Tomáš Mužík

Laser hard overlaying is an advanced, perspective technology with wide industrial applications, for example, dies. The aim is to improve surface properties like wear resistance using special layers of powder sintered or remelted by laser beam. At present, dies are manufactured by machining with following bulk heat treatment, which is an expensive process particularly due to use of expensive high-alloyed tool steels. Repairs performed using arc or plasma welding introduce a big amount of heat to the part, which can cause dimension changes and material degradation. These methods often fail also due to low weldability of the materials. An advantage of laser overlaying is minimization of these difficulties. The paper contains a comprehensive evaluation of several types of hard overlayed powder of H13 tool steel on a S355 structural steel and on H11 tool steel using a laser beam. Macro- and microstructure, hardness and fatigue resistance are evaluated, including fatigue damage mechanisms. In the case of the H13 welds on the S355 steel plate, the quality of the welds was mostly acceptable, without pores or segregate impurities and with a good interconnection between the weld track and base material. Results are completed with basic measurement of residual stresses using destructive strain-gauge methods. High tensile residual stresses of 1465 MPa were measured at the boundary of the first track of the single-layer overlay. Fatigue resistance is sensitive on surface and subsurface defects, which can significantly reduce endurance limit. Fatigue strength of specimens with the single layer overlay was considerably lower than fatigue strength of the S355 steel. The decrease was between 25% and 50%. In the case of overlay of H13 on H11 tool steel, the decrease in fatigue strength was between 25% and 30%.

Laser hard overlaying is an advanced, perspective technology with wide industrial applications, for example dies. The aim is to improve surface properties like wear resistance using special layers of powder sintered or remelted by laser beam. At present, dies are manufactured by machining with following bulk heat treatment, which is an expensive process particularly due to use of expensive high-alloyed tool steels. Repairs performed using arc or plasma welding introduce a big amount of heat to the part, which can cause dimension changes and material degradation, These methods often fail also due to low weldability of the materials. An advantage of laser overlaying is minimization of these difficulties. The paper contains a comprehensive evaluation of several types of hard overlayed powder of H13 tool steel on a S355 structural steel and on H11 tool steel using laser beam. Macro- and microstructure, hardness and fatigue resistance are evaluated including fatige damage mechanisms. Results are completed with basic measurement of residual stresses using destructive strain-gauge methods. Fatigue resistance is sensitive on surface and subsurface defects, which can significantly reduce endurance limit.

Railway wheels are usually attached to axles by press-fitting; therefore, the mechanical processes taking place during operation can result in failure, with fatal consequences for the axle seats. This manuscript describes the effect of laser hardening on the residual stress state, microstructural parameters (lattice defects—dislocations, crystallites, microstrains, etc.), and mechanical properties of laser-hardened EA1N steel railway axles under fatigue life conditions. Differences were found between ground, single-track, and multi-track hardened surfaces. Tensile residual stresses, low dislocation densities and hardnesses, and different microstructures (tempered cubic martensite) were found at the overlapped tracks and at the boundary of the heat-affected zone and bulk surface compared with the hardened zone. As a result, the surface treatment of axle seats by laser hardening improved the fatigue failure resistance compared with untreated seats. Optimal properties of the integrity of the axle seat surface were achieved, including fatigue resistance, which seems to be positively influenced mainly by sufficient hardness and the appropriate microstructure. The influence of the other investigated parameters was not evident, and was reduced by the presence of fretting corrosion and press-fitting.