To drive over pot holes and rough roads at speed without damage and yet give the driver a sensitive road feel would seem to be mutually exclusive goals. Powertrain and suspension components require toughness to handle the worst of everyday driving and high-precision mechanical characteristics to accurately transmit road conditions and driving dynamics to the driver. This paradox can be resolved with inductively hardened modern automotive components. The manufacturing of these high-performance components is accomplished by a sophisticated induction hardening machine with the flexibility to harden a variety of disparate parts.

Introduction

Today’s induction hardening machines handle a dynamic and continuously growing product spectrum while setting new records for shorter cycle times and higher throughputs. Fast, reliable and reproducible setup and change-over from one product run to the next is critical, particularly with today’s smaller lot sizes and flexible run scheduling.

Essential machine performance requirements include:

• A good price/performance ratio
• Fast and reliable setup
• Short machine delivery times
• Simple and quick installation and commissioning on the production floor
• Optimal, appropriate solutions for each application

Modular Machine Design

Induction hardening machines meet these diverse requirements by incorporating modular machine designs, where a base system can be configured with various modules for fast and easy adaptation to a variety of hardening jobs. Standard hardware and software interfaces (analogous to object-oriented programming) facilitate a plug-and-play approach to the machine configuration process, streamlining the engineering, manufacturing, installation and commissioning of these machines.

Basic Machine Variants

Depending on the requirements, machines are available with both manual and automated loading and unloading with one or more workstations for hardening and tempering. These machines feature an elegant, modern design that meets all the requirements regarding process visibility and maintenance access. Selected example configurations are described below.

Figure 1 shows a manual machine with two double workstations for hardening drive shafts (Fig. 2) with a length of 1,000 mm. In each station, two shafts are hardened simultaneously, while the other station is unloaded and reloaded. Both stations are supplied by a single power supply. Thus, the time for loading and unloading does not increase the cycle time. Depending on customer requirements, hardening depths in a range from 3-8 mm can be achieved by adjusting the relevant parameters like power, feed rate and frequency (Fig. 3).

Automatically chained machines with one workstation are used for hardening ball hubs, jointed parts (Fig. 4) or similar workpieces with a hardening zone. The hardening can be performed in a single shot or alternatively in the scanning process. The workpieces go into the machine by way of a plate conveyor with a separating device. From there, the machine transports them to the hardening station and (if needed) to cooling and/or blow-off stations before they are placed on a plate conveyor again at the machine outlet.

Figure 5 shows an automatically chained machine with two workstations for hardening workpieces with two hardening zones (e.g., axle journals or tripods, Figs. 6 & 7). For these workpieces, usually a shaft and the bearing races (contact surfaces) are hardened in the bell and/or in the tulip. These areas can be hardened with a single-shot or a scanning process. Alternately, the two stations can be used for hardening and subsequent tempering. Here as well, the intermediate positions are available for cooling and/or blow-off.  

Modular Structure

The hardening system is modular, meaning that the appropriate hardware for the customer’s special requirements can be selected for each individual machine module. With standardized interfaces, plug-and-play modules can be selected to make a multiple configurations. This also applies to the individual workstations. So, for example, during the hardening of axle journals or tripods, the hardening sequence for the shaft and bell/tulip can be freely selected. Also, bells/tulips can be hardened from below or from above (with workpiece turning) in the single-shot or scanning process. The transport direction of the workpieces through the hardening machine can be in either direction.

Different options for transport systems within the machine offer solutions for short or long workpieces. With options for two different control systems and multiple converter sizes, the base machine is quickly adapted to a given application.

Figure 8 shows the individual modules in the machine, emphasized in color, which can be expanded with further modules and various option packages.

Process Know-How and Energy Efficiency

Synergies are realized through a systems- engineering approach to machine and process development. This starts with the converter technology for optimal and energy-efficient heating of workpieces, selective application of the current exactly at the point in the workpiece to be heated by precisely adjusted inductors, and by exact parameter setting for the entire process.

The current generation of the digitally controlled frequency converter (power supply) features:

• Patented control algorithms for the converter for an automatic adaptation of the converter to various loads and for reducing losses in the converter[1]
• Fast reaction times for extremely short heating times < 1 second
• The ability to run continuously at 100% of rated power
• A larger frequency range
• Short-circuit resistance due to integrated overcurrent and overvoltage protection
• Robust monitoring and diagnostic capabilities

Energy efficiency has been improved by optimizing the medium-frequency equipment (better placement of individual components with respect to each other, better bus-bar and cable connections to the capacitors and more precise matching of the transformer to the inductor). Optimized inductors are essential for a reproducible and energy-efficient hardening process.

For further reduction of the energy consumption, if there are no workpieces at the loading position, the machines automatically switch to standby mode, in which all pumps and auxiliary equipment are shut off.

Accessibility and Maintenance Friendliness

Large glass doors in the front allow good observation of the process and also good access for maintenance work. A reduced workspace depth improves component access. Another safety door permits access from the rear.

Reduced Floor Space

The complete machine – control, converter, network transformer and capacitor cabinet – is set up on a common base frame. The machine footprint is reduced, and complete, fully assembled machines can be transported to the plant.

Quality

Patented net workpiece energy monitoring performs 100% online quality control of the hardening process in these machines.[2] Power generated by the IGBT power supply is necessarily subject to heat losses in the converter, busbars, capacitors, transformers and finally in the inductor. The net heating energy applied to the workpiece is less than the power-supply output energy. Conventional energy-monitoring schemes track only the power output by the converter and, therefore, do not account for system losses.

The SMS Elotherm patent describes a method with which the frequency-dependent losses are measured and considered. Ultimately, the power actually applied to the workpiece is integrated over the complete heating time (energy), recorded as a curve and monitored in real time. In this case, the energy applied to the workpiece is an absolutely reliable measurement for checking the heating quality. The smallest changes of the coupling gap between inductor and workpiece lead to clearly measurable changes in the energy values, and tolerance limits are user-adjustable. Changes of the coupling cap could, on one hand, be due to deformations of the inductor. On the other hand, these changes could be caused by tolerance deviations or cracks in the workpiece surface.

In addition to the heating (austenitizing), the hardening process consists of quenching with cooling medium. During quenching, it is a matter of running through a fast cooling curve by using adequate cooling-medium supply, which leads to the desired hardening microstructure in the material (martensitic microstructure). The quenching process is monitored by measuring the exact spray flow quantity and an appropriate temperature control for the quenching medium. Together with the net workpiece energy monitor, the complete hardening process is therefore monitored and recorded for each and every part.

Summary

Modular induction systems for hardening automotive components are setting manufacturers up for both current and future product requirements. This is true for complex and also simple components, with high reproducibility and process control using patented process technologies.

Modular system solutions are conceived for users with frequently changing hardening tasks and designed cost-effectively according to this. Using net workpiece energy measurement and other systems for quality monitoring, out-of-spec parts are recognized and rejected automatically, ensuring continuous operation without interruption. IH

For more information: Contact George Burnet, general manager of SMS Elotherm North America at tel: 724-553-3471; e-mail: g.burnet@sms-elotherm.com. Jochen C. Huljus is marketing project manager, and Dirk M. Schibisch is sales/marketing department manager for SMS Elotherm GmbH.