Cr Austenitic Steel Research Articles

Impact of the voids on the cracking behavior of the duplex oxide scale on the 18%Cr austenite alloy surface

This work investigated the oxide cracking behavior of 18%Cr austenitic steels. The role of voids at the duplex oxide layer interface has been discussed. Simulation results demonstrate that the stress concentration facilitates the crack nucleation at the void top. Interestingly, the interface of the duplex oxide layer exhibits better cracking resistance. Experiment results confirm that the voids facilitate cracking when the temperature drop is beyond 119 °C during oxidizing at 650 °C for TP347H alloy. By comparing, more than 290 °C of temperature drop is needed for the cracking on TP347HFG alloy, which is because the void diameter is small.

The effect of thermomechanical treatment on the microstructure and mechanical properties of high Mn–Cr austenitic steels

AbstractIt was shown that new high-manganese austenitic steels can be used as an alternative to conventional Ni–Cr austenitic steels especially in vacuum vessels. In this study, a nickel-free austenitic steel with chemical composition of 24 wt.% Mn, 10 wt.% Cr, 0.13 wt.% C and trace elements of Si, Ti, V, W, and Al was hot-rolled at 950 and 1100 °C to different strains. Microscopic observations, X-ray diffraction, tensile, impact and microhardness tests were performed to characterize the microstructural aspects and mechanical properties. A small amount of chromium carbide was found on the grain boundaries of this fully austenitic steel. The results showed that the microstructure and mechanical properties of hot rolled manganese austenitic steel are affected by the dynamic recrystallization and twinning phenomena. Also, by changing the grain morphology from equiaxed to elongated, strength and hardness increased, and the impact toughness and ductility were reduced.

Fatigue crack initiation and growth in 43Fe-25Ni-22.5Cr austenitic steel at a temperature of 700 °C

Crack initiation and short crack growth in highly-alloyed austenitic steel Sanicro 25 cyclically strained at a temperature of 700 °C has been studied. Specimens cycled to 10% of the fatigue life and to the fracture were inspected using scanning electron microscopy and electron backscatter diffraction. On the surface, thin lamellae were extracted site-specifically by focused ion beam and then they were further characterized by high spatial resolution scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. The preferential oxidation of grain boundaries was found to play a principal role in the initiation of multiple cracks. The oxide scale was analysed to show the distribution of individual elements during crack initiation and growth. Two types of crack paths were observed: intergranular due to the oxidation and transgranular accompanied by a cyclic plastic zone. Cracks preferentially initiated at the grain boundaries perpendicular to the stress axis and propagated in the bulk of the material. The role of the oxidation in high temperature fatigue crack initiation is discussed as well as the mechanisms of the crack initiation and growth.

Prediction of carbide free layer formation in Fe–Ni–Cr austenitic steel process heater tube

The kinetics of carbide free layer formation in a process heater tube made of Fe–Ni–Cr austenitic steel during high temperature service exposure has been studied. An integrated scanning electron microscopy–energy dispersive spectroscopy–electron backscatter diffraction system was used for microstructural and compositional analysis of the investigated alloy tubes. A diffusion based model has been proposed to estimate the surface temperatures based on the widths of the respective carbide free layers. The model would be useful to predict the chromium concentration profiles at various times to prevent catastrophic failure of the material.

Effect of Shot-peening on the Oxidation Behaviour of Boiler Steels

The presence of short diffusion paths is very important for rapid diffusion processes which are involved in forming protective oxide layers against high temperature corrosion, e.g. on boiler steels. Rapid diffusion paths can be produced by applying cold work such as shot-peening to the surface of the boiler steels prior to oxidation. The effect of shot-peening on oxidation behaviour was tested experimentally on 12 wt% Cr martensitic steel and 18 wt% Cr austenitic steel. Isothermal oxidation tests were performed at 700 and 750 °C. The surface treatment proved to be very effective in improving oxidation protection at 700 °C. Shot-peening the surface prior to the oxidation has an influential effect in changing the diffusion mechanisms of the elements involved in oxidation and changes the oxidation kinetics substantially at the applied conditions in this study.

Ultrasupercritical power plant development and high temperature materials applications in China

The rapid development of Chinese economy demands sustainable growth of power generation to meet industrial and domestic demand. The total installed capacity of electricity and annual overall electricity generation are now both the second highest in the world, close to those of the USA. Forecasts of China's electricity demand over the period 2010–20 are presented. Chinese power plants, like those worldwide, are facing demands to increase thermal efficiency and to decrease the emission of CO2, SOX and NOX. In light of the national resource of coal and electricity market requirements in the next 15 years, power generation – especially ultrasupercritical (USC) power plants with the steam temperature over 600°C – will undergo rapid development. The first 1000 MW USC power unit, with steam parameters 600°C, 26·25 MPa, entered service in November 2006. It is estimated that more than 350 USC power units will be installed in China by 2020. USC power plant designs will adopt a variety of qualified high temperature materials for boiler and turbine manufacturing applications. Among these materials, the modified 9–12%Cr ferritic steels, Ni–Cr austenitic steels and certain nickel base superalloys have received special attention in the Chinese materials market.

Effect of electron-beam irradiation temperature on irradiation damage of high Mn–Cr steel

The effect of electron-beam irradiation temperature on irradiation damage of a high Mn–Cr austenitic steel for structure material of nuclear and/or fusion reactors from the point of view of the reduced radio-activation was investigated by using the 1250 kV HVEM and an energy dispersed X-ray analyzer (EDX) in a 200 kV FE-TEM with beam diameter of about 0.5 nm. Void formation was not observed even at the elevated irradiation temperature. Dislocation loop growth was observed and the density and size of dislocation loop increased with irradiation dose. Irradiation-induced segregation of Cr and Mn at grain boundaries were observed by electron-beam irradiation condition. The amount of Mn segregation increased with irradiation temperature, however, the case of Cr was suppressed.

A new perspective for rapid nitriding in high Cr austenitic steels

A new perspective for rapid nitriding in high Cr austenitic steels

Low Temperature Fracture Behavior of 12%Cr-15%Mn Austenitic Steels

The primary objective of the present study is to investigate the low temperature toughness of high Mn–Cr austenitic steels which are being developed as one candidate alloy for the structural material of fusion reactor from the point of view of reduced radio-activation. In the present work, the effects of V, Ti and P on the low temperature toughness of the alloys with the basic composition of 12%Cr-15%Mn-(0.1–0.2%)C-(0.1–0.2%)N are investigated. Coarse precipitates of M23C6 on the grain boundaries caused intergranular fracture and tiny precipitates of VN within the grains enhanced cleavage fracture in aging treated alloys. The addition of Ti was beneficial for improving toughness due to the restraining of the tiny VN precipitation. The addition of P, however, deteriorated the low temperature toughness of the alloys. The toughness values of the 12%Cr-15%Mn materials were compared with those of JFMS (a modified 9Cr–2Mo ferritic stainless steel) and JPCA (modified 316 austenitic stainless steel).

Effect of niobium on creep and creep crack growth of cast Ni–Cr austenitic steel

An investigation of the effect of Nb on creep properties and creep crack growth rate in a 25Cr–35Ni–0·4C (wt-%) cast steel at 871 and 950°C was carried out. Tensile tests were also carried out at room temperature, 871, and 950°C. The tensile strength and elongation increased with an increase in Nb content at high temperatures. There existed an optimum Nb content for the creep properties and creep crack growth rate. Creep crack growth is controlled by creep deformation.MST/1222

Effect of niobium on creep and creep crack growth of cast Ni–Cr austenitic steel

An investigation of the effect of Nb on creep properties and creep crack growth rate in a 25Cr–35Ni–0·4C (wt-%) cast steel at 871 and 950°C was carried out. Tensile tests were also carried out at room temperature, 871, and 950°C. The tensile strength and elongation increased with an increase in Nb content at high temperatures. There existed an optimum Nb content for the creep properties and creep crack growth rate. Creep crack growth is controlled by creep deformation.MST/1222

Oxidation of a Mn‐Cr austenitic steel during creep testing and its interaction with the fracture

AbstractThe interaction between oxidation and creep rupture was studied in a 17 Mn‐10 Cr austenitic steel, of interest as structural material for the internal components of fusion reactors. The observation of the creep specimens tested in air at temperatures ranging from 773 to 973 K revealed the presence of an adherent oxide scale and of a ferritic phase underneath, which forms as a consequence of the Mn depletion of the austenitic matrix.The microstructure of the two layers was investigated by optical microscopy, SEM, X‐ray diffraction, EDS and magnetic permeability measurements. The scale has a complex structure, being composed mainly of manganese oxide. The ferritic layer is completely recrystallized and does not present grain boundary precipitates as the austenitic phase does.The effect of the surface modification on the creep rupture process is discussed in the light of a recent model of deformation‐oxidation interaction.

Structural and analytical electron microscopy of dislocation loops and precipitates formed during ageing of an α-bombarded MnCr austenitic steel

Solution annealed samples of the MnCr austenitic steel Nitronic 32 are uniformly implanted at 310 K with 500 atppm He corresponding to a dose of ≈ 0.1 dpa. Specimens are subsequently aged, one set at 773 K and another set at 1023 K; the evolution of the dislocation structure and the related segregation and precipitation phenomena are examined by TEM, EELS, and electron diffraction. Short annealing (2 h) at 1023 K lead to the formation of Frank interstitial dislocation loops which are found enriched in N and Cr. After longer annealing (25 h) at 1023 K, Cr2N precipitated on the loops, small He bubbles becomes visible in the matrix, whereas no lamellar precipitation is observed in contrast to unirradiated specimens. Losungsgegluhte Proben von austenitischem MnCr-Stahl Nitronic 32 werden bei 310 K mit 500 atppm He implantiert. Die entsprechenden Strahlungsschaden betragen ≈ 0,1 dpa. Die Proben werden anschliesend bei 773 und 1023 K warmebehandelt. Die Entwicklung der Versetzungsstruktur und die damit zusammenhangenden Segregations- und Ausscheidungsphanomene werden mittels TEM, EELS und Elektronenbeugung untersucht. Kurze Anlaszeiten (2 h) bei 1023 K fuhren zur Bildung von Frankschen Zwischengitterversetzungsringen, in denen N und Cr angereichert sind. Bei langerem Anlassen (25 h) bei 1023 K scheidet sich Cr2N an den Versetzungsringen aus und kleine He-Blasen werden in der Matrix sichtbar. Im Gegensatz zu unbestrahlten Proben wird keine lamellare Ausscheidung beobachtet.

Evaluation of anelasticity and plastic creep limit for alloy 800

The elevated temperature tensile, anelastic and creep properties of a precipitation strengthened NiCr austenitic steel (alloy 800) have been evaluated with respect to LMFBR application. These properties have been estimated and formulated for small strains and temperatures on the order of 823 K. It has been shown that despite suppression of the in-service stresses to beneath the alloy's yield strength, plastic deformation will occur at such temperatures and the stress below which plastic strain is effectively zero (plastic creep limit) is negligible for a reactor life time. The plastic creep limit at a given time and temperature markedly depends on the degree of precipitation hardening or prior cold deformation for alloy 800. The contribution from anelastic deformation is less than 0.05% and only becomes important for strict dimensional control and constrained parts.

The effect of plastic deformation on the formation of sigma phase in an austenitic stainless steel

A 24% Ni, 25% Cr austenitic steel containing 0.27% Ti has been solution-treated and quenched from 1150°C. The effect of prior cold-rolling on the aging processes at 750°C has been studied by transmission electron microscopy, comparative tests having been made on a Ti-free steel. Acceleration of ferrite and sigma-phase formation on recrystallization of the matrix was observed. It is suggested that the effect arises from the segregation of Ti atoms to dislocations, which element would otherwise promote the formation of these phases. The presence of serrated stress-strain curves in tensile tests at 704°C in the Ti-containing alloy supports this hypothesis. Only when the dislocation density is reduced by recrystallization is the free Ti content sufficient to promote the formation of the ferrite and sigma phases.

Factors Affecting the Precipitation of NbC on Frank Partial Dislocations in Fe–Ni–16% Cr Austenitic Steels

AbstractThe effect of variations in alloy composition, quench rate, and deformation on the formation of FPP (Frank partial precipitation) has been studied by means of hardness measurements and electron microscopy. Solute concentration is the most important factor controlling the maximum area of FPP. The numbers of precipitate particles increase with dislocation density but the number per unit length of dislocation is generally lower the higher is the dislocation density. The efficiency of nucleation by a dislocation is therefore not increased by the prior cold work and this is consistent with nucleation by climbed loops. The free-vacancy profile near grain boundaries is used to explain the FPP-free zone (PFZ) width, and this is controlled by the rate of precipitation. Silicon additions accelerate the nucleation of NbC and promote FPP formation.