Thomas Sourmail - Academia.edu (original) (raw)
Papers by Thomas Sourmail
Materials Science and Technology, 2002
ABSTRACT The creep rupture life and rupture strength of austenitic stainless steels have been exp... more ABSTRACT The creep rupture life and rupture strength of austenitic stainless steels have been expressed as functions of chemical composition, test conditions, stabilisation ratio, and solution treatment temperature. The method involved a neural network analysis of a vast and general database assembled from published data. The outputs of the model have been assessed against known metallurgical trends and other empirical modelling approaches. The models created are shown to capture important trends and to extrapolate better than conventional techniques.
Scripta Materialia, 2005
ABSTRACT The existence of an ordering reaction in FeCo alloys has recently been challenged. The c... more ABSTRACT The existence of an ordering reaction in FeCo alloys has recently been challenged. The compatibility of new and published results with the alternative explanation is examined. It is shown that the existence of an ordering reaction remains the best explanation for most observations to date.
Scripta Materialia, 2004
Boron segregation and precipitates were investigated using EFTEM. Chromium borides were identifie... more Boron segregation and precipitates were investigated using EFTEM. Chromium borides were identified. Comparison with published results suggests that segregation is mainly of non-equilibrium type. Non-uniform distribution of precipitates can be explained using nucleation theory if the segregation leads to grain-boundary boron concentration only slightly above the solubility limit.
Materials Science and Technology, 2004
Page 1. Stability of retained austenite in TRIP-assisted steels MY Sherif, C. Garcia Mateo, T. So... more Page 1. Stability of retained austenite in TRIP-assisted steels MY Sherif, C. Garcia Mateo, T. Sourmail and HKDH Bhadeshia A theory is developed for the quantitative representation of the strain induced transformation of retained ...
The significance of Bainite and Martensite start temperatures is unquestionable for the steel com... more The significance of Bainite and Martensite start temperatures is unquestionable for the steel community, therefore a significant amount of work has been devoted not only in understanding the physical mechanism lying beneath both transformations, but also to obtaining quantitatively accurate models for predicting both temperatures. Nowadays, with no limitations in computer power, rigorous and complex data analysis methods should be applied whenever it is possible. Thus, Neural Network analysis becomes a very attractive alternative, for being easily distributed, self-sufficient and for its ability of accompanying its predictions by an indication of their reliability
This report summarises the work carried out over the duration of the project. The aim of this pro... more This report summarises the work carried out over the duration of the project. The aim of this project was to investigate the fatigue and wear properties of Si-rich nanostructured bainitic steels (Nanobain). A first phase of the project consisted in the design of different possible alloy compositions, divided into a first series tailored for the manufacture of small components using gas quenching, and a second series for the manufacture of larger sections heat-treated in a salt bath. Kinetics and basic mechanical properties were investigated on laboratory casts. During this first part, an unprecedented combination of tensile strength and ductility was achieved on a newly designed composition (UTS 2.2GPa, total elongation > 20 %). Wear rates as measured in twin-disc tests were as little as 50 % of those achieved on standard high-hardness bainitic grades. In a second phase, two industrial heats were produced on the basis of results achieved during the first part of the project. Thes...
Materials Science and Technology, 2013
ABSTRACT Knowledge of the martensite start temperature in low and medium alloy steels is of criti... more ABSTRACT Knowledge of the martensite start temperature in low and medium alloy steels is of critical importance for steel makers, or any industry involved in the transformation of hardenable steels. Its determination is often carried out by length change measurements in quenching dilatometers. The present paper focuses on the meaning of the information obtained in these experiments and discusses two alternative methods and their respective benefits, particularly with respect to the phenomenon sometimes referred to as ‘slow start’ for the martensite transformation.
Materials Science and Technology, 2013
Solid State Phenomena, 2011
ABSTRACT Isothermal bainitizing of high carbon steel is used to obtain exceptionally high mechani... more ABSTRACT Isothermal bainitizing of high carbon steel is used to obtain exceptionally high mechanical characteristics together with near zero distortion during heat-treating. Such heat-treatments are often carried out at temperatures close to but above the martensite start temperature, where transformation kinetics are unfortunately very sluggish. The formation of a small amount of martensite prior to isothermal holding is known to accelerate transformation kinetics. However, the influence of the amount transformed remains to clarify, and the consequence on final properties have not been explored. In the following, the influence of partial martensite formation on the kinetics of bainite formation is investigated in 100Cr6 (AISI 52100) and similar grades. It is shown that, although the formation of martensite allows for shorter heat-treatment durations, it does not seem to affect the bainite kinetics, inasmuch as the bainite formation kinetics after formation of X% martensite are identical to that which would be expected if the first X% transformation had been obtained through isothermal bainitic transformation.
Solid State Phenomena, 2008
... However hammer press should be avoided as they imply complex tool design. ... [59] W. Püttgen... more ... However hammer press should be avoided as they imply complex tool design. ... [59] W. Püttgen, W. Bleck, B. Hallstedt, and PJ Uggowitzer, Solid State Phenomena 116-117 (2006) 177-180 [60] SFMedina, M.Chapa, P.Valles, A.Quispe, MIVega, ISIJ International, Vol.39, 1999, N ...
Acta Materialia, 2014
ABSTRACT The formation of austenite during both slow and fast heating (0.25–100 °C s−1) was inves... more ABSTRACT The formation of austenite during both slow and fast heating (0.25–100 °C s−1) was investigated for different microstructures of a selected low-alloy steel. With the simultaneous use of dilatometry and high-energy X-ray diffraction, it was possible to follow not only the global progress of the austenitization, but also the individual evolutions of each phase (ferrite, cementite and retained austenite if present in the initial microstructure). The results confirm earlier published data regarding the ease of austenitization of different initial microstructures (ferrite–pearlite, bainite and tempered martensite). More importantly, two stages were clearly identified, corresponding to the simultaneous transformation of ferrite and cementite, followed by the progressive disappearance of the remaining ferrite. While this is well known for ferrite–pearlite microstructures, it is not yet documented for bainite and tempered martensite. Microstructure evolution calculations based on a diffusion-controlled mechanism helped rationalize the differences observed between the three initial microstructures. In addition, they also strongly suggested the existence of a critical carbide size beyond which the second austenitization phase would correspond to carbide dissolution instead of ferrite transformation.
Materials Science and Engineering: A, 2015
ABSTRACT Nanostructured bainite is not a novel laboratory-scale steel anymore and the interest on... more ABSTRACT Nanostructured bainite is not a novel laboratory-scale steel anymore and the interest on the commercial production of these microstructures by steelmakers and end-users is now conceivable. These microstructures are achieved through the isothermal transformation of high-carbon high-silicon steels at low temperature, leading to nanoscale plates of ferrite with thickness of 20–40 nm and retained austenite. Nanostructured bainitic steels present the highest strength/toughness combinations ever recorded in bainitic steels (2.2 GPa/40 MPa∙m1/2) and the potential for engineering components is alluring. However, fatigue properties, responsible of the durability of a component, remain to be examined. In order to understand the role of the microstructure during the fatigue crack propagation, the crack path in three nanoscale bainitic structures has been analysed on the basis of the relationships between grain misorientations and grain boundaries by Electron Backscatter Diffraction. Active slip systems in bainitic ferrite and crack deflection at grain boundaries have been identified, while retained austenite is cast doubt on its role.
ISIJ International, 2014
ABSTRACT Abnormal grain growth (AGG) in austenitic state is studied in low alloy steel in relatio... more ABSTRACT Abnormal grain growth (AGG) in austenitic state is studied in low alloy steel in relation with precipitation state. It is observed that initial austenite grain size and precipitation state plays more important role in defining the normal or abnormal grain growth condition than the final one obtained after a heat treatment. Precipitate volume fraction evolution with time-temperature having similar quantity at the end but different initial grain sizes, showed different grain growth phenomenon. Arguments are presented to rationalize the presented experience. A simplified AGG model is applied to understand the effects of initial mean austenite grain size and precipitate size distribution on the subsequent AGG occurrence.
Materials Science and Technology, 2003
... Consequently, creep rupture is one of the typical metallurgical problems where physical model... more ... Consequently, creep rupture is one of the typical metallurgical problems where physical models do not yet describe the process as a whole. On the other hand, empirical models based on data analysis, eg Gaussian processes or arti®cial neural networks, are, at present, the ...
Abstract: The bainite start temperature B s is defined as the highest temperature at which ferrit... more Abstract: The bainite start temperature B s is defined as the highest temperature at which ferrite can transform by a displacive transformation. A common observation is that the bainite start temperature is very sensitive to the chemical composition, indicating that the ...
The present study is concerned with the potential of high carbon, high silicon steel grades isoth... more The present study is concerned with the potential of high carbon, high silicon steel grades isothermally transformed to bainite at low temperature (< 300 °C). A first part gives an overview of design principles allowing very high strength and ductility to be achieved, while minimising transformation duration. Wear and fatigue properties are then investigated for over ten variants of such material, manufactured in the laboratory or industrially. The results are discussed against published data. Tensile strength above 2 GPa are routinely achieved, with, in one case, an exceptional and unprecedented total elongation of over 20%. Bainite plate thickness and retained austenite content are shown to be important factors in controlling the yield strength, though additional, non negligible parameters remain to be quantified. Rolling-sliding wear performances are found to be exceptional, with as little as 1% of the specific wear rate of conventional bainitised 100Cr6. It is suggested that this results from the decomposition of retained austenite in the worn layer, which considerably increases hardness and presumaby introduces compressive residual stresses. Fatigue performance were slightly improved over 100Cr6 for one of the two industrially produced material, but significantly lower otherwise. Factors controlling fatigue resistance require further investigations.
Materials Science and Technology, 2002
ABSTRACT The creep rupture life and rupture strength of austenitic stainless steels have been exp... more ABSTRACT The creep rupture life and rupture strength of austenitic stainless steels have been expressed as functions of chemical composition, test conditions, stabilisation ratio, and solution treatment temperature. The method involved a neural network analysis of a vast and general database assembled from published data. The outputs of the model have been assessed against known metallurgical trends and other empirical modelling approaches. The models created are shown to capture important trends and to extrapolate better than conventional techniques.
Scripta Materialia, 2005
ABSTRACT The existence of an ordering reaction in FeCo alloys has recently been challenged. The c... more ABSTRACT The existence of an ordering reaction in FeCo alloys has recently been challenged. The compatibility of new and published results with the alternative explanation is examined. It is shown that the existence of an ordering reaction remains the best explanation for most observations to date.
Scripta Materialia, 2004
Boron segregation and precipitates were investigated using EFTEM. Chromium borides were identifie... more Boron segregation and precipitates were investigated using EFTEM. Chromium borides were identified. Comparison with published results suggests that segregation is mainly of non-equilibrium type. Non-uniform distribution of precipitates can be explained using nucleation theory if the segregation leads to grain-boundary boron concentration only slightly above the solubility limit.
Materials Science and Technology, 2004
Page 1. Stability of retained austenite in TRIP-assisted steels MY Sherif, C. Garcia Mateo, T. So... more Page 1. Stability of retained austenite in TRIP-assisted steels MY Sherif, C. Garcia Mateo, T. Sourmail and HKDH Bhadeshia A theory is developed for the quantitative representation of the strain induced transformation of retained ...
The significance of Bainite and Martensite start temperatures is unquestionable for the steel com... more The significance of Bainite and Martensite start temperatures is unquestionable for the steel community, therefore a significant amount of work has been devoted not only in understanding the physical mechanism lying beneath both transformations, but also to obtaining quantitatively accurate models for predicting both temperatures. Nowadays, with no limitations in computer power, rigorous and complex data analysis methods should be applied whenever it is possible. Thus, Neural Network analysis becomes a very attractive alternative, for being easily distributed, self-sufficient and for its ability of accompanying its predictions by an indication of their reliability
This report summarises the work carried out over the duration of the project. The aim of this pro... more This report summarises the work carried out over the duration of the project. The aim of this project was to investigate the fatigue and wear properties of Si-rich nanostructured bainitic steels (Nanobain). A first phase of the project consisted in the design of different possible alloy compositions, divided into a first series tailored for the manufacture of small components using gas quenching, and a second series for the manufacture of larger sections heat-treated in a salt bath. Kinetics and basic mechanical properties were investigated on laboratory casts. During this first part, an unprecedented combination of tensile strength and ductility was achieved on a newly designed composition (UTS 2.2GPa, total elongation > 20 %). Wear rates as measured in twin-disc tests were as little as 50 % of those achieved on standard high-hardness bainitic grades. In a second phase, two industrial heats were produced on the basis of results achieved during the first part of the project. Thes...
Materials Science and Technology, 2013
ABSTRACT Knowledge of the martensite start temperature in low and medium alloy steels is of criti... more ABSTRACT Knowledge of the martensite start temperature in low and medium alloy steels is of critical importance for steel makers, or any industry involved in the transformation of hardenable steels. Its determination is often carried out by length change measurements in quenching dilatometers. The present paper focuses on the meaning of the information obtained in these experiments and discusses two alternative methods and their respective benefits, particularly with respect to the phenomenon sometimes referred to as ‘slow start’ for the martensite transformation.
Materials Science and Technology, 2013
Solid State Phenomena, 2011
ABSTRACT Isothermal bainitizing of high carbon steel is used to obtain exceptionally high mechani... more ABSTRACT Isothermal bainitizing of high carbon steel is used to obtain exceptionally high mechanical characteristics together with near zero distortion during heat-treating. Such heat-treatments are often carried out at temperatures close to but above the martensite start temperature, where transformation kinetics are unfortunately very sluggish. The formation of a small amount of martensite prior to isothermal holding is known to accelerate transformation kinetics. However, the influence of the amount transformed remains to clarify, and the consequence on final properties have not been explored. In the following, the influence of partial martensite formation on the kinetics of bainite formation is investigated in 100Cr6 (AISI 52100) and similar grades. It is shown that, although the formation of martensite allows for shorter heat-treatment durations, it does not seem to affect the bainite kinetics, inasmuch as the bainite formation kinetics after formation of X% martensite are identical to that which would be expected if the first X% transformation had been obtained through isothermal bainitic transformation.
Solid State Phenomena, 2008
... However hammer press should be avoided as they imply complex tool design. ... [59] W. Püttgen... more ... However hammer press should be avoided as they imply complex tool design. ... [59] W. Püttgen, W. Bleck, B. Hallstedt, and PJ Uggowitzer, Solid State Phenomena 116-117 (2006) 177-180 [60] SFMedina, M.Chapa, P.Valles, A.Quispe, MIVega, ISIJ International, Vol.39, 1999, N ...
Acta Materialia, 2014
ABSTRACT The formation of austenite during both slow and fast heating (0.25–100 °C s−1) was inves... more ABSTRACT The formation of austenite during both slow and fast heating (0.25–100 °C s−1) was investigated for different microstructures of a selected low-alloy steel. With the simultaneous use of dilatometry and high-energy X-ray diffraction, it was possible to follow not only the global progress of the austenitization, but also the individual evolutions of each phase (ferrite, cementite and retained austenite if present in the initial microstructure). The results confirm earlier published data regarding the ease of austenitization of different initial microstructures (ferrite–pearlite, bainite and tempered martensite). More importantly, two stages were clearly identified, corresponding to the simultaneous transformation of ferrite and cementite, followed by the progressive disappearance of the remaining ferrite. While this is well known for ferrite–pearlite microstructures, it is not yet documented for bainite and tempered martensite. Microstructure evolution calculations based on a diffusion-controlled mechanism helped rationalize the differences observed between the three initial microstructures. In addition, they also strongly suggested the existence of a critical carbide size beyond which the second austenitization phase would correspond to carbide dissolution instead of ferrite transformation.
Materials Science and Engineering: A, 2015
ABSTRACT Nanostructured bainite is not a novel laboratory-scale steel anymore and the interest on... more ABSTRACT Nanostructured bainite is not a novel laboratory-scale steel anymore and the interest on the commercial production of these microstructures by steelmakers and end-users is now conceivable. These microstructures are achieved through the isothermal transformation of high-carbon high-silicon steels at low temperature, leading to nanoscale plates of ferrite with thickness of 20–40 nm and retained austenite. Nanostructured bainitic steels present the highest strength/toughness combinations ever recorded in bainitic steels (2.2 GPa/40 MPa∙m1/2) and the potential for engineering components is alluring. However, fatigue properties, responsible of the durability of a component, remain to be examined. In order to understand the role of the microstructure during the fatigue crack propagation, the crack path in three nanoscale bainitic structures has been analysed on the basis of the relationships between grain misorientations and grain boundaries by Electron Backscatter Diffraction. Active slip systems in bainitic ferrite and crack deflection at grain boundaries have been identified, while retained austenite is cast doubt on its role.
ISIJ International, 2014
ABSTRACT Abnormal grain growth (AGG) in austenitic state is studied in low alloy steel in relatio... more ABSTRACT Abnormal grain growth (AGG) in austenitic state is studied in low alloy steel in relation with precipitation state. It is observed that initial austenite grain size and precipitation state plays more important role in defining the normal or abnormal grain growth condition than the final one obtained after a heat treatment. Precipitate volume fraction evolution with time-temperature having similar quantity at the end but different initial grain sizes, showed different grain growth phenomenon. Arguments are presented to rationalize the presented experience. A simplified AGG model is applied to understand the effects of initial mean austenite grain size and precipitate size distribution on the subsequent AGG occurrence.
Materials Science and Technology, 2003
... Consequently, creep rupture is one of the typical metallurgical problems where physical model... more ... Consequently, creep rupture is one of the typical metallurgical problems where physical models do not yet describe the process as a whole. On the other hand, empirical models based on data analysis, eg Gaussian processes or arti®cial neural networks, are, at present, the ...
Abstract: The bainite start temperature B s is defined as the highest temperature at which ferrit... more Abstract: The bainite start temperature B s is defined as the highest temperature at which ferrite can transform by a displacive transformation. A common observation is that the bainite start temperature is very sensitive to the chemical composition, indicating that the ...
The present study is concerned with the potential of high carbon, high silicon steel grades isoth... more The present study is concerned with the potential of high carbon, high silicon steel grades isothermally transformed to bainite at low temperature (< 300 °C). A first part gives an overview of design principles allowing very high strength and ductility to be achieved, while minimising transformation duration. Wear and fatigue properties are then investigated for over ten variants of such material, manufactured in the laboratory or industrially. The results are discussed against published data. Tensile strength above 2 GPa are routinely achieved, with, in one case, an exceptional and unprecedented total elongation of over 20%. Bainite plate thickness and retained austenite content are shown to be important factors in controlling the yield strength, though additional, non negligible parameters remain to be quantified. Rolling-sliding wear performances are found to be exceptional, with as little as 1% of the specific wear rate of conventional bainitised 100Cr6. It is suggested that this results from the decomposition of retained austenite in the worn layer, which considerably increases hardness and presumaby introduces compressive residual stresses. Fatigue performance were slightly improved over 100Cr6 for one of the two industrially produced material, but significantly lower otherwise. Factors controlling fatigue resistance require further investigations.