Tsanka Dikova

Laser Surface Melting of Austenitic Cr-Ni Stainless Steel

The present paper deals with the microstructure and hardness distribution in width and in depth of the surface layer of steel Ch18N10T GOST (AISI 321, EN X6CrNiTi 18-10) after surface melting by continuous wave CO2 laser. Light microscopy, XRD analysis and Vickers hardness testing (HV5 and HV0,05) have been used in our research. Phase analysis shows disturbance of the mono-phase initial austenitic structure in the treated layer. The structure of the melted pool consists of austenite with dendrite morphology and δ-ferrite situated in the dendrites’ cores. The ferrite has been clearly identified by XRD analysis. As a result from fast heating and cooling, ferrite, obtained by diffusionless sliding mechanism, was observed along the austenite grains’ boundaries in the heat affected zone. The presence of small inclusions of supposed Ti carbide, non-identified by XRD analysis, was also observed. The durometric investigations show that the surface hardness in the melted zone is in the range 180-210 HV5 while that of the basic metal is about 270 HV5.

Mechanism of TiO2 Nanotubes Formation on the Surface of Pure Ti and Ti-6Al-4V Alloy

The present paper deals with the investigation of the mechanisms of TiO2 nanotubes formation on titanium surfaces during anodization process. The samples were made of pure Ti Grade-2 and Ti-6Al-4V alloy. They were grinded, etched with 0,5 wt. % HF acid and anodized. The anodization was done in electrolyte containing 0,5 wt. % HF acid using DC power supply with graphite electrode as cathode. The samples were investigated by SEM, EDAX and XRD analysis. The results show two different mechanisms of formation of TiO2 nanotubes on the surfaces of both materials. During the anodization process the oxide formations, obtained on the pure Ti surface after etching, are oxidized to nanorods; the area between them is also oxidized and connects them. This thin oxide layer grows in the metal depth while the nanorods are dissolved thus forming the porous sponge-like structure which is further transformed in tubular. While on the surface of Ti-6Al-4V alloy oxide nanonuclei originate which transform their shape from nanoseed to bowl-like with clearly pronounced bottom and walls, growing in tubular structures. The type of the material defines the surface morphology after etching. Thus obtained morphology influences on the processes running rate in different micro-regions determining origination of the titanium nanotubes on different stage as well as by different mechanism. The field-enhanced oxidation and field-enhanced dissolution are the main processes for formation of TiO2 nanotubes during anodization. In the regions with prevalent oxidation processes the TiO2 nanotubes are formed earlier while in the regions with dominant dissolution processes the TiO2 nanotubes are formed on the later stage.

Microstructure of surface layer of T1 and D2 steels after laser melting

Abstract The aim of this study was to investigate the in-depth structure transformations of the laser-melted layers of the steels mostly used in tool production – T1 and D2 steels. The experiments of laser surface melting of T1 and D2 steels were done with continuous wave CO2 laser (wavelength λ = 10.6 μm). The sample’s microstructure was investigated, and the EDX analysis was made by SEM JEOL JMS-35C. Our results show that the microstructural changes of the laser-melted layer of T1 steel were determined by the process of directed crystallisation from the bottom layer to the surface. There is considerable inhomogeneity, followed by the predominantly ongoing process of chemical liquation during crystallisation. Cellular structure, consisting high-carbon martensite and significant amount of retained austenite, prevails in the melted pool bottom. Austenite–carbide eutectic with possible presence of lamellar martensite is situated along the dendrites’ boundaries (in inter-dendrite spaces). The austenite–carbide quasieutecticum with carbide-type M6C possesses ‘skeletal’ morphology in these regions. In the laser melting of D2 steel, the phase transformations in the transition zone (liquid–solid state) were defined by running of reverse eutectic reaction at the places of overheated and partially melted carbide phases. During cooling of these regions, the inhomogeneous melt goes through changes that involve precipitation of oversaturated with carbon and chrome austenite and disperse microquasieutectic with carbides of the M7C3 type.

Factors Influencing on the Dimensions Accuracy in Laser Cutting

This research aims to establish the influence of different factors on the dimensions accuracy in laser cutting. Samples with different thickness were made of DC01 EN 10130 steel. On each sample 32 round or square holes were cut in 4 rows by different laser beam movement schemes linear, zig-zag and random. The tests were made by pulse CO2 laser with regimes, recommended for each sheet thickness, by the machine operating manual. The diameters of the round holes and the dimensions of the square holes along the laser head and the table axes were measured by a projector. The varying intervals of the dimensions and the shape deviations were calculated. It was established that three groups of factors influence on the dimensions accuracy in laser cutting: factors, related to the machine, laser parameters power, laser cutting speed and focus distance, determining the technological process parameters, and the material characteristics thermal physical properties and sample sizes. Factors, related to the machine, exert effect mainly on the intervals of dimensions varying, while the laser parameters and the material properties influence on the dimensions value independently of the cutting scheme. In the all laser cutting schemes the increasing of the sample thickness leads to the larger interval of the dimensions varying and increasing of the shape deviation. Nomenclature

Combined Plasma‐Arc Treatment And Surface Plastic Deformation Of Armco‐Fe

Present paper deals with the structure transformations and hardness changes of the surface layer of Armco‐Fe after combined Plasma‐Arc Treatment (PAT) and Surface Plastic Deformation (SPD). The SPD was realized by rotational‐progressive movement of spherical contra‐body. Optical microscopy, TEM and durometric investigations were used. The processes of structure hardening in the melted zone of surface layer after PAT were analyzed. The formation of high‐carbon martensite near former cementite inclusions as well incoherent secondary phase precipitations in heat affected zone were established. The hardness in the melted zone increases up to 320–350 HV while in the heat affected zone it smoothly decreases to 120 HV. The following SPD leads to deformation hardening of the samples’ surface layers. The investigations show that this is due to the phase cold hardening in precrystallization processes, incoherent secondary phase precipitation along the boundaries as well the higher dislocation density. After SPD, fo...

Method, Technology and Equipment for in Depth Surface Hardening

The present work deals with a new method, technology and equipment developed for “in depth surface hardening” of large rotary details and tools, made of constructional, carbon, alloyed or low-alloyed tool steels. According to the technical requirements the details and tools have to possess hardened layer with specific shape, depth in the range 20-35mm and surface hardness of 35-62 HRC. The “in-depth hardening” was performed by quenching and self-tempering. Preliminary volume heating and depth regulated local surface adjustable gas-flame heating were used. The gasflame heating was realized by gas-flame burners (CH4+O2) located near the details’ periphery, depending on the shape and depth of the hardened layer. The burners’ power and heating time were adjustable. The cooling was performed by showers located near the details’ periphery also depending on the shape of the hardened layer. Water or polymer water solution was used as cooling media and its capacity was adjusted in the beginning. Due to the heat accumulated during the preliminary heating and the heating up to the quenching temperature of the surface layer, after stopping the cooling, subsequent self-tempering was applied. All of the equipment for realizing the whole hardening cycle, temperature and time control and parameters of heating and cooling was developed. Series of different details were hardened: traversing wheels and gears, forming rolls, rolling disks etc., made of construction and tool steels. The hardened layers obtained were with specific shape, surface hardness in the range of 35-60HRC and depth of the hardened layer of 15- 25mm.

Structure and Properties of High-Chromium-Alloyed Upper-Eutectoid Steels after Hardening with Concentrated Energy Fluxes

The present paper deals with the structure and properties of two types of tool steels with high chromium content (12% Cr) hardened by means of Concentrated Energy Fluxes (CEF). The treatment conditions are chosen to ensure liquid state transformations. The melted zone of the surface layer features a quasi-ledeburite structure, consisting of inhomogeneous residual austenite and Cr-containing carbides of MmCn type. This austenite is strongly cold hardened and over-saturated with Cr and carbon. The micro-hardness of this region varies from 400 up to 800 HV0.1. The higher the energy density, the lower the hardness and the wider the modified layers. The lower hardness is due to the presence of nearly 100% austenite. Higher hardness was obtained in the heat affected zone. Carbides of M23C6, M7C3, M6C, M3C and МС types were identified. A scheme of carbide changes after treatment with CEF is given.

Structure Features of Martensite and Residual Austenite during Treatment with Concentrated Energy Fluxes

The paper deals with the structure features of Fe-C alloys quenched by means of laser, electron beam and plasma arc. The martensite and residual austenite obtained are highly inhomogeneous. Their morphology and distribution depend both on the initial state before quenching and on the kinetics of the temperature changes. Four different structures of martensite are observed – package, lamellar isothermal, lamellar thermo-kinetic and “feathery nest-like”. The new martensite structure observed, called by us “feathery nest-like”, is a result of explosive austenite-martensite transformation in pearlitic irons. It differs from the classic modification in its specific morphology. Low-carbon package martensite occupies the regions of the former ferrite grains. Its hardness reaches 1050-1150 HV0.1. In the regions of microstructure with increased carbon concentration lamellar martensite is observed. The residual austenite is with different proportion in relation to the martensite. In particular regions its quantity could reach 100%. It is characterized by a high quantity of imperfections and high mechanical properties. Its hardness reaches 450-500 HV0.1. The higher the power density and the lower the energy density of the concentrated energy flux, the higher the residual austenite quantity.