Martin Zupančič

Regular cambial activity and xylem and phloem formation in locally heated and cooled stem portions of Norway spruce

The effect of heating (23–25°C) and cooling (9–11°C) on regular cambial activity and xylem and phloem formation in the stem portion of Norway spruce was investigated. Adult trees were sampled at 21-day intervals during the 2005 vegetation period. Continuously elevated temperatures increased the rate of cell division in the first part of the growing season, but did not significantly prolong cambial activity at the end of the vegetation period in the heated tree. Low-temperature treatments shortened regular cambial activity and slowed down cell production. The xylem growth ring was wider in the heated sample and narrower in the cooled sample compared to the control. The temperature in the cambial region was only negligibly transferred along the stem from the site of its application. In general, the temperature in the cambium was affected by a long-term rise or drop in air temperatures. Both experiments affected the structure and width of phloem growth increments. The tangential band of the axial parenchyma was not continuous in the cooled sample. The number of late phloem cells was reduced in the cool-treated sample and increased in the heat-treated sample. Our experiments confirmed the effect of constantly increased or decreased temperatures on regular cambial activity in Norway spruce.

Influence of quenching process parameters on residual stresses in steel

Abstract The paper presents results regarding cooling intensities of quenching oil of domestic production and of polymeric water solutions. Numerous residual stresses measurements on heat-treatment steel specimens with different masses were made. To measure the residual stresses, the relaxation method was used, involving gradual mechanical removal of the hardened layer in which the deformation of the specimen was measured by resistance strain gauges. On the basis of the measured strain at a specific depth of material removal, it is possible to calculate the size or variation of residual stresses depending on the depth of the hardened layer. Numerous residual stresses measurements provided very important data on various steels when using different quenching agents.

A 548-year tree-ring chronology of oak (Quercus spp.) for southeast Slovenia and its significance as a dating tool and climate archive.

Tree-ring series of oak, from both living trees (Quercus petraea and Q. robur) and historic timbers in southeastern Slovenia were assembled into a 548-year regional chronology spanning the period A.D. 1456–2003. It is currently the longest and the most replicated oak chronology in this part of Europe located at the transition between Mediterranean, Alpine and continental climatic influence. The chronology correlated significantly with regional and local chronologies up to 700 km away in Austria, Hungary, Serbia, Czech Republic and southern Germany. It also showed good “heteroconnection”, i.e. agreement with chronologies of beech (Fagus sylvatica), ash (Fraxinus excelsior) and silver fir (Abies alba) in Slovenia. A preliminary dendroclimatic analysis shows that precipitation and temperature in June accounted for a high amount of variance (r2 = 0.51) in the tree-ring widths. The chronology thus contains considerable potential as a climate archive. We also present its use as a tool for the dating of wooden objects of the cultural heritage. Moreover, the chronology can be a point of reference for building tree-ring chronologies in neighboring regions.

The formation of a ligno-suberised layer and necrophylactic periderm in beech bark (Fagus sylvatica L.)

Beech (Fagus sylvatica L.) bark was wounded in early April of 1993 and tissue changes followed on days 7, 14, 21, 28, 35, 42, 49, 56, 84, 112, and 140. In 7 days, tissue at the wound surface became necrotic and discoloured. In 14 days the walls of the parenchyma cells immediately underneath the necrotic tissue became thickened and after 21 days became lignified. In 28 days these lignified cells showed intracellular-suberisation. In 42 days the ligno-suberised layer was continuous with the phellem of the superficial periderm. In 35 days we first noted differentiation of the necrophylactic phellogen under the ligno-suberised layer. In 49 days suberised phellem of the necrophylactic periderm differentiated. At 112 days the phellem of the necrophylactic periderm had coalesced with that of the surface periderm. In I40 days abscission of the wound rhytidome began.

Suitability assessment of replacement of conventional hot-working steels with maraging steel. Part I: Mechanical properties of maraging steel after precipitation hardening treatment

Abstract A study of the properties of maraging steel Thyssen 1.2799 in the production of die-casting tools for aluminium alloys has been undertaken. Beside the study of the mechanical properties described in Part I of the paper, analyses of the microstructure by means of optical microscopy and scanning electron microscopy were made, too. The precipitates having a size of the order of a few 10 nm and more could be identified. Relating of the mechanical properties to the microstructure analyses supported by a micro chemical analysis, the suitability of the maraging steel concerned for the production of die-casting tools for aluminium alloys was proved.

ANATOMY, CELL WALL STRUCTURE AND TOPOCHEMISTRY OF WATER-LOGGED ARCHAEOLOGICAL WOOD AGED 5,200 AND 4,500 YEARS

SUMMARY Evaluating the state of deterioration of water-logged archaeological wood is necessary in order to select treatments for its conservation and storage, particularly in the case of valuable archaeological artefacts. For this purpose archaeological wood of ash (Fraxinus sp.) and oak (Quercus sp.) buried in water-logged conditions at prehistoric settlements on the Ljubljansko barje (Ljubljana moor), Slovenia, aged approx. 5,200 and 4,500 years, was investigated by means of light microscopy (LM), transmission electron microscopy (TEM) and cellular UV-microspectrophotometry (UMSP). LM and TEM revealed that the ash wood aged 5,200 years was the least preserved. The secondary walls of fibres, vessels and parenchyma cells were considerably thinner than in normal wood, indicating distinct degradation. TEM and UMSP additionally revealed strong delignification of the remaining parts of the secondary walls of all cell types. The compound middle lamellae appeared structurally intact, but had lower UV-absorbance than normal wood of the same species. The cell corners were topochemically unchanged, as shown by high analogue UV-absorbance. The UV-absorbance maxima at a wavelength of 278 nm corresponded to those of hardwood lignins. The oak heartwood was generally better preserved than the ash wood. Within each species, the 4,500year-old samples generally appeared better preserved than those 5,200 years old.

Laser Shock Processing of the Maraging Steel Surface

Laser Shock Processing (LSP) is a process of laser treating of a surface with a pulsed beam of high power density. The process enables hardening of a thin surface layer; therefore, it is suitable for the improvement of fatigue strength of quality materials. Locally directed mechanical waves produce a considerably increased dislocation density in the thin surface layer, which affects the variations of microhardness and residual stresses. The magnitude and variation of the residual compressive stresses in the surface layer are favourable, which ensures higher fatigue strength. Laser shock processing (LSP) is more exacting than conventional shot peening, but it shows certain advantages such as better control of the surface state, processing of locally limited surfaces and a possibility to produce different transitions between the processed surface and the non-processed one. LSP has so far been tested and efficiently applied to various materials, including maraging steels. Relevant publications often deal with LSP mechanisms and the influence of the process on the dynamic strength of maraging steel, but less frequently the influence of individual characteristics such as the microstructure of matrix and of precipitated phases or residual stresses. The present paper deals with LSP of 12% Ni maraging steel. The material chosen is suitable for the production of complex structural parts and dies for die casting, which require high resistance of the material to thermo-mechanical loads. By means of measurement of the state before and after LSP, the value of the mean roughness Ra, surface defects and the variation of residual stresses in the thin surface layer were determined. After LSP of the surface, the influence of processing parameters such as laser-beam diameter and pulse density per unit of area was established.

Measurement and numerical analysis of surface residual stresses occurring under different quenching conditions

The paper presents the results of residual stresses modelling and experimental stress measurements after quenching of AISI 4140 heat-treatment steel specimens. In the research, cylindrical specimens of ? 40 ? 160 mm in size were used whereas in modelling specimens of the same diameter, i.e., ? 40 mm, but of infinite length were employed. One quenching agent was selected, i.e., a 15% polymer water solution. The influence of residual stresses due to martensitic transformation and temperature differences was investigated with different initial specimen temperatures above and under TAC1. Two different temperatures were chosen in order to reveal the nature and quantitative properties of temperature- and transformational-induced residual stresses. Analytical work was made by mathematical modelling of heat treatment which supposes that the rheological model of a continuum is thermo-elasto-plastic material. Austenitic-martensitic transformations are described with Koistinen-Marburger equation. Related experimenta...

Stress State of 12% Ni Maraging Steel After a Modified Procedure of Precipitation Hardening

Fatigue resistance of a surface layer of dynamically and thermomechanically loaded machine components and tool parts can be improved with a combination of mechanical hardening by cold deformation and thermal precipitation hardening. One of the mechanical processes of surface hardening is a relatively simple and accessible process of roto peening. The hardening effect of roto peening is related to a local plastic deformation of the surface layer, which results in a higher hardness and an increase of compressive residual stresses. The application of roto peening or related process is possible and efficient with maraging steels used in the production of mechanically loaded tools and machine components. It turned out that in such combined hardening processes applied to maraging steels certain limitations related to the temperature of precipitation annealing should be considered. In the opposite case, thermal hardening can annul all the advantages obtained in the previous mechanical hardening of the surface layer.

Non-Homogenous Precipitation Annealing of Maraging Steels

Nickel maraging steels in the as-delivered state show a microstructure consisting primarily of oversaturated lath martensite. The final mechanical properties are achieved with precipitation annealing, during which the dissolved alloying elements start to precipitate. The newly-formed precipitated phases efficiently hinder shifts of dislocations, which results in considerable changes of the mechanical properties. Strength and hardness thus essentially increase whereas the material shows a relatively high fracture toughness. The precipitated phases formed during precipitation annealing also cause some distortion of a workpiece and transformational residual stresses on the micro and macro scales. Linear strains after precipitation annealing range from -0.05% to -0.1% and depend primarily on temperature/time conditions during the precipitation process. The non-homogenous annealing such as laser annealing produces temperature-induced residual stresses which are combined with transformational stresses. These stresses are expected to be lower and much more different than the stresses occuring in comparable classical heat-treated steels, however they cannot be neglected due to the exceptional performance of maraging steels and also their employment with high-demanding products. The results of the residual stress measurements after laser-induced and classical heat treatments of Ni-Co-Mo maraging steel with 12% nickel are presented.