Eurotherm
Invensys Operations Management
.

Control Systems for the Heat Treatment Process

It is necessary to have a very diverse approach to control system design as heat treatment can be carried out in a wide variety of furnace or oven types, operating in either the batch or continuous mode.

Furnace sequence control

Whether furnaces are of the batch or continuous design, there is often an element of sequential control to be provided by the control system. Very often this is a secondary consideration to the process parameter control. For convenience, the sequencing can often be incorporated into the process controller itself.

  

The range of furnace dependent issues that will influence the sequence control requirement:

Work piece transfer

  • Furnace door actions

  • Furnace hearth or hood movement

  • Furnace transfer mechanisms

  • Quench transfer

  • Gas/Fan quench control

  • Heat shield door actions

  • Cooling water control

Control Systems

Temperature control and thermal profiling

Accurate and repeatable temperature control is at the heart of most heat treatment processes. It is important that special measures are taken to control the process to the metallurgical requirements of the component. This requires accurate control for both the programming setpoints and the steady state temperature control over a wide range of temperature setpoints and furnace loadings.

 

To ensure repeatable and consistent performance across the widest range of furnace use, Eurotherm build into their control solutions special routines for gain scheduling, overshoot inhibition and ramp/dwell transitions. Control structures must also be incorporated to ensure that temperature control complies with the metallurgical requirements of the work piece and where necessary the control system should be designed to accommodate separate work-piece thermocouples.

Furnace testing and audits

Most thermal processing equipment is audited to ensure that parts are processed in accordance with the applicable specification.

 

To achieve maximum furnace loadings and equipment utilization, temperatures in a defined workload region must be within given tolerance and Temperature Uniformity Surveys (TUS) are often carried out to determine the degree of compliance. Furnaces are grouped into different classes depending on the degree of (TUS) tolerance and workload treatments can be defined to be undertaken in furnaces of a particular accredited class.

 

Heat treatment furnaces are also controlled by a range of System Accuracy Tests (SATs), which define the type and accuracy requirements for control instrumentation and sensors.

 

It is important that consideration is given to the needs of required SAT and TUS compliance when designing control systems to meet the users furnace class and processing requirement. In many processes where there is considerable time delay between the process and the furnace, (such as a retort furnace or during low temperatures regions on vacuum furnaces), it may be required to control the furnace from ‘work-piece’ sensors.

Having strategies for Cascade or Override control and being able to implement effective routines for guaranteed soak times and holdback on thermal profiles are a necessary requirement of the control system design in these applications.

 

The thermal profiling requirement is defined by the metallurgical process and is very often determined through the use of ‘controlled’ recipes systems. It is important that an easy secure method is available that enables users and operators to set-up and run repeatable recipes, without the fear of unauthorized changes.

 

Gas atmosphere control

Since many heat treatment and surface chemistry processes rely on being performed in a gaseous atmosphere, it is important to include provision for this in the control system. It may be as simple as including timed or temperature driven events within the profile when gasses like Nitrogen, Argon and Hydrogen are required. Traditionally the control system does not provide feedback control on the atmosphere for these type of processes; typically only providing the capability to set fixed gas flow rates at defined parts of the cycle via simple On/Off flow or more complex, self contained, mass flow control devices.

 

With the advent of new low cost analyzers and the growing application of zirconia probes, more users are using analysis equipment to ensure their processes are maintaining the desired levels of oxidizing reduction.

 

Since many of the gasses are volatile, special care needs to be taken with control system design to ensure that the sequence and safety of gasses is maintained.

 

For many of the surface treatments it is also necessary to control the flow of enrichment gas or dilution air into the furnace against a defined setpoint for a defined period. Enrichment gases are used to provide the gaseous atmospheres for processes such as Carburizing and Nitriding. In these cases feedback control is used, and special algorithms are required to convert the output from zirconia probes into application specific functions.

 

The purpose for controlling the atmosphere in these applications is to gain a specific surface hardness and finish by diffusing nascent gas particles into the surface of the component. Various algorithms have been employed to determine the diffusion rate rather than applying fixed time/ temperature/ atmosphere profiles. Much of the new OEM equipment makes use of these diffusion calculations, while the majority of the installed base relies on simple carbon potential profiling. For example: Using 3 gas Infrared analysis to aid the carburizing processes adds a high degree of additional confidence to the conventional carbon potential control.

Vacuum atmosphere control

Vacuum furnaces are widely used in heat treatment, particularly in the Aerospace and Automotive industries. Almost all conventional heat treatment cycles can be carried out in a vacuum, such as: homogenization, stress relieving, normalizing, hardening, tempering, and annealing. Vacuum furnaces are also used for brazing and out gassing of material and are becoming more widely used for low-pressure carburizing.

 

Chambers typically operate in the atmospheric range from ambient atmosphere down to 10-9 millibars. Control systems need to accommodate the interface between complex vacuum gauges and the process sequence, taking particular account of:


● Pump start up and the pumping sequencing
● Vacuum pumping efficiency and chamber leak rate
● Partial pressure control and back fill control
● Vacuum out gassing and heater interlocks

Quenching systems

One of the most important aspects of heat treatment is the quench process. Many of the phase structure changes that occur in the treatment of alloys do so during quenching.
Once a component has been held for a desired period of time at the temperature required to effect the correct crystalline structure it must be quenched in air, oil, water or special polymer. Control systems need to accommodate the rapid transition from temperature control to quenching, since the rate of change of temperature during cooling greatly determines the micro grain structure of the component.

For example:
A component which consists of a 0.6% carbon steel, that has been heat treated above the Upper Critical Temperature to achieve a micro-grain structure consisting of Austenite, will convert to a material suitable for rework when air cooled in an annealing process (Pearlite with Ferrite). When satisfactorily rapidly quenched in oil the same component will harden by a different conversion of the micro-structure to Martensite, The component can be further processed in the tempering process to remove much of the brittleness. Variations in the quench process provide a range of material properties.

Industry regulations

Much of the heat treatment sector is regulated either by prescriptive specifications or by the demands of industrial quality procedures. Demands are made of treatment providers to show that the processes they adopt stand up to scrutiny under some type of audited environment. The audited environments tend to be industry specific with both global and regional variations.

The automotive industry have widely adopted the (Automotive Industry Action Group) recommendations contained in CQI-9 which refers to the SAE Pyrometry guide for heat treatment AMS2750D. Alternatively they rely on quality systems contained in TS16949. where the specification is based on business quality procedures and individual quality manuals, which show how process compliance is achieved and maintained. The aerospace industry have widely adopted the more prescriptive methods of accreditation which are covered in the Nadcap global specification with sections 7102 and the associated AMS2750D specifically applying to heat treatment. 

Data management

As part of the need to comply with industry regulations there is a demand on heat treatment suppliers to record and retain process information. The control system usually includes recording equipment, which enables data to be conveniently displayed, archived, saved and recalled. Specific rules are laid down in the prescriptive specifications or in quality manuals, which advise on the procedure for data management. Additionally, records must be maintained regarding the accredited status of the process plant detailing data on instrument calibration, information about process sensors and reports of temperature uniformity surveys. Recent trends towards digital data management have enabled Eurotherm to build solutions which encompass all these data management needs in a suite of complimentary products that meet the needs of both the process and analysis.