Karakterizacija željeznih lijevova ultrazvučnim ispitivanjem

This paper presents an overview of possibility of the cast iron characterization using ultrasonic testing that could eliminate most of the disadvantages of the metallographic method, but its usage is limited by the fact that velocity and attenuation of ultrasonic wave depend on technological singularities of cast iron production. Characterization of microstructural features and mechanical properties of cast iron is based on the ultrasonic testing parameters, primarily on ultrasonic wave velocity, which depends on the elastic modulus and density of the cast iron.Ovaj rad prikazuje pregled mogućnosti karakterizacije željeznih lijevova ultrazvučnim ispitivanjem koje bi moglo otkloniti većinu nedostataka metalografske metode, no njegovo korištenje ograničava činjenica da brzina i prigušenje ultrazvučnih valova ovise o tehnološkim posebnostima proizvodnje željeznih lijevova. Karakterizacija mikrostrukture i mehaničkih svojstava željeznih lijevova temelji se na parametrima ultrazvučnog ispitivanja, prije svega na brzini ultrazvučnih valova, koja ovisi o modulu elastičnosti te o gustoći željeznog lijeva

Corrosion investigations of Al-Si casting alloys in 0.6 M NaCl solution

This paper presents results of the corrosion investigations of specimens made from finished parts for the automotive industry, produced by high-pressure die casting and gravity die casting process of six Al-Si alloys (40000 series). Open circuit potential and potentiodynamic polarization measurements have been performed using a potentiostat with three-electrode set-up in 0.6 M NaCl naturally aerated solution. Microstructural characterization before and after electrochemical investigations has been carried out with optical microscope to establish the connection between microstructure and corrosion parameters of investigated alloys and to analyze and record surface changes of each sample due to electrochemical corrosion. All alloys show good corrosion resistance, which manifests with low values of corrosion rates, calculated from the corrosion current densities obtained from potentiodynamic polarization measurements. Differences in electrochemical behavior appear due to the distinctions in their chemical composition and microstructure. The type of casting process does not affect electrochemical behavior of Al-Si alloys

Prediction of Microstructure Constituents’ Hardness after the Isothermal Decomposition of Austenite

An increase in technical requirements related to the prediction of mechanical properties of steel engineering components requires a deep understanding of relations which exist between microstructure, chemical composition and mechanical properties. This paper is dedicated to the research of the relation between steel hardness with the microstructure, chemical composition and temperature of isothermal decomposition of austenite. When setting the equations for predicting the hardness of microstructure constituents, it was assumed that: (1) The pearlite hardness depends on the carbon content in a steel and on the undercooling below the critical temperature, (2) the martensite hardness depends primarily on its carbon content, (3) the hardness of bainite can be between that of untempered martensite and pearlite in the same steel. The equations for estimation of microstructure constituents’ hardness after the isothermal decomposition of austenite have been proposed. By the comparison of predicted hardness using a mathematical model with experimental results, it can be concluded that hardness of considered low-alloy steels could be successfully predicted by the proposed model

Mathematical modeling and computer simulation of microstructure transformations and mechanical properties during steel quenching : doctoral thesis

Cilj ove doktorske disertacije bio je istraživanje mehanizama i kinetike mikrostrukturnih pretvorbi te mehaničkih svojstava pri gašenju čelika, a u svrhu što točnijeg definiranja fizikalnih pojava pri gašenju čelika. Sve veći zahtjevi u svezi kvalitete mehaničkih svojstava strojnih dijelova ukazuju na nužnost dobrog poznavanja povezanosti mikrostrukture, kemijskog sastava te mehaničkih svojstava. Iako je jednostavan za izvođenje, proces gašenja čelika spada u jedan od fizikalno najkompleksnijih inženjerskih postupaka jer pri gašenju čelika nastaje više procesa koji se međusobno isprepliću: fizikalni procesi mikrostrukturnih pretvorbi, procesi izmjene, prijelaza i provođenja topline, procesi stvaranja deformacija i zaostalih naprezanja te procesi formiranja i rasta pukotina. Na temelju provedenih teorijskih istraživanja, u radu su predložene metode određivanja kinetičkih parametara izotermičkog raspada austenita u ferit, perlit te bainit te izrazi za predviđanje kinetike raspada austenita. Nadalje, predloženi su izrazi za predviđanje vrijednosti termodinamičkih konstanti raspada austenita u ferit, perlit i bainit na temelju kemijskog sastava podeutektoridnih čelika. Također, predloženi su izrazi za procjenu tvrdoće mikrostrukturnih sastojaka čelika: ferita, perlita, binita i martenzita. Vlastiti algoritam razvijen u svrhu predviđanja kinetike raspada austenita i tvrdoće mikrostrukturnih sastojaka čelika implementiran je u računalni program za 3‐D simulaciju ohlađivanja uzoraka, čime je omogućena 3‐D simulacija raspada austenita pri gašenju čelika. Za provjeru rezultata računalne simulacije mikrostrukturnih pretvorbi te tvrdoće pri gašenju čelika korišten je nisko‐legirani čelik za poboljšanje: 42CrMo4 (DIN). Rezultati računalne simulacije ukazuju na to da se razvijeni matematički modeli mikrostrukturnih pretvorbi te tvrdoće mogu uspješno koristiti pri predviđanju rezultata raspada austenita za vrijeme gašenja čelika.The scope of this doctoral thesis has been the investigation of mechanisms and kinetics of microstructure transformation as well as the study of mechanical properties, with the objective of a more accurate defining of physical phenomena during steel quenching. Increasing technical requirements, relating to the quality of mechanical properties of the engineering components, imply a deep understanding of relations among microstructure, chemical composition and mechanical properties. Although the process of steel quenching is simple to apply, it is one of the physically most complicated engineering processes, which involves many interacting processes: physical processes of microstructure transformation, processes of heat exchange, transfer and heat conduction, processes of generation of deformation and residual stresses, and processes of crack formation and its growth. Based on theoretical investigations, methods for determination of kinetics parameters of isothermal austenite decomposition into ferrite, pearlite and bainite have been proposed, as well as equations of austenite decomposition kinetics. Furthermore, on the basis of the chemical composition of hypoeutectioid steels, equations for the estimation of thermodynamic constants of austenite decomposition into ferrite, pearlite and bainite have been put forward. Equations for the evaluation of the microstructure constituents` hardness have been also presented. A proper algorithm developed in order to predict the austenite decomposition kinetics as well as the microstructure constituents` hardness has been implemented in the 3‐D computer program for a 3‐D simulation of the specimen`s cooling, whereby the 3‐D simulation of the austenite decomposition during steel quenching is enabled. Low‐alloy steel for tempering 42CrMo4 (DIN) has been applied for the verification of results obtained by the computer simulation. The results of the computer simulation show that developed mathematical models of microstructure transformations and hardness can be efficiently used for the prediction of austenite decomposition during steel quenching

Mathematical modeling and computer simulation of microstructure transformations and mechanical properties during steel quenching : doctoral thesis

Cilj ove doktorske disertacije bio je istraživanje mehanizama i kinetike mikrostrukturnih pretvorbi te mehaničkih svojstava pri gašenju čelika, a u svrhu što točnijeg definiranja fizikalnih pojava pri gašenju čelika. Sve veći zahtjevi u svezi kvalitete mehaničkih svojstava strojnih dijelova ukazuju na nužnost dobrog poznavanja povezanosti mikrostrukture, kemijskog sastava te mehaničkih svojstava. Iako je jednostavan za izvođenje, proces gašenja čelika spada u jedan od fizikalno najkompleksnijih inženjerskih postupaka jer pri gašenju čelika nastaje više procesa koji se međusobno isprepliću: fizikalni procesi mikrostrukturnih pretvorbi, procesi izmjene, prijelaza i provođenja topline, procesi stvaranja deformacija i zaostalih naprezanja te procesi formiranja i rasta pukotina. Na temelju provedenih teorijskih istraživanja, u radu su predložene metode određivanja kinetičkih parametara izotermičkog raspada austenita u ferit, perlit te bainit te izrazi za predviđanje kinetike raspada austenita. Nadalje, predloženi su izrazi za predviđanje vrijednosti termodinamičkih konstanti raspada austenita u ferit, perlit i bainit na temelju kemijskog sastava podeutektoridnih čelika. Također, predloženi su izrazi za procjenu tvrdoće mikrostrukturnih sastojaka čelika: ferita, perlita, binita i martenzita. Vlastiti algoritam razvijen u svrhu predviđanja kinetike raspada austenita i tvrdoće mikrostrukturnih sastojaka čelika implementiran je u računalni program za 3‐D simulaciju ohlađivanja uzoraka, čime je omogućena 3‐D simulacija raspada austenita pri gašenju čelika. Za provjeru rezultata računalne simulacije mikrostrukturnih pretvorbi te tvrdoće pri gašenju čelika korišten je nisko‐legirani čelik za poboljšanje: 42CrMo4 (DIN). Rezultati računalne simulacije ukazuju na to da se razvijeni matematički modeli mikrostrukturnih pretvorbi te tvrdoće mogu uspješno koristiti pri predviđanju rezultata raspada austenita za vrijeme gašenja čelika.The scope of this doctoral thesis has been the investigation of mechanisms and kinetics of microstructure transformation as well as the study of mechanical properties, with the objective of a more accurate defining of physical phenomena during steel quenching. Increasing technical requirements, relating to the quality of mechanical properties of the engineering components, imply a deep understanding of relations among microstructure, chemical composition and mechanical properties. Although the process of steel quenching is simple to apply, it is one of the physically most complicated engineering processes, which involves many interacting processes: physical processes of microstructure transformation, processes of heat exchange, transfer and heat conduction, processes of generation of deformation and residual stresses, and processes of crack formation and its growth. Based on theoretical investigations, methods for determination of kinetics parameters of isothermal austenite decomposition into ferrite, pearlite and bainite have been proposed, as well as equations of austenite decomposition kinetics. Furthermore, on the basis of the chemical composition of hypoeutectioid steels, equations for the estimation of thermodynamic constants of austenite decomposition into ferrite, pearlite and bainite have been put forward. Equations for the evaluation of the microstructure constituents` hardness have been also presented. A proper algorithm developed in order to predict the austenite decomposition kinetics as well as the microstructure constituents` hardness has been implemented in the 3‐D computer program for a 3‐D simulation of the specimen`s cooling, whereby the 3‐D simulation of the austenite decomposition during steel quenching is enabled. Low‐alloy steel for tempering 42CrMo4 (DIN) has been applied for the verification of results obtained by the computer simulation. The results of the computer simulation show that developed mathematical models of microstructure transformations and hardness can be efficiently used for the prediction of austenite decomposition during steel quenching

Artificial Neural Networks-Based Prediction of Hardness of Low-Alloy Steels Using Specific Jominy Distance

Successful prediction of the relevant mechanical properties of steels is of great importance to materials engineering. The aim of this research is to investigate the possibility of reducing the complexity of artificial neural networks-based prediction of total hardness of hypoeutectoid, low-alloy steels based on chemical composition, by introducing the specific Jominy distance as a new input variable. For prediction of total hardness after continuous cooling of steel (output variable), ANNs were developed for different combinations of inputs. Input variables for the first configuration of ANNs were the main alloying elements (C, Si, Mn, Cr, Mo, Ni), the austenitizing temperature, the austenitizing time, and the cooling time to 500 °C, while in the second configuration alloying elements were substituted by the specific Jominy distance. Comparing the results of total hardness prediction, it can be seen that the ANN using the specific Jominy distance as input variable (runseen = 0.873, RMSEunseen = 67, MAPE = 14.8%) is almost as successful as ANN using main alloying elements (runseen = 0.940, RMSEunseen = 46, MAPE = 10.7%). The research results indicate that the prediction of total hardness of steel can be successfully performed only based on four input variables: the austenitizing temperature, the austenitizing time, the cooling time to 500 °C, and the specific Jominy distance