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Laser powder bed fusion (L-PBF) was utilized to produce specimens in Ti-6Al-4V, which were subjected to a bi-lamellar heat treatment, which produces microstructures consisting of primary α-lamellae and a fine secondary α-phase inside the inter-lamellar β-regions. The bi-lamellar microstructure was obtained as (i) a direct bi-lamellar heat treatment from the asbuilt condition or (ii) a bi-lamellar heat treatment preceded by a β-homogenization. For the bi-lamellar treatment with β-homogenization, cooling rates in the range 1−500 K/min were applied after homogenization in β-region followed by inter-critical annealing in the α + β region at various temperatures in the range 850-950 °C. The microstructures were characterized using various microscopical techniques. Mechanical testing with Vickers hardness indentation and tensile testing was performed. The bi-lamellar microstructure was harder when compared to a soft fully lamellar microstructure, because of the presence of fine α-platelets inside the β-lamellae. Final low temperature ageing provided an additional hardness increase by precipitation hardening of the primary α-regions. The age hardened bi-lamellar microstructure shows a similar hardness as the very fine, as-built martensitic microstructure. The bi-lamellar microstructure has more favorable mechanical properties than the as-built condition, which has high strength, but poor ductility. After the bi-lamellar heat treatment, the elongation was improved by more than 250 %. Due to the very high strength of the as-built condition, loss of tensile strength is unavoidable, resulting in a reduction of tensile strength of ∼18 %.

This contribution addresses the effect of sub-zero Celsius treatments at cryogenic temperatures on the steel microstructure. It is shown that the formation of martensite, including the socalled isothermal, i. e. time dependent or thermally activated, martensitic product, can provide an explanation to the observations in the literature that both the temperature of cryogenic treatment and the holding time at cryogenic temperatures may have an influence on the performance of steel products in service. This review traces the most important stages in the development of cryogenic treatments along the last 95 years and describes it in parallel with the description of the current state of understanding of the kinetics of martensitic transformation in steel. In the last part of the contribution, the new insight is put into context with a practical example where various types of subzero Celsius treatments are applied.

Low temperature nitrided austenitic stainless steel exhibits strong orientation dependent case growth kinetics. Anisotropy effects during low temperature gaseous nitriding of single crystals of austenitic stainless steel with surface-normal orientations 〈100〉, 〈111〉 and 〈10 7 2〉 were investigated with thermogravimetry, X-ray diffractometry (XRD), glow discharge optical emission spectroscopy (GD-OES), nano-indentation and electron back-scatter diffraction (EBSD). Case growth kinetics were fastest for 〈100〉 and slowest for 〈111〉. Depth-profiling over the case depth was accomplished by successive layer removal combined with X-ray diffraction and nano-indentation. The results indicate that anisotropic case-growth kinetics is related to early nucleation of long-range ordered (LRO) over-stoichiometric γ’N−Me4N1+ x (Me=metal atoms) in 〈100〉, which is inseparable from short range ordered (SRO) expanded austenite, γN. Depth ranges of elasto-plastic and purely plastic accommodation of the nitrogen induced lattice expansion were distinguished. In the elasto-plastic region stress-gradient enhanced diffusion of N is observed.

Long-term oxidation behavior of Zr48Cu36Al8Ag8 BMG under atmospheric conditions at 600 K was investigated using in-situ X-ray diffraction analysis. Two oxide zones develop: an outer oxide zone (OOZ), consisting of a stratified distribution of CuOx, Ag, and ZrO2, and an inner oxide zone (IOZ) consisting of tetragonal ZrO2 showing Cu-enriched lines inclined to the surface. The stratified microstructure in the OOZ and the Cu enrichments in the IOZ are ascribed to micro-cracks resulting from the compressive stresses induced by ZrO2 formation. Segregation of noble elements towards these oxidation-generated free surfaces “repairs” the micro-cracks and can consistently explain the microstructural features.

Expanded austenite obtained by gaseous carburizing of stainless steel was investigated with X-ray diffraction to determine composition-depth and residual stress-depth distributions. Avoiding ghost stress effects in the analysis of X-ray diffraction data, the obtained composition- and stress-depth profiles are in excellent quantitative agreement with those obtained with other techniques. The residual stress-depth profile was attempted calculated from the composition-depth profile assuming elastic-plastic accommodation of the lattice expansion. In the model, composition-dependence of Young's modulus, yield stress and work hardening exponent were considered. Excellent quantitative agreement was achieved between the experimental and numerical residual stress-depth profiles.

The evolution of the substructure and the distribution of interstitial elements in lath martensite during tempering in soft martensitic stainless steel X4CrNiMo16-5-1 was studied with line profile analysis of diffractograms from energy dispersive synchrotron X-ray diffraction, local chemical analysis with atom probe tomography and orientation mapping with electron backscatter and transmission Kikuchi diffraction. Martensite formation occurred below 135 °C without auto-tempering and led to a dislocation density in martensite of 3.8 ∙ 1015 m−2, as determined from X-ray line profile analysis. On tempering, carbon and nitrogen segregated to low-angle and high-angle grain boundaries. Recovery commenced above 550 °C and led to a reduction in dislocation density to a steady value of 4 ∙ 1014 m−2 from 600 to 750 °C. Further tempering led to a second increase in dislocation density at room temperature, owing to the transformation of reverted austenite, formed above 650 °C, into martensite on cooling. It was observed that the recovery of martensite competes with the formation of reverted austenite. The interpretation of the coherently diffracting domain size obtained from X-ray line profile analysis was critically discussed in the context of the internal structure in martensite.

Surface engineering of Zr 51.3 Al 8.5 Cu 31.3 Ni 4 Ti 4.9 bulk metallic glass (BMG) by gaseous oxidizing below the glass-transition temperature is investigated as a means to introduce compressive residual stress in the surface region. The ZrCuAl-based BMG was exposed to an extremely low oxygen partial pressure of 10−41 bar at 600 K for 60 h. The oxidizing treatment led to the formation of an internal oxidation zone, consisting of finely dispersed nano-crystalline cubic ZrO 2 (c-ZrO2 ), metallic regions inclined with the surface and Cu-hillocks at the surface. The stresses introduced by the volume expansion associated with oxidation were evaluated from i) the lattice strains within c-ZrO2 , as determined with an X-ray diffraction (XRD) based method, and ii) strain-relaxation as a response to annular focused ion beam (FIB) milling, as monitored with digital image correlation (DIC). XRD analysis yielded -1.5 GPa (compressive stress) in the nano-crystalline c-ZrO2 , while the strain relaxation monitored with FIB-DIC analysis indicated compressive residual stresses of −1.4 GPa in the internal oxidation zone. The strains and stresses determined with the independent measurement methods are discussed. The quantitative macro-strains are discussed in relation to the microstructural features and stress relaxation mechanisms during evolution of the internal oxidation zone.

AISI 304L stainless steel in austenitized and in solution nitrided condition was severely mechanically deformed by surface roller burnishing. High-temperature solution nitriding was applied to achieve a nitrogen-concentration depth profile, leading to a depth-gradient in the austenite stability. X-ray diffraction, electron microscopy and hardness indentation were applied for characterization of the graded microstructures obtained by combining a composition profile and a deformation profile. While severe plastic surface straining of an austenitized specimen leads to a deformation-induced transformation of austenite into martensite, the solution nitrided specimen remains austenitic upon deformation, even in the region where nanocrystallization occurs. The deformation mechanisms operable in the nitrogen-stabilized austenitic stainless steel, i.e. twinning or dislocation glide, depend on the combination of applied plastic strain/strain rate, and the nitrogen-concentration dependent stacking fault energy.

Three nickel electrodeposits with comparable grain size were synthesized by tailoring the electrodeposition conditions. Thorough microstructural characterizations including electron backscatter diffraction, ion channeling contrast imaging, electron channeling contrast imaging, transmission Kikuchi diffraction, transmission electron and high annular dark-field imaging were applied. The deposits contain a high density of twin boundaries with similar microstructures in terms of grain boundary character. These materials were annealed at various temperatures to study the microstructural evolution, and hence, their thermal stability. The differences in the character of twin boundaries and morphology of the grain boundaries in as-deposited state and their influence on the microstructural evolution at elevated temperatures are analyzed. The importance of incoherent twin boundaries, and the interaction of mobile general high angle boundaries with stationary boundaries are discussed. Finally, an optimal design for high thermal stability is proposed, based on the mechanisms that were inferred from the results. Keywords: Thermal stability, Twin boundaries, Annealing, Grain boundary engineering, Nickel, Electrodeposition

Magnetometry was applied to investigate the formation of α/α´martensite in 13ferrous alloys during immersion in boiling nitrogen and during re-heating to room temperature at controlled heating rates in the range 0.0083-0.83 K s-1. Data showsthat in 3 of the alloys, those that form {5 5 7}γ martensite, no martensite developsduring cooling. For all investigated alloys, irrespective of the type of martensiteforming, thermally activated martensite develops during heating. The activationenergy for thermally activated martensite formation is in the range 8‒27 kJ mol-1and increases with the fraction of interstitial solutes in the alloy

Magnetometry was applied to quantify the fraction of austenite retained in Fe-C alloys subjected to various treatments. These treatments consisted of: (i) water quenching; (ii) water quenching followed by immersion in boiling nitrogen and again in water; (iii) as for (ii) but re-heating from 77 K at a rate of 0.0083 K s− 1; (iv) as for (iii) but (re-)heating at 0.167 K s− 1 interrupted by an isothermal step. Data was coupled with hardness measurements and demonstrates that the re-heating conditions from 77 K significantly influence the fraction of austenite retained at the end of the thermal cycle.

A face-centered cubic (FCC) phase formed during the carbo-oxidation of hexagonal close-packed (HCP) α-Titanium at 600 to 700 °C. The FCC crystal structure was confirmed using transmission electron diffraction, X-ray diffraction and high angle annular dark field scanning transmission electron microscopy. The a lattice parameter for the FCC resulting from the three methods was consistent with FCC sublattice of titanium atoms in δ-TiH2-x. The FCC phase is hypothesized to form partly due to hydrogen pick-up, the stress state in the foil caused by the mismatch between body centered cubic iron-stabilized β-Titanium and HCP α-Titanium and the tensile stress present in front of the oxygen concentration profile. A transformation of α-Titanium into δ-TiH2-x is accomplished by introducing Shockley partial dislocations on every 2nd closest paced plane, resulting in an orientation relation that has previously been observed for thermally induced FCC γ-Ti. The TiH2-x phase was shown to decompose upon vacuum annealing at 500 °C.

Energy dispersive synchrotron X-ray diffraction was applied to investigate in situ the evolution of lattice strains and stresses in austenite and martensite during quenching and tempering of a soft martensitic stainless steel. In one experiment, lattice strains in austenite and martensite were measured in situ in the direction perpendicular to the sample surface during an austenitization, quenching, and tempering cycle. In a second experiment, the sin2 ψ method was applied in situ during the austenite-to-martensite transformation to distinguish between macro- and phase-specific micro-stresses and to follow the evolution of these stresses during transformation. Martensite formation evokes compressive stress in austenite that is balanced by tensile stress in martensite. Tempering to 748 K (475 °C) leads to partial relaxation of these stresses. Additionally, data reveal that (elastic) lattice strain in austenite is not hydrostatic but hkl dependent, which is ascribed to plastic deformation of this phase during martensite formation and is considered responsible for anomalous behavior of the 200 γ reflection.

Chromium-rich nitride precipitates in production of nickel-free austenitic stainless steel plates via pressurised solution nitriding of Fe–22.7Cr–2.4Mo ferritic stainless steel at 1473 K (1200 °C) under a nitrogen gas atmosphere was investigated. The microstructure, chemical and phase composition, morphology and crystallographic orientation between the resulted austenite and precipitates were investigated using optical microscopy, X-ray Diffraction (XRD), Scanning and Transmission Electron Microscopy (TEM) and Electron Back Scatter Diffraction (EBSD). On prolonged nitriding, Chromium-rich nitride precipitates were formed firstly close to the surface and later throughout the sample with austenitic structure. Chromium-rich nitride precipitates with a rod or strip-like morphology was developed by a discontinuous cellular precipitation mechanism. STEM-EDS analysis demonstrated partitioning of metallic elements between austenite and nitrides, with chromium contents of about 80 wt.% in the precipitates. XRD analysis indicated that the Chromium-rich nitride precipitates are hexagonal (Cr, Mo)2N. Based on the TEM studies, (Cr, Mo)2N precipitates presented a (1 1 1)γ//(0 0 2)(Cr, Mo)2N, (Formula presented.)γ//(Formula presented.)(Cr, Mo)2N orientation relationship with respect to the austenite matrix. EBSD studies revealed that the austenite in the regions that have transformed into austenite and (Cr, Mo)2N have no orientation relation to the untransformed austenite.

Expanded austenite on stainless steel with a high interstitial nitrogen content is characterized by elasto-plastic accommodation of the large composition-induced lattice expansion leading to huge compressive residual stress. The elasto-plastic accommodation as well as the (steep) concentration profile has implications for the measurement strategy to determine lattice strains and associated residual stresses with X-ray diffraction. Lattice strain measurements were performed on nitrided as well as subsequently de-nitrided expanded austenite on AISI 316L stainless steel, for various grazing incidence angles. It is demonstrated that keeping the information depth constant by choosing appropriate combinations of grazing incidence and tilt angle leads to reliable results for the 111 reflection, while the 200 reflection should be avoided. Further, it is shown for the first time that the residual stresses in expanded austenite can be tailored by de-nitriding after nitriding, such that a condition of virtually zero stress at the surface is obtained.

A 3D elastoplastic phase-field model is used to study the effect of thermal cycling on martensitic transformation as well as on mechanical strengthening of both austenite and martensite in stainless steel. The results show that with an increasing number of thermal cycles, martensite becomes more stable. Increase in strain, plastic strain and strain hardening lead to strengthening of austenite.

Hexagonal e-iron nitride and e-iron carbonitride phases are formed on nitriding and nitrocarburizing of iron and steel surfaces and can exist in broad compositional ranges. Long-range nitrogen ordering and magnetic properties for e-iron nitrides and their dependence on composition have been the focus of several studies. So far, limited attention has been paid to the carbonitrides. In the current work, the effects of substitution of nitrogen by carbon on the interstitial ordering and magnetic properties in Fe2(C,N)1-z are explored using neutron diffraction, Mossbauer spectroscopy and vibrating sample magnetometry. Neutron diffraction patterns showed 001 and 301 superstructure reflections, confirming a previously proposed structural model in space group P 3 ¯ 1 m (compared to P6322 for the pure nitrides). On partial substitution of nitrogen by carbon in e-iron nitride the Curie temperature, the saturation magnetization and the hyperfine fields of the iron atoms are increased, while isomer shifts are decreased. The effects on the a and c lattice parameters indicate a change in interstitial ordering, which is related to more favorable interactions between a nitrogen and carbon atom than among nitrogen atoms. This interaction leads to additional interstitial (short-range) ordering and a decrease in the c lattice parameter, while the a lattice parameter is largely unaffected.

Additive Manufacturing (AM) enables the design of complex part geometries for high-temperature applications. Laser Powder Bed Fusion or Direct Energy Deposition of the nickel-based superalloy MAR-M247 poses a challenge in AM due to its poor weldability. Binder Jetting does not utilize a heat source and interaction with a liquid metal during build-up and thus has the potential to overcome this limitation. In this study, MAR-M247 was manufactured by Binder Jetting and subsequently characterized regarding density and microstructure. Combinations of two different Hot-Isostatic-Pressing (HIP) treatments (T = 1120–1180 °C, p = 1000–1500 bar, t = 4 h) and four different heat-treatments involving solution treatment (T = 1250 °C, t = 4 h) and different aging steps (T = 700–1000 °C, t = 12–24 h) were applied to study the densification and microstructural evolution of binder-jetted MAR-M247. The influence of the build direction in combination with HIP is studied concerning the resulting density and mechanical properties at room temperature. The results show that close-to-full densification can be achieved after HIP. Subsequent solution treatment and double-aging after HIP lead to a favorable bimodal microstructure. A process chain for binder-jetted MAR-M247 is presented, which yields tensile properties comparable to those of analogously post-processed cast material. Further possibilities of microstructural optimization and the design philosophy are discussed in the light of the Binder Jetting process-chain.

Low-temperature (470 °C) gaseous carburization is successfully employed in surface hardening of a CoCrFeNi high entropy alloy. After low-temperature gaseous carburization, a carburized case of expanded face-centered cubic (FCC) structure, i.e., a carbide-free supersaturated interstitial solid solution of carbon (maximum of ~3.0 wt%) in FCC, forms in the surface region of the high entropy alloy. Due to solid solution strengthening by interstitially dissolved carbon atoms and strain hardening caused by plastic accommodation of the lattice expansion, the surface hardness of high entropy alloy is significantly enhanced. The surface hardness reaches 1221 HV0.1N, which is about 5 times the hardness of the substrate.

The austenitization and inter-critical annealing of X4CrNiMo16-5-1 (1.4418) supermartensitic stainless steel were investigated in-situ with synchrotron X-ray diffraction (XRD), dilatometry and differential scanning calorimetry (DSC) under isochronal heating conditions. Austenitization occurred in two stages: the austenitization started at approx. 600 °C, decelerated at approx. 700 °C at 60 to 75 v.% of transformed austenite, and first resumed after heating for approx. 100 °C. This plateau in the transformation curve was more dominant for faster heating rates. Inter-critical annealing at 675 and 700 °C revealed, that austenite can to a certain extent be stabilized to room-temperature. There was good agreement for the transformation curves yielded by dilatometry and XRD. Some deviation occurred due to the different applied heating principles, different temperature monitoring and the impact of surface martensite formation on the XRD measurement. The applicable temperature range for DSC as well as the close proximity of the Ac1- and the Curie-temperature limited the usage of the technique in the present case.

Pack cementation was used to produce Fe1−xAl and Fe2Al5 diffusion coatings on ferritic-martensitic steel P91 and a Ni2Al3 diffusion coating on pure nickel. The performance of diffusion coatings against high-temperature corrosion induced by potassium chloride (KCl) was evaluated by exposing the samples at 600 °C for 168 h in static lab air under KCl deposit. In addition, a salt-free experiment was performed for comparison. Microstructure, chemical and phase composition of the samples were analyzed with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD) before and after the exposures. It was found that all the diffusion coatings formed protective oxides under salt-free exposure in air. Under the salt deposit, Fe1−xAl showed local failure while on large parts of the sample a protective layer had formed. Fe2Al5 was attacked over the entire surface and the dominant mode of attack was selective aluminum removal. Ni2Al3 showed excellent performance and no sign of attack was observed anywhere on the sample.

Nickel layers with 〈100〉, 〈210〉, 〈110〉 and 〈211〉 fiber textures were electrodeposited from additive-free Watts type electrolytes by adjusting both the pH and the applied current density. Quantitative crystallographic texture analysis by XRD was supplemented by micro-texture analysis applying EBSD. While XRD results correspond to absorption-weighted averages over the top part of the layer, EBSD on the cross section allowed studying the texture evolution as a function of distance to the substrate. Although layer growth started on amorphous substrates, implying that nucleation occurs unbiased by the substrate, often relatively strong fiber textures develop already at the early stage of growth. These fiber textures can further develop into other preferred fiber axis further away from the substrate. The experimental results demonstrate that already in an early stage of deposition there are major differences in the developing textures. The evolution of the substrate-adjacent textures into the texture of thick layers depends strongly on the deposition conditions.

The present work deals with the evaluation of the residual stress profiles in expanded austenite by applying grazing incidence X-ray diffraction (GI-XRD) combined with successive sublayer removal. Annealed and deformed (e eq=0.5) samples of stable stainless steel EN 1.4369 were nitrided or nitrocarburized. The residual stress profiles resulting from the thermochemical low-temperature surface treatment were measured. The results indicate high-residual compressive stresses of several GPa’s in the nitrided region, while lower-compressive stresses are produced in the carburized case. Plastic deformation in the steel prior to thermochemical treatment has a hardly measurable influence on the nitrogen-rich zone, while it has a measurable effect on the stresses and depth of the carbon-rich zone.

In the past decades, high nitrogen steels (HNS) have been regarded as substitutes for conventional austenitic stainless steels because of their superior mechanical and corrosion properties. However, the main limitation to their wider application is their expensive production process. As an alternative, high-temperature solution nitriding has been applied to produce HNS from three commercially available stainless steel grades (AISI 304L, AISI 316, and EN 1.4369). The nitrogen content in each steel alloy is varied and its influence on the mechanical properties and the stability of the austenite investigated. Both hardness and yield stress increase and the alloys remain ductile. In addition, strain-induced transformation of austenite to martensite is suppressed, which is beneficial for subsequent low-temperature nitriding of the surface of deformed alloys. The combination of high- and low-temperature nitriding results in improved properties of both bulk and surface.

A complete first-principles thermodynamic model was developed and applied to hexagonal close-packed structure e -Fe3N. The electronic structure was calculated using density functional theory and the quasiharmonic phonon approximation to determine macroscopic thermodynamic properties at finite temperatures was generalized in terms of the partition function for any lattice of interest. Specially, thermal expansion of the hexagonal close-packed e phase with two independent lattice parameters was studied by means of the present model and first-principles phonon calculations. The present predictions of thermal expansion of e -Fe3N are in good agreement with experimental data.

The carbon solubility in expanded austenite is investigated by controlled low temperaturegaseous through-carburizing of AISI 316 stainless steel thin foils with thermogravimetry and synchrotron powder diffraction analysis. Carburizing is carried out in C2H2–H2–N2 and CO–H2–N2 atmospheres at 380–420 °C and 465–470 °C, respectively. Hägg carbide (x-M5C2)develops when the carbon content in the expanded austenite exceeds the metastable solubility limit; the transformation of carbon expanded austenite into Hägg carbide occurs irrespective of carburizing temperature in the investigated temperature range (380–470 °C). The maximum solubility of carbon in expanded austenite (380 °C) is found to correspond to an occupancy (yC) of 0.220 of the interstitial octahedral sites of the austenite lattice (i.e., 4.74wt%C). Decomposition of Hägg carbide into M7C3 occurs upon prolonged carburizing treatment orthermal exposure in inert atmosphere (in situ synchrotron experiments).

The crystal structure and magnetic properties of austenitic stainless steel with a colossal interstitial content, so-called expanded austenite, are currently not completely understood. In the present work, the magnetic properties of homogeneous samples of expanded austenite, as prepared by lowerature nitriding of thin foils, were investigated with magnetometry and Mössbauer spectroscopy. At room temperature, expanded austenite is paramagnetic for relatively low and for relatively high nitrogen contents (yN = 0.13 and 0.55, respectively, where yN is the interstitial nitrogen occupancy), while ferromagnetism is observed for intermediate nitrogen loads. Spontaneous volume magnetostriction was observed in the ferromagnetic state and the Curie temperature was found to depend strongly on the nitrogen content. For the first time, X-ray diffraction evidence for the occurrence of long-range interstitial order of nitrogen atoms in expanded austenite was observed for high nitrogen contents.

Kurzfassung Die Austenit-zu-Martensit-Umwandlung unterhalb Raumtemperatur wurde in situ mit Magnetometrie in Fe-N-, Fe-Cr-C- und Fe-Cr-Ni-basierten Legierungen untersucht. Nach dem Abschrecken von der Austenitisiertemperatur auf Raumtemperatur wurden die Legierungen in siedendem Stickstoff abgeschreckt. Anschließend wurde die martensitische Umwandlung während der Erwärmung auf Raumtemperatur verfolgt. Verschiedene konstante Erwärmraten zwischen 0,5 K/min und 10 K/min wurden untersucht. Eine Probe wurde als Referenz schockartig in Wasser aufgeheizt, um zu überprüfen, ob die Martensitbildung bei einer hohen Erwärmrate unterdrückt werden kann. Thermisch aktivierte Bildung von Martensit während dem Erwärmen wurde für alle getesteten Legierungen mit einer von der Erwärmrate abhängigen Umwandlungskinetik überzeugend demonstriert. Außerdem konnte mittels Magnetometrie gezeigt werden, dass die Erwärmrate den Martensitanteil während der Wärmebehandlung beeinflusst. Die Aktivierungsenergie für thermisch aktivierte Martensitbildung wurde mit der Methode nach Kissinger zu 11–18 kJ/mol bestimmt und nimmt mit dem Anteil der Zwischengitterelemente zu.

Vibrating sample magnetometry, VSM, was applied to follow the evolution of the austenite-to-martensite transformation in AISI D2 tool steel at cryogenic temperatures and to determine the associated changes in magnetic hardness. Data shows that the transformation can progress during cooling from room temperature (RT) to −193 °C and, more surprising, during re-heating from −193 °C to RT, indicating the formation of, so called, isothermal or thermally activated martensite. The thermally activated conversion of austenite into martensite occurred in the temperature range −33 °C ≤ T ≤ −173 °C and was fastest at −113 °C. Additionally, VSM revealed that the magnetic hardness decreases in correspondence with the formation of martensite. Data is interpreted in terms of continuous growth of the (ferromagnetic) martensite units at cryogenic temperatures that reduces the number of (paramagnetic) austenite regions retained in the material. Eventually, the work is put in perspective to arrive at an understanding of the effect of cryogenic treatment on the microstructure and performance of martensitic steels.

Low-temperature gaseous carburization is a surface modification method for austenitic stainless steels. In order to investigate the effects of low-temperature gaseous carburization on the fatigue behavior of AISI 316L, fully reversed axial fatigue tests were performed at room temperature on specimens with various remaining case depths. The fatigue performance of AISI 316L could be significantly improved; a 15% higher endurance limit is achieved after low-temperature gaseous carburization. After removal of the outer, brittle part of carburized case by electropolishing, the improvement of the fatigue performance is reduced. Fractography showed that for untreated specimens, fatigue cracks always initiated on the surface. For the carburized specimens, however, the locations of crack initiation sites depend on the applied stress levels. Compressive residual stresses in the case move the crack initiation site to the sub-surface; the lower the applied stress, the deeper the initiation site. A quantitative analysis of the effect on fatigue behavior forms the basis for a life prediction model, which can accurately predict the fatigue life of AISI316L steel after low temperature carburization.

The present work investigates the response of different gaseous thermochemical treatments on selective laser melted (SLM) Ti-6Al-4V. The resulting microstructures after thermochemical treatment were investigated with X-ray diffraction, light optical microscopy, scanning electron microscopy and Vickers-microhardness indentation. Nitriding, performed at 1000–1050 °C resulted in a diffusion zone of nitrogen in solid solution and surface compound layers consisting of TiN (and Ti2N at 1000 °C). Below the compound layer Al-enrichment of the α-zone was observed. Carbo-oxidising in a CO atmosphere at 1000–1050 °C resulted in deep diffusion zones and thick compound layers of the ternary compound TiC1-xOx. Both surface hardness and layer depth were found to increase with temperature and treatment time. Chemically controlled carbo-oxidising, applying the gas redox system CO-CO2, was performed at temperatures in the range 850–1050 °C, resulting in carbo-oxides and formation of oxides with increasing Ti:O ratio with increasing temperatures (rutile and Magnéli phases). Nitriding followed by (carbo-)oxidising treatment resulted in higher surface hardness owing to the formation of mixed interstitial compounds TiC1-x-yNxOy in the compound layer. The compound layer grew into the Al-rich zone as elongated structures.The improvement of wear by nitriding, carbo-oxidising and duplex nitriding/(carbo-)oxidising on SLM Ti-6Al-4V was evaluated by dry sliding wear testing. Lowering of the wear volume by up to a factor of 450 compared to an annealed reference sample was realised. Carbo-oxidising in CO at 1000 °C offered the best wear resistance and resulted in a lowering of the friction coefficient, averaging μ = 0.22, compared to μ = 0.45 for an annealed reference sample.

A multi-physics thermo-chemical-mechanical 1-dimensional implicit finite difference model is presented to simulate the evolution of composition and residual stress profiles over the expanded austenite case developing during low temperature nitriding of austenitic stainless steels. The model combines 1-dimensional diffusion of nitrogen in the depth direction with a concentration-dependent diffusivity, elasto-plastic accommodation of the lattice expansion, stress gradient-induced diffusion of nitrogen, solid solution-strengthening by nitrogen and trapping of nitrogen by chromium atoms. The rate of the surface reaction governing the transfer of nitrogen from the gas to the solid is unknown and was evaluated by inverse modelling. The modelling was applied adopting the surface reaction rate as the only fitting parameter and taking mass-uptake curves (thermogravimetry) as the constraint, while all other data were taken from established literature values. Very good agreement is achieved between the predicted and experimental composition-depth profiles. Further, the applicability of the present model to plasma nitriding was verified by simulating (not fitting) the evolution of composition-depth profiles obtained after plasma nitriding of stainless steel. The good to very good agreement of the present model's predictions with experimental data for gaseous and plasma nitriding, indicates that the essential multi-physics influences and parameters are taken into account, with a minimum of adjustable parameters.

This research focuses on establishing the relationship between the SLM process parameters, microstructure and mechanical properties in 3D printed ti-6Al-4V. To this end modelling of the thermal history, reflecting the applied process parameters, is linked to the as-printed microstructure, which, in turn, is linked to experimentally determined mechanical properties. The very high cooling rates in the centre of the specimen, as predicted by modelling, lead to a fine, martensitic microstructure, while the slower cooling rates close to the support structure lead to a fine lamellar α+β microstructure. The tensile properties, in particular the ductility, were found to depend on the printing orientation and surface finish.

The kinetics of \( \left\{ {5 \,5 \,7} \right\}_{\upgamma} \) lath martensite formation in (wt%) 17Cr-7Ni-1Al-0.09C and 15Cr-7Ni-2Mo-1Al-0.08C steels was assessed with magnetometry at sub-zero Celsius temperatures. Samples were cooled to 77 K by immersion in boiling nitrogen to suppress martensite formation. Thereafter, thermally activated martensite formation was monitored during: (i) isochronal (re)heating at different heating rates; (ii) isothermal holding at temperatures between 120 and 310 K. The activation energy, \( {\text{E}}_{\text{A}} \), of thermally activated martensite formation was quantified from the results of both isochronal and isothermal tests by applying a Kissinger-like method. In addition, the isothermal data was interpreted applying the approach presented by Borgenstam and Hillert. The results of the independent quantification methods were consistent and indicated an \( {\text{E}}_{\text{A}} \) in the range 9–13 kJ mol−1. Thereafter, the two methods were applied to evaluate the data available in the literature. The overall analysis showed that \( {\text{E}}_{\text{A}} \) varies in the range 2–27 kJ mol−1 and increases logarithmically with the total fraction of interstitials in the steel.

This article addresses an investigation of the influence of plastic deformation on low temperature surface hardening by gaseous nitriding of three commercial austenitic stainless steels: AISI 304, EN 1.4369 and Sandvik Nanoflex® with various degrees of austenite stability. The materials were plastically deformed to different equivalent strains by uniaxial tension. Gaseous nitriding of the strained material was performed in ammonia gas at atmospheric pressure at 703 K (430 °C) and 693 K (420 °C) depending on the material. Microstructural characterization of the as-deformed states and the nitrided case included X-ray diffraction analysis, reflected light microscopy and microhardness. The results demonstrate that a case of expanded austenite develops and that, in particular, strain-induced martensite has a large influence on the nitrided zone. ▪

The present contribution showcases the possibility for developing new surface hardenable stainless steels containing strong nitride/carbide forming elements (SNCFE). Nitriding of the commercial alloys, austenitic A286, and ferritic AISI 409 illustrates the beneficial effect of having SNCFE present in the stainless steel alloys. The presented computational approach for alloy design enables “screening” of hundreds of thousands hypothetical alloy systems by use of Thermo-Calc. Promising compositions for new stainless steel alloys can be selected based on imposed criteria, i.e. facilitating easy selection of candidate alloys designed for low temperature surface hardening.

Made available in DSpace on 2018-12-11T16:38:31Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-08-01 Surface layers of expanded austenite resulting from nitriding typically exhibit large gradients in residual stress and composition. Evaluation of residual-stress profiles is explored by means of grazing incidence X-ray diffraction (GI-XRD), probing shallow depths, combined with successive layer removal. Several factors complicating the stress determination are analysed and discussed: (1) ghost stresses arising from a small variation in the shallow information depths probed with GI-XRD, (2) selection of the grain interaction model used to calculate the X-ray elastic constants for conversion of lattice strains into residual stress and (3) the composition dependence of these elastic constants. Technical University of Denmark, Department of Mechanical Engineering, Produktionstorvet 425 Sao Paulo State University (UNESP) Sao Paulo State University (UNESP)

Laboratory testing on selected alumina and silica-forming alloys was performed to evaluate their performance against high temperature corrosion induced by potassium chloride (KCl). The alloys studied were FeCrAlY, Kanthal APM, Nimonic 80A, 214, 153MA and HR160. Exposure was conducted at 600 °C for 168 h in flowing N2(g)+5%O2(g)+15%H2O(g) (vol.%) with samples covered under KCl powder. A KCl-free exposure was also performed for comparison. Corrosion morphology and products were studied with scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometry (XRD). It was observed that alloying with aluminum did not lead to the formation of protective alumina for the studied alloys. The silicon containing stainless steel 153MA showed an analogous performance to low-silicon austenitic stainless steels of similar chromium and nickel contents. For alloy HR160, a potassium-chromium-silicon-oxygen containing layer forms as the innermost corrosion product. The layer was uniformly distributed over the surface and appears to render some protection as this alloy exhibited the best performance among the investigated alloys. To reveal further aspects of the corrosion mechanism, Nimonic 80A was exposed in static laboratory air for the same duration and temperature with either KCl or K2CO3 deposits. Comparison of results obtained with these experiments showed that both potassium and chlorine can play a role in material degradation by KCl.

The kinetics of lath martensite formation in Fe–17·3 wt-%Cr–7·1 wt-%Ni–1·1 wt-%Al–0·08 wt-%C stainless steel was investigated with magnetometry and microscopy. Lath martensite forms during cooling, heating and isothermally. For the first time, it is shown by magnetometry during extremely slow isochronal cooling that transformation rate maxima occur, which are interrupted by virtually transformation free temperature regions. Microscopy confirms martensite formation after athermal nucleation of clusters followed by their time dependent growth. The observations are interpreted in terms of time dependent autocatalytic lath martensite formation followed by mechanical stabilisation of austenite during the transformation process.

A steel containing 16 wt.% Cr, 5 wt.% Ni and 3 wt.% Cu was transformed into martensite by applying isochronal, i.e. constant rate, cooling followed by isothermal holding. The formation of martensite was monitored with dilatometry. A series of retardations and accelerations of the transformation was observed during isochronal cooling for cooling rates ranging from 1.5 to 50 K min −1 . The cooling rate in the isochronal stage was observed to influence the transformation rate in the isothermal stage. Electron backscatter diffraction was applied to determine the morphology of the martensite, which was of lath type, and to investigate the microstructure of the material. No influence of the cooling rate on the scale of the microstructure was observed. The series of retardations and accelerations of the transformation is interpreted in terms of the combined effect of the strain and interfacial energy introduced in the system during martensite formation, which stabilizes austenite, and autocatalytic nucleation of martensite.

A series of nickel layers was electrodeposited at different current densities from a Watts type electrolyte containing the additive 2-butyne-1,4-diol in various concentrations. The internal structure of the nickel electrodeposits was systematically investigated applying complementary microscopic and diffraction based characterization methods involving both surface and cross section investigations with different resolution and statistics. The complementary information gathered from combining the various techniques of microstructure characterization, in particular, supported the investigation of twins and the preferred crystallographic orientations of grains in the electrodeposits. Additive-free deposition results in relatively large columnar grains with numerous growth twins of nanoscale dimensions. The presence of the additive in the electrolyte yields considerable grain refinement already at low additive concentration, but the preferred crystallographic orientation of grains changes gradually with increasing additive concentration and twins with nanoscale dimensions are detected also in these samples.

Ni-based alloys with high Cr contents are not only known for their excellent high temperature and hot corrosion resistance, but are also known for poor mechanical properties and difficult workability. Powder metallurgical (PM) manufacturing of alloys may overcome several of the shortcomings encountered in materials manufacturing involving solidification. In the present work, six PM Ni-based alloys containing 35 to 45 wt pct Cr and 3.5 to 6 wt pct Nb were produced and compacted via hot isostatic pressing. Samples were heat treated for up to 1656 hours at either 923 K or 973 K (650 °C or 700 °C), and the microstructures and mechanical properties were quantified and compared to thermodynamic calculations. For the majority of the investigated alloys, the high Cr and Nb contents caused development of primary populations of globular α-Cr and δ (Ni3Nb). Transmission electron microscopy of selected alloys confirmed the additional presence of metastable γ″ (Ni3Nb). A co-dependent growth morphology was found, where the preferred growth direction of γ″, the {001} planes of γ-Ni, caused precipitates of both α-Cr and δ to appear in the form of mutually perpendicular oriented disks or plates. Solution heat treatment at 1373 K (1100 °C) followed by aging at 973 K (700 °C) produced a significant strength increase for all alloys, and an aged yield strength of 990 MPa combined with an elongation of 21 pct is documented for Ni 40 wt pct Cr 3.5 wt pct Nb.

The present work deals with the evaluation of the residual-stress profile in expanded-austenite by successive removal steps using GI-XRD. Preliminary results indicate stresses of several GPa's from 111 and 200 diffraction lines. These stresses appear largest for the 200 reflection. The strain-free lattice parameter decayed smoothly with depth, while for the compressive stress a maximum value is observed at some depth below the surface. Additionally a good agreement was found between the nitrogen profile determined with GDOES analysis and the strain-free lattice parameter from XRD.

The austenite to martensite transformation in a semi-austenitic stainless steel containing 17 wt-%Cr, 7 wt-%Ni and 1 wt-%Al was investigated with vibrating sample magnetometry and electron backscatter diffraction. Magnetometry demonstrated that, within experimental accuracy, martensite formation can be suppressed on fast cooling to 77 K as well as on subsequent fast heating to 373 K. Surprisingly, martensite formation was observed during moderate heating from 77 K, instead. Electron backscatter diffraction demonstrated that the morphology of martensite is lath type. The kinetics of the transformation is interpreted in terms of athermal nucleation of lath martensite followed by thermally activated growth. It is anticipated that substantial autocatalytic martensite formation occurs during thermally activated growth. The observation of a retardation of the transformation followed by a new acceleration during slow isochronal (i.e. at constant rate) cooling is interpreted in terms of the combined effect of the strain energy introduced in the system during martensite formation, which thermodynamically and/or mechanically stabilises austenite, and autocatalytic nucleation of martensite.