Anna Kisko - Academia.edu (original) (raw)
Papers by Anna Kisko
In this thesis, the influence of reversion and recrystallization annealing on microstructure and ... more In this thesis, the influence of reversion and recrystallization annealing on microstructure and mechanical properties was studied in metastable austenitic low-Ni high-Mn stainless steels, some alloyed with up to 0.45 wt.% Nb. Further, the effect of the various microstructures created by reversion and recrystallization on strain-induced martensite transformation in tensile testing was investigated. The aim was to achieve excellent combinations of strength and ductility in the steels and to improve understanding of the behaviour of ultrafine-grained austenitic stainless steels during deformation. All the steels were cold-rolled up to 60% thickness reduction producing up to 60% strain-induced α’-martensite in the austenitic structure. Annealing was carried out using a Gleeble thermomechanical simulator between 450–1100 °C for durations of 0.1–1000 s. The resultant microstructures were examined using different research equipment and methods. Regardless of the amount of Nb alloying, she...
Materials Science and Engineering: A, 2017
The effect of hydrogen on the fracture and impact toughness of ultra-high-strength steels at sub-... more The effect of hydrogen on the fracture and impact toughness of ultra-high-strength steels at sub-zero temperatures in the transition temperature region has been investigated with arctic applications in mind. Two types of as-quenched microstructure were studied, i.e. autotempered martensite and a mixture of martensite and bainite, both having yield strengths close to 1000 MPa. These were charged with hydrogen using passive cathodic protection and then tested in both the charged and uncharged condition at sub-zero temperatures. Hydrogen contents were measured with meltextraction. Fractography, kernel average misorientation measurements and cohesive zone modelling were used to analyse the results considering the degree and the active mechanisms of hydrogen embrittlement. It is shown that hydrogen embrittlement is present at sub-zero temperatures, causing an increase in fracture toughness reference temperature T 0 and a small decrease in deformation capability. The relationship between the T 0 and the impact toughness transition temperature T 28J , which, in the case of ultra-high-strength steel, deviates from that observed for lower strength steels, is proposed to be affected by the hydrogen content.
Materials Science Forum, 2013
The influences of the heating rate and annealing duration on martensite formation and its reversi... more The influences of the heating rate and annealing duration on martensite formation and its reversion to austenite have been investigated in a 60% cold-rolled metastable high-manganese austenitic Type 204Cu stainless steel. A Gleeble 3800 thermomechanical simulator was used for dilatometric measurements. Cold-rolled steel pieces were either heated up to 1000 °C at various heating rates between 5 °C/s and 150 °C/s followed by quenching to room temperature, or heated and held at temperatures in the range of 450 620 °C for different durations between 0.1 600 s. In heating experiments, dilatation curves revealed an expansion of a specimen starting around 550 °C followed by contraction, both processes depending on the heating rate. These dimensional changes could be correlated to the formation and reversion of a ferromagnetic phase, α-martensite. Some martensite was also formed during isothermal holding in connection with tempering of the pre-existing α-martensite before the following reve...
Metals, 2020
Strength properties of annealed austenitic stainless steels are relatively low and therefore impr... more Strength properties of annealed austenitic stainless steels are relatively low and therefore improvements are desired for constructional applications. The reversion of deformation induced martensite to fine-grained austenite has been found to be an efficient method to increase significantly the yield strength of metastable austenitic stainless steels without impairing much their ductility. Research has been conducted during thirty years in many research groups so that the features of the reversion process and enhanced properties are reported in numerous papers. This review covers the main variables and phenomena during the reversion processing and lists the static and dynamic mechanical properties obtained in laboratory experiments, highlighting them to exceed those of temper rolled sheets. Moreover, formability, weldability and corrosion resistant aspects are discussed and finally the advantage of refined grain structure for medical applications is stated. The reversion process has...
Materials Science Forum, 2012
ABSTRACT The annealing behavior of cold rolled Type 430 ferritic stainless steel is the subject o... more ABSTRACT The annealing behavior of cold rolled Type 430 ferritic stainless steel is the subject of this paper. The steel was cold rolled 79%, then heated at 6 °C/s to the soaking temperature of 841 °C, which is just below the Ae1 temperature. During heating, specimens were quenched from selected temperatures between 650 and 841 °C and after various times at 841 °C. These quenched samples underwent metallographic examination and micro-hardness determination. The results indicated that under the prevailing experimental conditions, the hardness appeared to correlate strongly with the extent of recrystallization. The kinetics of recrystallization appeared to originate in the cold worked state, where three kinds of grain were found: (i) smooth elongated, featureless of α-fiber orientation {001}<100>; (ii) irregular fishbone grains of the γ-fiber orientations {111}<112> plus {111}<110>; and (iii) twisted grains of the η-fiber orientation {001}<100>. It was found that the twisted grains of the η-fiber were the first to recrystallize, with the fishbone grains of the γ-fiber second, and the smooth elongated, featureless grains of the α-fiber last. It was found that the grains of the α-fiber orientation {001}<100> and the η-fiber orientation {001}<100> were replaced with grains of the γ-fiber orientations as recrystallization progressed. These results are discussed in terms of recrystallization and texture development.
Materials Science Forum, 2013
The influences of the heating rate and annealing duration on martensite formation and its reversi... more The influences of the heating rate and annealing duration on martensite formation and its reversion to austenite have been investigated in a 60% cold-rolled metastable high-manganese austenitic Type 204Cu stainless steel. A Gleeble 3800 thermomechanical simulator was used for dilatometric measurements. Cold-rolled steel pieces were either heated up to 1000 °C at various heating rates between 5 °C/s and 150 °C/s followed by quenching to room temperature, or heated and held at temperatures in the range of 450 -620 °C for different durations between 0.1 -600 s. In heating experiments, dilatation curves revealed an expansion of a specimen starting around 550 °C followed by contraction, both processes depending on the heating rate. These dimensional changes could be correlated to the formation and reversion of a ferromagnetic phase, α'-martensite. Some martensite was also formed during isothermal holding in connection with tempering of the preexisting α'-martensite before the following reversion, as established by magnetic measurements. Tempering of martensite was revealed by microhardness behaviour, X-ray diffraction analysis and transmission electron microscopy.
In this thesis, the influence of reversion and recrystallization annealing on microstructure and ... more In this thesis, the influence of reversion and recrystallization annealing on microstructure and mechanical properties was studied in metastable austenitic low-Ni high-Mn stainless steels, some alloyed with up to 0.45 wt.% Nb. Further, the effect of the various microstructures created by reversion and recrystallization on strain-induced martensite transformation in tensile testing was investigated. The aim was to achieve excellent combinations of strength and ductility in the steels and to improve understanding of the behaviour of ultrafine-grained austenitic stainless steels during deformation. All the steels were cold-rolled up to 60% thickness reduction producing up to 60% strain-induced α’-martensite in the austenitic structure. Annealing was carried out using a Gleeble thermomechanical simulator between 450–1100 °C for durations of 0.1–1000 s. The resultant microstructures were examined using different research equipment and methods. Regardless of the amount of Nb alloying, she...
Materials Science and Engineering: A, 2017
The effect of hydrogen on the fracture and impact toughness of ultra-high-strength steels at sub-... more The effect of hydrogen on the fracture and impact toughness of ultra-high-strength steels at sub-zero temperatures in the transition temperature region has been investigated with arctic applications in mind. Two types of as-quenched microstructure were studied, i.e. autotempered martensite and a mixture of martensite and bainite, both having yield strengths close to 1000 MPa. These were charged with hydrogen using passive cathodic protection and then tested in both the charged and uncharged condition at sub-zero temperatures. Hydrogen contents were measured with meltextraction. Fractography, kernel average misorientation measurements and cohesive zone modelling were used to analyse the results considering the degree and the active mechanisms of hydrogen embrittlement. It is shown that hydrogen embrittlement is present at sub-zero temperatures, causing an increase in fracture toughness reference temperature T 0 and a small decrease in deformation capability. The relationship between the T 0 and the impact toughness transition temperature T 28J , which, in the case of ultra-high-strength steel, deviates from that observed for lower strength steels, is proposed to be affected by the hydrogen content.
Materials Science Forum, 2013
The influences of the heating rate and annealing duration on martensite formation and its reversi... more The influences of the heating rate and annealing duration on martensite formation and its reversion to austenite have been investigated in a 60% cold-rolled metastable high-manganese austenitic Type 204Cu stainless steel. A Gleeble 3800 thermomechanical simulator was used for dilatometric measurements. Cold-rolled steel pieces were either heated up to 1000 °C at various heating rates between 5 °C/s and 150 °C/s followed by quenching to room temperature, or heated and held at temperatures in the range of 450 620 °C for different durations between 0.1 600 s. In heating experiments, dilatation curves revealed an expansion of a specimen starting around 550 °C followed by contraction, both processes depending on the heating rate. These dimensional changes could be correlated to the formation and reversion of a ferromagnetic phase, α-martensite. Some martensite was also formed during isothermal holding in connection with tempering of the pre-existing α-martensite before the following reve...
Metals, 2020
Strength properties of annealed austenitic stainless steels are relatively low and therefore impr... more Strength properties of annealed austenitic stainless steels are relatively low and therefore improvements are desired for constructional applications. The reversion of deformation induced martensite to fine-grained austenite has been found to be an efficient method to increase significantly the yield strength of metastable austenitic stainless steels without impairing much their ductility. Research has been conducted during thirty years in many research groups so that the features of the reversion process and enhanced properties are reported in numerous papers. This review covers the main variables and phenomena during the reversion processing and lists the static and dynamic mechanical properties obtained in laboratory experiments, highlighting them to exceed those of temper rolled sheets. Moreover, formability, weldability and corrosion resistant aspects are discussed and finally the advantage of refined grain structure for medical applications is stated. The reversion process has...
Materials Science Forum, 2012
ABSTRACT The annealing behavior of cold rolled Type 430 ferritic stainless steel is the subject o... more ABSTRACT The annealing behavior of cold rolled Type 430 ferritic stainless steel is the subject of this paper. The steel was cold rolled 79%, then heated at 6 °C/s to the soaking temperature of 841 °C, which is just below the Ae1 temperature. During heating, specimens were quenched from selected temperatures between 650 and 841 °C and after various times at 841 °C. These quenched samples underwent metallographic examination and micro-hardness determination. The results indicated that under the prevailing experimental conditions, the hardness appeared to correlate strongly with the extent of recrystallization. The kinetics of recrystallization appeared to originate in the cold worked state, where three kinds of grain were found: (i) smooth elongated, featureless of α-fiber orientation {001}<100>; (ii) irregular fishbone grains of the γ-fiber orientations {111}<112> plus {111}<110>; and (iii) twisted grains of the η-fiber orientation {001}<100>. It was found that the twisted grains of the η-fiber were the first to recrystallize, with the fishbone grains of the γ-fiber second, and the smooth elongated, featureless grains of the α-fiber last. It was found that the grains of the α-fiber orientation {001}<100> and the η-fiber orientation {001}<100> were replaced with grains of the γ-fiber orientations as recrystallization progressed. These results are discussed in terms of recrystallization and texture development.
Materials Science Forum, 2013
The influences of the heating rate and annealing duration on martensite formation and its reversi... more The influences of the heating rate and annealing duration on martensite formation and its reversion to austenite have been investigated in a 60% cold-rolled metastable high-manganese austenitic Type 204Cu stainless steel. A Gleeble 3800 thermomechanical simulator was used for dilatometric measurements. Cold-rolled steel pieces were either heated up to 1000 °C at various heating rates between 5 °C/s and 150 °C/s followed by quenching to room temperature, or heated and held at temperatures in the range of 450 -620 °C for different durations between 0.1 -600 s. In heating experiments, dilatation curves revealed an expansion of a specimen starting around 550 °C followed by contraction, both processes depending on the heating rate. These dimensional changes could be correlated to the formation and reversion of a ferromagnetic phase, α'-martensite. Some martensite was also formed during isothermal holding in connection with tempering of the preexisting α'-martensite before the following reversion, as established by magnetic measurements. Tempering of martensite was revealed by microhardness behaviour, X-ray diffraction analysis and transmission electron microscopy.