Steady-state creep in a 25 wt % Cr-20 wt % Ni austenitic stainless steel

Steady-state creep behaviour of a 25 wt % Cr-20 wt % Ni stainless steel without precipitates was studied in the stress range 9.8 to 39.2 MPa at temperatures between 1133 and 1193 K. The results of stress-drop tests indicate that, in the steady-state creep region, diffusion-controlled recovery creep is dominant. Such recovery creep can be accounted for in terms of the composition of the internal stress, _i=_s+_c, except in the case of fine-grained specimens where d<80 m, whered is the mean grain diameter, _s is possible to reduce easily and is comparable to the driving stress for creep, and _c is the persistent stress field due to metastable substructure. In the fine-grained specimens, it is suggested that the steady-state creep is dominantly controlled by grain boundaries.     

The structure of splat-cooled Fe-20% Cr-25% Ni austenitic steel

A controlled atmosphere splat quenching gun has been used to produce splats of Fe-20% Cr-25% Ni. Three types of structure were observed by thin foil electron microscopy, namely high-angle cellular, low-angle cellular, and linear arrays of dislocation loops, which were determined primarily by the heat transfer conditions. In the thin, most rapidly cooled areas (lift-off regions) high-angle cellular structures were observed which were largely free of defects. As the cooling rate decreases there was a greater tendency for low-angle cellular structures to form, but at intermediate cooling rates bands of dislocation loops were observed. These are explained in terms of solute segregation and vacancy coalescence along 〈100〉_gg directions in the austenite.     

The influence of antimony-additions on the creep behaviour of a 25wt% Cr-20wt% Ni austenitic stainless steel

The effect of antimony on the creep behaviour (dislocation creep) of a 25 wt% Cr-20 wt% Ni stainless steel with ~ 0.005 wt% C was studied with a view to assessing the segregation effect. The antimony content of the steel was varied up to 4000 ppm. The test temperature range was 1153 to 1193 K, the stress range, 9.8 to 49.0 MPa, and the grain-size range, 40 to 600m. The steady state creep rate, , decreases with increasing antimony content, especially in the range of intermediate grain sizes (100 to 300m). Stress drop tests were performed in the secondary creep stages and the results indicate that antimony causes dislocations in the substructure to be immobile, probably by segregating to them, reducing the driving stress for creep.Nomenclature _a Creep stress in a constant load creep test without stress-drop - _A Initial applied stress in stress-drop tests - Stress decrement - ( _A-) Applied stress after a stress decrement, - t _i Incubation time after stress drop (by the positive creep) - _C Strain-arrest stress - _i Internal stress - _{s s}-component (= _i- _c) - Steady state creep rate (average value) in a constant load creep test - Strain rate at time,t, in a constant load creep test - New steady state creep rate (average value) after stress drop from _A to ( _A-) - Strain rate at time,t, after stress drop.     

Deformation mechanisms in an austenitic stainless steel (25Cr-20Ni) at elevated temperature

The creep behaviour at elevated temperature of an austenitic stainless steel (25Cr-20Ni), both with and without antimony additions, has been reanalysed. Formerly, the creep behaviour was interpreted by considering creep mechanisms based on diffusional (Coble) creep and threshold stresses. In the present paper, it is proposed that an alternative mechanism of grain boundary sliding, accommodated by slip in grain boundary mantle regions, can in fact be used to describe more accurately the creep behaviour. Quantitative predictions, based on phenomenological equations for describing creep controlled by grain boundary sliding, are made of the influences of grain size, stress and antimony addition on creep rates, and of the influence of grain size on the activation energy for creep of 25Cr-20Ni stainless steel. Comparison of these predictions with those based on creep models incorporating only diffusional flow are made. Furthermore, the existence of a threshold stress in creep of single-phase, massive materials is strongly questioned.     

Creep of austenitic 20% Cr/35% Ni stainless steels at low stresses

The present work comprises measurements of the secondary creep-rate at different stress levels with rates between about 2×10^–5 %/h and 10%/h and the grain-boundary sliding at 700° C in two austenitic 20 wt % Cr/35 wt % Ni stainless steels. One alloy was a pure 20 wt % Cr/35 wt % Ni steel, whereas the other contained about 0.5 wt % Ti and 0.5 wt % Al so that it precipitated during creep at 700° C. Special care was taken to assure equivalent microstructure in the specimens and precise creep conditions so as to obtain accurate and reproducible creep-rates. Both materials exhibited decreasing stress-dependence of the creep-rate at low stresses. Neither the stress-dependence of the creep-rate, nor the absolute creep-rate was consistent with diffusion-creep. The amount of grain-boundary sliding was measured separately by means of scribed grid lines on the creep specimens for the pure material at stresses above the creep yield. The values for the component of the creep-rate due to grain-boundary sliding coincide very well with the extrapolated line of the low-stress branch of the creep-rate/stress curve. All these results taken together suggest that the most likely explanation of the creep yield in 20 wt % Cr/35 wt % Ni steels is the one based upon grain-boundary sliding.     

Creep of austenitic stainless steel welds

Austenitic stainless steels (SS) find extensive application in power, petrochemical and nuclear industries in view of their excellent elevated temperature mechanical properties, corrosion resistance, formability and weldability. However, they are susceptible to hot cracking during fusion welding. To avoid this problem, chemical composition of the welding consumable is generally adjusted to promote primary ferrite mode of solidification and retain about 3 to 10%δ-ferrite in the as-welded condition. The duplex microstructure of the weld metal undergoes transformation to carbides and a variety of intermetallic phases during elevated temperature service and causes deterioration in the mechanical properties. This paper presents a comprehensive review of the current understanding of the solidification microstructures, ageing processes and their influence on the creep behaviour of types 308 and 316 SS weld metals. The effects of varying chemical composition,δ-ferrite content, electrode coating and welding processes on creep strength and ductility are examined. Current trends in the design of welded components for creep application are also discussed.     

Effect of aging treatment on the microstructures and mechanical properties evolution of 25Cr-20Ni austenitic stainless steel weldments with different Nb contents

The microstructure evolutions and the mechanical properties of the 25Cr-20Ni austenitic stainless steel weld metals with different Nb contents were investigated during the long term aging treatment at 700~?C.M_(23)C_6,Nb(C,N),α-Cr phase and Nb-nitride phase(Z phase)were observed in the microstructures of the aged weld metals.The results showed that theα-Cr phase precipitated in the interdendritic regions of the weld metals after being exposed to~ 700?C for 500 h and the element Nb accelerated the precipitation of theα-Cr phase significantly.The density of theα-Cr phase decreased with the increase of the distance away from the primary Nb(C,N).Additionally,theα-Cr phase showed a crystallographic relationship with the austenitic matrix,■.It was observed that the Z phase precipitated in the periphery of the Nb(C,N)and may replace the Nb(C,N)after long term exposure to high temperature.The transformation of the Nb(C,N)into Z phase suggested that the Z phase had a higher stability than the Nb(C,N)par~ticles at 700?C for long term aging.The tensile strength of the Nb-bearing weld metal showed a continuous decrease at the initial stage of the aging treatment and then went up slightly with the prolonged aging time.However,the elongations and the impact energies of the weld metals decreased monotonously with the increase of the aging time.     

Observation of hydrogen effects on fatigue crack growth behaviour in an 18Cr-8Ni austenitic stainless steel

The hydrogen effect on crack growth behaviour in a type 304 austenitic stainless steel was investigated and the following results were obtained. The crack growth rate in hydrogen gas is accelerated compared with that in air. In order to clarify the mechanism of the acceleration, the growth behaviours of a crack propagating in a grain and propagating along the boundary to be a fracture facet were investigated. Slip behaviour, opening displacement and fractography showed that the slip-off mechanism in fatigue crack growth is valid even in hydrogen gas. Hydrogen mainly affects slip behaviour such that slip bands concentrate at a crack tip and result in acceleration of the growth rate. The facets are not significantly responsible for the acceleration. The ratio of facets to the entire area is low, and a crack nearly compensates for the temporary acceleration by the facets with subsequent deceleration.     

Creep of low-activated Fe-12Cr-20Mn-1W austenitic corrosion-resisting steel

Conclusions 1. Creep resistance of the Fe-12Cr-20Mn-1W steel, containing 0.1-0.26 wt. % C and also small additions of Ti, B, and P in the range 823–973 K is similar to the creep resistance of the Fe-Cr-NI steels of the 304 type, 800 alloy, and also the Fe-Cr-Mn-Ni steel of the ÉP838 type.2. An increase of the carbon content in the range 0.1–0.26 wt. % in the Fe-12Cr-20Mn-1W alloys slightly reduces the minimum creep rate 873 K and does not effect this parameter at 973 K.3. The efficiency of the effect of carbon on the minimum creep rate of the Fe-12Cr-20Mn-1W steels is evidently lower than the 800 alloy.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 2, pp. 81–85, March–April, 1993.     

The influence of grain size,stress and temperature on the steady state creep of a 25 Cr-20 Ni austenitic stainless steel without precipitates

This work was performed in order to study the steady state creep behaviour of a modified 25 Cr-20 Ni stainless steel which has no precipitates. The test temperature range was 1171 to 1211 K, the stress range 4.9 to 19.6 MPa, and the grain size was 40 to 600m. The steady state creep rate, , decreases with increase in grain size, especially at the lowest stress; is proportional to 1/d ² at 4.9 MPa, whered is a mean grain diameter. The variation of with grain size is smaller in the middle and coarse-grained specimens than in the fine-grained specimens, the stress exponent,n, gradually decreases from ~ 4 to ~ 1.5 with reducing stress, but in the middle- and coarse-grained specimens, a discontinuous point appears on a log versus log plot. The activation energy for the steady state creep of the coarse-grained specimens tends to be larger than that of the fine-grained specimens, and the tendency is remarkable in the higher stress level. It is indicated that the creep mechanisms in the fine-grained specimens are essentially different from those in the coarse-grained specimens, and that the creep at the lowest stresses and smallest grain size is similar to that predicted by a vacancy creep model involving grain-boundary diffusion.     

23Cr-14Ni高氮奥氏体不锈钢σ相析出行为EI北大核心CSCD

采用经验公式、热力学计算方法、Gleeble热/力模拟实验技术,结合光学显微镜、扫描电镜及透射电镜分析,研究了23Cr-14Ni高氮奥氏体不锈钢中σ相的析出行为。结果表明,23Cr-14Ni高氮奥氏体不锈钢中σ相可在960~1030℃析出,高于1050℃溶解。σ相析出具有异常快速的动力学特征,在经过1030℃保温1 min固溶处理后,σ相可直接从奥氏体晶界快速析出,析出先于碳氮化物相。σ相析出动力学行为及相对碳氮化物的析出次序和传统奥氏体不锈钢显著不同。铬、锰、钼元素含量较高且钼元素在晶界处偏聚提高了σ相平衡析出温度,是加速σ相析出的主要原因。     

Interaction of 18Cr-10Ni stainless steel with liquid aluminium

The dissolution of an 18Cr-10Ni stainless steel in liquid aluminium at 700 to 850 ° C was found by the rotating disc technique to be non-selective and diffusion controlled. Experimentally determined values of the parameters characterizing the dissolution run are presented. In the case of saturated aluminium melts two intermetallic layers were found to form between the steel and the melt material at 700 °C. The compact layer adjacent to the steel surface is probably a solid solution based upon the Fe₂Al₅ compound. Its thickness,x, tends with increasing time to the limiting valuex _max = 10m. The porous layer adjacent to the melt material is probably a solid solution based upon the FeAl₃ compound. After a certain period of non-linear growth its thickness,Y, increases with time,t, according to the linear law:Y = 1 × 10^–8 t + 6 × 10^–6 m. The time dependence of the total thickness of both layers is well described in terms of the paralinear kinetics. In the case of undersaturated aluminium melts the formation of a single-phase intermetallic layer, 3 to 11 m thick, was observed at 700 ° C for 100 to 600 sec. The steel-to-aluminium transition joints with good mechanical properties were made by interaction of a solid steel material with liquid aluminium.     

Creep properties of two 25Cr-20Ni cast alloys in air and under carburizing conditions

The creep behaviour of two cast austenitic 25Cr-20Ni steels (HK30 and HK40) has been investigated in air and in a carburizing environment at 1000°C. In the carburizing environment the 1% creep strength is much less than in air and this also applies to the creep rupture strength for short and medium test times. For tests lasting longer than about 4000 h, however, the creep rupture strength was found to exceed the corresponding value in air. HK40 (0.46%C) was found to have a higher 1% creep strength and creep rupture strength than HK30 (0.30%C); the difference has been related to the different microstructures of the two casts.     

Creep crack propagation in austenitic stainless steel at elevated temperatures

Creep crack propagation behavior at high temperature was investigated for type 304 stainless steel. The present experiment reveals that creep crack propagation is explained better in terms of K than in terms of σ_net. The rate of the creep crack propagation is represented by the Arrhenius equation. The activation energy is higher in the present experiment compared with the case of fatigue. Main crack extends by means of joining micro-cracks initiated at vicinity of the main crack tip. Creep rupture is occured when the value of stress intensity factor reaches its critical value which increases with decreasing temperature but independent of stress level. It is found that the creep rupture time is expressed as a function of initial stress and initial crack size, and good agreement is obtained between observed and calculated times to rupture.     

Transformation-induced plasticity in Fe-17.20% Cr-7.34% Ni steel

The characteristics of transformation-induced plasticity of Fe-17.20% Cr-7.34% Ni steel were studied. The maximum value of fracture elongation occurred at 20° C in the temperature rangeMs (–196° C) toMd (75° C), and this maximum elongation was brought about by the delay of necking. The percentage of martensite per unit tensile strain after the martensite transformation was then 2.20     

Dynamical behaviour and microstructural evolution of a nitrogen-alloyed austenitic stainless steel

The aim of this work is to study the mechanical properties of a nitrogen austenitic stainless steel (Uranus B66) and their relation to its microstructural evolution. Quasi-static (10⁻³ s⁻¹) and quasi-dynamic (1 s⁻¹) compression tests have been carried out with a universal servo-hydraulic testing machine. Dynamic (>10³ s⁻¹) compression tests have been performed on a classical split-Hopkinson bar apparatus. These tests, which cover a wide range of plastic strain, show that the material has a high-strain hardening rate, a good ductility and a great strain rate sensitivity. The temperature sensitivity has been determined over a large range, going from 77 K to 673 K. Transmission electron microscopy (TEM) observations have been conducted in order to correlate the microstructure to the mechanical behaviour. Uranus B66 undergoes basically the same structure evolution during both quasi-static and dynamic compression tests. The plastic deformation is governed initially by planar gliding, followed by mechanical twinning when the dislocation density is saturated.     

Cold-worked state and annealing behaviour of austenitic stainless steel

An investigation was made into the structural changes accompanying cold working and annealing treatments in seven austenitic stainless steels. The materials studied included five laboratory alloys and two commercial grades of austenitic stainless steels (types 304 and 316). X-ray line profile analysis showed that the stacking-fault energies of the seven steels ranged from 8 MJ m^–2 to 68 MJ m^–2. Transmission electron microscopy (TEM) was used extensively to characterize the cold-worked and annealed states, Measurements of the resistivity change were performed to characterize the recovery and recrystallization behaviours. The cold-worked structure was found to be related to the stacking-fault energy. Dislocations tended to be arranged in planar arrays and to be confined in the original slip planes in alloys of low stacking-fault energy. Dislocation arrangement was less uniform and more random for steel of high stacking-fault energy. In none of the cases studied was the stacking-fault energy high enough to allow the cross-slip necessary to generate the dislocation cell structure often seen in other metals. Isochronal annealing of the steels reveals a distinguishable stage of resistivity recovery prior to recrystallization, which was attributed to the annihilation of vacancies and removal of carbon from the solid solution. A second stage of resistivity drop (above 500° C) resulted from recrystallization. The temperature for the start of recrystallization was found to be related to stacking-fault energy.