Tibor Kvačkaj

Effect of thermomechanical processing on the microstructure and mechanical properties of Nb–Ti microalloyed steel

Abstract Based on analysis of austenite deformation behaviour during thermomechanical processing of Nb–Ti microalloyed steel, the rolling schedules were designed to produce (i) recrystallized austenite, (ii) unrecrystallized austenite, and (iii) ferrite-pearlite. The effects of austenite conditioning on the final ferrite-pearlite microstructure and mechanical properties of steel were investigated. To rationalise the variation in final ferrite grain size with different thermomechanical processing schedules, it is necessary to consider the ferrite grain growth in addition, to the density of ferrite nucleation sites. Mechanical properties were the means to evaluate the variation in austenite solutioning and deformation conditions introduced into individual applied rolling schedules. The benefit of tensile tests, especially yield strength and ductility values, in determining the optimum deformation schedule and coiling condition for given steel is demonstrated.

Effect of various processing conditions on the tensile properties and structural developments of EN AW 2014 aluminium alloy

The paper deals with the influence of processing conditions on tensile properties, hardness and structure development of the aluminium alloy EN AW 2014, depending on various processing conditions (as-rolling, quenching, severe plastic deformation SPD by equal channel angular pressing ECAP and ageing). The evaluated tensile properties showed that ECAP has the highest effect on the material properties of aluminium alloy EN AW 2014. Severe plastic deformation by means of ECAP caused rapid increase of strength and only partial decrease of ductility was achieved. Strengthening of material is caused by grains refinement and strain hardening of solid solution. Based on the results above, the tensile properties, hardness and structure development of the EN AW 2014 alloy are discussed.

Nanostructure Formation and Properties in Some Al Alloys after SPD and Heat Treatment

Influence of SPD process realized by ECAP on structural formation and mechanical properties was searched. Samples after ECAP were heat treated at various temperature and time conditions. Investigation material bases were high purity aluminium and aluminium alloys EN AW 6082, EN AW 2014. The best material properties are describing in dependence on experimental conditions.

Wear Mechanism of Chromium Pre-alloyed Sintered Steel

The paper is focused on the wear mechanism of a chromium pre-alloyed Fe (Cr Mo) C sintered steel. The wear behaviour of the sintered steel is investigated through pin-on-disk tests. Two different processing conditions have been used, involving a slow cooling rate of 0.05 K/s and a process called sinter hardening with an average cooling rate of 6 K/s from the sintering temperatures (1453 and 1513 K). The microscopic investigations reveal deformed layers and tracks along the direction of sliding during wear. Particular attention has also been paid to the friction coefficient and to the role of porosity on wear.

Controlled Rolling of Hastelloy-N

This paper provides information of the uniform grain structure after application of hot and cold rolling conditions and recrystallization processes followed by various annealing times in NiMoCr alloy. The different structure would result in high temperature mechanical properties such as yield strength, tensile strength, ductility, fatigue, and creep strength. The microstructural changes as results from all tested programs were investigated by optical microscope. The obtained results showed that the uniformity of recrystallized structure relates strongly to amount of deformation during hot and cold rolling and period of annealing time. It was also found that the uniformity of recrystallized structure increased with higher deformation and longer annealing times. The formability was tested and the base deformation resistances (BDR) were also calculated.

From Micro to Nano Scale Structure by Plastic Deformations

Nowadays, the strategy for improving of mechanical properties in metals is not oriented to alloying followed by heat treatment. An effective way how to improve the mechanical properties of metals is focused on the research looking for some additional structural abilities of steels. Structural refinement is one of the ways. Refinement of the austenitic grain size (AGS) carried out through plastic deformation in a spontaneous recrystallization region of austenite, formation of AGS by plastic deformations in a non-recrystallized region of austenite will be considered as potential ways for AGS refinement. After classic methods of plastic deformations, next structure refinement can be obtained by an application of severe plastic deformation (SPD) methods.

Influence of Deformation Temperature and Time on the Mechanical Properties of Pulsation Deformed Stainless Steel

Pulsation forging of metallic materials (also referred to as stresscycling) is a novel forming method based on the application of a variable pulsation frequency during the metal forming process. This paper presents some new results obtained after pulsation forming of steel grade Crl8Nil0. The previous works given by the present authors suggested that the value of summary deformation of 25 % (at the pulsation frequency of 30 Hz) was obtained with the very good compromise of yield strength (Rp 0,2), tensile strength (R„,), elongation (A5) and contraction (Z). Value of geometrical parameter Θ of 20 % or the highest value of Θ is found to improve the flow of material at pulsation forming process in a sense of the lower plastic deformation heterogeneity. The experimental results were given with respect to the effect of deformation temperature (from 1193 to 1123 K) and deformation time (from 2 to 10 sec.) at constant heating temperature (1373 K) and constant pulsation frequency (30 Hz) on mechanical characteristic (Rpo, 2; Rm; A5; Z) of stainless steel grade CrlSNilO. The relationships between strength plastic values and geometrical parameters Θ were also investigated.

Influence of Plastic Deformation on Creep Behaviour of NiMoCr Alloy

Creep behaviour of nickel-base solid solution strengthened NiMoCr alloy after different applied hot and cold rolling conditions was investigated. The results of creep tests showed that the creep characteristics including strain rate, fracture strain, and lifetime were greatly dependent on the plastic deformation conditions carried out prior to the creep. Failure process including crack nucleation and crack propagation was strongly dependent on grain size. Fine recrystallized structures have provided much less resistance to creep deformation compared to coarse grain size. A comprehensive study of creep deformation as a function of plastic deformation conditions of NiMoCr alloy is also presented.

Influence of SPD by ECAP on Cu Properties

Equal channel angular pressing (ECAP) is a material processing method for developing an ultrafine-grained (UFG) structure by introducing severe plastic deformation (SPD) in a bulk material with no changes in its cross-section. Numerous analytical and numerical studies on equal channel angular pressing have been performed in recent years. The present work focuses on the effects of die geometry width is defined by the angle between two channels Φ, angle on outer corner of die Ψ (or radius R) and angle within internal corner (or radius r) of die on average effective strain after one pass route. Next, there are analyses of strength properties, plastic properties, fracture mechanism, as well as analyses of Cu structure evolution after SPD by ECAP technology, in the paper. The sixteen passes through the ECAP matrix were realized using route C. The following experimental results and their analyses, the biggest increase of strength and microhardness was proved already after 4th pass. Valuation of fracture surfaces shows that after 12th pass plastic fracture is transformed from transcrystalline ductile mixed fracture. After 4th pass, the avarage grain size decreased from initial approximate size 7 µm to 200 nm, whereby the average grain size was changeless after subsequent deformations. Possible mechanism of high-angle boundary nanograins evolution consists of formation of cell structure, subgrains that transform with the increase of deformation into nanograins with big-angle misorientation.

Mechanical and Thermal Properties of Central Former Material for High-Current Superconducting Cables

Ongoing projects to realize high-current superconducting cables for transport or magnet applications need to incorporate a high number of coated conductor tapes. Several design layouts published, such as conductor-on-round-core cable, stacks, or Roebel-Rutherford, can achieve this. Design layouts proposed by the research institutes ENEA and KIT use a central former having grooves along the length where stacks of tapes or Roebel strands can be embedded. The former material is a substantial fraction of the cable cross section influencing the overall performance of the cable. In this work, materials used as the central former are investigated after severe plastic deformation, using equal channel angular pressing (ECAP) and equal channel angular rolling (ECAR) processes. Applying these methods allows producing continuously long-length profiles. Materials under investigation are aluminum alloy EN AW 6063 and oxygen-free high-conductivity copper. The influences of substructural characteristic obtained by ECAP and ECAR technology on mechanical properties, as well as thermal and electrical conductivity, at operational cryogenic temperatures, at 77 K and 4.2 K, are observed.