Resistance to appearance and propagation of cracks in hard-faced layers of hot work tool steels

Uspešno projektovanje alata u savremenoj industriji zahteva detaljno poznavanje karakteristika materijala i procesa u kojima će dati alat (ili konstrukcija) biti eksploatisani. Kada je reč o alatima za kovanje u toplom stanju, s obzirom na njihove radne uslove i stalno prisutna udarna opterećenja i visoke temeprature, podrazumeva se da se oni izrađuju od čelika koje odlikuje visoka jačina i tvrdoća, koje su postojane i na povišenim temperaturama. Pored toga, veoma važna karakteristika jeste i otpornost materijala na pojavu i rast prslina koje dovode do otkaza alata. To dalje vodi ka zastoju u proizvodnji, povećanju troškova i vremena izrade proizvoda, padu produktivnosti i dr. Međutim, budući da se alati neminovno oštećuju, kada do toga dođe, postavlja se pitanje da li alat zameniti novim ili reparirati postojeći? Današnja težnja industrije je u velikoj meri zasnovana i na samoodrživosti što podrazumeva težnju ka sposobnosti kompanije da problemi koji nastanu budu rešeni u okviru postojećih kapaciteta. S obzirom na to, reparatura alata zavarivanjem ili navarivanjem se ističe kao jedan od najefikasnijih načina. Naravno, podrazumeva se da navarivanje nije idealan proces, jer se njegovom primenom umnogome utiče na svojstva pojedinih zona materijala koji se regeneriše, ali niz prednosti tog procesa ga nameće kao nezaobilaznog u ovoj grani industrije. Zbog toga je neophodno što bolje poznavati proces i ponašanje materijala pri navarivanju i biti u mogućnosti da se predvide karakteristike materijala u zoni navarivanja, tj. otpornost materijala u radnim uslovima alata i, ako je to moguće, predvideti radni vek regenerisanog alata. Cilj ove doktorske disertacije je da, na jedan sistematičan način, analizom niza karakteristika materijala dobijenih opsežnim eksperimentalnim ispitivanjem, ukaže na uspešnost primene navarivanja za reparaturu oštećenih alata izrađenih od termopostojanih čelika. U radu su analizirana dva čelika koja se najčešće koriste za izradu alata za toplo kovanje i shodno njima, dva dodatna materijala. Radom je obuhvaćeno i propisivanje tehnologije navarivanja ploča određene debljine iz kojih su pripremani uzorci za različita ispitivanja. Ona su obuhvatila ispitivanje na zatezanje na sobnim i povišenim temperaturama, ispitivanje tvrdoće i određivanje mikrostrukture različitih zona navara, ispitivanje udarne žilavosti i određivanje trajne dinamičke izdržljivosti za osnovne materijale. Kao najvažnija ispitivanja, čiji parametri se mogu koristiti u proceni radnog veka navarenog alata, izdvajaju se ispitivanja rasta zamorne prsline (da/dN) u različitim zonama navarenih ploča. Ta ispitivanja su praćena optičkom i SEM metalografijom koje su poslužile za potvrdu zaključaka donetih na osnovu eksperimentalno dobijenih rezultata. Na kraju je izvedena i numerička analiza odgovarajućih modela u softverskom paketu ANSYS a dobijeni rezultati su upoređivani sa eksperimentalnim. Predložena tehnologija navarivanja, kao i koraci sprovedeni pri ispitivanju i na kraju dobijeni rezultati, mogu veoma korisno da posluže firmama koje se bave kovanjem i u svojim pogonima imaju alate izrađene od ovih i sličnih čelika.Successful design of tools in contemporary industry requires detailed knowledge of materials' characteristics and processes in which the given tool (or a structure) would be exploited. When the subject matter are forging tools for hot working, taking into account their operating conditions - constantly present impact loading and high temperatures, one assumes that they are being manufactured from steels characterized by high strength and hardness, which remain stable at elevated temperatures, as well. Besides that, their very important characteristics should be resistance to appearance and growth of cracks that might lead to tool failure, which would then lead to production down-times, an increase in costs and time of manufacturing, productivity decay etc. However, considering that tools are ineviatably being damaged during the exploitation, the question arises whether to replace the damaged tool by the new one or to repair the existing. Nowadays, aspiration in the industry is largely based on self-sustainability, which means striving for the company's ability to solve the emerging problems within existing capacity. Considering that, the tool reparation by welding or hard-facing becomes prominent as one of the most attractive ways for solving those problems. Hard-facing, of course, is not an ideal process, since its application strongly affects properties of certain zones of the material being repaired, but a whole series of this process' advantages are imposing it as unavoidable in the tool manufacturing industry. Due to those reasons, one must know, as best as possible, the hard/facing process, material behaviour during it and has to be able to predict the material properties in the hard-faced zone, i.e. material strength in the tool operating conditions and, if possible, to predict the repaired tool's service life. The objective of this doctoral dissertation is to show, in a systematic way, by analyzing a number of material characteristics, obtained through extensive experimental testing, the success of application of hard-facing for repair of the damaged tools made of thermally stable steels. Two types of steels were analyzed, which are used for hot forging and, accordingly, the two filler metals. That also included prescribing the hard-facing technology of plates of a certain thickness, from which the samples for experiments were prepared. Investigations included the tensile test at room and elevated temperatures, hardness testing and determination of microstructure of different zones of the hard-faced layers, impact toughness tests, as well as determination of the permanent dynamic durability of the two base metals. As the most important tests, the parameters of which can be used for estimates of the hard-faced tool's service life, were recognized as the fatigue crack growth investigation (dа/dN) in different zones of the hard-faced layers. Those tests were accompanied by the optical and SEM metallography, which served for verification of experimentally obtained results. In the end, the numerical investigation was performed of the corresponding models in the software package ANSYS and obtained results were compared to the experimental ones. The proposed hard-facing technology, the steps that were taken during the investigation, as well as obtained results, can be of great use to the forging companies that have, in their plants, tools made of these or similar steels

Analysis of the cause of the girth gear tooth fracture occurrence at the bucket wheel excavator

Premature damages and fractures of components and structures of bucket-wheel excavators at open-pit mine often occur in exploitation, caused either by inadequate design or insufficient knowledge of the material properties, welded joints and flaws in component production technology. The bucket-wheel excavator, TAKRAF SRs 2000 x 32/5.0 was employed on the excavation of barren soil for 5.000 h (a few weeks more than a year after the assembly) when the fracture of the tooth of the girth gear, which enables the circular motion of the upper structure of the bucket-wheel excavator, occurred. The gear was, according to the manufacturer's certificate, made of the cast steel GS 40 MnCrSi3 V. The paper presents calculations of the stress variations cycles' number for one tooth, as well as of the fracture mechanics parameters - the critical stress intensity factor and critical crack length. It was established that the fracture of the tooth occurred due to an initial crack existing in its base, which originated during the gear's manufacturing, i.e. due to the so-called "manufacturing-in defect"

Prediction of service life of components and structures of hydro power plants during the design, prototyping and service period

During the rehabilitation of turbine and hydromechanical equipment at hydro power plant 'Djerdap 1' results of tests and researches carried out in order to analyze the condition of vital components and structures during service led to the conclusion that the components and structures could be made only if parameters of fracture mechanics were applied during the design and prototyping phase, because by doing so the occurrence of fatigue fracture and/or degradation of parent material and welded joints due to variable loading, corrosion, erosion and cavitation would be prevented, therefore the integrity of material would be maintained. Experimental determination of the fatigue crack propagation rate as an important property of development of the fatigue process during the action of the variable load, from the initial to the critical length, enables the prediction of the stable crack propagation period, or in other words of the service life of vital components and structures of turbine and hydromechanical equipment. Taking into account the fact that cracks are most severe of all defects, the results obtained for cracks could safely be applied for other types of defects which often occur in welded joints. This paper contains the methodological approaches for the assessment of integrity and evaluation of service life of components and structures of turbine and hydromechanical equipment during the design, prototyping and service period based on history of fatigue loading and application of fracture mechanics parameters

Hydro-meteorological data quality assurance and improvement

Advances in measurement equipment and data transfer enabled easy and economic automatic monitoring of various hydro-meteorological variables. The main characteristic of such automatic monitoring systems is that they do not rely on human activities, but only on electronic devices. Even if those electronic devices are of highest quality and accuracy, and properly tuned to specific problem, the reliability of measured values relyieson many other factors and unexpected or undesired occurrences, like modification of measurement micro location, power supply shortages or surges, etc. The sampled and acquired data values have to be additionally checked, validated and sometimes improved or cleared before further use. This paper presents an innovative approach to data validation and improvement through the framework generally applicable to all hydrological data acquisition systems. The proposed framework can incorporate any number of validation methods and can be easily customized according to the characteristics of every single measured variable. The framework allows for the self-adjustment and feedback to support self-learning of used validation methods, same as expert-controlled learning and supervision. After data validation, for low-scored data, its value quality can be improved if redundant data exist, so framework has the data reconstruction module. By applying different interpolation techniques or using redundant data value the new data is created same as accompanying metadata with the reconstruction history. After data reconstruction, the framework supports the data adjustment, the post-processing phase where the data is adjusted for the specific needs of each user. Every validated and sometimes improved data value is accompanied with a meta-data that holds its validation grade as a quality indicator for further use.

DEM-based GIS algorithms for automatic creation of hydrological models data

In this paper the authors discuss the Digital Elevation Model (DEM) based GIS algorithms, applicable for automatic creation of input data needed by hydrological models. The expected source of DEM data are standard digital data sets that a modeler can purchase from the official data sources. The DEM has to be cleared from errors and prepared for standard hydrology usage as the depression less version, or a version of DEM where only small depressions are filled-in, while larger depressions are extracted as separate objects. The slopes and aspects are calculated from such DEM. The flow accumulation image is created using slopes and aspects and using information about large ponds. From flow accumulation image the surface drainage network is extracted with user-specified density. Based on slopes and aspects, the catchment is delineated into subcatchments, the areas drained by each river segment. Finally, the model-specific parameters like length of flow segment along the terrain and along the river to the subcatchment outlet, slope of those segments, subcatchment shape factor, or subcatchments mean or weighted slope are calculated. Paper explains the used algorithms and emphasizes the problems one can encounter during the usage of DEM data using examples from River Drina catchment. Finally, the authors comment on overall usability of presented GIS algorithms, especially if purchased data are of low quality.

Working life estimate of the tubular T-joint by application of the LEFM concept

© 2018 The Authors. The crack growth in tubular joints usually occurs along the weld's toe. That is the point where the chord and brace intersect. The semi-elliptical crack appears in this area from the initial flaw that was created during the welding. Sensitivity to fatigue depends on combination of cyclic loading, initial defects, environmental influences and the hot spot stresses, which are result of the walls' bending during the loading of a structure. The principles of the linear elastic fracture mechanics (LEFM) are applied here to crack growth in the tubular T-joint, subjected to axial load, in-plane and anti-plane bending. Influences of the level and type of loading, as well as of the joint's geometrical characteristics, on the fatigue crack propagation and consequently on the working life of the welded joint, are considered. Based on the conducted analysis, which implies a set of assumptions, one can draw sufficiently relevant conclusion on the remaining working life of the tubular T-joint. The assumptions included: the crack shape is semi-elliptical, there is only one crack propagating through the tube wall, the cyclic plastic zone at the crack tip is small with respect to other geometrical variables and the crack grows only if the difference between the stress intensity factor values at maximal and minimal loads is greater than the stress intensity factor necessary for the fatigue crack growth initiation. Results are presented in the form of diagrams from which can be seen that for the same load level the longer working life is achieved for the axial load of the joint than for the in-plane bending, while the values for the anti-plane bending lie between these two limiting results

The contact and compacting pressures influences on the quality of the friction welded joint

The theoretical and experimental analyses of the friction welding pressure influence on the plastic deformation level and the quality of the friction welded joint are presented in this paper. The joint of the tempering and the High-Speed steel was realized by the friction welding. The objective was to relate the two basic process parameters - the friction and compacting pressures - to plastic deformation parameters during the friction welding of two the steels. The fact that materials are dissimilar additionally complicates the welding procedure and its analysis. The friction welding is a specific and complex process, since in the joint zone material is heated and plasticized with necessary action of the multi-step pressure to realize the joint. The total deformations in the axial and radial directions are directly dependent on the applied welding pressure. Considering that geometry and shape of the friction welded joint directly depend on the friction pressure, some welded joints' basic shapes obtained with various pressures are presented. The experimental investigation was conducted on cylindrical samples made of the two steels and the analysis of results served for establishing the influences of the friction and compacting pressures on changes of the steel samples dimensions and shapes.

The influence of heat input on the toughness and fracture mechanism of surface weld metal

© 2018 The Authors. Surface welding is a way to extend the exploitation life of damaged parts and constructions and the heat input has a major influence on the weldment properties. In this paper is shown the influence of the heat input on the toughness and the fracture mechanism of the surface welded joint. Surface welding of high carbon steel with self shielded wire was conducted with three different heat inputs (6kJ/cm, 10 kJ/cm and 16 kJ/cm). Total impact energy, crack initiation and crack propagation energy were estimated at room temperature, -20 o C and -40 o C. Fracture analysis of fractured surfaces was also conducted and it has been found that increasing of heat input leads to an increase of share of transgranular brittle fracture, what is in complete accordance with the obtained energy values. Based on all obtained results, the optimum value of heat input for welding procedure applied was defined

The contact and compacting pressures influences on the quality of the friction welded joint

The theoretical and experimental analyses of the friction welding pressure influence on the plastic deformation level and the quality of the friction welded joint are presented in this paper. The joint of the tempering and the High-Speed steel was realized by the friction welding. The objective was to relate the two basic process parameters - the friction and compacting pressures - to plastic deformation parameters during the friction welding of two the steels. The fact that materials are dissimilar additionally complicates the welding procedure and its analysis. The friction welding is a specific and complex process, since in the joint zone material is heated and plasticized with necessary action of the multi-step pressure to realize the joint. The total deformations in the axial and radial directions are directly dependent on the applied welding pressure. Considering that geometry and shape of the friction welded joint directly depend on the friction pressure, some welded joints' basic shapes obtained with various pressures are presented. The experimental investigation was conducted on cylindrical samples made of the two steels and the analysis of results served for establishing the influences of the friction and compacting pressures on changes of the steel samples dimensions and shapes.

The effect of heat input on the fracture behaviour of surface weld metal of rail steel

Surface welding of rail steel with self-shielded wire was conducted with different heat inputs, and the influence of welding heat input on the total impact energy and its components, crack growth rate and fracture mechanism was systematically investigated. It is shown that toughness decreases as heat input increases, but, with a temperature decrease, these differences are not so marked. An increase of heat input leads to increasing the share of transgranular brittle fracture, what is in complete accordance with the obtained energy values. It is established that fatigue life increases when increasing the welding heat input, while resistance to crack growth decreases in the final deposit layer up to the HAZ at all heat inputs. Based on obtained results, the optimal value of heat input is defined for the selected welding procedure