Flame hardening is a heat treatment process widely used on parts made from mild steels, alloy steels, medium carbon steels and cast iron. This process involves heating the metal’s surface directly with oxy-gas flames until it reaches its austenitisation temperature. At this critical temperature, the surface structure transforms while the core remains softer and unchanged. Immediate quenching is then required to achieve the desired hardness, as the rapid cooling solidifies a hard surface layer, greatly enhancing the metal’s resistance to wear and corrosion. Before hardening, the steel surface typically consists of austenite or ferrite, which are converted into martensite through flame hardening.
Flame hardening can be applied either differentially, focusing on specific areas or uniformly across the entire surface of a workpiece. The success of this process is influenced by several factors, including flame intensity, heating duration, quenching speed, temperature and the material’s elemental composition. The flame is generated using gases that can achieve high, stable temperatures, most commonly a mixture of oxygen and acetylene.
One of the key advantages of flame hardening is its ability to enhance wear resistance, reduce processing times and minimise distortion while maintaining cost effectiveness. However, there are also significant challenges. The open flame introduces fire hazards and the hardened martensite can become brittle if overheated, leading to potential cracking and flaking. Furthermore, flame hardening is inherently less precise than other case hardening methods such as induction hardening or boronising, and the process can lead to oxidation or decarburisation of the material.
Accurate temperature measurement is not just important but critical for the success of flame hardening. The precise control of the surface temperature directly impacts the quality of the hardened layer. However, conventional infrared sensors often fall short in this application as the flame can interfere with the infrared signal, leading to inaccurate temperature readings. Additionally, there is a risk that the sensor might mistakenly register the flame’s temperature rather than the workpiece itself.
Thermal monitoring in flame hardening
When hydrocarbon gases burn, the infrared energy emitted comes not only from the flame itself but also from the by-products of combustion, such as water vapour and CO2. These gases emit infrared radiation across various wavelengths, which can interfere with accurate thermal measurements taken through the flame. Traditional infrared devices measure the flame, not the part.
To obtain precise thermal measurements in such conditions, it is crucial to use a wavelength region where both water vapour and CO2 have high transmittance. This wavelength allows infrared radiation to pass through with minimal absorption. Additionally, it must be far removed from the intense infrared energy emitted by the flame to avoid interference and ensure accurate readings.
The wavelength band around
Cost-effective solutions for reliable flame-hardening process control
Detecting infrared radiation within this specific wavelength typically requires the use of cooled detectors. While these cameras offer high-precision thermal imaging by reducing sensor noise through cryogenic cooling, they are expensive due to their complex systems and require regular, costly maintenance. Additionally, they have longer startup times and are larger and heavier, which makes them less practical for large-scale process automation.
In contrast, the test CTLaser MT pyrometer provides accurate temperature measurement through flames with a wide temperature range from 200 to 1650°C. Its robust stainless steel housing, dual laser aiming system and versatile analogue and digital output options ensure precise targeting and better control of the flame hardening process, helping to prevent overheating and brittleness.
For imaging needs, the Xi410 MT infrared camera, equipped with a 3,9 µm filter, offers an affordable alternative. Although uncooled, it is recommended to use the Xi410 MT in conjunction with the test CTLaser MT pyrometer for enhanced temperature accuracy. The PIC Connect software facilitates this by allowing the pyrometer’s 4-20 mA output to correct temperature offsets in the infrared camera.
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