Nitriding and nitrocarburizing are surface treatments intended to increase surface hardness and to improve other properties of steels such as those listed in Table 1. One of the appealing attributes of these processes is that rapid quenching is not required; hence, dimensional changes are kept to a minimum. Nitriding and nitrocarburizing benefits include:
- Ability to streamline the manufacturing cycle by eliminating processing steps
- Exceptionally high surface hardness
- Resistance to wear and anti-galling properties (good in poor lubricating conditions)
- Minimal distortion and deformation compared with carburizing/hardening
- Resistance to tempering; resistant to softening up to nitriding temperature at which conventional steels soften
- Stable nitrided case
- Improved fatigue life and other fatigue-related properties such as fatigue strength (resistance to dynamic loading)
- Reduction in notch sensitivity
- Marked resistance to corrosion in several common media (except for nitrided stainless steels)
- Small volumetric changes (some growth does occur)
Nitriding and nitrocarburizing surface treatments have been used increasingly over the past decade. Non-gaseous processing methods, such as salt and plasma nitriding, have not reduced the interest in gaseous nitriding/nitrocarburizing, primarily due to the flexibility in processing parameters, which allows achieving an optimized microstructure to meet treated component application requirements. These one-step processes are replacing two- or three-step conventional processes, such as carburizing or carbonitriding followed by grinding and/or plating, due to benefits such as minimal dimensional change and high surface hardness. However, there are currently a substantial number of furnaces in use that do not have adequate process measurement and control systems, which are required to meet the demands of today's customers.
Innovative commercial heat treat companies are improving furnace atmosphere control by adapting state-of-the-art temperature and atmosphere controls, which provide continuous, precise regulation of the process in progress (Fig.1). For example Service Heat Treating's Vice President of Operations Jim Gallos says the company has grown substantially by adopting such equipment.
Three major benefits of control systems based on automatic nitriding potential measurement compared with traditional open-loop furnace control, where predetermined amount of gas is pumped into the furnace over certain amount of time are:
- Reliable measurement of nitriding potential, which enables a repeatable process and prevents the degradation of processed part quality caused by hard to control circumstances, such as failing seals
- Automatic control, which offers the ability to use just the right amount of gasses, lowers the processing cost and minimizes emissions
- Optimal nitriding potential level, which allows processed parts to meet case specifications and controls the growth of the white layer
Limitations of existing control methods
Several methods of automated measurement of nitriding potential or ammonia (NH3) dissociation percentage in a heat-treat atmosphere are being used, usually consisting of photometry of ammonia in the infrared range while ammonia is still in gaseous form. The principal of infrared photometry is based on measuring the intensity of infrared light transmitted, absorbed or reflected from a gas sample and comparing the results with a reference light intensity. Disadvantages of this type of process include interference of other gases in the mixture, drift of the instrument, the need for frequent calibration and expensive span gas. One of the biggest obstacles of using infrared nitriding potential measurement in the heat-treat production environment is the necessity to have extremely rigorous maintenance procedures in place to keep the optics and sensors clean.
Another automatic nitriding-potential measurement method involves determining the individual gas concentration in a mixture of gases by a thermal reaction heat measurement, where the heat is generated by ammonia being burned on a catalyst. This approach is also difficult to perform reliably in an industrial setting.
Another method involves determining the concentration of gases that are a product of dissociated ammonia (such as hydrogen) and calculating actual dissociation. This approach also uses sensors that can drift, which gives false readings, and that are not always able to endure the harsh furnace environment.
One of the most widely used nitriding potential measurement of the gas mixture is a manual sampling method. This is performed by capturing a predetermined amount of gas in a special ammonia dissociation measurement burette, then dissolving ammonia in the gas form into a liquid form, and determining the concentration of ammonia in solution by visually measuring the level of water. Problems with this measurement method are fragile equipment, difficult visual extrapolation and interpretation of the water level measurement, which make manual burette measurements challenging to use in an industrial setting, but it remains a measurement of choice for a wide variety of furnace operators.
New automatic measurement system
A new automatic nitriding-potential measurement system (Fig. 2) based on dissolving ammonia in water has been designed to improve existing furnace controls. The system is installed and operating at Service Heat Treating, a commercial heat treating company in Milwaukee, Wis. In addition to sensing nitriding potential, the system also generates process alarms and provides closed-loop ammonia control within the heat-treating furnace.
System description and operation
Increasing process-quality challenges require operator-friendly measurement devices, which removes operator-induced variability, while requiring minimal maintenance. The automatic system (Fig. 3) performs the measurement and allows the operator to adjust process variables manually or automatically according to a pre-set program.
The system-engineering goal was to design a robust, stable system that could survive harsh plant environments, while providing continuous, accurate and easy-to-define process measurement. The high accuracy and repeatability requirements of Service Heat Treating's processes presented even greater challenges.
The nitriding potential-measurement system is set-up to automatically sample the furnace atmosphere. The main measurement chamber is equipped to receive the true processing atmosphere that exists in the nitriding/nitrocarburizing furnace. The measurement chamber is also equipped with water inlet and outlet lines and atmosphere exhaust line. Water for precision measurement is supplied from a specially designed constant low-pressure water vessel. The water is used to process furnace atmosphere gas samples and to calculate the nitriding potential. During the measurement cycle, the water flow and volume required for full ammonia dissolution is measured using electronic sensors, which generate signals that are converted by a digital controller into process information. The measurement result is automatically logged and instantaneously reported to a furnace operator. The information can be presented in multiple formats, such as nitriding potential, percent residual and percent dissociated ammonia.
Nitriding-potential information is sent to the nitriding/nitrocarburizing control system, which allows adjustment of gases flowing in the furnace based on the input. An automatic self-calibration procedure is a unique characteristic of the system, which is especially critical when processing expensive workloads. Automatic self-calibration can be performed according to a pre-set schedule, or initiated by the operator as needed, does not require certified gases and is based on a precise measurement of the known vessel volume.
Multiple sensors increase reliability
Automatic nitriding-potential measurement offers the use of multiple sensors to augment existing sampling or in-situ systems for increased accuracy and control reliability. This aspect of the nitriding-potential measurement determined using the new automatic gas dissolution in water method is comparable to ammonia measurement results achieved using other measurement methods.
This combination system with redundant capability (Fig. 4) offers high reliability and continuous availability compared with a single sensor control system. Sensing deviation from other sampling or in situ automatic systems can be also minimized by using the multisensor system's self-calibration feature.
The new measurement system is suitable for use with integral quench furnaces, fluidized bed furnaces, pit furnaces and retort furnaces, and can be used to monitor and control ammonia dissociator performance. Process gas-sampling frequency is adjustable and can be set up by the operator to achieve optimal control over the full processing cycle.
Process measurement and control examples
Commonly nitrided and nitrocarburized parts include extrusion dies (Fig. 5) and components made of plain or medium carbon, low-alloy steels used in automotive, agricultural, hand tool, lawn and garden and medical application. Figure 6 shows a nitrocarburized medium carbon, low-alloy pin used in an automotive application.
To establish a baseline of the processes being measured and controlled, a number of tests were performed in a batch furnace using the new ammonia measurement and control system. Furnace temperature and ammonia dissociation information (Fig. 7) were recorded for two process cycles. The ammonia flow in the furnace was kept constant, while the temperature was set at 1100 F (595 C) during the first cycle and 1250 F (680 C) during the second cycle.
Tests were performed (Fig. 8) to determine the optimal amount of ammonia to produce the desired processed part characteristics. When the dissociation of ammonia is decreasing, the possibility exists that excess ammonia gas is being supplied to the furnace. Control of gas flow in this case will allow realizing a substantial gas savings.
Flow control can be implemented after understanding the process dynamics. Part configuration and surface area must be taken into consideration when selecting the control algorithms. For the two loads made of the same material and processed at the same temperature, the load with the larger surface area will tend to reduce nitrogen activity more rapidly, as more ammonia molecules dissociate at the larger steel surface area per unit of time. Figure 9 shows a chart of the controlled ammonia dissociation during two consecutive batch processing cycles. The microstructure of the part processed using constant ammonia flow method is shown on the left side of Fig.10 and that of the part processed using reduced ammonia flow is shown on the right side of Fig. 10. IH