Impact Of Delivery And Control In Vacuum Carburizing
The article "New Technology Is The Next Step In Vacuum Carburizing" (Industrial Heating, October 2002) introduced a completely new vacuum carburizing approach, known commercially as VringCARB(tm), that utilizes a high purity liquid napthene hydrocarbon as the carburizing agent. Since that time, significant continuing development has been performed. A key portion of that work relates directly to the patent-pending liquid injection and control system and the benefits provided in the areas of purity, delivery and control of the vacuum carburizing process. A comparison with current vacuum carburizing standards is presented to provide a better perspective of the new approach.
Propane is the dominant carburizing gas utilized by the industry today, although acetylene and gas blends are also being aggressively promoted together with other alternatives. Each approach seeks to address two key issues; first is the tendency for the process to form soot (carbon drop-out), and second, the fundamental difficulty of getting a "pure" carburizing media to the parts in a uniform manner (i.e., case variations part to part in a load).
Each of these issues will be discussed, but it is important to acknowledge that current processes do carburize and are currently running production at some level in the industry today. However, as is the goal of any new technology, this article describes how the VringCARB approach expands and/or extends the performance potential for vacuum carburizing.
Current vacuum practices involve a range of techniques developed over the years, all of which are basically intended to attempt to get the carbon to the workpieces in a uniform manner before it either drops out as soot or is evacuated (removed from the chamber by the vacuum pump).
A contrast to conventional atmosphere gas carburizing is useful to better understand this issue. At atmospheric pressure, a fan is used to mix and circulate the atmosphere throughout the workload. Many designs provide a gas inlet directly above the negative pressure side of the circulation fan to force fresh carburizing gas directly into the load. The fan also provides complete remixing of "fresh" gas with the older "spent" (carbon has been released to the workload) gas. The result is the fan provides the mechanical force to get carbon into the most dense load configurations. In low-pressure vacuum carburizing, no fan is used because the impact of a fan moving gas molecules at 10 torr is extremely limited since there is about 1% of the normal gas carburizing atmosphere pressure (1% of the gas molecules) in the work zone.
For this reason, current vacuum carburizing methods typically use multiple inlets or some form of header system to attempt to direct the gas (propane, acetylene, or blends) to all areas of the workload. These distribution plenums can be thought of as ring burners or sprayers to expand or spread out the gas penetration. As designs are enhanced (i.e., addition of "pulse" or "sweep"), gas distribution techniques can help get the fresh gas to remote areas of the load. Typically furnaces are "tuned" by changing the gas flows entering the heated zone. If load configurations are frequently changed or if significant "shadowing" of parts in a load is present, then part loading is often reduced to allow the remaining parts the chance to be fully exposed to the gas. When the parts do not see the gas completely, less case depth occurs, and variations in hardness may result.
The next step in delivery technology
To improve carbon delivery to the load, a liquid injection delivery system was developed that allows precise rapid metering of the gas from multiple injectors with microprocessor control. This approach allows continuous programmable direction of uniform, fresh gas flow at varying positions within the furnace hot zone.
The injection system is comprised of three main components as described in Fig. 1. The injectors are rugged and precise components that are also highly reliable in operation. The injector assembly mounts to the furnace with a simple "KF" fitting and can be quickly disassembled if necessary. This design is typical of Surface's move to proven modularized designs on all new developments. Wiring for the injector is accomplished by the VringCARB Injector Termination Board (Fig. 2).
To provide control, the fuel injectors interface directly to the microprocessor. The injector control board (Fig. 3) was developed by Surface Combustion specifically for the application. This VringCARB board controls any number of injectors up to eight. Programming the board is performed with a set of switches and once set, the board knows how many injectors are available and how to fire each injector at the appropriate time. The change in burst pulse widths is automatically controlled. Even if manually turning off an injector, the board simply reallocates the liquid to the remaining active injectors. Like the injector itself, the board requires 12 volts dc for power and a few interface signals to the main furnace programmable logic controller (PLC).
The injector control board technology controls the burst, or "on time", of the injectors, thereby controlling the pulse width and penetration distance. In addition, the injectors change their burst time throughout the cycle. This changing in burst time results in a fresh gas distribution that can be repositioned throughout the furnace. When multiple injectors are used, fresh gas is dispersed in a highly uniform manner. The control logic is built into a microprocessor, and distribution of gas flow to deep locations within the workload is automatic and repeatable.
Figure 4 shows the injector arrangement in a furnace. Liquid cyclohexane, rich in carbon, is held back by the fuel injectors and uniformly dispersed in the furnace chamber.
Users of atmosphere gas carburizing equipment always ask the question: How do I know what is happening in a vacuum carburizing furnace? By contrast, most vacuum heat treaters don't ask this question. The experienced vacuum heat treater will assume gas additions for partial pressure or gas quenching are in fact working. On the other hand, the typical gas carburizing heat treater wants data about what's going on in the furnace, such as what are the actual carburizing gas furnace constituents (CO2, CO, O2, dew point, etc.) and is the quench OK (are the agitators running)?
Today's standard for control of vacuum carburizing processes is largely based on empirical results rather than direct monitoring or control of key cycle parameters such as carbon potential in the furnace. Control systems use PLC-based algorithms that calculate time to reach a target case depth based on furnace temperature and saturation carbon conditions in the furnace.
For a given part, the boost-diffuse cycle is calculated based on the desired carbon case depth. This cycle control system assumes that all conditions to achieve success are met if the furnace temperature and flow of carburizing gas is maintained. Unlike atmosphere carburizing, there are no oxygen probes or gas analyzers that ensure proper atmosphere conditions are being met during the process cycle. In vacuum carburizing, it is not possible to simply input a carbon set point and control the atmosphere to a percent carbon level below saturation. Because direct measurement and monitoring of the process atmosphere conditions is not performed, "process" upsets cannot be recorded by the PLC during the process, and therefore, can only be detected in post-process quality control.
The Next Step In Control Technology
To address these issues, Surface Combustion developed a reliable, patent-pending in-situ sensor for vacuum carburizing. This device is not an oxygen probe or a system that extracts a sample from the furnace and pumps it to an analyzer. The device looks right into the furnace to determine an optimum and sufficient level of vacuum carburizing gas.
With this technology added to the vacuum carburizing furnace, the heat treater will know there is sufficient fresh carburizing gas in the furnace to maintain the necessary carbon level at the part surface. Before the carburizing cycle has begun, the sensing system reads "zero." During preheating of the workload when the furnace is under a hard vacuum, the sensor monitors the furnace chamber and through self-diagnostics should also read zero. When the carburizing boost portion of the cycle has started, the sensor moves swiftly upscale indicating the presence of carburizing gas. Typical spent gas from the carburizing process is hydrogen. However, the system does not see the presence of hydrogen, and is therefore able to monitor the remaining carburizing gas. The system can determine quite easily if excess gas is available, or in the case of high surface area production loads, if an insufficient gas situation is occurring. At the end of the boost cycle, when the carburizing gas is removed from the furnace to allow diffusion, the sensor ensures that in fact the gas has left and verifies that the chamber is truly void of carburizing gas.
The sensor technology is also ideal for high temperature carburizing. Boosting temperature in vacuum carburizing is easy since the equipment is typically built for high temperature operation. This is a highly desirable attribute of vacuum carburizing because, as temperatures are raised, cycle times are substantially decreased. At elevated temperature, carbon diffusion is happening very fast, and maintaining carbon at the surface of the part is even more critical. Carbon is diffusing away from the surface at higher rates of speed and the carburizing boost cycles are short. Therefore, with carb cycles at 10 to 15 minutes, not having sufficient gas during this period of time could be disastrous. The sensor ensures that sufficient gas is available during these short duration, high carburizing rate times.