Defect and Diffusion Forum Vol. 403

Abstract: This paper explores the thermochemical treatment of cemented carbides (CC), specifically the boriding process. Six different types of CCs with different size of tungsten carbide (TC) grains were chosen as experimental materials. They contained binders of different chemical compositions. In five CCs, the binders were pure metals: cobalt (four of them) and nickel (one of them). In the six one, the binder was a complex Ni-Co-Cr-based alloy. Samples of the different types of CCs were prepared by grinding and polishing and then half of them underwent boriding process. The experiment aimed to find how boriding affects the final properties of CCs and their structure. Microstructural changes in the materials were examined using X-ray diffraction and optical and electron microscopy. Changes in mechanical properties and wear resistance were evaluated using hardness testing and the Ball on Disk test. The experimental results, for example, shown that CC with nickel binder had lowest wear resistance from all tested sorts of CC.

Abstract: During the processing of Ti6Al4V alloy by a 3D printing method SLM (selective laser melting), high internal stresses are developed in the material as a result of high temperature gradients between microvolumes of the powder melted at a given time and the already solidified material. Cooling rates thus reach up to the order of 10⁸ °C·s^-1. At such rates, a diffusionless martensitic transformation occurs, which also contributes to internal stresses in the material. High internal stresses can be a problem already during production; they can manifest themselves by cracking of products, deformations of thin parts, etc. Even in defectless products, internal stresses negatively affect their properties; in particular, they reduce ductility. Therefore, it is desirable to include a heat treatment after the 3D printing, which would reduce the stresses and transform the metastable martensitic structure into a stable one. As a result of the heat treatment, the ductility increases at the expense of strength. The subject of this paper is to find such heat treatment regime that provides the best combination of mechanical properties.

Abstract: Despite the fact that the sub-zero treatment has widely been employed in various industrial branches more than 100 years, metallurgical principles of microstructural alterations was discovered only over past several years. Many experimental works have been done in order to describe what happens in the microstructures of various steels when they are subjected to the treatment within a temperature range 4 - 273 K and for different durations of this treatment. The obtained results infer that the changes in microstructure and thus in mechanical properties depend not only on the treatment regime used, but they are closely related to the steel chemistry. The current paper summarizes the findings obtained by sub-zero treatments of two different ledeburitic tool steels (AISI D2 and Vanadis 6). The changes in retained austenite characteristics, alterations in martensitic structures, variations in carbide characteristics and modifications in precipitation behaviour are demonstrated and discussed.

Abstract: Ti-Al-Si alloys are materials for high-temperature applications. They are characterized by low density, good mechanical properties and excellent resistance against oxidation in comparison with other commonly used alloys, for example nickel alloys or stainless steels. The preparation of Ti-Al-Si is very problematic due to high melting points of the intermediary phases, the high reactivity of melt with the melting crucibles and with the atmosphere in the furnace or formation of the cracks and pores during the process. Powder metallurgy seems to be a promising method for preparation of Ti-Al-Si alloys but there are still many complications. In this work, Ti-Al-Si alloys were prepared by unconventional powder metallurgy techniques and the aim of this work was to describe the problems during the sintering of these materials and their solution.

Abstract: Multicomponent boron-based diffusion layers are capable to provide a wider variety of surface improvements compared to pure boriding. In this research, we consider a way to increase mechanical properties of carbon steels by using two-component thermal-chemical treatment (TCT) such as boroaluminizing (B+Al), and boronickelizing (B+Ni). Diffusion treatment of steel surface was carried out by pack method in powder mixtures, and pastes, containing the above-mentioned elements and sodium fluoride as an activator. The exposure time was 3 hours, the treatment temperature was 950 °C. Pure boriding was conducted additionally to compare with two-component methods. The metallographic analysis revealed diffusion layers with a tooth-like structure after boriding and B+Ni. Typical composition of boride layer with iron boride FeB as an outer phase and Fe₂B as an inner one was obtained after the first method. EDS analysis revealed a small amount of Ni (less than 1%) in boronickelized layer. Although XRD analysis revealed Ni₂B, Fe₃Ni₃B besides iron borides and carboborides after B+Ni. Another structure was obtained after B+Al – namely a layered microstructure with outer softer iron aluminides and iron borides beneath. The thickest layer was obtained after boriding with the thickness of 140-160 μm, where higher value corresponds to low-carbon steel and vice versa. While for boroaluminizing the layer thickness was around 120-140 μm and for boronickelizing - 60-100 μm. Microhardness profiles differ significantly depending on the TCT method. For instance, initial high values followed by a drastic drop of hardness for boriding and boronickelizing. Wave type profiles characterize the microhardness distribution on boroaluminized samples. Wear tests indicated that samples with boride layer were the most wear resistant and the least resistant were the samples after B+Al. Fe-Me-B layers with the tooth-like structure are superior to the ones with the layered structure.

Abstract: Iron aluminides have been considered as materials resistant against high temperature oxidation in air and sulphur-containing environments. Previous research of our team proved that the oxidation resistance in the air can be significantly improved by the addition of silicon. Fe-Al-Si alloys have also very good mechanical properties at high temperatures. However, the resistance in the environments simulating combustion gasses have not been studied yet. This work focuses on the oxidation resistance in carbon dioxide, which is the main component of the combustion gasses. It was found that the Fe-Al-Si, Fe-Al-Si-Ni and Fe-Al-Si-Ti alloys have lower oxidation resistance in carbon dioxide containing atmosphere than in the air due to carbon diffusion to the material and even to the formation of carbides below the oxide layer. It leads to the spallation of the oxide layer, especially in FeAl20Si20Ni20 alloy.

Abstract: Maraging steels are interesting for research after heat treatment, from which name is derived "maraging" – martensite-aging. After solution annealing and precipitation hardening the X3NiMoCoTi 18-9-5 alloy has excellent mechanical properties (tensile strength reaches up to 2000 MPa and hardness is 50-55 HRC), it is ductile and well weldable. The advantage of these materials is the possibility to be manufactured not only by conventional methods but also by modern additive manufacturing (AM) methods. One of which is selective laser melting (SLM). In this paper, the influence of heat treatment on the final microstructure and mechanical properties of the 3D-printed X3NiMoCoTi 18-9-5 maraging steel is investigated.

Abstract: The present paper explores the effects of deep cryogenic treatment (DCT) on the properties of WC-Co cemented carbides. The investigation involved four different cemented carbide (CC) grades. Two of them were coarse-grained WC with grain sizes larger than 6 μm and binder fractions of 10 and 15 wt. %. The other two were fine-grained with WC grains of 0.5-0.8 μm and the same binder fractions of 10 and 15 wt. %. Their specimens were ground and polished to prepare them for DCT. In each specimen, one half of this polished surface was used for testing the properties of the CC before cryogenic treatment. The post-DCT properties were then determined on the other half. Properties of the cemented carbides prior to and after DCT were studied using optical and scanning electron microscopy, X-ray diffraction, hardness testing according to Vickers scale followed by calculation of fracture toughness K_IC and a ball-on-disk test of the wear resistance of the surface. One of the findings was that cryogenic treatment led to a decrease in residual stresses and to lower fracture toughness K_IC in the CC.

Abstract: The paper summarizes the findings of a study of properties and behavior of laser hardened surfaces. The influence of hardening on the service life and tribological resistance of molds and other machine parts will be presented. Furthermore, the influence of process parameters on residual stresses, their direction and magnitude, and related deformation, fatigue behavior under cyclic loading, etc. are described.