Vacuum carburising and high pressure gas quenching technology in automotive industry

Abstract

As the conventional gas carburising furnaces and oil quenching baths used for powertrain parts in the automotive industry have become outmoded, introduction of new equipment and technology is of increasing urgency. Recent demands to improve the mechanical properties of parts and productivity while minimising environmental impact have made vacuum carburising/high pressure gas quenching technology an attractive alternative. However, problems with over carburising and soot in vacuum carburising, and the low cooling ability and high running cost of high pressure gas quenching constitute barriers to adoption of this technology. Attempts to solve these problems by experiments and numerical simulation are reported.     

Plasma carburising: exotic with potential

Abstract

Low pressure carburising in combination with high pressure gas quenching has gained an increasing importance in the case hardening of highly stressed components. With the introduction of acetylene as carburising gas and the further development of the systems engineering, this technology has found broad acceptance in the heat treatment industry. Plasma carburising with methane as process gas was first introduced more than 20 years ago but has only been utilised in a few special cases on an industrial scale. The advantage of this type of low pressure carburising is the opportunity to achieve a partial case hardening by simply mechanical masking of the parts to be treated. The ALD Own & Operate GmbH Companies have a 10 year of practical experience with the partial plasma carburising process in the serial production of automotive and aerospace parts. By means of two examples the plasma processes and furnace equipment as well as the masking and the results of the treatment will be presented. Other possible applications of this heat treatment method will also be given.     

Effect of alloying elements on heat treatment response of aluminium high pressure die castings

Abstract

High pressure die cast (HPDC) aluminium components that respond to age hardening cannot normally be solution treated at high temperatures because the presence of internal porosity and entrapped gases leads to the formation of surface blisters. Parts may also become dimensionally unstable due to swelling. These factors that prevent heat treatment present significant limitations to the utilisation of HPDC components. Now it has been found that blistering and dimensional change can be avoided by using much shorter solution treatment times and lower temperatures. Experiments with alloys 360 (Al–9·5Si–0·5Mg) and 380 (Al–8·5Si–3·5Cu) have shown that strong responses to age hardening are still possible following these modified solution treatments. In the current paper, the role of critical alloying elements is considered in both current specification Al–Si–Cu–(X) alloys, and also in newly developed alloy compositions. It is shown that 0·2% proof strengths over 400 MPa may be readily achieved by heat treating conventionally produced die castings.     

On assessment of processing variables in vertical centrifugal casting technique

Abstract

The aim of the present study is to investigate the influence of the vertical centrifugal casting technique over mechanical and metallurgical properties of a hypereutectic Al–18Si alloy. Due to the inherent vibration of the centrifugal casting technique, and in order to study and understand the individual effects of the equipment vibration and the centrifugal force itself (pressure or fluid dynamics), as well as the combined effect of both, three different tests were performed: gravity casting, gravity casting with vibration and centrifugal casting. It was concluded that the metallurgical and mechanical properties of castings obtained by the centrifugal casting process depend on the combined effect of the centrifugal pressure and/or fluid dynamics and on the inherent vibration of the technique itself. Correlations between the different casting techniques and obtained mechanical and metallurgical properties are presented.     

Generation of compressive residual stresses by high speed water quenching

Abstract

According to the literature, rapid water quenching can create compressive residual stresses (RSs) near the surface and thereby produce a significant increase in the fatigue limit. This technique is called 'intensive quenching'. In the present paper, some results from a research project will be presented. The project was initiated by the technical committee 'quenching' of the German Heat Treatment Association to deal critically with this issue.

The focus of this paper is on the influence of the martensite transformation on RS generation. A process window was determined, within which one needs to quench fast enough, to get the surface temperature below the M_S temperature, before the maximum temperature gradient in a cylinder is reached, in order to get compressive RSs on the surface.     

Determination of the metal/die interfacial heat transfer coefficient and its application in evaluating the pressure distribution inside the casting during the high pressure die casting process

Abstract

High pressure die casting (HPDC) experiments were conducted on a 650 t cold chamber die casting machine to study the interfacial heat transfer behaviour between casting and die. A 'step shape' casting and two commercial alloys namely ADC12 and AM50 were used during the experiments. Temperature and pressure measurements were made inside the die and at the die surface. The metal/die interfacial heat transfer coefficient (IHTC) was successfully determined based on the measured temperature inside the die by solving the inverse heat transfer problem. The IHTC was then used as the boundary condition to determine the 3-D temperature field inside the casting. Based on the predicted temperature distribution, the pressure distribution inside the casting was evaluated by assuming that the transferred pressure from the plunger tip of the injection side to the casting is primarily influenced by the solid fraction of the casting. Reasonable agreement was found between the determined pressure values and the measured pressures at the die surface of the casting.     

Simulation of high pressure die filling of a moderately complex industrial object using smoothed particle hydrodynamics

Abstract

The present study reports on the extension of smoothed particle hydrodynamics (SPH) of high pressure die casting to realistic three-dimensional components. Predictions of the isothermal filling of a moderately complex die in 3D demonstrates the importance of flow separation off corners, edges and faces with high curvature and the non-intuitive order of fill resulting from complicated back flows. The free surface behaviour involves significant transient void formation and free surface fragmentation. The predictions are shown to be insensitive to the Reynolds numbers. Their accuracy is confirmed by comparing simulations with coarser and finer resolutions.     

Numerical simulation of molten metal flow under vacuum condition in high pressure and low pressure die casting process

Abstract

The vacuum analysis algorithm was developed to simulate the total system of high pressure die casting process including vacuum vent, cavity and plunger area. The various vacuum degrees (760, 650, 500, 250 and 60 mmHg) were artificially applied in cavity. The filling behaviours of molten metal under the applied vacuum conditions were simulated and compared with those of experiment. The filling amount in cavity was increased with the increase of applied vacuum pressure during partial shot experiments. The simulated filling behaviours of molten metal were relatively well agreed with those of experiment. Through the results of fluid flow simulation, the relationship of filling length and filling velocity with the variation of vacuum pressure was analysed respectively. And it applied to a real die casting product and the internal gas quantity of product was significantly reduced by modification of vacuum gate system.     

Application of novel heat treatments for aluminium high pressure die castings to industrially produced components

Abstract

High pressure die casting (HPDC) is widely used as a cost effective way to mass produce metal components that are required to have close dimensional tolerances and smooth surface finishes, accounting for ～50% of the aluminium castings produced worldwide. These components are not considered to be heat treatable by conventional means because the high temperatures involved with solution treatment cause surface blistering and dimensional instability. A new heat treatment procedure involving a truncated solution treatment at lower than conventional temperatures alleviates this problem and can significantly improve mechanical properties, in many cases, doubling the 0·2% proof strength after artificial (T6) aging. This may enable current HPDC parts to be redesigned to use less metal while still achieving the required performance. The cost of heat treatment can be easily offset by the reduction in metal content and productivity improvements which result in an overall lower cost of the part. The new process also creates opportunities to substitute for some other cast or wrought products with aluminium HPDC parts of lower weight and lower cost. Application of this heat treatment technology to a range of industrially produced HPDC components is discussed and the cost advantages are briefly considered.     

Control of weld composition when welding high strength aluminium alloy using the tandem process

Abstract

A new cost effective process for generating different weld element compositions has been examined. Utilising tandem welding technology, different series aluminium filler wires were mixed in a single weld pool with the result that the composition of the principal alloy elements, copper and magnesium were accurately controlled. Thermodynamic modelling was then used to predict an optimum weld bead composition for eliminating solidification cracking when welding Al2024. In order to validate the predicted target composition, the tandem process was used to control the composition of the weld bead. The presented results show that using this system to deposit a controlled ternary composition weld, solidification cracking was eliminated when welding highly constrained test pieces. In contrast, cracking was evident when using commercially available binary filler wires under the same conditions.     

Control of nitrogen content and porosity in gas tungsten arc welding of high nitrogen steel

Abstract

In welding of high nitrogen steel (HNS), it is essential to control the nitrogen content and porosity in the weld metal. In this paper, the influence of shielding gas composition and heat input on the nitrogen content and porosity in the weld metal of HNS was investigated by gas tungsten arc welding. The experimental results indicate that the weld nitrogen content increases as N₂ in the shielding gas is increased in the same heat input of welding. The weld nitrogen content decreases with increasing the heat input for pure argon used as a shielding gas, whereas it increases with increasing the heat input for the shielding gas including some nitrogen. The nitrogen pore can be avoided when the nitrogen content in the shielding gas is <4% in the heat input range of 528–2340 J mm^–1.     

Hydrodynamical phenomena in the process of laser welding and cutting

Abstract

The main objective of the paper is to outline the 'bridges' existing between the outcomes of fundamental researches and the results of investigations in the field of industrial laser materials processing (LMP). An analysis is presented on the models based on non-stationary hydrodynamic phenomena caused by deeply penetrating high power CW laser beam into materials. This is typical of laser welding (LW) and laser cutting (LC). A physical analysis pertaining to melt removal and melt layer instability mechanisms of gas jet assisted CW–CO₂ laser fusion cutting is presented. The models deliberated here are melt squeezing out by gas pressure gradient, melt dragging by the friction force between melt surface and gas flow, formation of moving shelves at the cutting front. In case of high laser intensity, radiative flux interacts with material causing dynamical thermal transport onto the surface and phase transition at solid–liquid–gas interfaces. The solution is based on the non-stationary variables. Under these conditions the Mach number varies significantly due to laser intensity associated with laser flux energy instabilities. The connection among material surface temperature, laser intensity, laser flux and pressure in the plasma cloud is brought out. In addition, novel mechanisms based on hydrodynamics are proposed.     

Development of low spatter CO2 arc welding process with high frequency pulse current

Abstract

Metal transfer phenomena and spatter generation in CO₂ arc welding with a solid wire were investigated, and a low spatter welding process using a high frequency pulse rectangular current was developed. The optimal conditions of high frequency pulse CO₂ arc welding were determined to be a peak current of 450–550 A and pulse frequency of 450–750 Hz. These high frequency pulse currents influenced the droplet oscillation due to resonance between the applied pulse frequency and the natural frequency of the droplet. A droplet was regularly transferred by 9–11 pulses, and the average interval of metal transfer was ～16 ms, which was half of that in conventional CO₂ arc welding. The average droplet weight is 34 mg, showing a large reduction in comparison with that of the conventional method. As a result, the total spatter weight was reduced by 70% in comparison with the conventional method, and particularly large spatters more than 0·5 mm in diameter were reduced from 25 to 3 mg s^?1.     

Advantages of low pressure carburising and high pressure gas quenching technology in manufacturing

Abstract

Over the last decade, vacuum carburising in combination with high pressure gas quenching (HPGQ) has become a preferred technology in gear industry mainly in Europe and North America. Driven by cost savings in manufacturing of gears and shafts, the heat treatment process has gone into the focus of the manufacturers. The potential of savings in heat treatment is huge because the new technology allows the integration into the manufacturing chain of gears and shafts. With vacuum heat treatment furnaces is it possible to implement this integration into manufacturing. The advantages of vacuum technology are in particular: the absence of surface oxidation, the cold wall technology, the gas quenching technology, the reduced logistics, flexible reaction on the needs of production and the control of distortion. In parallel to the development of the new heat treat process, a second point came into the focus of manufacturers: the choice of material. The industry recognised that by choosing a slightly higher alloyed material, significant savings in the entire manufacturing chain can be realised: smaller grinding stock, faster carburising cycles, gas quenching with control of distortion, and ultimately the reduction/elimination of grinding- and straightening operations. Vacuum furnaces are flexible in their reaction to the production requirements. Only with these types of heat treatment furnaces is it possible to switch them off after use and save immediately energy and costs. This benefit was essential for the industry in 2008/09 during the world economic crisis. In the past, the automotive industry in Europe and North America mostly ran on conventional pusher type furnaces which must be kept on operating temperature even if only 50% of installed production capacity is needed. The vacuum furnace of type ModulTherm is a multi-chamber system and each chamber can be switched On or Off according to the production plan which finally saves a huge amount of cost for energy. This paper presents the advantages of the vacuum carburising technology with high pressure gas quenching. The author will demonstrate with examples and comparisons the benefits of vacuum technology and the successfully integration of heat treatment in the manufacturing chain.     

工业机器人精密减速器输入轴的真空低压渗碳高压气淬工艺

针对轴齿类零件高温渗碳淬火热处理畸变问题,研究了工业机器人减速器精密轴齿类零件真空低压渗碳和高压气淬工艺。结果表明,在升温阶段采用阶梯式升温保温方式,强渗阶段以乙炔-氮气交替脉冲进行强渗和扩散,淬火冷却阶段精确控制氮气压力1.8 MPa(18 bar)并使之稳定,可使轴齿类零件的总畸变量控制在0.005~0.015 mm。实际生产结果表明,轴齿类零件采用真空低压渗碳和高压气淬技术,渗碳层中的马氏体为1级,残留奥氏体和碳化物为1~2级,心部组织为1~2级。批量生产的减速器精密轴齿表面硬度、心部硬度和有效硬化层深度均值分别为59.7 HRC、38.6 HRC和0.681 mm,全部满足技术要求。     

热处理工艺对高压气瓶用34CrMo4钢屈强比的影响

为了改善高压气瓶用34CrMo4钢屈强比较高的问题,分别研究了调质处理(QT)、在浓度为7.5%PAG水溶性淬火剂中的淬火+回火(Q₁T)、亚温淬火+回火(IT)和淬火+亚温淬火+回火(QIT)4种不同热处理工艺对34CrMo4钢屈强比的影响,以及屈强比与微观组织之间的关系。结果表明:采用QT工艺得到回火索氏体组织,屈强比最高;采用Q₁T工艺得到较粗的回火索氏体组织,屈强比较高;采用IT工艺得到回火索氏体+块状及板条状铁素体两相组织,屈强比较低;采用QIT工艺得到回火索氏体+均匀分布的板条铁素体两相组织,屈强比最低。试样的组织为硬相回火索氏体上分布着软相铁素体时,有较低的屈强比。     

大长径比零件高压气淬温度场数值模拟

针对大长径比零件淬火均匀性和变形控制这一难点问题,采用数值模拟方法研究高压气淬技术对大长径比零件温度场的影响规律。结果表明,气体的淬火压力越大、流速越快、换热能力越强,工件冷却速度越快。采用上下交替吹风的静态交变流型不会对工件的冷速产生明显影响,但有利于改善温度场均匀性,且上下交替吹风时间间隔越短,工件的温度场越均匀。     

Numerical simulation of high pressure gas quenching of H13 steel

Aided by the computational fluid dynamics software package FLUENT flow and heat-transfer model has been established to simulate the high pressure gas quenching process of a large H13 die. The complicate geometry mesh of finite volume method (FVM) simulation was exactly built according to the practical chamber set-up of vacuum furnace. The velocity and temperature distribution of gas, as well as the temperature field in H13 die, were obtained by simulation. The validation of simulation results has been carried out by comparing the simulated cooling curves of certain points inside the die with the measured ones. It can be found that the temperature depended thermal physical properties of gas and H13 die, such as thermal conductivity, specific heat and viscosity, have dramatic effects on the accuracy of simulation results. The possible improvement of the numerical simulation based on the detailed discussion is also elucidated.     

Three-dimensional turbulent weld pool convection in gas metal arc welding process

Abstract

A three-dimensional model has been developed to study turbulent fluid flow and heat transfer in a gas metal arc weld pool. The phase change process during melting and solidification is modelled using the enthalpy–porosity technique. Mass and energy transports by droplet transfer are considered through a thermal analysis of the electrode. The droplet heat addition into the molten pool is considered to be in the form of a volumetric heat source distributed in an imaginary cylindrical cavity within the weld pool ('cavity' model). A two-equation k-&epsi; model capable of addressing turbulent weld pool convection, taking into account the morphology of the phase change, is presented. The weld pool dynamics and geometry (shape and size) in a moving gas metal arc welding (GMAW) process are studied and the effects of enhanced diffusivities on the turbulent weld pool are discussed. The predicted weld pool geometry using laminar and turbulent models is also compared with corresponding experimental post-weld sections.     

Numerical simulation on vacuum solution heat treatment and gas quenching process of a low rhenium-containing Ni-based single crystal turbine blade

Numerical heat-transfer and turbulent lfow model for an industrial high-pressure gas quenching vacuum furnace was established to simulate the heating, holding and gas fan quenching of a low rhenium-bearing Ni-based single crystal turbine blade. The mesh of simpliifed furnace model was built using ifnite volume method and the boundary conditions were set up according to the practical process. Simulation results show that the turbine blade geometry and the mutual shielding among blades have signiifcant inlfuence on the uniformity of the temperature distribution. The temperature distribution at sharp corner, thin wal and corner part is higher than that at thick wal part of blade during heating, and the isotherms show a toroidal line to the center of thick wal. The temperature of sheltered units is lower than that of the remaining part of blade. When there is no shelteration among multiple blades, the temperature distribution for al blades is almost identical. The lfuid velocity ifeld, temperature ifeld and cooling curves of the single and multiple turbine blades during gas fan quenching were also simulated. Modeling results indicate that the loading tray, free outlet and the location of turbine blades have important inlfuences on the lfow ifeld. The high-speed gas lfows out from the nozzle is divided by loading tray, and the free outlet enhanced the two vortex lfow at the end of the furnace door. The closer the blade is to the exhaust outlet and the nozzle, the greater the lfow velocity is and the more adequate the lfow is. The blade geometry has an effect on the cooling for single blade and multiple blades during gas fan quenching, and the effects in double layers differs from that in single layer. For single blade, the cooing rate at thin-waled part is lower than that at thick-waled part, the cooling rate at sharp corner is greater than that at tenon and blade platform, and the temperature at regions close to the internal position is decreased more slowly than that close to the surface. For multiple blades in single layer, the temperature at sharp corner or thin wal in the blade that close to the nozzles is much lower, and the temperature distribution of blades is almost paralel. The cooling rate inside the air current channel is lower than that of at the position near blade platform and tenon, and the effect of blade location to the nozzles on the temperature ifeld inside the blade is lower than that on the blade surface. For multiple blades in double layers, the lfow velocity is low, and the lfow is not uniform for blades in the second-layer due to the shielding of blades in the ifrst-layer. the cooling rate of blades in the second-layer is lower than that in the ifrst-layer. The cooling rate of blade close to the nozzles in the ifrst-layer is the higher than that of blade away from the nozzles in the second-layer, and the temperature distribution on blades in the same layer is almost paralel. The cooling rate in thin wal position of blade away from the nozzles is larger than that in tenon of the blade closer to the nozzles in the same layer. The cooling rate for blades in the second-layer is much lower both in thin wal and tenon for blades away from the nozzles.