The beginners guide to Heat Treatment ControlBeginners Level (Heat Treatment)

© Peter Sherwin | Eurotherm | V2 2014

Introductory Guide

IntroductionThe purpose of this guide is to introduce the basics of Heat Treatment control and is aimed at new Engineers supplying control systems to the Heat Treatment Industry.

This guide is part of as a series of six separate publications which cover the following:

1. The Beginners Guide to Heat Treatment Control

2. Understanding CQI9 (intermediate) 3. Understanding AMS2750E (intermediate)4. Practical PID for Heat Treaters (intermediate)5. Advanced Control methods (expert)6. Business Performance and algorithms

(expert)

Guide OverviewThese guides have been produced to help answer some of the common questions asked about Heat Treatment Control systems.

There are three levels:

Beginners - aimed at recent recruits or Engineers new to providing control systems into the Heat Treat Industry and who want to learn more about the role of control systems in the Heat Treatment process.

Intermediate – specific guides on industry standards for both suppliers and end users. These guides have been produced to explain the key points in the standard and provide an overview of how your instrument manufacturer can help you meet the requirements of the standard.

Expert – these guides are a deep dive into the detail of control and how modern instruments and associated algorithms can directly help Business Performance. These guides are for Engineers with prior knowledge of control systems.

Hyperlinks (links to websites) are included in the guides to provide quick access to more detailed information. 2

Contents1. Chef2. Heat Treater3. Heat Control

Energy Source

Electric Gas

SensorProgramPID

4. Red Tape – Regulations & AMS2750

RegulationsAMS2750

5. Watch-out6. Support

Thermocouple management

Calibration of instruments

Record Keeping TUSProgrammers

Appendix – Heat Treat ApplicationsUseful LinksFurther information

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1 Chef

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1. ChefThink of heat treaters as glorified chefs!

A chef takes raw inedible food and creates a

sumptuous meal through careful mixing of ingredients and application of a known level of heat for a defined period of time.

Introduction

Interesting fact - Eurotherm controllers areused in pizza ovens!

2 Heat Treater

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2. Heat TreaterThe heat treater takes widgets that are

pretty useless in terms of strength properties andchanges the structure through the

applicationof heat and additives (usually gas mixtures) for a known time and temperature.

Hardening Treatments To develop useful engineering propertiessuch as strength and wear resistance.• Direct Hardening• Case Hardening (Carburizing, Nitriding e

tc.)

Softening TreatmentsTo facilitate a manufacturing process or toeliminate stresses remaining at the end of amanufacturing process.• Annealing• Normalizing• Stress Relieving

3 Heat Control

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3. Heat Control – Energy Source

Obviously the temperature scale is different thanthat of the chef, but the principles remain the same.If your temperature is too low or time too short youundercook the food, or for the heat treater thematerial properties are not correct. If you burn yourfood then you might have to throw it away, it’s the

same principle in heat treat, if the temperature is too highor your process time is too long then the parts can bescrap.

The heat applied is either through electric heatingelements or gas fired burners.

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3. Heat Control – ElectricThe electric heater is regulated by devices that can adjust the electrical supply to the elements.

Common types of Power Control (in order of increasing precision and speed of control):

1. Contactor2. Solid State Relay (SSR)3. Silicon Controlled Rectifier / Thyristor (SCR)

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3. Heat Control – GasGas Fired systems are controlled by sending appropriate signals to solenoid valves (for

pulse fired burners) or to actuating devices for

conventional gas-fired burners.

The actuating device controls the valve position of the

supplied combustion air and sometimes the gas lines

as well.

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3. Heat Control – SensorSo how do we control the required energy input into the furnace? We need a sensor to detect the

temperature.

This is typically a thermocouple – the construction of this

sensor is based on an assembly of dissimilar metals that when heated creates an electrical signal. The thermocouple creates a mV (millivolt) reading and the magnitude of this reading is related to the temperature level.

The sensor is connected to a control or indicating device which can automatically interpret and convert this mV signal into a well known temperature scale (the calculation is based on the type of Thermocouple). The scale used is typically degrees Fahrenheit (°F) for

the United States and degrees Centigrade (°C) for Canada, Europe, India etc.

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3. Heat Control – ProgramThe sensor is connected to an indicator or a controller device to indicate the actual temperature. If the unit is a controller then the information from the sensors PV (Process Value) can be compared to the required SP (Setpoint value) and the controller willthen send an output signal to the heating device to raise or lower the temperature.

Programmers are enhanced control devices which can offer automatic profiling of the Setpoint over a time period. This might be just to ramp up to temperature and soak or dwell at the temperature for a defined time (electronic timer), or, ramp to intermediate temperature, preheat, raise again, soak, and perform end of cycle actions. Sophisticated programmers can turn on and off events during each stage (or segment) of the Process cycle.

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3. Heat Control – PIDHow do we ensure that the temperature required is achieved in minimum time (to avoid wasted

energy) and maintained to fine tolerances so Heat Treatspecifications can be adhered to?

It is common to use PID control. PID controllers are sophisticated instruments that automatically adjust the heat input to ensure the process stays at the desired Setpoint.

PID stands for:1. Proportional2. Integral3. Derivative

Basically this type of control reacts when the Process

Value (PV) is close to the desired Setpoint (SP) and through a set of calculations adjusts the energy

input into the process. The idea is that you can achieve stable control with correctly set PID constants.

4Red Tape

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4. Red tape – RegulationsSo is it that simple for heat treaters - just time, temperature and right additives / atmosphere mix and an advanced Programmer?

Well… yes and no. The process is fairly straightforward

and each material has a set defined process cycle, but

there are local authority laws, regional legislative requirements, government regulations and industry standards that all apply to heat

treat.

Two key industries that drive Heat Treatment improvements are Aerospace and Automotive.

They both have heat treat standards (Nadcap

Aerospace & CQI9 Automotive) - at the core of both specifications is the AMS2750 Pyrometry (or Thermal) Control specification.

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4. Red tape – AMS2750What is AMS2750E? It is the latest issue of an SAE specification that covers pyrometric (temperature measurement) requirements for thermal processing equipment.

The standard covers:• Temperature Sensors – e.g. Thermocouples• Instrumentation – e.g. PID Controllers / Programmers• Thermal Processing Equipment – e.g. Furnace work zone uniformity• System Accuracy tests (SAT)• Temperature Uniformity Surveys (TUS)

Basically the standard prescribes both initial and ongoing accuracy and calibration requirements of thermocouples, accuracy, calibration and makeup of instrumentation as well as requirements for furnace work zone uniformity.

Furnaces are classified based on Temperature Uniformity of the workzone. The higher class furnaces (e.g. Class 1; +/-5°F) have the best temperature uniformity and smallest spread of temperature throughout the workzone.Instrumentation types vary from A through to E. The A instrumentation provides more information about the temperature within the furnace by including thermocouples in a number of locations – conversely the E instrumentation only has one thermocouple.

5Watch Out

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5. Watch outSo what are the key issues to watch out for in AMS2750E to avoid heat treaters falling foul of the standard.

Thermocouple Management If a heat treater has multiple furnaces each with its own thermocouple setup, this presents an issue of managing the calibration and replacement periods for each thermocouple type.

Calibration of Instruments Multiple instruments across many furnaces require a comprehensive Calibration plan.

Record keeping Not only do you have to make sure you ensure you keep to your calibration periods and ensure that TUS, SAT’s are all done on time, the tests all need to be documented to specific requirements.. This is in addition to running batches to the correct Process parameters and securely recording that data.

6Support

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6. Thermocouple Management

Ad-hoc Thermocouples are changed infrequently and generally when there is a process problem. Sometimes the thermocouple will be in the furnace for a number of years. This would not meet the AMS2750 requirements.

Manual Manual thermocouple management adheres to the AMS2750 requirements for calibration and replacement of thermocouples. Requires a lot of faith in manual record keeping to ensure you continue to meet the requirements.

Automated The furnace control/data system has some intelligence that can track and alert on calibration and replacement periods for thermocouples. The Aerodaq is a Paperless Chart Recorder and Thermocouple Management system that provides auto-tracking of periodic inspections.

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6. Calibration of instruments

Ad-hoc Calibration of instruments may be on an infrequent basis, usually annually.Typically issues occur at these annual checks and either recalibration of units or replacement isnecessary. Cannot meet the AMS2750 standard with this practice.

Manual Manual system that conform to AMS2750. Intervals are set with an internal department or external supplier. Typically there is a delay from calibration of instruments to receipt of calibration certificates.

Automated Using cloud based software your maintenance periods are aligned either with your internal group or with an external calibration services supplier. Records are updated in real-time and available for review at any time, Online tracking systems are available to provide this information, please see eos for more info.

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6. Record Keeping – TUS (Temperature Uniformity Surveys)

Paper Charts Accuracy is maintained on paper recording systems to conform to AMS2750. The individual channel data is printed on a paper chart at intervals of 2 minutes or less. Significant time is taken to manually judge max and min levels from a list of numbers and then apply correction factors.

Digital Records Accuracy is maintained to conform to AMS2750 requirements with data recorded in a secure format. Electronic Paperless chart recorders capture the information, this is usually exported to a spreadsheet for analysis which makes it easier to apply correction factors but this data is not always secure.

Reporting Software Provides secure links with digital/paperless chart recorders so no extraneous manipulation of base data is possible. The report is setup to guide the user through creating the TUS report to meet all the requirements of AMS2750. Reduces time to prepare an accurate report.

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6. Process Performance - Programmers

Manual Timer (Clock)The contract review process will dictate the processing parameters (time @temperature etc.). If you rely on a manual timer to control the process dwell/soak period then you can have variability in process time that might not meet the desired specification.

TimerFor simple single setpoint time and temperature setups then an electronic timer can reduce the variability that a manual timer introduces. You can be certain that the dwell time is consistent.

ProgrammerMore complicated processes (requiring preheats etc.) require a more sophisticated timer system – this is where the programmer is appropriate. Consistent results are achieved with the ability to hold a number of prewritten programs in a single device.

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Appendix – Heat Treat Applications

Common MaterialsSteelGlassCopper and BrassCeramicAluminumComposite MaterialsPrecious Metals

Typical Processes Common Product AreasAnnealing

NormalizingStress RelievingAgingQuenchingTemperingFlame HardeningInduction HardeningNeutral Hardening

Case Hardening Carburizing Carbonitriding Nitrocaburizing NitridingCold and Cryogenic Treatment

For general information:en.wikipedia.org/wiki/Heat_treating

Automotive PartsAppliancesSpecialty SteelsPipe & TubeFastenersMachineryToolsPowdered MetalsWeapons & ArmamentsFarm & Heavy Equipment

FoundryDefence EquipmentForgingConstruction MaterialsAerospace ComponentsCan & ContainerWireMedical ProductsMarine and Shipbuilding

Useful LinksCarbon (Oxygen Probes)Carbon Probe SensorsSimple Electronic TimersElectronic Timer Cycle (Setpoint) ProgrammersProgrammer (3504 Programmer)Temperature and Process Controllers2-Loop Controllers (3500 series)Process RecorderPaperless chart (6000 series chart recorders)Controller and Recorder InstrumentPID Controller / Recorder (nanodac)AMS2750E InformationRegulatory Compliance (AMS2750E)Calibration ServicesEurotherm Calibration ServicesThermocouples, SAT and TUS ManagementAerodaq (thermocouple management)Data Managementeos (data storage solutions)Field Test Instrument / Survey Recorder6100A TUS Portable Temperature Survey RecorderTUS Auto Report GeneratorAMS2750E TUS report software

Further informationSAE AMS2750E Standardstandards.sae.org/ams2750eNADCAP Auditswww.pri-network.orgAIAG CQI9 Standardwww.aiag.org

© Peter Sherwin | Eurotherm | 2014