Showing papers on "Tempering published in 2017"

High dislocation density–induced large ductility in deformed and partitioned steels

Abstract: A wide variety of industrial applications require materials with high strength and ductility. Unfortunately, the strategies for increasing material strength, such as processing to create line defects (dislocations), tend to decrease ductility. We developed a strategy to circumvent this in inexpensive, medium manganese steel. Cold rolling followed by low-temperature tempering developed steel with metastable austenite grains embedded in a highly dislocated martensite matrix. This deformed and partitioned (D and P) process produced dislocation hardening but retained high ductility, both through the glide of intensive mobile dislocations and by allowing us to control martensitic transformation. The D and P strategy should apply to any other alloy with deformation-induced martensitic transformation and provides a pathway for the development of high-strength, high-ductility materials.

Thermal and mechanical stability of retained austenite surrounded by martensite with different degrees of tempering

Abstract: The mechanical and thermal stability of austenite in multiphase advanced high strength steels are influenced by the surrounding microstructure. The mechanisms underlying and the relations between thermal and mechanical stability are still dubious due to the difficulty of isolating other factors influencing austenite stability. In this work, martensite/austenite microstructures were created with the only significant difference being the degree of tempering of the martensite matrix. Hence, the effect of tempering in martensite is isolated from other factors influencing the stability of austenite. The thermal stability during heating of retained austenite was evaluated by monitoring phase fractions as a function of controlled temperature employing both dilatometry and magnetometry measurements. The mechanical stability was studied by performing interrupted tensile tests and determining the remaining austenite fraction at different levels of strain. The thermal stability of this remaining austenite after interrupted tests was studied by subsequent reheating of strained specimens. The results are evidence for the first time that thermal recovery of martensite during reheating assists austenite decomposition through shrinkage and softening of martensite caused by a reduction of dislocation density and carbon content in solid solution. This softening of martensite also leads to a subsequent reduction of austenite mechanical stability. Additionally, remaining austenite after pre-straining at room temperature was thermally less stable than before pre-straining, demonstrating that plastic deformation has opposing effects on thermal and mechanical stability.

Tempering of Lath Martensite in Low and Medium Carbon Steels: Assessment and Challenges

Abstract: This paper is a directed review of the microstructures and the mechanisms, by which they are formed by tempering of lath martensite in low and medium carbon steels. As-quenched martensitic structure is first reviewed, then low-temperature tempering (LTT) that produces ultrahigh strength and good toughness, and high-temperature-tempering (HTT), which can produce excellent combinations of strength, ductility, toughness, and good resistance to hydrogen embrittlement in sour gas and oil environments, are covered. It is well known that hardness decreases continuously with increasing tempering temperature, but the superimposed multiple mechanisms of microstructural changes by which the softening and its retardation occur during various stages of tempering have received little attention on the integrated scales of microstructure, substructure, and nanostructure. Thus, earlier descriptions of the reasons for softening need deeper examination. In particular, the reasons for the changes in the very fine crystal size and high dislocation density of as-quenched lath martensite produced by high temperature tempering, sometimes producing a low dislocation density, un-recrystallized, fine lath-morphology ferritic grain size, sometimes producing an equiaxed, recrystallized ferritic grain size, are not clear. Deformation behavior and mechanical properties as related to the residual components of martensitic microstructure that survive tempering are discussed. Questions remain and the use of newer techniques, such as Electron Back Scatter Diffraction and atom probe tomography, in addition to light microscopy, transmission electron microscopy, Mossbauer effect spectroscopy, and Mechanical Spectroscopy used in characterizing quench and tempered structures are noted.

Carbide precipitation in Nb-V-Ti microalloyed ultra-high strength steel during tempering

Abstract: The microstructure, precipitates and mechanical properties of Nb-V-Ti microalloyed ultra-high strength steel under different tempering temperatures are investigated. With the increasing tempering temperature, the width of martensitic laths gradually increased with reduced dislocations within them, and the volume fraction of retained austenite first increased and then decreased. Furthermore, the Charpy impact toughness, reduction of area and elongation are enhanced at the sacrifice of the strength. Four kinds of precipitates are identified at different tempering temperatures, namely M 3 C, M 2 C, M 23 C 6 and M 7 C 3 . M 3 C precipitated when tempering at 200–400 °C, and it was replaced partially by M 2 C when the temperature is elevated to 500 °C, at which M 23 C 6 also precipitated. For a higher temperature, above 600 °C, M 3 C disappeared, and some M 2 C transformed into M 7 C 3 . The other two types of MC precipitates are found to be the stable phases existing over different tempering temperatures. The correlation between the mechanical properties and microstructure was established, and the precipitation mechanisms of carbides at the different tempering temperatures are explained.

Microstructure-mechanical property relationship and austenite stability in medium-Mn TRIP steels: The effect of austenite-reverted transformation and quenching-tempering treatments

Abstract: In the present study, we fundamentally explore the reasons underlying differences in mechanical properties in hot-rolled 0.2C-1.6Al-6.1Mn-Fe TRIP steels subjected to different heat treatments. Comparing with austenite reverted transformation annealing (ART) process, quenching and tempering (QT [ART (UTS: 885–945MPa, TEL: 13–28%)]. In the ART process, long time annealing led to excessive C and Mn enrichment in austenite, which rendered austenite too stable and deteriorated TRIP effect. Furthermore, long time annealing reduced dislocation density and led to low work-hardening rate. The Q&T process enabled appropriate enrichment of elements and hence desired stability for significant TRIP effect to be observed. Thus, the steel quenched from 625 °C exhibited best combination of mechanical properties (UTS: 1038 MPa, TEL: 42%, UTS×TEL: 43.6 GPa%) because of significant contribution of TRIP effect and high dislocation density in austenite.

Effect of normalization and tempering on microstructure and mechanical properties of V-groove and narrow-groove P91 pipe weldments

Abstract: In Very High Temperature Reactor (VHTR), The Nb-V modified 9Cr-1Mo (P91) creep strength enhanced ferritic (CSEF) steel is currently considered as a candidate material for reactor internals and reactor pressure vessels (RPVs). After the welding of P91 steel, the inhomogeneous microstructure of weldment is a serious issue because it promotes the well-known Type IV cracking in P91 weldments. The present research work is focused on how the microstructure evolve in various zone of P91 pipe weldment during the sub-critical post weld heat treatment (PWHT) and normalized and tempered (N&T) heat treatment. The effect of PWHT and N&T heat treatment are also considered on tensile properties and hardness variation of P91 weldments. To characterize the sample scanning electron microscope (SEM), X-ray diffraction (XRD) and optical micrograph was used. It was observed that the N&T heat treatment provides the homogeneous microstructure compared to PWHT. The superior mechanical properties was also measured in N&T condition compared to PWHT. Study of fracture surface morphology of tensile tested specimen in different heat treatment condition is also presented.

Unraveling the Initial Microstructure Effects on Mechanical Properties and Work-Hardening Capacity of Dual-Phase Steel

Abstract: Ferritic-martensitic, dual-phase (DP) microstructures with different size, morphology, and distribution of martensite were produced by altering the initial microstructures using heat treatment and thermomechanical processing routes. It was revealed that the strength, ductility, and work-hardening rate of DP steels strongly depend on the volume fraction and the morphology of the martensite phase. In this regard, the fine-grained DP microstructure showed a high work-hardening ability toward an excellent combination of strength and ductility. Such a microstructure can be readily obtained by intercritical annealing of an ultrafine grained (UFG) microstructure, where the latter can be produced by cold-rolling followed by tempering of a martensite starting microstructure. Conclusively, the enhancement of mechanical properties of DP steels through microstructural refinement was found to be more beneficial compared with increasing the volume fraction of martensite. Finally, it was also demonstrated that the work-hardening rate analysis based on the instantaneous (incremental) work-hardening exponents might be an advantageous approach for characterizing DP steels along with the conventional approaches.

Characterization of microstructure of HAZs in as-welded and service condition of P91 pipe weldments

Abstract: Steels 9-12% Cr, having the high creep rupture strength are advocated for the modern low polluting thermal power plants. In the present investigation, the P91 pipe weldments have been characterized for microstructural responses in as-welded, post-weld heat treatment (PWHT) and ageing conditions. The PWHT of welded samples were carried out at 760 °C for time of 2 h and ageing at 760 °C for 720 h and 1440 h, respectively. The effect of time has been studied on precipitates size, distribution of precipitates and grain sizes present in various zones of P91 steel weldments. The impact toughness and hardness variation of heat affected zone (HAZ) have also been studied in as-welded condition as well as at different heat treatment condition. A significant change was observed in grain size and precipitates size after each heat treatment condition. The maximum impact toughness of HAZ was obtained after PWHT at 760 °C for 2 h. The main phase observed in weld fusion zone in as-welded, PWHT and ageing conditions were M23C6, MX, M7C3, Fe-rich M3C and M2C. The unwanted Z-phase (NbCrN) was also noticed in weld fusion zone after ageing of 1440 h.

Precipitation Behavior of Carbides in H13 Hot Work Die Steel and Its Strengthening during Tempering

Abstract: The properties of carbides, such as morphology, size, and type, in H13 hot work die steel were studied with optical microscopy, transmission electron microscopy, electron diffraction, and energy dispersive X-ray analysis; their size distribution and quantity after tempering, at different positions within the ingot, were analyzed using Image-Pro Plus software. Thermodynamic calculations were also performed for these carbides. The microstructures near the ingot surface were homogeneous and had slender martensite laths. Two kinds of carbide precipitates have been detected in H13: (1) MC and M6C, generally smaller than 200 nm; and (2) M23C6, usually larger than 200 nm. MC and M6C play the key role in precipitation hardening. These are the most frequent carbides precipitating at the halfway point from the center of the ingot, and the least frequent at the surface. From the center of the ingot to its surface, the size and volume fraction of the carbides decrease, and the toughness improves, while the contribution of the carbides to the yield strength increases.

Role of evolving microstructure on the mechanical properties of electron beam welded ferritic-martensitic steel in the as-welded and post weld heat-treated states

Abstract: The microstructure and mechanical properties of electron beam welded joints of reduced activation ferritic-martensitic steel in the as-welded and post-weld heat treatment (PWHT) states have been explored. The as-received base metal (BM) was in normalised and tempered condition. The PWHTs employed include post-weld direct tempering (PWDT) at 760 °C/90 min/air cooling and (ii) re-austenitizing at 980 °C/30 min/air cooling+ tempering at 760 °C/90 min/air cooling (PWNT). The BM microstructure was composed of fully tempered lath martensite with prior austenite grain and martensite lath boundaries decorated with M 23 C 6 type carbides whereas intra-lath regions majorly displayed MX type carbides. In the as-welded state, the fusion zone (FZ) contained martensite in coarse grains and small amount of δ-ferrite with no evidence for precipitation of M 23 C 6 and MX either in intra- or inter-granular regions. The heat affected zone (HAZ) was made up of martensite in fine grains without any δ-ferrite and with subtle variations in microstructure across the HAZ. The as-welded joints exhibited high hardness in the FZ and HAZ due to the occurrence of martensite during the weld thermal cycle. The impact toughness of the as-welded joint was inferior compared to that of the BM due to the combined influence of the martensite, coarse grains and presence of δ-ferrite in the weld zone. Tensile strength of as-welded joint was higher than that of BM. PWHTs were beneficial in decreasing the hardness in the FZ and HAZ. PWDT could not fully eliminate the pronounced variation of hardness observed in the transverse section of welded joint. Though the impact toughness of the weld joint was improved marginally compared to as-welded state after PWDT, it was much lower than that recorded in the case of BM. PWNT treatment minimised the variation in hardness across the transverse section of weld joint and the impact toughness surpassed than that achieved in BM. The tensile properties of BM, welded joints in as-welded and in PWHT conditions were determined at room temperature and correlated with the prevailing microstructures.

Characterization and evaluation of mechanical properties of CSEF P92 steel for varying normalizing temperature

Abstract: The microstructural and mechanical properties of creep enhanced ferritic (CSEF) steels are affected by various parameters, solutionizing temperature is one of them. In the present investigation, the effect of normalizing temperature on the microstructural and mechanical properties of cast and forged (C&F) P92 steel were carried out. Grade P92 steel is considered as a candidate material for Thermal and Nuclear power plants at temperatures of up to 650 °C. In this study, C&F P92 steel was subjected to various normalizing temperatures (from 950 °C to 1150 °C). For microscopic characterization, Optical microscope and Field emission scanning electron microscope (SEM) were used. The grain size, precipitate size, area fraction of precipitates and Cr/Fe were calculated from micrographs. The normalized specimens were tested for tensile strength, hardness, and toughness. Considering observation for the optimum combination of strength, ductility, and toughness, the normalizing at 1000 °C and tempering at 760 °C has been suggested for C&F P92 steel.

Welding metallurgy of martensitic advanced high strength steels during resistance spot welding

Abstract: The paper addresses the process–microstructure–performance relationships in resistance spot welding of a martensitic advanced high strength steel. Significant softening was observed in the heat affected zone (HAZ) due to allotriomorphic ferrite formation in the inter-critical HAZ and tempering of martensite in sub-critical HAZ (SCHAZ), with the latter plays more important role in mechanical properties of the spot welds. The strain concentration associated with the HAZ softening promotes initiation of pullout failure from the soft SCHAZ. While, the peak load in the interfacial failure mode is governed by the fusion zone size, that of the pullout mode is significantly affected by the HAZ softening. To improve weldability of martensitic steels, the HAZ softening should be minimised via modifications in welding process or steel chemistry.

Microstructure characterization and charpy toughness of P91 weldment for as-welded, post-weld heat treatment and normalizing & tempering heat treatment

Abstract: The effect of weld groove design and heat treatment on microstructure evolution and Charpy toughness of P91 pipe weldments was studied. The P91 pipe weldments were subjected to subcritical post weld heat treatment (760 °C-2 h) and normalizing/tempering conditions (normalized-1040 °C/40 min, air cooled; tempered 760 °C/2 h, air cooled) were employed. The influence of subsequent PWHT and N&T treatment on the microstructure of various zone of P91 pipe weldments were also investigated. The present investigation also described the effect of PWHT and N&T treatment on hardness, grain size, precipitate size, inter-particle spacing and fraction area of precipitates present in each zone of P91 pipe weldments. The result indicated great impact of heat treatment on the Charpy toughness and microstructure evolution of P91 weldments. The N&T treatment was found to be more effective heat treatment compared to subsequent PWHT. Charpy toughness value was found to be higher for narrow-groove design as compared to conventional V-groove design.

Microstructural characterization, strengthening and toughening mechanisms of a quenched and tempered steel: Effect of heat treatment parameters

Abstract: A quenched and tempered steel for a large bearing ring was investigated. The heat treatment experiments were designed by using the L 9 (3 4 ) type orthogonal form. Based on these conditions, a better combination of mechanical properties was obtained. The results showed that the quenching and the tempering temperatures were the most influential factors on the strength and toughness. The dislocation strengthening and the solid solution strengthening of the dissolved alloying carbides are the main mechanisms of increasing the strength by decreasing the tempering temperature and increasing the quenching temperature, respectively. The stripped carbides and long chain carbides strongly influence the differences in the tensile strength of the steels under different processes. The toughness AKv at −20 °C was increased by 42.2 J when the quenching temperature increased from 800 to 900 °C. The stripped undissolved carbides at lower quenching temperature promoted crack propagation and cleavage fracture and thus decreased the toughness of the steel. The AKv was increased by 47.4 J when the tempering temperature increased from 550 to 650 °C. The long chain carbides distributed along the grain boundary and the martensitic laths with a high density of dislocations at the lower tempering temperature decreased the toughness. Oil quenching can improve both the strength and toughness by refining the martensitic microstructure.

Effect of tempering temperature and inclusions on hydrogen-assisted fracture behaviors of a low alloy steel

Abstract: Abstact The tensile properties and fracture behaviors of the pre-charged hydrogen low alloy steel subjected to various tempering temperatures were investigated by slow strain rate tensile tests. Hydrogen was introduced into the samples by electrochemical method in 0.5 mol L−1 NaOH solution with 1 g L−1 CH4N2S for 24 h at room temperature (298 K). The results show that with an increase in tempering temperature, the resistance to hydrogen embrittlement increases. For the sample tempered at 200 °C, hydrogen-assisted microcracks initiate at the lath boundaries and the interfaces between inclusions and matrix. Moreover, the void around the inclusion initiates at the matrix-inclusion interface and grows in the form of the interface decohesion for hydrogen free sample, while the void also starts at the interfaces and propagates into the steel matrix along vertical tensile stress direction in the presence of hydrogen. Fractography reveals that for hydrogen-charged samples, the crack originates from mixed O-Si-Al-Ca inclusions in the samples tempered at 200 °C and 400 °C, while it is not located at inclusions in the sample tempered at 650 °C.

The evolutions of microstructure and mechanical properties of 2.25Cr-1Mo-0.25V steel with different initial microstructures during tempering

Abstract: The evolutions of microstructure and mechanical properties during tempering at 700 °C, of normalized and oil-quenched 225Cr-1Mo-025V steel samples to simulate the central and surface parts of the industrial heavy wall forgings, respectively, have been investigated It is found that the normalized sample has a granular bainite microstructure and the oil-quenched sample has a lath bainite microstructure After 05 h of tempering, the normalized sample has a higher strength and ductile-to-brittle transition temperature (DBTT) than the oil-quenched sample because of the strengthening effect of the undecomposed martensite-austenite (M-A) constituents and the presence of coherent tiny VC type precipitates in granular bainite However, when the tempering time is increased from 05 to 128 h, the strength as well as the DBTT of the normalized sample decreases more pronounced than that of the oil-quenched sample This is attributed to the synergistic effect of the decomposition of M-A constituents, growth of VC type precipitate in the normalized sample, and the increase in the effective grain size in the oil-quenched sample

Effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel

Abstract: The effect of tempering on the microstructure and mechanical properties of a medium carbon bainitic steel has been investigated through optical microscopy, electron back-scattered diffraction, transmission electron microscopy and X-ray diffraction analyses. A nano-level microstructure containing plate-like bainitic ferrite and film-like retained austenite is obtained by isothermal transformation at Ms+10 °C followed by tempering within 240–450 °C. Results show that the sample tempered at 340 °C occupies the optimal balance of strength and toughness by maintaining a certain level of plasticity; samples tempered at 320 °C and 360 °C with low and high yield ratio come second. The microstructure of the steel is not sensitive to tempering temperatures before 360 °C. When the temperature is increased to 450 °C, the significantly coarsened bainitic ferrite plate and the occurrence of a small quantity of carbide precipitation account for its low toughness. The amount of retained austenite increases with the tempering temperature before 400 °C, followed by decreasing with further increase in the temperature. This behavior is related to the competition between retained austenite further transforming into bainite and decomposing into carbide during tempering.

Hardfacing of AISI H13 tool steel with Stellite 21 alloy using cold metal transfer welding process

Abstract: Cold metal transfer (CMT) welding is a new gas metal arc welding process having several advantages such as low heat input and spatter free welding. This makes it of great advantage in weld cladding applications. In this study, hardfacing of AISI H13 die steel with Stellite 21 alloy has been carried using CMT process. Coatings were deposited on H13 substrate in annealed as well as quenched & tempered (Q&T) condition at room temperature as well as with a preheat of 400 °C. The Q&T substrate with and without preheat and the annealed substrate without preheat were found to be susceptible to underbead cracking upon Stellite deposition. The cracking in the heat affected zone (HAZ) was due to formation of brittle martensite upon rapid cooling which is associated with formation of high tensile residual stresses at the bead toe. The annealed substrate with preheat of 400 °C showed the least cracking tendency. The cracking tendency was investigated by studying the variation of the microstructure and microhardness along the depth of HAZ. The dilution levels based on Fe content was found to be 3–4%, which was considerably lower than that of conventional arc welding deposits. The Stellite coated H13 plate (annealed with preheat) could be successfully subjected to quenching & tempering heat treatment to restore the properties of the substrate without introducing any defects.