How can one ever forget the smell of burning shoe leather? In the early 1970s, The Doctor, with the soles of his shoes literally melting away, was working atop a pusher carburizer to change one of those newfangled oxygen-probe sensors.

It was summer. It was hot. It was humid. It was Iowa. The job had to be done every few hours or so – such was the life expectancy of an oxygen probe in those days. The salesman working next to me had it tougher, though. He was wearing a three-piece suit!

Throughout his career, The Doctor has been in the right place at the right time in history more times than not. Such was the case here – to be present at the dawn of the introduction and commercialization of the oxygen probe into the heat-treatment industry. As the phrase goes, we’ve come a long way, baby. Let’s learn more.

More about Oxygen Probes

We continue our discussion from the June issue about measuring and controlling furnace atmospheres by looking at devices other than oxygen probes.

Maintenance Tips for Sampling Systems

Certain other types of furnace-atmosphere measurement and control devices (see online content) require pulling a gas sample from the furnace. Preventive-maintenance programs are common to all heat-treatment operations, but nowhere is it more important than for gas-sampling systems.

By using existing inputs and outputs on controllers and PLCs, operators can set specific timers and counters, which are extremely useful when determining when certain maintenance tasks should be planned or considered. With an established baseline, quick evaluation of real-time versus historical information can identify problem situations. Taken a step further, notification software can be used to automate e-mails and text messages to key personnel.

Here are several items and a few tips to keep sampling systems running trouble-free.

• Sampling lines
  • Install, inspect and replace the line and analyzer filters as necessary.
  • Verify sample flow (many analyzers are flow-dependent).
  • Use flowmeters at the furnace sample port or at the analyzer (or both).
  • Provide a way to clean out the sample port (i.e., rod out if carbon buildup occurs).
  • Do not over-pressurize/backpressure the analyzer (many cells are flow- and pressure-dependent). Install metering valves if necessary.
  • Purge the sample lines and analyzer with nitrogen when not in use.
• Analyzers
  • Calibrate (i.e., zero or span) the infrared and thermal-conductivity analyzers with nitrogen and a span gas (typically from a cylinder filled with a gas of known composition).
  • Span with outside air or a moisture generator.
  • Span with lithium-chloride or potassium-chloride salt solutions.
  • Some units (Alnor® dew pointers) require factory calibration only.
• Oxygen probes
  • Check surface-carbon values against shim stock.
  • Measure temperature near the probe.
  • Measure and correct for CO content of the furnace atmosphere.
  • Confirm reference air and burnout air are being supplied unimpeded.
  • Confirm proper probe location and insert depth.
  • If hot insertion or removal is required, follow manufacturer’s recommendations with respect to prescribed speed of movement of the probe. Slower is always better.

Infrared (NDIR) Analyzers

Infrared analysis uses light in the infrared spectrum to analyze a gas sample and determine the percentage of each constituent in the furnace atmosphere. Single gas (carbon monoxide) or multiple (3-8) gas analyzers are used to detect these percentages.

The amount of carbon dioxide in the furnace is an indirect way of measuring the carbon potential of the atmosphere, and charts/tables can be used to correlate CO₂ readings with dew-point or millivolt signals from oxygen probes. For example, the CO₂ value – similar to water vapor and oxygen – varies based on furnace temperature and carbon potential but will typically lie between 0.25 and 0.50%. Multiplying the CO₂ value by 100 can roughly approximate the dew point (in Fahrenheit).

Today, three-gas infrared analyzers (Fig. 1a) are popular and used to monitor the atmosphere produced by generators (Table 1) and furnaces. The analyzers work on the principle that individual gases absorb infrared radiation of very specific wavelengths. The amount of absorption increases with gas concentration. The unit operates under the principle that a gas sample passes through a cell where a light emits infrared energy of known wavelength. The sensor converts measured infrared energy into an electrical signal. These values are usually compared to the values obtained with a reference gas. Infrared analyzers are known for their fast response and are easily calibrated.

Summing Up

While manual control methods are highly effective when properly employed and continue to be used throughout the heat-treatment industry, the future lies in the precision and customizability of automated atmosphere-control methods. By utilizing specialized process controllers and software, these systems help enhance productivity through real-time monitoring and control, leading to reduced scrap and freeing up operators for other tasks.

Automated systems also form the basis for electronic data acquisition that improves process traceability and makes it much easier to analyze data in making decisions for improving processes. Preventive-maintenance programs often grow from automated methods, reducing unplanned downtime. Instrumentation upgrades associated with automation bring many additional benefits, including:

• Increased focus on operation-specific features, such as carburizing, vacuum heat treating and nitriding.
• Recipe control and management, leading to fewer entry points and operator requirements.
• Potential for greater operating ranges due to the ability to manage multiple parameters at the same time.

What will the future hold with respect to atmosphere control? Artificial intelligence and real-time load-monitoring devices seem to be only a step away. As technology improves, processes improve. While the initial cost of control upgrades may be seen as significant, quality demands and assurance of absolute process repeatability make them a necessity. 

Dew Point

Dew point is defined as the temperature that water vapor starts to condense. In simplest terms then, a dew-point analyzer (Fig. 1b) measures the amount of water vapor present in the furnace atmosphere (Table 2). This information can then be used to determine the carbon potential of the atmosphere (Table 3). It is considered an indirect-measurement technique if it involves pulling a gas sample from the furnace into the instrument.

If performed properly, dew point is a simple and accurate atmosphere measurement technique and indicates the condition inside the atmosphere generator or heat-treat furnace. The sample port (Fig. 2) should keep the sample line free of carbon and include a way to open and close the gas flow (soot). It should also include the capability to filter and cool the gas sample. The port should be designed in such a way as to keep the port open (i.e., the ability to “rod out” the line).

Dew point will help tell you if the reaction is stable or unstable (constant dew point or changing dew point over time). It can tell you when the catalyst bed in your endothermic gas generator is starting to soot and if there is a water leak, an air leak or non-uniformity (“breathing”) of the atmosphere inside your furnace.

The types of dew-point analyzers available include capacitance sensors, chilled mirrors and older-model fog chambers (Alnor®, Dew Cup). Condensation is a problem for all dew-point devices if the sample temperature is less than the dew point of the gas.

One solution is to heat trace the sample lines. Also, if the ambient temperature exceeds 40°C (105°F), as it often does in heat-treat shops, many dew-point analyzers will not give consistently accurate readings unless special precautions are taken (such as keeping the unit in the office and bringing it out on the shop floor for only a short period of time).

Interpretation of Data

In order to interpret furnace-atmosphere data correctly, it is important to collect data in real-time. However, it is of critical importance to understand the exact furnace operating conditions at the time the data was collected (e.g., zone temperatures and gas flows, furnace pressure, exhauster settings, fan rotation and speed, etc.).

In more than one instance, data was being analyzed and adjustments made based on improper sample port locations, instrumentation that was not properly calibrated, filters that were dirty and/or sample ports not extending fully into the furnace chamber. Mistakes such as these can be devastating to productivity, quality and equipment.

Process Control

In its basic form, process control applied to heat treatment is controlling temperature to maintain a desired setpoint. Process control can then be extended to other variables and in its most complex form to a lights-out operation where all mechanical, electrical and control functions are being handled by instrumentation. Here are some simple examples.

Generator Control

For gas generators, maintaining the quality of the gas (i.e., temperature, gas composition, dew point) using microprocessor controls and sensors to regulate the addition of enriching gas or dilution air is key. Control systems can incorporate fuel injection, which performs the mixing and control in one package with very little adjustment or maintenance required.

Carburizing Control

Carburizing requires precise management of the atmosphere composition to achieve the desired metallurgical results in the parts being processed. This includes such items as heat-up and soak times, temperature, carbon potential (single or boost /diffuse), cool down and quench.

The carburizing atmosphere is typically controlled by use of an oxygen probe. The carbon algorithm (using sensor millivolts and furnace temperature) used when determining carbon potential available in the furnace atmosphere assumes a consistent gas composition of the natural gas coming into the building. The equation also assumes a known CO value. If the composition of the natural gas or air changes, it not only influences the quality of the endothermic gas but also changes the amount of additive or enriching gas. The calculation of surface carbon from the millivolts and temperature monitored by the probe could be off. The automation component comes in when the calculation in the controller is automatically adjusted based on the three-gas calculation of carbon potential, thus providing a more accurate reading of carbon. This is commonly referred to as IR compensation.

Periodic verification of the process is typically performed using dew point, shim stock, shim coils or portable gas analyzers. If possible, shim stock (the only “true” test of carbon available in the furnace atmosphere) should always be used for verification.

Nitriding Control

Nitriding is another process that requires precise management of its variables. To achieve this, multivariable control systems (Fig. 3) monitor temperature, atmosphere, nitriding potential, flow rate and pressure to provide both safe and precise control of compound (i.e., white) layer and diffusion-zone variations in material. Load and furnace types are controlled automatically as well.

Automated control for both nitriding and nitrocarburizing is based on temperature, time, flow and measurement of hydrogen, oxygen, carbon monoxide (CO) and CO₂. In nitriding, hydrogen is the process variable that must be measured as a result of the reaction of NH3 during dissociation.

It is possible to define the material, group and class. Determination of the desired K_N and K_C (Equation 1, Tables 4-5) based on the required epsilon layer and allowable porosity can be found in AMS 2759/12 (latest revision).

K_N = p(NH₃)

                                                     p(H₂)                              (1)

    where:  p = gas partial pressure

Ferritic Nitrocarburizing (FNC) Control

FNC control (Fig. 4) is similar to nitriding but with the addition of a carbon-bearing gas. When controlling FNC, hydrogen is one of a number of variables that must be considered. With the use of CO₂ (common in pit-type furnaces), there is a reaction with hydrogen that produces CO to create carbon activity, K_C (Equation 2).

K_C = p(CO₂ ) p(H₂)

                                            P(CO) p(H₂0)                               (2)

    where:  p = gas partial pressure

CO and CO₂ are both reliably measured with infrared technology. It should be noted that the infrared-cell technology must be compatible with NH3 and water for the FNC process. Oxygen is measured using zirconia- or lambda-probe technology. As with nitriding, it is important to meter the gas flows when using nitrogen and any other diluents. When using endothermic gas with 40% hydrogen present, it is important to meter and compensate for the gas flow and hydrogen content. With these variables being considered, K_N and K_C are calculated to allow for more precise control of the nitrogen and carbon compound layers.

Sample lines and filters to the H₂ analyzer and oxygen cell must be maintained on a regular basis. Calibration of the instrumentation and analyzers must be completed at manufacturer-required intervals. In addition, the mechanical assembly of the system must take into account the need for filtration and ease of maintenance. For example, sample systems must be supplied with drip legs in order to remove any moisture that might be generated during the pre-oxidation process.

When nitriding stainless steel and using a pre-activation process (typically involving polyvinylchloride), filtration and heat tracing must be included in the exhaust piping. Lines and filters must be cleaned for each run. Filtration systems should be designed with pressure-indication and automatic-bypass modes. FNC processes, in particular, are more challenging due to the formation of ammonium carbonate in the exhaust.