For mainstream AA6xxx series alloys the continuous homogenizing concept is the first choice. The majority of these billets are homogenized in one of the 120 continuous furnaces manufactured by HERTWICH.
All Hertwich homogenizing plants are configured to meet individual requirements of clients, in terms of throughput, floor area and technical features.
Batch type homogenizing is still the best suited technology for various alloys which require long holding times, e.g. 6 – 12 hours and for large-only log diameter production, as well as for frequent temperature changes or intermittent production.
Hertwich has in recent years supplied several batch homogenizing plants build upon latest state-of-the-art technology, which is instrumental for outstanding metallurgical aspects and productivity.
Batch homogenizing process:
During homogenization in a conventional batch furnace the temperature cycle of a log heavily depends on its position in the batch.
Logs at the edge of a batch are more intensely exposed to the convection from heated gases and therefore reach the set temperature ahead of logs in the centre of a batch.
With the new generation of batch furnaces with reversing air some 20% faster heating is achieved. The reversing airflow allows very narrow temperature deviation between outer and inner logs.
Important of course is an intense air flow and best heat distribution to achieve near uniform log temperature throughout the stack, which is enhanced by an improved air guide system.
During cooling (in the cooling chamber) the variation of log temperatures within a batch is even more severe.
The different cooling rates result in variations to the grade of heterogenization.
Continuous homogenizing process:
The schematic below shows the temperature cycle in a Continuous Homogenizing Plant. Logs travel through the furnace and the cooling station in one plane.
The furnace comprises a heating and a holding compartment. In the heating compartment logs are uniformly heated to the required temperature. At the end of the heating compartment the temperature of each and every log is verified by sting thermocouples.
Logs then pass to the holding compartment of the furnace where they are precisely kept at the set temperature throughout the holding time, adjustable from 2 to 9 hours. Upon entering the cooling station logs are cooled according to the selected regime.
In the air cooler the cooling speed is regulated over the fan speed.
Parameters of heating, holding and cooling are exactly the same for every log.
Usually logs are heated swiftly to homogenizing temperature within say 1,5 to 4 hours, depending on log diameter. Holding times may vary, depending on alloy and recipe.
Some plants have been built to meet different, special heating requirements, for instance a plant in Norway processes hard ZnMg-alloys. This furnace is capable of slow heating in the critical temperature range to avoid development of cracks due to inner tensions. (green line (b) vs. standard red line(a))
Throughout holding all logs are kept precisely on the set temperature, over the entire log length. Temperature deviations are within 2°K, and guaranteed within +3°K. Actual log temperature for each log is verified with several sting thermocouples in strategic places, i.e. prior entering the holding zone as well and before transfer to the cooling station. Temperature control is supported by a separate algorithm.
Cooling after homogenizing is as important as heating and holding to achieve best possible extrudability and mechanical properties of billets. Many different cooling regimes are in use, depending on metallurgical requirements and clients recipes, hence a wealth of different coolers have to date been built. Log temperature uniformity in a continuous cooler is typically superior to that in a batch type cooler.
In accordance with this variety of cooling regimes, actual cooling stations vary much in design and construction. We distinguish between open and closed type coolers, even coolers with built in heating devices.
For certain countries with cold climate HE has built cooling stations with integrated waste-heat recovery. Thereby cooling air is re-circulated until it is suitable for heating of buildings.
Major benefits of continuous homogenizing:
- Perfectly uniform metallurgical property of the billet due to precise and reproducible temperature regime during heating, holding and cooling.
- Lowest Labour Cost
As opposed to a batch furnace which requires one or two operators per shift (building of log stacks, transporting of the batches, removal), operating labour for a continuous Homogenizing Plant is practically NIL.
Logs, stripped from the casting pit are entered at the Log Laydown Station. After homogenizing heat-treated logs automatically arrive at the billet saw. Plant operation, including heat-treatment and cooling as well as all integrated log handling operations, is fully automatic.
No operator is required.
- Best Log Straightness
Perfect straightness of logs is an important issue whenever a hot shear is used at the extrusion press. A continuos homogenizing furnace will turn bent logs into straight ones, effectively reducing the reject rate.
- Low Downtime & Maintenance
Plant availability is 99%. As opposed to a batch type furnace, which is typically subjected to periodic temperature change cycles, the continuos homogenizing furnace practically operates on a set temperature level in continuous operation, which means much reduced thermal strain and wear on the equipment.
- Control Software
An important factor is the extensive automation of a HERTWICH Plant. Operation is in principle fully automatic with no human intervention required. Re-start of the plant is equally fully automatic. This completely eliminates the possibility of human error during any restart. Malfunctions are reported on screen, with all necessary information for easy fault finding and fast correction of the cause.
- Low Energy Consumption
An electric heated continuous homogenizing furnace consumes around 210 kWh of electric energy per ton of aluminium. A gas-heated furnace of same size with recuperative burners requires 230kWh natural gas and 35 KWh electric energy per ton of aluminium.
Specific electric power consumption is considerably lower than that of a batch type furnace.