PSI - Issue 42

Ghosh et al. / Structural Integrity Procedia 00 (2022) 000 – 000 Ghosh et al. / Structural Integrity Procedia 00 (2022) 000 – 000 Ghosh et al. / Structural Integrity Procedia 00 (2022) 000 – 000

920 1. Introduction Mechanical structures and components like those employed in offshore applications, bridges, rails, railway wheels, engine components, load bearing parts of automobiles, transportations systems etc., often must endure long service lives, equivalent to 10 8 -10 10 loading cycles in fatigue and sometimes even beyond their original design lives taking into account both environmental as well as economic considerations. This has led to a growing interest in fatigue behaviour of structural materials under very high cycle loading conditions to ensure their long-term safety aspects (Sakai et al. (2009)). In this context, the recent development of tough, ultrahigh strength steels has shown a great potential in the field of high-end equipment manufacturing, thereby fulfilling the demands of lightweight engineering and performance upgrade. However, very high cycle fatigue (VHCF) failure has become a key issue because of the inherent low defect tolerance of high/ultrahigh-strength steels (Nakajima et al. (2010), Nie et al. (2013)). In these steels, cracks tend to initiate at interior inclusions in the VHCF regime, even at a stress level below the conventional endurance limit. Hence, to ensure the long-term stability and safety of many engineering components and structures manufactured from these advanced ultrahigh-strength steels, a thorough evaluation of their VHCF properties is of utmost importance. Moreover, the fracture surface of a failed VHCF sample often shows a unique fine-granular appearing area at the site of crack nucleation at the interior inclusion. Such a unique feature has attracted the attention of many researchers to investigate the associated VHCF failure micro-mechanism/s. In recent years, a significant research effort has been directed towards the development of advanced high strength steels with excellent combinations of strength, toughness and ductility. The novel processing route of direct quenching and partitioning (DQ&P) is now a well-established process imparting a significant balance of ultrahigh-strength and reasonable ductility in advanced high-strength steels, besides imparting excellent low temperature toughness (Ghosh et al. (2021), Ghosh et al. (2022)). In this process, small fractions of austenite are partially or fully stabilized down to room temperature, often divided as thin interlath films and/or tiny pools in the martensitic matrix (Speer et al. (2003), Ghosh et al. (2022)). Whilst the martensitic matrix provides the required high strength, a small fraction of finely divided austenite stabilized between the martensitic laths is expected to provide desired uniform elongation and work hardening characteristics via the transformation induced plasticity effect. In this paper, investigations were carried out to understand the micro-mechanisms associated with crack initiation in the VHCF regime for a DQ&P processed 0.4 wt.% C steel using the ultrasonic-fatigue testing technique. The specimens were subjected to VHCF testing (19 kHz) at a stress ratio of R = − 1 and thoroughly investigated in respect of fractographic features using a field emission scanning electron microscope (FE-SEM) to identify the crack initiation sites and determine the cause of VHCF failure. An account of the microstructural characteristics of the crack initiation region, analysed using FE-SEM and transmission electron microscopy (TEM), along with associated micro-mechanisms is presented in this paper. 2. Experiment Details 2.1. Test material In this study, a 0.4 wt.% carbon (C) steel containing 0.75 wt.% silicon (Si) was designed along with different contents of manganese (Mn), chromium (Cr), and nickel (Ni). A 70-kg vacuum-cast steel ingot of this steel was procured from OCAS, Belgium. The final composition details of the steel are presented in Table 1. Table 1: Chemical composition (wt. %) of the experimental steel. C Si Mn Al Cr Ni 0.41 0.68 2.04 0.03 0.99 0.49 2.2. Material processing A novel processing route involving multiple passes of thermomechanically controlled rolling and subsequent direct quenching and partitioning (DQ&P) treatment was employed to impart ultrahigh-strength level in the steel alongside reasonable ductility and toughness. Fig. 2a represents a schematic presentation of the designed DQ&P schedule. The first three hot rolling passes were conducted well above the no-recrystallization temperature ( T nr ) with ~0.2 strain in each pass. The second stage comprised of three controlled rolling passes in the no recrystallisation regime (below the T nr temperature). The finish rolling temperature was chosen well the above the A r3 temperature to avoid any strain-induced ferrite formation. Immediately after the finish rolling pass, the rolled plate with the final thickness of about ~15 mm was quenched in a tank of water close to the desired quench stop temperature ( T Q = 150  C) and then subjected to partitioning treatment by quickly inserting the sample in a 1. Introduction Mechanical structures and components like those employed in offshore applications, bridges, rails, railway wheels, engine components, load bearing parts of automobiles, transportations systems etc., often must endure long service lives, equivalent to 10 8 -10 10 loading cycles in fatigue and sometimes even beyond their original design lives taking into account both environmental as well as economic considerations. This has led to a growing interest in fatigue behaviour of structural materials under very high cycle loading conditions to ensure their long-term safety aspects (Sakai et al. (2009)). In this context, the recent development of tough, ultrahigh strength steels has shown a great potential in the field of high-end equipment manufacturing, thereby fulfilling the demands of lightweight engineering and performance upgrade. However, very high cycle fatigue (VHCF) failure has become a key issue because of the inherent low defect tolerance of high/ultrahigh-strength steels (Nakajima et al. (2010), Nie et al. (2013)). In these steels, cracks tend to initiate at interior inclusions in the VHCF regime, even at a stress level below the conventional endurance limit. Hence, to ensure the long-term stability and safety of many engineering components and structures manufactured from these advanced ultrahigh-strength steels, a thorough evaluation of their VHCF properties is of utmost importance. Moreover, the fracture surface of a failed VHCF sample often shows a unique fine-granular appearing area at the site of crack nucleation at the interior inclusion. Such a unique feature has attracted the attention of many researchers to investigate the associated VHCF failure micro-mechanism/s. In recent years, a significant research effort has been directed towards the development of advanced high strength steels with excellent combinations of strength, toughness and ductility. The novel processing route of direct quenching and partitioning (DQ&P) is now a well-established process imparting a significant balance of ultrahigh-strength and reasonable ductility in advanced high-strength steels, besides imparting excellent low temperature toughness (Ghosh et al. (2021), Ghosh et al. (2022)). In this process, small fractions of austenite are partially or fully stabilized down to room temperature, often divided as thin interlath films and/or tiny pools in the martensitic matrix (Speer et al. (2003), Ghosh et al. (2022)). Whilst the martensitic matrix provides the required high strength, a small fraction of finely divided austenite stabilized between the martensitic laths is expected to provide desired uniform elongation and work hardening characteristics via the transformation induced plasticity effect. In this paper, investigations were carried out to understand the micro-mechanisms associated with crack initiation in the VHCF regime for a DQ&P processed 0.4 wt.% C steel using the ultrasonic-fatigue testing technique. The specimens were subjected to VHCF testing (19 kHz) at a stress ratio of R = − 1 and thoroughly investigated in respect of fractographic features using a field emission scanning electron microscope (FE-SEM) to identify the crack initiation sites and determine the cause of VHCF failure. An account of the microstructural characteristics of the crack initiation region, analysed using FE-SEM and transmission electron microscopy (TEM), along with associated micro-mechanisms is presented in this paper. 2. Experiment Details 2.1. Test material In this study, a 0.4 wt.% carbon (C) steel containing 0.75 wt.% silicon (Si) was designed along with different contents of manganese (Mn), chromium (Cr), and nickel (Ni). A 70-kg vacuum-cast steel ingot of this steel was procured from OCAS, Belgium. The final composition details of the steel are presented in Table 1. Table 1: Chemical composition (wt. %) of the experimental steel. C Si Mn Al Cr Ni 0.41 0.68 2.04 0.03 0.99 0.49 2.2. Material processing A novel processing route involving multiple passes of thermomechanically controlled rolling and subsequent direct quenching and partitioning (DQ&P) treatment was employed to impart ultrahigh-strength level in the steel alongside reasonable ductility and toughness. Fig. 2a represents a schematic presentation of the designed DQ&P schedule. The first three hot rolling passes were conducted well above the no-recrystallization temperature ( T nr ) with ~0.2 strain in each pass. The second stage comprised of three controlled rolling passes in the no recrystallisation regime (below the T nr temperature). The finish rolling temperature was chosen well the above the A r3 temperature to avoid any strain-induced ferrite formation. Immediately after the finish rolling pass, the rolled plate with the final thickness of about ~15 mm was quenched in a tank of water close to the desired quench stop temperature ( T Q = 150  C) and then subjected to partitioning treatment by quickly inserting the sample in a 1. Introduction M chanical structures and c mponents like those e ploy d in offshore applications, bridges, rails, railway wheels, engine components, load bearing parts of automobiles, transportations systems etc., often must endure long service lives, equivalent to 10 8 -10 10 loading cycles in fatigue and sometimes even beyond their original design lives taking into cc nt both environment l as well as economic considerations. This has led to a growing interest in fatigue behaviour of structural materials under very high cycle loading conditions to ensure their long-t rm safety aspects (Sakai et al. (2009)). In this context, the recent development of tough, ultrahigh strength steels has shown a great potential in the field of high-end equipment manufa turing, thereby fulfilling the demands of lightweight engineering and performance upgrade. However, very hig cycle fatigue (VHCF) f ilure has become a key issue because of the inh rent low defect tolerance of high/ultrahigh-strength steels (Nakajima et al. (2010), Nie et al. (2013)). In these steels, cracks tend to initiate at interior inclusions in the VHCF regime, even at a stress level below the conventional endurance limit. Hence, to ensure the long-term stability and safety of many engineering components and structures manufactured from these ad anced ultrahigh-strength steels, a thorough evaluation of their VHCF properties is of utmost import nce. Moreover, the fracture surf ce of a failed VHCF sample often shows a unique fine-granular appearing area at the site of crack nucleation at the interior inclusion. Such a unique feature has attracted the attention of many researchers to investigate the associated VHCF failure micro-mechanism/s. In recent years, a significant research effort has been directed towards the development of advanced high strength steels with excellent combinations of strength, toughness and ductility. The novel processing route of direct quenching and partitioning (DQ&P) is now a well-established process imparting a significant balance of ultrahigh-strength and reasonable ductility in advanced high-strength steels, besides imparting excellent low temperature toughness (Ghosh et al. (2021), Ghosh et al. (2022)). In this process, small fr ctions of austenite are partially or fully stabilized down to room temperature, often divided as thin interlath films and/or tiny pools in the martensitic matrix (Speer et al. (2003), Ghosh et al. (2022)). Whilst the martensitic matrix provides the required high strength, a small fraction of finely divided auste ite stabilized between the martensitic laths is ex ected to provide desired uniform elongation and work hardening characteristics via the transformation induced plasticity effect. In this paper, investigations were carried out to understand the micro-mechanisms associated with crack initiation in the VHCF regim for a DQ&P processed 0.4 wt.% C steel using the ultrasonic-fatigue testing technique. The specimens were subje ted to VHCF testing (19 kHz) at stress ratio of R = − 1 and thoroughly investigated in respect of fractographic features using a field emission scanning electro microscope (FE-SEM) to identify the crack initiation sites and determine the cause of VHCF failure. An account of the microstructural characteristics of the crack initiation region, analysed using FE-SEM and transmission electron microscopy (TEM), along with associated micro-mechanisms is presented in this paper. 2. Experiment Details 2.1. Test material In this study, a 0.4 wt.% carbon (C) steel containing 0.75 wt.% silicon (Si) was designed along with different contents of anganese (Mn), chromium (Cr), and nickel (Ni). A 70-kg vacuum-cast steel ingot of this steel was procured from OCAS, Belgium. The final composition details of the steel are presented in Table 1. Table 1: Chemical composition (wt. %) of the experimental steel. C Si Mn Al Cr Ni 0.41 0.68 2.04 0.03 0.99 0.49 2.2. Material processing A novel processing route involving multiple passes of thermomechanically controlled rolling and subsequ nt direct quenching and partitioning (DQ&P) treatment was mployed to impart ultrahigh-strength level in the steel alongside reasonable ductility and toughness. Fig. 2a represents a schematic presentation of the designed DQ&P schedule. The first three hot rolling passes were conducted well above the no-recrystallization temperature ( T nr ) with ~0.2 strain in each pass. The second stage comprised of three controlled rolling passes in the no recrystallisation regime (below the T nr temperatur ). The finish rolling temperature was chosen well the above the A r3 temperature to avoid any strain-induced ferrite formation. Immediately after the finish rolling pass, the rolled plate wi h the final thickness of about ~15 mm was quenched in a tank of water close to the desired quench stop temperature ( T Q = 150  C) and then subjected to partitioning treatment by quickly inserting the sample in a 1. Introduction Mechanical structures and components like those employed in offshore applications, bridges, rails, railway wheels, engine components, load bearing parts of automobiles, transportations systems etc., often must endure long service lives, equivalent to 10 8 -10 10 loading cycles in fatigue and sometimes even beyond their original design lives taking into account both environmental as well as economic considerati ns. This has led to a growing interest in fatigue behaviour of structural materials under very high cycle loading conditions to ensure their long-term safety aspects (Sakai et al. (2009)). In this context, the recent development of tough, ultrahigh strength steels has shown a great potential in the field of high-end equipment manufacturing, thereby fulfilling the demands of lightweight engineeri g and performance upgrade. However, very high cycle fatigue (VHCF) failure has become a key issu because f the inherent low defect tolerance of high/ultrahigh-strength steels (Nakajima et al. (2010), Nie et al. (2013)). In these steels, cracks tend to initiate at interior inclusions in the VHCF regime, even at a stress level below the conventional endurance limit. Hence, to ensure the long-term stability and safety of many engineering components and structures manufactured from these advanced ultrahigh-strength steels, a thorough evaluatio of th ir VHCF properties is of utmost importanc . Moreover, the fracture surface of a failed VHCF sample often shows a unique fine-granular appearing area at the site of crack nucleation at the interior inclusion. Such a unique feature has attracted the attention of many researcher to investigate the associ ted VHCF failure micro-mechanism/s. In recent years, a sig ificant research effort has been directed towards the development of advanced high strength steels with excellent combinations of strength, tou ness and ductility. Th novel processing route of direct quenching and partitioning (DQ&P) is now a well-established process imparting a significant balance of ultrahigh-strength and reasonable ductility in advanced high-strength steels, besides imparting excellent low temperature toughness (Ghosh et al. (2021), Ghosh et al. (2022)). In this process, small fractions of austenite are partially or fully stabilized down to room temperature, often divid as thin interlath film and/or tiny pools in the martensitic matrix (Speer et al. (2003), Ghosh et al. (2022)). Whilst the martensitic matrix provides the required high strength, a small fraction of finely divided austenite stabilized between the martensitic laths is expected to provide desired uniform elon ti and work hardening characteristics via the transformation induced plasticity effect. In t is paper, investigations were carried out to understand the micro-mechanisms associated with crack initiation in the VHCF regime for a DQ&P processed 0.4 wt.% C steel using the ultrasonic-fatigue testing technique. The specimens were subjected to VHCF testing (19 kHz) at a stress ratio of R = − 1 and thoroughly investigated in respect of fractographic features using a field emission scanning electron microscope (FE-SEM) to identify the crack initiation sites and determine the cause of VHCF failure. An account of the microstructural characteristics of the crack initiation region, analysed using FE-SEM and transmission electron microscopy (TEM), along with associated micro-mechanisms is presented in this paper. 2. Experiment Details 2.1. Test material In this study, a 0.4 wt.% carbon (C) steel containing 0.75 wt.% silicon (Si) was designed along with different contents of anganese (Mn), chromium (Cr), and nickel (Ni). A 70-kg vacuum-cast steel ingot of this steel was procured from OCAS, Belgium. The final composition details of the steel are presented in Table 1. Table 1: Chemical composition (wt. %) of the experimental steel. C Si Mn Al Cr Ni 0.41 0.68 2.04 0.03 0.99 0.49 2.2. Material processing A novel processing route involving multiple passes of thermomec anically controlled rolling and subsequent direct quenching and partitioning (DQ&P) treatment was employed to impart ultrahigh-strength level in th steel alongside reasonable ductility and toughness. Fig. 2a represents a schematic presentation of the designed DQ&P schedule. The first three hot rolling passes were conducted well above the no-recrystallization temperature ( T nr ) with ~0.2 strain in each pass. The second stage comprised of thr e controlled rolling passes in the no recrystallisation regime (b low the T nr temperature). The finish rolling temperature was chosen well the above the A r3 temperature to avoid any strain-induced ferrite formation. Immediately after the finish rolling pass, the rolled plate with the final thickness of about ~15 mm was quenched in a tank of water close to the desired quench stop temperature ( T Q = 150  C) and then subjected to partitioning treatment by quickly inserting the sample in a 1. Introduction Mechanical structures and components like those employed n offshore applications, bridges, rails, ra lway wheels, engine components, load bearing parts of automobiles, transportations systems etc., often must endure long service lives, equivalent to 10 8 -10 10 loading cycles in fatigue and sometimes even beyond their original design lives taking into account both environmental as well as economic considera i ns. This has led to a growing interest in fatigue behaviour of structural materials under very high cycle loading conditions to ensur their long-term safety aspects (Sakai et al. (2009)). In this context, the recent development of tough, ultrahigh strength steels has shown a great potential in the field of high-end equipment manufacturing, thereby fulfilling the demands of lightweight engineeri g and performance upgrade. However, very high cycle fatigue (VHCF) failure has become a key issu because f the inherent low defect tolerance of high/ultrahigh-strength steels (Nakajima et al. (2010), Nie et al. (2013)). In these steels, cracks tend to initiate at interior inclusions in the VHCF regime, even at a stress level below the conventional endurance limit. Hence, to ensure the long-term stability and safety of many engineering components and structures manufactured from these advanced ultrahigh-strength steels, a thorough evaluation of th ir VHCF properties is of utmost importanc . Moreover, the fr cture surface of a failed VHCF sample often shows a unique fine-granular appearing area at the site of crack nucleation at the interior inclusion. Such a unique feature has attracted the attention of many researcher o investigate the associ ted VHCF failure micro-mechanism/s. In recent years, a s g ificant research effort has be n directed towards the development of advanced high strength steels with excellent combina ons of strength, tou ness and ductility. Th novel processing route of direct quenching and partitioning (DQ&P) is now a well-established process imparting a significant balance of ultrahigh-strength and reasonable ductility in advanced high-strength steels, besides imparting excellent low temperature toughness (Ghosh et al. (2021), Ghosh et al. (2022)). In this process, small fractions of austenite are partially or fully stabilized down to room temper ture, often divid as thin i terlath film and/or tiny pools in the martensitic matrix (Speer et al. (2003), Ghosh et al. (2022)). Whilst the m rtensitic matrix provides the required high strength, a small fraction of finely divided austenite stabilized between the martensitic laths is expected to provide desired uniform elonga i and work hardening characteristics via the transfor ation induced plasticity effec . In t is paper, investigations were carried out to u derstand the micro-mechanisms associ ted with crack initiation in the VHCF regime for a DQ&P processed 0.4 wt.% C steel using the ultrasonic-fatigue testing technique. The specimens were subjected to VHCF testing (19 kHz) at a stress r tio of R = − 1 and thoroughly investigated in respect of fractographic features using a field emission sc ning electron microsc pe (FE-SEM) to identify the crack initiation sites and determine the caus of VHCF failure. An account of the microstructural characteristics of the crack initiation region, analysed using FE-SEM and transmission electron microscopy (T M), along with ssociated micro-mechanisms is presented in this paper. 2. Experiment Details 2.1. Test material In this study, a 0.4 wt.% carbon (C) steel containing 0.75 wt.% silicon (Si) wa designed along with different contents of manganese (Mn), chromium (Cr), and nickel (Ni). A 70-kg vacuum-cast steel ingot of this steel was procured from OCAS, Belgium. The final composition details of the steel are presented in Table 1. Table 1: Chemical composition (wt. %) of the experimental steel. C Si Mn Al Cr Ni 0.41 0.68 2.04 0.03 0.99 0.49 2.2. Material processing A novel processing route involving multiple passes of thermomechanically controlled rolling and subsequent direct quenching and partitioning (DQ&P) treatment was employed to impart ultrahigh-strength level in th steel alongside reasonable ductility and toughness. Fig. 2a represents a schematic presentation of the designed DQ&P schedule. The first three hot rolling passes were conducted well above the no-recrystallization temperature ( T nr ) with ~0.2 strain in each pass. The second stage comprised of thr e controlled rolling passes in the no recrystallisation regime (b low the T nr temperature). The finish rolling temperature was chosen well the above he A r3 temperature to avoid any strain-i duced ferrite formation. Immediately after the finish rolling pass, the rolled plate with the final thickness of about ~15 mm was quenched in a tank of water close to the desired quench stop temperature ( T Q = 150  C) and then subjected to partitioning treatment by quickly inserting the sample in a Ghosh et al. / Structural Integrity Procedia 00 (2022) 000 – 000 1. Introduction Mechani al structures and components like those employed n offshore applications, bridges, rails, railway wheels, engine components, load beari g parts of automobiles, transportations system etc., often must endure long service lives, equivalent to 10 8 -10 10 loading cycles in fatigue and sometimes even beyond their original design lives taking into account both environmental as well as economic considera i ns. This has led to a growing interest in fatigue behaviour of structura materials und r very high cycle loading conditions to ensure their long-term saf ty aspects (Sakai et al. (2009)). In this context, th rec nt development of tough, ultrahigh strength steels has shown a great potential in the field of high-end equipment manufacturing, thereby fulfilling the demands of lightweight engineeri g and performance upgrade. How ver, v y high cycle fatigue ( HC ) fa lure has become a key issu because of the nherent low defect tolerance of high/ultrahigh-strength steels (Nakajima et al. (2010), Ni et al. (2013)). In these steels, cracks tend to initiate at interior inclusions in the VHCF regime, even at a stress level below the conventional endurance limit. Hence, to ensure the long-term stability and safety of many engineering components and structures m nufactured from t ese advan ed ultrahigh-strengt steels, a thorough evaluation of their VHCF properties is of utmost importanc . Moreover, the fr cture surface of a failed VHCF sample often shows a unique fine-granular appearing area at the site of crack nucleation at the interior inclusion. Such a unique feature has attracted the attention of many researcher to investigate the associ ted VHCF failure micro-mechanism/s. In recent years, a significant research effort has be n direct toward the development of advanced high strength steels with excellent combinations of strength, toughness and ductility. The novel processing route of direct quenchin and partitioning (DQ&P) is now a well-established process imparting a significant balance of ultrahigh-strength and reasonable ductility in advanced high-strength steels, besides imparting excellent low temperature toughness (Ghosh et al. (2021), Ghosh et al. (2022)). In this process, small fractions of austenite are partially or fully stabilized d wn to room temper ture, often divide as thin interlath films and/or tiny pools in the martensitic matrix (Speer et al. (2003), Gh sh et al. (2022)). Whilst the martensitic matrix provi es the required high strength, a small fraction of finely divided austenite stabilized between the martensitic laths is expected to provide desired uniform elongati and work hardening characteristics via the transformation induced plasticity effec . In t is paper, investigations were carried out to understand the micro-mechanisms associ ted with crack initiation in the VHCF regime for a DQ&P processed 0.4 wt.% C steel using the ultrasonic-fatigue testing technique. The specimens were subjected to VHCF testing (19 kHz) at a stress ratio of R = − 1 and thoroughly investigated in respect of fractographic features using a field emission sc ning electron microsc pe (FE-SEM) to identify the crack initiation sites and determine the cause of VHCF failure. An account of the microstructural characteristics of the crack initiation region, analysed using FE-SEM and transmission electron microscopy (TEM), along with ssociated micro-mechanisms is presented in this paper. 2. Experi ent Details 2.1. Test material In this study, a 0.4 wt.% carbon (C) steel containing 0.75 wt.% silicon (Si) was designed along with different contents of manganese (Mn), chromium (Cr), and nickel (Ni). A 70-kg vacuum-cast steel ingot of this steel was procured from OCAS, Belgium. The final composition details of the steel are presented in Table 1. Table 1: Chemical composition (wt. %) of the experimental steel. C Si Mn Al Cr Ni 0.41 0.68 2.04 0.03 0.99 0.49 2.2. Material processing A novel processing ro te involving multiple passes of thermome anically controlled rolling and subsequent direct quenching and partitioning (DQ&P) treatment was employed to impart ultrahigh-strength level in th steel alongside reasonable ductility and toughness. Fig. 2a represents a schematic presentation of the designed DQ&P schedule. The first thr e hot rolling passes were conducted well above the no-recrystallization t mperature ( T nr ) with ~0.2 strain in each pass. The second stage comprised of thr e cont oll d rolling passes in the no recrystallisation regime (b low the T nr temperature). The finish rolling temperature was chosen well the above the A r3 t mperature to avoid any strain-i duced ferrite formation. Immediately after the fin sh rolling pass, the rolled plate with the final thickness of about ~15 mm was quenched in a tank of water close to the desired quench stop temperature ( T Q = 150  C) and then subjected to partitioning treatment by quickly inserting the sample in a Ghosh et al. / Structural Integrity Procedia 00 (2022) 000 – 000 1. I troduction Mechanical structures and components like those employed in offshore applicati s, bridges, rails, railway wh els, engine components, load bearing parts of automobiles, transportations systems etc., often must endure long ervice lives, equivalent to 10 8 -10 10 loading cycles in fatigue and sometimes even beyond their original design lives taking to account both environment l as well as economic considerations. This has led to a growing inter st n fatigue behaviour of st uc ura mat rials under very high cycle loading conditions to ensure their long-term safety aspects (Sakai et al. (2009)). In this c ntext, th recent developmen of tough, ultrahigh str gth steel has shown a great potential in the field of high-end equipm t manufacturing, thereby fulfilling the demands of lightweight engi eri g and p rforman e upgrade. How ver, ve y high cycle fatigu ( H ) fa lure has become a key issu because of the inherent low d fect tolerance of high/ultrahigh-strength steels (Nakajima et al. (2010), Ni t al. (2013)). In these steels, cracks tend to initiate at interior inclusions n the VHCF r gime, even at a stress level below the conventional endura ce limit. Henc , to ensure the long-term st bility and safety of many engine ring components and structures manufactured from these advanced ultrahigh-strength steels, a thorough evaluation of th ir VHCF properties is f utmost importanc . Moreover, the fracture surfa e of a failed VHCF sample often shows a unique fine-granular appearin area t the site of crack nucleati n at the interi r inclusion. Such a u ique feature h s attracted the attention of many researchers to investi ate the associated VHCF failure micro-mech ism/s. In recent y ars, a signific nt research effort has been directed towa ds the development of advanc d high strength steels wi h exc llent combinations of strength, toughness nd ductility. The novel processing route of direct quenchin and partitioning (DQ&P) is n w a well-established process imparting a significant balance of ult ahigh-strength and reas able ductility in advanced high-strength steels, besides imparting excell nt low temperature toughness (Gh sh et al. (2021), G osh et al. (2022)). In this proc ss, small fractions of austenite are partially or fully stabilized down to room temperature, often divided as thin interlath films and/or tiny pools in the martensitic matrix (Speer et al. (2003), Ghosh t al. (2022)). Whilst the m tensitic matrix provides the required high strength, a small fraction of finely divided austenite stabilized between the martensitic laths is expected to provide desired uniform elon i and work hardening charac eristics via the transformation induced plasticity effec . In t is paper, investigations were carried out to understand the mic o-mechanisms associated with crack initiation in the VHCF regime for a DQ&P processed 0.4 wt.% C steel using the ultrasonic-fatigue testing technique. The specimens were subjected to VHCF te ti g (19 kHz) t a stress ratio of R = − 1 and thoroughly investigated in respect of fractographic features using a field emission sc ning electron microsc pe (FE-SEM) to identify the crack initiation sites and determine the cause of VHCF failure. An account of the microstructural characteristics of the crack initiation region, analysed using FE-SEM and transmission electron microscopy (TEM), along with ssociated micro-mechanisms is presented in this paper. 2. Experiment Details 2.1. Te material In this study, a 0.4 wt.% carbon (C) steel containing 0.75 wt.% silicon (Si) was designed along with different contents of manganese (Mn), chromium (Cr), and nickel (Ni). A 70-kg vacuum-cast steel ingot of this steel was procured fro OCAS, Belgium. The final composition details of the steel are presented in Table 1. Table 1: Chemical composition (wt. %) of the experimental steel. C Si Mn Al Cr Ni 0.41 0.68 2.04 0.03 0.99 0.49 2.2. Mat rial processing A ovel p ocessing route nvolving multiple pass s of thermomec anically controlled rolling and subsequent direct quenching and partitioning (DQ&P) treatment was employed to impart ultrahigh-strength level in th steel alongside reasonable du tility and toughness. Fig. 2a represents a schematic presentation of the designed DQ&P sch dule. T e first t r e hot r lling passes were conducted well above the no-re ry tallization t mper ture ( T nr ) with ~0.2 strain in each pass. The second stage comprised of three controlled rolling pass s in the o recrystallisation regime (below the T nr temperature). The finish rolling temperature was chosen well the above the A r3 temperature to avoid any strain-i d ced f rrite formation. Immediately after the fin sh rolling pass, the rolled plate with the final thickness of about ~15 mm was quenched in a tank of water close to the desired quench stop Ghosh et al. / Structural Integrity Procedia 00 (2022) 000 – 000 1. Introduction Mechanical structures and components like those employ d n offshore applications, bridges, rails, railway wh els, engine components, load beari g parts of automobiles, transportations systems etc., often must endure long service lives, equivalent to 10 8 -10 10 loadi g cycles in fatigue and sometimes ven beyond their original design lives taking i to account both environmental as well as economic consid ra i ns. This has led to a growing interest in fatigue behaviour of structura materials under very high cycle loading conditions to ensur their long-t rm safety aspects (Sakai et al. (2009)). In this context, th recent development of tough, ultrahig - strength steels as shown a great potential in the field of high-end equipment manufacturing, thereby fulfilling th demands of lightweight engi eri g and performance upgrade. How ver, v y hig cycle fatigue ( HC ) failure has become a key issu because of the nh rent low defect t lerance of high/ultrahigh-strength steels (Nakajima t al. (2010), Ni et al. (2013)). In these steels, cracks tend to initiate at interior inclusions in the VHCF regime, even at a stress level below the conventional endura ce limit. Hence, to ensure the long-term stability and s fety of many engineering components and structures manufactured from these advan ed ultrahigh-strengt steels, a thorough evaluation of th ir VHCF properties is f utmost import nc . Moreover, the fracture surfa e of a failed VHCF sample often shows a unique fine-granular appearin area at the site f crack nucleati n at the interior inclusion. Such a unique feature has attracted the attention of many researchers to investigate the associated VHCF failure micro-mecha ism/s. In recent y ars, a s g ific nt research effort has been dir cte towards the development of advanced high strength steels with exc llent combina ons of strength, toughness and ductility. The novel processing rout f direct quenching and partitioning (DQ&P) is now a well-established process imparting a significant balance of ult ahigh-strength and r asonable ductility in advanced high-stre gth steels, besides imparting excell nt low temperature toughness (Ghosh et al. (2021), G osh et al. (2022)). In t is process, small fr ctions of austenite ar partially or fully tabilized down to room temperature, often divide as hin interlath film and/or tiny pools in the martensitic matrix (Speer et al. (2003), Ghosh et al. (2022)). Whilst the m rtensitic matrix provides the required high strength, a small fraction of finely divided austenite stabilized between the martensitic laths is expected to provide desired uniform elon and work hardening characteristics via the transf rmation induced plasticity effec . In t is paper, investigations were carried out to understand the micro-mechanisms associated with crack initiat on in the VHCF regim for a DQ&P processed 0.4 wt.% C steel using the ultrasonic-fatigue testing echnique. The specimens were subjected to VHCF testi g (19 kHz) at a stress r tio of R = − 1 and tho oughly investigated in respect of fractographic features using a field emission sc ning electron microsc pe (FE-SEM) to identify the crack initiation sites a d determine the cause of VHCF failure. An account of the microstructural characteristics of the crack initiation region, analysed using FE-SEM and transmission electron microscopy (TEM), along with ssociated micro-mechanisms is presented in this paper. 2 Experiment Details 2.1. Te t material In this study, a 0.4 wt.% carbon (C) steel containing 0.75 wt.% silicon (Si) wa designed long with different contents of anganese (Mn), chromium (Cr), and nickel (Ni). A 70-kg vacuum-cast steel ingot of this steel was procured from OCAS, Belgium. The final composition details of the steel are presented in Table 1. Table 1: Chemical composition (wt. %) of the experimental steel. C Si Mn Al Cr Ni 0.41 0.68 2.04 0.03 0.99 0.49 2.2. Material processing A novel processing route involving multiple passes of therm me anically controlled rolling and subsequ nt direct quenching and partitioning (DQ&P) treatment was employed to impart ultrahigh-strength level in th steel alongside reasonable ductility and toughness. Fig. 2a represents a schematic presentation of the designed DQ&P schedule. The first t r e hot rolling passes were conducted well above the no-re rystallization t mperature ( T nr ) with ~0.2 strain in each pass. The second stage comprised of thr e cont olled rolling passes in the no recrystallisation regime (b low the T nr temperatur ). The finish rolling temperature was chosen well the above the A r3 temperature to avoid any strain-i duced ferrite formation. Immediately after the finish rolling pass, the rolled plate wi h the final thickness of about ~15 mm was quenched in a tank of water close to the desired quench stop Sumit Ghosh et al. / Procedia Structural Integrity 42 (2022) 919–926

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