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Basing on the SYSWELD-driven FEM simulations and dilatometric experiments, we propose a method to predict structural changes in laser hardened austempered 30HGSNA steel containing 18.9 vol% of retained austenite. The ability to predict effects through simulation is very valuable for the design of unconventional hybrid treatments, especially when the initial microstructure is not ferritic-pearlitic. For 2 out of 3 evaluated variants, there was a satisfying prediction of a hardened – transition zone border with the prediction error in the 1.5–8.0 % range. In addition to the methodology adopted in the simulation, the impact of the increased heating rate of the austenitic transformation under continuous heating conditions was also described. The progressive shift of A c1 and A c3 temperatures towards higher values occurring at heating rates of tens of °C per second and higher was explained by the accumulation of atomic jumps. Mutual influence of the heating rate and retained austenite on A c1 temperature and hardness decrease of tempered bainite below A c1 temperature were shown. Austempered steel's hardness after rapid reheating to laser hardening temperatures and microstructural changes were presented. [Display omitted] • PN 30HGSNA steel with submicron bainite microstructure was laser hardened. • SYSWELD simulations were used to design heat treatments imitating laser hardening conditions. • SYSWELD, dilatometry and machine learning allowed prediction of hardening results. • Simulations aimed to predict depth of border between hardened and transition zones. • For 2/3 variants the error of prediction was in the range of 1.5–8.0 %. [ABSTRACT FROM AUTHOR]

Abstract Laser surface hardening, used to achieve hardened surface without affecting bulk properties of steels, generally employs continuous-wave laser to do the job. The purpose of this paper is to systematically investigate the use of different pulsed lasers for surface hardening of 50CrMo4 steel. A continuous-wave laser and various pulsed lasers with pulse duration ranging from fs to ms were used for the experiment. It was found that millisecond laser utilizing about 9 times lower power is as effective as continuous-wave laser for surface hardening. It produced an average surface hardness of ~719 HV (2.7 times higher than base material hardness) and ~200 μm hardened depth, which is comparable with continuous-wave laser hardening. Similarly, nanosecond laser could induce both surface hardening effect and material removal depending on the parameters used. However, a shallow hardened depth (of mere ~80 μm) was achieved compared to continuous-wave laser. Furthermore, femtosecond and picosecond lasers did not produce any observable surface hardening effect; instead they resulted in direct surface ablation. Graphical abstract Unlabelled Image Highlights • Millisecond laser of 9 times lower power is as effective as cw laser in surface hardening and produces hardness as high as 719 HV. • Nanosecond laser is less effective than cw laser in surface hardening. It produces shallow hardened depth (~80 μm) with hardness of about 642 HV. • Femtosecond and picosecond lasers result in no hardening of surface. [ABSTRACT FROM AUTHOR]

A numerical model is presented for evaluation of residual stresses following laser surface treatment of mechanical components with arbitrary geometry. Following on from previous temperature and microstructural models, stress evaluation is performed by considering the resulting deformation from thermal expansion, elastic and plastic deformation, and microstructural changes. A 3.3 kW diode laser with wavelength of 930 nm and 34 mm × 2 mm rectangular spot is utilized to perform heat treatment experiments on an AISI 9810 steel cam, with x-ray diffraction measurements performed before and after laser exposure to determine circumferential and axial surface stresses. Verification of model accuracy is performed by comparing calculated stresses with the measured values. The influence of incident laser fluence and scanning velocity on the hardened depth and residual stress state is then investigated numerically for the same component. It is found that higher laser fluence, or an increase in exposure velocity at constant fluence, leads to an increase in the hardened depth and a reduction in compressive residual stresses. [ABSTRACT FROM AUTHOR]

The paper presents a modelling of the laser hardening process by a high-power diode laser (HPDL). Through numerical implementation into the finite element method (FEM) code ABAQUS, the model is used in the computer simulation of two case studies of laser hardening selected for experimental validation. In the experiment, 100 × 100 × 15 mm cuboid samples made of 50CrV4 steel were subjected to laser hardening with significantly different sets of applied technological parameters (laser beam power, laser beam velocity) but still aiming at attaining a comparable maximum temperature on the sample surface. The simulation considers two alternative approaches to microstructure evolution and subsequent material hardness determination: one relying on the heating rate dependent austenitisation temperatures ( Ac 1 and Ac 3 ) governing microstructure transformation kinetics and the other neglecting heating rate dependence. Physical objectivity of the computed results is verified based on the corresponding temperature field measurements on the sample surface during heat treatment process and hardness measurements through the thickness of the laser-hardened sample. The experimental validation clearly proves that considering austenite kinetics at a high temperature change rate in computer simulation is definitely more physically congruent. In the study of the applied process parameters impact, the effect of a higher temperature change rate on austenite kinetics is shown by the temperature shift of austenite and ferrite to austenite start formations. From the investigation of the effect of different heat inputs providing the same maximum temperature on the sample surface it results that deeper area of increased hardness is established when less laser beam power and velocity are applied. [ABSTRACT FROM AUTHOR]

Abstract: Diode laser surface treatment of three different railroad steels, namely pearlitic railroad-rail steel (PRS), ferritic–pearlitic railroad-wheel steel (FPRS) and bainitic railroad-rail steel (BRS), has been investigated. Power-ramping experiments, with in-situ surface temperature measurement, were initially carried out to assess surface temperature effects on treated layers. Influence of laser energy on characteristics of treated layers like surface topology, case-depth, microstructure and micro-hardness was comprehensively analyzed. Results revealed that the hardened layer depth is governed by both the process parameters as well as the nature of the substrate steel. With melting, significant reduction in hardness was noted owing to an increase in residual austenite and coarsening of martensitic microstructure. Unlubricated dry-sliding wear tests performed under close to real rail/wheel conditions revealed substantial improvement in wear resistance of hardened/melted rail steels compared to the parent substrates. The observed differences in characteristics of the laser-treated layers and their tribological behavior have been correlated with the chemistry and prior microstructure of the parent substrates. [Copyright &y& Elsevier]

Abstract: Discrete laser spot transformation hardening is a process that creates isolated laser hardening spots, usually distributed in a certain pattern, on a component surface, covering only a fraction of the surface region that is being treated. The process offers several unique advantages for tribological applications, including improved lubrication conditions and wear performance and increased productivity. However, very limited information is available on the appropriate selection of processing parameters to achieve optimal results. In this paper, discrete laser spot hardening of AISI O1 tool steel has been studied using a pulsed Nd:YAG laser. Effect of various laser processing parameters, including laser pulse energy, pulse duration and defocus distance, on characteristics of the laser treated spots are investigated. Maps are experimentally established for processing parameter selections in discrete laser spot transformation hardening of the AISI O1 tool steel. Results show that the maximum diameter and depth of transformation hardening zones with no surface melting increase with the increase of laser pulse energy. However, they are not markedly affected by laser pulse duration. On the other hand, longer pulse durations at a given pulse energy reduce the size of softening zone surrounding the central hardening zone and are thus more favourable for most practical applications. Short laser pulse durations below 8ms tend to produce shallower hardening zones and are not recommended for wear applications. [Copyright &y& Elsevier]

Abstract: A predictive model for residual stresses induced in a laser hardened workpiece of AISI 4140 steel with no melting has been developed and experimentally verified. A transient three-dimensional thermal and kinetic model is first solved to obtain the temperature and solid phase history of the workpiece, which is then sequentially coupled to a three-dimensional stress model to predict residual stresses. The phase transformation strains are added to the thermal strains at each time step during the heating and cooling cycles to obtain the resultant residual stresses in the workpiece. The importance of considering phase transformation has been explained through the comparison of the magnitudes of residual stresses with and without the inclusion of phase transformation kinetics. The model predicted strong compressive residual stresses of about 200 MPa in the heat affected zone due to austenite-to-martensite transformation. The predictions matched well with the X-ray diffraction measurements. [Copyright &y& Elsevier]

Laser hardening is a process to improve surface functionalities by controlling the surface properties of targeted steel, such as hardness, residual stress and microstructure. This study aims in investigating the hardness and grain size based on multi-pass laser hardening. This was conducted on normalized AISI 4140 steel with a controlled maximum temperature in each pass below melting point with different parameter combinations, followed by the Vickers hardness test, electron microscopic investigations for determining the grain size of the martensite and electron probe microanalysis for determining quantitative carbon content of the phase-transformed surface zone. The results showed that with increasing number of passes the hardness and the hardening depth increased. Meanwhile, the grain size of martensitic grains decreased with cycle numbers and with the distance to surface after multi-cycle re-austenitization. With increasing number of paths, the grain size distribution and the carbon content distribution, which were initially inhomogeneous due to ferrite and pearlite segregation lines, became more homogenous. These findings show that multi-pass laser hardening may be promising regarding the functional properties due to increasing the hardening depth and grain refinement as well as homogenization of the martensite grains. • Multi-pass laser hardening broadens laser-hardened and grain-modified zone. • Rapid cyclic austenitization and quenching refine martensite grains. • Cyclic austenitization homogenizes carbon distribution of ferrite-perlite steel. [ABSTRACT FROM AUTHOR]

Abstract: The aim of the present work is to assess the possibility of predicting the thickness of the hardened layers produced by High Power Diode Laser (HPDL) heat treatment. Instrumented indentation by cylindrical flat punch was carried out in order to evaluate the effectiveness of the surface-hardening heat treatments on milled AISI 1040 steel substrates by varying laser operational parameters as well as the morphology of steel substrates. In particular, the trend in load-penetration depth were obtained, supporting the identification of the depth at which the hardened layer starts transferring the deformation to the softer substrate, that is, the Critical Transition Depth (CTD) in addition to identifying the overall degree of hardening reached by the laser treated material. On the other hand, hardness profiles and the thickness of hardened layers were also monitored by using combination of Vickers micro-hardness tests and standard metallographic characterizations. Analytical examinations and simple mathematical simulations of the experimental results coming from indentation tests and conventional testing procedures showed good correlation between the extent of the heat affected zone (HAZ), hardness and CTD. In conclusion, this study demonstrates that indentation tests are an effective, sensitive, fast, and low-cost characterization technique for heat treated steels. [Copyright &y& Elsevier]

A study of the surface hardening process of the 42CrMo4 steel alloy by means of an oscillating laser beam has been carried out. For this purpose, a fiber coupled high power diode laser source and a galvanometric mirror scanner have been used. The study is focused on determining the influence of the oscillation frequency parameter on the temperature, geometry, microstructure, microhardness and stress state of the treated region, as well as its relationship with the key parameters of the laser hardening process. The results confirm that the surface hardening process of the 42CrMo4 alloy by means of an oscillating laser beam is an alternative technique to the conventional laser surface hardening methods based on fixed optical guidance systems. On the other hand, the selected frequency of oscillation has a significant influence on the treated region, mainly on the temperature, and, consequently, on the geometry, microstructure and stress state of the same. The selection of a high oscillation frequency, able to maintain the material temperature above that of austenization and below that of melting, is decisive to ensure the stability and uniformity of the treatment. • 42CrMo4 steel was surface treated by an oscillating beam from a scanner and diode laser. • Oscillation frequency has a direct influence on temperature and material microstructure. • Threshold frequency for a uniform treatment is not fixed, depends on key process parameters. • High oscillation frequencies permit to reduce the oscillation in surface temperature. • Oscillation temperature between austenization and melting points permits a uniform treatment. [ABSTRACT FROM AUTHOR]

Laser hardening is usually performed in air. Use of a controlled gas atmosphere might produce a direct reaction of the gas with surface and thus provide an opportunity to engineer the surface microstructures. However, few studies are reported on direct gas-assisted laser hardening process. In this paper, a direct gas-assisted laser hardening is demonstrated on AISI 1020 steel using an ytterbium-doped fiber laser in presence of four different gases – air, argon, carbon dioxide and propane. All hardening trials were conducted at same laser parameters and only the gas was changed to evaluate its effect on hardness and microstructure of the steel. Results show that argon had slightly higher hardening effect than air as it prevents surface oxidation. Propane produced a very high surface hardness (914 HV) compared to conventional laser hardening process (395 HV). The ultrahigh hardness is due to diffusion of carbon into steel from decomposed propane. Increasing the concentration of propane contributed to an increment in hardness primarily due to large amount of carbides in near surface region. Contrarily, carbon dioxide yielded a lower surface hardness (350 HV) despite its high carbon content. This is attributed to the oxidizing effect of carbon dioxide, which produces decarburization during laser processing. The study reveals the potential of using direct gas-assisted laser hardening technique to alter surface mechanical properties of steel. Unlabelled Image • Laser hardening under atmosphere of air, argon, propane and CO 2 is demonstrated. • Argon produced 16% higher hardness than in air due to reduction in oxidation. • Propane yielded ultra-high hardness (914 HV) near the surface region. • CO 2 resulted in 11% lower hardness than in air due to surface decarburization. • Use of controlled gas during laser hardening can modify steel surface properties. [ABSTRACT FROM AUTHOR]