It’s tough to case harden low carbon steels. Simply put, more carbon equals more hardness, a feature that even the casual observer can appreciate. One solution to this processing puzzle is to introduce more carbon. In essence, the low carbon surface is infused with a supplementary carbon source. Consequently, as the additional carbon is artificially introduced, the super-heated workpiece is slowly cooled. Let’s examine the intricacies of this process.
Carburization Facilitated Case Hardening
Just to underscore the issue, it’s hard to case harden a low carbon steel when carbon is the required process instigator. Logically, we need more carbon. We need to add carbon-rich iron shavings, carbon powder, or some other sooty medium to the heat treatment procedure. Cycled several times, the carbon medium binds itself to the surface of the carbon-low alloy until it’s case hardened. Referred to as a cementing or carburizing stage, the operation basically enhances the technique by introducing the case hardening element.
Furnishing the Carbon Medium
Depending on the furnace configuration, several carbon supplementing techniques are on hand at this stage of the project. A gas carburization process injects carbon monoxide into a heated furnace. Alternatively, there are molten liquid carbons and salted baths, which diffuse the element into the surface of the workpiece. Deposited and cemented, the slow cooling surface assumes its case hardened characteristics. Finally, we’ll use pack carburizing as a work-in-motion example, a method that illustrates the finer points of the carbon supplementation cycle.
Pack Carburization Proceduralism
Uniformly heated until the steel part glows red-orange, the component is immersed in its carburizing medium. This time, instead of a salted bath or a gaseous environment, it’s a bone charcoal mix that covers the processing part. Still furnace-hot, the workpiece is run through this cycle several times. The charcoal is binding itself to the low steel component, cementing the carbon to the part’s surface. We’re not quite done just yet. The charcoal detritus needs to be rinsed and the part hardened via a water cooling station. Finally, the annealing stage takes place, which means the subtly altered microcrystalline surface should be left to slowly cool. Further tempering work is typically carried out before the case hardening project is considered complete. Typically, the additional stages call for reheating, cooling, and water or oil quenching.
Here’s a process that can’t run on heat alone. A secondary element is required to facilitate low carbon case hardening, a literal second element. Supplied as a supplementary reservoir of sooty carbon, as a gaseous carbon monoxide compound, or simply as wood charcoal, the subsidiary medium increases surface carbon content, then ultimately cements that element to the low carbon alloy until it’s properly case hardened.
Vacuum heat furnaces perform expeditiously as atmosphere controlled heat treatment chambers. Drained of process-influencing oxygen and filled instead with an inert gas, the furnace stops scaling, stops parts discolouration, and generally improves the mechanical and physical characteristics of any processed workpiece. What happens if this sealed chamber develops a leak? The vacuum is tainted and the heat treatment process is compromised. We need to find that leak, and fast.
Fundamental Leak Management Tips
Sudden leaks are often easier to diagnose, for they’re often preceded by some other equipment defect. However, a chamber breach that develops slowly is one that will likely go undetected for some time. An unpredictability factor, therefore, exists unless a proactive maintenance strategy is adopted. In short, a scheduled leak testing check and repair program should be monitoring vacuum integrity.
A Deeper Investigation
A niggling thought occurs: if the leak is small enough, can it be ignored? The short answer is always a firm, “No.” The workpiece isn’t experiencing the full vacuum. That inert gas isn’t uniformly distributed, the reactive heating elements are showing signs of attack, and the pumping mechanism is running continuously. The leak, even if it’s a pinprick, must be found and sealed. At this point, a background history is handy. Is this a newer or older model? Has it been in service for years? Perhaps the seals have failed before? Consult past maintenance documentation, and look for a pattern.
Active Leak Detection Methods
If you remember the old method of finding a hole in the walls of a bicycle inner tube, then you’ll know what’s coming next. In this case, however, the technology takes the principle to a whole other level. Advanced solvent test mediums are sprayed on suspect surfaces. Sealed once more, the vacuum is applied to the chamber. If the pressure gauge drops and the solvent indicates a leak, then a temporary putty seal is created. Tested again, the leak is said to be found if the gauge now holds steady. Alternatively, and this technique works best for those pinhole leaks mentioned earlier, a qualified operator sets up a helium mass spectrometer, a test instrument that traces small quantities of free-floating helium.
Solvent tests and inert gas spectrometer checks are effective vacuum heat furnace leak detection solutions. However, a predictive maintenance program can be just as effective, especially when the documentation held in that program contains the history of the chamber. Used as a tool to mark areas where wear and fatigue are occurring, the pattern detected by a scheduled maintenance plan isn’t quite as active as a spectrometer, but it can provide valuable clues, signs that point to certain seals and surfaces.