Carbide Precipitation in a Low Alloyed Steel during Aging Studied by Atom Probe Tomography and Thermodynamic Modeling

Carbide precipitation in martensitic low alloyed steels contributes to the mechanical properties through precipitation hardening. A high number density of carbides is desired to maximize the hardening effect, which is achieved through the precipitation of carbides on the dislocations in the martensitic structure. In this study, the nucleation, growth, and coarsening of vanadium and molybdenum carbides during aging at 600 °C for periods up to four weeks were investigated. The work covers characterization with atom probe tomography, which showed that the nucleation of V and Mo rich MC/M2C carbides takes place on dislocations. The growth of these carbides proceeds by the diffusion of elements to the dislocations, which has been modeled using Dictra software, confirming the rate of the reaction as well as the depletion of carbide formers in the matrix. For longer aging times, particle coarsening will decrease the number density of particles with a transition from dislocation-based carbides to separate rounded carbides.

Studies on the Mechanical Properties of C45 Steel with Martensitic Structure after a High Tempering Process

This article describes the studies of the mechanical properties of the martensitic structure of C45 steel, obtained as a result of heat treatment by a corresponding cooling method. This steel was subjected to high tempering, within the temperature range of 500÷700o C (every 50o C) and for various exposure times, from 15 minutes to 23 hours. Moreover, martensitic steel was subjected to non-standard tests by quenching at a temperature of 850 oC for 20 minutes and then tempering it for 1 hour, within the temperature range of 50÷800o C (50o C). The resulting steel structure variants were subjected to strength tests, that is, to tensile and hardness tests and also to tests on the micro-structure. The results of these studies are presented and discussed, in detail, in terms of their practical application.

Intelligent Materials and their Direction of Use

The paper presents the manufacturing process of two classes of material that feature intelligent precursors, used for welding/cladding, and deposit layers with viable physical proprieties, that can be predetermined by a precise and well-balanced programming of the process parameters. The fields of use for these materials are those of manufacturing welded structures that have equal resistant joints, made from stainless steel with austenitic-ferrite-martensitic structure or mixes of the above mentioned structures, with the possibility of machining, in welded state, namely with self-hardening proprieties during exploitation. The methods of securing the above mentioned materials is programming, according to exploitation requirements, of the preheating and in between rows temperature, as well as the dilution degree of the filler into the base material. Results obtained by structural or morphological changes of the crystalline grains are determined by perturbing the equilibrium state and/or precipitation of secondary phases. The field of use of the methods above mentioned are valid in the interval: 30-55 HRC, namely 500-1200 N/mm2.

Spinning Process of D406A Ultra-High Strength Steel

D406A steel is a medium-carbon low-alloy steel, which has excellent comprehensive mechanical properties. It is widely used in the production of missiles and rocket barrels. In this paper, the spinning forming limit test and the intermediate heat treatment process of ultra-high-strength steel were used to explore the effect of spinning process and heat treatment on the properties of spinning parts. The research results showed that the reduction amount of the material made the material thinning rate approach the limit thinning rate. The final blank wall thickness was reduced from 15 mm to 3.0 mm when the cracking occurred. It was calculated that the material's power spinning limit thinning rate was 80%. The ferrite matrix after spinning showed a streamline distribution characteristic perpendicular to the thinning direction, and the precipitated carbides were uniformly distributed on the surface of the matrix, which had the characteristics of deformation and extension along the streamline. After the heat treatment, the structure of the spinning parts changed continuously. When the structure was quenched and tempered, the martensitic structure can be obtained, and the tempered martensitic structure was smaller. Furthermore a test piece for ultra-high-strength steel spinning technology has been developed, and the solutions discussed for flanging defects in the actual spinning process, and test data for the actual production of ultra-high-strength steel spinning parts accumulated.

Changes of Structure and Physical-Mechanical Properties in Alloy Steels Thermochemically Treated by Plasma Nitriding

Plasma nitriding is a method of surface modification using a glow discharge technology to introduce nitrogen into the surface of a metal, which subsequently diffuses into the material. The main advantages of plasma nitriding over conventional nitriding processes are: reduced cycle time, controlled growth of the surface layer, elimination of white layer, reduced distortion, no need of finishing, pore-free surfaces and mechanical masks instead of copper plating. The process is especially suitable for complex parts that are intensively solicited by wear, fatigue, contact pressure, shocks, possibly also to corrosion. It is applied in order to bring the metallic products in a state favourable from the point of view of structure, chemical composition and internal stress state. The paper presents an analysis of the structures and characteristics of a widely used structural steel 39 - CrAl6. As a result of plasma nitriding, a surface layer with high wear and fatigue resistance was created on the surface of the material. Also, between the surface layer and the base material was interposed a hard substrate with high wear resistance having a bainitic / martensitic structure.

INFLUENCE OF ELECTROLYTIC-PLASMA SURFACE HARDENING ON THE SURFACE PROPERTIES OF BANDAGE STEEL

This work is devoted to the research of the influence of the technological parameters of electrolytic-plasma surface hardening on the structure and tribological properties of the surface of samples of the retaining steel Mark 2. Electrolytic-plasma surface hardening was carried out in an electrolyte from an aqueous solution of 10% urea and 10% sodium carbonate. According to the result of metallographic and X-ray diffraction analysis, it was determined that the phase composition of steel Mark 2 after processing varies, and fine martensite with a small amount of troostite and iron oxide is formed on the surface of the samples. Tribological experiments of samples without lubrication were carried out. These experiments have shown that all the samples studied have an increased wear resistance, which may be associated with the formation of a fine-grained martensitic structure. It was shown that from the point of view of the complex of the properties obtained, and the most promising is electrolytic-plasma action with a treatment time of 4 s.

Robotic Spot Welding of DOCOL 1200M Steel

The paper presents robotic spot resistance welding technologies for DOCOL 1200M steel with a thickness of 1.8 mm. DOCOL 1200M steel with a martensitic structure is intended mainly for the production of car bumpers, side beams and other elements ensuring the safety of the user of motor vehicles. The test joints were made on a robotic station equipped with a KUKA KR180 robot and a welding from ARO. The obtained welded joints were subjected to macro and microscopic metallographic tests, hardness measurement and strength tests. It has been shown that for properly selected resistance spot welding, DOCOL 1200M steel joints with a satisfactory strength level can be obtained.

Effect of Microstructure on Hydrogen Permeation in EA4T and 30CrNiMoV12 Railway Axle Steels

A comparative study was conducted to reveal the effect of microstructure on hydrogen permeation in the EA4T and 30CrNiMoV12 railway axle steels. Unlike the EA4T with its sorbite structure, 30CrNiMoV12 steel shows a typical tempered martensitic structure, in which a large number of fine, short, rod-like, and spherical carbides are uniformly dispersed at boundaries and inside laths. More importantly, this structure possesses plentifully strong hydrogen traps, such as nanosized Cr7C3, Mo2C, VC, and V4C3, thus resulting in a high density of trapping sites (N = 1.17 × 1022 cm−3). The hydrogen permeation experiments further demonstrated that, compared to EA4T, the 30CrNiMoV12 steel not only delivered minimally effective hydrogen diffusivity but also had a high hydrogen concentration. The activation energy for hydrogen diffusion of the 30CrNiMoV12 steel was greatly increased from 23.27 ± 1.94 of EA4T to 47.82 ± 2.14 kJ mol−1.