Design for Application

Applications - Case Study

This collection of applications considers the supply of vacuum furnaces to three different customers and highlights how the design of the furnaces was tailored to suit the particular manufacturing process or required performance specification of the finished component. Each customers' particular requirements were satisfied by mixing and matching design specification options from those offered within our standard range of vacuum furnaces.

Introduction

A single chamber vacuum furnace can be considered as being made up of six major subdivisions

Vacuum chamber
Working hot zone
Vacuum pumping leg
Gas cooling system
Control
Power supply

CVE have developed a number of construction and design options within each of these subsystems that allow us to tailor the final furnace installation to best suit a customers particular requirements for process performance, workflow control and cost.

The selection of applications detailed below illustrates how the design flexibility built into our standard VLFC vertical top-loaded furnace range has been successfully employed at three of our customer installations.

Application 1 – Hardening and tempering of tool steel roll cutters up to 1 metre in length

In this application the customers primary requirement was for minimum distortion of the cutters during the hardening and tempering process. A CVE VLFC 2442 vertical furnace was chosen with a hot zone deep enough to allow the components to hang vertically, thus limiting the tendency for the parts to bow during the heating cycle.

The hot zone consisted of wide band graphite elements running round the working zone. These elements were divided into three separate control zones, each with its own thermocouple and slave PID controller, ensuring that the components followed, within very close tolerances, the master thermal profile generated by the programme controller.

The thermal insulation consisted of foil faced rigid graphite board which gave the user good thermal efficiency, resistance to erosion and wear during the gas quenching phase of the cycle, low maintenance and economical capital outlay.

Another major customer requirement was for the roll cutters' machined surfaces to remain clean and bright during heat treatment. A low vacuum (10-2 mbar) environment proved satisfactory, the furnace being equipped with a mechanical pump and roots blower combination.

The rapid and even cooling of the parts was critical to ensure metallurgical transformation without exceeding the specified distortional tolerances. This was achieved with the CVE 6 bar overpressure gas cooling system, comprising an internal heat exchanger, fan, fan motor and Nitrogen cooling gas. Nitrogen is injected into the hot zone very evenly through nozzles positioned regularly to cover the full 360° around the complete length of the working volume. A quench pressure of 3 bar was found to be adequate for the process.

An additional requirement was to carry out a low temperature tempering operation as part of the total cycle. The solution was to vent the overpressure of cooling gas to atmosphere and to employ vacuum pumps to then evacuate the chamber which was then back filled with Nitrogen to restore the pressure to 850 mbar. A secondary fan re-circulated the inert gas over the elements and load. This convective heating ensured rapid and uniform heating of the load to tempering temperature.

Application 2 – Long gas burners formed from sections vacuum brazed together in very clean conditions to maintain fine internal gas passageways

Due to the nature of the burner design no particles or contaminants could be tolerated within the furnace environment which might cause blockages of the fine passageways within the burner assemblies. The customers need for a very clean processing environment and optimum vacuum performance led to the choice of a CVE VLFC 2436 vertical furnace with vacuum chamber fabricated from stainless steel. The outgassing characteristics of the vessel's inner wall were much lower than the epoxy paint finish found in a carbon steel chamber which helped to achieve ultimate vacuum pressure in the shortest time.

The customer requirement for total cleanliness within the furnace chamber determined that a hot zone of 'all metal' construction be employed. This consisted of wide band molybdenum elements controlled in three zones, as in Application 1 above, with an insulation pack of radiation shields manufactured from molybdenum and stainless steel. Although expensive and thermally less efficient than a graphite hot zone it more than met the customer's cleanliness requirements.

In order to achieve the required vacuum level (10-5 mbar range) necessary for successful brazing the vacuum pumping leg included a diffusion pump, above which was mounted an optically dense water cooled baffle thus ensuring that the low risk of back streaming of diffusion pump oil was reduced even further.

As the need for rapid gas cooling of the load was not a metallurgical requirement in this application a gas cooling pressure of 850 mbar was employed to give an acceptable floor to floor turn round time.

In order to comply with the customers quality standards the control system was enhanced with a software package allowing data storage, quality reporting and two-way communication with the furnace allowing the furnace to be operated either at its own control panel or via a PC situated in a remote office.

Application 3 – Sintering graphite based powder which had been compacted into containers

The sintering of carbon in a powder form meant that a graphite construction within the CVE VLFC 2442 furnace hot zone was perfectly compatible with the process. The chance for carbon pick up and particulate matter within the hot zone had no impact upon the process.

The sintering operation required very low levels of oxygen and oxygen bearing gasses within the chamber. Working with compacted powder meant that atmospheric gasses within the interstices of the powder had great difficulty in escaping. Outgassing levels were very high, peaking excessively at particular temperatures during the ramp up to processing temperature. In order to cope with this outgassing a high vacuum pumping system with almost twice the throughput capacity of that fitted to standard configuration furnaces was specified. In the early stages of heating the furnace ramped to temperature using pressure rather than temperature control.

At a pre-set temperature during the cycle a partial pressure of Hydrogen gas was introduced into the chamber. The reducing effect of this Hydrogen being beneficial to the sintering process.

As the rate at which the load cooled did not affect the properties of the sintered product and was only a function of production turn round time an 850 mbar inert gas cooling system was selected for the furnace.

Conclusion

The three applications discussed above serve to illustrate the need to carefully select a vacuum furnace specification which is tailored to meet the requirements of a particular process. This overcomes the danger of trying to accommodate widely varying process requirements in one machine which can often lead to compromises in construction resulting in none of the treatments being carried out to their optimum potential.

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