Recent progress in visualization and digitization of coherent transformation structures and application in high-strength steel

High-strength steels are mainly composed of medium-or low-temperature microstructures,such as bainite or martensite,with coherent transformation characteristics.This type of microstructure has a high density of dislocations and fine crystallographic structural units,which ease the coordinated matching of high strength,toughness,and plasticity.Meanwhile,given its excellent welding perform-ance,high-strength steel has been widely used in major engineering constructions,such as pipelines,ships,and bridges.However,visual-ization and digitization of the effective units of these coherent transformation structures using traditional methods(optical microscopy and scanning electron microscopy)is difficult due to their complex morphology.Moreover,the establishment of quantitative relationships with macroscopic mechanical properties and key process parameters presents additional difficulty.This article reviews the latest progress in microstructural visualization and digitization of high-strength steel,with a focus on the application of crystallographic methods in the development of high-strength steel plates and welding.We obtained the crystallographic data(Euler angle)of the transformed microstruc-tures through electron back-scattering diffraction and combined them with the calculation of inverse transformation from bainite or martensite to austenite to determine the reconstruction of high-temperature parent austenite and orientation relationship(OR)during con-tinuous cooling transformation.Furthermore,visualization of crystallographic packets,blocks,and variants based on actual OR and digit-ization of various grain boundaries can be effectively completed to establish quantitative relationships with alloy composition and key process parameters,thereby providing reverse design guidance for the development of high-strength steel.

Microstructure Evolution of Heat‑Affected Zone in Submerged Arc Welding and Laser Hybrid Welding of 690 MPa High Strength Steel and its Relationship with Ductile–Brittle Transition Temperature

The comparative study of submerged arc welding(SAW)and laser hybrid welding(LHW)was carried out for a 690 MPa high strength steel with thickness of 20 mm.Microstructure and ductile–brittle transition temperature(DBTT)evolution in welded zone were elucidated from the aspect of crystallographic structure,particularly,digitization and visualization of 24 variants.The impact toughness of each micro zone in LHW joint is better than that of SAW,in which the DBTT of equivalent fusion line and heat-affected zone(HAZ)can reach−70 and−80℃,while that of SAW is only−50℃.LHW technology induces narrowing of the HAZ and refining of the microstructure obtained in weld metal and HAZ.Meanwhile,the austenite grain size and transformation driving force in the coarse grained heat-affected zone(CGHAZ)are reduced and increased,respectively.It makes variant selection mechanism occurring in CGHAZ of LHW dominate by close-packed plane grouping,which promotes lath bainite formation with high density of high angle grain boundary,especially block boundary dominated by V1/V2 pair.While for SAW,the lower transformation driving force inferred from the large amount of retained austenite in CGHAZ induces Bain grouping of variants,and thus triggers the brittle crack propagating straightly in granular bainite,resulting in lower impact toughness and higher DBTT.

Origin and Classification of Geothermal Water from Guanzhong Basin,NW China:Geochemical and Isotopic Approach

Combined with tectonic evolution, a multi-isotopic method （δD, δ^18O, ^87Sr/^86Sr and ^14C） and hydrochemistry data have been used to study the origin and classification of geothermal water in the Guanzhong Basin. The study shows that geothermal water of Xianli terrace primarily came from north- west direction when accepting recharge. A small amount supply source of geothermal water in Xi＇an City is from Qinling Mountain and the principal supply source comes from the west direction, but geothermal water of Chang＇an District mainly accepts supply from Qinling Mountain. Based on geothermal environ- ment is open or not, the degree of water-rock interaction, and the origin of geothermal water, geothermal water of the study area can be divided into four types： A, geothermal water of Gushi depression, perfect closed thermal environment and significant water-rock interaction, belonged to residual sedimentary wa- ter origin; B, geothermal water of Xianyang City, good closed environment and relatively significant water-rock interaction, belonged to residual sedimentary water origin mixed with fossil leaching water; C, geothermal water of Xi＇an City, half closed environment and some water-rock interaction, belonged to fossil leaching water origin; D, geothermal water of Chang＇an District, open environment and mixed with modern precipitation, belonged to fossil leaching water origin.