Božo Smoljan

Main features of designing with brittle materials

Brittle material behavior and mode of failure are contrasted with those characteristics of ductile materials. The stochastic nature of brittle fracture, which results from the random occurrence of fracture-initiating microstructural imperfections, necessitates a probabilistic fracture mechanics approach to design with brittle materials. It is also clearly shown which main properties of brittle materials have to be optimized to improve the reliability of mechanically loaded components made of brittle materials. Important features of designing with brittle materials are elucidated and illustrated by an exemplary design calculation of a ceramic disc spring. It is shown how even environmentally induced subcritical crack growth, characteristic of ceramic materials, can be adequately accounted for in the assessment of reliability.

An analysis of application of modified Jominy-test in simulation of cold work tool steels quenching

The possibility of application of modified Jominy-test in computer simulation of quenching of cold work tool steels has been investigated. Because of high hardenability of cold work tool steels there are limits in application of original Jominy-specimen in simulation of quenching of steels. The modified Jominy-test was designed for prediction of hardenability of cold work tool steels. The characteristic cooling time, relevant for results of quenching, was predicted by computer simulation of quenching of both of JM ® -specimen and of cylindrical specimen. Modified Jominy-test can be applied in simulation of quenching of high hardenability steel more successfully than by original Jominy-test.

An Analysis of Relationships Between Behavior and Microstructure Constitution of Hot-Work Tool Steel

Influence of microstructure on mechanical behavior of hot-work steels is analyzed on submicro, micro, and macro scale. Except for suitable microstructure, phase distribution, and microstructure refinement, proper alloy chemistry and melting practice are important for hot-work steel properties. Methods of determination of working stress of hot-work steels have been presented. In optimization and selection of hot-work steels, proper failure criteria are based on predicted specific failure micromechanism of tools and dies in application. Failure micromechanisms have to be modeled and numerically defined in terms of time and temperature. Hot-work steel development has to be based on simultaneous optimization of materials properties and tool performance. In modern optimization and selection of hot-work steels, proper failure criteria have to be recognized based on probable failure micromechanisms in tool or die applications.

Mathematical modelling and computer simulation of mechanical properties of quenched and tempered steel

A mathematical model and method of computer simulation for the prediction of mechanical properties of quenched and tempered steel were developed. Numerical modelling of hardness distribution in as-quenched steel components was performed based on experimental results of Jominy test. Modified Jominy test was designed for hardenability prediction of high-hardenability steels. Hardness of quenched and tempered steel was expressed as a function of maximal hardness of actual steel and representatives of chemical diffusivity of steel according to the time and temperature of tempering. Yield strength and fracture toughness distributions were estimated using the Hahn−Rosenfield approach. Fatigue resistance was estimated based on predicted microstructure and hardness. Distribution of other relevant mechanical properties was found out based on predicted as-quenched and tempered hardness of steel. Experimental investigation was performed on high-hardenability steel for tools and dies. The established procedure was applied in computer simulation of mechanical properties of a quenched and tempered steel workpiece.

Mathematical Modeling and Computer Simulation of Fatigue Properties of Quenched and Tempered Steel

The engineering and economical aspects of the optimization of steel shaft quenching and tempering were investigated. The mathematical model and method of computer simulation for the prediction of fatigue properties of quenched and tempered steel were developed. Computer simulation of the fatigue properties of quenched and tempered steel was applied in the optimization of quenching and tempering of steel shafts. The proper heat treatment process was accepted based on economical analysis. Fatigue properties of quenched and tempered steel were predicted based on microstructure composition and yield strength. Microstructure composition and yield strength were predicted based on as-quenched hardness. The distribution of as-quenched hardness in the workpiece was predicted through computer simulation of steel quenching using a finite volume method. The as-quenched hardness was estimated based on time of cooling and on Jominy test results. It was taken into account that the mechanical properties of quenched steel directly depend on the hardness, degree of hardening, and microstructural constituents. Using a numerical simulation of microstructure and mechanical properties, it was found out that the investigated shaft has the best fatigue properties in cases when the shaft was machined or formed on proper geometry before proper quenching and tempering. Through economical analysis of the investigated shaft manufacturing, it was found out that the most suitable shaft manufacture process is to manufacture the shaft from the quenched and tempered bar. But in this case, heterogeneous microstructures of ferrite, perlite, bainite, and martensite with very low fatigue limits could appear at some critical locations.

Computer Simulation of Quenched Steel Working Stress

The possibility of the application of a modified Jominy-test in the computer simulation of quenching of steels for tools and dies has been investigated. The performance of an investigated modified Jominy-test in the simulation of quenching of tools and dies was estimated by a comparison of the cooling curves of a modified Jominy-specimen (JMr ; -specimen) and a cylindrical specimen. The hardness distribution in a quenched specimen was estimated based on a relevant cooling time from 800 to 500°C, as well as on the results of a modified Jominy-test. The characteristic cooling time, relevant for the results of quenching, was predicted by the computer simulation of quenching of both the JMr ; -specimen and cylindrical specimen. Mechanical properties of quenched steel directly depend on the degree of quenched steel hardening. The algorithm of the estimation of yield strength and fracture toughness on the base of steel hardness is established in this paper. Using the established algorithm, the mechanical properties of quenched and tempered die steel were estimated by computer simulation. It was found that the modified Jominy-test can be applied in the simulation of quenching of high hardenability steel more successfully than by the original Jominy-test.

Evaluation of Steel Hardenability by JM® -test

The modified Jominy-test was designed for prediction of hardenability of high-hardenability tool steels and possibility of application of modified Jominy-test in computer simulation of quenching of high-hardenability tool steels has been investigated. Because of high hardenability there are limits in application of original Jominy-specimen in simulation of quenching of steels. The performance of investigated modified Jominy-test in simulation of quenching of high-hardenability tool steels was estimated by comparison of cooling curves of modified Jominy-specimen (JM®-specimen) and cylindrical specimen. The influence of dimension of JM®-specimen on cooling curves has been investigated. The time of cooling, t8/5 relevant for results of quenching was predicted. Modified Jominy-test can be applied in simulation of quenching of steel with higher hardenability rather than original Jominy-test.

An analysis of induction hardening of ferritic ductile iron

Achievements of the induction hardening of ferritic ductile iron were investigated. Ductile iron is not advisable for use in induction hardening because of the small carbon content in the metal matrix of ferritic ductile iron. The carbon content in the metal matrix of ductile iron can be increased by additional preparation of metal matrix before final induction heat hardening. Wear resistance of the induction hardened ferritic ductile iron can increase as result of increased carbon content of the metal matrix and higher hardness after induction hardening. Some heat pretreatments for metal matrix preparation were applied before the induction hardening of ferritic ductile iron. The process parameters of the induction hardening heat pretreatment were analyzed and optimized. According to recommended elemental composition of ferritic ductile iron and required mechanical properties, the process parameters of the investigated induction heat pretreatment were optimized. The efficiency of pretreatment processes of induction hardening was analyzed. Applicability and manufacture ability of engineering components by proposed heat pretreatments were investigated. The limitations of the investigated heat pretreatment applications were estimated by the comparison of mechanical properties of heat-treated specimens.

Measuring quality related costs

The cost aspect of heat treatment quality management has been presented. Quality management cannot be successful without quality cost management. The activity of heat treatment quality system as cost centers have been defined. The heat treatment quality costs have been classified in categories.

Mathematical Modeling and Computer Simulation of Steel Quenching

The purpose of this research is to upgrade the mathematical modeling and computer simulation of steel quenching. Based on theoretical analyses of physical processes that exist in quenching systems, the mathematical model for steel quenching is established and computer software is developed. The mathematical model of steel quenching is focused on physical phenomena, such as heat transfer, phase transformations, mechanical properties, and generation of stresses and distortions. The numerical procedure of computer simulation of steel quenching is divided into three parts: numerical calculation of transient temperature field, numerical calculation of phase change, and numerical calculation of the mechanical behaviors of steel during quenching. The numerical procedure is based on the finite volume method. Physical properties that were included in the model, such as heat conductivity coefficient, heat capacity, and surface heat transfer coefficient, were obtained by the inversion method based on the Jominy test results. By the completed algorithm, 3-D situation problems, such as the quenching of complex cylinders, cones, spheres, etc., can be simulated. The established model of steel quenching can be successfully applied in the practical usage of quenching.