Plastic deformation and fracture behaviors of nitrogen-alloyed austenitic stainless steels

The plastic deformation and fracture behaviors of two nitrogen-alloyed austenitic stainless steels, 316LN and a high nitrogen steel (Fe–Cr–Mn–0.66% N), were investigated by tensile test and Charpy impact test in a temperature range from 77 to 293 K. The Fe–Cr–Mn–N steel showed ductile-to-brittle transition (DBT) behavior, but not for the 316LN steel. X-ray diffraction (XRD) confirmed that the strain-induced martensite occurred in the 316LN steel, but no such transformation in the Fe–Cr–Mn–N steel. Tensile tests showed that the temperature dependences of the yield strength for the two steels were almost the same. The ultimate tensile strength of the Fe–Cr–Mn–N steel displayed less significant temperature dependence than that of the 316LN steel. The strain-hardening exponent increased for the 316LN steel, but decreased for the Fe–Cr–Mn–N steel, with decreasing temperature. Based on the experimental results and the analyses, a modified scheme was proposed to explain the fracture behaviors of austenitic stainless steels.     

Modeling and experimental study of long term creep damage in austenitic stainless steels

One of the main challenges for some reactors components in austenitic stainless steels at high temperature in-service conditions is the demonstration of their behavior up to 60 years. The creep lifetimes of these stainless steels require on the one hand to carry out very long term creep tests and on the other hand to understand and to model the damage mechanisms in order to propose physically-based predictions toward 60 years of service. Different batches of austenitic stainless steels like-type 316L with low carbon and closely specified nitrogen content, 316L(N), are subjected to numerous creep tests carried out at various stresses and temperatures between 525 °C and 700 °C up to nearly 50 • 10³ h.Interrupted creep tests show an acceleration of the creep deformation only during the last 15% of creep lifetime, which corresponds to macroscopic necking. The modeling of necking using the Norton viscoplastic power-law allows lifetime predictions in fair agreement with experimental data up to a transition time of about ten thousand hours which is temperature dependent. In fact, one experimental result together with literature ones, shows that the extrapolation of the ‘stress–lifetime’ curves obtained at high stress data leads to large overestimations of lifetimes at low stress. After FEG–SEM observations, these overestimates are mainly due to additional intergranular cavitation as often observed in many metallic materials in the long term creep regime. The modeling of cavity growth by vacancy diffusion along grain boundaries coupled with continuous nucleation proposed by Riedel is carried out. For each specimen, ten FEG–SEM images (about 250 observed grains) are analyzed to determine the rate of cavity nucleation assumed to be constant during each creep test in agreement with many literature results. This measured constant rate is the only measured parameter which is used as input of the Riedel damage model. Lifetimes for long term creep are rather fairly well evaluated by the lowest lifetime predicted by the necking model and the Riedel model predictions. This holds for experimental lifetimes up to 200,000 h and for temperatures between 525 °C and 700 °C. A transition time as well as a transition stress is defined by the intersection of the lifetime curves predicted by the necking and Riedel modelings. This corresponds to the change in damage mechanism. The scatter in lifetimes predicted by the Riedel model induced by the uncertainty of some parameter values is less than a factor of three, similar to experimental scatter. This model is also validated for various other austenitic stainless steels such as 304H, 316H, 321H (creep rupture data provided by NIMS). A transition from power-law to viscous creep deformation regime is reported in the literature at 650 °C–700 °C for steel 316H. Taking into account the low stress creep rate law, it allows us to predict lifetimes up to 200,000 h at very high temperature in fair agreement with experimental data.     

The recovery creep of niobium-stabilised austenitic stainless steels

The creep properties of niobium-stabilised stainless steels of carbon contents in the range 0.01 to 0.05% carbon can be accounted for by the general recovery theory of creep. The high stress dependencies of recovery and creep rate can be adequately explained through an internal friction stress or impedance term, retarding recovery. Measurement of this friction term by dislocation density and stress relaxation techniques provides the correct stress dependencies when applied to the modified recovery theory.     

The multiaxial creep ductility of austenitic stainless steels

Calculations of creep damage under conditions of strain control are often carried out using either a time fraction approach or a ductility exhaustion approach. In practice, calculations of creep damage are further complicated by the presence of multiaxial states of stress. In the case of the time fraction approach, there are a number of models that can be used to predict the effect of state of stress on creep rupture strength. In particular, Huddleston developed a model from data on stainless steels. The R5 procedure uses a ductility exhaustion approach to calculate creep damage and includes a model for use under triaxial states of stress. The aim of this paper is to describe the development of this model, which is based on considerations of cavity nucleation and growth and was developed from multiaxial creep data on Type 304 and 316 steels.     

Impression creep deformation behaviour of 316LN stainless steel

The impression creep deformation behaviour of 316LN SS was investigated from microstructure, substructure, microhardness and profilometry studies of the creep deformed region. Impression creep tests were conducted on 316LN SS in the temperature range of 923–973?K, at different punching stresses in the range of 472–760?MPa. The impression creep deformation was characterised by a hemispherically shaped plastic zone which developed around the indentation. The study revealed the distinct regions under the punch undergoing deformation to different extents. The deformation was found to occur predominently on (111) planes. The dislocations in the highly deformed region were well dispersed in the matrix. The size of the plastic zone was estimated to be ～1·5 times the diameter of the indenter based on the microhardness and profilometry studies. The critical spacing to be maintained between the adjacent indentations was estimated to be >5 times the diameter of indenter.     

The formation of multipoles during high-temperature creep of austenitic stainless steels

It is shown that multipole dislocation configurations can arise during power-law creep of certain austenitic stainless steels. These multipoles have been analysed in some detail for two particular steels (Alloy 800 and a modified AISI 316L) and it is suggested that they arise either during instantaneous loading or during the primary creep stage. Trace analysis has shown that the multipoles are confined to {1 1 1} planes during primary creep but are not necessarily confined to these planes during steady-state creep unless they are pinned by interstitials.     

Influence of deformation on the transformation of austenitic stainless steels

Two commercial austenitic stainless steels of type 304 and 321 were deformed by reduction in thickness at room temperature or at liquid nitrogen temperature and subsequently annealed in the temperature range 473 to 1073 K. Microstructural changes were examined by electron microscopy. The deformation product in both the steels are different. An attempt has been made to correlate the mechanical properties with the microstructural changes.     

Effects of nitrogen on hydrogen embrittlement in AlSl type 316, 321 and 347 austenitic stainless steels

Hydrogen embrittlement of AlSl type 316, 321 and 347 stainless steels with nitrogen alloying has been studied by a tensile test through cathodic charging. The results show that addition of nitrogen improved resistance to hydrogen cracking regardless of the failure mode. Fracture surfaces of cathodically charged steels showed intergranular brittle zones on each side of the fracture surfaces. AlSl type 316 with nitrogen alloying stainless steel is more resistant to hydrogen embrittlement than AlSl type 321 with nitrogen alloying steel, whereas AlSl type 347 with nitrogen alloying steel is susceptible to hydrogen embrittlement. Nitrogen alloying of stainless steel increased the mechanical properties in hydrogen environments by increasing the stability of austenite.     

Nanoindentation creep deformation behaviour of high nitrogen nickel-free austenitic stainless steel

Nanoindentation tests of the high nitrogen nickel-free austenitic stainless steel (HNS) were performed with peak load in a wide range of 100–600?mN to investigate the nanoindentation creep deformation behaviours. The results of the nanoindentation creep tests have demonstrated that the load plateaus, creep strain rate and creep stress of the cold-rolled HNS are larger and its creep stress exponent is smaller than the solution-treated HNS. The analysis reveals that the obvious creep deformation behaviour in the cold-rolled HNS arises from the rapidly relaxed dislocation structures in the initial transition regime, while the small creep deformation behaviour of the solution-treatedHNS is mainly attributed to that the stable dislocation structures for the intensive interactions between dislocations.     

Effects of deformation mode on surface roughening of austenitic stainless steels

Abstract

Free surface roughening of polycrystalline austenitic stainless steels during tensile and deep drawing deformation is investigated using scanning white light interferometry. A fractal analysis method is applied to analyse the surface profile data in two directions which are perpendicular to each other. It is shown that the surface profile is fractal within the length and is on the same order of magnitude as the grain size. The Hurst exponent H of the two deformation modes in different directions stabilises at H≈0·85. While the correlation lengths are different, the value in the longitudinal direction is about two times larger than the one in the transverse direction. The relationship between root mean square roughness and strain appears to be linear, and the roughening rate in tensile deformation is larger than the one in deep drawing deformation.     

Study on creep deformation mechanism of 316LN stainless steel from impression creep tests

Abstract

Impression creep tests were carried out on 316LN stainless steel (SS) at various temperatures in the range 898–973 K. The stress dependence of the steady state impression velocity followed the power-law with stress exponent n?=?6. The temperature dependence of the steady state impression velocity obeyed Arrhenius type rate equation. The apparent activation energy for creep deformation (Q_c) was estimated to be 500 kJ mol^?1. Based on the n and Q_c values, it is concluded that the rate controlling mechanism is dislocation creep.     

Tensile ductility and deformation mechanisms of a nanotwinned 316L austenitic stainless steel

A nanotwinned 316 L austenitic stainless steel was prepared by means of surface mechanical grinding treatment.After recovery annealing,the density of dislocations decreases obviously while the average twin/matrix lamella thickness still keeps in the nanometer scale.The annealed nanotwinned sample exhibits a high tensile yield strength of 771 MPa and a considerate uniform elongation of 8%.TEM observations showed that accommodating more dislocations and secondary twinning inside the nanotwins contribute to the enhanced ductility and work hardening rate of the annealed nanotwinned sample.     

Microstructural evaluation of fatigue damage in SA533-B1 and type 316L stainless steels

Transmission electron microscopy (TEM) examinations were made on fatigued SA533-B1 low alloy steel and Type 316L stainless steel specimens with the intention to investigate the mis-orientation changes among dislocation cells and the evolution of dislocation structures. Contrary to what might be expected for the cell structures, no clear relationship between fatigue damage and the mis-orientation changes of cell walls (or subgrain boundaries) was found in the fatigued samples of SA533-B1 steel (a bcc structure); however, significant changes of dislocation structures were observed in the fatigued samples of Type 316L stainless steel (an fcc structure). This could be accounted for by their different structures as well as complicated defect structures such as subgrain boundaries, small carbides, and dislocations inhomogeneously distributed in the SA533-B1 steel. It is interesting to note that at room temperature dislocations of fatigued SS316L specimens were observed to arrange themselves on {111} slip planes, in contrast, at 300°C the dislocations tend to move from their slip planes into subgrain boundaries in the surface layers rather than in the cross sectional layers.