Influence of initial quenching on the microstructure and mechanical properties of quenched and partitioned ferritic stainless steels

Modern steel industry has great interest in developing new advanced high-strength steels, especially for the automotive industry. The need for stronger and more ductile sheet steels has led to development of novel heat treatments such as quenching and partitioning. The Q&P heat treatment provides an opportunity of manufacturing strong steels without sacrificing their formability. However, there is limited research conducted on the microstructure evolution of many alloys potential for Q&P such as stainless steels. This study evaluates the selection for the optimal quench interruption temperature during Q&P of ferritic stainless steels. The paper compares different simulation models for optimizing the Q&P-process. Q&P was applied to two AISI 420-type stainless steels EN 1.4021 and EN 1.4034 to assess the simulation results. Microstructure analyses with X-ray diffraction and electron microscopy revealed that simulated values overestimate the retained austenite fractions after Q&P due to formation of Cr-rich carbides. Mechanical tests showed that Q&P is applicable to grade EN 1.4021 stainless steel, whereas EN 1.4034 fractured in a brittle manner under tensile load. Electron microscopy revealed intergranular fracture type and concentration of Cr-rich carbides at parent austenite grain boundaries in EN 1.4034. These results suggest that impurities may expose stainless steels to temper embrittlement during partition.publishedVersionPeer reviewe

Dynamic softening kinetics of Al0.3CoCrFeNi high-entropy alloy during high temperature compression and its correlation with the evolving microstructure and micro-texture

To establish the characteristics and kinetics of dynamic softening in a Al0.3CoCrFeNi high–entropy alloy (HEA), isothermal compression tests were carried out in a suitable temperature range of 1273–1423 K at 10−2 and 10−1 s−1 in accord with our previous study. It was found that the discontinuous dynamic recrystallization (DRX) was the dominant microstructural reconstitution mechanism. The conditions of critical stress/strain for the onset of dynamic recrystallization were determined using the Poliak–Jonas analytical criterion. Further, a kinetic model was established based on the Avrami-type function in order to be able to predict the volume fraction of DRX. The DRX volume fraction expectedly increased with strain. The microstructural investigation of the isothermally compressed specimens revealed a good agreement with the proposed DRX kinetics model and validated its accuracy. Additionally, the evolution of DRX with strain was characterized by interrupting the test carried out at 1323 K/10−1 s−1 at different strains. The progress of DRX evolving as increased formation of new recrystallized grains further corroborated the predictions of the kinetic model. The micro-texture analysis revealed random texture in the recrystallized grains, whereas the unrecrystallized grains had shown their preferred orientation towards the fiber texture.publishedVersionPeer reviewe

Characterization of hot deformation behavior of Al0.3CoCrFeNi high entropy alloy and development of processing map

This study presents the characteristics of hot deformation behavior of a Al0.3CoCrFeNi high entropy alloy in the temperature and strain rate ranges of 1023–1423 K and 10–3–10 s–1, respectively. The constitutive flow behavior was modeled based on the hyperbolic–sinusoidal Arrhenius–type equations and a mathematical relation was used to observe the influence of true – strain on material constants. To define hot workability of the alloy, a processing map was developed based on the principles of dynamic materials model. Accordingly, a dynamic recrystallization (DRX) domain in the temperaure and strain rate ranges of 1273–1423 K and 10–2–2 x 10–1 s–respectively, with a peak efficiency of ~45 % at 1423 K/6 x 10–2 s–1, was identified as prudent for processing. At lower temperatures (1048–1148 K) and strain rates (10–3–3x10–3 s–1), a dynamic recovery (DRV) domain was identified with a peak efficiency of 38% at 1123 K/10–3 s–1. A large instability regime occurred above 3x10–1 s–1 with an increased tendency of adiabatic shear bands. It extended to lower strain rates 10–2–10−1 s−1 at temperatures <1123 K, manifested by localized shear bands and grain boundary cracking . At low strain rates (5x10–3–10–3 s–1) and temperatures (1148 – 1298 K), particle stimulated nucleation of new DRX grains occurred at B2 precipitates, though the efficiency of power dissipation dropped sharply to 9%.publishedVersionPeer reviewe

Impact-abrasive and abrasive wear behavior of low carbon steels with a range of hardness-toughness properties

This work investigates steels for mining wear applications involving abrasive and impact-abrasive conditions. The study comprises four low carbon steels with a range of hardness-toughness combinations: a commercial grade martensitic steel, the same steel heat treated to lower bainite, a commercial TRIP steel (tensile strength grade 700 MPa), and a quenching-partitioning (QP) steel. The steels were subjected to crushing pin-on-disc (CPOD) and slurry-pot wear tests, offering reasonably high-stress abrasive and impact-abrasive conditions, respectively. The results showed that the best performer in both studied wear conditions is the martensitic steel due to its higher initial hardness. Nevertheless, the performance benefit of this steel was of lesser magnitude in the slurry-pot than in the CPOD tests. On the other hand, the TRIP steel showed poor ranking in the CPOD tests but outperformed the QP and lower bainite steels in the slurry-pot tests. Detailed surface and subsurface wear damage investigations were conducted to study the wear responses of the microstructural constituents of the steels to explain their wear behavior in different wear conditions.acceptedVersionPeer reviewe

Directed energy deposition of AA7075 - effect of TiC nanoparticles on microstructure

AA7075 alloy is a high-strength aluminum alloy with properties enhanced by heat treatments. However, like most high-strength aluminum alloys, AA7075 is non-weldable, as it suffers from hot cracking when it is welded or additively manufactured with fusion techniques. A proposed way to reduce the hot cracking tendency is by refining the microstructure by adding nucleation enhancers. In this study, AA7075 powder feedstock was functionalized with 1.7 and 3.4 vol.% TiC nanoparticles, printed with laser-directed energy deposition (DED), subjected to T6 heat treatment, and characterized with optical and electron microscopy, electron backscatter diffraction (EBSD), and hardness measurements. Although TiC was not homogeneously distributed in the aluminum matrix, the addition of TiC successfully suppressed hot cracking by inhibiting dendritic growth produced by increased and more uniform nucleation, which resulted in refined equiaxed grains, and thus enhanced the printability of the material.publishedVersionPeer reviewe

Effects of strain rate and adiabatic heating on mechanical behavior of medium manganese Q&P steels

In this work, the mechanical behavior and properties of four different multiphase steels was studied in tension at strain rates of 10−4, 10−2, 0.5 and 800 s−1. The four materials include a medium manganese (3%) steel grade overcritically and intercritically annealed and Q&P heat treated and two industrially produced TRIP-assisted steels, DH800 and TRIP700 steels, which have different retained austenite morphology. The temperature and strain of the specimens were studied using high speed infrared thermography (IRT) and digital image correlation (DIC). The mechanical response of the Q&P steels had considerably higher tensile strength than the two industrially produced steels. The Q&P steel with a higher austenite volume fraction strain hardened significantly more than the other steels. The DH800 steel and the intercritically annealed Q&P steel heated less with ΔT of 25 °C during uniform deformation than the TRIP700 steel and the overcritically annealed Q&P steel with ΔT of 35 °C. However, the industrially produced steels DH800 and TRIP700 had higher uniform elongation of 0.12 mm/mm and 0.14 mm/mm whereas the Q&P steels reached only 0.09 mm/mm, meaning that the heating rate of the Q&P steels was considerably steeper. In addition, the stronger necking of the DH800 and TRIP700 steels led to much higher maximum temperatures before failure (max. 260 °C) than those observed for the Q&P steels (max. 140 °C). The Taylor-Quinney coefficients of the Q&P steels were large in the beginning of the plastic deformation (0.65–0.95) but decreased as a function of plastic deformation, whereas the Taylor–Quinney Coefficients of the DH800 and TRIP700 steels were lower (0.5–0.6) but increased gradually as a function of plastic deformation.publishedVersionPeer reviewe

Enhancing the cavitation erosion resistance of AISI 420-type stainless steel with quenching and partitioning

Stainless steels are commonly used in hydraulic components where they may be sus-ceptible to cavitation erosion. In this study the cavitation erosion resistance of AISI 420-type stainless steel is examined after quenching and partitioning (Q&P) heat treatment. Q&P-samples were prepared with varying heat treatment parameters, and their initial properties were examined with X-ray diffraction and hardness measurements. Reference samples were also prepared with quenching and tempering and quenching without parti-tioning. The samples were eroded for 6 h with an ultrasonic cavitation erosion device, and their mass losses were measured. The eroded areas were examined with scanning electron microscopy, optical profilometry, and magnetic induction measurements. The results suggest that the cavitation erosion resistance of the examined stainless steel can be significantly enhanced with Q&P. This enhancement of cavitation erosion resistance results from high initial hardness and retained austenite fraction of the steel. During cavi-tation erosion the retained austenite can absorb cavitation bubble collapse energy by transforming into strain induced martensite, which increases the hardness of the steel and generates expansion of the lattice. This expansion additionally hinders crack propa-gation at grain boundaries, which reduces the formation of initial cavitation damage.publishedVersionPeer reviewe

High-speed laser cladding of chromium carbide reinforced Ni-based coatings

Fusion-bonded and low-diluted overlay welded coatings are frequently very thick (>1mm). High-speed laser cladding is a novel process capable of producing thin fusion-bonded and low-diluted coatings with high coverage rates and low heat input. In this study, for the first time, high-speed laser cladding was used to fabricate relatively thin Ni-based coatings reinforced with chromium carbides onto low-alloy structural and quenched and tempered steels. Obtained coatings were characterized with X-ray diffraction (XRD), optical (OM), and scanning electron microscopy (SEM), as well as electron backscatter diffraction (EBSD). Mechanical and wear properties were tested with Vickers microhardness measurements and three-body dry-sand rubber wheel abrasion tests (RWAT). It was shown that high-speed laser cladding produces 0.2-0.3 mm thick coatings, which consist of ultrafine-substructured hypereutectic M7C3 structures reinforced with coarser primary Cr3C2 particles. Coatings with hardness up to 1300 HV0.05 exhibited high wear resistance in low-stress three-body abrasion. Coatings developed can be used as alternatives for hard-chrome plated coatings.Peer reviewe

Occurrence of dynamic strain aging in intercritically annealed low carbon high aluminum medium manganese steels

Medium manganese steel grades heat treated with the quench and partition (Q&P) treatment have shown promising results of strength and formability, but the number of studies on high aluminum medium manganese Q&P steels still remains rather limited. Consequently, this study investigates the mechanical properties, behavior and microstructures of a low carbon-high aluminum medium manganese (3%) steel after intercritical annealing and subsequent Q&P heat treatments with varying heat treatment parameters. Samples that were intercritically annealed above 760°C showed an ultimate tensile strength (UTS) of around 1500 MPa with uniform elongation values of approximately 10 %. On the other hand, the samples annealed at 740°C showed somewhat lower UTS values (in the range of 1000...1200 MPa) but much higher uniform and total elongations, i.e., considerably improved formability. However, the stress-strain curves of the samples annealed at 740°C showed rather severe serrations that can be connected to the dynamic strain aging (DSA) phenomenon, which limits the usability of the steel especially in applications where good surface quality of the deformed (sheet) is required. Therefore, the main focus of this article is on the samples annealed at temperatures close to A1, where DSA serrations tend to appear. The evidence gathered from these investigations leads to the conjecture that the DSA serrations are due to the free carbon and nitrogen originating from the dissolution of M(C,N) and M2(C,N) type carbides, which were formed during the intercritical annealing at temperatures producing less than 50 percent of the austenite phase. Other reasons for the occurrence of DSA in the current test materials can be the scarcity of martensite and the morphology of the retained austenite.publishedVersionPeer reviewe

The effect of scrap originating trace elements on the properties of low alloyed steels

The present intention to reach fossil-free steel manufacturing will inevitably result in an increase in the use of steel scrap as a raw material for steel production. Consequently, the amounts of elements, seen as impurities, will increase in steels. This has already been seen in electric arc furnace (EAF) processed steels, where the Cu and Sn levels have doubled in some cases after 1980’s. This may cause problems, as it is well-known, that some impurity elements have harmful effects on the properties of steel. This has been widely studied in low-alloy steels containing chromium and molybdenum which are widely used in components for the petroleum and electrical power generation applications. However, limited number of studies have been performed on formable steel grades, and the published reports/articles have mostly concentrated on the effects of P and B. Thus, there is still a need to understand the roles of other impurity elements. In the present study, a formable C-Mn steels containing additions (either individually or in combination) of Cu and Sn is investigated. The samples were cold rolled and annealed following typical time-temperature profiles of modern continuous annealing lines. Mechanical and forming properties (incl. bending and cupping tests) are determined as well as elemental profile analysis is conducted. The results identify that minor additions of impurity elements, in this case Cu and Sn, does not affect the mechanical and forming properties of low alloyed formable steel grades considerably.publishedVersionPeer reviewe