Multi-strand heat treatment furnace gives high throughput strip processing

Azmüsebat, one of Turkey’s biggest razor blade producers, has increased throughput at its factory in Istanbul with the installation of a second specialised heat-treatment furnace from UK manufacturer Carbolite.

The furnace, which is designed to harden and temper four strands of thin stainless steel strip simultaneously in a continuous process, joins an existing unit with similar specifications built by Carbolite three years ago. Processing four strands of strip simultaneously has been found to bring considerable cost/throughput benefits compared with single-strand equipment. When strip 22.5mm wide x 0.1mm thick is processed at 12.5 metres per minute, typical throughput is 50kg/hour.

The equipment is designed to produce sufficient hardness in the strip for it to retain a sharp edge without being brittle and hard to work. The material also has to remain flat at the end of the process, despite the potential for distortion. The 6000mm long hardening section has a maximum temperature of 1150°C and is divided into six independently controlled zones, allowing a temperature profile to be created, generally between 750°C and 1100°C. Temperature uniformity at 1000°C is guaranteed to be better than ±10°C. The power rating allows a fast heat-up to operating temperature from cold.

Each strand runs in a rectangular stainless steel tube, which is designed to minimise friction and is also tensioned to ensure it remains straight when hot. The strip and tubes are protected from corrosion and discolouration by injecting a nitrogen and hydrogen gas mixture into the tubes. After passing through a quench unit, the four strands are tempered in a 1000mm-long section with a maximum operating temperature of 500°C. This section incorporates four special heat transfer plates, designed to give rapid heating and to maintain product flatness and straightness. It also has two heating zones to give accurate control of temperature. After tempering, the four strands are cooled again and coiled.

The furnace has separate control and over-temperature instruments operating off independent thermocouples fitted to each heating zone. The controller is a microprocessor-based PID unit with dual display of set-point and measured value.

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