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Effects of CO2 Laser Surface Hardening in the Unlubricated Wear of Ductile Cast Iron Against Alumina
Abstract
The microstructure of a ferrito-pearlitic ductile cast iron has been modified by CO2 laser surface hardening. Analysis of the laser-processed surfaces showed a dramatic increase in microhardness. Dry sliding wear of laser-treated specimens against an alumina counterbody has been investigated by ''ball-on-disk'' testing. The evolution of the wear coefficient, as well as metallographic observations, revealed an oxidational wear mechanism. The wear resistance of the laser-treated samples was significantly enhanced. The laser-treated cast iron has a better resistance to abrasion and plastic enhanced. The laser-treated cast iron has a better resistance to abrasion and plastic deformation. The improvement of the wear resistance was due to the fine and homogeneous microstructure produced after laser-treatment. Wear plots showing the evolution of wear coefficient with normal load, sliding velocity, and humidity have been established. The wear of the laser-treated cast iron is not influenced by the variation of operating conditions (normal load, sliding velocity, and relative humididy).
... Researchers have reported that the laser-hardened layers have high wear resistance and are suitable for use in vehicle dies. Papaphilippou et al. [12] applied a surface hardening process using the CO 2 laser to change the microstructure of ferro-pearlitic spherical cast iron. In the study, it was reported that the laser-applied surfaces of the material have higher surface hardness and, as a result, better resistance to abrasion and plastic deformation. ...
This study describes the laser surface hardening process effect on microstructures produced and their wear behavior. Ductile iron samples were austempered at three different austempering temperatures: 232 C/288 C/398 C respectively. Then, the laser surface hardening process was applied on each sample. Different laser gaps were designed: 1.5mm/3mm/4 mm. The microstructures were observed using optical microscopy and tribo-tests were run using a UMT-3 tribo tester. A Rockwell hardness tester was used to measure the hardness after heat-treatment. Optical microscopy and SEM were used to observe the different microstructures and their distribution on the worn surface. The results showed that the laser processing generated ledeburite, martensite, or tempered bainite microstructures. Vickers hardness tests were carried out on these microstructures. The original needle-like microstructures were observed on the samples with 4 mm laser gap but not with 1.5 mm or 3 mm laser gap. This is because part of the 4 mm laser gap is beyond the laser heat effected zone. Generally, severe ploughing wear and smearing wear were observed on the tempered bainite zone but not the bainite zone. Debris was generated during the tribo-tests and led to three body abrasive wear which not only resulted in higher wear loss, but also polished the worn surface.
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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.
The sliding wear characteristics of cast irons having a range of compositions and initial graphite forms have been determined in both as-cast and laser-surface-melted conditions using a pin-on-ring test configuration. Observed differences in equilibrium wear behaviour between the as-cast alloys were principally in the mild-to-severe transition load and the nature of the severe wear process. Such effects are interpreted in terms of the mean interparticle spacing of graphite in the microstructure which determines the relative propensity for subsurface crack propagation during wear. The ledeburitic structures produced by laser surface melting of the cast iron substrates acted to stabilize a regime of mild equilibrium wear with substantially lower wear rates than for the mild oxidative wear of the as-cast microstructures. Metallographic observations of the laser-melted layer have identified a wear process consisting of fine polishing abrasion.
The unlubricated wear of ductile cast iron sliding against an alumina counterbody under mild pin-on-disc testing conditions was studied. Wear plots showing the evolution of wear coefficient with normal load, sliding velocity and humidity have been established. The evolution of the wear coefficient as well as metallographic observations revealed a three stage oxidational wear mechanism. A decrease in humidity resulted in an increase of wear coefficient, mainly owing to abrasive debris behaviour. The removal of wear debris during testing helped in understanding its role in the wear process of ductile cast iron. This role appeared to be both detrimental under dry and beneficial under humid conditions respectively.
Laser induced microstructural modification of ductile iron has been studied as a function of processing parameters such as power density and beam–substrate interaction time. The high energy density CO2 laser source changes and refines the microstructure of the near surface layer, leading to enhanced hardness and wear resistance. Two basic types of microstructure are produced by laser processing, depending on the solidification/cooling rate of the melted zone. High solidification/cooling rates produce a mechanically metastable austenitic matrix, and a dendritic microstructure with interdendritic cementite films. Low solidification/cooling rates produce a very hard, lamellar ferrite+cementite microstructure. A mixed microstructure of intermediate hardness is produced at intermediate cooling rates. Detailed optical and electron microscopy studies of these microstructures as a function of laser processing parameters are reported.MST/701
The surfaces of nodular and gray cast iron specimens have been modified by CO2 laser processing for enhanced hardness and erosion resistance. Control of the near-surface microstructure was achieved primarily
by controlling resolidification of the laser melted layer through variations in laser beam/target interaction time and beam
power density. Typical interaction times and power densities used in this study were 5 msec and 500 kW/cm2. Analysis of the laser melted surface showed a dramatic increase in hardness and a greatly refined microstructure. Depending
on the processing parameters, two basic kinds of microstructure can be produced in the laser hardened layer—a feathery microstructure
with a very high hardness (up to 1245 HV) and a dendritic microstructure with a metastable, fully austenitic matrix and a
lower hardness (600 to 800 HV). Erosion testing was done in a rotating paddle device using slurries of SiO2 or SiC in water. Weight loss and crater profile measurements were used to evaluate the erosion characteristics of the various
microstructures. Both ductile and gray cast iron showed marked improvement in erosion resistance after laser processing.
Pin-on-disc wear and friction of hypereutectic ductile iron, the type employed for automotive components, was investigated at sliding speeds of 5 and 7.5 m s−1, before and after laser surface treatment by CO2 continuouswave and Nd-YAG pulsed lasers. A significant increase in transition load and wear resistance upon laser treatment has been attributed to the ultrafine microstructure and high hardness; laser-melted ledeburite was superior to martensite by transformation hardening. Wear rate at a specific contact pressure and sliding speed bears a log-linear relationship with the harmonic mean of tensile and fatigue stress of ductile irons. The role of lubrication by graphite during mild wear and plastic deformation in severe wear of pearlitic ductile iron, and its enhanced resistance to plastic flow on laser melting, have been confirmed. The coefficient of friction of a ductile iron pin sliding on a steel disc before and after laser melting has been determined and compared with that of white iron of identical composition and structure obtained by conventional chilling.
Spot Laser Hardening,” (submitted for publication to the
- C Papaphilippou
Laser Surface Modification of Ductile Iron
- C H Chen
- Rigsbeej P M Juc