Short crack growth during low cycle fatigue in a Duplex Stainless Steel

M. BALBI, ; H. KNOBBE; S. HEREÑU; M. KÜBBELER; M.C. MARINELLI; U. KRUPP; I. ALVAREZ

Simposio; International Symposium on Fatigue Design & Material Defects Norwegian University of Science and Technology NTNU; 2011

Norwegian University of Science and Technology, NTNU

 Few investigations have examined the evolution of the surface damage before microcracking be-gins [1,2]. Recently, the kinetics of microcrack growth during cycling has been studied in aged standard DSS. In this case, microtwins on the ferrite phase are responsible for crack initiation [3 The materials studied in this investigation are the duplex steel 1.4462 (German designation) and the superduplex stainless steel type S32750 manufactured in the form of hot-rolled and solution annealed (as-received) cylindrical bars. The materials were studied in two different thermal condi-tions: AR and aged 475¨¬C for 100 h. When comparing hardness values in these conditions in the ferrite phase, a substantial increase in the aged samples can be evidenced due to the spinodal de-composition: 470 HV against 350HV in the AR material. The ¥ã phases do not change; i.e 350HV in both conditions. Cylindrical and cylindrical slightly shallow-notched specimens were manufactured and polished in the test section to achieve a perfectly smooth surface The notch focuses the fatigue damage in the zone of observation, (stress concentration factor is 1.06 at the notch tip). Mechanical and electrolytical polish was given to the surface of the shallow notch to improve the observation of micro-crack nucleation and growth. The central part of the notch was monitored during the test us-ing a powerful optical system consisting of a CCD camera with and objective of 50X, ¡¾ 1¥ìm FD and 13mm WD and a 12X ultra zoom. Another part of the specimens were used for rotating bending testing to study the effect of the embrittling heat treatment on the fatigue limit of the duplex steel. Fatigue life has been earlier measured on standard smooth cylindrical specimens in the AR and in the aged conditions subjected to constant plastic strain range of 0.3%. The fatigue life in both conditions is very close with an uncertainty of 10% [4]. The microcrack nucleation and propagation has been studied in the shallow-notched specimens in both thermal conditions. In the aged material microcracks nucleate at slip markings in the ferritic phase.  Few investigations have examined the evolution of the surface damage before microcracking be-gins [1,2]. Recently, the kinetics of microcrack growth during cycling has been studied in aged standard DSS. In this case, microtwins on the ferrite phase are responsible for crack initiation [3 The materials studied in this investigation are the duplex steel 1.4462 (German designation) and the superduplex stainless steel type S32750 manufactured in the form of hot-rolled and solution annealed (as-received) cylindrical bars. The materials were studied in two different thermal condi-tions: AR and aged 475¨¬C for 100 h. When comparing hardness values in these conditions in the ferrite phase, a substantial increase in the aged samples can be evidenced due to the spinodal de-composition: 470 HV against 350HV in the AR material. The ¥ã phases do not change; i.e 350HV in both conditions. Cylindrical and cylindrical slightly shallow-notched specimens were manufactured and polished in the test section to achieve a perfectly smooth surface The notch focuses the fatigue damage in the zone of observation, (stress concentration factor is 1.06 at the notch tip). Mechanical and electrolytical polish was given to the surface of the shallow notch to improve the observation of micro-crack nucleation and growth. The central part of the notch was monitored during the test us-ing a powerful optical system consisting of a CCD camera with and objective of 50X, ¡¾ 1¥ìm FD and 13mm WD and a 12X ultra zoom. Another part of the specimens were used for rotating bending testing to study the effect of the embrittling heat treatment on the fatigue limit of the duplex steel. Fatigue life has been earlier measured on standard smooth cylindrical specimens in the AR and in the aged conditions subjected to constant plastic strain range of 0.3%. The fatigue life in both conditions is very close with an uncertainty of 10% [4]. The microcrack nucleation and propagation has been studied in the shallow-notched specimens in both thermal conditions. In the aged material microcracks nucleate at slip markings in the ferritic phase.  Few investigations have examined the evolution of the surface damage before microcracking be-gins [1,2]. Recently, the kinetics of microcrack growth during cycling has been studied in aged standard DSS. In this case, microtwins on the ferrite phase are responsible for crack initiation [3 The materials studied in this investigation are the duplex steel 1.4462 (German designation) and the superduplex stainless steel type S32750 manufactured in the form of hot-rolled and solution annealed (as-received) cylindrical bars. The materials were studied in two different thermal condi-tions: AR and aged 475¨¬C for 100 h. When comparing hardness values in these conditions in the ferrite phase, a substantial increase in the aged samples can be evidenced due to the spinodal de-composition: 470 HV against 350HV in the AR material. The ¥ã phases do not change; i.e 350HV in both conditions. Cylindrical and cylindrical slightly shallow-notched specimens were manufactured and polished in the test section to achieve a perfectly smooth surface The notch focuses the fatigue damage in the zone of observation, (stress concentration factor is 1.06 at the notch tip). Mechanical and electrolytical polish was given to the surface of the shallow notch to improve the observation of micro-crack nucleation and growth. The central part of the notch was monitored during the test us-ing a powerful optical system consisting of a CCD camera with and objective of 50X, ¡¾ 1¥ìm FD and 13mm WD and a 12X ultra zoom. Another part of the specimens were used for rotating bending testing to study the effect of the embrittling heat treatment on the fatigue limit of the duplex steel. Fatigue life has been earlier measured on standard smooth cylindrical specimens in the AR and in the aged conditions subjected to constant plastic strain range of 0.3%. The fatigue life in both conditions is very close with an uncertainty of 10% [4]. The microcrack nucleation and propagation has been studied in the shallow-notched specimens in both thermal conditions. In the aged material microcracks nucleate at slip markings in the ferritic phase.