Heat treating in an atmosphere furnace is not without complication; the one constant thing in atmosphere heat treating is that the composition of the furnace atmosphere is constantly changing. We must use measurement and control devices to ensure the proper atmosphere composition necessary to achieve the desired metallurgical quality and mechanical/physical properties throughout the workload. In our effort to support our customer’s process, Prolific Heating International Co., Ltd (PHI) is cooperating with Shin-Ei Netsukeisou from Japan, which has been manufacturing measuring instruments for controlling atmosphere furnaces for many years, and with over 90% of its customers are well-known companies from heat treatment industry.
Dew Point Analyzer
– Sample is required
Infra-Red CO2 Analyzers
– Sample is required
– Sample is not required
Dew point is defined as the temperature that water vapor starts to condense. In simplest terms then, a dew-point analyzer measures the amount of water vapor present in the furnace atmosphere. This information can then be used to determine the carbon potential of the atmosphere. It is considered an indirect-measurement technique if it involves pulling a gas sample from the furnace into the instrument.
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.
O2 Sensors are directly inserted into the furnace without sampling is required. While the O2 sensors are used together with the CP converter to calculate O2 values into Carbon Potentials (CP) Values in the furnace. The device looks similar to a thermocouple for measuring temperature and typically sits inside the furnace. In whatever location, the oxygen probe measures minute changes in oxygen concentration of the furnace atmosphere.
A difference in partial pressure of oxygen in the furnace atmosphere and the partial pressure of oxygen in the room air induces a voltage across the electrodes in the probe. At any given temperature, there is a known relationship between the probe millivolt output and the oxygen potential of the atmosphere. The oxygen potential can be directly related to the carbon potential. Hence, monitoring the furnace temperature and the probe output can control the carbon potential of the furnace atmosphere.
The oxygen probe uses a conductive ceramic sensor, most often manufactured from zirconium oxide (Zr2O3). The operating range of the probe is normally 650-980˚C (1200-1800˚F). Oxygen probes can be used for a variety of atmosphere compositions, but they need to be calibrated for the specific one in use. They are fast-response devices and subject to contamination by carbon, zinc, and certain stop-off paint vapors. When used in carbonitriding applications, the presence of ammonia might shorten the life of the probe.
The current trend is to use oxygen probes in combination with three-gas analysis (CO, CO2, CH4) equipment to calibrate the probe to a known CO value and to monitor the amount of free hydrocarbon gas in the furnace atmosphere.
Source: The Heat Treat Doctor®
Furnace Atmosphere Control Methods (Part 1)
Infrared CO2 Analyzers are to retrieve samples from the furnace, while the CP Converter is used to measure CO2 values and display CP values.
The 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).
Nowadays, three-gas infrared analyzers are popular and used to monitor the atmosphere produced by generators and furnaces. The analyzers work on the principle that individual gases absorb infrared radiation of very specific wavelengths. The number 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.
Source: THE HEAT TREAT DOCTOR®
Furnace Atmosphere Control Methods (Part 2)
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 soaks times, temperature, carbon potential (single or boost /diffuse), cool down and quench. The carburizing atmosphere is typically controlled by the use of an oxygen probe.
Nitriding is another process that requires precise management of its variables. To achieve this, multivariable control systems 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.
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