Laser hardening of aluminum bronzes

The susceptibility of two-phase aluminum bronzes to harden by quenching is well known. This work deals with the features of hardening from liquid or solid phases of several bronzes with 9-11% Al under the action of CO₂ continuous wave laser beam with a power of 1300 W coupled with a table in x-y-z coordinates digitally controlled. In order to determine the hardening conditions, the influence of the processing parameters on the geometry of the hardened layer and surface hardness was analyzed. By the microstructural analysis of the hardened layer, the hardening degree measured by the microhardness meter and structural changes were correlated. The microhardness profile on the depth of the heat-affected zone was traced. The range of the analyzed bronzes points out the influence of the initial condition and of the Ni, Fe, and Mn alloy elements contents on the microstructure, size, and hardness of the hardened layer. The effect of subsequent tempering was investigated.     

Predictive modeling of multi-track laser hardening of AISI 4140 steel

Laser hardening provides benefits over the conventional hardening processes, including minimum distortion in the parts and the absence of a quenchant. This process is also faster than conventional hardening processes and can be used for selective hardening of specific areas of components. One known problem with laser hardening in steels, however, is back tempering when a large area is hardened by multiple, overlapping passes. This study focused on the development of a numerical model to predict the back tempering in multi-track laser hardening. A tempering model was combined with existing models of thermal behavior and phase change kinetics, which were developed earlier in the authors’ group, to predict three-dimensional hardness profiles after multiple track laser hardening. The combined model was first validated through multi-track laser hardening tests and then used to predict and optimize the laser hardened case depth in multi-track laser hardening of AISI 4140 steel. The predictions and parameters optimized to obtain maximum case depth with the least variation along width of the hardened zone were experimentally verified. Case depths up to 2 mm were obtained with 5 mm overlapping of laser tracks.     

Laser surface hardening of AISI 01 tool steel and its microstructure

Abstract

The effects of laser surface hardening on AISI 01 tool steel samples were studied by changing the laser operating parameter combinations and the initial steel microstructure. Both melted and solid state transformed regions were produced, and then studied using optical microscopy, analytical electron microscopy, X-ray diffraction, and measurements of micro hardness to investigate the hardening mechanisms and the development of compressive residual stresses. The results indicate that hardened case depths up to 0·6 mm can be obtained using a laser beam operated at a power of 500 W and a scan rate of 2·1 mm s^?1, but that different amounts of retained austenite and undissolved carbides are observed for different beam powers. Quenched and tempered AISI 01 steel samples, with initial hardness values in the range 30–40 HRC, are better suited for laser surface hardening compared with the samples with initial hardness of 48–50 HRC, because the formation of an over tempered region adjacent to the hardened zone can be avoided.

MST/901     

Characterization of laser hardened steels by laser induced ultrasonic surface waves

Ultrasonic surface waves are suitable for the characterization of surface hardened materials. This is shown on laser hardened turbine blades. The martensitic microstructure within the surface layer of surface hardened steels has a lower surface wave propagation velocity than the annealed or normalized substrate material. Because the propagation velocity depends on the ratio of layer thickness to wavelengthd/, its measurement allows the determination of the hardening depth. If the surface wave frequency is high enough, the surface wave propagates mainly within the hardened layer. A correlation of the surface wave velocity to the surface hardness has been found. Because the variation of the surface velocity in hardened steels is small, a high measurement accuracy is necessary to obtain the interesting hardening parameters with sufficient certainty. Therefore, a measuring arrangement has been developed where laser pulses, guided by optical fibers to the surface hardened structure, generate simultaneously surface wave pulses at two different positions. The two ultrasonic pulses are received by a piezoelectric transducer. The surface wave velocity is obtained from the time delay between these pulses which is determined by the cross-correlation method. To evaluate simultaneously surface waves with different penetration depths from the same signal acquisition, digital filtering has been used in connection with the cross-correlation.     

Dot Matrix Hardening of Steels Using a Fiber Optic Coupled Pulsed Nd:YAG Laser

This paper describes a novel process called “Dot Matrix Hardening” as applied to Ol, D2 and AISI 4340 steels. This process uses a pulsed laser (particularly an Nd:YAG laser) to create a uniform distribution of transformation-hardened spots to cover only a certain percentage of the desired surface. Due to significantly reduced energy input, wear resistance can be imparted to thin and intricate parts without distortion. In addition, with the use of a coupled fiber optic beam delivery system, this process provides greater flexibility compared to conventional CO₂ laser hardening for a number of applications. The use of an Nd:YAG laser also eliminates the need of absorptive coating required for hardening with a CO₂ laser. With optimized processing parameters, a relatively uniform hardened layer is obtained within the hardened spot, with a thickness of about 60 um and hardness values around 800 HV₁₀₀. The sliding wear test results show that the wear resistance of Ol samples with only 20-40% area coverage of laser-hardened spots is similar to the 100% covered laser dot hardened sample as well as the furnace hardened (Re 60) sample.     

A semi-empirical method for predicting hardened case depths in laser heat treating

Some predictions on the hardness and hardening depths on laser heat treatment of steels can be obtained when specific characteristics of both laser processes (heating and cooling rates) and laser heat treated steels (microhardness profiles) are taken into account. Some controlled surface temperature laser heat treatments have been carried out with a medium power c.w. CO₂ laser on a medium carbon steel (AISI/SAE1045), allowing these predictions to be tested. In particular, knowing the surface temperature has enabled an analytical algorithm to be used to describe thermal processes and a simple exponential expression to be employed to carefully predict the hardened case depth.     

Laser Surface Alloying of (Pre-placed) SiC and Ni-SiC Coating on Commercially Pure Titanium

Results of laser alloying of 100% SiC and 50% Ni + 50% SiC on commercially pure titanium were presented in this investigation. The high hardness Hv800-1200 obtained at 100% SiC and 50% Ni + 50% SiC alloying conditions were due to the presence of various intermetallic phases such as TiC, TiSi, Ti₅Si₃ and NiTi₂. These intermetallic phases present in the laser alloyed surface were validated by EDXRD analysis and the diffusion of Ni, Si, C in titanium responsible for these phase formations was identified by SIMS study. The alloyed layer microstructure consists of dendrites and its density level depends on laser processing conditions. At low level power density the alloyed layer depth was about 0.5 mm with a constant hardness level, whereas at higher level powerdensity the depth of alloyed layer touched a maximum of 1.6 mm with large fluctuation in hardness.     

Laser Surface Hardening of 9CrSi Steel

The effects of laser hardening parameters such as beam power,beam diameter and scanning rate on microstructure and mardness of 9CrSi steel were investigated.The microstructure of the surface layer of 9CiSi steel was changed from pearlite to martensite,retained austenite and carbide by laser hardening .The depth of the hardened layer increased with increasing laser energy density and the surface hardeness increased by 3-5times as high as the untreated steel.The laser hardened surface had good wear resistance due to martensite and carbide in the surface layer.The wear mode at low speed was abrasive,while the wear mode at high speed was adhesive.     

Surface Hardened Corrosion Resistant Rolling Element Bearings

Through hardening bearing steels with low alloy content are sometimes surface hardened by induction hardening to implement compressive residual stresses into the surface. This method is not regularly applied for steels with high alloy content especially not for applications in the aerospace and space industry. Here carburizing grades are used if compressive residual stresses are required. For a protection against corrosion chromium plating is applied or a through hardening chromium steel is chosen which offers no compressive residual stresses. This work deals with the evaluation of surface hardening and corrosion resistant bearing components. For this the steels AISI 440C and Cronidur30 were investigated.     

Laser surface melting of low and medium carbon steels: influence on mechanical and electrochemical properties

The influence of a laser melting on both mechanical and electrochemical properties was studied in two carbon steels. Geometry and microstructure of the melt zone were first observed by optical and scanning electron microscopy, in various experimental conditions: with different interaction times and power density, for simple and multiple track melting. A large hardening was induced by the severe quenching resulting from rapid solidification; therefore the presence of hard points on the surface of the material may increase lifetime of specimens and improve tribological properties. This hardening created by the laser melting did not produce deleterious modifications of surface roughness or corrosion behaviour, at least in mildly corrosive saline solutions.     

Continuous Wave ND:YAG Laser Welding of Sand-Cast ZE41A-T5 Magnesium Alloys

A continuous wave 4 kW Nd:YAG laser system was used to weld 2-mm butt joints of sand-cast ZE41A-T5 magnesium alloys at a power of 2.5 kW, welding speed of 6.0 m/min, and defocusing distance from - 2 to + 3 mm for the material in the machined surface conditions. It was found that the adjustment of defocusing distance greatly influences the establishment of conduction or keyhole mode welding. Conduction welding is obtained at a power density of 4.0 × 10⁵ W/cm². Keyhole welding is reached at a threshold irradiance of 1.5 × 10⁶ W/cm². The fusion zone consists of refined equiaxed grains formed through cellular growth in the Zr-containing magnesium alloys. The partially melted zone is rather narrow, only a few grains wide. No grain growth or coarsening but softening is observed in the heat affected zone (HAZ). The weld defects observed include three main types: imperfect shape, cavities, and weld cracks. The mechanisms of their formations are discussed. In addition, the original cast quality was found to have a significant influence on the formation of defects such as underfill, surface depression, porosity, and burn-through during laser welding.     

Surface Topography Analysis When High-Speed Rough Milling Hardened Steel

This study is part of a bigger picture on investigating three main cutter path strategies—raster, single-direction raster, and offset—in order to evaluate the feasibility of employing high axial depths of cut (10 mm ≤ A_d ≤ 20 mm) when high-speed rough milling hardened AISI H13 hot work tool steel with the aim of achieving high volume of metal removed with short machining time. Here, comparative studies were made of the surface topography maps induced at various axial depths of cut in order to gain an in-depth understanding of their effects on the surface texture obtained via the parametric study of alternative cutter path strategies. Previous work has shown that the use of an offset cutter path strategy when high-speed rough milling hardened steel using an axial depth of 15 mm resulted in the lowest tool life, as compared with the use of raster and single-direction raster strategies.[1]This article also describes a novel approach on improving the offset cutter path strategy by selecting the proper entrance and exit conditions to improve surface texture. The effects of an offset strategy and a modified offset strategy are investigated in terms of surface roughness and surface texture.     

Laser Nitriding of Ti-5.0Al-2.i5Sn

Laser surface treatment provides excellent wear resistance with good oxidation and corrosion resistance. Laser surface nitriding is one such technique resulting in high surface hardness to a depth of a few microns. This can be carried out in pure nitrogen and dilute nitrogen environments. This paper investigates the effect of laser nitriding on Space Shuttle Main Engine (SSME) Ti-5.0Al-2.5Sn alloy under pure nitrogen environment. The nitriding was carried out using 3 kW CW CO2 laser at different laser powers 900 W, 1.0 kW, and 1.2 kW with scan speeds 0.5 m min^-1, 1.0 m min^-1 and 1.5 m min^-1 respectively. Optical microscopic and Vickers hardness tests were conducted on the test specimen to reveal the effect of laser nitriding in melt zone of laser nitrided trail. The extra high surface hardness of 3785 VHN at 25-50 (m depth was observed using the laser variable 1.0 kW laser power, 1.0 m min^-1 speed and 3 mm beam dia. This may be attributed to the TiN dendrite formation. The melt zone of laser nitrided trail at other processing parameters shows fine needlelike structure of alpha prime with larger grain size and alpha in the heat affected zone with smaller grain size, with an average hardness 450 VHN. This present investigation shows that the surface of the nitrided trail is free from any cracks, even under the pure nitrogen atmosphere for all laser processing conditions.     

Recrystallisation of austenite grain when non-isotherm steel working: Effects of thermal kinetics and deformation-based mechanisms

The effects of high temperature deformation on the recrystallisation of austenite grains and hardening occurring during hot forging of steels were studied. Three commercial steels containing various carbon weight percentages were heated beyond the austenitising temperature and free forged up to desired deformation ratios. The specimens were then air-, or oil-cooled. Two zones were distinguished according to the grain-size: a zone with fine grains, associated with highest plastic deformation and, a zone with coarse grains located within the subsurface layers. Unexpectedly, the highest values of microindentation hardness were obtained in the coarse-grain zone. Consequently, the interaction between the grain-size gradient induced by thermal kinetics of cooling and the local hardening governed by dislocation kinetics was studied by means of microindentation hardness inspections. Analysis of stress–strain curves confirmed that while forging enhances mechanical strength, it has a detrimental effect on ductility of steel.     

Endurance of VT1-0 Titanium Alloy Subjected to Solid-Solution Surface Hardening

We study the influence of solid-solution hardening (by the method of thermal-diffusion saturation from a rarefied oxygen-containing atmosphere) of the surface layers of VT1-0 titanium alloy on its fatigue resistance in bending with rotation. It is shown that the dependence of fatigue limit on the degree of surface hardening has a maximum whose value depends on the depth of the hardened zone. We managed to increase the original fatigue limit of the alloy by 38% for a relative increment of the surface hardness K of 70% and a depth of the hardened zone l of 30 μm. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 3, pp. 119–122, May–June, 2005.     

Optimization of Process Variables in Gas Carburizing—An Experimental Investigation with AISI 3310 Steel Material

Gas carburizing is the most widely used process for surface hardening of critical components used in automobiles, heavy duty machines, etc. In this process the surface composition of the carbon steel changes by diffusion of carbon and results in a hard outer surface with good wear resistance properties. Fatigue behavior of case hardened parts depends largely on the correct combination of its magnitude and depth of hardness penetration without undue distortion. An industrial survey indicates that there is a rejection of 10-12% of case hardened components due to defects like low hardness and strength after hardening, distortion, and warping. Optimal control of the process parameters is the way to ensure the quality of the surface hardened part. This paper is the result of the investigation carried out using limit design concept called Taguchi's Technique in the optimization of process variables in gas carburizing.     

On selective laser melting of ultra high carbon steel: Effect of scan speed and post heat treatment

Selective laser melting is a laser‐based additive manufacturing process applying layer manufacturing technology and is used to produce dense parts from metallic powders. The application of selective laser melting on carbon steels is still limited due to difficulties arising from carbon content. This experimental investigation aims at gaining an understanding of the application of the process on ultra high carbon steel, which is a special alloy with remarkable structural properties suitable for different industrial applications. The feedstock ultra high carbon steel (2.1% C) powder, 20 μm to 106 μm was prepared by water atomizing technique. This powder was used for the selective laser melting to build specimens 10×10×40 mm in dimensions. To decrease the thermal stresses during layer by layer building, laser scanning was done through 5×5 mm random island patterns while layer thickness was 30 μm. Laser beam diameter, maximum power output, layer thickness and scan speed range were 0.2 mm, 100 W, 30 μm and 50–200 mm/s respectively. The process was done inside high purity nitrogen environment, with less than 0.5% oxygen content. The results illustrate the influence of scan speed from 50 to 200 mm/s on product geometry and dimensions, surface roughness, internal porosity and cracks, microstructure and surface hardness. The effect of post heat treatment by heating and holding for one hour (annealing) at different temperatures of 700°C, 750°C, 950°C is studied. The results indicate that selective laser melting is able to produce near to 95% density of ultra high carbon steel parts with acceptable geometry and surface quality. Porosity cracks, and microstructure formed during the process could be controlled through proper selection of process parameters and post heat treatment. Industrial ultra high carbon steel products can be rapidly fabricated by selective laser melting.     

Yb:YAG laser welding of TRIP780 steel with dual phase and mild steels for use in tailor welded blanks

Advanced high strength steels (AHSS) are essential to meet the demands of safety and fuel efficiency in vehicles. In this paper, we present the results of laser welding of two AHSS steels, TRIP780 and DP980. A 2 kW Trumpf TRUDISK 6002^® Yb:YAG laser beam was utilized to join 1 mm thick TRIP780 with 1.5 mm thick DP980 and 1 mm thick mild steel. Optical metallography was used to characterize the weld profile and microstructures. Microhardness, tensile and fatigue tests were performed to evaluate the mechanical properties. Results indicate that the laser welds exhibit excellent strength and hardness with minimal defects which are attributed to the high beam quality, disk type of laser. In addition, there is a distinct effect of pre-straining of TRIP780 steels on the energy absorption.     

45 钢表面激光相变硬化改性组织及耐蚀性能

目的为了改善45钢表面状态,提高其表面性能,采用CO2激光器对其表面进行激光相变硬化处理。方法利用带有能谱的扫描电子显微镜(SEM/EDS)、盐雾试验机等,对激光相变硬化层组织及耐蚀性能进行了观察和分析。结果激光相变硬化层由熔化区、相变硬化区和热影响区三部分组成,其组织依次为:混合马氏体+未溶碳化物、针状马氏体、残余奥氏体。随扫描速度增加,耐蚀性先变好而后变差。结论激光相变硬化处理可改善45钢的表面性能,显著提高其耐蚀性能。