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.     

The precipitation of niobium carbide at grain boundaries in an austenitic stainless steel

Electron microscopy has been used to show that the precipitation of niobium carbide at the grain boundaries of an austenitic stainless steel can occur in a spatially non-random fashion. Preferential nucleation and growth of carbides occurs on grain-boundary defect structures. Where present, these defect structures include extrinsic grain-boundary dislocations and topographical discontinuities. An example of the precipitation of niobium carbide on an intrinsic dislocation array is also shown.     

一种新型核级奥氏体不锈钢焊接接头的晶间腐蚀研究

利用"不锈钢硫酸-硫酸铜腐蚀实验"、光学显微镜和扫描电镜等测试方法分别对未敏化和经敏化处理(650℃×100 h)的800H和新型Cr18Ni30Mo2Al3Nb合金焊接接头的抗晶间腐蚀性能进行研究,结果表明,800H和Cr18Ni30Mo2Al3Nb接头焊缝组织均为单一的奥氏体基体,800H合金中TiN缺陷处易引起点蚀,而Cr18Ni30Mo2Al3Nb无明显点蚀现象;对比腐蚀失重、腐蚀深度等实验结果,未敏化和敏化态Cr18Ni30Mo2Al3Nb焊接接头的抗晶间腐蚀性能明显优于相同状态的800H。     

Phase stability in austenitic stainless steel weld deposits

Abstract

Recent theories of precipitation, nucleation, and growth are discussed in relation to the phase stability of two steel weld deposit materials. The materials consist of regions of δ-ferrite surrounding austenite grains. Copious precipitation occurs in the δ-ferrite. Nucleation and growth intragranular precipitation models are shown to represent the real situation for M₂₃C₆, σ, and spherical Laves phase, whereas needle-shaped Laves phase appears to be governed by a boundary assisted growth mechanism. Needle-shapes are produced by small microcompositional changes occurring in the Laves phase. These changes cause low energy facets to form, with the resulting deviation from spherical shape and change in growth mechanism.

MST/614     

Grain-boundary structure in an austenitic stainless steel

This paper considers to what extent the grain-boundary structure in a commercial material may be understood in terms of modern structural theories of crystal boundaries. It is shown that in one particular state (partially recrystallized) 78% of the boundaries, examined by transmission electron microscopy, could be said to contain the type of structure predicted by the theoretical approach (i.e. intrinsic dislocation arrays).This paper goes on to examine not only the equilibrium component of the boundary structure (intrinsic dislocation arrays) but also looks at the perturbations created in this component by the presence of other, non-equilibrium components (such as topographical discontinuities, precipitates and extrinsic dislocations produced by the dissociation of run-in matrix dislocations).     

Effect of additions of Ti,B and Ce on microstructural stability,creep strength and creep damage in austenitic stainless steel

Abstract

Optical and transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis of bulk extracted precipitate residues were carried out on long term (more than 80 000 h) creep tested (at 1023 K) type 304 austenitic stainless steels with different levels of Ti content to assess the microstructural stability and creep strength. B and Ce were added to the steels to suppress the creep cavitation. Finer Ti(C,N) particles with higher density and narrower size distribution were observed in steels with a higher Ti content, resulting in an increase in the creep rupture strength. However, higher Ti content increased the intergranular precipitation of the σ phase on longer creep exposure, resulting in the increase in creep cavitation and in the decrease in creep rupture strength. The study indicated an optimum level of Ti and {C + (6/7)N} content with the Ti/{C + (6/7)N} ratio close to the stoichiometric value of the Ti(C,N) precipitate particles that should also be close to their solubility limit at the solution heat treatment temperature.     

Anelasticity and creep transients in an austenitic steel

The extent of the anelastic strain imposed during the creep of a Type 316 stainless steel has been examined in detail and related to the structure of the dislocations within the material. The stress dependence of the anelastic strain recovery has been used to show that the intergranular carbide particles present gave rise to a small frictional stress. The accummulation of creep strain on reloading has been examined and shown to take place only after the anelastic strain has been re-imposed.     

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.     

Predicting effect of upward temperature excursions on long-term creep of an austenitic steel

Abstract

A procedure was demonstrated for predicting the effect of upward temperature excursions on the long-term creep of a type 316 steel. The procedure exploits the existence of regimes of stable creep at 600–750°C, from which creep rates can be used to predict long-term strain accumulation or rupture, without the problems associated with the history dependence of creep rupture data for solution treated steel. Pre-aging can be used to remove the potential for transient metallurgical strengthening and ensure stable creep. The excursion tests were run under the relevant conditions and the overall creep rate determined for a few cycles, for comparison with isothermal behaviour. Creep strains caused by the 1 h excursions were consistent with stable creep at the excursion temperature. Thus, the transient increased strain rate observed after excursions must have been compensated by a transient depression of strain rate on reaching the excursion temperature.

MST/580     

Large-strain Bauschinger effect in austenitic stainless steel sheet

Large-strain Bauschinger effect in cold-rolled austenitic stainless steel sheet is investigated after large amounts of prestrain. The material is prestrained in uniaxial tension, and the tensile properties of the prestrained material are measured in different angles with respect to the prestraining direction. By comparing the differences in the yield stresses in different orientations, the effect of prestraining on material anisotropy is studied. The method is applied to AISI 304-type stainless steel sheet. The test results are analyzed using a combined isotropic–kinematic hardening model. The results indicate that this kind of material shows a considerable Bauschinger effect. Transient and permanent softening is observed in the experiments. The experimental Young's modulus also seems to decrease with prestrain.     

On the influence of deformation rate on intergranular crack propagation in Type 304 stainless steel

The influence of deformation rate on the stable intergranular crack propagation behaviour of Type 304 stainless steel, as reflected in the crack width, length, and angular orientation parameters was examined. Specimens deformed to failure in the slow tension and creep-rupture modes at 650°C were studied. The results indicate that a rapid, step-wise crack propagation between grain-boundary triple junctions does not occur for these specimens, but that the triple junctions do provide a significant barrier to crack propagation. The crack angular orientation and width, as a function of deformation rate, were concluded to be the parameters which reflect the crack growth rate for the test conditions employed in this work.     

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.     

Influence of temperature on multiaxial creep behaviour of 304HCu austenitic stainless steel

Effect of temperature on multiaxial creep behaviour of 304HCu austenitic stainless steel has been investigated. The multiaxiality was introduced by incorporating notches in smooth specimens. Creep rupture life increased with notch acuity ratio having a saturation/decline tendency. Notch strengthening increased with temperature, stress and notch sharpness. Multiaxial ductility decreased rapidly with notch sharpness and tended towards saturation. Fracture mode was found to change from transgranular ductile to intergranular creep depending on the stress, temperature and notch sharpness. Finite element analysis of notched specimens along with orientation imaging microscopic study was carried out to assess the deformation and damage at different normalised stress ratio. A temperature independent unique master plot for multiaxial rupture life as a function of stress has been established.     

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.     

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.