Thomas Garcin

In Situ Measurements of Grain Growth and Recrystallization by Laser Ultrasonics

Laser ultrasonics for metallurgy (LUMet) is an innovative sensor technology for in-situ measurement of microstructure evolution during thermomechanical processing. This unique sensor has been attached to a Gleeble 3500 thermomechanical simulator for dedicated laboratory studies during processing of steel, aluminum, magnesium and titanium samples. Advanced processing software has been developed for the measurement of grain size and texture evolution from laser ultrasonic signals. Results of austenite grain growth measurements in low carbon steels will be described to demonstrate the capabilities of the LUMet technique. Further, applications of the system to measure recrystallization of ferrite and austenite formation during intercritical annealing simulations of dual phase steels will be presented. The ability to rapidly acquire data both during a single test and for multiple conditions over a range of conditions from different samples has important implications on expediting process modelling and alloy design. Although certain limitations exist, the LUMet technique offers a very reliable characterization platform with a number of potential applications in metallurgical process engineering.

Reverse Austenite Transformation and Grain Growth in a Low-Carbon Steel

The mechanisms controlling the reverse austenite transformation and the subsequent grain growth are examined in a low-carbon steel during slow continuous heating. The ex-situ metallographic analysis of quenched samples is complemented by in-situ dilatometry of the phase transformation and real-time laser ultrasonic measurements of the austenite grain size. Although the initial state of the microstructure (bainite or martensite) has only limited impact on the austenite transformation temperature, it has significant influence on the mean austenite grain size and the rate of grain growth. The coarsening of austenite islands during reverse transformation occurring from the martensitic microstructure is responsible for a large austenite grain structure at the completion of the austenite formation. On the other hand, a much finer austenite grain size is obtained when the austenite transforms from the bainite microstructure. Upon further heating, the rate of austenite grain growth is limited by the presence of nanometric precipitates present in the bainite microstructure leading to a significantly finer austenite grain size. These results give important guidance for the design of thermomechanical-controlled processing of heavy-gage steel plates.

Microstructure model for the heat-affected zone of X80 linepipe steel

An important aspect of the integrity of oil and gas pipelines is the heat-affected zone (HAZ) of girth welds where the microstructure of the as-hot rolled steel is altered with potentially adverse effects on the HAZ properties. Therefore, it is critical to evaluate the HAZ microstructure for different welding scenarios. Here, an integrated microstructure evolution model is proposed and applied to the HAZ of an X80 linepipe steel. The model considers dissolution of Nb-rich precipitates, austenite grain growth and austenite decomposition into ferrite and bainite. Microstructure maps showing the fraction of transformation products as a function of distance from the fusion line are obtained and used to compare the effect of different welding procedures on the HAZ microstructure.

FORMATION OF MARTENSITE/AUSTENITE (M/A) IN X80 LINEPIPE STEEL

Linepipe steels are usually microalloyed with Nb to promote the formation of complex microstructures that lead to the required mechanical properties. In particular, Nb in solution affects significantly the austenite decomposition kinetics and the resulting microstructure. A systematic study has been carried out to quantify the influence of Nb on the austenite decomposition kinetics in X80 linepipe steel. Continuous cooling transformation tests were conducted with a Gleeble 3500. The transformation products include ferrite, granular and upper bainite and M/A (martensite/ retained austenite) constituents. For this study optical microscopy was used to investigate the formation of M/A constituents that critically determine the fracture toughness. A relation between M/A and the surrounding microstructure is observed. In combination with an existent model for the prediction of the microstructure evolution during weld thermal cycles, the area fraction, size and morphology of M/A can be predicted for the simulated HAZ, based on the prior austenite grain size, cooling rate and amount of Nb in solution.Copyright

Microstructure Engineering of High-Performance Steels

Microstructure evolution during steel processing assumes a critical role in tailoring mechanical properties, e.g., the austenite–ferrite transformations are a key metallurgical tool to improve properties of advanced low-carbon steels. Microstructure engineering is a concept that links the processing parameters to the properties by accurately modeling the microstructure evolution. Systematic experimental laboratory studies provide the basis for model development and validation. Laser ultrasonics, dilatometry, and a range of microscopy techniques (including electron backscatter diffraction mapping) are used to characterize recrystallization, grain growth, phase transformations, and precipitation in high-performance steels. Based on these studies, a suite of state variable models is proposed. Examples of their applications are given for intercritical annealing of advanced automotive steel sheets and welding of high-strength line pipe grades. The extension of state variable models to the scale of the microstructure is illustrated using the phase field approach. Here, the emphasis is placed on simulating the austenite decomposition into complex ferrite–bainite microstructures. The challenges and opportunities to develop next-generation process models will be discussed.

An Integrated Model to Predict Microstructure and Mechanical Properties in the Heat Affected Zone for X80 Linepipe

There is a complex interplay between welding procedures and the steel chemistry which determines the final engineering performance of the heat affected zone in a large diameter girth weld. This work uses a combination of experimentally determined thermal histories from laboratory scale single and dual torch multi-pass gas metal arc welding (GMAW) with phenomenological models to predict microstructure and mechanical properties in the heat affected zone (HAZ) of an X80 steel. The integrated model consists of sub-models for austenite grain growth, dissolution of Nb based precipitates and austenite decomposition. These models have been calibrated with detailed experimental studies using a Gleeble 3500 thermomechanical simulator. The models are fully integrated so that the austenite grain size and the Nb solid solution level are used as inputs into the austenite decomposition model where these two factors strongly affect the final microstructure. The decomposition model includes ferrite and bainite models with suitable criteria for transition from one model to the other and a simple first order empirical relation to predict the final fraction of martensite/retained austenite (MA). The integrated model has been applied to a variety of thermal scenarios which are derived from experimental measurements of thermal histories including dual torch conditions where, for example, the Nb solid solution level has to be tracked through both thermal excursions into austenite. Using the integrated model, microstructure maps of the HAZ can be generated for the different welding scenarios.

The Effect of Microstructure on Tensile Behaviour of X80 Microalloyed Steel

A high degree of work hardening is desirable for steels to be employed in strain-based pipeline designs. In an effort to enhance work hardening characteristics, this study was conducted to determine the effect of thermal treatment on microstructural development and the subsequent relationship between microstructure and tensile behaviour of high strength microalloyed line pipe steel. A series of thermal schedules was applied to X80 steel samples using a Gleeble thermo-mechanical simulator in order to generate a variety of microstructures. The microstructures were quantified by calculating the phase fraction of individual phases using scanning electron microscopy (SEM). A focused ion beam (FIB) instrument was used to prepare electron transparent samples of specific grains that were characterized using transmission electron microscopy (TEM). The X80 microstructures were composed mostly of bainitic and ferritic grains with isolated pockets of martensite and M-A islands due to local carbon segregation. The effect of thermal treatment on microstructural evolution was determined based on varying the interrupt temperature, re-heat temperature and hold time at elevated temperatures. The overall effect of microstructure on the mechanical properties was evaluated, with a particular focus on hardness values and the shape of the stress-strain curves. The effect of thermal history and microstructure development on the work hardening characteristics was also determined.Copyright