Showing papers on "Tempering published in 2011"

Tempering of Martensite in Dual-Phase Steels and Its Effects on Softening Behavior

Abstract: The isothermal and nonisothermal tempering of martensite in dual-phase (DP) steels was investigated mainly by analytical transmission electron microscopy, and the effect on softening behavior was studied. The isothermal tempering resulted in coarsening and spheroidization of cementite and complete recovery of laths. However, nonisothermal tempering manifested fine quasi-spherical intralath and platelike interlath cementite, decomposition of retained austenite, and partial recovery of laths. The distinct characteristic of nonisothermal tempering was primarily attributed to the synergistic effect of delay in cementite precipitation and insufficient time for diffusion of carbon due to rapid heating that delays the third stage of tempering. The finer size and platelike morphology of cementite coupled with partial recovery of lath resulted in reduced softening in nonisothermal tempering compared to severe softening in isothermal tempering due to large spheroidized cementite and complete recovery of lath substructure. The substitutional content of precipitated cementite in nonisothermal tempering was correlated to the richness of particular steel chemistry. Softening resistance during nonisothermal tempering was related to DP steel chemistry, i.e., Cr and Mn content. Fine cementite and less decomposed martensite in rich chemistry confer high resistance to softening compared to leaner chemistries, which indicated severe decomposition of martensite with coarser cementite.

A new effect of retained austenite on ductility enhancement in high-strength quenching–partitioning–tempering martensitic steel

Abstract: A high-strength martensitic steel treated by a quenching–partitioning–tempering process is presented to examine the effect of retained austenite on ductility enhancement in martensitic steels. Results from X-ray diffraction line profile analysis (XLPA) indicate that the average dislocation density in martensite during uniform deformation is lower than before deformation, which effectively intensifies the deformation ability. The average dislocation density in retained austenite rapidly increases with increased strain and exceeds that in martensite. Based on the XLPA results, a new effect of austenite on the ductility enhancement is proposed: the austenite phase can continuously absorb ample dislocations from neighbouring martensite laths. This effect is indirectly verified by transmission electron microscopy.

Comparison of various 9–12%Cr steels under fatigue and creep-fatigue loadings at high temperature

Abstract: The present article compares the cyclic behaviour of various 9–12%Cr steels, both commercial grades and optimized materials (in terms of creep strength). These materials were subjected to high temperature fatigue and creep-fatigue loadings. TEM examinations of the microstructure after cyclic loadings were also carried out. It appears that all the tempered ferritic–martensitic steels suffer from a cyclic softening effect linked to the coarsening of the subgrains and laths and to the decrease of the dislocation density. These changes of the microstructure lead to a drastic loss in creep strength for all the materials under study. However, due to a better precipitation state, several materials optimized for their creep strength still present a good creep resistance after cyclic softening. These results are discussed and compared to the literature in terms of the physical mechanisms responsible for cyclic and creep deformation at the microstructural scale.

The influence of tempering temperature on the reversed austenite formation and tensile properties in Fe–13%Cr–4%Ni–Mo low carbon martensite stainless steels

Abstract: The influence of tempering temperature on the reversed austenite formation and tensile properties are investigated in Fe-13%Cr-4%Ni-Mo low carbon martensite stainless steel in the temperature range of 550-950 degrees C. It is found that at the temperatures below 680 degrees C, the reversed austenite formation occurs by diffusion. Amount of the reversed austenite is determined I:IN the tempering temperature and the holding time. The segregation of Ni is the main reason for the stability of the reversed austenite. When the temperatures are above 680 degrees C, the reversed austenite formation proceeds by diffusionless. The reversed austenite will transform back to martensite after cooled to room temperature. The tensile properties are most strongly influenced by the amount of the reversed austenite obtained at room temperature. The excellent combination of good strength and ductility is at 610 degrees C. (C) 2011 Elsevier B.V. All rights reserved.

Effect of Heat Treatment on Microstructure and Property of Cr13 Super Martensitic Stainless Steel

Abstract: The microstructures and mechanical properties of Cr13 super martensitic stainless steel after different heat treatments were studied. The results show that the structures of the steel after quenching are of lath martensite mixed with a small amount of retained austenite. With the raising quenching temperature, the original austenite grain size increases and the lath martensite gradually becomes thicker. The structures of the tempered steel are mixtures of tempered martensite and reversed austenite dispersed in the martensite matrix. The amount of reversed austenite is from 7.54% to 22.49%. After different heat treatments, the tensile strength, the elongation and the HRC hardness of the steel are in the range of 813–1070 MPa, 10.1%–21.2% and 21.33–32.37, respectively. The steel displays the best comprehensive mechanical properties after the sample is quenched at 1050 °C followed by tempering at 650 °C.

Thermal stability of retained austenite in bainitic steel: an in situ study

Abstract: The tempering of two-phase mixtures of bainitic ferrite and carbon-enriched retained austenite has been investigated in an effort to separate the reactions that occur at elevated temperatures from any transformation during cooling to ambient conditions. It is demonstrated using synchrotron X-radiation measurements that the residue of austenite left at the tempering temperature partly decomposes by martensitic transformation when the sample is cooled. It is well established in the published literature that films of retained austenite are better able to resist stress or strain-induced martensitic transformation than any coarser particles of austenite. In contrast, the coarser austenite is more resistant to the precipitation of cementite during tempering than the film form because of its lower carbon concentration.

High strength-elongation product of Nb-microalloyed low-carbon steel by a novel quenching–partitioning–tempering process

Abstract: In this article, a novel quenching–partitioning–tempering (Q–P–T) process was applied to a Fe–0.25C–1.5Mn–1.2Si–1.5Ni–0.05Nb (wt%) hot-rolled steel, and its optimized parameters were obtained by a Gleeble-3500 thermal simulator and salt baths, respectively. Mechanical property results of the as-treated Q–P–T samples show that the Nb-microalloyed low-carbon steels subjected to Q–P–T processes cover a wide spectrum of strength (1200–1500 MPa) and elongation (14–18%), and exhibit excellent product of strength and elongation (21,000–22,000 MPa%). Microstructural characterization indicates that high strength results from dislocation-type martensite laths and dispersively distributed fcc NbC or hcp ɛ-carbides in martensite matrix and good ductility is attributed to transformation induced plasticity (TRIP) effect from plenty of retained austenite flakes between martensite laths.

Effect of Heat Treatment on Mechanical Properties and Microstructure of NST 37-2 Steel

Abstract: Engineering materials, mostly steel, are heat treated under controlled sequence of heating and cooling to alter their physical and mechanical properties to meet desired engineering applications. In this study, the effect of heat treatment (annealing, normalising, hardening, and tempering) on the microstructure and some selected mechanical properties of NST 37-2 steel were studied. Sample of steel was purchased from local market and the spectrometry analysis was carried out. The steel samples were heat treated in an electric furnace at different temperature levels and holding times; and then cooled in different media. The mechanical properties (tensile yield strength, ultimate tensile strength, Young’s modulus, percentage reduction, percentage elongation, toughness and hardness) of the treated and untreated samples were determined using standard methods and the microstructure of the samples was examined using metallographic microscope equipped with camera. Results showed that the mechanical properties of NST 37-2 steel can be changed and improved by various heat treatments for a particular application. It was also found that the annealed samples with mainly ferrite structure gave the lowest tensile strength and hardness value and highest ductility and toughness value while hardened sample which comprise martensite gave the highest tensile strength and hardness value and lowest ductility and toughness value.

Microstructural evolution of AISI 4340 steel during Direct Metal Deposition process

Abstract: In the current investigation AISI 4340 steel was laser deposited on a rolled mild steel substrate by Direct Metal Deposition (DMD) technology. The microstructural investigation of the clad was performed using optical and electron microscopes and X-ray diffraction techniques. The microstructure consisted of ferrite, martensite and cementite phases. Two types of martensite, lathe-type and plate-type, were observed in the microstructure. Decrease in microhardness values from the top layer to the alloy layer proves that the degree of tempering of the martensite phase increases in the same direction. The lattice parameters of the identified phases were found to be shorter than those reported in literature. The reported parameters in literature are from samples processed under equilibrium conditions.

Influence of heat treatments on toughness and sensitization of a Ti-alloyed supermartensitic stainless steel

Abstract: Supermartensitic steels are a new class of martensitic stainless steels developed to obtain higher corrosion resistance and better toughness through the reduction of carbon content, and addition of Ni and Mo. They were developed to more critical applications or to improve the performance obtained with conventional grades AISI 410, 420, and 431. In this study, the influences of the tempering parameters on the microstructure, mechanical properties (hardness and toughness), and sensitization of a Ti-alloyed supermartensitc stainless steel were investigated. The material showed temper embrittlement in the 400–600 °C range, as detected by low temperature (−46 °C) impact tests. The degree of sensitization measured by double loop reactivation potentiodynamic tests increased continuously with the increase of tempering temperature above 400 °C. Healing due to Cr diffusion at high tempering temperatures was not observed. Double tempered specimens showed high amounts (>20%) of reverse austenite but their toughness were similar to specimens single tempered at 625 and 650 °C.

TiC precipitation induced effect on microstructure and mechanical properties in low carbon medium manganese steel

Abstract: The precipitation behavior of titanium treated by different processes and its effect on microstructure and mechanical properties has been investigated in low carbon medium manganese steel. It is found that the formed precipitates during both tempering and reheating-quenching processes are TiCs. The size of them mainly ranges from 1 to 18 nm and 1 to 36 nm, respectively. And tempering treatment especially promotes a peak of TiC in diameter from 1 to 5 nm, but it also boosts parts of precipitates nucleating along the grain boundaries. The precipitation of TiC is useful for the refinement of austenite grains and this effect depends on the reheating temperature. The mean grain size can be refined to 7.8 μm when the reheating temperature is 900 °C. Furthermore, it is found that the mechanical properties are determined by the reheating-quenching process. However, the tempering has nearly no influence on it when the sample is treated by the same reheating temperature. It is concluded that dislocation strengthening mechanism induced by TiC precipitation is the dominant factor for the changes of both strength and toughness.

HAZ Microstructures and Local Mechanical Properties of High Strength Steels Resistance Spot Welds

Abstract: The heat affected zones (HAZ) of two Dual Phase steels spot welds, DP450 and DP980, were investigated experimentally with a Gleeble 3500 thermomechanical simulator. The thermal cycles experienced locally were identified by finite element analysis of the resistance spot welding process and their evolution with an increasing sheet thickness was highlighted within a usual range of [1.0–3.0] mm. The cooling rates are significantly lower in the case of thick sheets and this promotes the occurrence of diffusional phase transformations. HAZ microstructures and constitutive behaviours could be characterized with the Gleeble specimens. Experimental simulations were run in the subcritical and the coarse grain temperature ranges (700°C and 1200°C respectively) in order to address the main transformations for both steels. DP450 shows little sensitivity to the subcritical thermal cycles while DP980 exhibits significant softening. This is in accordance with their respective base metal martensite content and the occurrence of martensite tempering. On the contrary, DP450 microstructures and mechanical properties are strongly sensitive to the investigated range of coarse grained HAZ thermal cycles while the evolution is less pronounced in the case of DP980, which is related to their respective hardenability.

Microstructure evolution in a 3%Co modified P911 heat resistant steel under tempering and creep conditions

Abstract: Microstructure evolution was studied in a 3%Co modified P911 heat resistant steel during creep tests at 873 and 923 K to failure, which occurred in 4103 and 4743 h, respectively. The tempered martensite lath structure consisted of packets, blocks and laths. The average spacing of high-angle boundaries and the mean transverse lath size were about 1.9 μm and 360 nm, respectively. Various second phase particles precipitated upon tempering. Fine M(C, N) carbonitrides with an average size of about 30 nm were homogeneously distributed throughout the tempered martensite laths, while relatively coarse M 23 C 6 carbide particles (average size 120 nm) were located at internal boundaries. The tempered martensite lath structure was rather stable upon aging for about 4 × 10 3 h. The boundary precipitates of M 23 C 6 and Laves phases, which appeared during creep tests, exerted a high pinning pressure on low-angle lath boundaries and high-angle packet/block boundaries. The growth of martensite structural elements during the tests correlated with the coarsening of second phase particles. Quantitative relations of pinning and driving pressures for low- and high-angle boundaries are discussed.

Role of Nb in low interstitial 13Cr super martensitic stainless steel

Abstract: The effect of adding 0.1 wt% Nb to low interstitial (N 0.01 wt%, C 0.01 wt%) 13Cr super martensitic stainless steel (SMSS) on solid phase transformation and microstructures achieved by normalizing and tempering was investigated using dilatometer, electron backscattered diffraction (EBSD), transmission electron microscope (TEM), X-ray diffraction (XRD), and its consequence on mechanical properties was examined to clarify the role of Nb in low interstitial martensitic stainless steel. Nb was found to retard kinetics of reversed austenite formation during tempering and to suppress the occurrence of Cr rich precipitates. The measurement of mechanical properties shows that while the strength properties were significantly increased by nano-scale precipitates enriched in Nb in the steel with 0.10 wt% Nb, the ductility and toughness properties were restored by optimum volume fraction of retained austenite. Excellent strength and adequate toughness properties were obtained by tempering the steel with 0.10 wt% Nb and low interstitial (N 0.01 wt%, C 0.01 wt%) steel at 600 °C.

Microstructure and mechanical properties of high boron white cast iron with about 4 wt% chromium

Abstract: The microstructure and mechanical properties of high boron white cast irons with about 4 wt% chromium before and after treating with rare earth magnesium alloy were studied in this article. The experimental results indicate that the cast irons comprise a dendritic matrix and interdendritic eutectic borides M2B and M′0.9Cr1.1B0.9 that distributed in the form of continuous network in as-cast condition. The matrix is made up of fine pearlite in the alloys with and without modification, but the grain size of the matrix is decreased greatly after modification. After water quenching at 1,303 K and tempering at 473 K, the matrix of the alloy mostly changes to lath-type martensite. For the alloy without modification the boride morphology remains almost unchanged after heat treatment. And a secondary precipitation of M23(C,B)6 compound appears in the central region of dentritic matrix grains. The morphology of the eutectic borides is changed to the form of isolated blocks after heat treatment and there is only little intragranular M23(B,C)6 particles in the matrix are found in the alloy modified with rare earth magnesium alloy. The modification by rare earth magnesium alloy can refine the primary austenite and the eutectic borides. Combined with a high austenitizing temperature the modification can improve the morphology of the borides which results in the improvement of toughness and tensile strength.

On the Precipitation Sequence in a 10%Cr Steel under Tempering

Abstract: Precipitation sequence under tempering in a 10%Cr steel has been considered in details Two different precipitation processes have been identified, concurrently First, at 350°C, a dispersion of M2C phase with orthorhombic lattice appears At 525°C, the M2C phase is displaced by the formation of M23C6 carbide However, this process remains uncompleted Dissolution of M2C phase occurs completely due to the formation and growth of more stable M6C phase as a result of tempering in the range of 650 to 770°C Second, Nb-rich and V-rich M(C,N) carbonitrides precipitate in the temperature interval of 500–770°C These dispersoids play a role of heterogeneous nucleation sites for precipitations of M23C6 and M6C carbides

Development of New High-Strength Carbide-Free Bainitic Steels

Abstract: An attempt was made to optimize the mechanical properties by tailoring the process parameters for two newly developed high-strength carbide-free bainitic steels with the nominal compositions of 0.47 pct C, 1.22 pct Si, 1.07 pct Mn, 0.7 pct Cr (S1), and 0.30 pct C, 1.76 pct Si, 1.57 pct Mn, and 0.144 pct Cr (S2) (wt pct), respectively. Heat treatment was carried out via two different routes: (1) isothermal transformation and (2) quenching followed by isothermal tempering. The results for the two different processes were compared. The bainitic steels developed by isothermal heat treatment were found to show better mechanical properties than those of the quenched and subsequently tempered ones. The effect of the fraction of the phases, influence of the transformation temperatures, the holding time, and the stability of retained austenite on the mechanical properties of these two steels was critically analyzed with the help of X-ray diffraction, optical metallography, scanning electron microscopy, and atomic force microscopy. Finally, a remarkable combination of yield strength of the level of 1557 MPa with a total elongation of 15.5 pct was obtained.

Microstructure evolution and kinetic analysis of DM hot-work die steels during tempering

Abstract: Utilizing the hardness measurement, an attempt is made to study the tempering kinetics of DM hot-work die steels, combined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results demonstrate that the increasing of tempering temperature will result in precipitation and coarsening of the intra-lath spherical carbides. Meanwhile, it is significantly observed the dissolution of needle-like Mo2C carbides with increasing of tempering temperatures. The growth mechanism of carbides is coarsening of large spheroids after complete and impingement precipitation on dislocations. In addition, the activation energy ΔH of DM steels during additional tempering between 620 °C and 700 °C is slightly higher than that of the diffusion of usual alloying elements (e.g. Cr, V, Mo and W) in ferrite, which may be attributed to a higher Mn content in DM steels that dissolves into carbides and retardsthe coarsening of carbides at high temperature.

Effect of processing parameters on the evolution of dislocation density and sub-grain size of a 12%Cr heat resistant steel during creep at 650 °C

Abstract: The influence of hot-deformation and tempering temperature on the microstructure evolution of a 12%Cr heat resistant steel during short-term creep at 80–250 MPa and 650 °C was investigated. Quantitative determination of dislocation density and sub-grain size in the initial microstructure and after creep was investigated by STEM-HAADF. A correlation between microstructure evolution and creep response is established. All crept samples showed a significant increase of sub-grain size and a reduction of dislocation density. Hot-deformed samples showed better creep strength than non hot-deformed samples due to homogenization of the microstructure. The tempering temperature affected the dislocation density and the sub-grain size evolution, influencing the creep behavior.

Atom Probe Tomography Analysis of Precipitation during Tempering of a Nanostructured Bainitic Steel

Abstract: Carbon distribution during tempering of a nanostructured bainitic steel was analyzed by atom probe tomography (APT). Three different types of particles are detected on samples tempered at 673 K (400 °C) for 30 minutes: lower bainite cementite with a carbon content of ~25 at. pct, e-carbides with a carbon content close to 30 at. pct, and carbon clusters, small features with a carbon content of ~14 at. pct indicative of a stage of tempering prior to precipitation of e-carbide. After tempering at 773 K (500 °C) for 30 minutes, the e-carbide-to-cementite transition was observed. Solute concentration profiles across carbide/ferrite interfaces showed the distribution of substitutional elements in e-carbide and cementite for all the tempering conditions.

Effect of Tempering on Microstructure and Mechanical Properties of Boron Containing 10%Cr Steel

Abstract: The effect of heat treatment on the microstructure and the mechanical properties of a 10%Cr steel with 0.008% boron was examined. The microstructure and the mechanical properties of this steel subjected to the normalizing were studied after tempering under different conditions. The layers of retained austenite are located along the lath boundaries. The formation of M23(B·C)6 phase having film-like shape takes place on interface boundaries of retained austenite/martensite during tempering at 525°C. As a result, the steel exhibits brittle fracture with a low value of Charpy V-notch impact toughness of 6 J/cm2. Particles of the M23(B·C)6 phase are highly resistant against the spheroidizing. The tempering at 770°C only leads to the coagulation of these particles; when the fraction of M23(B·C)6 phase significantly decreases while the fraction of M23C6 carbides increases. The tempered at 770°C steel exhibits a high value of Charpy V-notch impact toughness of 260 J/cm2. The effect of boron additives on the phase composition and the brittleness of high-chromium steels is discussed.

Mechanism exploration of an ultrahigh strength steel by quenching–partitioning–tempering process

Abstract: The mechanical property of a Nb-microalloyed steel with different heat treatment processes are investigated. The sample with good strengthening-ductility match after quenching–partitioning–tempering (Q–P–T) 2-step process is selected as further microstructure characterization by means of scanning electron microscope (SEM) equipped with electron backscattered diffraction (EBSD) mapping device and transmission electron microscope (TEM). The results indicate that the Q–P–T 2-step sample shows best mechanical property due to its optimized microstructure with multiphase and multiscale, i.e., lath martensite (submicron-scale in thickness), retained austenite (nano-scale in thickness for interlath film-like type and submicron in diameter for island-like type) and carbide precipitates (nano-scale in diameter). This microstructure feature is named as multiphase-metastable-multiscale (3M) configuration. The idea of the 3M microstructure is proposed to explain the reason why Q–P–T steels possess good mechanical properties.