J. Krawczyk

Fracture toughness of steels with nickel content in respect of carbide morphology

Abstract This paper is focused on the influence of Ni addition on the microstructure and fracture toughness of structural steels after tempering. Nickel is known to increase the resistance to cleavage fracture of steel and decrease a ductile–brittle transition temperature. The medium carbon, low alloy martensitic steels attain the best combination of properties in low tempered condition, with tempered martensite, retained austenite and transition carbides in the microstructure. In the present research, four model alloys of different Ni contents (from 0·35 to 4·00%) were used. All samples were in as quenched and tempered condition. Quenching was performed in oil at room temperature. After quenching, samples were tempered at 200°C for 2 h. An increase in nickel content in the investigated model structural steels causes a decrease in ε carbide volume fraction in their microstructure. Cementite nucleates independently in the boundaries of martensite laths and in the twin boundaries in the areas where the ε carbide has been dissolved. It was stated that stress intensity factor KIc significantly decreases in the case of the presence of dispersive elongated cementite precipitations at the boundaries of the prior austenite grains.

Influence of the Ageing Temperature on the Selected Mechanical Properties of the Ti6Al7Nb Alloy

The results of investigations of the influence of the ageing temperature on the selected mechanical properties i.e. hardness, fracture toughness (examined by the linear elastic fracture mechanics - KIc test) and impact strength (KV) of two-phase Ti6Al7Nb alloy, are presented in the hereby paper. Investigations were performed in the ageing temperatures range: 450÷650°C of the alloy previously undercooled from the selected heating temperature (in two-phase range) - equal 970°C. The heating temperature was determined on the basis of the dilatometric curve of the alloy heating in the system ΔL = f ((T), where: ΔL – change of the sample length, T – temperature, which was then differentiated in the system: ΔL/ΔT = f (T). The dilatometer L78 R.I.T.A of the LINSEIS Company was used in the tests. Investigations of the alloy microstructure in the ageing temperatures range 450÷650°C were carried out by means of the light microscope Axiovert 200 MAT of the Carl Zeiss Company. It was found that nearly equiaxial grains of the primary α phase occur in the microstructure (of the volume fraction app. 30%) and that the volume fraction of the new lamellar α phase - formed from the supersaturated β phase - increases. With an increase of the alloy ageing temperature, in the mentioned above range, a small increase of its hardness from 305 to 324HV as well as a decrease of stress intensity factor KIc from 67.3 to 48.6 MPa x m1/2 and impact strength (KV) from 40.2 to 31.3 J. The impact tests results were supplemented by the fractographic documentation. It was found, that the characteristic features of the fractures of impact test samples do not exhibit essential differences in dependence of the ageing temperature and material hardness. The fractographic investigations were performed by means of the scanning electron microscope NovaNanoSEM 450.

The Role of Microstructure in High Temperature Tribology of Iron Alloys

The present paper describe the issue of tool materials wear in a high temperature conditions. The investigations were performed at the cast steel tool material at the tribological contact to the structural steel. The investigations aim was to determine the role of microstructure in a tribological properties between the structural steel and tool material. The results of such investigation could be referenced to the industry conditions and could answer about the problems of tool materials wear. The observations of the wear mechanisms were referred to the microstructure of the mill rolls. The laboratory tests ware aimed at evaluating the thermal treatment modification effect on the cast steels properties. A significant role of the morphology of ledeburitic cementite and secondary cementite on the tribological properties was exhibited. The investigations assumed the presence of an austenitic matrix with primary and secondary cementite. Influence of varying morphology carbides was described. in the cast steel microstructure. The investigation results make possible to point to a direction of carbide morphology change with the purpose of obtaining the assumed properties of hot operation tools.

T-Shape Connector Hydroforming Process Analysis

The work is focused on hydroforming of T-shape connector for high temperature applications. A seamless part for use in industrial applications was formed in a high pressure liquid extrusion process. Due to the occurrence of faults in the final products, numerical simulations were conducted to reveal the possible sources of such failures. The numerical simulation took into account precisely determined boundary conditions allowing proper selection of processing parameters. The microstructure of charge material as well as that of the final product was also examined. Numerical simulations of the investigated extrusion process showed the possibility of obtaining good quality product, however, the quality of final part is strongly influenced by properly designed heat treatment schedule.

Dynamic Recrystallization in Titanium Alloys

The results of the investigations on five different titanium alloys were presented in this paper. Two two-phase α+β alloys (Ti-6Al-4V and Ti-6Al-2Sn-4Zr-6Mo) as well as two β alloys (Ti-10V-2Fe-3Al and Ti-3Al-8V-6Cr-4Mo-4Zr) were studied. Moreover, Ti-48Al-2Cr-2Nb intermetallic alloy (γ alloy) was also investigated. All investigated alloys were tested in compression on Gleeble thermomechanical simulator under various strain rate and temperature conditions. Metallographic observations of the microstructure of tested samples allowed determining thermomechanical conditions under which dynamic recrystallization or recovery processes occurred in the investigated alloys. The obtained results also showed the importance of the influence of strong exothermic effect resulting from deformation process on recovery and recrystallization processes in these alloys. The methodology of distinguishing subgrains formed during recovery process from the grains resulting from recrystallization was also presented in this study.

Influence of the Ageing Temperature on the Microstructure and Selected Mechanical Properties of Ti13Nb13Zr Alloy

Investigation results concerning changes occurring in the microstructure of the near β titanium alloy, grade: Ti13Nb13Zr, when the ageing temperature was changed within the range 350÷650°C, are presented in the paper. These changes were assessed on samples previously quenched in water from the selected soaking temperature in the single-phase β range (800°C). Microstructures were investigated by means of the optical microscopy and scanning electron microscopy (SEM). Phases occurring in the alloy at various ageing temperatures were identified with using the XRD analysis. Microstructure investigations were supplemented by hardness measurements by means of the Vickers apparatus, type HP0 250. In addition, for the selected ageing temperatures, the Charpy impact tests were performed on the Ti13Nb13Zr alloy samples. These tests were carried out on the Charpy tester of a potential energy of 150 J. The results of Charpy impact tests were supported by the fractographic documentation of samples fractures. The fractographic investigations were performed by means of the scanning electron microscope NovaNanoSEM 450.The obtained results constitute the valuable supplementation of the data bases concerning microstructures and mechanical properties of near β titanium alloys.

Characteristics of the G200CrNiMo4-3-3 Cast Steel in as Cast State

The subject of investigations constituted the G200CrNiMo4-3-3 cast steel applied for metallurgical rolls. This is the material, which microstructure can be modified by the heat treatment. The presented hereby work concerns the initial state (as cast condition) of the investigated cast steel. This cast steel constitutes the initial material for the heat treatment. The morphology of carbide precipitates, including the cementite precipitated on grain boundaries of the primary austenite, hypereutectoid cementite precipitated in the Widmannstӓtten system and lederburitic cementite, were characterised in the study. The results of investigations of mechanical and fractographic properties of the G200CrNiMo4-3-3 cast steel, were also presented. Among others, the values of fracture work in the impact test, fracture toughness (KIc) and wear resistance were given. The crack pathway together with the hypereutectoid cementite fraction role in its development, was presented. The obtained investigation results, presented in the hereby paper, will constitute the comparison base for the assessment of the heat treatment influence on the microstructure and properties of the tested G200CrNiMo4-3-3 cast steel.