Building an Atmosphere Furnace Solution

SIMPLE to build using pre-defined building blocks

SCALABLE solution fits the requirements of any plant

The most ACCURATE atmosphere control solution available

Control Systems

Thermochemical heat treatments are widely used in the aircraft and automotive industry manufacturing process. Many components such as gears, shafts and bearings, as well as a host of sub-parts, are subject to some case hardening technique and there is a world-wide installed base of furnace equipment.

Eurotherm supply a wide variety of control solutions into batch and continuous carburizing furnace applications.

The scope of supply may vary but there are some key common requirements:

Temperature programming and control

Atmosphere (Carbon Potential) programming and control of case depth diffusion control

Atmosphere probes and atmosphere probe diagnostic features

Gas control (sequence and mass flow control) for some specific applications

Quench programming and control

Furnace sequence control

Furnace safety alarms

Recipe control and programme management

Data management

Power control and energy management

Furnace diagnostics and maintenance

Furnace mimics and screen navigation

Furnace Temperature Control and Programming

Optimum temperature control is achieved using an independent furnace sensor. The heat control output can either be connected to gas burners or thyristors. In some applications a cooling output may also be connected to a circulation fan or an exhaust damper.

It is normal for many instances of the furnace programme to be available for operator selection against a component or batch reference.

Password protected multi-segment programmes enable an optimum profile to be achieved with full traceability.

Furnace Atmosphere Control

A zirconia probe is used to measure very low concentrations of % oxygen in the furnace, typically less than 1 x 10 ^-20. To supply the required species carbon it is usual for the furnace to be supplied with a base endothermic carrier gas which conditions the furnace, at say 20% CO for a methane based carrier gas.

In the atmosphere loop, the controller uses the carbon potential calculation based on the known oxygen reading to increase the carbon potential by allowing additional carburizing gas (ie methane) to enter the furnace.

Conversely, to decrease the carbon potential, controlled air is introduced into the furnace. Automatic probe cleaning functions ensure that the reading is accurate, while probe health and sooting alarms warn of a deterioration in the probe performance.

Discrete control solutions smaller image

Typical discrete control solution

Typical integrated solution

Gas IR Control

The Carbon potential calculation using an oxygen probe is based on furnace gas equilibrium conditions. Equilibrium conditions rarely exist during short or medium carburizing cycles.

As a consequence, atmosphere carbon potential, based on an oxygen probe, is often overstated. Furthermore, the carbon potential is only calculated from a single gas constituent, the oxygen content of the furnace atmosphere, and a fixed value for the carrier gas CO.

THE SOLUTION FOR OPTIMUM ACCURACY

Three gas infrared with automatic probe compensation

A three gas infrared system calculates, with higher accuracy, the carbon potential using furnace realtime CO, CO₂ and CH₄ values. The Infrared system then calculates the Process Factor or CO Factor based on the IR Carbon % and sends this new Process Factor, via a special communications link, to update the Eurotherm atmosphere carbon controller’s Process Factor. As a result, the atmosphere controller now reads the same as the calculated IR carbon. The oxygen probe may be sooted or even failing and the infrared system will compensate.

The system combines the convenience and response of an oxygen probe with the superior accuracy of multi-gas infrared measurement to provide the very best control available.

Benefits

Accurate calculations of atmosphere carbon potential - Three times more accurate than a probe

Automatic oxygen probe compensation

Verification of oxygen probe accuracy and performance

Easy identification of furnace atmosphere problems and furnace condition

Reduction of scrap and rework from high accuracy process control & repeatability

Diffusion Control

Traditionally carburizing is carried out against a timed setpoint profile where the time periods for the different temperature/carbon stages are selected against empirical post-process material results.

This method provides a steel/alloy dependent recipe, which gives good repeatable performance but heat treaters tend to process at the high end of the case depth tolerance to ensure good repeatable results.

Since the purpose of the process is to provide components with a defined effective case depth, usually with a minimum tolerance, other methods, which allow effective case depth to be selected as the controlling setpoint, have become more widely available.

The case depth diffusion solution is available as a function block within the Eycon ^TM Visual Supervisor. This uses an algorithm based on carbon

A typical diffusion cycle

potential, temperature, material specification and process factors to determine the carbon setpoint profile.

In this case the traditional recipe hands control to the dynamic on-line diffusion calculation, which completes the carburizing cycle to the required effective case depth.

Benefits

Reduced cycle time

Real time calculation of case depth profile