Effect of Agitation Fan on Oxygen Probe

Most carburising furnace users know the importance of burning off (or purging) the oxygen probe. This ensures that soot doesn’t build up between the ceramic and steel tubes which can have a drastic affect on the lifetime of the sensor. However, it is not well understood that there are good burn offs and bad burn offs, and a bad burn off can actually reduce the lifetime of a sensor. This is why key metrics should be measured during the burn off process (see more in my blog post from 2017: www.supersystemseurope.com/benefits-oxygen-probe-maintenance).

During a recent project I had some time to experiment with the impact of the agitation fan on the burn-off. I noticed a very significant effect as you can see from the charts below. Figure 1 is the mV response of the oxygen sensor during a burn-off with the fan on. Figure 2 shows the mV response during a burn-off with the fan off.

This simple test showed that the fan is preventing the burn-off air from travelling all the way to the tip of the sensor. If the air was travelling all the way, as in the second example, we would see a far lower mV due to the inverse relationship between mV and oxygen. In this case, the oxygen probe was installed in the roof of the furnace, in close proximity to the fan. The user might consider trying a different spot in the furnace for the oxygen probe, or even temporarily automatically inhibiting the agitation fan during burn off (for a short time only). This could end up drastically improving the life and performance of the oxygen probe.


Figure 1. Fan on. Weak mV response (mV drops only to ~850mV) and temperature increase.

Figure 2. Fan off. Strong mV response and smaller temperature increase.

Temperature Uniformity Surveys (TUS) with Super Systems Europe

Put simply, a Temperature Uniformity Survey (TUS) ensures that components see the same temperature regardless of where they are in the furnace.

AMS 2750e requires heat treaters to classify their furnaces between +/- 3°C (class 1) and +/-  28°C (class 6), whereas CQI-9 (table A) requires heat treaters to meet a (quite generous) uniformity of +/- 15°C. Note that ‘uniformity’ is generally understood to mean the total deviation around the set point, versus ‘spread’ which has no relation to the set point. It is common to survey a furnace with a good spread (e.g. 10°C), but with a poor uniformity, due often to a poorly positioned control thermocouple.

A TUS can take a furnace out of service for a considerable amount of time, so it is worth maximising that downtime by performing other tests during the TUS. Our engineers will often tune burners and analyse the furnace atmosphere with a PGA3510 (if it is safe to gas up the furnace) during the TUS.

There are two ways to go about TUS with Super Systems Europe:

1) Order an SSi SDS datalogger with SDS reporter and training by an qualified SSi Calibration Engineer. Read more about the SDS datalogger here: www.supersystemseurope.com/product/dataloggers/portable-touch-screen-data-logger.

2) Have an SSi calibration engineer visit your site to perform the TUS for you.  Read more about our calibration services here: www.supersystemseurope.com/services/calibration-services/

Please contact us with any questions or enquiries!

Quenching Norwegian Style

My final visit of the year was to Norway to perform AMS2750E compliant TUSs on tempering furnaces. This was how we cooled our TUS fixture:


Improving Process Quality and Reducing Downtime with SSi Gas Analysers

As an industry, heat treatment is probably fairly conservative when it comes to technological progress, but I’m pleased to work for a manufacturer that is constantly expanding, innovating, and developing its product range. In 2001, SSi produced the first PGA3000 to verify and troubleshoot the a gas carburising furnace’s carbon potential. Today, SSi produces (what I think is) the heat treatment industry’s most comprehensive range of gas analysers, comprising:

  • SGA (new) single gas analysers available in OEM or wall mounted versions.
  • PGA portable multi gas analysers.
  • MGA multi gas analysers available in OEM, wall mounted, or rack mounted (new) versions.
  • MZA multi gas, multi zone gas analysers available in wall mounted or Rittal cabinet versions.
The MZA has proven to be an incredibly successful system because it cost-effectively provides valuable process and troubleshooting data for up to 8 zones from a single sensor. Installation and commissioning of a 4-zone system is typically completed in 2-3 days.

Pictured is a recently completed MGA and MZA installation at a UK aerospace manufacturer. The system measures and automatically corrects the Carbon Potential on three pit furnaces and three endothermic gas generators.

Below are some recently installed systems that demonstrate the versatility of the current range of gas analysers:

4 Zone MZA Rittal Cabinet for a global bearing manufacturer

Pictured is the second MZA cabinet designed and built for installation at a bearing manufacturer in Poland. 9205 controllers were installed alongside the MZA to provide automatic correction of COF and highly precise, stable temperature and carbon potential control.

Rack mounted MGA to replace a Siemens Ultramat 23

SSi developed a 19″ rack mounted MGA due, partly, to multiple requests to replace old Siemens Ultramat (and others) gas analysers. As SSi manufactures the complete analyser, including the 3-gas IR sensor, we can provide rapid, cost effective calibration and repair services meaning that lifetime costs are significantly lower than our competitors.

4 Zone standard MZA for a global fastener manufacturer

An MZA system was recently installed and commissioned in Holland. The CQI-9 compliant system automatically monitors continuous furnaces and endothermic gas generators. It saves process engineers many hours (and many miles) per week versus manual checks.

Please contact us to discuss any of the standard or custom systems mentioned in this blog post!

Upgrading A Vacuum Furnace Control System

There is often a tough decision when a vacuum furnace reaches a certain age. Do you scrap it and replace it with a state-of-the-art new furnace? Or do you commit time and resources to upgrade the existing furnace’s control system? Sometimes the driver for this decision may dictate that a furnace is replaced, but there is often a false assumption that an ‘old furnace can not be taught new tricks’. In reality, mechanically, the furnace may be in reasonable condition. The real source of the issue is unsupported, unreliable, out-of -date, non-compliant, and in the worst case, unsafe control technology. Increasingly demanding customers, combined with standards that require more traceability, force heat treaters to ask the question: do we buy a new vacuum furnace or upgrade our existing vacuum furnace control system?

A seemingly complex vacuum furnace actually consists of relatively few components: the shell, hot zone, heating systems, cooling systems, diffusion pump, vacuum pumping system, pipework and valves. Degrading hot zones, cooling jackets, failing diffusion/vacuum pumps can be cleaned, repaired, rebuilt or replaced.

Vacuum Furnace Uniformity

Many vacuum furnaces struggle with passing temperature uniformity survey (TUS), especially where uniformity requirements are tight and strictly enforced in the aerospace sector. Even with routine hot-zone maintenance, this may be inevitable. TUS failures are commonly cited as a reason to discard a furnace. However, even minor modifications to the existing control and heating system can result in drastic improvements.

Heat chambers traditionally have three to five distinct zones with the same number of variable reactance transformers (VRTs). Each VRT’s output is proportional to a command signal and generates from a single silicone-controlled rectifier (SCR), split into three to five distinct signals. Each is manually trimmed via adjustable rheostats. Optimal rheostat settings often vary between temperature ranges which limits the overall uniformity capabilities of the furnace. Huge improvements are achieved by installing an independent SCR per VRT and trimming, through an SSi 9220 for example, the outputs from the controller to the SCR during tuning.

Example of electric heating modification which resulted in a spread reduction at 1050 deg from 10 deg C to 3 deg C. Click to enlarge.

SSi controllers can scale each output at specified temperatures, which improves uniformity at the required set points. It also means we can center the load TC delta around the set point which is a simple but effective way to quickly improve uniformity, since it is deviation from set point that determines total uniformity, not simply the spread between hot and cold TCs.

Read more

Portable Three Gas Analyser: Top 3 Uses

The PGA 3510 portable three/four gas furnace analyser for the carburising/nitriding process is one of SSi’s most popular products. This is largely because it displays and records data that is critical for verifying process quality and maintaining equipment. Below are top three ways SSi Europe engineers use the PGA:

1. Verifying carbon potential (carburising) and dissociation (nitriding).

The large majority of carburising processes are controlled with an in-situ oxygen probe (e.g. the SSi Gold Probe!), a carbon potential PID control loop that adjusts air and/or gas additions using a proportional valves you can see in the image. See a simplified diagram above. Theoretically, with stable carbon monoxide and carbon dioxide values, the carbon potential value on the controller/regulator is the carbon potential of the furnace atmosphere. In practice there are many potential reasons that a controller/regulator’s actual carbon potential value could be false or inaccurate. These include unstable or rich/lean carrier gas being produced by an endogas generator or nitrogen/methanol supply, furnace leaks, oxygen  probe damage or degradation, contaminated reference air, improper installation, calibration status of the controller and many more. A verification is critical to ensuring that the carbon potential on the controller is the carbon potential of the atmosphere in the heat treatment furnace, and is a requirement of heat treatment standard CQI-9.

The same principle applies to gas nitriding where often the only method of atmosphere control is a periodic burette inspection which indicates the % of NH3 which has dissociated. The PGA can be used to verify the burette reading and also provides a Kn value. Where a diluent gas is used (e.g pre-cracked NH3 or nitrogen), burette readings are less useful but the PGA automatically calculates the Kn if the user enters gas flows. With a nitrocarburising atmosphere, the PGA can calculate both Kn and Kc.

2. Identifying furnace air ingress or radiant tube cracks.

Read more

Preventive and Predictive Oxygen Probe Maintenance

Note that, if not burned off, soot will gather around the probe tip and eventually cause sensor failure.

This morning, we visited a commercial heat treatment supplier to follow up on a staff training session we performed around 1 year ago. The Maintenance Manager asked us if we had noticed that their probe consumption had dropped since the training. He is responsible for 6 Ipsen sealed quench furnaces with SSi Redundant Probe Systems fitted, i.e. 12 oxygen probes in total on site. The furnaces tend to run at pretty high carbon potentials (near to the soot limit), so probes were regularly filling with soot and being returned to us for cleaning, testing and re-certification.  Dani, in the workshop, had been advising during his test reports that the burn-off did not seem to be effective, so during the training we advised that burn-off air pumps were inspected. Some of them were replaced in order to get higher flow during burn-off.

We also advised that regular burn-off checks are performed. During the burn-off, it is important to measure the response of the zirconia element and the probe’s temperature rise or fall. Air should pass the tip in significant volumes (typically between 3-6 l/min), so the mV should drop to around 200 mV (remember, as air/oxygen % goes up, mV goes down). If this is not happening, there will not be enough oxygen present to burn-off the soot. The temperature of the tip (only measurable from an internal TC) is a good indication that burn-off is occurring, and that the flame tip is beyond the probe sheath. You will see a rise of between 5 and 25 deg C if soot is being burned correctly. If the temperature rise is higher than this, the flame tip is too close to zirconia element. Over time this will affect the probe readings and in the worst case scenario, cause a material failure, so burn-off airflow should be adjusted.

Extreme example of damage to the tip of an oxygen probe sheath due to overheating during burn-off.

By training operators, replacing a few burn-off pumps, and regularly checking the burn-off performance, the heat treater has significantly reduced the furnace running costs (by thousands of £s), but more importantly, reduced reworks and unplanned furnace downtime.

Those are a couple of simple items. By combining those with quarterly impedance and recovery time tests (which are performed automatically in SSi controllers but can also be performed manually), we can start to analyse the level of deterioration/degradation of the sensor. Those values (see image below) can be recorded and charted in a SCADA system. When the trend begins to accelerate, or the values exceeds pre-defined tolerances, we know that the probe is beginning to deteriorate so know when to return it for inspection.

Probe data, recorded and charted in a SCADA system as part of predictive maintenance plan.

Many furnaces have inadequate reference and burn-off air supplies, which may be reducing the life of the oxygen probes. It is definitely worth checking them to ensure that they are not the cause of furnace problems or quality issues. Let us know if we can help with training or advice!


Refurbished Stock

We have added a clearance section which will be updated to include products that have been refurbished, repaired, and/or re-certified in our UK workshop.

Product performance and quality is guaranteed for 12 months (unless otherwise stated) but there may be cosmetic damage to the product or its packaging.

You can visit the clearance page here: www.supersystemseurope.com/clearance-items.

Please contact me at workshop@supersystemseurope.com to discuss any of the items listed!