Insight to the influence of Ti addition on the strain-induced martensitic transformation in a high (about 7 wt.%) Mn stainless steel

Tiivistelmä The kinetics of strain-induced martensite (SIM) formation in a Ti-bearing 201L stainless steel were evaluated and compared to the existing results of two conventional stainless steel grades; i.e., 201L and 304L AISI. The effects of strain rate and rolling pass reduction on the kinetics of SIM formation during cold rolling were investigated. The Ti-microalloying was found to be intensifying the transformation due to lowering the stacking fault energy. It was seen that decreasing the rolling pass reduction strongly affected the variation of SIM volume fraction. Furthermore, a close correlation between the hardness and strain-induced transformation was found arising from microstructural evolution during the cold rolling process. Three stages in the hardening behavior were detected associated with lath-type martensite formation, transition stage of martensite laths break up and formation of dislocation-cell-type martensite

The effect of phase stability on the grain growth behavior of beta titanium alloys

Abstract The grain growth kinetics of two beta titanium alloys, Ti–4733 and Ti–5553 was studied over a temperature range of 850 °C–1000 °C. The more stable alloy i.e. Ti–4733, showed a smaller average grain size and slower growth rate compared to Ti–5553 at a same annealing condition. The results showed that the uniformity of grain size decreased initially dawn to a minimum and then increased with increasing temperature. The grain growth exponent (n) and activation energy (Q) were calculated and it was revealed that n is mainly affected by temperature and Q is generally influenced by time. The n value for Ti–4733 was found to be lower than that for Ti–5553 while a higher Q was calculated for Ti–4733. The lower n values and the higher Q were attributed to the solute drag effect and the high Mo content with low diffusivity in Ti matrix

Heterogeneous multiphase microstructure formation through partial recrystallization of a warm-deformed medium Mn steel during high-temperature partitioning

Abstract A novel processing route is proposed to create a heterogeneous, multiphase structure in a medium Mn steel by incorporating partial quenching above the ambient, warm deformation, and partial recrystallization at high partitioning temperatures. The processing schedule was implemented in a Gleeble thermomechanical simulator and microstructures were examined by electron microscopy and X-ray diffraction. The hardness of the structures was measured as the preliminary mechanical property. Quenching of the reaustenitized sample to 120 °C provided a microstructure consisting of 73% martensite and balance (27%) untransformed austenite. Subsequent warm deformation at 500 °C enabled partially recrystallized ferrite and retained austenite during subsequent partitioning at 650 °C. The final microstructure consisted of a heterogeneous mixture of several phases and morphologies including lath-tempered martensite, partially recrystallized ferrite, lath and equiaxed austenite, and carbides. The volume fraction of retained austenite was 29% with a grain size of 200–300 nm and an estimated average stacking fault energy of 45 mJ/m2. The study indicates that desired novel microstructures can be imparted in these steels through suitable process design, whereby various hardening mechanisms, such as transformation-induced plasticity, bimodal grain size, phase boundary, strain partitioning, and precipitation hardening can be activated, resulting presumably in enhanced mechanical properties

A new combinatorial processing route to achieve an ultrafine-grained, multiphase microstructure in a medium Mn steel

Abstract A new combination of factors enhancing the stabilization of austenite, including pre-existed austenite among quenched martensite, prior deformation, and partitioning at high temperatures is employed to create a multi-component refined microstructure in a medium Mn steel (Fe–4Mn–0.31C–2Ni–0.5Al–0.2Mo, wt.%). The microstructure evolution and phase fraction during the processing are systematically investigated using various characterization methods. The microstructure of the specimen after 0.4 strain deformation of 73% martensite–27% austenite at 250 °C and subsequent partition-annealing at 600 °C for 20 min was composed of several phases including tempered martensite, fresh martensite, pearlite, 10% of retained austenite (RA) and undissolved cementite. By increasing the annealing temperature, the pearlitic transformation was suppressed, whereas recrystallization of the deformed martensite and carbide dissolution occurred following annealing at 650 °C for 20 min resulting in an ultrafine-grained microstructure composed of equiaxed ferrite, 32% RA along with some fresh martensite during final cooling and few carbide precipitates. The results demonstrate that the combinatorial approach accelerated partitioning of alloying elements from martensite and carbides to largely pre-existing austenite is responsible for the improved austenite stabilization during intercritical annealing of the deformed dual-phase specimens. However, competitive processes are also enhanced so that the RA content is not increased by deformation

Promising bending properties of a new as-rolled medium-carbon steel achieved with furnace-cooled bainitic microstructures

Abstract The bending properties of a new thermomechanically processed medium-carbon (0.40 wt% C) low-alloy steel, intended for the slurry transportation pipeline application, have been investigated. The studied material was hot-rolled to 10 mm thick strips followed by direct quenching to two different quench-stop temperatures (QST) of 560 °C and 420 °C. The samples were subsequently cooled slowly to room temperature in a furnace, producing two different bainitic microstructures. In general, the final microstructures on the centerline consisted of different bainitic features with yield strengths of a ~700 MPa and ~1200 MPa for QST 560 °C and 420 °C, respectively. To determine the factors affecting bendability, as determined by three-point brake press bending tests, local microstructural features and texture were characterized with transmission and scanning electron-microscoy and macrohardness tests. Detailed quantitative microstructural evaluation of both subsurface and mid-thickness regions revealed that the bainitic sample produced at the higher temperature (QST 560) consisted of almost equal amounts of bainite types B1 and B2, where B1 has bainitic sheaves with a low dislocation density and intralath cementite, and B2 a very dislocation-dense morphology with mainly interlath cementite. In the QST 420 sample, the high dislocation density components B2 bainite and martensite were dominant, although martensite was only present near the strip surfaces. Different post-rolling cooling conditions did not change the general crystallographic theme but resulted in a slight increase in the texture intensity of the QST 420 sample. Neither the concave nor convex subsurface regions showed significant changes in texture after bending. The more favorable distributions of microstructural components and textural component intensities in the QST 560 sample resulted in higher elongation to fracture and work-hardening capacity, resulting a smaller minimum usable punch radius than for the stronger QST 420 sample. Fractographic examination of the cracked surfaces revealed that cracks developed by shear band formation followed by surface roughening, which promoted subsequent void and micro crack nucleation and growth

On the role of grain size on slurry erosion behavior of a novel medium-carbon, low-alloy pipeline steel after induction hardening

Abstract Grain refinement has been widely used to enhance the hardness and toughness properties of metallic materials. However, the effect of prior austenite grain refinement on the final martensitic microstructure and wear performance of steels is not yet fully understood. In this study, induction hardening treatment with heating rates in the range 50–500 °C/s to the peak temperatures of 900 and 1000 °C followed by water quenching has been employed to produce through-hardened microstructures in a new medium-carbon, low-alloy steel intended as a slurry transportation pipeline material. The results revealed that in the range of achieved prior austenite grain size i.e. 2–15 μm, during different heating paths, the final martensitic microstructures experienced only a slight difference in the size of blocks and level of hardness. The mean hardness, hardness homogeneity, and grain structure uniformity were highest with a heating rate of 50 °C/s, especially for those samples which were re-austenitized at the peak temperature of 900 °C. A pin-mill type of high-speed slurry-pot wear tester was used to evaluate the slurry erosion behavior of the steel. It was found that prior austenite grain size in the above-mentioned range had no significant effect on the final microstructure and hardness value, however, the slight difference in martensite block size did notably influence the work hardening behavior and consequently the wear mechanism of the samples during the tests

Rapid tempering of a medium-carbon martensitic steel:in-depth exploration of the microstructure – mechanical property evolution

Abstract Rapid tempering is a remarkable sustainable and energy-efficient way to modify properties of as-quenched martensite, particularly its toughness and ductility. In this work, tensile and fracture toughness properties, and microstructural evolution of a medium-carbon (0.4 wt%), low-alloy steel under different rapid tempering circumstances are discussed. The hot-rolled and direct-quenched specimens were subjected to rapid tempering treatments (heating rate of about 90 °C/s) to the different tempering temperatures (320–720 °C). As a reference material, one sample was subjected to conventional tempering at 420 °C for one hour. The results indicated that the mechanical properties and microstructural features are influenced considerably by tempering temperature rather than the holding time. Despite the short tempering duration, the strength of tempered martensite dropped (from a max. tensile strength of ∼ 2100 MPa to a min. of ∼ 1100 MPa) while ductility increased with increasing tempering temperature (from a tensile elongation of 4% to ∼ 15%). However, the rapidly tempered sample at 420 °C experienced a loss of fracture toughness as a result of coarse stick-like cementite precipitation. Microstructural observations revealed that, even with a short time frame, tempering temperature plays a significant role in the resulting cementite morphology. At lower tempering temperatures, the cementite precipitates have a stick-like structure, while at higher temperatures, they become more globular/spheroid-like. This change in cementite morphology led to an enhancement in fracture toughness. Overall, the results indicated that, by a proper design of the rapid tempering process, an excellent combination of tensile properties and fracture toughness can be obtained compared to conventional tempering while significantly saving time and energy

Improving the yield strength of an antibacterial 304Cu austenitic stainless steel by the reversion treatment

Abstract As an implant material, Cu-bearing austenitic stainless steels can possess the antibacterial property, but their mechanical strength is low. In order to improve the yield strength of a 304Cu (17%Cr–7%Ni–3%Cu) alloy through substantial grain refinement, a research investigation has been taken up to conduct the reversion annealing treatment comprising a heavy (71%) cold rolling reduction followed by annealing at various temperatures (650–950 °C) and durations (1–5400 s). The microstructure evolution was examined by electron backscatter diffraction and further characterized by magnetic measurements, and mechanical properties were determined by tensile and hardness testing. The precipitation of Cu was confirmed by transmission electron microscopy. It was found that the reversion of deformation-induced martensite to austenite took place by the shear mechanism, followed by subgrain formation and continuous recrystallization resulting in quite non-uniform grain size distribution. The finest reversed grains were around 0.6 μm in size, but also much larger austenite grains and a small fraction of unreversed martensite existed in the final structure despite annealing at least up to 800 °C. Coherent Cu particles were observed after aging for 1.5 h at 700 and 650 °C, while the yield strength could be improved to 507 and 791 MPa, respectively, i.e. by ~2–3 times that of the annealed steel. The ductility of the steel remains still high, the fracture elongation being 36%