Vacuum Furnaces: Buy New or Upgrade 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.

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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.

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Preventive and Predictive Oxygen Probe Maintenance

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

Since starting SSi Europe in 2003, I have always tried to help our customers extend the lifetime of their furnace’s oxygen probes. The most important factor, in my experience, is staff training.

This morning, we (myself and Karina Frampton, Project Administrator) 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:

Please contact me at to discuss any of the items listed!