Monitoring fatigue degradation in austenitic stainless steels

Abstract

A B S T R A C T During cyclic loading of austenitic stainless steel, microstructural changes occurred, which affected both mechanical and physical properties. For certain steels, a strain-induced martensitic phase transformation was observed. The investigations showed that for the given material and loading conditions the volume fraction of martensite depended on the cycle number, temperature and initial material state. It was found that the martensite content continuously increased with the cycle number. Therefore, the volume fraction of martensite was used for indication of the fatigue usage. The temperature dependence of the martensite formation was described by a Boltzmann function. The martensite content decreased with increasing temperature. Two different heats of the austenitic stainless steel X6CrNiTi18-10 (AISI 321, DIN 1.4541) were investigated. The martensite formation rate was much higher for the cold-worked material than for the solution-annealed one. All applied techniques, neutron diffraction and advanced magnetic methods allowed the detection of martensite in the differently fatigued specimens.Keywords austenitic stainless steel; lifetime extension; low-cycle fatigue; non-destructive testing; nuclear safety; strain-induced martensite. N O M E N C L A T U R Ea = lattice constant A, B, t o = parameters of the Boltzmann function for data analysis bcc = body centred cubic lattice B x , B z = axial and radial components of the magnetic induction c α , (c α ) o = volume fraction of martensite for the fatigued (strained) and initial material state D = usage factor, ratio of the applied cycle number to an averaged cycle number representing the technical crack initiation DMC = neutron diffractometer for cold neutrons used for determination of the volume fraction of martensite EC = eddy current fcc = face centred cubic lattice FERROMASTER ® = device for measurement of the magnetic susceptibility or permeability FLUXGATE = high sensitive magnetometer for measuring the remaining magnetic induction (remanence) GMR = Giant Magneto-Resistance sensor used for eddy current impedance measurement with high local resolution ICP-OES = inductive coupled plasma emission photometry used for the determination of the chemical composition of steels LCF = low-cycle fatigue M d = starting temperature for the strain-induced martensite formation

Evaluation of the strain-induced martensitic transformation by acoustic emission monitoring in 304L austenitic stainless steel: Identification of the AE signature of the martensitic transformation and power-law statistics

“…In this sample, almost all transformation is completed gradually by small events. This result is in agreement with previous works [1,11,14]. Kalkhof showed that for the given material and loading conditions the volume fraction of martensite depended on the cycle number.…”

“…Since the displacive MT modifies the volume (with ␥ → ␣ martensite a volume expansion occurs [2]), internal stresses are generated that can affect the lifetime of components subjected to fatigue. Thus the monitoring of MT is of societal interest and many non-destructive techniques have been applied, among which eddy current testing [13,14] and acoustic emission [15,16]. The aim of this study is to investigate MT by AE and MT in 304L steel during fatigue tests, using two kinds of analysis: pattern recognition and power-laws statistics, which we shortly describe hereunder.…”

Comparative study on biaxial low‐cycle fatigue behaviour of three structural steels

“…From the comparative studies in this paper, it is shown that the AISI 303 stainless steel undergoes a significant additional hardening under non‐proportional loading. Through a close comparison with the cyclic behaviour of AISI 304 stainless steel, 4–6,19,20 which has a similar increase in hardening, it is suggested that this occurs because of the gamma to alpha prime phase transformation. AISI 303 stainless steel is likely to be unstable under low‐cycle fatigue and some gamma to alpha prime phase transformation may take place under cyclic loading.…”

“…Among the structural steels, stainless steels have very special cyclic behaviour and microstructural changes under cyclic loading, which affect both mechanical and physical properties. In the literature, several researchers (for example, McDowell et al , 4 Kalkhof et al , 5 Chen et al 6 ) published research data about the transformation of gamma to alpha prime phase and additional hardening of AISI 304 stainless steel. But no research paper was found for AISI 303 stainless steel with a higher carbon content, to the knowledge of the authors.…”

Sensitivity of the magnetization curves of different austenitic stainless tube and pipe steels to mechanical fatigue

“…The corresponding changes of magnetic properties can be detected by measuring the eddy current impedance, the magnetic permeability and the remanence field. These methods have been extensively investigated by several research groups in the last decade [5][6][7][8][9][10][11][12][13]. It was shown that in meta-stable ASS a 0 -formation can be used as an indication of the fatigue state, if the conditions listed below are fulfilled [4]:…”