Plastic Deformation of Duplex Stainless Steel

Doktorsavhandling, 1995

Duplex austenitic/ferritic stainless steels are increasingly used as an alternative to conventional austenitic grades. Main advantages of the duplex grades are good resistance to corrosion cracking and corrosion fatigue in environments that contain chloride. One drawback of the duplex material is the susceptibility of the ferritic phase to a spinodal decomposition in the temperature range 250-500oC. The decomposition of the ferrite leads to a significant hardening and embrittlement of this phase. This thesis treats the deformation properties of duplex stainless steel and particular attention is paid to the influence of the spinodal decomposition of ferrite. Evaluation of cyclic and monotonic deformation behaviour, fatigue crack growth and low cycle fatigue properties and impact toughness is included. Duplex stainless steels with different amounts of austenite and ferrite were studied and compared to single-phase austenitic and ferritic stainless steels. By including single-phase materials, understanding of the properties of the duplex stainless steel was facilitated. The monotonic yield stress of the investigated steels clearly deviate from the classic linear law of mixture, a law predicting a linear change in yield stress with increasing ferrite content. The explanation for the deviation is partly to be found in the presence of internal stresses, allowing early yielding of the duplex grades. A marked anisotropy of the mechanical properties is evident from the tensile tests. Further, annealing at 475oC that hardens the ferritic phase leaves the austenite unaffected. The results from the tensile tests are discussed in terms of phase and grain distribution, contiguity and load/strain transfer. The influence of grain size and nitrogen content has been studied on two austenitic stainless steels. The grain size dependence of the yield stress was found to obey the well-known Hall-Petch relation. The cyclic yield stress of the investigated steels follow the classic linear law of mixture, i.e. vary linearly with increasing ferrite content. The austenite cyclically softens at low plastic strain amplitudes and hardens at higher. The duplex and ferritic steels show no or only little cyclic hardening/softening. Low cycle fatigue properties of the austenitic single-phase steel and a duplex steel having 25.alfa./75.gamma. have been determined. The austenitic grade shows better fatigue properties at high plastic strain amplitudes as compared to the duplex grade. It was found, in conjunction with earlier observations in the literature, that cracks initiated in the ferritic phase of the duplex material at high strains. Near-threshold crack growth properties were investigated, including crack closure measurements and quantitative fractography of the fracture surfaces. The duplex steels exhibit higher effective crack growth threshold levels .DELTA.Kth,eff, while the single-phase materials have higher crack closure levels Kcl. These results are discussed in terms of phase boundary interaction and fracture surface roughness. The monotonic and cyclic plastic zones accompanying fatigue cracks in austenite and ferrite were measured by stylus profilometry (monotonic zone) and nanoindenter technique (cyclic zone). The measured plastic zone sizes were rationalised by using cyclic stress-strain data of the bulk material and models on strain distribution ahead of a fatigue crack. A duplex grade with 25.alfa./75.gamma. showed a technically useful toughness still after 100 hours of annealing at 475oC. This indicates that a careful choice of the phase balance of duplex stainless steel may allow use at higher temperaturesthan 300oC.