Hardening of 316L stainless steel by laser surface alloying

The goal of this study is to investigate different hardening routes for 316L stainless steel by laser surface alloying. We have investigated the formation of iron-chromium carbides by SiC or carbon incorporation, the alloying with submicronic particles of TiC and the precipitation of titanium carbide from mixtures of Ti and SiC. For each hardening route we present the microstructures and the hardness of the processed surface alloys and the conditions leading to the best compromise between highest hardness, best homogeneity and lowest occurrence of cracks. From these results it can be reasoned that hardening by iron-chromium carbides is the best hardening route and that this surface alloy might be a good candidate for tribological applications.     

Development of microstructure in laser surface alloying of steel with chromium

Laser surface alloying (LSA) was used to formin situ Fe-Cr-C alloys on AISI 1018 steel substrates. Chromium powders of different particle sizes were mixed together to obtain optimum packing density of the powder deposited on the substrate. The surface was then melted using a 2kW CW carbon dioxide laser. The processing conditions were related to solute (chromium) content, microstructural refinement of the laser alloyed zone and the heat affected zone (HAZ). The microstructure of the laser surface alloyed region was investigated by optical, scanning and transmission electron microscopy, and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying element. This process produced a fine distribution of complex type of carbide precipitate in the martensite-ferrite matrix because of the high cooling rate. An alloy of this composition does not show any retained phase. The nature of alloying and chemical diffusion profile as a function of intertrack separation distance affects the final content of alloying element in the surface layer.     

Thermal oxidation-resistant surface alloys processed by laser alloying on 35 NCD 16 ferritic steel

The hardness and the high-temperature oxidation resistance of the low-alloyed ferritic steel 35 NCD 16 (Fe-0.38 C-1.8 Cr-4 Ni, wt%) were increased by laser surface alloying of Cr₃C₂ or Cr₃C₂ and SiC. The obtained surface alloys always exhibit primary dendrites ( Fe solid solution with Cr and Si + martensite) and an interdendritic eutectic containing Fe + martensite + M₇C₃ carbides (M=Fe, Cr). In the first case, the formation of iron chromite, FeCr₂0₄, in contact with the coating accounts for the good oxidation resistance. In the second case, the beneficial influence of silicon lies in the formation in the presence of oxygen of a thin chromia scale only, with local silicon enrichment, showing excellent barrier properties. Kinetic and structural observations are discussed in the light of thermodynamics and diffusion processes.     

Microstructures of titanium-aluminides produced by laser surface alloying

The microstructures of Ti-Al layers (from 43–80 at %Al) produced by laser surface alloying of titanium substrate with a powder feed technique have been investigated. The laser processing parameters were; 1.8 kW laser power, 3 mm beam diameter, 7 mm s^–1 traverse speed, and values of powder flow rates of aluminium ranging from 0.07–0.11 g s^–1. The microstructures were dendrites of ₂ and interdendritic regions of ₂+ in the Ti-43 at %Al alloy; dendrites of either ₂ or ₂+ with interdendritic in the Ti-50 at %Al alloy; dendrites of ₂+ with interdendritic in the Ti-55 at %Al alloy; single phase in the Ti-60 at %Al alloy and TiAl₃ dendrites and Al solid solution in the interdendritic regions in the Ti-80 at %Al alloy. The microstructures were fine and comparable to those produced by other methods of rapid solidification processing. The microstructures of the Ti-50 and Ti-55 at %Al alloys were in agreement with the existence of the peritectic reactions:L + andL + , in the Ti-Al system.     

Development of wear resistant composite surface on mild steel by laser surface alloying with silicon and reactive melting

The present study concerns laser surface alloying with silicon of mild steel substrate using a high-power continuous wave CO₂ laser with an objective to improve wear resistance. The effect of surface remelting using nitrogen as shrouding environment (with and without graphite coating) on microhardness and wear resistance has also been evaluated. Laser surface alloying leads to formation of a defect free microstructure consisting of iron silicides in laser surface alloyed mild steel with silicon and a combination of silicides and nitrides when remelted in nitrogen. Carbon deposition prior to remelting leads to presence of a few martensite in the microstructure. A significant improvement in microhardness is achieved by laser surface alloying and remelting to a maximum of 800 VHN when silicon alloyed surface is melted using nitrogen shroud with carbon coating. A detailed wear study (against diamond) showed that a significant improvement in wear resistance is obtained with a maximum improvement when remelted in nitrogen atmosphere followed by carbon coating.     

Transport phenomena in laser surface alloying

A three dimensional, transient model is developed for studying heat transfer, fluid flow and mass transfer for the case of a single-pass laser surface alloying process. The numerical study is performed in a co-ordinate system fixed to the laser which moves with a constant scanning speed. The coupled momentum, energy and species conservation equations are solved using a finite volume technique. Phase change processes are modelled using a fixed-grid enthalpy-porosity technique, which is capable of predicting the continuously evolving solid-liquid interface. The three-dimensional model is able to predict the species concentration distribution inside the molten pool during alloying, as well as in the entire cross section of the solidified alloy. Corresponding experimental results show a good qualitative agreement with the numerical predictions with regard to pool shape and final composition distribution.     

Formation of hard microcrystalline layers on stainless steel by laser alloying

Abstract

One possibility for surface treatment using lasers is to cause significant changes in surface composition and microstructure. However, techniques for changing composition remain poorly developed and the mechanisms controlling final microstructure are still not fully understood. A very simple technique for supplying alloying additions has been used in the present work to modify a ferritic stainless steel and has been shown to be capable of causing large changes in composition. As-alloyed layers remain fairly inhomogeneous and a second melting is necessary, at which time treatment conditions can be selected to obtain the required microstructural state. There is a relationship between the formation of the microcrystalline layers observed and the cooling rate and the formation of the microcrystalline layers can be analysed in terms of continuous crystal nucleation. As expected, the abrasive wear resistance of the surface layers improved as a result of the increase in hardness.

MST/909     

Non-equilibrium solidification by laser surface alloying of molybdenum with nickel: Structure and properties of alloyed layers

Surface alloying was carried out on molybdenum using continuous wave Nd : YAG and CO₂ lasers. The alloying element (nickel) was deposited on the molybdenum surface by plasma spraying. Microstructure evolution and phase identification was investigated by means of optical and scanning microscopy and X-ray microanalysis. Layers with an average content of about 33% at. of nickel were obtained. Rapid solidification with fine scale microstructures and non-equilibrium nickel rich phases was reported. Molybdenum oxides with a suggested composition of the MoO₂ were also identified. In addition the results of investigations of the alloyed layer microhardness and the layer resistance to erosion in an electric arc have been presented.     

Phase composition of the surface layers of high-speed cutting steel after electrospark alloying and laser treatment

Due to the metallurgical processes occurring under the action of laser irradiation, which are accompanied by convectional mass transfer, phase formation, and the interaction of alloying elements with the base material and elements of the environment, tungsten carbide fully dissolves in the steel melt in the case where alloying was carried out with alloys based on this carbide. Moreover, the refractory component of tungsten-free electrode materials stayed completely unaltered. by partially losing a non-metal. These transformations occur under nonequilibrium conditions. To improve the tribotechnical properties and durability of a high-speed cutting steel upon cutting, it is reasonable to alloy its surface layer with elements and compounds which form stable oxides strongly bound to the base. Frantsevich Institute of Problems of Materials Technology, the Ukrainian Academy of Sciences, Kiev. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 36, No. 1, pp. 83–86, January–February, 2000.     

Ni-based superalloy surface alloying by double-glow plasma surface alloying technique

The double-glow surface alloying technique, also called the Xu-Tec/Xu-Loy process, is a novel technique in the field of surface alloying. This technique allows alloy layers with unique physical, chemical and mechanical properties, such as nickel-based alloy layers, stainless-steel layers and age-hardened surface high-speed steel layers to be formed at the surface of treated metallic materials. In this paper, recent research of the application of the double-glow plasma surface alloying technique in the formation of corrosion resistance alloy layers is briefly reviewed. The results of a study of Ni-Cr-Mo-Nb and Ni-Cr-Mo-Cu corrosion-resistant alloying layers as well as composite alloying layers with an electric brush plating Ni interlayer are reported.     

低碳钢的激光除锈机理及表面性能研究

激光除锈技术是一种新型、绿色环保的除锈方法。对一些在极易生锈的工作环境中的低碳钢构件,采用激光除锈技术代替传统除锈方法,有广阔的发展前景。激光除锈技术主要利用辐射在锈蚀表面的激光能量高、脉冲短的特点,使锈蚀温度很快达到熔点以上。但在除锈的同时,会有部分激光透过锈蚀层直接与金属基底作用,以及辐射在锈蚀表面的激光也会通过热传导将部分能量传递到金属基底表面。本文采用实验分析手段,对金属基底表面的微观组织、力学性能、硬度等进行对比研究。结果表明,所采用的激光除锈工艺在获得良好的除锈效果情况下,对金属基底没有造成损伤,对金属基底表面性能没有产生显著影响.     

Improving the wear resistance of AA 6061 by laser surface alloying with NiTi

To improve the wear resistance of a popular aluminum alloy AA 6061, a 1.5 mm thick hard surface layer consisting of Ni–Al and Ti–Al intermetallic compounds was synthesized on the alloy by laser surface alloying technique. NiTi powder was preplaced on the aluminum alloy substrate and irradiated with a high-power CW Nd:YAG laser in an argon atmosphere. With optimized processing parameters, a modified surface layer free of cracks and pores was formed by reaction synthesis of Al with Ni and Ti. X-ray diffractometry (XRD) confirmed the main phases in the layer to be TiAl₃ and Ni₃Al. The surface hardness increased from below 100 HV for untreated AA 6061 to more than 350 HV for the laser-treated sample. Accompanying the increase in hardness, the wear resistance of the modified layer reached about 5.5 times that of the substrate.     

Influence of laser surface alloying with chromium and nickel on corrosion resistance of type 304L stainless steel

Abstract

The influence of laser surface alloying (LSA) with Cr and Cr + Ni on the corrosion behaviour of type 304L stainless steel (SS) was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in chloride (0·5M NaCl) and acidic (1 N H₂SO₄) media. Surface alloying was carried out by laser cladding type 304L SS substrate with premixed powders of AISI type 316L SS and the desired alloying elements. The results indicated that Cr surface alloyed specimen exhibited a duplex (γ + α) microstructure with Cr content of ～24 wt-%, whereas Cr + Ni surface alloyed specimen was associated with austenitic microstructure with Cr and Ni contents of ～22 wt-% each. The potentiodynamic polarisation results in chloride solution indicated that LSA with Cr + Ni considerably enhanced the pitting corrosion resistance compared with LSA with Cr alone. In acidic media, such beneficial effects were not observed. Electrochemical impedance spectroscopy results showed an increase in semicircle arc for both chloride and acidic media for both Cr and Cr + Ni clad samples indicating improvement in the oxide film stability compared with untreated specimen. The polarisation resistance was higher and capacitance values of the laser clad specimen were lower than those in the untreated specimen. The microstructural changes and compositional variations produced by LSA are correlated to the corrosion behaviour.     

Processing aspects of laser surface alloying of titanium with aluminium

Abstract

An investigation has been made of the laser surface alloying of titanium using a continuous feed of aluminium powder. By means of a continuous wave CO₂ laser operating at 1·8 kW power, with a beam diameter of 3 mm, ranges of traverse speed (from 3 to 20 mm s^?1) and aluminium powder feed rate (from 0·03 to 0·11 g s^?1) were used to produce a series of alloyed zones with aluminium contents in the range ~20–80 at.-%. Conditions for obtaining reasonable homogeneity and reproducibility of composition were determined.

MST/1353     

High-temperature oxidation resistance improvement of titanium using laser surface alloying

Commercially pure titanium substrate was coated with aluminium by flame spraying or arc spraying techniques and was irradiated with a laser to form a laser-alloyed zone (LAZ). The microstructure of the LAZ was investigated. High-temperature oxidation tests were carried out in air at different high temperatures and the characteristics of the oxide layer were investigated using Auger electron spectroscopy. The aqueous corrosion tests were carried out in 3.5% NaCl solution at room temperature. The laser-processed material showed considerable improvement in the high-temperature oxidation-resistant property while the aqueous corrosion-resistant property of the LAZ was found to remain almost unaffected.