Showing papers on "Austenite published in 1984"

Chromium depletion in the vicinity of carbides in sensitized austenitic stainless steels

Abstract: The analytical electron microscope (AEM) was used to examine the microstructure of type 316LN stainless steel alloys which had been annealed for 50 to 300 hours in the temperature range 600 to 700 °C. The M23C6 carbide chemistry and distribution are described as a function of heat treatment.X-ray spectroscopy in the AEM revealed significant chromium depletion at grain boundaries in the vicinity of carbides for samples aged at 50 and 100 hours at 650 °C and 100 and 300 hours at 700 °C, with lower grain boundary chromium values observed at 650 °C than at 700 °C. The width of the chromium depleted zone normal to the grain boundaries increased with increasing annealing time and/or temperature. Measurements of chromium concentration along the grain boundaries away from a carbide were made after aging at 700 °C for 100 hours, and the chromium level rose steadily until the bulk value was reached at a distance of ~3μm from the carbide. The width of the chromium depleted zone normal to the boundaries in the same sample was an order of magnitude less. Some molybdenum depletion was also found at the grain boundaries, and the Mo-depletion profiles were in form and extent similar to the chromium results. Simple thermodynamic models were used to calculate the equilibrium value of chromium at the carbide-matrix interface, and the chromium distribution along and normal to the grain boundaries. The results of these models agreed well with the AEM results, and the agreement can be improved by considering the effect of electron probe configuration on the AEM measurements. The calculated thermodynamic data and the AEM results were related to the corrosion behavior of the alloys. The occurrence of severe asymmetries in some concentration profiles normal to the grain boundaries, which increased with increasing annealing temperature or time, was shown to be due to boundary movement during the discontinuous precipitation of M23C6 carbides.

High temperature oxidation of iron-manganese-aluminum based alloys

Abstract: The isothermal oxidation resistance of high purity iron-manganese-aluminum alloys containing from 0 to 40% manganese and from 0 to 15% aluminum was investigated at 600, 800, and 1000°C in pure oxygen at a pressure of 200 torr for periods up to 100 hr They were subsequently examined using SEM and metallographic techniques, and an oxide map showing the alloy structure and general oxidation behavior at 800°C was produced Scales formed on alloys which contain insufficient aluminum to form protective alumina have structures which depend largely upon the concentration of manganese in the alloy Alloys which contain more than 75% manganese form manganese rich scales, whereas alloys which contain lower levels of manganese form scales that are composed almost entirely of the oxides of iron Small manganese oxide nodules grow through the alumina scales which form on alloys containing in excess of 9% aluminum The most oxidation resistant alloys, having compositions within the range Fe-(5–10)%Mn-(6–10)% Al, develop continuous protective alumina scales and are totally ferritic Austenite is detrimental to the oxidation resistance of duplex alloys as it promotes the breakdown of preexisting alumina scales and the growth of bulky manganese rich oxides Small additions of chromium are beneficial and reduce the concentration of aluminum required to form protective alumina scales

Solubility of niobium carbide and niobium carbonitride in alloyed austenite and ferrite

Abstract: The available data on the solubility of niobium carbide and niobium carbonitride in plain carbon and alloyed austenite has been analyzedvia dilute solution thermodynamics with a view to establishing a consistent set of interaction parameters for predicting austenite + niobium carbonitride equilibria. The computation algorithm includes the prediction of phase mass fractions as a function of alloy composition and temperature between 900° and 1300 °C (tie lines). Analogous ferrite equilibria are included.

Diffusion coatings of steels: Formation mechanism and microstructure of aluminized heat‐resistant stainless steels

Abstract: The effect of base‐alloy composition on the microstructure and mechanical and thermal stabilities of aluminum diffusion coatings has been studied for 316, 310, and I800H stainless steels, by optical, microprobe, transmission/scanning transmission electron microscopy, and microhardness testing. In all the diffusion aluminized alloys, two distinct coating layers form: an outer aluminide layer and an inner, interdiffusion layer. The substrate austenite stability is the single most important parameter affecting the thickness, phase distribution, and microchemistry of these two layers. TEM/STEM analyses showed that the interdiffusion layer is a ‘‘natural composite’’ made up of a uniform dispersion of the hard nickel aluminide phase (B2) in a soft ferrite matrix. Formation of this layer involves ‘‘ferritization’’ of the substrate, a process akin to pearlitic transformation in carbon steels. The interdiffusion layer demonstrated high hardness with good mechanical integrity and its thermal stability with the subs...

Embrittlement of Steels Occurring in the Temperature Range from 1 000 to 600°C

Abstract: Characteristic features of the embrittlement occurring in the temperature range from 1 000 to 600°C in various kinds of steels were examined by hot stage tensile test as well as fractographic analysis, and the mechanism of this embrittlement is discussed.The embrittlement is caused by the slower strain rate of the tensile test and fracture occurs along the austenite grain boundary by either grain boundary sliding or by localization of strain in the film-like proeutectoid ferrite produced by the γ-α transformation.Steels such as ferritic steel and electrolytic iron do not have this embrittlement, while carbon steels containing 0.05 to 0.4% carbon and fully austenitic steels reveal this embrittlement. In the case of carbon steels, sulfur, nitrogen and niobium are detrimental elements while aluminum and phosphorus suppress it depending on their content and state of existence in the steels.The stress-strain analysis by an Instron type machine shows that the restoration process either by dynamic recovery or by dynamic recrystallization plays an important role in this embrittlement. Another finding is that proeutectoid ferrite deforms preferentially in the austenite plus ferrite region because of very low levels of flow stress and work hardening rate in this ferrite.Thus, factors governing this embrittlement are the degree of ease of the recrystallization in austenitic steel relating to the grain boundary sliding, and the formation of film-like proeutectoid ferrite produced by the γ-α transformation in the case of carbon steels. Grain boundary precipitates such as sulfides and carbonitrides act as nuclei of voids and thus promote this embrittlement.

Austempered ductile iron—process control and quality assurance

Abstract: Recent interest in the development of austempered ductile cast irons has resulted in considerable study of the physical metallurgy and mechanical properties of these high strength, high toughness cast irons. Equally important is the identification of process control and quality assurance factors to achieve the desired properties successfully and consistently. In this study, aspects of austempered ductile iron quality control are reviewed including the production of quality ductile iron that will respond to austempering heat treatments, heat treatment process control variables to achieve the desired properties, and non-destructive techniques for quality assurance. A distinction is made between austempered ductile irons transformed at high temperatures and austempered/bainitic ductile irons transformed at low temperatures. Austempering response is a complex interaction between all heat treatment variables and chemical composition on a macroscopic and microscopic level. Alloying elements are essential to provide sufficient hardenability (or austemperability) for heavy section heat treatment. Austenitizing temperature as well as austempering time and temperature affect the transformation response for a given alloy. In addition, segregation causes a non-uniform transformation response within the material on a microscopic level. In many cases the final properties of austempered ductile iron can be directly related to the amount of stabilized (retained) austenite present in the final structure. Non-destructive techniques to measure stabilized austenite are discussed and evaluated. Problems associated with dimensional control for critical tolerance components are highlighted.

Ginzburg-Landau theory and solitary waves in shape-memory alloys

Abstract: A one-dimensional dynamic Ginzburg-Landau theory of the martensitic phase transition in shape-memory alloys is established. The nonlinear equations of motion yield solitary wave solutions of kink and of soliton type. The kink solutions which cannot move without external force represent single domain walls either between austenite and martensite or between two martensite variants. The soliton solutions correspond to a matrix of austenite or of martensite containing a moving sheet of the other phase. The velocity of the solitons depends on their amplitude. In the static case they reduce to the critical nucleus. The energy of each type of solitary waves is calculated.

Laser processing of cast iron for enhanced erosion resistance

Abstract: The surfaces of nodular and gray cast iron specimens have been modified by CO2 laser processing for enhanced hardness and erosion resistance. Control of the near-surface microstructure was achieved primarily by controlling resolidification of the laser melted layer through variations in laser beam/target interaction time and beam power density. Typical interaction times and power densities used in this study were 5 msec and 500 kW/cm2. Analysis of the laser melted surface showed a dramatic increase in hardness and a greatly refined microstructure. Depending on the processing parameters, two basic kinds of microstructure can be produced in the laser hardened layer—a feathery microstructure with a very high hardness (up to 1245 HV) and a dendritic microstructure with a metastable, fully austenitic matrix and a lower hardness (600 to 800 HV). Erosion testing was done in a rotating paddle device using slurries of SiO2 or SiC in water. Weight loss and crater profile measurements were used to evaluate the erosion characteristics of the various microstructures. Both ductile and gray cast iron showed marked improvement in erosion resistance after laser processing.

Method of processing a nickel/titanium-based shape memory alloy and article produced therefrom

Abstract: There is disclosed a method of processing a nickel/titanium-based shape memory alloy. The method comprises overdeforming the alloy so as to cause at least some amount of nonrecoverable strain, temporarily expanding the transformation hysteresis by raising the austenite transformation temperature, removing the applied stress and then storing the alloy at a temperature less than the new austenite transition temperature. There is also disclosed an article produced from this method.

Effect of Prior Cold Work on the Martensite Transformation in SAE 52100

Abstract: Numerous publications refer to the phase transformations and properties of SAE 52100 steel, and this paper concerns itself with the effect of prior cold deformation on the martensitic hardening response. TheAc1 and Ac3 temperatures are lowered due to cold work as is theMs with a resultant increase in the retained austenite content for a given hardening cycle. Significantly, the prior cold deformation results in a refinement of the austenite grain size. The low angle dislocation cells produced by the cold deformation recover during the heating to the austenitizing temperature to form fine ferrite subgrains. The intersections of the fine ferrite subgrains with the spheroidal carbides in the soft annealed microstructures are preferential sites for nucleation of austenite. This results in finer austėnite grains, which produces accelerated carbide dissolution and austenite alloy enrichment compared to un worked, soft annealed structures. The mechanism for the accelerated austenitization is significant in predicting heat treatment response from published phase transformation data for SAE 52100 steel.

Tough, wear- and abrasion-resistant, high chromium hypereutectic white iron

Abstract: A tough, wear- and abrasion-resistant, high chromium, hypereutectic white iron alloy, with a composition by wt.% of >4.0 C, 25-45 Cr, 0-15 Mn, 0-10 Mo, 0-10 Ni, 0-2 B and 0-5 of at least one of Ti, W, Ta, V and Nb, subject to the proviso that Ti + W + Ta + V + Nb = 15 max., the balance, apart from incidental impurities, being Fe. The recommended super-heating range is 20-100oC above the liquidus for a particular composition, for pouring prior to casting to a minimum thickness of 10 mm. The microstructure is characterised by a volume fraction > 20% of primary, acicular, M7C3 - type carbides of mean cross-sectional dimension of 75mu max. in a eutectic matrix of eutectic and secondary carbides and austenite and/or martensite. The alloy in its softest condition, exhibiting stabilised austenite, substantially free of martensite, has a hardness of about 450 HV; in the as-cast state, the hardness typically ranges from 600-700 HV; after appropriate heat treatment, (e.g. heating at 900oC for 2-3 hours, followed by furnace or air cooling), the hardness range can be increased to 850-900 HV. The hardened alloy can be applied to such wear-and abrasion-resistant components as grizzly bars, hammer tips in hammer mills, sinter blow bars, coal and bone pulverisers, slurry spray nozzles and lining plates in chutes and crushers. The above components are typically assembled in the form of composites, with a copper liner at the interface, either by vacuum brazing pre-cast alloy on to, or casting in situ on or around, a strong and tough metallic substrate.

Cleavage fracture of A 533 B pressure vessel steel in martensitic condition

Abstract: Measurements of microscopic cleavage fracture stress σF* and fracture toughness K 1C at low temperatures have been made on A 533 B, in an autotempered martensitic condition, to assess the effect of austenitizing temperature on fracture in a system where the fracture mode is transgranular quasicleavage for all austenitizing temperatures. A wide range of prior austenite grain sizes (and hence packet sizes) has been studied, and detailed microstructural information has been obtained in an attempt to gain a clear understanding of the mechanisms of cleavage fracture in this steel. The microscopic cleavage fracture stress is found to be independent of austenitizing temperature, and in the temperature range from −100 to −160°C it is also independent of temperature. At −196°C, the value of σF* falls slightly. The temperature independence of σF* strongly suggests a tensile stress controlled fracture criterion. The coarse grained condition exhibits higher strength, slightly higher fracture toughness, and a ...

Hydrogen effects on phase transformations in austenitic stainless steels

Abstract: The effects of hydrogen and stress (strain) on the phase transitions of a variety of stainless steels (316, 321, 347) were investigated. Hydrogen was introduced by severe cathodic charging at room temperature. X-ray diffraction was employed to reveal the transformations occurring in thin surface layers. After charging expanded ∈ phase is always present,α′ martensite content increases during ageing and the final content depends on the stability of the austenite. The broadening of diffraction peaks of austenite after cathodic charging is caused by nonuniform distribution of hydrogen. The state of hydrogen distribution in the steel and the relationship between internal stresses, surface cracking and phase transition is discussed.

Structure of Austenite of Carbon Steels in High Speed Hot Working Processes

Abstract: Synopsis : In this investigation, the recrystallization behaviour of austenite of carbon steels due to hot working has been studied. To investigate the change of structure of austenite during hot rolling of strip, a newly developed testing machine which enables simulations of hot rolling of plate, strip and wire rod has been used. The results of the experiments have been evaluated to make a calculation model of grain size and fraction recrystalized. The feature of this calculation model is that the effect of prior working on the recrystallization behaviour at next deformation can be quantitatively taken into consideration by means of dislocation density of material. The accuracy of the model has been examined by experiments of single pass rolling in laboratory and by wire rod rolling in practice. A reasonable agreement is obtained in most cases. The model is used to estimate the influence of rolling temperature, speed of strip, initial grain size, and schedule of reduction on the grain size after final rolling of hot strip.

Influence of nitrogen alloying on hydrogen embrittlement in AISI 304-type stainless steels

Abstract: Hydrogen embrittlement of AISI 304-type austenitic stainless steels has been studied with special emphasis on the effects of the nitrogen content of the steels. Hydrogen charging was found to degrade the mechanical properties of all the steels studied, as measured by a tensile test. The fracture surfaces of hydrogen charged specimens were brittle cleavage-like whereas the uncharged specimens showed ductile, dimpled fracture. In sensitized materials transgranular cleavage mode of fracture was replaced by an intergranular mode of fracture and the losses of mechanical properties were higher. Nitrogen alloying decreased the hydrogen-induced losses of mechanical properties by increasing the stability of austenite. In sensitized steels the stability of austenite and nitrogen content were found to have only a minor effect on hydrogen embrittlement, except when sensitization had causedα′-martensite transformation at the grain boundaries.

Grain boundary segregation of copper, tin and antimony in C-Mn steels at 900°C

Abstract: The segregation of copper, tin and antimony to austenite grain boundaries at 900° C has been investigated in C-Mn steels using a scanning Auger microprobe (SAM). The specimens for microanalysis were prepared in a manner such that the prior austenite grain boundaries could be exposed by fracturing at room temperature in the UHV chamber of the SAM unit. Initial bulk concentrations ranged between 600 and 2600 ppm Cu, 50 and 360 ppm Sn and 8 and 35 ppm Sb. Significant enrichment of copper, tin and antimony was detected along the austenite grain boundaries. The grain boundary concentration of copper and tin was found to vary depending upon the initial bulk concentration while the average concentration of antimony at the grain boundaries was found to be approximately 1 at % for all of the heats studied. For heats in which a significant level of copper segregation was detected, a relationship of at % Cu = at % (Sn+Sb) at the austenite grain boundaries was observed. Deformation at 900° C prior to fracture in UHV was found to be necessary to promote segregation. Samples that were annealed at 900° C but not hot worked did not exhibit evidence of copper, tin or antimony segregation. These results have been interpreted in terms of the effects of deformation on segregation kinetics, and were correlated with hot ductility measurements made at 900° C.

Hydrogen Induced Slow Crack Growth in Stable Austenitic Stainless Steels

Abstract: The behavior of hydrogen induced slow crack growth in type 310 and type 16-20-10 stable austenitic stainless steels along with type 321 unstable austenitic stainless steel were investigated. It was found that slow crack growth could occur in all three types of stainless steels, and the threshold values wereK H/Kc = 0.55, 0.7, and 0.78 for type 321, 310, and 16-20-10 stainless steel respectively, when charged under load. Slow crack growth could also occur if the precharged specimens were tested under constant load in air. No slow crack growth occurred in the precharged and then out-gassed specimens. This indicates that delayed cracking in stable austenitic stainless steels is induced by hydrogen. Since there is no hydrogen induced α’ martensite in type 310 and 16-20-10 stainless steel, the existence of a’ martensite is not necessary for the occurrence of slow crack growth in the austenitic stainless steels, although it can facilitate slow crack growth. The mode of hydrogen induced delayed fracture in either the stable or unstable austenitic stainless steel is correlated with theK, value; the fracture surface is changed from ductile to brittle asK 1 is decreased.

Grain Refinement of Nb Steels by Control of Recrystallization during Hot Rolling

Abstract: Control rolling of Nb-bearing steels is employed to produce the uniform fine-grained product needed to meet the most stringent strength and toughness specifications. To produce such structures consistently requires a thorough understanding of the effects of compositional and process variables on austenite grain refinement at every stage of the hot rolling process. These effects were examined by means of a laboratory simulation of plate rolling. It was found that limiting the reheated grain size to about 100 μm (by lowering the reheat temperature or by adding Ti to the steel) is essential to assure complete recrystallization and refinement of the initial grain structure without an excessive amount of reduction at high temperatures. Because of the slow cooling rate at the slab center, this initial breakdown stage of rolling must be followed by a hold to allow the slab to cool to the lowest temperature at which recrystallization during rolling will be complete. This procedure will assure that the second roughing stage will produce the finest and most uniform recrystallized structure possible. This structure transforms to a very uniform ferrite-pearlite with a grain size of 10 to 15 μm. If, however, the fine recrystallized austenite is further rolled below the recrystallization temperature (control rolled) it will produce flattened austenite grains that transform to ultra-fine ferrite 4 to 6 μm in diameter. The uniformity of the final structure depends critically on the state of the austenite prior to grain flattening. The austenite must be completely recrystallized, since partially recrystallized regions produce duplex ferrite. Two attempts to change the control-rolling temperature range by modifying composition were unsuccessful. Increasing the Nb content from 0.05 to 0.10 to promote higher-temperature precipitation and boundary pinning did not raise the temperature at which grain flattening began. The addition of Ti to a Nb steel to remove soluble nitrogen and thereby prevent formation of NbCN in favor of the lower-temperature NbC did not promote recrystallization to lower temperatures.