Identical output

by Dr. Hans-Georg Bittner, WTH-Wärmetechnik Vertriebsgesellschaft mbH

Hans-Georg Bittner developed the first fully automatic hardening unit for saw blades. This machine, built by WTH, turns out premium quality, blade for blade. At the same time it’s easy on the environment.

For years now it has been possible to harden large, flat products such as saw blades with a minimum of warping using watercooled quenching units made of metal. Market demands, however, have made it necessary to optimize this process. Larger dimensions and thinner gauges have to be treated at consistent high quality. What’s more, the hardness spectrum has shifted toward lower-alloy steels. A further development of the quenching technology has made it possible to temper steel not only at superb quality but in a process that’s also kind to the environment.

This improved process is based on a combination of a rollover furnace and optimized rapid-quenching units. In the rollover furnace the saw blade is heated up to temperatures of between 800 and 1200 degrees Celsius. It is then cooled down at a predefined speed in the water-cooled, rapid-quenching units. The metal components do not warp in this process and decarburization and oxidation effects at the edge are minimal.

The leap from manual to machine processing

In this heat treatment process it is not just a question of product hardness. The desired structure should be distributed as uni-formly as possible throughout the cross-section of the saw blade. Another decisive aspect in the hardening process for highly tempered metals is keeping the times as identical as possible. This is because at these high temperatures even just a few seconds can have a considerable impact on quality, structure and warping of the workpiece. In the quenching unit the saw blades are treated for periods accurate down to the second. This makes sure that the characteristics and hardness qualities in a given lot will be exactly identical.

This unit made it possible to take the leap from manual to fully automatic hardening. The multi-stage heat treatment process has been reduced to a single machine. This also brings with it the advantage that it does entirely without environmentally dubious quenching media such as oil or highly toxic cyanide salt. What’s more, the reject rate in this process goes toward zero.

Accurate timing was the challenge

One of the major challenges to be mastered in development work was to control — as a factor of time — the pressure the quenching plates exert on the blank. This makes it possible to start at high pressure and then taper off in a second phase. It is exactly this attenuation of pressure that lets the blanks be heat treated with minimal distortion even at unfavorable dimensions — and that at identical quality.

A further hurdle in development was the transition from a protective atmosphere furnace to an open furnace — the rollover furnace. The carbon-poor layer created in the open furnace process must not be allowed to become overly thick. That is why we developed the time patterns in the atmospheric furnace quite specifically for this application.

Moving from the furnace to the quenching press is the third critical area in the system. Here the workpiece, at a temperature of 800 to 1200 degrees Celsius, has to be mounted and chucked in the press, by way of the rapid transfer device, within five seconds. That is why the quenching unit, acting as the cooling unit, is permanently connected with the rollover furnace by way of the rapid take-off device. This configuration arose from the realization that the properties of the blank being worked will depend on the temperature and the velocity at which it passes through the austenitization zone of the rollover furnace.

Hardening with steel or with copper

The material from which the quenching plates are manufactured also plays an important role in the quality of the heat-treated workpiece. Several factors have to be considered when selecting the material. The fundamental principle is that the thermal resistance, thermal conductivity and malleability have to be in a perfect relationship one to another. Finally the quenching platewill have to be fitted with cooling channels. Depending on the nature of the saw blade to be treated, we select one of two materials that satisfy these requirements: steel or copper.

Copper is used where rapid cooling is needed. That is the case, for example, when treating the base material for tungsten-tipped saw blades intended to cut wood. Copper is also used where there are severe demands in regard to cooling uniformity. Developed for this purpose was a quenching technology using a special copper alloy and equipped with an entirely new, high-speed cooling technique. This combination makes it possible to cool materials uniformly and in very short periods of time. In highspeed steel saw blades intended for use with metal, which can cool down more slowly, the steel that has been used exclusively to date is sufficient. In the current facility we use, for example, ribbed steel quenching plates. If demands change, we can quickly switch over to copper plates. And thus the system permits greater product variety.

Automation using perfect components

As has already been pointed out, the great advantage of this system is that we have a fully automated machine, handling everything from loading and heating through to hardening and unloading. A fully automated plant like this will achieve profitable operation, however, only if it is built around highly reliable components. When, in addition, the question of pan-European delivery capabilities arose and the standards that this requires, that’s when we approached Bosch Rexroth. There we found the required know-how, product variety standardized across numerous technological sectors, and the critical reliability level.

Now everything that moves in all the handling operations is moved by Rexroth components. In the loading and discharge area RTC pneumatic cylinders with integral guides raise and lower vacuum gripper systems in order to move the saw blades into the system and remove them. Here we can fully exploit the clear advantages of the RTC rodless thrust cylinder: great loading capacities with a space-saving design together with precision and repetition accuracy at low cost.

All the technology — including the valve systems, the vacuum components and compressed air treatment — is taken from the Rexroth product line. The platforms that move the as yet untreated saw blades through the rollover furnace run on ballbearing rails. Each eight-ton platform, driven by Rexroth servo motors, runs on profiled rails with toothed segments below. Behind that, pneumatic cylinders with a belt-driven electromechanical linear module (MKR) pull the incandescent workpiece across rollers to beneath the press. The quenching plates themselves are moved by a Rexroth hydraulic drive under PLC control and provide us with the required flexibility and precise repeatability. In this way we can apply pressure to the hot workpiece in several stages. These might, for example, be five seconds at 100 bar, two seconds at 20 bar.

These components also help us make a contribution to environment protection, workplaces that are harmless to employee health, production optimized in business terms, and product quality.

Author:

Dr. Hans-Georg Bittner, Managing Director, WTH — Wärmetechnik Vertriebsgesellschaft mbH

www.wth-wth.de

Pages: »1 »2