Kofi Annan - Academia.edu (original) (raw)

Papers by Kofi Annan

Research paper thumbnail of Resistance spot welding of a thin 0.7 mm EN10130: DC04 material onto a thicker 2.4 mm 817M40 engineering steel

Journal of the Southern African Institute of Mining and Metallurgy, 2021

The effects of welding current, electrode force, and welding time in a resistance spot weld were ... more The effects of welding current, electrode force, and welding time in a resistance spot weld were studied to investigate the effectiveness of welded joints between a thin EN10130: DC04 material and a thicker 817M40 part, through analysis of the microstructural and mechanical properties. All welded specimens were subjected to tensile testing at room temperature (25°C) and sub-zero temperature (-46°C) to test the strength of the welded joints. No full button failure was observed at either room temperature or sub-zero temperature after optimization of the weldng parameters. The fusion zone was observed to consist mainly of martensitic phase, due to rapid quenching, while the HAZ was composed of clusters of martensite in a ferrite and bainite matrix. The base 817M40 metal remained fully ferritic after welding. The hardness was found to increase with increasing welding current. An increase in nugget size, indicating good fusion of the weld, was observed with an increase in the welding cur...

Research paper thumbnail of Austenite Grain Growth Kinetics after Isothermal Deformation in Microalloyed Steels with Varying Nb Concentrations

ISIJ International, 2018

Grain growth equation constants n, Q and A for Nb bearing steels with the Nb varying from 0.002 w... more Grain growth equation constants n, Q and A for Nb bearing steels with the Nb varying from 0.002 wt% to 0.1 wt%, were experimentally determined under reheating and high temperature hot rolling roughing conditions. The constants from these treatments were then used to develop constitutive equations that incorporate the initial grain size D o and a Nb-effect for grain growth predictions in these steels. Comparative analysis of the results showed that the values of the constants generated under rough rolling deformation conditions were slightly higher than those generated under reheating conditions. The activation energy for grain boundary migration Q was found to be in the range of 256 to 572 kJ/mol, the exponential constant n ranged from 2.6 to 6.5 and the material and processing condition's constant A was found to range from 5.23 × 10 11 to 4.96 × 10 28 in all cases as a function of the Nb content. Analysis of the influence of the initial grain size D o showed that any contribution of D o can be neglected unless it is equal or more than 70 percent of the average size of the measured austenite grain size D. A logical degree of precision in predicting austenite grain growth in microalloyed steels with different Nb contents, has been achieved in the current work.

Research paper thumbnail of Effect of hot working characteristics on the texture development in AISI 430 and 433 ferritic stainless steel

Un ni iv ve er rs si it ty y o of f P Pr re et to or ri ia a Acknowledgement My first and foremos... more Un ni iv ve er rs si it ty y o of f P Pr re et to or ri ia a Acknowledgement My first and foremost thanks go to the Almighty God for giving me grace to go through this study successfully. I would like to thank the many people who were so helpful and kind to me during my time at University of Pretoria. First, I wish to express my deepest gratitude to my supervisors, Professor W.E. Stumpf and Dr. C.W. Siyasiya. They provided an excellent environment to work in, generously extending immeasurable help, experience, direction and kindness throughout my studies. I would also like to thank the Advanced Material Division (AMD) of MINTEK for providing me with the assistantship in my hot rolling and texture measurement by bearing all the costs of such work. I am grateful to Mr. Richard Couperthwaite for all the help with the texture measurement at MINTEK. I am also very grateful to the Industrial Metals and Minerals Research Institute of University of Pretoria for provision of equipment for my studies. Columbus Stainless Steels needs a special mention and appreciation for provision of materials for the studies. I also thank the Centre for Materials Engineering of the University of Cape Town and in particular Dr. Sarah George for the high strain rate tests on the Gleeble 3800 TM. I would like to thank my fellow students who provided a great atmosphere for me at Tuks and Pretoria in general. I wish I could name them all as they all deserve it, but I would like to pick out Richard Nkhoma, Given Maruma, and Bundjoko Kweto. Finally, I would like to thank my family especially my wife, Celestine and my daughter Efua, for all of their support.

Research paper thumbnail of Effect of hot rolling conditions on ridging in 16wt% Cr ferritic stainless steel sheet

Journal- South African Institute of Mining and Metallurgy

of the textures obtained from different hot-rolling conditions Conditions Description Observed te... more of the textures obtained from different hot-rolling conditions Conditions Description Observed textures A High finishing temp., high strain rate, long interpass time {100}<001>, {111}<011>, B High finishing temp., short interpass time, low strain rate {100}<001>, {111}<123>, {554}<225> C High finishing temp., low strain rate, long interpass time {100}<011>,{100}<001>, {211}<111> {111}<123>, {554}<225>, {211}<011> D Low finishing temp., low strain rate, short interpass time {100}<011>, {100}<001> {211}<011>, {211}<111>

Research paper thumbnail of Evaluation of Heat Treatment Parameters on Microstructure and Hardness Properties of High-Speed Selective Laser Melted Ti6Al4V

Metals

This study presents the investigation on how heat treatment parameters, which are temperature, co... more This study presents the investigation on how heat treatment parameters, which are temperature, cooling method, and residence time, influence the microstructural and hardness properties of Ti6Al4V components produced on Ti6Al4V substrate using high speed selective laser melting technique. Heat treatment was performed on the produced samples before they were characterized for microstructure and hardness. The microstructure of the as-built sample contained large columnar β-grains that were filled with martensite α’ phase and had a high hardness of 383 ± 13 HV. At 1000 °C and residence time of maximum 4 h, better heat treatment parameters were seen for the selective laser melting (SLM) produced Ti6Al4V sample since an improved lamellar α + β microstructure was obtained at this condition. This microstructure is known to have improved tensile properties.

Research paper thumbnail of Sustaining the Earth in the New Millenium: The UN Secreatary-Gener Speaks Out

Environment, 2000

... We must pre-serve our forests, fisheries, and the diversity of living species, all of which a... more ... We must pre-serve our forests, fisheries, and the diversity of living species, all of which are close to collapsing under the pressure of human consumption and destruction. ... emissions credits for cli-ing countries where these would reduce pre-existing levels of green-ssions. ...

Research paper thumbnail of Resistance spot welding of a thin 0.7 mm EN10130: DC04 material onto a thicker 2.4 mm 817M40 engineering steel

Journal of the Southern African Institute of Mining and Metallurgy, 2021

The effects of welding current, electrode force, and welding time in a resistance spot weld were ... more The effects of welding current, electrode force, and welding time in a resistance spot weld were studied to investigate the effectiveness of welded joints between a thin EN10130: DC04 material and a thicker 817M40 part, through analysis of the microstructural and mechanical properties. All welded specimens were subjected to tensile testing at room temperature (25°C) and sub-zero temperature (-46°C) to test the strength of the welded joints. No full button failure was observed at either room temperature or sub-zero temperature after optimization of the weldng parameters. The fusion zone was observed to consist mainly of martensitic phase, due to rapid quenching, while the HAZ was composed of clusters of martensite in a ferrite and bainite matrix. The base 817M40 metal remained fully ferritic after welding. The hardness was found to increase with increasing welding current. An increase in nugget size, indicating good fusion of the weld, was observed with an increase in the welding cur...

Research paper thumbnail of Austenite Grain Growth Kinetics after Isothermal Deformation in Microalloyed Steels with Varying Nb Concentrations

ISIJ International, 2018

Grain growth equation constants n, Q and A for Nb bearing steels with the Nb varying from 0.002 w... more Grain growth equation constants n, Q and A for Nb bearing steels with the Nb varying from 0.002 wt% to 0.1 wt%, were experimentally determined under reheating and high temperature hot rolling roughing conditions. The constants from these treatments were then used to develop constitutive equations that incorporate the initial grain size D o and a Nb-effect for grain growth predictions in these steels. Comparative analysis of the results showed that the values of the constants generated under rough rolling deformation conditions were slightly higher than those generated under reheating conditions. The activation energy for grain boundary migration Q was found to be in the range of 256 to 572 kJ/mol, the exponential constant n ranged from 2.6 to 6.5 and the material and processing condition's constant A was found to range from 5.23 × 10 11 to 4.96 × 10 28 in all cases as a function of the Nb content. Analysis of the influence of the initial grain size D o showed that any contribution of D o can be neglected unless it is equal or more than 70 percent of the average size of the measured austenite grain size D. A logical degree of precision in predicting austenite grain growth in microalloyed steels with different Nb contents, has been achieved in the current work.

Research paper thumbnail of Effect of hot working characteristics on the texture development in AISI 430 and 433 ferritic stainless steel

Un ni iv ve er rs si it ty y o of f P Pr re et to or ri ia a Acknowledgement My first and foremos... more Un ni iv ve er rs si it ty y o of f P Pr re et to or ri ia a Acknowledgement My first and foremost thanks go to the Almighty God for giving me grace to go through this study successfully. I would like to thank the many people who were so helpful and kind to me during my time at University of Pretoria. First, I wish to express my deepest gratitude to my supervisors, Professor W.E. Stumpf and Dr. C.W. Siyasiya. They provided an excellent environment to work in, generously extending immeasurable help, experience, direction and kindness throughout my studies. I would also like to thank the Advanced Material Division (AMD) of MINTEK for providing me with the assistantship in my hot rolling and texture measurement by bearing all the costs of such work. I am grateful to Mr. Richard Couperthwaite for all the help with the texture measurement at MINTEK. I am also very grateful to the Industrial Metals and Minerals Research Institute of University of Pretoria for provision of equipment for my studies. Columbus Stainless Steels needs a special mention and appreciation for provision of materials for the studies. I also thank the Centre for Materials Engineering of the University of Cape Town and in particular Dr. Sarah George for the high strain rate tests on the Gleeble 3800 TM. I would like to thank my fellow students who provided a great atmosphere for me at Tuks and Pretoria in general. I wish I could name them all as they all deserve it, but I would like to pick out Richard Nkhoma, Given Maruma, and Bundjoko Kweto. Finally, I would like to thank my family especially my wife, Celestine and my daughter Efua, for all of their support.

Research paper thumbnail of Effect of hot rolling conditions on ridging in 16wt% Cr ferritic stainless steel sheet

Journal- South African Institute of Mining and Metallurgy

of the textures obtained from different hot-rolling conditions Conditions Description Observed te... more of the textures obtained from different hot-rolling conditions Conditions Description Observed textures A High finishing temp., high strain rate, long interpass time {100}<001>, {111}<011>, B High finishing temp., short interpass time, low strain rate {100}<001>, {111}<123>, {554}<225> C High finishing temp., low strain rate, long interpass time {100}<011>,{100}<001>, {211}<111> {111}<123>, {554}<225>, {211}<011> D Low finishing temp., low strain rate, short interpass time {100}<011>, {100}<001> {211}<011>, {211}<111>

Research paper thumbnail of Evaluation of Heat Treatment Parameters on Microstructure and Hardness Properties of High-Speed Selective Laser Melted Ti6Al4V

Metals

This study presents the investigation on how heat treatment parameters, which are temperature, co... more This study presents the investigation on how heat treatment parameters, which are temperature, cooling method, and residence time, influence the microstructural and hardness properties of Ti6Al4V components produced on Ti6Al4V substrate using high speed selective laser melting technique. Heat treatment was performed on the produced samples before they were characterized for microstructure and hardness. The microstructure of the as-built sample contained large columnar β-grains that were filled with martensite α’ phase and had a high hardness of 383 ± 13 HV. At 1000 °C and residence time of maximum 4 h, better heat treatment parameters were seen for the selective laser melting (SLM) produced Ti6Al4V sample since an improved lamellar α + β microstructure was obtained at this condition. This microstructure is known to have improved tensile properties.

Research paper thumbnail of Sustaining the Earth in the New Millenium: The UN Secreatary-Gener Speaks Out

Environment, 2000

... We must pre-serve our forests, fisheries, and the diversity of living species, all of which a... more ... We must pre-serve our forests, fisheries, and the diversity of living species, all of which are close to collapsing under the pressure of human consumption and destruction. ... emissions credits for cli-ing countries where these would reduce pre-existing levels of green-ssions. ...