Roman Šturm

Influence of laser surface melt-hardening conditions on residual stresses in thin plates

Abstract Wear resistance of products made of gray cast iron with a ferrite-pearlite microstructure of the matrix may be considerably increased by the process of laser remelting of the surface. Products with comparatively thin walls (up to 5 mm), however, seem to be problematic since the stresses introduced during laser heat treatment may produce deformations of the product. This paper provides the results of research into various methods of laser beam guidance along the workpiece surface with various degrees of overlapping of remended traces and the influence of various machining conditions in laser machining or through thickness variations in residual stresses in the workpiece. Residual internal stresses were measured by a strain gage by means of the relaxation method where the stressed modified surface layer was electro-chemically removed. The size of deformations occurring during heat treatment of rest pieces was assessed. On this basis internal stress intensity was accessed. The efficiency of the process itself of laser remelting of gray cast iron in increasing wear resistance is thoroughly described by an analysis of microstructure and confirmed by through-thickness microhardness measurement of the modified layer.

Comparison of measured and calculated thickness of martensite and ledeburite shells around graphite nodules in the hardened layer of nodular iron after laser surface remelting

Abstract The application of laser surface remelting to nodular iron 400-12 causes the material to undergo microstructural changes. Because of the rapidity of laser beam action on the surface of the specimen material, melting occurs only in a thin surface layer. After the laser beam has passed along the specimen, thanks to very rapid heat transfer from the surface to the interior of the specimen, we can achieve very rapid solidification of the melted pool and self-hardening of the rest of the modified layer. The process of rapid heating above the melting-point temperature and the temperature of the austenitic transformation creates conditions for the formation of metastable microstructures that can offer important technological properties such as high hardness, good wear and corrosion resistance. A newly created austenite–cementite microstructure with the presence of graphite nodules in the remelted layer and a martensite–ferrite microstructure with graphite nodules in the hardened layer have been observed. Microscopy of the hardened layer was used to analyse the occurrence of martensite shells or ledeburite shells around the graphite nodules in the ferrite matrix. The size of the shells was later supported by simple diffusion calculations.

Laser surface melt-hardening of gray and nodular irons

Abstract The procedure of laser heat treatment by surface remelting has recently been increasingly used on products from gray and nodular irons for modifying the surface layer. Since in laser treatment the surface or surface layer is heated above the melting temperature, greater depth of the austenite transformation in the material is achieved, which contributes to the formation of a modified layer deeper than that obtained by conventional hardening procedures. For the case of laser surface-melt hardening, a mathematical model was used to determine the temperature cycles in gray and nodular irons and the depth of the modified layer. The mathematically obtained results are critically assessed and compared with the results in the experiments. The microstructural changes which can be predicted from the temperature cycles on heating and cooling are confirmed by microhardness measurements.

Determination of Creep Properties From Small Punch Test

The small punch test is one of the techniques, which can directly assess the current material properties of components on situ. The main advantages of this method are that only small amount of material is needed and no repair is required afterwards. In addition, the test itself is rather simple to perform. Since early 1980’s, the technique has been developed and applied to nuclear reactors, electric power plants for safety assessment. European researchers have carried out the pioneer work on small punch testing at the creep region. Recently the European Code of Practice documents are available for both high and low temperature properties, which summarizes the international experiences in last 20 years and provides a guide line to perform small punch test for metallic materials. This paper gives an overview of the research work on determination of creep properties from small punch test. Verification is performed by comparison of creep properties derived by small punch test and uni-axial tests. An agreement is found and shows that small punch test is able to predict reasonable creep properties of materials. Considerations how to improve the approach are also addressed. Further work is needed as available tests are still limited. This paper makes a starting point that others can improve further.Copyright

Deformation of specimen during laser surface remelting

During laser surface remelting, thermal expansion in heating, and contraction of material in cooling, and due to microstructure changes in the heated surface layer, the specimen deforms. This induces volume changes during the process of remelting the thin surface layer and causes internal stresses. By measuring the deformation of the specimen during the remelting process as well as after the laser beam interaction, i.e., to the moment when the specimen cools down to the ambient temperature, it is possible to follow the progress of deformation in the specimen material. Detailed information on the deformation events in the specimen material during the laser remelting process of a thin surface layer enables the engineer to optimize the process variables in terms of amount of deformation or magnitude of residual stresses on completion of cooling.

An Electromagnetic Sensor with a Metamaterial Lens for Nondestructive Evaluation of Composite Materials.

This paper proposes the study and implementation of a sensor with a metamaterial (MM) lens in electromagnetic nondestructive evaluation (eNDE). Thus, the use of a new type of MM, named Conical Swiss Rolls (CSR) has been proposed. These structures can serve as electromagnetic flux concentrators in the radiofrequency range. As a direct application, plates of composite materials with carbon fibers woven as reinforcement and polyphenylene sulphide as matrix with delaminations due to low energy impacts were examined. The evaluation method is based on the appearance of evanescent modes in the space between carbon fibers when the sample is excited with a transversal magnetic along z axis (TMz) polarized electromagnetic field. The MM lens allows the transmission and intensification of evanescent waves. The characteristics of carbon fibers woven structure became visible and delaminations are clearly emphasized. The flaws can be localized with spatial resolution better than λ/2000.

Influence of laser beam guiding and overlapping on residual stress in remelting process

Abstract The laser remelting process is a very promising hardening method for ferrous and non-ferrous alloys which do not show a homogeneous microstructural state after being heat treated in a traditional way. Consequently, the alloys concerned do not achieve the required hardness of the surface layer and do not provide the required wear resistance. The present paper deals with residual stress and strain changes in thin flat specimens of nodular cast iron 500-7 in the laser remelting process. In the process of gradual remelting of the specimen surface, i.e. during its heating and cooling, volume changes in the specimen occur due to thermal and microstructural changes. To find out more about volume changes in the thin surface layer of specimens, the latter were monitored by strain measurement at the lower side of the specimens during the remelting process and by residual stress measurement in the thin surface layer after the remelting process. The strain was measured using a resistance rosette, placed on the opposite side of the remelted layer, and the temperature was measured in the same place simultaneously. In laser remelting of a workpiece, such remelting conditions should ensure that a uniform thickness of the modified layer is obtained, in spite of several laser beam passages across the workpiece surface. Owing to the multiple passage of the laser beam across the workpiece surface, the state of internal stress in the workpiece changes and, consequently, undesired strain may occur. Such strain may result in greater consumption of material and additional machining which, in turn, may result in higher cost of the machined parts. The results obtained confirmed that on the basis of knowing the strain occurring during the remelting process and the residual stress, the process could be successfully optimised. On the basis of knowing the strains occurring during the remelting process and the residual stress, optimal laser surface remelting conditions were chosen, while the method of guiding the laser beam over the surface of the flat samples and the degree of overlap were varied.

Residual stress state after the laser surface remelting process

Residual stresses are a result of elasto-plastic deformations induced in the workpiece material during the heat treatment process. The extent and magnitude of internal stresses depend on temperature conditions in heating and cooling and physical properties of the workpiece material. This contribution discusses the extent and distribution of residual stresses after laser remelting a thin surface layer on ductile iron 80-55-06 (ASTM specification) or Gr 500-7 according to ISO. Residual stresses are not only induced by temperature differences but also result from stresses due to microstructural changes between the surface and the core of the specimen subsequent to cooling to the ambient temperature. The distribution and extent of residual stresses in the remelted thin surface layer depend mostly on melt composition and cooling conditions. Different rates of solidification and subsequent cooling of the remelted layer are reflected in the volume proportions of the created cementite, residual austenite, and martensite in the microstructure. The rate of heating and cooling of the thin surface layer is a function of laser power, beam diameter on the workpiece surface, and interaction time. In addition, the number of passes of the laser beam over the workpiece surface and different degrees of laser trace overlapping were increased to see how these can affect the thermal conditions in the workpiece. To determine the residual stresses, the relaxation method was used. This is based on measuring the specimen strain during electrochemical material removal.

Microstructure variations in the laser surface remelted layer of nodular iron

Laser remelting process is a very promising hardening method for ferrous and nonferrous alloys because it does not show a homogeneous microstructural state after having been heat-treated in a traditional way. The paper deals with residual stresses in the surface layer of thin flat specimens of nodular cast iron 500-7 in the laser remelting process. Volume changes of the specimen occur due to thermal and microstructural changes in the process of gradual remelting of the specimen surface. For a better knowledge of volume changes in the thin surface remelted layer of the specimen, the latter was evaluated by residual-stress measurement. Such remelting conditions in laser remelting of a workpiece should be ensured so that a uniform thickness of the modified layer can be obtained despite several laser-beam passages across the workpiece surface. The state of internal stress in the workpiece changes because of a multiple laser-beam passage across the workpiece surface. Laser operating parameters were chosen on the basis of knowing the remelted geometry of one laser trace, while the overlapping degree of laser tracks was varied.

Cavitation erosion of the calcium carbonate deposits

The paper presents the research of erosion effects caused by acoustic cavitation on metallic and enamelled specimens coated with calcium carbonate - scale. The deposition of calcium carbonate was performed under controlled laboratory conditions. The deposition took place at the same integral conditions to the specimens of different surface roughness. Cavitation as a cause of erosion was generated in a vessel with ultrasonic excitation. Simultaneously with the monitoring of cavitation above the specimens, the destruction of scale on the surface of specimens was analysed. The results of experiments indicate a characteristic effect of cavitation intensity on the destruction of scale surface layer. The intensity of cavitation erosion is proportional to the voltage on the sonotrode or the intensity of pressure fluctuations, and depends on the roughness of the surface of specimens, on which the scales were deposited.