Showing papers on "Tempering published in 2003"

Boron solubility in Fe–Cr–B cast irons

Abstract: Boron solubility in the as-cast and solution treated martensite of Fe-Cr-B cast irons, containing approximately 1.35 wt.% of boron, 12 wt.% of chromium, as well as other alloying elements, has been investigated using conventional microanalysis. The significant microstructural variations after tempering at 750 degreesC for 0.5-4 h, compared with the original as-cast and solution treated microstructures, indicated that the matrix consisted of boron and carbon supersaturated solid solutions. The boron solubility detected by electron microprobe was between 0.185-0.515 wt.% for the as-cast martensite and 0.015-0.0589 wt.% for the solution treated martensite, much higher than the accepted value of 0.005 wt.% in pure iron. These remarkable increases are thought to be associated with some metallic alloying element addition, such as chromium, vanadium and molybdenum, which have atomic diameters larger than iron, and expand the iron lattice to sufficiently allow boron atoms to occupy the interstitial sites in iron lattice. (C) 2002 Elsevier Science B.V. All rights reserved.

Modeling the microstructure–mechanical property relationship for a 12Cr–2W–V–Mo–Ni power plant steel

Abstract: For a 12Cr–2W–V–Mo–Ni power plant steel, the microstructure was characterized and the 0.2% proof strength was quantitatively related to its microstructure and chemical composition. The microstructural features including martensite lath width, carbide size and dislocation density were measured from transmission electron microscopy and the equilibrium concentrations of alloying elements dissolved in the matrix after tempering were calculated using the ‘Metallurgical Thermodynamic Data Bank’ (MTDATA). The strengthening contribution to the 0.2% proof stress was evaluated from each individual strengthening mechanism by using measured microstructural parameters. The modeled 0.2% proof stress is compared with the experimental result and a reasonable agreement is reached by root mean-square summation, while the simple linear summation gives an overestimated result. The accuracy and limitation of the present modeling is also discussed.

Static strain aging phenomena in cold-rolled dual-phase steels

Abstract: The effect of low-temperature aging, with aging temperatures up to 170°C, on a cold-rolled CMn−CrMo dual-phase (DP) ferrite-martensite steel was investigated. This material was processed using three different intercritical annealing treatments, leading to DP structures with different microstructures and properties. It has been found that both the aging in the ferrite phase and the tempering in the martensite play an important role in the mechanical behavior of the material with regard to the strain aging phenomena. The yield stress increase accompanying the aging phenomenon revealed three separate aging stages. In the present study, those stages were determined to be the result of the pinning of dislocations in the ferrite, the C-cluster formation, or low-temperature carbide precipitation in the ferrite and the volume contraction of the martensite due to formation of low-temperature carbides, leading to the relief of residual stresses in the ferrite. In the absence of a clear yield point, a new method is proposed to measure the increase in yield stress due to aging only.

Low cycle fatigue behavior of a multiphase microalloyed medium carbon steel: comparison between ferrite–pearlite and quenched and tempered microstructures

Abstract: In an attempt to improve fatigue and fracture resistance, a multiphase (ferrite–bainite–martensite) microstructure was developed in a V-bearing medium carbon microalloyed steel using a two-step cooling and annealing (TSCA) treatment following finish forging. The monotonic, cyclic stress–strain and low cycle fatigue behavior of this steel are reported. These results are compared with those of ferrite–pearlite and tempered martensite microstructures obtained by air cooling (AC) and quenching and tempering (Q&T), respectively. The tensile properties of the multiphase microstructure are superior to those of the ferrite–pearlite and the Q&T microstructures. Under cyclic loading, the ferrite–pearlite microstructure showed hardening at higher total strain amplitudes (≥0.7%) and softening at lower total strain amplitudes (<0.7%). The quenched and tempered and the ferrite–bainite–martensite (TSCA) microstructures displayed cyclic softening at all total strain amplitudes employed. Despite the cyclic softening, the ferrite–bainite–martensite structure was cyclically stronger than the ferrite–pearlite and the Q&T microstructures. Bilinearity in the Coffin–Manson plots was observed in Q&T and the multiphase TSCA conditions. An analysis of fracture surface provided evidence for predominantly ductile crack growth (microvoid coalescence and growth) in the ferrite–pearlite microstructure and mixed mode (ductile and brittle) crack growth in Q&T and the multiphase TSCA microstructures.

Influence of the heat treatment on the corrosion resistance of the martensitic stainless steel type AISI 420

Abstract: The martensitic stainless steel type AISI 420 is widely used for applications like cutlery, plastic molds, structural parts and medical devices [1]. This grade of steel is particularly important, because it is suitable to hardening after heat treatment such as quenching and tempering. After hardening it is possible to combine high strength, toughness and corrosion resistance. Regarding corrosion resistance it is well known that a minimum of 11% of chromium is necessary to attain corrosion resistance by the formation of the native protective oxide film [2], and for the martensitic grade the chromium must be dissolved into the matrix. Therefore, the corrosion resistance of martensitic stainless steels grade is sensitive to the carbide volume fraction dissolved on matrix after austenitizing for quenching and is close related to the carbide precipitation during tempering [3, 4]. Under such considerations, the heat treatment is an important processing step to control the corrosion resistance of this steel. Taking into account the importance on combining high strength and corrosion resistance, the present work present results concerning a detailed study on the influence of the hardening and tempering heat treatment cycles on the corrosion resistance of the martensitic stainless steel type AISI 420. The material used as reference was received as annealed bar with a ferritic matrix containing M23C6 chromium carbides with a homogeneous dispersion, as expected from the phase equilibrium [5]. The annealed state is considered here as reference for the highest volume fraction of chromium carbides. To study the corrosion resistance, the secondary M23C6 carbide fraction was varied, by dissolution, using oil quenching from austenitizing temperatures ranging from 900 ◦C to 1100 ◦C, for 1 h. Additionally tempering treatments were performed to study the influence of the carbide precipitation. The heat treatment response was evaluated by Rockwell C hardness. The corrosion resistance was evaluated by mass loss, by unit area, using a 0.5 M H2SO4 solution at room temperature. These tests were carried out between 10 to 180 min, and after the experiments the corroded surfaces were examined at a stereomicroscope. The influence of the austenitizing temperature on hardness after quenching is shown on Fig. 1. The hardness increases when the temperature raises up to 1050 ◦C and lowers for 1100 ◦C. The hardness increase is a consequence of the M23C6 carbide dissolution that increases the carbon supersaturation and the lattice distortion of the martensite [6]. The retained austenite fraction at 1100 ◦C is high enough to decrease the as quenched hardness [7]. Fig. 2 shows that the corrosion resistance is strongly influenced by the austenitizing temperature, and, therefore, by the carbide volume fraction. There is a decrease of the corrosion resistance with the increase of the austenitizing temperature up to 1075 ◦C for the temperature of 1100 ◦C the corrosion resistance increase. Considering that the corrosion resistance should enhance with the increase of the chromium content dissolved into the ferritic matrix, this is an unexpected result. Therefore, there must be another mechanism controlling the corrosion resistance that superimposes the beneficial aspect of the carbide dissolution. This behavior must be explained as a consequence of the increase of the internal martensite lattice stresses [8]

Effects of microstructural change on fracture characteristics in coarse-grained heat-affected zones of QLT-processed 9% Ni steel

Abstract: This study investigates the correlation between the microstructural change and fracture characteristics in the coarse-grained heat-affected zones (CGHAZs) of the newly developed quenching, lamellarizing and tempering (QLT)-processed 9% Ni steel. The microscopic fracture behaviors of the various sub-zones within the HAZs including local brittle zone (LBZ) were estimated using simulated HAZ specimens. Both results of Charpy impact tests and in situ scanning electron microscopy (SEM) observations on simulated CGHAZ specimens show that the inter-critically reheated coarse-grained HAZ (IC CGHAZ) is a primary LBZ of this steel at cryogenic temperature, but not at room temperature. Microstructural analysis suggests that, unlike in other studies, the cryogenic LBZ phenomenon of the IC CGHAZs cannot be explained simply by the amount of martensite–austenite (M–A) constituents, but is mainly associated with the carbon contents in them. From all results obtained, a mechanism for microscopic toughness change among the CGHAZs is proposed and discussed.

Modelling and characterisation of V4C3 precipitation and cementite dissolution during tempering of Fe-C-V martensitic steel

Abstract: The precipitation and Ostwald ripening behaviour of V4C3 (plate shaped) particles during the tempering of a ternary Fe - C - V martensitic steel have been characterised and modelled, taking account of local equilibrium, the capillarity effect, and simultaneous cementite enrichment and dissolution. Particles of V4C3 are represented as parabolic cylinders of revolution, with the tip radius chosen to yield the maximum lengthening rate. Transmission electron microscopy has been used to validate the theory; measurements of the average length, volume fraction, and number density of particles showed good agreement with experimental observations.

Influence of high temperature exposure on the mechanical behavior and microstructure of 17-4 PH stainless steel

Abstract: The mechanical behavior and microstructural evolution of 17-4 PH stainless steels in three conditions, i.e. unaged (Condition A), peak-aged (H900) and overaged (H1150), exposed at temperatures ranging from 200 to 700°C were investigated. The high-temperature yield strength of each condition decreased with an increase in temperature from 200 to 400°C except for Condition A at 400°C with a longer hold time where a precipitation-hardening effect occurred. At temperatures from 500–700°C, the decrease in after-exposure hardness of Condition A and H900 at longer exposure times was caused by a coarsening effect of copper-rich precipitates. A Similar microstructural change was also responsible for the hardness of H1150 exposed at 700°C decreasing with increasing exposure time. Scanning electron microscopy (SEM) observations indicated that the matrix structures of Condition A and H900, when exposed at 600°C and above, exhibited lamellar recrystallized α-ferrite in the tempered martensite and the size and quantity of these lamellar ferrite phases increased with exposure time. X-ray diffraction (XRD) analyses showed that the reverted austenite phase in H1150 that formed during the over-aging treatment was stable and hardly affected by deformation at temperatures of 200–400°C.

Optimization of mechanical properties of an HSLA-100 steel through control of heat treatment variables

Abstract: HSLA-100 steel is widely used in Navy vessels, where the requirement is a steel with good weldability possessing a high strength together with a high degree of low-temperature toughness property. However, an increase in strength is usually accompanied by a decrease in toughness. Hence there is a need for optimization of the properties. The present work attempts to optimize the mechanical properties of an HSLA-100 steel through control of heat treatment process parameters. Initially single factor experiments are carried out over a wide range of tempering time/temperature combinations to identify the heat treatment zone where the optimum combination of properties is likely to be obtained. Further experiments are done in this zone through statistical design of experiments. The present work involves quantification of properties by: (i) classical curve fitting technique with data obtained from single factor experiments; and (ii) forming regression equations from 2 2 factorial design of experiments. Subsequently maximization of low-temperature impact property has been done within the experimental region by Steepest Ascent method, and finally optimum combination of the properties of the present steel has been obtained by Grid Search technique with a constraint on yield strength. Transmission Electron Microscopy studies are made with a view to understanding and correlating the mechanical properties with the microstructures. # 2002 Elsevier Science B.V. All rights reserved.

Coarsening kinetics of multicomponent MC-type carbides in high-strength low-alloy steels

Abstract: Morphology and coarsening kinetics of MC-type carbide (MC-carbide) precipitating during the tempering process have been investigated in V- and Nb-bearing Cr-Mo martensitic steels. Detailed transmission electron microscopy (TEM) observations show that the addition of V and Nb stabilizes the B1-type MC-carbide instead of L’3-type M2C-carbide. The morphology of the MC-carbide is characterized as disk-like with Baker and Nutting orientation relationships with the matrix. When the specimens are fully solution treated followed by quenching, the MC-carbide precipitates as a multicomponent system with continuous solid solution of VC, NbC, and MoC. The V-, Nb-, and Mo-partitioning control the lattice parameter of MC-carbide and consequently affect the coherency between MC-carbide and the matrix. The coherent MC-carbide grows into an incoherent one with the progress of tempering. The numerical analysis on TEM observations has shown that the coarsening kinetics of MC-carbide is equated to (time)1/5 criteria, while the coarsening kinetics of the coexisting cementite is equated to (time)1/3 criteria. It is thus suggested that the Ostwald ripening of MC-carbide is controlled by pipe diffusion of V, Nb, and Mo along dislocations. It has been confirmed that the coarsening rate of the multicomponent MC-carbide is affected by V, Nb, and Mo content. Applying the thermodynamic solution database, the rate equation for MC-carbide coarsening can be expressed as a function of V, Nb, and Mo content, and the activation energy for pipe diffusion can be estimated as ΔQ v: ΔQ Nb: ΔQ Mo=1:3.9:0.6.

High cycle fatigue behaviour of a multiphase microalloyed medium carbon steel: a comparison between ferrite–pearlite and tempered martensite microstructures

Abstract: To improve toughness and fatigue strength, a mutiphase (ferrite (F)–bainite (B)–martensite (M)) microstructure was developed in a V-bearing medium carbon microalloyed (MA) steel through a two-step cooling process that was followed by an annealing (two-step cooling and annealing (TSCA)) treatment In the present paper, the high cycle fatigue (HCF) response determined in terms of the endurance limit, long crack fatigue threshold (ΔKth), crack closure and fatigue crack growth rate (FCGR) in a material that has a multiphase microstructure is presented and compared with those of the same material with a ferrite–pearlite (F–P) and a tempered martensite (T–M) microstructure obtained by air-cooling (AC) and quenching and tempering (Q&T), respectively Long crack fatigue threshold (ΔKth) and crack closure were evaluated using a dynamic compliance (DYNACOMP) measurement technique The fatigue limit of the F–B–M and the T–M microstructures (∼400 MPa) was greater than that of the F–P microstructure (∼340 MPa) At load ratios less than 05, the threshold for long crack growth was lower for the F–B–M microstructure compared with that of the F–P microstructure This is attributed to the reduced roughness-induced crack closure (RICC) contribution to the threshold in the former multiphase microstructure A quantitative analysis of the near-threshold fracture surfaces validated the above conclusion Fatigue crack growth rate in the Paris regime was found to be independent of the microstructure but dependent on the load ratio

Relationship between nanohardness and microstructures in high-purity Fe-C as-quenched and quench-tempered martensite

Abstract: The relationship between the nanohardness and the microstructures in the Fe–C martensite was studied to understand the contributions of the matrix and the grain boundary to the macroscopic strength. As-quenched martensite was examined for five kinds of Fe–C alloys with various carbon contents in the range of 0.1–0.8 mass%, while quench-tempered martensite was investigated for an Fe–0.4% C alloy. The ratio of the nanohardness to the macrohardness Hn/Hv was much smaller for the Fe–C martensite than those for the single crystals, indicating that there is a significant grain-boundary effect for the martensite. The ratio Hn/Hv of the as-quenched martensite decreased with an increase in the carbon content since the size of the block structure decreased with increasing carbon content. For the quench-tempered specimens, a significant reduction of the grain-boundary effect occured at the tempering temperature of 723 K. It is mainly due to the depression of the locking parameter caused by the disappearance of the film-like carbides on the boundaries.

Effect of Co Addition on Microstructure in High Cr Ferritic Steels

Abstract: Co is one of the interesting alloying elements in advanced high Cr ferritic steels used to improve their creep properties at elevated temperatures. However, it has been reported that Co bearing tends toward rapid deterioration in creep property at long-term testing. In this study, microstructures were studied in detail by comparing two kinds of 9% Cr ferritic steels with 3% Co and Co free for better understanding difference in those creep behaviors.Creep property of the steel with 3% Co was superior to that of the steel without Co within this study, as many researchers pointed out. Adding Co certainly suppressed δ-ferrite formation and there was large difference in prior-austenite grain size between two steels after tempering. In addition to this macroscopic difference, there was also remarkable change in precipitation behavior between them. It was found that heterogeneous precipitation of inter-metallic compounds and MX type carbonitrides was observed in Co free steel, while any extreme localization of such precipitation were not seen in 3% Co steel.It was deduced that such microstructural difference in connection with δ-ferrite formation between two steels was an important factor to understand better high temperature creep properties in 3% Co steel rather than those in Co free steel.

Modeling and Measurement of Residual Stresses in a Bulk Metallic Glass Plate

Abstract: The recent advent of multi-component alloys with exceptional glass forming ability has allowed the processing of large metallic specimens with amorphous structure. The possibility of formation of thermal tempering stresses during the processing of these bulk metallic glass (BMG) specimens was investigated using two models: (i) instant freezing model, and (ii) viscoelastic model. The first one assumed a sudden transition between liquid and elastic solid at the glass transition temperature. The second model considered the equilibrium viscosity of BMG. Both models yielded similar results although from vastly different approaches. It was shown that convective cooling of Zr41.2Ti13.8Cu12.5Ni10Be22.5 plates with high heat transfer coefficients could potentially generate significant compressive stresses on the surfaces balanced with mid-plane tension. The crack compliance (slitting) method was then employed to measure the stress profiles in a BMG plate that was cast in a copper mold. These profiles were roughly parabolic suggesting that thermal tempering was indeed the dominant residual stress generation mechanism. However, the magnitude of the measured stresses (with peak values of only about 1.5% of the yield strength) was significantly lower than the modeling predictions. Possible reasons for this discrepancy are described in relation to the actual casting process and material properties. The extremely low residual stresses measured in these BMG specimens, combined with their high strength and toughness, serve to further increase the advantages of BMGs over their crystalline metal counterparts.

High strength seamless steel pipe excellent in hydrogen-induced cracking resistance and its production method

Abstract: The present invention relates to a high strength seamless steel pipe excellent in hydrogen-induced cracking resistance, characterized by consisting of, by mass %, C: 0.03 - 0.11 %, Si: 0.05 - 0.5 %, Mn: 0.8 - 1.6 %, P: 0.025 % or less, S: 0.003 % or less, Ti: 0.002 - 0.017 %, Al: 0.001 - 0.10 %, Cr: 0.05 - 0.5 %, Mo: 0.02 - 0.3 %, V: 0.02 - 0.20 %, Ca: 0.0005 - 0.005 %, N: 0.008 % or less and O (Oxygen): 0.004 % or less, and the balance Fe and impurities, and also characterized in that the microstructure of the steel is bainite and/or martensite, ferrite is precipitated at grain boundaries and yield stress is 483 MPa or more. Further, to ensure high strength of the steel, the seamless steel pipe preferably contains, by mass %, at least one of Cu: 0.05 - 0.5 % and Ni: 0.05 - 0.5 %. To produce the above-mentioned steel pipe, it is desirable to limit a starting temperature of quenching after rolling, a cooling rate and a tempering temperature. By this configuration a seamless steel pipe having an yield stress of 483 MPa or more and excellent HIC resistance, which is suitable for a pipeline, can be provided.

On the nature of internal interfaces in tempered martensite ferritic steels

Abstract: This paper presents results on the evolution of microstructure in 12% Cr tempered martensite ferritic steels during heat treatment and creep. Prior austenite grain size and micro grain size measurements were performed using optical microscopy, and scanning and transmission electron microscopy. Moreover, orientations of micro grains are determined using TEM and SEM. Prior austenite grain sizes, micro grain dimensions and orientation relationships between micro grains are interpreted in terms of processes associated with the formation of martensite and ferrite during heat treatment and creep. The elongated micro grains (in the interior of prior austenite grains) can have high- or low-angle boundaries. The hierarchical evolution of internal interfaces is described. Micro grain boundaries most often represent (i) variant boundaries separating individual former martensite variants and (ii) subgrain boundaries (associated with recovery processes during tempering and creep). The present results also sho...

Influence of thermomechanical treatments on the microstructure and mechanical properties of HSLA-100 steel plates

Abstract: The influence of thermomechanical treatment (TMT), i.e., controlled rolling and direct quenching, as a function of rolling temperature and deformation on the microstructure and mechanical properties of HSLA-100 steel have been studied. The optical microstructure of the direct quenched (DQ) and tempered steel rooled at lower temperatures (800 °C and 900 °C) showed elongated and deformed grains, whereas complete equiaxed grains were visible after rolling at 1000 °C. The transmission electron microscope (TEM) microstructure of the 800 °C rooled DQ steel showed shorter, irregular, and closer martensite laths with extremely fine Cu and Nb(C,N) precipitates after tempering at 450 °C. The precipitates coarsened somewhat after tempering at 650 °C; the degree of coarsening was, however, less compared to that of the reheat-quenched (RQ) and tempered steel, indicating that the DQ steel was slightly more resistant to tempering. Similar to the RQ steel, at a 450 °C tempering condition, the DQ steel exhibited peak strength with extremely poor impact toughness. After tempering at 650 °C, the toughness of the DQ steel improved significantly, but at the expense of its strength. In general, the strength of the DQ and tempered steel was good and comparable to that of the RQ and tempered steel, although, its impact toughness was marginally less than the latter. The optimum combination of strength and toughness in the DQ steels was achieved after 900 °C rolling with 50 pct deformation, followed by direct quenching and tempering at 650 °C (yield strength (YS)=903 MPa, ultimate tensile strength (UTS)=928 MPa, and Charpy V-notch (CVN) strength=143 J at −85 °C).

High toughness steel material and method of producing steel pipes using same

Abstract: A steel material and a steel pipe made by using the same are provided which are to be used in severe oil well environments. Such a highly tough oil well steel pipe can be produced by rolling the base material, quenching the rolling product from the austenite region and tempering the same so that the relationship between the content of Mo [Mo] in the carbides precipitated at austenite grain boundaries and the austenite grain size (according to ASTM E 112) can be defined by the formula (a) given below. In this manner, steel pipes suited for use even under oil well environments becoming more and more severe can be produced while satisfying the requirements that the cost should be rationalized, the productivity improved and energy saved. [Mo]≦exp( G− 5)+5 (a)

An Analysis of Combined Cyclic Heat Treatment Performance

Abstract: The strengthening of AISI 4140 steel by cyclic heat treatment has been investigated. The applied cyclic heat treatment was consisted of both, temperature cycling with diffusional transformation and double quenching with diffusionless transformation. Both, temperature cycling and second quenching increase yield strength and Charpy-V toughness as measures of steel strength and toughness. The increase of yield strength and Charpy-V notch toughness are directly in relation of structure fineness. Much better mechanical properties are obtained by the appropriate cyclic heat treatment than by the quenching and tempering of steel AISI 4140.

Study of cast microalloyed steels

Abstract: Improved grades of cast steels were prepared by microalloying C–Mn–Cr steels (0.15–0.22C, wt-%) with V (0.1)+Ti (0.01) or V (0.11)+Nb (0.034)+Ti (0.01) combinations. When quenched and tempered an ultimate tensile strength (UTS) of the order of 784–1078 MPa (80–110 kg mm2) and an elongation ranging from 10 to 35% were achieved. However, the YS/UTS ratio was usually of the order of 0.9, which indicates that these steels did not undergo enough plastic deformation before failure under tensile stress. Examination by TEM of the as quenched as well as the quenched and tempered samples yielded the following information. All the as quenched steels had lath martensite morphology with retained austenite entrapped in the interlath region. After water quenching from 950°C an appreciable volume fraction of undissolved precipitates remained in the steels. Retained austenite broke down during tempering at 400°C and above producing a chain of rod like carbides. During tempering partial recrystallisation of the lat...

Influence of thermal history on precipitation of hardening phases in tempered martensite 10%Cr-steel X12CrMoWVNbN 10-1-1

Abstract: Precipitation of hardening phases was studied in the cast and the forged version of a tempered martensitic 10% Cr-steel by EDX-analysis of extracted precipitates in the scanning transmission electron microscope. It was found that the type of V-rich precipitates varies with thermal history. Tempering of the martensitic structure at temperatures ≤690 and ≥730 °C produces M 2 X and MX, respectively, as major V-rich phases (M: metallic elements, X: C, N) with M 2 X being lower in V-content and smaller in size than MX. Creep for 7.7 kh at 650 °C changes the major V-rich phase from M 2 X to MX and also leads to local appearance of relatively large particles of modified V-containing Z-phase. The evolution from M 2 X to MX and Z-phase is expected to be accompanied with decrease in creep resistance.