Laser Transformation Hardening Of Steel Research Articles

Status of laser transformation hardening of steel and its alloys: a review

The state of laser transformation hardening in surface modification of steel and its alloys is reported in this review paper. The laser hardening process involves the use of high-intensity laser radiation to heat the surface of steel into the austenitic region rapidly. Due to high rates of heat transfer, steep temperature gradients result in rapid cooling by conduction. This causes the transformation from an austenite to a martensite microstructure without the need for external quenching and generates a hard wear-resistant surface. The introduction of high-power lasers, such as neodymium-doped yttrium aluminum garnet (Nd:YAG), diode and fiber lasers, has made laser-assisted hardening an attractive candidate for surface hardening processes. Recently, fiber lasers appeared as a competitor to Nd:YAG and diode lasers for material processing. This review paper is a synopsis of the fundamentals of laser hardening, outlining some of its benefits compared with those of conventional hardening techniques and its industrial applications. A selective review of the experimental and numerical research carried out on steel in this area is also presented. Finally, it is concluded that the overall state of research on laser hardening in surface engineering of steel is well developed, and a still-growing industrial application is observed.

Theoretical and experimental studies on laser transformation hardening of steel by customized beam

An exact solution was developed for a quasi-steady-moving-interface heat transfer problem in laser transformation hardening by a beam customized to a flat-top rectangular shape. The absorption and release of the latent heat were taken into account. The solution provided the body temperature, heating rate, and subsequent cooling rate. Using the IT and CCT diagrams, the rapid austenitization and pearlitic/martensitic phase transformation were determined for prediction of the hardening effect. In the experimental study, a diffractive optical kinoform (DOK) was used to customize a conventional Gaussian laser spot beam to a flat-top rectangular beam. A reasonable agreement between the theoretical predictions and the experimental measurements was obtained.

Henry Granjon Prize Competition 2006 Co-Winner, Category A “Joining and fabrication technology” Models for Diode Laser Transformation Hardening of Steels

Diode laser beam transformation hardened surfaces have been studied in order to develop models for estimating the hardness and depth of the transformed region. The surfaces of low alloy and martensitic stainless steels have been transformation hardened in inert and oxidising atmospheres to establish the dependence of processing parameters and oxidation on the absorptivity of steel surfaces. It was found that the absorptivity varied between 35 to 85%, depending on the oxide formation on the surface. A model for estimating the depth of the hardened region was developed. The model utilises data established by absorption experiments, and the variation of absorptivity with processing parameters is approximated in the model. Validation was carried out by measuring the hardened depth from transverse cross sections of processed samples and comparing the results to data obtained by the model. A good estimation of the hardened depth was achieved when the traverse rate was equal to or above 3.3 mm s−1. Hardening experiments were performed to 34 commercial steel grades. 7 steels were studied in detail to establish the phase transformations occurring during the hardening process. Based on these experiments, a model for calculating the surface hardness was developed. The model takes data from equilibrium diagrams. A kinetic term to express the rate of change during hardening was introduced to relate equilibrium data to the non-equilibrium process. A good correlation between the model and experimental results was achieved with all materials, excluding martensitic stainless steels. Developed models are applicable, within the established limits, for most commonly hardened steels and processing parameters. The presented model for calculating the hardened depth is not bound to the laser and beam type used in the experiments. Input data for estimating the absorptivity and heat flow is based on the beam-material interaction time and laser power on work piece surface. It can, thereby, easily be converted to be used with other diode laser sources. Correspondingly, input data for the hardness calculations is obtained from pyrometric measurement and is not, thereby, limited to the laser type, wavelength, beam profile or processing parameters.

Modelo numérico tridimensional para la simulación de procesos de tratamiento superficial de materiales con láser

A numerical model for the predictive assessment of laser transformation hardening of steels and other surface treatments both in metallic and non-metallic materials has been developed. Starting from material and geometry data, the problem of heat diffusion from the thermal source provided by the laser is solved with complete space and temperature-dependence. The developed 3D simulation model has been successfully checked against theoretical and experimental results both for metallic and non-metallic materials. Typical examples of its application have been the thermal surface hardening of steels and the analysis of thermal fields in low diffusivity materials (as optical glass) in laser polishing applications.

Laser transformation hardening of steel: effects of beam mode, beam size, and composition

Laser transformation hardening experiments have been carried out on three model steel alloys of nominal composition Fe–0·4C, Fe–0·4C–1·0Mn, and Fe–0·4C–1·0Si (wt-%). A 1 kW continuous wave laser with a near Gaussian beam and a 5 kW continuous wave laser fitted with a beam integration device have been employed. Thus, it has proved possible to isolate the effects of alloy chemistry and beam mode on the kinetics of the hardening process. The dimensions of the hardened zones have been characterised and the results compared with calculations made using process models. It is shown that the results from experiment and theory are in broad agreement. Hardened zones resulting from the uniform source had a larger width/depth ratio than those resulting from the Gaussian source, although the effect was not as enhanced as suggested by theory. A comparison of the Fe–C, Fe–C–Mn, and Fe–C–Si specimens revealed that manganese and silicon increased the width of the transformed zone, and that this effect was more significant at higher processing speeds.