Quenched And Tempered Research Articles

Effects of the microstructure and reversed austenite on the hydrogen embrittlement susceptibility of Ni-Cr-Mo-V/Nb high-strength steel

The effects of the microstructure and reversed austenite (RA) on the hydrogen embrittlement (HE) behavior were investigated in NiCrMoV/Nb high-strength steel prepared using three different heat treatments. The quenching and tempering (QT) steel showed a noble HE resistance for the lowest hydrogen diffusion and maximum apparent hydrogen concentration compared to quenching (Q) steel and quenching, lamellarizing, and tempering (QLT) steel. Thermal desorption spectroscopy (TDS) analysis confirmed that RA had an activation energy of 32.5 kJ/mol which indicates a stable trapping site. The QLT steel containing more RA exhibited a higher HE susceptibility, suggesting that RA degrades the HE resistance.

Enhanced Spring Steel’s Strength Using Strain Assisted Tempering

Spring steels are typical materials where enhancement of mechanical properties can save considerable mass for transport vehicles, in this way the consumption of fuel or electric energy can be decreased. A drastic change in both the resulting microstructure and mechanical properties could be achieved due to the inclusion of strain into the tempering process after quenching. The strain assisted tempering (SAT) technology was applied, i.e., the process of quenching and following a sequence of tempering operations alternating with strain operations. After the first tempering, controlled deformation by rotary swaging was carried out with a strain of 17% (strain rate is about 120 s−1). Considerably higher strength parameters after SAT compared to conventional quenching and tempering (QT) technology were nevertheless accompanied by enhanced notch toughness at the same time by the decrease of elongation and reduction of area. However, by optimizing the process it is was also possible to achieve acceptable values for those parameters. Remarkable differences are visible in resulting microstructures of compared samples, which were revealed by metallographic analysis and X-ray diffraction measurement. While the standard microstructure of tempered martensite with transition carbides was observed after QT processing, carbideless islands with nanotwins occurred in martensitic laths after SAT processing.

Thermal Fatigue Behavior of AISI H13 Hot Work Tool Steel Produced by Direct Laser Metal Deposition

Direct laser metal deposition (DLMD) is an additive manufacturing technique getting growing attention thanks to the possibility of producing very complex parts in a short time and in a cost‐effective manner. The possible applications of this technology are tools with conformal cooling channels and claddings for dies and molds reparation. One of the damaging mechanisms of tools is thermal fatigue (TF) cracking, leading to surface deterioration and, consequently, the processed parts. Herein, the TF behavior of DLMD‐H13 submitted to two different heat treatments, namely direct tempering (T) and quenching and tempering (QT), is investigated. T does not significantly change the solidification microstructure after DLMD, whereas QT produces a more homogenous tempered martensite microstructure. A customary laboratory test is developed to induce TF damage under a cyclic temperature variation between 630 and 60 °C. The results evidence that the T‐H13 has a slightly better TF resistance with respect to QT‐H13 due to the higher tempering resistance of T‐H13 with respect to QT‐H13. Thus, according to TF test results, direct tempering can be preferred to quench and tempering since the elimination of quenching can decrease the costs of production as well as distortions‐related issues, increasing the competitiveness of DLMD.

Comparative study on strength of TMCP and QT high-strength steel butt-welded joints

The strength of high-strength steel butt-welded joints with a similar steel grade may be quite different due to their discrepancies in the processing methods of high-strength steel. The process methods mainly include quenching and tempering (QT) and thermo-mechanically controlled process (TMCP). These differences in the strength of high-strength steel butt-welded joints with different processing methods of steel have not been quantitatively determined and considered in several design codes, which may lead to the unsafe design for the high-strength steel butt-welded joints. This research demonstrated a comparative and quantitative study on the strength of double-V butt-welded joints made of QT550, QT690, TMCP 550, and TMCP690. The true stress-strain behavior of various zones within these welded joints, including the hardened heat affected zone (HHAZ), softened HAZ (SHAZ), weld metal (WM), and base metal (BM), were investigated based on the test and simulation results of the round notched specimens and the flat grooved specimens in tension. The developed true stress-strain models were validated against the test results of high-strength steel butt-welded joints. The strength of QT and TMCP high-strength steel butt-welded joints were then numerically analyzed considering the effect of different heat inputs of 1.0, 1.5 and 1.9 kJ/mm. The results show that strength ratios of SHAZ to BM of butt-welded joints made of TMCP steel were lower than those made of QT steel when the heat input was <1.9 kJ/mm. The strength of each zone in the TMCP high-strength steel butt-welded joint may not necessarily decrease with increasing heat inputs.

Structural behaviour of high strength steel hexagonal hollow section stub columns under axial compression

This paper presents a comprehensive experimental investigation into the structural behaviour of the high strength steel (HSS) hexagonal hollow sections (HexHS) stub columns under concentric compression. A total of 18 HSS HexHS stub columns encompassing three fabrication routes were performed. The HSS plates used to fabricate the specimen were delivered in Quenched and Tempered (QT) condition. The material properties within the cross sections of the HSS HexHS were measured. Initial local geometric imperfections for all the examined stub columns were measured. The cross-section slenderness yield limits in the existing design codes of the EN 1993-1-12, ANSI/AISC 360-16, AS 4100 and ASCE/SEI 48-11 were compared and assessed against the experimental results. It was found that the current limits for internal compression plates in HSS rectangular and square sections cannot be extended to cover the cross-section classification for HSS HexHS. In terms of the cross-sectional resistance predictions, all the design codes tend to under-estimate the resistance of HSS HexHS under axial compression. For sections failed by local buckling prior to the attainment of yield load, the AS 4100 yields more accurate predictions compared with those predictions based on the other design codes. Compared with the design approach of Direct Strength Method (DSM), Continuous Strength Method (CSM) provides more consistent and satisfactorily predictions, particularly for stocky sections due to the consideration of strain hardening.

Microstructure of Ultrafine Acicular Bainite and Mechanical Properties of 3Cr2MnNiMo Mold Steel during Austempering

Herein, the effect of austempering treatment on martensite/bainite multiphase microstructure evolution and mechanical properties of 3Cr2MnNiMo mold steel are investigated, and the mechanical properties are compared with the conventional quenching and tempering (QT) treatment. It indicated that the morphology of martensite/bainite can be effectively tailored by changing the austempering temperature. With the increase of temperature from 280 to 360 °C, the microstructure changes from tempered martensite + bainite + blocky retained austenite (RA) to martensite + coarse bainite plate + blocky RA and irregular martensite/austenite (M/A) island. When the temperature reaches 420 °C, the martensite content reaches the maximum. The bainite morphology is surprisingly transformed from coarse bainite plate to ultrafine acicular shape owing to the reduction of the bainitic driving force and the sharp increase of carbon diffusion coefficient. Moreover, blocky RA and M/A islands disappear and are replaced by nanoscale filmy RA. The alternating distribution of variants has a refinement effect on the microstructure and the effective grain size reaches a minimum value. Compared with the conventional QT treatment, the tensile strength of austempering at 420 °C increased by 702 MPa, but the toughness and ductility levels are almost the same.

Effect of line energy density in repairing 34CrNiMo6 steel by electron beam remelting

The role of the line energy density of electron beam remelting in repairing damaged 34CrNiMo6 parts was investigated. Remelting technology can be used to rapidly repair defects with superior performance, but the value of the heat input during operation was difficult to control, which has a substantial impact on the microstructure and mechanical properties of the parts. The line energy density Q was introduced to calculate the experimental parameters. The microstructure was observed at different regions of the sample, and its mechanical properties were analyzed. From the results, the following conclusion was drawn. An appropriate increase of line energy density could increase the depth and width of remelting to improve the remelting efficiency. Under a low line energy density (Q1), the mainly thermal cycle on 34CrNiMo6 steel was tempering rather than the preheating. With the increase of energy density, the preheating effect of ex-remelting came into sight, and the martensite of repaired zone with energy density Q2 and Q3 became coarse. As to the enhanced diffusion of carbon, the amount and size of carbides precipitated from ferrite increased. The microhardness decreased with the increase of energy density, the max value was about 660 HV (Q1) and the min was 330 HV (Q3). Tensile properties in different Q were similar, the max tensile strength σb was 845 MPa by remelting. The samples after quenching and tempering (QT) exceeded the forging standard even for the lowest strength value (1013 MPa). The fracture mode of remelted samples was brittle fracture in low energy density (Q1), and changed to quasi-cleavage fracture at energy density Q2 and Q3. All the modes of the QT samples were ductile fractures.

Study on Thermal Process and Microstructure & Properties of High Strength Automotive Steel

This paper studies how the Nb content affected mechanical properties and microstructures of automotive steel processed by quenching and tempering (Q-T) and quenching-partitioning-tempering (Q-P-T) with the help of tensile testers, X-ray diffractometers, scanning electron microscopes, transmission electron microscopes and other equipment. It was demonstrated that when the Nb content remained the same, Agt (total elongation at maximum force), A (elongation after fracture) and R m·A (the product of strength with ductility) of the steel processed by Q-P-T were higher than those of the steel processed by Q-T; and when the steel was treated by the same thermal process, as the Nb content increased, its R p0.2 (yield strength) and R m (tensile strength) would grow, while A gt, A and R m·A would rise first and then decline. No matter if treated by Q-T or Q-P-T, the microstructures of the steel containing 0% and 0.03% of Nb separately were always lath martensite and film-like retained austenite located between laths. When processed by Q-P-T, the steel containing 0.03% Nb saw the largest strength-ductility product. This was because Nb could help refine the sizes of the original austenite grain and the lath martensite and precipitate nano-sized NbC; and the Q-P-T process was more effective in thickening the film-like retained austenite in steel and promoting the dispersed precipitation of the NbC.

Testing, numerical modelling and design of Q690 high strength steel welded T-section stub columns

This paper presents a comprehensive experimental and numerical investigation into the material properties and stub column behaviour of Q690 high strength steel (HSS) welded T-sections. A total of 16 concentrically loaded stub column tests were performed. The Q690 HSS welded T-sections were fabricated from 6 mm and 10 mm thick plates, delivered in quenched and tempered (QT) conditions by means of gas metal arc welding (GMAW). Material properties measurements were carried out on the coupon specimens taken from the parent plates. Initial local geometric imperfection measurements were conducted. In conjunction with experimental tests, finite element (FE) models were developed to replicate the test results and failure modes. Afterwards, the validated FE models were adopted to conduct parametric studies to supplement the experimental data by generating further structural performance data covering a broader range of cross-section slenderness. Cross-section slenderness limits set out in design codes such as EN 1993-1-12, ANSI/AISC 360-16, AS 4100 and design methods of DSM as well as CSM were evaluated against the experimental and numerical data. It was found that the current limits are generally accurate and safe in three design codes and the cross-section slenderness of DSM is accurate and comparably conservative. Cross-section capacities predictions obtained from EN 1993-1-12, ANSI/AISC 360-16, AS 4100, DSM and CSM were also compared with the tests and numerical results. It is shown that the established local buckling design provisions in EN 1993-1-12 and ANSI/AISC 360-16 result in more precise and consistent predictions compared with AS 4100, DSM and CSM.

Surface degradation of nitrided hot work tool steels under repeated impact-sliding contacts: Effect of compound layer

Wear behaviour of quenched and tempered (QT) hot work tool steels (Uddeholm QRO90) was investigated against SAE 52100 grade bearing steel balls after gas nitriding using a dedicated laboratory scale impact-sliding wear test rig. Gas nitriding was employed in a fluidised bed reactor under two alternative regimes: i. “High Temperature Nitriding (HTN)” carried out at 510 °C and ii. “Low Temperature Nitriding (LTN)” carried out at ≤ 400 °C. The HTN process resulted in the formation of ∼2 μm thick external compound layer, whereas the LTN processed steels were free of any surface compound layer formation. After the impact-sliding wear tests employed at room temperature (RT), the prevailing wear mechanisms of the examined steels were assessed as tribo-oxidation and fatigue wear. The testing at 600 °C induced different wear mechanisms for the HTN and the LTN steels. While tribo-oxidation and fatigue wear were preserved for the HTN steel, plastic deformation dominated the wear that progressed on the compound layer free surface of the LTN steel. Impact-sliding wear testing at 600 °C showed that the wear rate of LTN > HTN steels, as opposed to the wear rate at RT where wear rate of HTN > LTN.

On the influence of heavy warm reduction on the microstructure and mechanical properties of a medium-carbon ferritic –pearlitic steel

Abstract The current study on a medium-carbon steel (0.36% C) with deformation-induced spheroidized cementite produced by heavy warm deformation (HWD) was carried out in order to investigate an alternative microstructural route when compared to conventional quenching and tempering (QT) or soft annealing (SA) processes. After austenite deformation, the HWD samples were cooled and heavily deformed at temperatures below the γ–α transformation followed by a simulated coiling treatment. In particular, the study addresses the influence of adiabatic heating due to the heavy deformation on the microstructure and properties. The mechanical properties after the various treatments show that the strength – ductility relation after HWD and QT are superior to those after continuous cooling (CC) or soft annealing (SA). In the microstructures obtained from the CC and SA treatments we observed lamellar pearlite which deteriorated the mechanical properties. A similar effect was observed after the adiabatic HWD treatment, which also entails some lamellar pearlite. HWD at 670 °C with subsequent coiling produced a microstructure with dispersed spheroidized cementite distributed homogeneously in the fine-grained ferrite matrix. The mechanical properties of the HWD samples are well comparable to those observed after the QT process owing to the resemblance in microstructure.

Submerged arc welding process peculiarities in application for Arctic structures

&lt;abstract&gt; &lt;p&gt;The paper focuses on the submerged arc welding (SAW) process in application to structures for Arctic conditions. One of the technical challenges for modern Arctic structures is to produce high-quality welds since a weld is usually the weakest part of any structure. Welding is especially difficult for the high strength steels (HSS), which are used in structures for weight-reduction purposes. The objective of the study is to explore the usability, development possibilities and parameters of SAW process for welding of thick cold-resistant HSS plates. Meeting this objective required in-depth understanding of the welding and material science background, which includes the quality requirements of weld joints intended for Arctic service as described in various standards, properties of cold-resistant HSS and description of testing methods used to validate welding joints for low temperature conditions. The study describes experimental findings that improve understanding of SAW process of thick quenched and tempered (QT) and thermo-mechanically processed (TMCP) HSS plates. Experiments were conducted to develop SAW procedures to weld several thick (exceeding 25 mm) high strengths (580–650 MPa tensile strength) cold-resistant (intended operational temperature at least −40 ℃) steel grades. The welds were evaluated by a wide range of industrial tests: analyses of chemical, microstructural and mechanical properties; hardness tests; and cold resistance evaluation tests: the Charpy V-notch impact test and the Crack tip opening displacement (CTOD) test. Acceptable welding parameters and recommendations were developed, and the results of the experiments show that high quality welds can be obtained using heat input up to 3.5 kJ/mm.&lt;/p&gt; &lt;/abstract&gt;

Abnormal relation between tensile and fatigue strengths for a high-strength low-alloy steel

The microstructure, tensile and fatigue strengths of the high-strength 35CrMo steel with quenched and tempered (QT) and hot-rolled (HR) states were investigated. There is not a commonly linear relation between tensile and fatigue strengths, namely, the HR steel with higher tensile strength has lower fatigue strength, which is mainly attributed to microstructural inhomogeneity of the steel and different damage mechanisms under fatigue and tensile loads. In this work, an estimation method of fatigue strength for steels with dual phase is suggested.

Fracture behaviors at inclusions of very-high-cycle fatigue in newly developed clean bearing steel

Bearing steel is essential for the driving parts of the rotating body, but fisheye crack of inclusions such as TiN, Al2O3 and Cr matrix is formed in the very-high-cycle fatigue (VHCF) regime. Therefore, research on the development of a clean bearing steel that reduces the size of inclusion such as TiN or that does not contain inclusion has been made recently. In this study, the results of long-term attempts by the maker to improve the performance and quality of bearing steel for very long life were quantitatively analyzed from the viewpoint of fracture mechanics. In other words, the phase transformation properties, VHCF characteristics with ultrasonic nanocrystal surface modification (UNSM) treatment and fracture mechanics analysis of this clean bearing are compared quantitatively with those of other common bearing steels from the viewpoint of engineering and industrial applications. The quenching and tempering (QT) specimens of the clean bearing showed fisheye cracks and duplex [Formula: see text]–[Formula: see text] curves in the VHCF regime. The fisheye crack of the clean bearing steel was an internal fatigue crack caused by a small inclusion mainly composed of Al2O3 in the VHCF region. However, fisheye cracking due to TiN and Cr matrix, which was observed in other common bearing steels, did not occur in the clean bearing steel due to the efforts of the developers of new clean bearing.

Effect of martensite pre-quenching on bainite transformation kinetics, martensite/bainite duplex microstructures, mechanical properties and retained austenite stability of GCr15 bearing steel

In this work, effect of martensite pre-quenching on the subsequent bainite transformation kinetics of GCr15 bearing steel has been investigated by means of dilatometry. The results show that the pre-quenched martensite is significantly accelerate nucleation of subsequent bainite transformation in comparison with direct bainite austempering. Meanwhile, specimen of martensite pre-quenching at 200 °C has the shortest incubation period of bainite transformation at 240 °C. Moreover, the transformation rate of bainite transformation after martensite pre-quenching is faster than direct bainite austempering in the initial growth stage, and decrease with the decreasing of martensite pre-quenching temperature. Compared with the conventional quenched and tempered (QT) specimen, the tensile strength is slightly decrease from 2170 MPa to 2143 MPa, but the impact toughness is obviously increase from 43 J to 71 J in specimen heat-treated by martensite pre-quenching at 200 °C and subsequent bainite transformation at 240 °C. In addition, the mechanical stability of retained austenite in all martensite pre-quenching specimens are higher than that of QT specimen.

Residual mechanical properties of Q890 high-strength structural steel after exposure to fire

Q890 high-strength structural steel has a promising application in engineering structures. However, little research has focused on its post-fire residual mechanical properties, which restricts its application extent. The present study aims to investigate the residual mechanical properties of Q890 high-strength quenched and tempered (QT) steel after exposure to fire. A series of experimental tests are conducted to obtain the post-fire stress-strain curves and microstructures of Q890 steel, considering the influence of temperature (room temperature to 1000 °C) and cooling methods (cooling in air and water). The residual elastic modulus, yield stress, ultimate tensile strength, ultimate strain, and microstructures were analyzed and discussed. A comparison of residual mechanical properties between Q890 steel and other high-strength QT structural steels was performed. Experimental results show that the post-fire mechanical properties are significantly dependent on temperature and cooling methods. Finally, empirical equations are proposed to predict the residual mechanical properties of Q890 steel, providing a basis for evaluating the post-fire performance of steel structures.

Comparison of the Impact Wear Performances of Quenching and Partitioning and Quenching and Tempering Steels

A low‐carbon, high‐strength steel is treated by different quenching and partitioning (QP) and quenching and tempering (QT) routes in a salt bath furnace, and its wear performance is evaluated by impact abrasive wear tests. It is observed as compared with the traditional QT steel; the QP steel manifests better wear performance at the quenching temperature of 220 °C. Stable film‐like retained austenite (RA) and fine martensite laths improve the wear resistance of the QP steel at the quenching temperature of 220 °C, whereas unstable blocky RA formed in the QP steel at the quenching temperature of 190 °C decrease the wear resistance. In addition, the lower critical impact stress for crack initiation at the higher impact energy decreases the wear resistance; however, the relative wear resistance is improved greatly at the higher impact energy due to the better fracture toughness of the QP steel. Moreover, the correlation of wear loss, hardness, and KIC is modeled to compare the wear resistances of the test steel after different heat treatments.

A critical review on wear and corrosion of carbide free bainitic steel

Different research on wear and corrosion of CFB (carbide free bainitic steel) over the past two decades has enabled a better understanding and advanced applications of CFB steel. With the help of advanced testing techniques, and equipment the detailed study of microstructure and nanostructured steels has been possible. The earlier years were focused on comparison of the bainite steel with the steel which is quenched and tempered (QT) for identifying the wear tendencies and comparison between carbon free bainite and martensite for the corrosive behavior of the carbide free bainitic steel. Later the trend shifted towards the nanostructured analysis from microstructure analysis with the improvement in the instruments. Advancements like HT-CAT i.e., high temperature continuous abrasion tester have an important role to play, the mechanism of material removal rate studied. There has also been development in alloying the bainite to reduce the corrosion rate and give more strength. Current trends in the form of study of wear and corrosion of carbide free bainitic steel and future needs are discussed hereby.

Process Routes of Low‐Ni Liquefied Natural Gas Tank Steel with Excellent Cryogenic Toughness

Herein, several treatments, including modified quenching and tempering (QT) treatment, ultrafast cooling and tempering (UFC‐T) treatment, and ultrafast cooling, intercritical quenching, and tempering (UFC‐LT) treatment, are used to prepare low‐Ni steels and compared with conventional QT treatment. All the three treatments enable low‐Ni steel to achieve equivalent or better cryogenic toughness than 9% Ni steel. The toughening effect of modified QT treatment is better than that of UFC‐T treatment, which is attributed to the increase in density of high‐angle grain boundaries (HAGBs). For modified QT treatment, fine equiaxed prior austenite grains and weak variant selection of martensite transformation lead to higher density HAGBs. Although the coarse elongated prior austenite grains in UFC‐T treatment are beneficial to Bain grouping and not conducive to matrix refinement, the intercritical quenching step in UFC‐LT treatment further increases the density of HAGBs due to the jagged prior austenite grain boundaries (PAGBs) and fresh martensite formed along PAGBs. Therefore, UFC‐LT treatment achieves high‐density HAGBs and excellent cryogenic toughness similar to modified QT treatment. Due to the wider tempering temperature process window, UFC‐LT treatment is the optimal process route for low‐Ni liquefied natural gas tank steel.

Processing and cooling effects on post-fire mechanical properties of high strength structural steels

High strength structural steel (HSSS) can be divided into TMCP (Thermo-Mechanical Controlled Process) and QT (Quenched and Tempered) types according to the manufacturing process. Most previous studies on the post-fire mechanical properties of HSSS were conducted on QT HSSS, while few studies were conducted on TMCP HSSS. In order to identify the effect of manufacturing process on the post-fire mechanical properties of HSSS, experimental studies were carried out on TMCP Q550 and TMCP Q690 HSSS and the results were compared with the previous studies on corresponding QT Q550 and QT Q690 HSSS. The static tensile tests were conducted at ambient temperature on TMCP Q550 and TMCP Q690 HSSS specimens heated up to nine pre-selected temperatures ranging from 200 to 900 °C and then cooling in air and water respectively to simulate various post-fire situations. It can be found that the heating and cooling process had little influence on elastic modulus but significant effect on strength and ultimate elongation once the heated temperature exceeded 600 °C, and the cooling way and manufacturing process became critical once the heated temperature exceeded 700 °C. In general, the post-fire strength and ultimate elongation was always less for TMCP HSSS than QT HSSS.