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Internal nitridation kinetics were determined for a UNS N08810/800H alloy using a general model of the form $$x^{n}=kt$$ . Nitridation behavior was studied at service-relevant temperatures 800 °C to 1000 °C in a 95 pct $$\hbox {N}_2/5\,{\text{pct}}$$ $$\hbox {H}_2$$ atmosphere for times 50 to 750 hours. Optical and scanning electron microscopy were used for microstructural characterization and measurement of nitride penetration. AlN, $$\hbox {Cr}_2\hbox {N}$$ , and CrN were formed, and the experimentally observed precipitation sequence was consistent with equilibrium calculations for this alloy using Thermo-Calc. A combination of diffusivity data determined using DICTRA and experimentally verified equilibrium calculations showed that Wagner’s analysis for internal oxidation kinetics was valid for AlN penetration. Parabolic kinetics closely approximated measured AlN penetration. This suggests that extension of AlN penetration models to other temperatures and Fe-Ni-Cr-Al alloy systems is reasonable. $$\hbox {Cr}_2\hbox {N}$$ penetration did not conform to Wagner’s analysis. Deviation from parabolic behavior was evident, and general model penetration predictions for $$\hbox {Cr}_2\hbox {N}$$ were experimentally validated. Using the experimentally determined models, time–temperature–precipitation diagrams for AlN and $$\hbox {Cr}_2\hbox {N}$$ penetration were constructed.

Low-cycle fatigue (LCF) failures can be expected when rebars are subjected to a small number of cycles at large stress/strain amplitudes. Major seismic events are often preceded or followed by other events of smaller/larger magnitude. During each event, the residual fatigue life of the steel reinforcement may reduce, eventually leading to failure of the bars due to the cumulative damage. Strain ageing may reduce even more the residual fatigue life of steel. This paper presents the results of an experimental testing campaign conducted on samples fabricated from New Zealand Grade 300E steel. A benchmark LCF life was obtained for 12-mm steel reinforcing bars subjected to axial-strain-controlled completely reversed cyclic tests at amplitudes ranging between 0.78 and 2.75%. The reduction in fatigue life attributed to strain ageing was obtained by comparing the benchmark fatigue life with that of steel reinforcing bars precycled up to 33% and 66% of the benchmark life and artificially strain aged for an equivalent period of 1 year at 15 °C. The Coffin-Manson and Koh-Stephens models were employed to evaluate the experimental results. The study demonstrates that, depending on the strain amplitude, strain ageing can cause a reduction in total and residual fatigue life ranging from 20 to 70%. Strain ageing should not be ignored during the assessment of earthquake-damaged steel reinforcing bars.

Grain size variations are common in thermomechanically processed alloys with non-uniform cold work. A method to produce samples with grain size gradients was developed using Incoloy 800H. Two tensile samples with non-uniform gages were designed with finite element analysis and manufactured. Measured strain profiles were consistent with designs, and maximum von Mises strains of $$({18.6 \pm 6.1})$$ and $$({13.6 \pm 4.6})$$ pct were obtained. After annealing, an area $${40 \, {\hbox {mm}} \times 5}\, {\hbox {mm}}$$ was mapped by electron backscatter diffraction. Totals of 2849 and 2569 grains were identified after merging twins. Both samples had duplex grain structures as defined in ASTM E1181. Grains were binned into 4 mm strips to evaluate the spatial grain size distribution. Grain size gradients of 0.0081 and 0.0112 $${\hbox {mm}^2/\hbox {mm}}$$ were obtained. Simulated grain growth of the linear gradient microstructure was consistent with accelerated growth predictions. This new method of making samples will enable laboratory studies of gradient grain size effects in realistic industrial alloy microstructures. Further, samples could be used for parallel, single specimen experiments on phenomena that depend on grain size such as complexion transitions and fatigue. Most importantly, high-throughput parallel testing of microstructures enabled by our method could accelerate materials discovery and qualification in fields such as high-entropy and nuclear alloys.

A total of fifty-eight Incoloy 800H samples were creep tested between temperatures of 1023 K and 1293 K, stresses of 14.1 to 105 MPa, and average grain sizes of 87.7 to 315 µm. Combined with data obtained by the National Institute for Materials Science (NIMS), a deformation mechanism map (DMM) for Incoloy 800H was produced. Optimization of the fit of the constitutive creep equations to the experimental data was performed using a global search iterative numerical optimization tool called a genetic algorithm (GA). It was found that the data were well represented by both high-temperature and low-temperature power-law creep mechanisms, but the extent of the influence of diffusion-based creep mechanisms, most specifically Coble creep, will require further investigation. A training and test method was performed to validate the solution and to test the extrapolability of the dataset. It was determined that the extrapolability of the data in all directions of the DMM was generally low.

An aircraft fuel line was found to have fractured and caused an in-flight fire. A brief history of the general problem of cracking in brazed stainless steel fuel injection lines places this failure in context. The fracture mechanism was likely to be fatigue due to vibrations. Other factors contributed to the failure, such as an inappropriate braze repair and incorrect bend radius and location dimensions. The effectiveness of current airworthiness directives and manufacturer service bulletins are discussed. Alternative designs are suggested to prevent recurrence.

The 2010 and 2011 Christchurch seismic events have highlighted the limitations of the current knowledge in assessing the residual capacity of earthquake-damaged reinforced concrete (RC) buildings. An important challenge during the assessment phase was determining the residual ductility and the remaining low-cycle fatigue life of damaged rebars. Low-cycle fatigue is a possible failure mechanism of steel reinforcing bars when subjected to large-amplitude cyclic loads, such as due to earthquakes. While a single seismic event may not cause rebar failure, the low-cycle fatigue life will be reduced due to plastic strain. Also, New Zealand (NZ)-manufactured Grade 300E is prone to strain ageing. This phenomenon causes a change in mechanical properties, such as increase in yield and ultimate tensile strength, return of a discontinuous yield point, reduction in ductility and rise in the ductile/brittle transition temperature, and must be considered in damage assessment. This paper discusses the effects of strain ageing on the monotonic and cyclic steel mechanical properties. Low-cycle fatigue tests were conducted on Grade 300E steel rebars. Reinforcing bar samples were subjected to constant and fully-reversed strain amplitude cycles. Strain amplitudes ranged from 0.5% to 3%. The strain-fatigue life curve for the un-aged steel was determined. The strain ageing effects on the fatigue life of Grade 300E were then investigated. Specimens were cyclically tested up to the 33% and 66% fatigue life previously determined and “artificially” aged at 100°C. Finally, they were cyclically tested until failure. The experimental data were analyzed and low-cycle fatigue models were calibrated using the Coffin-Manson empirical relationship. Fatigue lives of the un-aged and aged samples were then compared. Preliminary observations suggested that strain-ageing triggers a premature crack initiation which propagates until failure.

Modern seismic codes are based on capacity design and the hierarchy of strength philosophy that allows inelastic response in case of severe earthquakes. Thus, in most traditional systems, e...

A hybrid material has been developed to improve creep performance in pressurized metallic pipes subjected to high-temperatures. Model materials were selected for an investigation of reinforcement design parameters in architectured materials. Brass pipes (65 wt.% Cu/35 wt.% Zn) with austenitic stainless steel reinforcement were pressurized and creep rupture tested at 673 K. Compared to unreinforced pipes of equal dimensions, a 47-times reduction in the effective strain rate was observed with a 50° reinforcement angle. A ‘neutral angle’ of 54.7 ± 1.5° was determined experimentally, where tangential (hoop) and longitudinal stresses on the pipe can be balanced and strains minimized. For initial angles below the neutral angle, creep strain was shown to facilitate a shift in orientation towards the neutral angle. For an initial angle of 42°, this shift towards the neutral angle resulted in instantaneous creep rate dropping from 170% of the mean creep rate to 60% of the mean creep rate over 820 h, when the final angle was measured to be 50°. A high-temperature prototype (tungsten braid oriented at 53° over a 253MA stainless steel pipe) was shown to give a creep life extension in excess of 300-times at 1313 K. Keywords: Hybrid, Creep, Architectured reinforcement, Neutral angle

A highly carburized HP40-Mod alloy ethylene pyrolysis tube was characterized by means of scanning electron microscopy with backscatter electron imaging, electron back-scattered diffraction, energy dispersive spectroscopy, and etching using the NACE International Standard Test Method for evaluation of carburization of ethylene pyrolysis tubes. The response of the tube to eddy current non-destructive testing was measured using the carburization crawler under development at Quest Integrity NZL Ltd. The matrix was significantly depleted of chromium, as low as 4 wt% Cr at the inner wall. M23C6 carbides transformed to M7C3 at the inner wall region and NbC carbides partially transformed to the chromium-rich η-carbide at the outer wall region, both of which likely contributed to the chromium depletion of the matrix. The present results indicate that the NACE etchant attacks the austenitic matrix where chromium content is below approximately 12wt%. A comparison to other ex-service tubes indicates that matrix chromium content and the location of the M23C6/M7C3 transformation front are useful microstructural characteristics for interpreting eddy current NDT response.

This article describes the root cause failure analysis of a mechanical vapor recompressor (MVR) impeller used in a dairy processing plant. An impeller blade was thrown during commissioning after approximately 150 h of service. Visual examination of the blade’s fracture surface indicated that the crack growth mechanism was fatigue and this was confirmed by scanning electron microscope (SEM) examinations. A detailed investigation was commissioned to determine the source of the oscillatory stresses that must have been present to cause fatigue. A vibration analysis indicated large torsional oscillatory stresses, leading to fatigue, were induced by excitation of the 1st and 2nd torsional vibration modes of the MVR impeller and motor system. The excitation was induced by the control system for the motor variable speed drive (VSD).

A comprehensive, critical, and up to date review is presented for the proeutectoid cementite transformation in steels. It is believed that many of the new findings, features, and concepts presented here for this classic phase transformation in steels serve as a model which may be more broadly applicable to test against many other phase transformations systems as well. There were a number of early investigations of cementite morphology, and this review considers those early results in light of many newer studies that provide critical new insight into cementite morphologies in both two and three dimensions. A number of different orientation relationships (ORs) between proeutectoid cementite and the austenite matrix from which it forms have been reported in the literature, in some cases leading to confusion, and they are critically evaluated here, as are the habit plane, growth direction, and interfacial structure of various morphologies of proeutectoid cementite. Quantitative experimental a...

This paper describes the three-dimensional (3-D) microstructure of a pearlite colony in a carburized electrolytic iron. The original data was obtained by serial sectioning methods around 1960, and is revisited with modern 3-D visualizations. After a brief historical introduction, the structure of the pearlite colony is discussed, using surface renderings and blue-red stereoscopic images.

The crystallography and morphology of proeutectoid grain boundary cementite precipitates were studied in an isothermally transformed Fe-1.3%C-12%Mn steel. Grain boundary cementite precipitates develop with complex variations of a morphology that can be described as fern-like dendrites growing preferentially within and along austenite grain boundaries. The dendritic morphology was made most readily apparent by deep etching specimens of the alloy whose growth had been arrested after relatively short times during the solid state austenite→cementite transformation. Trace analysis revealed that these precipitates usually have no crystallographically preferred primary or secondary dendrite arm growth directions. Electron Backscattered Diffraction pattern analysis showed that most of the grain boundary cementite precipitates approximated one of the known cementite–austenite crystallographic orientation relationships (OR) with at least one of its adjacent austenite grains. Occasionally, the grain boundary cementite exhibited an OR that achieved ‘best fit’ directions between the cementite and austenite in both adjacent austenite grains.

Three-dimensional (3D) analysis techniques were used to examine a model Fe–1.34% C–13.0% Mn alloy to reveal connectivities of proeutectoid cementite precipitates whose growth was arrested at an early stage. The present results indicate that grain boundary cementite precipitates nucleate at austenite grain boundary edges and corners, then grow and spread on the grain boundary faces. No cementite precipitates were found to connect solely to austenite twin boundaries, which appear to act as barriers to precipitate growth rather than as potential nucleation sites. Cementite precipitates were all connected to austenite grain boundaries or cementite grain boundary precipitates, confirming 3D observations made earlier on a specimen transformed at a longer isothermal transformation time. Widmanstatten lath precipitates appear to emanate only from grain boundary cementite precipitates. While edges of several Widmanstatten plate precipitates were observed to intersect with areas of ‘clean’ austenite grain boundary, they may or may not nucleate directly on austenite grain boundaries.

Electron backscatter diffraction was used in conjunction with deep etching to examine the relationship between the crystallography and three-dimensional morphology of cementite precipitates in an Fe–1.34 wt% C–13.1 wt% Mn steel. Orientation relationships (ORs) between more than 200 proeutectoid Widmanstatten cementite precipitates and the surrounding austenite were determined to be on or very near either the well-known Pitsch OR or the Farooque–Edmonds OR. Scanning electron microscopy of the same specimens after deep etching was used to determine the three-dimensional morphology of each of the precipitates for which an OR was determined. These precipitates could be identified as either monolithic plates or conglomerates of laths. Results show that monolithic plates consistently exhibit the Pitsch OR and conglomerates of laths have the Farooque–Edmonds OR, indicating that the precipitate morphologies are dictated by their orientation relationships.

Serial sectioning and computer-aided visualization of three-dimensional reconstructions were used to reveal the three-dimensional morphology, connectivity and distribution of proeutectoid cementite precipitates in a model Fe–1.34% C–13.0% Mn alloy. Only dendritic grain boundary precipitates and Widmanstatten precipitate morphologies were observed in the present alloy. Unlike earlier morphological studies of similar alloys, intragranular precipitates were not observed. Scanning and transmission electron microscopy of deeply etched specimens corroborated and augmented these results. Three-dimensional analysis has also made possible the first quantitative three-dimensional measurements of cementite precipitates and yielded a more precise classification of cementite precipitates than was possible with classical two-dimensional techniques.

The proeutectoid precipitation of Widmanstatten cementite has recently been shown to occur in two morphologies, either as broad monolithic plates or as aggregates of narrow laths, in a Fe-13 wt% Mn-1.3 wt% C steel, see Figure 1. Each of these morphologies correlates to a specific orientation of the cementite crystal within the austenite matrix; precipitates with a Pitsch orientation relationship adopt the monolithic plate morphology while precipitates with a Farooque-Edmonds orientation relationship develop as laths. In this paper, we examine the morphology and interfacial structure of these Widmanstatten cementite laths.The typical three-dimensional morphology of Widmanstatten cementite laths can be revealed by etching with a 10% nitric acid solution in methanol. This procedure selectively etches the austenite matrix without affecting the cementite precipitates (see Figure 1). By similarly etching an electropolished thin foil TEM sample, the three-dimensional morphology and internal structure of entire electron transparent cementite precipitates can be examined.