Abstract

Taking energy from renewable sources such as wind, water or sun becomes more and more important in the world. In that context, the wind power Industry becomes the main supplier of renewable electric energy. Because of the high operation costs, the modern wind power plants require proper operation throughout many years. To meet this requirement, all the parts, subassemblies used for manufacturing of wind power plants must be of high quality and durability. In the paper, we will describe furnaces and heat treatment processes that can be used In the production of large-size gearboxes, which are one of main elements of wind power plants construction.

Introduction

Renewable energy is becoming more and more frequently used worldwide. It is a good alternative to solve the problem of continuous increasing of the electric energy demand, decrease the quantity of carbon dioxide (CO₂) that the industry emit into the atmosphere and generate thousands of jobs opportunities. But not only this - the air, water, sun, etc., are sources that will now go away, so we can decrease or eliminate our dependency on the fossil fuel sources.

Of all renewable energy, wind energy is growing the fastest, more than 30% per year. The energy policies in the biggest economies in the world are promoting wind energy instead of the traditional ways to generate power. For example, the USA plans to generate 20% of the nation's electricity by wind energy until 2030. In that context, the main wind turbines manufacturers are already in business or they have plans to open new factories in the US, China or India. All this means new opportunities of business and new jobs for different industrial sectors.

The wind energy market represents a huge challenge for the heat treatment industry because of the gears and bearings necessary in the gearbox. With a gearbox you convert slowly rotating, high torque power which you get from the wired turbine rotor - into high speed, low torque power, which you use for the generator. Gearboxes are one of the most expensive, critical and important components of the wind turbine system. Hence the manufacturers invest in research of materials and heat treatment technologies looking for improving the gears quality, reliability, performance and durability.

In general, when we talk about gears for wind turbine gearboxes, we are talking about big energy transmission components. Usually for the biggest wind turbines we will need biggest gears. As we know, the heat treatment of gears is one of the processes during the manufacturing of gears. However, in the case of wind turbines, due to the fact that companies expect a wind turbine service life of up to 30 years and are pushing for warranties up to 10 years for gearboxes, it has become an increasingly important step in the manufacturing process. Currently, the gearbox cost represents approximately 12% of the overall cost. Figure 1 shows the wind turbine components and their share in the overall cost.

It is well known that gearbox is the component where more failures and breakdowns occur [4]. As a consequence of the continuous change at the wind's force, the gearbox should withstand high dynamics loads that can produce pitting in the gears-wheel and bearing breakdowns. The number of gear problems seem to be increasing and the size of the problem is showing that the knowledge of design, lubrication and heat treatment is insufficient.

Process, Requirements and Materials

The materials used in large gears are of Chromium-Nickel-Molybdenum Steels and Nickel-Molybdenum Steels. For example: planetary and bull gears, and pinions carburised are made of alloy steels as 18CrNiMo7-6, and the case depth is defined by HV550. The charge are getting at carburising temperature among 900-940?C, then are hardened at temperature of 830-840?C, and then tempering at 160-180?C to get the case depth required among 1.6-4.5 mm.

For other hand, alloys steels as 31CrMoV9 and 34CrNiMo6 are nitrided using the Floe (2 stages) process to get a case depth of 0.50-0.75 mm.

Large Charge Equipment - Technology Choices

To treat large gears the following equipment currently is used:

Batch Atmosphere Integral Quench Technology: In this furnace, parts are discharged from the furnace into a vestibule that covers an oil quench tank. Because the furnace atmosphere also flows through the vestibule, parts can be kept free of oxidation prior to quenching. Sealed-quench batch furnaces are capable of processing many different types of loads with widely varying case depth requirements.

Its application in large gears is relatively now because of the cost of the investment which is much more expensive than pit furnace or rotary hearth furnace. That way if the customer is interested in keeping a good quality, regardless of the cost of investment, and eliminating any possibility of decarbusizing, the batch atmosphere integral quench furnace is the solution. The Elterma AFS 34 furnace with useful dimensions of 1400 x 1000 x 200 mm (H x W x L) is shown in Figure 2.

Figure 3 shows the complete line of batch furnace integral quench. It is equipped with an endothermic atmosphere generator, tempering furnace and washer machine. Also, a loading and/or unloading mobile car is integrated in this line.Batch atmosphere integral quench furnace is equipped with a carbosystem temperature and carbon potential control system.

Pit Furnace: Pit Furnace is the most popular solution for carburising of large power transmission components. It can be electrically heated or gas fired. Some of the reasons why customers may prefer pit furnace are: they are tight, easy to operate and maintain, and they can be loaded with large charges.

The only problem of pit furnace appears during the quenching since the loads have to be moved from the protective atmosphere into the quenching tank. As a result, parts will be covered by an adherent black scale, which, depending on the needs of the application, may have to be removed by shot blasting or acid pickling [1]. However, this problem can be minimised with the installation of a bell that will cover the load during the transfer to the quenching tank. Also the bell will reduce the possible decarburization than can appear during the transfer to values practically depressible.

In the case of wind energy gears, we can offer pit furnace with the following useful sizes: Diameter - 1500, 1800, 2000, 2300, 2500 and 3050 mm and depth - 1500, 2000, 2500, 3000 and 3200 mm. However, customised versions of the furnace chamber dimensions as requested can be produced by Elterma.

Figure 4 shows an electrically heated Pegat type pit furnace with useful dimensions of 1800 mm in diameter x 3000 mm in depth. The load gross weight is 8000 kg. This furnace is operating in one of the main wind turbines manufacturers worldwide. From the economical point of view, the pit furnace is much cheaper than batch atmosphere integral quench furnace.

Auxiliary equipment that should be incorporated into the carburising pit furnace production tine includes: pit furnace for tempering and annealing, spray washing and spray-dip washing equipment, quenching tanks for hardening in hot oil and water, atmosphere generators, control cabinets and special instrumentation. Figure 5 shows a complete Elterma's pit furnace line for large gears used in wind turbine gearboxes.

Elterma also supplies pit furnaces for nitriding. Nitriding is a low-distortion hardening melted, since no quenching is required, where the whole heat treatment process is made below the transformation temperature. In terms of process parameters, it is relatively easy to control. The advantage of nitriding over carburising is that the piece will finish with a very low residual stress as well as a low distortions for case depth up to 0.75 mm. Zero Flow is a new technology for nitriding applications that is being offered by Elterma and Seco/Warwick. Traditional nitriding has very limited control over the growth of the nitrided layer. Now, a new economical, environmentally friendly version of the gas nitriding process allows a substantial reduction in the consumption of industrial gases compared with currently used processes and allows maintenance of full control over the kinetics of the layer growth.

Rotary Hearth Furnace Technology: The construction of this furnace combines advantages of both continuous and chamber furnaces. Elterma's rotary furnace ensure continuous work flow in a compact design that uses a minimum of floor space. The operation of this furnace is flexible, has a high temperature uniformity, an overpressure control, a control over the hearth rotation speed and can be fully automatic. Rotary hearth furnace can work together with a manipulator which charge and discharge automatically the hardening machine.

Figure 6 shows a rotary hearth furnace with a useful chamber of 2000 mm in diameter.

Pusher Furnace Technology: Pusher-type furnaces are by far, the most widely used continuous furnaces for gaseous carburisation. These furnaces are quite versatile and, depending on the size and shape of parts and on permissible distortion, parts may be loaded randomly and free quenched in an elevator-type quench tank. In many cases, washing and tempering equipment is incorporated to provide a fully automated heat-treating line.

Elterma's pusher furnaces can incorporate any or all of the following equipment, depending on the customer's requirements: loading-unloading stand, pusher hardening furnace, transport car, oil or water quench tank, direct feeder; tempering furnace, load cooling tunnel, roller table, atmosphere gas control cabinet, washer, dryer, etc. The Elterma APF-520 type pusher furnace is shown in Figure 7. This furnace was designed for annealing of low carbon steel at temperature of up to 880?C.

Control System

Elterma furnace control system covers all the functions connected with every mechanism and the power supply of particular energy receivers installed in the furnace. Set point process parameter values can be introduced, indicating the temperature and carbon potential course over a specified time.

The control system provides full visualisation of the furnace operation, and signals failure conditions. Standard equipment applied in Elterma's retort pit furnace includes carbon potential and temperature control system based on oxygen probe. In case of retort pit type furnace, it is equipped with cascade furnace temperature control. This provides minimised heat-up time of the load and required temperature control accuracy.

Basic functions of the control system include programmed control of processes as well as archiving of all necessary heat treatment process parameters. The control systems of individual furnaces can be connected to a SCADA computer system (Supervisory Control and Data Acquisition). This system provides archiving of all the necessary process parameters and databases for the materials, heat treated parts, process recipes and for finished heat treatment process.

The CarbonSystem offered by Elterma is showed in Figure 8, above, and it enables the following functions to be performed:

??/strong> Programming of process recipes
??/strong> On-line calculation of carbon diffusion during carburising
??/strong> Calculation of the furnace atmosphere parameters
??/strong> Graphic presentation of calculated carbon profile predicted hardness profiles, current process status on Fe-C diagram and iron oxidation diagram
??/strong> Alarm handling, and
??/strong> Scheduling of service Inspections.