Analysis of Microstructure and Mechanical Properties in As-Built/As-Cast and Heat-Treated Conditions for IN718 Alloy Obtained by Selective Laser Melting and Investment Casting Processes

In this work, new customized heat treatments for selective laser melted (SLM) parts in IN718 alloy were analyzed. This was done through the evaluation of the mechanical properties and advanced characterization of the phases and microstructure obtained in as-built condition and after the application of standard and tailored heat treatments. The microstructure and mechanical properties were compared and discussed with results reported in the literature. Finally, strengthening mechanisms of IN718 alloy processed by SLM and its differences with mechanisms that occur in investment casting were analyzed. Both processes generate quite different microstructures, investment casting is composed mainly by a dendritic structure, and SLM is characterized by columnar and cellular structures with very thin cells. Due to the fine and homogeneous microstructure obtained from SLM processing and its specific strengthening mechanisms, it is not necessary to apply homogenization and solution stages as in standard heat treatment used for this type of alloy in casting or wrought. The pre-heating and process parameters selected, in combination with a direct stepped aging (at 720 °C/620 °C), provide the material with its best mechanical properties, which are superior to those obtained by standard heat treatment (AMS 5383F) applied to investment casting of IN718 alloy.

Improving the wear resistance of steel products by using non-standard heat treatment modes. Foundryproductionandmetallurgy

The use of non-standard modes of heat treatment increases the density of dislocations in the crystal structure of the α-phase and increases the wear resistance of carbon, low-alloy steels under various friction conditions, which is comparable to the results when heated to a standard temperature (Ac3 + 30–50 °C). The preliminary extreme heating temperature is determined. After requenching at standard temperature and low tempering, the wear resistance of steels under various types of friction increases by up to 40 % compared to standard quenching.

Non-standard modes of heat treatment and their influence on the wear resistance of steel products

The wear resistance of low-alloy carbon steels after non-standard heat treatment, which increases the density of dislocations in the crystal structure of the a-phase, under different friction conditions is investigated. Keywords: wear resistance, heat treatment, hardness, carbon and low-alloy steels, dislocation density. [email protected]

Effects of the γ″-Ni3Nb Phase on Fatigue Behavior of Nickel-Based 718 Superalloys with Different Heat Treatments

The effects of the γ″-Ni3Nb phase on fatigue behavior of nickel-based 718 superalloys with standard heat treatment, hot isostatic pressing + solution treatment + aging, and hot isostatic pressing + direct aging were investigated by scanning electron microscope, transmission electron microscopy, and fatigue experiments. The standard heat treatment, hot isostatic pressing + solution treatment + aging, and hot isostatic pressing + direct aging resulted in the formation of more and smaller γ″ phases in the matrix in the nickel-based 718 superalloys. However, the grain boundaries of the hot isostatic pressing + direct aging sample showed many relatively coarse disk-like γ″ phases with major axes of ~80 nm and minor axes of ~40 nm. The hot isostatic pressing + direct aging sample with a stress amplitude of 380 MPa showed the longest high cycle fatigue life of 5.16 × 105 cycles. Laves phases and carbide inclusions were observed in the crack initiation zone, and the cracks propagated along the acicular δ phases in the nickel-based 718 superalloys. The precipitation of fine γ″ phases in the matrix and relatively coarse γ″ phases in the grain boundaries of the hot isostatic pressing + direct aging sample can hinder the movement of dislocation.

Microarc surface alloying of tool steels

A new method of accelerating of steel diffusion saturation during thermo-chemical treatment – microarc surface alloying is proposed. The steel product is placed in a metal container filled with coal powder, and heated by passing an electric current. Powder having microarcs, which are concentrated around the surface of the product with the formation of zones of local gas discharge. This significantly speeds up the diffusion saturation. After carburizing of steel 20Cr13 is formed a diffusion layer thickness of 13-15 μm microhardness of 10.5 to 12.5 HPa, located below the eutectoid area a thickness of 300 μm and a hardness of 6.5 GPA. After boriding of steel Cr12V1 after standard heat treatment, the surface layer with a thickness of 170-180 μm consists of a base with microhardness of 9.3-9.6 GPA with inclusions of microhardness of 14.5-15.0 GPA. After boriding of steel 5CrNiMo formed a layer thickness of 250-260 μm with a composite structure consisting of sections of the eutectoid structure of microhardness of 7.0-8.0 GPA, surrounded by boride eutectic microhardness 12.0-12.5 GPA. The composite structure provides the combination of very hard boride eutectic and eutectoid plastic mixture. After boriding of steel W6Mo5Co5 after standard heat treatment, the surface layer thickness of 230-240 μm consists of a base of microhardness 11,0-11,2 GPA with inclusions of microhardness 13,5-14,5 GPA. Given the high intensity of diffusion saturation, microarc surface alloying is recommended for surface hardening of tool steels.

Effects of HIP Process on the Microstructure and Mechanical Properties of a Cast Superalloy

The effects of HIP process on microstructure and mechanical properties of IN792 cast superalloy were studied. The results showed that HIP process produced more uniform and finer cubic γ′ than standard heat treatment. The difference of the mechanical properties should be caused by the microstructure changes. HIP process leads the homogeneous distribution of γ′ at dendritic arm and interdendritic area, and improved UTS and YS of tested alloy at 550°C. However, it played no role in increasing UTS and YS at room temperature and stress-rupture lives of 760°C/662MPa and decreased stress-rupture lives of 982°C/186MPa.

Microstructure Evolution and Stress Rupture Properties of DD6 Single Crystal Superalloy after Overheating

The second generation single crystal superalloy DD6 after standard heat treatment was respectively overheated at 1100°C, 1150°C, 1200°C, 1250°C, 1300°C, 1320°C for 1h and air cooled. The effect of overheating on the microstructure and stress rupture properties at 980°C/250MPa of the alloy was investigated. The results showed that the size of γ′ phase was slightly increased overheating at 1100°C, 1150°C and 1200°C. The size of γ′ phase had a big increase and its size distribution was very uneven after overheating at 1250°C. The small part of γ′ phase has serrated γ′/γ phase surface as a result of un-completely solution and the irregular small γ′ phase was in the majority when overheated at 1300°C. While all the irregular small γ′ phase precipitated again after completely solution when overheated at 1320°C. There was no fine second γ′ phase in the γ matrix channel of the alloy after standard heat treatment and overheating at 1320°C. But the fine second γ′ phase precipitated in the γ matrix channel after overheating at every temperature of 1100 °C~1300°C. No obvious change of the stress rupture life was found after overheating at 1100°C, 1150°C, 1200°C and 1250°C. The stress rupture life considerably reduced after overheating at 1300°C, whereas slightly reduced after overheating at 1320°C. The appearance of the raft had almost no change after overheating at 1100°C. With increasing of overheating temperature from 1100°C to 1250°C, the length of raft became shorter and the width thickening. The γ phase formed the wavy raft after overheating at 1300°C and 1320°C and the thickness of latter was larger than that of the former. Finally, the relationship between the microstructural evolution and stress rupture properties of the alloy after overheating was discussed.

Influence of Heat Treatment Temperature on the Microstructures and High Cycle Fatigue Properties of DD6 Single Crystal Superalloy

The effect of heat treatment temperature on the microstructure and the high cycle fatigue property of DD6 single crystal superalloy was investigated. After standard heat treatment, the alloy was hold at 1200°C and 1300°C for 1h respectively, and then air-cooled. The results showed that after heat treatment at 1200°C for 1h, the size of γ′ particles became slightly larger and quite unevenly, at the same time, the serrated γ/γ′ interface appeared. After heat treatment at 1300°C for 1h, the small irregular γ′ particles re-precipitated. In addition, a small number of fine secondary γ′ particles were also observed in the matrix channel of the alloy after primary aging of 1200°C/1h and 1300°C/1h. The high cycle fatigue properties of the alloy by standard heat treatment were higher than that of the alloy by 1200°C/1h and 1300°C/1h, but the decreased degree of 1200°C/1h was much bigger than that of 1300°C/1h. Analysis on fracture surfaces of the alloy at 800°C demonstrated a quasi-cleavage mode.