The Effect of Heat Treatment on Microstructure, Microhardness and Pitting Corrosion of Ti6Al4V Produced by Electron Beam Melting Additive Manufacturing Process

There has been limited studies on corrosion behaviour of post-processed Electron Beam Melted (EBM) Ti6Al4V, given that the factors affecting corrosion resistance of AM Ti6Al4V remain unclear. This paper proposes using heat treatment method to improve the pitting corrosion resistance of EBM Ti6Al4V. Different treatment profiles alter the microstructure of EBM Ti6Al4V. A clear trend is observed between microhardness and α lath width. As-printed EBM Ti6Al4V exhibits an inferior pitting potential, while heat treatment provided a significant improvement in the corrosion resistance. This study finds that the β phase fraction is a better indicator than the α lath width for pitting corrosion resistance. Solution air-cooled & ageing heat treated EBM Ti6Al4V exhibits good mechanical and corrosion properties, and even performs better than commercial cast Ti6Al4V.

Microstructure evolution of low alloy wear resistant steels during heat treatment procedure

Microstructure evolution of low alloy wear resistant steels during heat treatment procedure was studied in this paper. The results showed that During furnace cooling in homogenizing, Chromium/iron, Niobium, Vanadium and other hardly soluble carbides formed. But Chromium/iron carbides could resolve into austenite during quenching procedure, while the other carbides barely changed. Carbon addition grew the carbides into shuttle shapes and inflated the austenite grains. But Ni addition broadened the martensite lath width without dilating the austenite grains. And it hardly influenced the carbides formation. Vanadium addition seemed that the martensite lathes were cut into several discontinues sections. With the temperature rising, the boundaries got blurred, which might correlated with the decomposing of retained austenite.

Effect of Heat Treatments on Microstructural Evolution and Tensile Properties of 15Cr12MoVWN Ferritic/Martensitic Steel

In this study, the phase transformation temperature of 15Cr12MoVWN ferritic/martensitic steel was determined by differential scanning calorimetry to provide a theoretical basis for the design of a heat treatment process. An orthogonal design experiment was performed to investigate the relationship between microstructure and heat treatment parameters, i.e., normalizing temperature, cooling method and tempering temperature by evaluating the room-temperature and elevated-temperature tensile properties, and the optimum heat treatment parameters were determined. It is shown that the optimized heat treatment process was composed of normalizing at 1050 °C followed by air cooling to room temperature and tempering at 700 °C. Under the optimum heat treatment condition, the room-temperature tensile properties were 1014 MPa (UTS), 810.5 MPa (YS) and 18.8% (elongation), while the values are 577.5 MPa (UTS), 469 MPa (YS) and 39.8% (elongation) tested at 550 °C. The microstructural examination shows that the strengthening contributions from microstructural factors were the martensitic lath width, dislocations, M23C6, MX and grain boundaries of prior austenite grain (PAG) in a descending order. The main factors influencing the tensile strength of 15Cr12MoVWN steel were the martensitic lath width and dislocations.

Improvement of toughness and hardness in BR1500HS steel by ultrafine martensite

To obtain the ultrafine martensite for BR1500HS ultra-high-strength steel, a new preparation process of cyclic heat treatment (CHT) with descending heat temperature and holding time along with cycle steps was developed. First, a series of thermal dilation tests were conducted with the temperature range of 420–730°C and the heat rate of 5 K/s on a Gleeble-3800 thermomechanical physical simulator. According to the experimental data, the temperature ranges and the optimal holding time to completely austenitize BR1500HS were determined. Then, to confirm the optimal parameters of CHT for BR1500HS, several tests with various temperatures and cycle steps were conducted and analyzed by optical microscope and scanning electron microscope. Subsequently, the CHTs with decreasing heating temperature and holding time were studied due to the increasing internal energy of steel along with the CHT process. The lath width was measured as a criterion to evaluate the refinement degree in this article. After several loops of heat treatment, the lath width is reduced to 0.268 µm. Finally, the hardness evolution of the specimens subjected to CHT in this study was analyzed and compared with the lath width test results, which justified the effectiveness of the new developed process.

The Effect of Lath Martensite Microstructures on the Strength of Medium-Carbon Low-Alloy Steel

Different austenitizing temperatures were used to obtain medium-carbon low-alloy (MCLA) martensitic steels with different lath martensite microstructures. The hierarchical microstructures of lath martensite were investigated by optical microscopy (OM), electron backscattering diffraction (EBSD), and transmission electron microscopy (TEM). The results show that with increasing the austenitizing temperature, the prior austenite grain size and block size increased, while the lath width decreased. Further, the yield strength and tensile strength increased due to the enhancement of the grain boundary strengthening. The fitting results reveal that only the relationship between lath width and strength followed the Hall–Petch formula of. Hence, we propose that lath width acts as the effective grain size (EGS) of strength in MCLA steel. In addition, the carbon content had a significant effect on the EGS of martensitic strength. In steels with lower carbon content, block size acted as the EGS, while, in steels with higher carbon content, the EGS changed to lath width. The effect of the Cottrell atmosphere around boundaries may be responsible for this change.

Heat treatment of electron beam melted (EBM) Ti-6Al-4V: microstructure to mechanical property correlations

Purpose The paper presents a wide range of post processing heat treatment cycles performed to Electron Beam Melted (EBM) Ti6Al4V alloy and establishes correlations of heat treat process to microstructure and mechanical property (microhardness). The research also identifies the optimal heat treatment to obtain the best microstructure and mechanical properties (hardness and tensile). Design/methodology/approach Rectangular bars fabricated using EBM was used to study the different heat treatment cycles. A variety of heat treatments from sub ß-transus, super ß-transus, near ß-transus and solution aircool plus ageing were designed. After the heat treatment process, the samples were analysed for, α lath width, prior ß grain size, microhardness and nanohardness. Tensile tests were done for the heat treated samples showing most refined α lath structure with uniform globular grains. Findings A clear correlation was observed between α lath width and the microhardness values. The solution aircooled plus aged samples exhibited the best refinement in α-ß morphology with uniform equiaxed grains. The tensile properties of the solution aircooled plus aged samples were comparable to that of the EBM printed samples and better than ASTMF1472 specifications. Originality/value There is hardly any prior work related to post processing heat treatment of EBM built Ti6Al4V other than HIP treatments. The variety of heat treatment cycles and its influence in microstructure and properties, studied in this research, gives a clear understanding on how to tailor final microstructures and select the optimal heat treatment process.

Combination of Microstructural Investigation and Simulation during the Heat Treatment of a Creep Resistant 11% Cr-Steel

This work deals with the microstructural evolution of creep resistant martensitic/ferritic 11% Cr-steel during thermomechanical treatment from an experimental as well as modeling point of view. The creep resistance of this material group is highly dependent on the precipitate status. The initial precipitate status is controlled by the chemical composition of the alloy and the heat treatment after casting or hot rolling. It is therefore of utmost interest to understand and model the precipitate kinetics during this process. Once the microstructural evolution has been modeled successfully, only minimum effort is required to computationally test variants in the composition or heat treatment in order to optimize the process. In this work, the material was hot rolled, austenitized and subsequently annealed. All heat treatments have been performed during dilatometry tests. In order to investigate the microstructural evolution during the process, specimens were extracted at definite stages of the treatment. The specimens were then investigated applying various microscopical techniques in order to quantify the microstructural features (grain size, martensite lath width and precipitate data). The experimental data were then compared to thermodynamic simulations (MatCalc). General data such as nucleation sites for precipitates were taken from literature, grain size and martensite lath widths from the experimental data. Simulations include equilibrium calculations and precipitate kinetic simulations. In general, the simulations showed good agreement with the experimental findings, with minor room for improvements. The work thus lays a solid ground for future improvements of the heat treatment process.

Predicting Microstructure From Thermal History During Additive Manufacturing for Ti-6Al-4V

Due to the repeated thermal cycling that occurs with the processing of each subsequent layer, the microstructure of additively manufactured parts undergoes complex changes throughout the deposition process. Understanding and modeling this evolution poses a greater challenge than for single-cycle heat treatments. Following the work of Kelly and Charles, a Ti-6Al-4V microstructural model has been developed which calculates the phase fractions, morphology, and alpha lath width given a measured or modeled thermal history. Dissolution of the alpha phase is modeled as 1D plate growth of the beta phase, while alpha growth is modeled by the technique of Johnson–Mehl–Avrami (JMA). The alpha phase is divided into colony and basketweave morphologies based on an intragranular nucleation temperature. Evolution of alpha lath width is calculated using an Arrhenius equation. Key parameters of the combined Kelly–Charles model developed here are optimized using the Nelder–Mead simplex algorithm. For the deposition of two L-shaped geometries with different processing parameters, the optimized model gives a mean error over 24 different locations of 37% relative to experimentally measured lath widths, compared to 106% for the original Kelly–Charles model.

Effect of Lath Width and Dislocation Density on Resistance to Irradiation of Warmly Deformed SCRAM Steel

The martensitic lath width (0.83 ± 0.45μm ∼ 0.48 ± 0.14 μm) and dislocation density (1.3 ± 0.3 × 1015 m−2 ∼ 6.4 ± 1.6 ×1015 m−2) change of Super-clean Reduced Activation Martensitic (SCRAM) steel caused by warm deformation on Gleeble-3500 thermo-simulation machine have been examined. The irradiation-induced helium bubbles and hardening were observed in all the specimens after helium implantation to 1e + 17/cm2 at 723 K. The helium bubbles became smaller and more numerous while the distribution was more homogeneous when the lath width decrease and dislocation density increase. The nano-indentation hardness indicated that the sample, the martensitic lath width is 0.83 ± 0.45μm and the dislocation density is 1.3 ± 0.3 × 1015 m−2, exhibited the maximum nano-indentation variation (ΔH) and the ΔH decreased with the lath width decreasing and dislocation density increasing. The hardening occurred in all helium implanted samples can mainly be ascribed to helium bubbles.

Effect of Annealing Temperature on Microstructures and Properties of Warmly Deformed SCRAM Steel

Effect of annealing temperature on microstructures and properties of warmly deformed SCRAM (Super-clean Reduced Activation Martensitic) steel on Gleeble-3500 thermo-simulation machine was investigated. The results showed that an increase in the annealing temperature can result in increasing the martensitic lath width from 0.48 um to 0.65 um and decreasing the dislocation density from 6.4×1015m-2to 2.8×1015m-2in SCRAM steel. The specimen exhibited high reduction of area and total elongations when the annealing temperature is up to 600 oC. The tensile fracture surface observation indicated that dimples became more uniform and deeper and cleavage fracture traces disappeared with the annealing temperature increasing. The irradiation-induced helium bubbles and hardening were observed in all the specimens after helium implantation to 1e + 17/cm2at 450 oC. The helium bubbles became larger but less when the annealing temperature increased. The optimal annealing temperature is 450 oC in this experiment.