Magnetic Evaluation of Crystallographic Texture in Steels

Researchers: Dr Jun LiuLink opens in a new windowLink opens in a new window, Professor Claire DavisLink opens in a new windowLink opens in a new window

Anisotropy is a fairly common phenomenon in steels such that the properties of a material vary with directions along which they are measured. This can be either a desirable or unwanted phenomenon. In either case, it is imperative to be able to non-destructively and accurately evaluate the anisotropy in steels. Magnetic non-destructive evaluation (NDE) is gaining increasing interest and recognition as a powerful NDE tool and a high value-added complement to conventional techniques for microstructural characterisation and evaluation of properties in steels. Most magnetic NDE systems have directionality in measurement built in them through the application of the magnetic field(s) along certain directions and measuring the responses along the same directions and hence are sensitive to magnetic anisotropy.

This project aims to develop an electromagnetic modelling tool and testing methods for NDE of crystallographic texture in steels. Our initial objective is to link magnetic permeability, an important magnetic property that affects electromagnetic sensor signals and is very sensitive to microstructures, to the crystallographic texture in advanced steels. A finite element microstructure model considering crystallographic texture and grain size has been developed for forward predictions of anisotropic magnetic permeability given a microstructure and corresponding crystallographic texture. The next objective is to solve the inverse problem: extracting crystallographic texture given the antistrophic magnetic permeability measurements by electromagnetic (EM) sensors. We are looking to validate the both the forward and the inverse model for a variety of steels representative of a range of strength and complexity of crystallographic texture including e.g. electrical steels and interstitial free steels. The ultimate objective of the inverse problem is to extract full orientation distribution functions given measurements of magnetic anisotropy.

Near Solidus Forging of High-Value Steels

Researcher: Dr Carl Slater

With the ever-increasing drive for lightweighting comes the increased need for high-strength materials. These materials are both hard to machine and also are comparably expensive to commodity grades of steel. Work on Near Solidus Forging (NSF) is being carried out in collaboration with Mondragon University to replace conventional Hot Forging (HF). NSF offers the benefit of working at temperatures where the material is super soft and much more complex shapes can be achieved with much lower loads (usually a decrease from 3000 to 300t load hammer). The near-net shape processing that NSF offers reduces the amount of material removed compared to the extensive machining needed via conventional methods.

Work at WMG is looking at the super high deformation temperatures (>1400 °C) and the microstructural evolution that takes place. Aspects such as adiabatic heating, segregation, incipient melting and strain inhomogeneity are all taken into consideration in order to be able to predict the optimal NSF process window.

Non-Destructive Evaluation of Steel Microstructures Using Electromagnetic Sensors

Researchers: Dr Jun LiuLink opens in a new windowLink opens in a new window, Professor Claire DavisLink opens in a new windowLink opens in a new window

A variety of electromagnetic (EM) sensors/techniques have been developed or commercialised for evaluating/monitoring microstructure, mechanical properties or creep damage in steels during industrial processing, heat treatment or service exposure in a non-destructive and non-contact fashion. These sensors/techniques are sensitive to different microstructures based on the principle that microstructural changes in steels alter their electrical and magnetic properties. The major challenge is the interpretation of the EM sensor signals and the correlation to microstructure and mechanical properties, which is currently more or less qualitative, empirical and/or general.

The present project aims to establish a quantitative link between the selected microstructural parameters of interest and relevant EM properties to help identify the most appropriate EM properties to be measured for greatest sensitivity to the features of interest. This will determine the type of sensors or techniques to be used for a given application, and enable accurate evaluation of the microstructural parameters and microstructural feature distribution. Major/minor magnetic hysteresis loop measurements will be carried out to measure various magnetic properties of model and/or commercial steels with different microstructural feature distributions, e.g. precipitate size and inter-particle spacing distribution. Major/minor loop models are being developed to consider the microstructural parameter distribution, which can, in turn, be inversely evaluated by fitting with experimental measurements. EM sensor outputs are also modelled by Finite Element method using Comsol Multiphysics to look at the link between EM sensor signals and relevant EM properties. To improve understanding of the fundamentals we also observe the magnetic domain structure (i.e. magnetic microstructure) and domain processes (i.e. development of domains in an applied magnetic field) to look at their interactions with microstructural features.

PhD/ENGD Projects

Projects being undertaken by our postgraduate students.

Product Uniformity Control (PUC)- Electromagnetic Sensors

Researcher: Dr Lei (Frank) Zhou, Research Fellow, with Prof Claire Davis

Participants: Tata Steel, ArcelorMittal, Thyssenkrupp Steel, Salzgitter, Swerea KIMAB AB, TNO, Chalmers University of Technology, SSSA, CEIT, The University of Warwick, The University of Manchester, WMG Universite Joseph Fourier Grenoble, CEA, Cedrat.

This is an EU funded RFCS (Research Fund for Coal and Steel) project with an overall objective to optimise the product (steel strip) uniformity by the employment of online systems that continuously and non-destructively measure electromagnetic (EM) and ultrasonic parameters that relate to the steel microstructure.

Electromagnetic sensors have previously been shown to be able to quantitatively monitor phase transformation in steels (e.g. austenite to ferrite) and are sensitive to other microstructural changes e.g. decarburisation in rail and rod steels and thermal exposure for power generation steels. In order to use online systems to assess microstructural inhomogeneity then detailed understanding of the sensitivity of sensors signals to microstructural changes is required. The main role of the University of Warwick in this project involves modelling of the EM inductance properties for microstructures in steels.

The research focuses on the link between the microstructure and the EM signal via the influence of the materials' electromagnetic properties. Finite element models (in 2D and 3D) will be established to model the effect of microstructure features (including phase balance, grain size, texture, dislocation density, precipitates, and applied / residual stress) on the EM properties (primarily the relative permeability). Initially, models for individual microstructural parameters will be developed with complex multi-component microstructures then being considered.

Rapid Alloy Processing

Researcher: Dr Sarah Connolly, Jacky Zhu

Project partners: Tata Steel Europe, Liberty Speciality Steels

Rapid alloy prototyping (RAP) allows for fast investigation of systematic variations to, or completely new, alloy systems through the combination of small scale processing, mechanical testing and simulation-based modelling. WMG’s RAP facilities aim to replicate all stages of processing within the steel plant, from the production of material, thermo-mechanical processes to develop a final product, application of coatings, mechanical testing and microstructural characterisation.

Enabling testing within research laboratories and virtual environments rather than in the steel plant means 100 samples can be produced and tested up to 100 times quicker and a lot less raw material is wasted. Overall, this means newer and better steel products can reach customers much more quickly. Whilst this offers incredible benefits in the array of steels that can be developed, the level of detail investigations can reach, and the time and cost savings, there are also some inherent limitations that need to be addressed.

RAP within a lab, when compared to a foundry environment, has very different levels of accuracy, control and cleanliness. Whilst all are beneficial to the scientific understanding of the process, they do lead to differences from plant material. Small scale testing methods, as utilised within RAP to gain the maximum amount of data possible from the smaller amounts of each alloy produced, also require validation against commercial standard testing methods. It is also anticipated that small volumes of material will have a much faster cooling rate than the large or even continuous casts in plant. The reduction in thickness through rolling or forging will be smaller than that achieved in plant. The combination of the effects of the two must be sought, and accounted for in process parameters.

A large part of the commission process for RAP, therefore, is understanding these differences, benchmarking both the production material and testing methods to those used by industry in order to validate the small scale route for use in product development. Validation must comprise chemical, microstructural and mechanical properties both in the final state and through process to identify points of deviation and any measures that can be introduced to successfully replicate plant material. The use of precise heat treatments will be vital in controlling the final microstructure and adapting to precisely match commercial material.

Ongoing, the process will be used to answer scientific questions surrounding the manufacturing of current products, optimise grades in terms of properties and cost reduction and allow a less risk averse approach to new alloy development.

Simulation of the Rapid Solidification Rates Seen in Thin Slab and Strip Casting of Steel

Researcher: Dr Carl Slater

Part of a EPSRC sponsored project “ASSURE – Advanced Steel Shaping Using Reduced Energy”

Since the implementation of continuous casting for steels in the 1960’s, the use of this technique has increased dramatically. Today about 95% of the steel produced worldwide is fabricated by this method. In particular thin slab cast direct rolling and strip casting offers major advantages in term of efficiency through the reduced capital needed for reheating and reduced rolling requirements. As production of steels moves closer to net shape casting, a dramatic increase in the initial cooling rates seen during solidification is observed. For thick and thin slab casting typical cooling rates are 12 and 50 °C/s respectively. Whereas strip casting can see cooling rates up to 1700 °C/s at the strip surface. Whilst these rates have been produced in full-scale casters, reproducing these controllably on a lab-based scale is difficult. Within this project several techniques have been explored in order to replicate the cooling conditions of these accelerated casting processes.

The figure (right) shows an example of one of these techniques: using a Gleeble 3500, tungsten electrodes are used to pass a large electrical current through a steel sample encased in an alumina crucible, which heats the sample at a rate of around 50 °C/s to just above the liquidus temperature. The sample is then controllably cooled at rates up to 100 °C/s. The advantage of this technique over the conventional splat or immersion test is the ability to have direct temperature measurement of the molten steel (compared to the extrapolated values obtained from the other methods). In addition to this controllable thermal profiles can be applied post solidification, such as reheats/dwells, to study the formation of precipitates. This method also incorporates the electromagnetic stirring seen in practise as well as the use of much larger samples compared to the other techniques.

Alongside the experimental investigations modelling is being carried out using both COMSOL (a multiphysics simulation software) and ThermoCalc (for phase prediction) to understand the thermal profiles in the samples and to predict the segregation behaviour.

Links ASSURE: http://gow.epsrc.ac.uk/NGBOViewGrant.aspx?GrantRef=EP/M014002/1