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.     

Weld decay-resistant austenitic stainless steel by grain boundary engineering

This paper presents an example of grain boundary engineering (GBE) for improving intergranular-corrosion and weld-decay resistance of austenitic stainless steel. Transmission and scanning electron microscope (TEM and SEM) observations demonstrated that coincidence site lattice (CSL) boundaries possess strong resistance to intergranular precipitation and corrosion in weld decay region of a type 304 austenitic stainless steel weldment. A thermomechanical treatment for GBE was tried for improvement of intergranular corrosion resistance of the 304 austenitic stainless steel. The grain boundary character distribution (GBCD) was examined by orientation imaging microscopy (OIM). The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of CSL boundaries indicated a maximum at the small roll-reduction. The corrosion rate was much smaller in the thermomechanical-treated specimen than in the base material for long time sensitization. The optimum thermomechanical treatment introduced a high frequency of CSL boundaries and the clear discontinuity of corrosive random boundary network in the material, and resulted in the high intergranular corrosion resistance arresting the propagation of intergranular corrosion from the surface. The optimized 304 stainless steel showed an excellent resistance to weld decay during arc welding.     

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).     

On the variation of grain size and fractal dimension in an austenitic stainless steel

Samples of an AISI type 316L stainless steel were subjected to different treatments to promote changes in their microstructure. The specimens were heated in a box furnace set at four different temperatures for 30 min and cooled in air to room temperature by placing them in water after the cycle was completed. The samples were prepared following standard metallographic procedures, the microstructure was revealed with an electrolytic etchant, and the average grain size in each sample was determined by the mean line intercept technique. Images from the microstructures were digitized and fed into a personal computer for their fractal analysis by box counting. Two different approaches were used to obtain the fractal dimension of the structure, yielding to similar values in both cases. It was found that the fractal dimension of the microstructure increased with the reduction in grain size.     

Interaction between precipitation,normal grain growth,and secondary recrystallisation in austenitic stainless steel containing particles

Abstract

With the aid of various complementary methods of microstructural analysis, the precipitation, grain growth, and secondary recrystallisation behaviour of an 15Cr–15Ni–1·2Mo–Ti–B (wt-%) austenitic stainless steel were studied over prolonged periods of time in the temperature range 600–1300°C. The experimental results showed that several types of precipitates were present in the material, and that the dissolution temperatures of each of these correlated with the type and extent of grain growth which was observed. It was, therefore, concluded that in the present study secondary recrystallisation was caused directly by the interaction of precipitates with grain boundaries. Furthermore, secondary recrystallisation produced a strong, predominantly {122{ 〈012〉 texture which has not previously been reported.     

A new etching route for revealing the austenite grain boundaries in an 11.4% Cr precipitation hardening semi-austenitic stainless steel

A detailed metallographic characterization of a precipitation hardening semi-austenitic stainless steel is described. A new etching procedure based on the Lichtenegger and Blöch color etching solution, which is frequently used in duplex stainless steels to differentiate delta ferrite from austenite, has been used to differentiate martensite, austenite and the Chi-phase in this stainless steel. By changing the etching conditions, this etchant now reveals the austenite grain boundaries when the steel is in the austenitic state. Moreover, this solution is able to reveal also the prior austenite grain boundaries when the steel is in its martensitic state. This etching procedure represents a great advantage because it reveals, at the same time, different features of the microstructure.     

A study on the mechanism of serrated grain boundary formation in an austenitic stainless steel

Measurement of the activation energy for the formation of serrated grain boundaries (GB) has been carried out to understand its underlying formation mechanism in an AISI 316 stainless steel. The apparent incubation time necessary to initiate grain boundary serration was obtained at different aging temperatures, and the apparent activation energy for serration was carefully calculated from the Arrhenius relationship between incubation time and aging temperature. The activation energy for GB serrations in this alloy was measured to be approximately 148 ± 20 kJ mole⁻¹, which is consistent with the activation energy for lattice diffusion of carbon in γ-iron (142 kJ mole⁻¹). This result indicates that GB serration could be controlled essentially by the lattice diffusion of carbon to grain boundaries. Based on the through-thickness observation of serrated GBs, a straight boundary began to serrate from the surface at an early stage of the aging treatment, and then the serrated parts propagated throughout the entire grain boundary.     

Study of copper precipitation behavior in a Cu-bearing austenitic antibacterial stainless steel

Copper (Cu) precipitation behavior in a type 304 Cu-bearing austenitic antibacterial stainless steel was studied by analyses of variations in micro-hardness, electrical resistivity, electrochemical impedance and lattice constant of the steel, complemented with transmission electron microscopy (TEM) observation, showing more or less changes on these properties of the steel with different aging time. It was found that both micro-hardness and electrical resistivity measurements were relatively sensitive and accurate to reflect the Cu precipitation behavior in the experimental steel, indicating the beginning and finishing points of the precipitation, which are more simple and effective to be used for development of the new type of antibacterial stainless steels.     

Influence of alloying on the free energy of austenitic grain boundaries in steel

We compute the conditional energy σ, of austenitic grain boundaries and analyze changes in boundary energy at elevated temperatures for low-carbon, chrome-manganese and chrome-manganese-molybdcnum steels. It is established that σ, varies within the range 1.0–1.3 J/m² and that, in almost all cases of alloying, the effect caused by a decrease in the surface energy of iron is so strong that even in the presence of chorophobic elements σ, decreases at elevated temperatures. For multicomponent steels, we analyze variations of the temperature coefficients of boundary energy. Philadelphia, United States of America. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 4, pp. 63–68, July–August, 1996     

Influence of alloying on the free energy of austenitic grain boundaries in steel

We compute the free energy of boundaries of austenitic grains and analyze the effect of alloying elements (Mo, Ni, V, W, Nb, Ce, Cr, Ti, Al, Si, B, and Cu) on the boundary energy of low-carbon, chromemanganese, and chrome-manganese-molybdenum steels. By using regression analysis, we develop interpolational models for the qualitative and quantitative evaluation of the effect of each element on the free energy of grain boundaries. Philadelphia, USA. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 2, pp. 24–34, March–April, 1996.     

Influence of alloying on the free energy of austenitic grain boundaries in steel

We compute the free energy of austenitic grain boundaries for steels whose carbon content is equal to 0.2%, for chrome-manganese steels with various concentrations of molybdenum and phosphorus, and for chrome-manganese-molybdenum steel. On the basis of regression analysis, we develop an interpolational model which enables one to estimate (both qualitatively and quantitatively) not only the effects of each element on the free energy of grain boundaries but also changes in these effects caused by the presence of other elements with different surface activities. It is shown that changes in the influence of alloying elements on the boundary energy of multicomponent systems can be explained by the interaction of different elements. Philadelphia, USA. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 3, pp. 55–62, May–June, 1996.     

Effect of grain boundary orientation on the sensitization of austenitic stainless steel

The effect of grain misorientation on the sensitization of grain boundaries in austenitic stainless steel was investigated by sensitizing samples consisting of a large number of 50–80 μm size grains that were sintered to flat, 10 mm2 single crystals. Seven different sensitization treatments were employed and samples were intergranulary corroded in the modified Strauss test. X-ray pole figures were obtained for each sample and were used to identify the grain misorientations that were resistant to sensitization. In general, macroscopic grain boundary geometry could not explain the sensitization behaviour of most grain boundaries. Nevertheless, the Σ = 9 boundary was found to be especially resistant to sensitization. Results suggest that grain misorientation primarily affects the growth of sensitization rather than its nucleation. Finally, the crystallographic plane of the grain boundary appears to have an effect on sensitization. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of grain refinement on strain hardening and fracture in austenitic stainless steel

The present study aims to investigate the effect of grain refinement on strain hardening behaviour and fracture surface characteristics in 316LN austenitic stainless steel (ASS). The ASSs with varying grain sizes were obtained through 90% cold rolled reduction and subsequently phase reversion annealing treatment. The results showed that the grain refinement from coarse-grained (CG) structure to ultrafine-grained (UFG) structure increased the yield strength whilst maintaining a reasonable ductility. The strain hardening curves in all the samples were divided into three stages. The fractures in all the samples were ductile fracture with dimples. The subtle differences in the strain hardening behaviour and fracture surface characteristics among the samples with various grain sizes from CG structure to UFG structure were influenced by the deformation mechanisms of austenite.     

Effect of grain boundary microstructure on fatigue crack propagation in austenitic stainless steel

The effect of grain boundary microstructure on fatigue crack propagation in austenitic stainless steel was investigated in order to control fatigue crack propagation. The fraction of low-Σ coincidence boundaries in specimens was controlled by thermomechanical processing. The specimen with the higher fraction of low-Σ boundaries (73%) showed the lower propagation rate of fatigue crack than the specimen with the lower fraction of low-Σ boundaries (53%). The ratio of intergranular fracture segments to the total crack length was lower for the specimen with the higher fraction of low-Σ boundaries. Moreover, the roles of grain boundaries in the fatigue crack propagation were investigated in connection with grain boundary microstructure, i.e., the character distribution and geometrical configuration of grain boundaries. It is evidenced that the approach to grain boundary engineering is applicable to controlling fatigue crack propagation in austenitic stainless steel.     

Effect of pre-strain on grain size distributions in 316H austenitic stainless steel

An experimental study addressing the effect of tensile deformation on recrystallized grain size has been undertaken to explore the conditions leading to abnormal grain growth in Type 316H austenitic stainless steel. Following a solution heat treatment, a Type 316H stainless steel has been subjected to various tensile deformations up to a maximum of approximately 50% strain and then heated at a temperature of 1150 °C for 0.5 h followed by furnace cooling. A fraction of abnormally large grains is observed following a prior strain of approximately 20%. The results are presented, in terms of standard statistical analysis, and also graphically. The graphical presentation provides a clear, visual appreciation of uni- and bi-modal distributions, which may be of general help in other analyses of this nature.     

Modelling primary recrystallization and grain growth in a low nickel austenitic stainless steel

This paper deals with the recrystallization and grain growth processes of a low nickel stainless steel. Samples of steel sheets with various cold rolling degrees were annealed at different temperatures and the recrystallization and grain growth kinetics have been studied. The grain size of the samples has been determined via automatic image analysis and transformed to 3-D values according to the Saltykov model. The experimental data have been analysed according to a modified model developed using the statistical approach by Abbruzzese and Lucke for the grain growth. This approach supplies a unified equation describing at the same time primary recrystallization and grain growth. The values of the dislocation density obtained from the comparison of theoretical predictions and experimental data of the grain mean radius are properly correlated to the mechanical properties of the steel.     

Effects of aging temperature and grain size on the formation of serrated grain boundaries in an AISI 316 stainless steel

The effects of aging temperature and grain size on the formation of serrated grain boundaries have been investigated in an AISI 316 stainless steel. Grain size increased slightly over aging temperature ranges of 650–870 °C, resulting in predominantly serrated grain boundaries. However, when the temperature exceeded 880 °C, the grain size significantly increased, and grain boundary serration was not observed. The initial grain size also had an influence on the occurrence of grain boundary serration. For specimen having a large initial grain size of about 200 μm, no serrated grain boundary formed after aging treatment at 760 °C. Serrated grain boundaries were observed when “normal” initial grain sizes 55 μm were employed. It was found that the frequency of low angle boundaries markedly increased as the initial grain size increased from 55 to 200 μm. From the results obtained, it is possible to describe that the grain boundary serration could be considered as a spontaneous reaction that aims to reduce the total free energy of the system, and form a new interface of lower free energy. We proposed that the competition between grain growth and grain boundary serration during aging treatment reduces the total free energy of the alloy system: at temperatures exceeding 880 °C, the dominant process is the grain growth, while grain boundary serration predominates over the range of 650–870 °C.     

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.