Unfortunately, variation exists in the carburizing process. This can be due to differences in the alloy content of products being treated (heat treaters are encouraged to treat batches from the same material lot), surface conditions of products from pre-processing and, obviously, the cycle selected in the furnace. Assuming the material alloy and jigging style is appropriate, this update concentrates on how to minimize variation in the heat-treat cycle.
Atmosphere Furnace Construction
The batch integral quench (BIQ or sealed quench) is the workhorse of the heat-treat industry with thousands installed around the globe. Subtle differences exist in the construction of these furnaces, typically large work-volume furnaces in North America with slightly smaller versions in Europe and Asia. This type of furnace has a basic construction of a heating zone and quenching zone with an inner door separating these two chambers, and it usually has either a pusher system or chain system to transfer between the chambers.
Furnace Carburizing Cycle
Noting the subtle differences in construction, heat-treat carburizing cycles actually tend to be very similar from furnace to furnace. Modern-day cycles include a ramp-up stage. This might be at a defined rate (e.g., degrees/minute), a preheat stage to ensure uniformity of heat across all components and then a boost/diffuse/equalize carburizing process. A programmer will usually be used to control the furnace temperature, carbon potential and time duration of these stages. The use of a programmer ensures repeatability of the process, and the modern cycle does not succumb to the variability that can be encountered by running a completely manual process.
Let’s now look at the main elements of control: temperature, atmosphere and time.
The alloy content of the steel, section and shape of the products to be treated, and microstructure requirements all impact the temperatures used in the carburizing process throughout the boost, diffuse and equalize stages. Once the desired temperatures are set (and programmed into a recipe or setpoint program), then the ability of the furnace to match the running Process Value (PV) to the desired Setpoint (SP) will be determined by the effectiveness of the heating system and controller working in tandem.
Correctly set PID or fuzzy logic control can be used to speed up the approach to setpoint and maintain tight temperature control throughout the desired process time. The use of overshoot inhibition (or cutback) reduces the possibility of overshoot of the process. The heating system can be gas (with control achieved through actuating between low-fire and high-fire states) or electric with (SCR-type control allowing precise control of power output throughout the entire range of operation). Gas is common in North America.
The maintenance of the burner system is incredibly important to reduce variability in temperature control. Quality systems utilizing AMS 2750E pyrometry standards help to maintain good temperature control and uniformity within the operating work zone.
Most BIQ furnaces today operate with some form of sensing device to indicate the level of carbon available in the furnace heating zone. The most common is the carbon or oxygen probe. This sensor produces an electrical signal (mV) relative to the amount of oxygen in the furnace and through calculations interprets this reading into a carbon potential (%Carbon) reading.
The carbon potential can be “boosted” by the additional of carbon-bearing gases into the atmosphere (e.g., methane, propane) and reduced by small injections of air.
The boost, diffuse and equalize stages will have a defined carbon level setpoint, which is influenced by a balance of speed of processing, microstructure requirements and the life of furnace furniture. For example, high carbon levels will improve the speed of processing but will reduce the life of the fan, burner tubes, oxygen probes, etc., and may cause undesirable microstructural features (including retained austenite and carbides) depending on the carbon level.
Again, once the desired setpoint is programmed, the function of the atmosphere control system is to match the PV to the SP required.
Correctly set PID settings can improve the speed and accuracy of matching PV to SP. In practice, this would be through using actuator devices or solenoid valves on the gas atmosphere lines.
The oxygen probe readings (mV) are usually compensated by a process factor or CO factor (used in most probe calculations). This is due to the inherent variability in the probe as a standalone device. To calculate this factor, an independent check on the atmosphere is used. Dew-point meters, gas infrared analyzers and shim strips are frequently utilized, with the ultimate check being a carbon analysis of the surface of the product being treated.
The development of more sophisticated analyzers (e.g., gas 3-IR) have enabled not only a double-check of the atmosphere to set the factor but also to calculate a precise infrared carbon potential (IR %Carbon). This value can be used to update the process factor/CO factor and allows for a redundant method of carbon control if the primary oxygen probe fails.
The majority of programmers are set up to control the time at each stage of the process. If there is a defined ramp-rate, the time will depend on the ability of the furnace to recover temperature but will not exceed the ramp-rate specified. In the boost, diffuse and equalize stages, the programmer will soak the products for a defined time before sequencing onto the next stage.
Unfortunately, the carburizing response of the furnace will differ over time mainly due its design. The brickwork inside the heating chamber will become saturated with carbon over time and then requires burnout (many companies will perform this on a weekly basis), and the furnace will require reconditioning before being put back into production. The results of the repeated conditioning, burnout, reconditioning cycle leads to variation in the carburizing response. This is generally picked up by either low case depths or progressively deeper case depths achieved over time before the furnace is taken offline for burnout. The age of the brickwork can also have an effect on the carburizing response, with longer and longer times needed to achieve the same cycle when the brickwork is overdue for complete renewal (it is not uncommon for a furnace to be re-bricked after five years).
This variation in time has to be manually altered on the programmer and is usually based on feedback from quality inspection. Intervals for conditioning and burnout are set based on historical performance (usually weekly intervals) and not on actual usage/output of the furnace.
The effect of this variation can lead to case depths not achieving specification (too shallow or too deep). This can result in rework, scrap or sometimes a possible concession obtained through dialog with the customer. If the case depth is too deep, then the process has been run for a longer time than necessary, incurring additional operating costs.
Real-Time Carburizing, and the Human Element
Real-time carburizing offers the advantage of utilizing advanced control to automatically adjust the previously used fixed-times set in programmers. Capturing information from the carbon sensors, the advanced programmer calculates the case-depth profile in real or near-real time. Typically, programs are then based on the case-depth required rather than entering fixed time. This is usually entered as a "% of case-depth achieved" in each of the stages (boost, diffuse, equalize).
A number of these types of systems are available on the market today and are used in a wide range of industries to reduce variability, improve quality results and reduce operating costs.
Certain industry horror stories have developed because some automated systems have soaked products for excessive time without alerting any operators, or gas-analysis pipework has become clogged and reported inaccurate results.
To counter these rare situations, recent developments from Invensys Eurotherm have enabled advanced control systems to become more intelligent and capture the habits of truly experienced operators. The following three steps are implemented:
1. Utilize furnace-specific Gas-3IR panels to remove the ongoing maintenance issues associated with multi-furnace analyzers.
2. Integrate PAC control (programmable automation controller) to enable sophisticated automation.
3. Add condition-based limits to each segment step with automatic alarming if these limits are exceeded.
Think of this as a solution to mimic an experienced operator looking at a chart and deciding when to force a programmer to progress from boost to diffuse or manually add/subtract fixed time on the programmer at the request of the quality department based on case-depth results. All of this is done automatically and within your predefined rules.
In this connected world, it is now possible to get control-system alerts through voicemail, texts and/or e-mail. With the soon-to-be release of smart watches, how long before “wearable technology” becomes commonplace on factory floors?
Peter Sherwin is the marketing manager for Invensys Eurotherm and looks after heat-treat business development in North America. He has spent nearly 25 years associated with the heat-treat industry, working for both captive and commercial heat treaters in Europe and Asia before moving to Colorado.