Image Credit: Maksim Safaniuk/Shutterstock.com
One of the most common approaches for hardening both iron and steel, relatively soft materials, is to perform heat treatment under endothermic atmospheres. In terms of heat-treating metal alloys, there are many methods for, of which some incorporate surface hardening and annealing or case-hardening, but in general, the purpose of the treatment is designed to improve the hardness or durability of the material.1
Heat treatment has several stages, which must be completed in order to achieve these increased hardness properties for alloys. Generally, these are divided into annealing, quenching, and tempering. The first of these, annealing, involves the heating of the metal first (to reach a given temperature, for a certain length of time) and subsequently using a controlled rate of cooling. The second of these, quenching, includes rapid temperature reduction of the material which then makes the alloy very hard. This hardness caused by the quenching can often lead to brittle materials, and in these instances the method of tempering can be used to restore elasticity.2
Throughout all of the stages, it is essential to retain careful control of conditions in order to retain maximum control of the final material properties, which additionally includes the properties of the surrounding environment. Part of maintaining this control over heat treatment processes is the use of endothermic gases, such as CO, H2, and N2.3
The endothermic environment is maintained to the right environmental conditions in order to begin and continue the hardening process. This then assists in the process of ensuring that the correct chemical reactions occur so as to preserve the valued physical properties which the metal has. In processes like carburizing or carbonitriding, atmospheres like this can also be used as carrier gases for other species. Part of the hardening process involves the decomposition of the carbon-rich gases, which causes migration onto the surface layers of the metal.4
CO2 concentrations need to be controlled in addition to ensuring the correct concentrations of endothermic gases in heat treatment for their thermal properties, as excessive CO2 levels can lead to unwanted oxidation reactions. Metals such as iron can be similarly affected by O2. In addition, there are certain benefits to controlling excessive CO levels, as this is often produced from side reactions between hydrocarbon gases and oxygen, but can be involved in ‘carbon reversal’ processes that lead to the production of soot.5
It is imperative to retain highly monitored and controlled gas concentrations, for heat treatment in endothermic conditions, in addition to a way of monitoring potential fluctuations, in the concentrations that are consistent and reliable over a large range in temperature.
NDIR, or non-dispersive infrared gas sensors are well-suited for the detection of a range of hydrocarbon gases as well as being ideal for the detection of many of the species involved in endothermic protection. This is because they absorb IR light very strongly, which renders it a highly sensitive detection technique.
Furnace Sensors and Feedback
Since opening in 1980, Edinburgh Sensors have been a market-leaders in both the design and manufacture of NDIR sensors.6 A range of gas monitors is on offer which can detect one gas type at a time, with built-in microcontroller processing. The latter allows onboard corrections for changes in pressure and temperature conditions.
Edinburgh Sensors, which are well-suited for the detection of CH4, CO2, and CO, offers the GasCard NG7 and Guardian NG NDIR-based gas monitors as part of their range.8 Both of these models of monitor are highly sensitive, and are capable of detecting CO2 concentrations between 0 – 5000 ppm and CH4 and CO levels between 0 – 100 %.
In addition, the Guardian NG8 offers an accuracy of ± 2 % across the full detection range as well as temperature compensation between 0 – 40 °C. Measurements herein are not affected or compromised by humidity conditions in the 0 – 95 % relative humidity. The housing of the sensor is an IP54 compliant casing, which possesses a convenient monitor for displaying current and historical gas concentrations, alongside built-in alarms which offer in-situ programming on the device.
The low response time (T90 < 30 s from sample inlet) has the result of making the Guardian NG the ideal candidate when it comes to providing continual monitoring and live feedback, on even minute condition changes within the heat treatment chambers.
Gascard NG. Image Credit: Edinburgh Sensors
The GasCard NG7 is available in two variants. One of these comes with external housing, in order to facilitate simple mounting and installation.9 In addition, the GasCard series comes with field-replaceable IR sources. This facilitates a hands-off maintenance approach, in that it does not require site visits, and offers similarly impressive accuracy over the full range (± 2 %) as does the Guardian NG. A highly adaptable system, the GasCard is available with a wide range of connection types. This range includes R323 and USB, which translates into the utility of the sensor in detection and control systems, for environmental systems. Indeed, despite the gases which are produced as part of the heat treatment process, this then means that the feedback produced from the GasCard is useful, in keeping environmental conditions constant whilst ensuring and maintaining consistent and optimum process control for the best results.
Edinburgh Sensors additionally offers pre- and post-sales support, for process design and integration into existing feedback systems, and can assist in the designing of bespoke and tailor-made solutions for any specific gas sensing requirements, as and when they are necessary.
References and Further Reading
- Oberg, E. (Ed.). (1920). "Heat-treatment of steel: a comprehensive treatise on the hardening, tempering, annealing and casehardening of various kinds of steel, including high-speed, high-carbon, alloy and low-carbon steels, together with chapters on heat-treating furnaces and on hardness testing. The Industrial Press.
- Gopalan, R., & Narayan, P. (2011). Review of thermo-physical properties, wetting and heat transfer characteristics of nanofluids and their applicability in industrial quench heat treatment. Nanoscale Research Letters, 6(1), 334.
- Dawes, C., Tranter, D. F., & Smith, C. G. (1980). Reappraisal of Nitrocarburizing and Nitriding When Applied To Design and Manufacture of Non-Alloy Steel Automobile Components. Studies in Surface Science and Catalysis, 1693(April), 60–68. https://doi.org/10.1179/030716979803276390
- Christiansen, T. L., & Somers, M. A. J. (2009). Low-temperature gaseous surface hardening of stainless steel: The current status. Zeitschrift Fuer Metallkunde/Materials Research and Advanced Techniques, 100(10), 1361–1377. https://doi.org/10.3139/146.110202
- Edenhofer, B., Gräfen, W., & Müller-Ziller, J. (2001). Plasma-carburising - A surface heat treatment process for the new century. Surface and Coatings Technology, 142–144, 225–234. https://doi.org/10.1016/S0257-8972(01)01136-7
- Edinburgh Sensors (2019) https://edinburghsensors.com/about/about-us/
- Gascard NG, (2019), https://edinburghsensors.com/products/oem/gascard-ng/
- Guardian NG (2019) https://edinburghsensors.com/products/gas-monitors/guardian-ng/
- Boxed GasCard (2019) https://edinburghsensors.com/products/oem/boxed-gascard/
This information has been sourced, reviewed and adapted from materials provided by Edinburgh Sensors.
For more information on this source, please visit Edinburgh Sensors.