character as a means of investigating the contribution of coalescence/grain growth to coarsening.

https://link.springer.com/content/pdf/10.1007/s11661-004-0112-x.pdf

A. Iza-Mendia

Papers

Analysis of different acicular ferrite microstructures in low-carbon steels by electron backscattered diffraction. Study of their toughness behavior

Study of the ferrite grain coarsening behind the transformation front by electron backscattered diffraction techniques