Showing papers on "Tempering published in 1998"

Mechanical performance of quartz‐tempered ceramics: part i, strength and toughness*

Abstract: The effect of quartz temper on the physical and mechanical properties of clay ceramics and the elucidation of the underlying mechanisms that are responsible for these effects are presented here. Characteristics studied included bulk density, open and closed porosity, density of impervious portion and fracture morphology. Mechanical behaviour was studied by measuring energy dissipation during fracture, Young's modulus, initial fracture toughness and strength in flexure. The significant increase in toughness with quartz volume fraction is explained by the development of a model that accounts for the crack distribution around the grains. The archaeological implications of the work are discussed on the basis of all the parameters that might affect the potter's choices of raw materials.

Enhancement of the mechanical properties of a low-carbon, low-silicon steel by formation of a multiphased microstructure containing retained austenite

Abstract: Dual-phase and transformation-induced plasticity (TRIP)-assisted multiphase steels are related families of high-strength formable steels exhibiting excellent mechanical characteristics. This study shows how a ferrite-bainite-martensite microstructure containing retained austenite can improve the mechanical properties of a cold-rolled low-carbon, low-silicon steel. Such a multiphased microstructure is obtained by a heat treatment involving intercritical annealing followed by a bainite transformation tempering. Depending on the heat-treatment parameters, the samples present a variety of microstructures. Due to the presence of retained austenite, some samples exhibit a TRIP effect not anticipated with such a low silicon content. A composite strengthening effect also results from the simultaneous presence of a ductile ferrite matrix with bainite and martensite as hard second phases. A true stress at maximum load of 800 MPa and a true uniform strain of 0.18 can be obtained by forming a ferrite-bainite-martensite microstructure containing up to 10 pct of retained austenite. These properties correspond to a favorable evolution of work hardening during plastic deformation.

Evolution of microstructure in a modified 9Cr–1Mo steel during short term creep

Abstract: An investigation of the effect of creep exposure on the microstructure of a 9Cr–1Mo alloy for steam tubing was performed. The samples were machined from a tube, austenised at 1323 K for 15 min and air cooled to room temperature, followed by tempering at 1023 K for 1 h. Creep tests were performed at 848, 873, 898 and 923 K for different loading conditions. The conventional power law was used to describe the minimum creep rate dependence on applied stress; the stress exponent was found to increase when temperature decreased. Transmission electron microscopy (TEM) of the crept samples showed that during creep both subgrain and particle size increased; the statistical analysis of the dimensions of the precipitates revealed a bimodal distribution of particles that coarsen during creep exposure at testing temperatures. A linear dependence of subgrain size on the inverse of the modulus compensated stress was used to describe the softening of the dislocation substructure. A similar relationship was found to be also valid for particle carbides.

Ultra-high strength ausaged steels with excellent cryogenic temperature toughness

Abstract: An ultra-high strength, weldable, low alloy steel with excellent cryogenic temperature toughness in the base plate and in the heat affected zone (HAZ) when welded, having a tensile strength greater than about 830 MPa (120 ksi) and a microstructure comprising (i) predominantly fine-grained lower bainite, fine-grained lath martensite, fine granular bainite (FGB), or mixtures thereof, and (ii) up to about 10 vol % retained austenite, is prepared by heating a steel slab comprising iron and specified weight percentages of some or all of the additives carbon, manganese, nickel, nitrogen, copper, chromium, molybdenum, silicon, niobium, vanadium, titanium, aluminum, and boron; reducing the slab to form plate in one or more passes in a temperature range in which austenite recrystallizes; finish rolling the plate in one or more passes in a temperature range below the austenite recrystallization temperature and above the Ar3 transformation temperature; quenching the finish rolled plate to a suitable Quench Stop Temperature (QST); stopping the quenching; and either, for a period of time, holding the plate substantially isothermally at the QST or slow-cooling the plate before air cooling, or simply air cooling the plate to ambient temperature.

Thermal tempering simulation of glass plates: inner and edge residual stresses

Abstract: Narayanaswamy's model is used to describe the thermomechanical behavior of glass. It includes both stress relaxation (to take into account the viscous aspect of glass) and structural relaxation (to take into account the structure state of glass). The necessary thermal and mechanical characteristics are given for the float soda-lime silicate glass. The thermal tempering of thin glass plates is simulated. Transient and residual stresses are given for the inner part of the plate. Computational results are compared with experimental results of previous works. This comparison validates Naranaswamy's model associated with material characteristics given previously. The edge effect (variation of stresses close to the edges) is described for thin plates. The thermal tempering of thick plates is simulated, and computational results are validated with optical measurements and a fractographic analysis.

Dependence of fracture toughness of austempered ductile iron on austempering temperature

Abstract: Ductile cast iron samples were austenitized at 927 °C and subsequently austempered for 30 minutes, 1 hour, and 2 hours at 260 °C, 288 °C, 316 °C, 343 °C, 371 °C, and 399 °C. These were subjected to a plane strain fracture toughness test. Fracture toughness was found to initially increase with austempering temperature, reach a maximum, and then decrease with further rise in temperature. The results of the fracture toughness study and fractographic examination were correlated with microstructural features such as bainite morphology, the volume fraction of retained austenite, and its carbon content. It was found that fracture toughness was maximized when the microstructure consisted of lower bainite with about 30 vol pct retained austenite containing more than 1.8 wt pct carbon. A theoretical model was developed, which could explain the observed variation in fracture toughness with austempering temperature in terms of microstructural features such as the width of the ferrite blades and retained austenite content. A plot of K 2 against σ y (X γ, C γ)1/2 resulted in a straight line, as predicted by the model.

Effect of direct quenching on microstructure and mechanical properties of copper-bearing high-strength alloy steels

Abstract: This study aims to investigate the effects of direct quenching on microstructural modification and mechanical properties of copper-bearing high-strength alloy steels. Two direct quenched and tempered (DQ&T) steel plates were rolled at different finish rolling temperatures, and their microstructures and mechanical properties were compared with those of a reheat quenched and tempered (RQ&T) steel plate. The as-quenched microstructure of the DQ plates consisted of refined lath martensite with high density of dislocations, which acted as preferred precipitation sites for NbC or e -Cu particles during tempering. These fine precipitates were not coarsened much up to the tempering temperature of about 650°C, and thus played a role in improving the tempering resistance. Especially in the DQ&T plate quenched at 760°C and tempered at 660°C, yield strength reached 1050 MPa, and Charpy impact energy at −18°C showed 140 J, indicating the potent effect of the DQ&T process. These findings indicated that the copper addition and the application of the DQ&T process to low-carbon alloy steels contributed to the production of steel plates with excellent strength and toughness.

Evolution of secondary phases 12% Cr steel during quenching and tempering

Abstract: Secondary phase evolution in the 12 wt.% Cr steel austenitized at 1100 and 1330°C and subsequently tempered at 750°C was investigated. To describe the microstructures and identify the secondary phases, methods of light microscopy and transmission electron microscopy including electron diffraction and microanalysis were used. It was found that oil quenching from 1100°C leads to formation of autotempered martensite containing intragranular (Fe4Cr)3C and MN particles. After oil quenching from 1330°C the microstructure consists of martensite, δ-ferrite, intragranular MN particles, and M23C6 particles precipitating at the γ/δ interfaces. In microstructures of tempered states Cr-rich M23C6, Ti-rich MN, and V-rich MC particles were found. It was shown that quenching temperature and/or appearance of δ-ferrite in the microstructure do not significantly affect the evolution of secondary phases during tempering.

Microstructural influence on fatigue properties of a high-strength spring steel

Abstract: A study has been made to investigate the fatigue properties of a high-strength spring steel in relation to the microstructural variation via different heat treatments. Rotating–bending fatigue and fatigue crack growth (FCG) tests were conducted to evaluate the fatigue properties, and a transmission electron microscope (TEM) equipped with an energy dispersive X-ray (EDX) unit was used to characterize the tempered microstructure. The results indicate that the fatigue endurance σf increases with increasing tempering temperature, reaching a maximum at 450°C, then decreases. The increase of σf is mainly attributed to the refined distribution of precipitation, together with the structural uniformity of tempered martensite. The softening of tempered martensite due to excessive precipitation accounts for the decrease of σf. By contrast, the FCG results show an insensitivity of the stage-II growth behavior to the microstructural changes for the whole range of tempering temperature tested. The insensitivity is interpreted in terms of the counterbalancing microstructure-dependent contributions to the FCG behavior.

Cutting performance and microstructure of high speed steels: Contributions of matrix strengthening and undissolved carbides

Abstract: While it is accepted that both the hot strength of the matrix and the amount of undissolved carbides are important for the cutting performance of high speed steels, the relative weights of their contributions are unknown. In this work, they are separately identified and a model is presented that provides a quantitative prediction of tool life (solely in uninterrupted cutting) on the basis of microstructural and compositional data over a wide range of alloy compositions and cutting speeds. The model seems to describe the individual contributions to tool life well enough to serve as a guide in alloy development. The model has been developed using 13 different steels, spanning the entire range of customary compositions. It is based on the following parameters: volume fractions and compositions of undissolved carbides; precipitates formed during tempering (secondary hardening) and during operation (tertiary precipitates); and, finally, residual solute in the matrix. Tool life is modeled as a linear combination of contributions from the undissolved carbides and from the precipitate population, including a contribution due to the action of Co, and with an additional term due to solute strengthening of the matrix. The weight factors are determined by multiple linear regression analysis. They reflect the relative importance of each contributing factor, and their dependence on cutting speed can be interpreted in terms of the change in operative wear mechanism with tool temperature.

The effect of tempering temperature on mechanical properties and microstructure of low alloy Cr and CrMo steel

Abstract: Two low alloy Cr and CrMo steels with similar levels of carbon, manganese and chromium have been studied to determine the effect of tempering temperature on the mechanical properties and microstructure. The quenching and tempering of steels were carried out using a high-speed dilatometer. The steels were quenched at the average cooling rate of 30 K s-1 in the temperature range from 1123 to 573 K by flowing argon and tempered at 673, 823 and 973 K. The martensite of steels formed during quenching was of entire lath morphology with 2 vol% retained austenite. It was found that after tempering at 973 K the Cr steel contained only orthorhombic cementite, while the CrMo steel contained the cementite and hexagonal Mo2C particles in the ferrite matrix. At the same tempering conditions, the CrMo steel shows higher strength but lower ductility as compared to those of Cr steel. It is shown that this difference results from finer prior austenite grain, substructure within matrix and precipitate dispersion strengthening, primarily by Mo2C. Transmission electron microscopy (TEM) bright- and dark-field micrographs as well as selected area diffraction pattern analysis of orientation relationship showed that the cementite precipitated from the ferrite matrix. Fractography analysis showed that the morphology fracture surface was changed by increasing tempering temperature. Tempering at 973 K obtained ductile fracture by the microvoid coalescence mechanism.

Effect of austenitizing conditions on the impact properties of an alloyed austempered ductile iron of initially ferritic matrix structure

Abstract: The effect of austenitizing conditions on the microstructure and impact properties of an austempered ductile iron (ADI) containing 1.6% Cu and 1.6% Ni as the main alloying elements was investigated. Impact tests were carried out on samples of initially ferritic matrix structure and which had been first austenitized at 850,900, 950, and 1000°C for 15 to 360 min and austempered at 360°C for 180 min.

Glass for chemical tempering and glass base for magnetic recording medium

Abstract: PROBLEM TO BE SOLVED: To produce glass for chemical tempering capable of producing glass of high homogeneity, having the low melting temperature and exhibiting excellent weather resistance even after chemical tempering by specifying the glass composition consisting of SiO2, Al2O3, Li2O, Na2O, ZrO2, ZnO and P2O5. SOLUTION: The composition of this glass is set by maintaining the content of ZrO2 for increasing viscosity at high temperatures within a relatively low level and by including ZnO and P2O5 in a preferred quantity respectively for increasing resistance to weather and for promoting chemical tempering. The glass contains 58-65 wt.% SiO2, 8-15 wt.% Al2O3, 4-10 wt.% Li2O, 9-13 wt.% Na2O, 0.5-2 wt.% ZrO2, 2-5 wt.% ZnO and 0.5-2 wt.% P2O5. The glass is prepared by throwing a raw material mixture into a platinum crucible, by heating it in an electric furnace at 1,450°C for 6 h, by molding the resulting glass into a preferred product form and by the ion-exchange treatment through dipping the glass in a mixed fused salt consisting of 40 wt.% sodium nitrate and 60 wt.% potassium nitrate and through retaining the glass at 380°C for 3 h. COPYRIGHT: (C)2000,JPO

Statistical modelling of intergranular brittle fracture in a low alloy steel

Abstract: The intergranular brittle fracture (IBF) behaviour of a low alloy steel 16MND5 (A508 Cl. 3) was investigated. A temper embrittlement heat treatment was applied to the material to simulate the effect of local brittle zones (ghost lines) which can be found in the as-received material condition. An increase in the Charpy V toughness transition temperature and a significant decrease in the fracture toughness measured on CT-type specimens were observed in the embrittled material, as compared to the reference material which was submitted to the same austenitizing and tempering heat treatment, but which was not subjected to the temper embrittlement treatment. Tensile tests on notched specimens were carried out to measure the Weibull stress and scatter in the results. A statistical model, the Beremin model, originally proposed for brittle cleavage fracture was applied to IBF. It is shown that this model is not able to fully account for the results, in particular for the existence of two slopes in a Weibull plot. Systematic fractographic observations showed that the low slope regime in this representation was associated with the existence of MnS inclusions initiating brittle fracture, while the larger slope was related to microstructural defects. Initiation of IBF from MnS inclusions can occur when the material is still elastically deformed while the second population of microstructural defects is active in the plastic regime. A modified statistical model based on the Beremin model and taking into account these specific aspects is proposed in the framework of the weakest link theory. The parameters of this model are identified from test results on notched specimens. It is shown that this model is able to predict the temperature dependence of fracture toughness and the scatter in the experimental results.

Microstructural change of a 5% Cr steel weld metal during tempering

Abstract: The microstructure of an ultra-low carbon 5% Cr steel weld metal was studied in welded and tempered conditions using optical microscopy and analytical electron microscopy. The tempering was carried out at 400, 500, 600, 700 and 750°C for 4 h after welding. Transmission electron microscopy (TEM) showed that martensite in the weld metal started to recrystallize at 500°C and had changed to a coarse polygonal ferrite after tempering at 750°C. Except for some cementite in bainite, the main precipitates in the weld metal were M7C3, M2X and M23C6. M2X was not stable and transformed to M23C6 during tempering at temperatures above 600°C. The process of the microstructural development in the 5% Cr weld metal is discussed.

Steel for oil well excellent in sulfide stress corrosion cracking resistance

Abstract: PROBLEM TO BE SOLVED: To produce steel for an oil well having excellent SSC resistance and >=110 ksi YS and to provide a method for inexpensively producing it. SOLUTION: This steel for an oil well is composed of low alloy steel contg., by weight, 0.2 to 0.35% C, 0.2 to 0.7% Cr, 0.1 to 0.5% Mo and 0.1 to 0.3% V, in which the total of precipitated carbides is 2 to 5 wt.%, and, among which, the ratio of MC type carbides is 8 to 40 wt.%, and, this steel for an oil yell is producible by subjecting the steel having the above chemical compsn. only to quenching from the temp. equal to or above the A3 transformation point and tempering at >=650 deg.C to the temp. equal to or below the Ac1 transformation point.

Wear resistant, powder metallurgy cold work tool steel articles having high impact toughness and a method for producing the same

Abstract: A hot-worked, fully dense, wear resistant, vanadium-rich, powder metallurgy cold work tool steel article having improved impact toughness. This is achieved by controlling the amount, composition and size of the primary carbides and by insuring that substantially all the primary carbides remaining after hardening and tempering are MC-type vanadium-rich carbides. The article is produced by hot isostatic compacting of nitrogen atomized powder particles.

Production of high strength steel excellent in sulfide stress cracking resistance

Abstract: PROBLEM TO BE SOLVED: To improve sulfide stress cracking resistance while maintaining high strength by subjecting a steel of specific composition to heating up to specific temperature, to holding for specific time, and to hardening, then subjecting the steel to reheating up to specific temperature at specific heating velocity, to holding for specific time, and to rehardening, and further carring out tempering. SOLUTION: The steel has a composition consisting of, by weight, 0.2-0.35% C, 0.05-0.5% Si, 0.1-1% Mn, 0.3-1.2% Cr, 0.2-1% Mo, 0.005-0.5% Al, 0.005-0.5% Ti, 0.0001-0.01% B, 0.05-0.5% Nb, 0-0.5% V, 0-1% W, 0-0.5% Zr, 0-0.01% Ca, and the balance Fe with inevitable impurities and satisfying Ti+0.5Zr=0.005 to 0.5%. The steel is heated up to 1,050 to 1,300 deg.C, held for >=10 min, and hardened. Subsequently, the steel is reheated up to 900 to 1,150 deg.C at (20 to 50) deg.C/sec heating rate in the temperature range between 700 and 1,150 deg.C, held for <=3 min, and rehardened. Then, tempering is performed.

Investigation of fracture toughness of deep carburised M50NiL steel under different tempering temperatures

Abstract: Fracture toughness of carburised M50NiL steel at different tempering temperatures has been investigated using a new evaluation method involving the ‘pop in’ phenomenon which occurs when a crack propagates through the carburised layer. An increase in tempering temperature resulted in an increase infracture toughness, and the rate of increase of the fracture toughness of the core was much higher than that of the surface. The changes in microstructure, residual stress, and retained austenite through the carburised layer were found to be the main factors causing the occurrence of pop in and causing the fracture toughness of the core to be higher than that of the surface.

Cast cold tool and method for producing the same

Abstract: A cast cold tool is obtained through the steps of forming a casting by founding a molten steel consisting by weight percentage of 0.5 to 0.8% of C, not more than 1.0% of Si, 0.25 to 1.50% of Mn, 4.0 to 8.0% of Cr, 1.0 to 5.0% of Mo, one or both of 0.2 to 1.0% of V and 0.2 to 2.0% of Nb, opptionally not more than 2.5% of W, not more than 2.5% of Ni, and the balance being Fe plus incidental impurities, subjecting the casting to solid solution treatment to decrease primary carbides precipitated in the casting to not more than 1%, preferably to extinguish completely, and subjecting the solid-solution treated casting to quenching and tempering treatment to give predetermined toughness and hardness to the casting.

Steel wire and method of manufacturing the same

Abstract: A steel wire of pearlite structure containing 0.8-1.0 mass % of C and 0.8-1.5 mass % of Si is disclosed. In the cross section of the steel wire the difference in average hardness between a region up to 100 μm from the surface thereof and a deeper region is within 50 in micro-Vickers hardness. The steel wire is manufactured by working a wire rod having the abovementioned chemical composition through shaving, patenting and drawing processes, then strain-relief annealing the resultant wire, and thereafter subjecting the thus annealed to a shot peening process. The steel wire has a high heat resistance and a high fatigue strength, and can be produced through a drawing process without applying a quenching and tempering process.