Hot Deformation Behaviour and Processing Map of Cast Alloy 825

Munir Al-Saadi,Fredrik Sandberg,Pär G Jönsson,Christopher Hulme-Smith

doi:10.1007/s11665-021-05957-0

Munir Al-Saadi, Fredrik Sandberg + Show 2 more

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https://doi.org/10.1007/s11665-021-05957-0

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Abstract

Alloy 825 is a nickel-based alloy that is commonly used in applications where both high strength and corrosion resistance are required, such as tanks in the chemical, food and petrochemical industries and oil and gas pipelines. Components made from Alloy 825 are often manufactured using hot deformation. However, there is no systematic study to optimise the processing conditions reported in literature. In this study, a processing map for as-cast Alloy 825 is established to maximise the power dissipation efficiency of hot deformation in the temperature range of 950 to 1250 °C at an interval of 50 °C and strain rate range of 0.01, {text{s}}^{ - 1} to 10.0, {text{s}}^{ - 1} to a true strain of 0.7 using a Gleeble-3500 thermomechanical simulator. The processing conditions are also correlated to the Vickers hardness of the final material, which is also characterised using optical microscopy and scanning electron microscopy, including electron backscattered diffraction. The true stress-true strain curves exhibit peak stresses followed by softening due to occurrence of dynamic recrystallization. The activation energy for plastic flow in the temperature range tested is approximately 450,{text{ kJ mol}}^{ - 1}, and the value of the stress exponent in the (hyperbolic sine-based) constitutive equation, n = 5.0, suggests that the rate-limiting mechanism of deformation is dislocation climb. Increasing deformation temperature led to a lower Vickers hardness in the deformed material, due to increased dynamic recrystallization. Raising the strain rate led to an increase in Vickers hardness in the deformed material due to increased work hardening. The maximum power dissipation efficiency is over 35%, obtained for deformation in the temperature range 1100-1250 °C and a strain rate of 0.01, {text{s}}^{ - 1}-0.1, {text{s}}^{ - 1}. These are the optimum conditions for hot working.

Highlights

Alloy 825 is a solid solution hardening nickel-based alloy (Ref 1) that is used widely in various applications at elevated temperatures under high stress and in corrosive environments, such as tanks in the chemical and oil and gas industries, agitators and heat-exchanger systems (Ref [1, 2])
A coincidence site lattice value A critical stress, MPa A critical strain The strain hardening rate, MPa Peak stress, MPa Peak strain Zener-Hollomon parameter, s -1 Dynamic Materials Modelling A total deformation power applied to the material, Joule Power dissipation into plastic flow, Joule Power dissipation into metallurgical phenomena The parameter of the efficiency of power dissipation,% Metallurgical instability during plastic flow Temperature-dependent strength parameters True stress, MPa Strain rate, sÀ1 Deformation temperature, K The universal gas constant, 8:314 J molÀ1KÀ1 The apparent activation energy for deformation, J molÀ1 Material constants Strain rate sensitivity Electron Backscattered Diffraction Orientation imaging microscopy Grain Orientation Spread, ° Particle Stimulated Nucleation
The focus of this study is to explore process conditions under which a homogeneous microstructure can be achieved from typical continuously cast feedstock during hot forging processes that are used in industry to manufacture components from alloy 825

Summary

Introduction

Alloy 825 is a solid solution hardening nickel-based alloy (Ref 1) that is used widely in various applications at elevated temperatures under high stress and in corrosive environments, such as tanks in the chemical and oil and gas industries, agitators and heat-exchanger systems (Ref [1, 2]). During hot working of nickel-based superalloys and other austenitic alloys, the dominant metallurgical phenomenon is usually dynamic recrystallization, which counteracts the strain hardening caused by the mechanical working of the material and leads to grain refinement (Ref [3,4,5]). Creep and adiabatic heating through lattice friction affect the response of the material during hot working (Ref 9) All of these processes are dependent on the deformation temperature and can be represented as a function of both temperature and processing parameters. The focus of this study is to explore process conditions under which a homogeneous microstructure can be achieved from typical continuously cast feedstock during hot forging processes that are used in industry to manufacture components from alloy 825 This complements existing studies on similar (but not identical) alloys (Ref 13)

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