Experimental determination of the critical strain of fracture of oxide scale in hot finishing mill conditions using the sigma test (original) (raw)

Critical Failure Strain of Oxide Scale in Boiler Austenitic Tubes

2018

Article history: Received 8 March 2018 Received in revised form 12 April 2018 Accepted 13 April 2018 Available online 16 April 2018 Exfoliation of oxide scales is known to be associated with the stress and strain developed during the oxide growth. This paper presents a simple procedure to estimate the oxide scale growths and critical failure strains in boiler austenitic tubes over a period of time. In this work, a classical heat flow formula and the relationship between the Larson Miller Parameter (LMP) and the scale thickness were utilized. An approach called Advance Oxide Scale Failure Diagram (A-OSFD) was adopted. The oxide scale failure diagram would provide a general guidance to the power plant engineers to estimate the critical strains.The technique may be used as a supplementary condition monitoring tool for oxide scale growths and to evaluate the critical failure strains.

Modelling of oxide failure in hot metal forming operations

2003

Oxide scale can affect many aspects of hot metal forming operations, such as heat transfer, friction and the final surface finish of the rolled product. Surface oxide scale is always present on the steel slab and sometimes on cold rolls. Therefore, the study of the thermo-mechanical behaviour of oxide scale, particularly under conditions as close as possible to the steel manufacturing process is very important. In order to undertake a detailed study of oxide scale behaviour, several hightemperature testing techniques were applied. First, high-temperature tensile tests were carried out to investigate the brittle fracture and cracking of the surface oxide scales. Second, high-temperature compression tests were developed and the results obtained revealed many different failure mechanisms that are present in the compressed oxide scale. Finally, a tensile-compressive test was developed and performed under thermal conditions which were as close as possible to hot rolling. The best results...

Modeling of Residual Stress in Oxide Scales

1998

The magnitude of the residual stress in an oxide scale, and how this varies with temperature, is of major importance in understanding the failure mechanisms of oxide scales. This stress encompasses both growth stresses introduced at the oxidation temperature and thermal-expansion-mismatch stresses induced on heating and cooling, as well as any externally applied stresses or stress relaxation which takes place in the scale/substrate system. Although some of these components are reasonably well understood (e.g., thermal stresses), growth stresses and the relaxation of the total scale stress by creep or fracture processes are much less well understood. In this study a model has been developed to predict stress generation and relaxation in oxide scales as a function of time and temperature for both isothermal exposure and cooling to room temperature. The model determines growth stress and thermal-stress generation in the scale and how this is balanced by stresses in the substrate. The substrate stresses are then allowed to relax by creep and the scale stresses recalculated. This model accurately predicts the room-temperature scale stresses for a range of scale/alloy systems. The model can be used to show how the scale stress depends on oxidation temperature, cooling rate, substrate, and scale thickness. The model predictions are discussed in light of experimental observations for alumina scales on FeCrAlY.

Examination of Oxide Scales of Hot Rolled Steel Products

ISIJ International, 2005

Porosities in the scale layer and wavy scale-steel interfaces are two common artefacts generated during metallographic preparation of oxide scale samples. This paper presents the techniques used by the authors to effectively remove porosities in the scale and reduce the waviness of the scale-steel interface so that the true scale structures can be examined. The techniques have been successfully applied to the examination of oxide scales on hot-rolled steel with various thicknesses and structures.

The structure and properties of mill scale in relation to easy removal

2001

Oxide scale must be removed before cold drawing wire, otherwise it will cause bad surface quality, inferior die life and many wire ruptures. The nature of oxide scales and the methods of scale removal are reviewed, with particular emphasis on mechanical descaling. This is the major concern of the research, therefore a cantilever bending test has been developed to assess the ease of removal of the scale on commercial steel rod surface in the laboratory. A scanner method and a Finite Element model have been developed to evaluate the critical strain for scale cracking and removal after cantilever bending. Scanning electron microscopy (SEM), together with electron backscattered diffraction (EBSD), energy diffraction spectrum (EDS) and X-ray element mapping analysis, was used to characterize the scale before and/or after bending test. The effects of laying temperature, cooling conditions, ageing time, relative humidity and temperature, and coil positions on scale cracking and removal beh...

Study of the morphology of oxide scale formedon hot-rolled steel

2014

Mechanism of oxide scales formation on steel during hot rolling process is delicately determined and their structures are extremely complex. This work is part of larger studies made to understand the oxide scale behavior. Therefore, the morphology of oxides is determined by optical microscopy. Identification of the mechanical properties of oxide scales is achieved by micro-hardness measurement. The work has revealed a variation of microstructure in several layers of oxide. It was obtained that the oxide scales consisting mainly of wustite FeO, magnetite Fe3O4 and hematite Fe2O3 owing to the formation of voids and cracks in the scales, especially on the outer layer where it is high porous. The intermediate layers is thicker than others oxide layers. The outer layer has a lowest hardness and highest porosity.

The effect of temperature on oxide scale adherence during descaling operations

Surface Technology, 1984

The oxide adherence and initial growth kinetics of oxide scales on type 1018 steel have been investigated at temperatures of 900, 1000 and 1100 °C. The adherence was measured using a controlled water spray quench. Mass gain versus time curves and metallographic observations were used to study the kinetics and adherence at each temperature. The steel exhibited exfoliation at 900 and 1100 °C but showed remarkable adherence at 1000 °C. The behavior was explained in terms of the interplay of classical oxidation theory and increased plasticity at elevated temperatures.

Assessment of oxide scales influence on wear damage of HSM work rolls

Wear, 2007

Improvement of hot rolling process involves the lengthening of rolling campaigns. This can be achieved through the wear resistance improvement of work roll grades. The wear damage observed in service on the hot working rolls is the result of a complex interaction between oxidation, friction and thermal fatigue. An approach is to assess the coupled effect of oxidation and friction on the overall tribological behaviour at high temperature. A first step consists in a good knowledge of the material behaviour against each stress. Tribological behaviour of roll grades at high temperature is indeed strongly affected by the nature, the thickness and the thermo-mechanical properties of their oxide scales involved into the contact. All these parameters depend on roll grade microstructure. An experimental methodology has also been performed to assess the influence of the roll grade microstructure on their tribological behaviour at high temperature. It constitutes the subject of the present paper. Results have focussed on high temperature friction tests and on the analysis of oxide morphologies (through TGA tests and environmental SEM, ESEM, observations). Nevertheless this approach does not aim to reproduce strictly rolling conditions, but only to analyse the chemical and mechanical response of roll grades regarding thermal and mechanical stresses involved in hot rolling. Material investigations have shown heterogeneous oxidation according to the different microstructural states and the environmental testing conditions. A particular scale roughness, depending on grades and oxidation conditions, and influencing the tribological behaviour, has been brought out.

Microstructural investigation of the breakdown of the protective oxide scale on a 304 steel in the presence of oxygen and water vapour at 600°C

Corrosion Science, 2006

This article focuses on the detailed microstructural investigation of the oxide scale formed on a 304 steel in the presence of oxygen and water vapour (40%) at 600°C. The work has been carried out using a combination of microanalytical techniques including FIB, TEM, EDX and electron diffraction. The local breakdown of the initially protective oxide scale and the growth of island/crater oxide morphology are described. Special consideration is given to the influence of the microstructure of the steel and the oxide scale on the breakdown behaviour.