Microstructure and Strengthening Mechanisms in an HSLA Steel Subjected to Tempforming

An effect of tempforming on the microstructure, the carbide precipitation, and the strengthening mechanisms of high-strength low-alloyed steel has been analyzed. The quenched steel was subjected to 1 h tempering at a temperature of 873 K, 923 K, or 973 K followed by plate rolling at the same temperature. Tempforming resulted in the formation of an ultrafine grained lamellar-type microstructure with finely dispersed carbides of (Nb,V)C, Fe3C and Cr23C6. A decrease in tempforming temperature resulted in a reduction of the transverse grain size from 950 nm to 350 nm. Correspondingly, the size of Fe3C/Cr23C6 particles decreased from 90 nm to 40 nm while the size of (Nb,V)C particles decreased from 17 nm to 4 nm. Refining the tempformed microstructure with a decrease in thetempforming temperature provided an increase in the yield strength from 690 MPa to 1230 MPa.

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.

A study on use of wobble features in laser welding of low alloy steel joint with butt joint configuration

Laser welding is modern digital welding process, which thanks to several advantages over traditional welding processes, is gaining ever growing role in manufacturing. The process has still some weaknesses. The better the beam quality the smaller the focal point, the actual welding tool, diameter is. Typically, because of this the welding of joints with lesser quality e.g. larger air gap is difficult or even impossible. So-called beam manipulation opens opportunities to deal with the problem. The dynamic beam manipulation gives opportunities to control the weld dimensions during the welding process by the requirements of individual locations of weld joint. This study used the two dimensional scanner to manipulate beam during welding with so called wobble function. Four different wobble configurations were tested in welding of low-alloyed steel with different joint qualities. The wobble typically made the welds wider, provided typically higher heat input and thus lowered the hardness of the joint. Wobble increased typically the root quality, but there are differences between different wobble parameters. It was possible to weld joints with wider air gaps in the selected material thickness, but the wider air gap and wobble caused finally, when wide enough the sagging of the joint.

Microstructure and Mechanical Properties of the 55CrMoV4 Steel Exposed to Boriding and Nitriding Treatments

In this study boriding and nitriding treatments were carried out on 55CrMoV4 low alloyed steel. The thermochemical treatments were carried out in solid medium by the powder technique at 900℃ for 4 hours for boriding treatment and at a temperature of 550℃ for 12 hours for nitriding treatment. The phases analysis of the boride and nitrite layers formed on the surface was carried out by optical microscopy (OM), and X-ray diffraction (XRD). The results of the surface analysis show that the boride and nitride layers a presence of FeB, Fe2B, CrN, Fe3N and Fe4N compounds. The thickness of boride layers and nitride layers was found to be 55 and 12 µm, respectively. Microhardness of boride and nitride layers are between 800 HV0.2 and 1200 HV0.2. Corrosion tests by immersion in a 1M HCl solution have shown the beneficial effect of boriding and nitriding treatments on treated steels. Increase in corrosion resistances was observed after nitriding and boriding treatment steel 55CrMoV4 was around 6 times.

Sulfidation of Ceramic-Based Coatings Deposited on Low-Alloyed Steel 16Mo3 Exposed at High Temperature

This study was carried out to evaluate a series of ceramic coatings in a sulfidation atmosphere containing 99% synthetic air and 1% H2S (vol.%) under a flow rate of 50 mL/min. The study was carried out at 773.15 K for 336 hours (heating rate 278.15 K/min). Chemically resistant glass enamels based on SiO2-B2O3-TiO2-Na2O compounds were deposited on the surface of 16Mo3 (16M) low-alloyed steel. Kinetic data were recorded periodically every 168 hours; macro- and microanalyses using scanning electron microscopy (SEM), phase analyses using x-ray diffractometry (XRD) and chemical composition using energy-dispersive x-ray spectrometry along with EDS x-ray mappings were carried out to observe the degradation process of the sulfidized coatings. The results indicated that some of the ceramic coatings exposed in the harsh atmosphere at 773.15 K for 336 hours showed a high degree of protection.