Quenching is an essential stage in heat treatment technology. The rapid cooling of a hot workpiece stops unacceptable low-temperature processes, including the microcrystalline alterations that would otherwise compromise the procedure. Hardness quotients drop while overall material toughness rises, all by applying a well-controlled quench. However, there are trade-offs to be made when steel parts are exposed to fluid-contact work. Among them, quench cracking can frustrate the finest heat treatment operations.
What is Quench Cracking?
By “trade-offs,” we’re referring to the hardness-to-toughness quotient that occurs when the steel part is rapidly cooled. Done properly, a desired amount of hardness is offset by an equally important measure of material ductility. Otherwise, the steel would fracture as soon as it was put to work. But this is a dynamic stage. The fluid sizzles, the metal cools right away, and there are energetic forces in play as the part returns swiftly to room temperature. Sometimes, even in a well-regulated operation, the conflicting energies tear at the metal so strongly that it fractures.
Material Expansion and Contraction
In this case, with steel as the workpiece, the part expands as it reaches its maximum transformative phase. By lowering it into a cooling medium, the metal instantly contracts. Quench cracking in steel parts can take place when those conflicting energies rupture the metal’s microcrystalline form. But wait a moment, the clashing thermal loads are dynamic, but are they the sole reason for this undesirable phenomenon? After all, steel is graded to handle the heat, plus the cooling medium that quenches the component, so why is a known contraction incident cracking the steel?
Eliminating Processing Errors
The steel part should be lowered promptly into the quenching pool. If it enters the water or oil too slowly, then cooling takes place in the submerged material, not the still super-heated metal. Furthermore, the quenching station must insert the workpiece at a planned angle of attack. If the part enters the medium at a poorly assigned approach angle, then additional stresses are likely. The steel cracks. Even the quenching medium influences an acceptable cooling curve. Oil and water are both popular cooling fluids, but then there’s brine water, polymers, and more. By selecting the correct fluid, we ensure a safely regulated heat dissipation factor.
In the processing line, equipment misalignments and timing woes cause quench cracking in steel components. Overheated steel also fractures, with the cooling medium pushing the alloy beyond its material limits. The quench fluid is next, then there’s the geometry of the component, its profile and sectional mass. Avoid this expensive mistake by planning for all of these crack-propagating factors.
There’s nothing simple about heat treatment technology. Even the various microcrystalline forms of steel make the process hard to regulate. Among these forms, martensitic stainless steel ranks as a super-hard alloy. It features a dense tetragonal crystalline structure, but where does this dense configuration occur in the heat treatment process? These and other questions will be addressed in the following post. Here, the stainless steel workpiece is approaching a furnace right now.
Directing Austenitic Tranformations
Heated in a furnace until the stainless steel piece radiates 670°C of raw thermal energy, the stainless steel is quickly quenched. As the heat is rapidly absorbed by the quenching medium, the atomic structure reorganizes inside the metal. Instead of a cubic, face-centred crystalline structure, the carbon atoms in the alloy move inwards. The hard yet ductile tetragonal arrangement relies on the fast, diffusionless trapping of the carbon, plus a high quantity of added chromium. Of course, having produced martensitic stainless steel, what comes next in the heat treatment operation?
Martensitic Stainless Steel: Heat Treatment Provisos
Quenching rates and carbon content are the two process-influencing variables to address this time. Additionally, certain alloying metals are known to hamper the hardening phase. By adding more carbon to the steel, there’s more material to relocate into the tetragonal matrices, which means the stainless steel becomes harder. However, alloying metals can attenuate the process. The result is transformation retardation and excessive grain growth. It’s the same for quenching, with certain quench rates causing undesirable carbide precipitates. In plain English, bereft of engineering complexities, this hardened stainless steel type is heat treatment sensitive, so all process variables must be rigorously managed.
Regulating the Process Properly
Employ the normal heat treatment methods, but beware of the control variables used during the various operations. Air cooling works well as a hardening mechanism, but oil quenching is also a recognized hardening solution, one that avoids carbide precipitates. In the meantime, over in the furnace, the next batch of martensitic stainless steel components are being heat treated in the furnace. They’re thermally subjected to between 925°C and 1060°C of furnace heat, at which point they gain corrosion resistant vigour and strength. That’s a product strength, the notion that different quantities of applied thermal energy make it easy to produce completely different strength ratios.
If there’s one phase of the process that receives more attention than the furnace work, it has to be the tempering of the alloy. Granted, martensitic stainless steel is clearly a super-hard alloy, but that hardness quotient also equals an undesirable quantity of process brittleness. Carefully managed tempering work is required to balance that renowned quality against an equally substantial amount of material ductility.
Counterintuitive consequences have been known to compromise heat treatment work. By way of illustration, steel strengthening operations should logically produce strong workpieces. However, there are times when these mathematically predictable results go out the window. Are there strange laws of probabilities in effect? Or is there a straightforward reason for heat treatment induced weakness? Uncertainty factors are not acceptable, not in heat treatment work.
Holding Up the Process for Scrutiny
Think of a heat treatment procedure as a two-stage operation. The component is heated, then it’s cooled. Like a sawtooth graph, thermal energy is the initial ramp, then the downward slope of the triangle is created by the cooling procedure. It should be obvious at this point in the proceedings that the workpiece is about to become weaker. It has to be transformed and softened so that it can be worked and homogenized. For that latter procedure, the microcrystalline structure of the ferrous alloy is redistributed and altered. Meanwhile, the apex of the triangle is imminent.
Peak Transformative Thresholds
The highest temperature in the furnace stops just short of allowing the steel particles to move freely. At approximately 720°C, the grain is about to transform. So where is the weakness introduced? Well, depending on the heat treatment technique, several zones on this graph can weaken the steel part. If the metal isn’t hot enough, carbon can’t be absorbed and diffused. Then there’s substandard stress relieving work, where the process uses heat to normalize internal stresses. Unaddressed, those stresses will turn a strong steel part into a fracture-prone one.
Tempering and Quenching Issues
Exposed to an advanced alloy toughening stage, the steel hardens. It’s brittle, but a tempering operation removes that weakness. Likewise, quench-hardened components can be brittle, and it’s the tempering phase that again removes this overly rigid substructure. Using the correct temperature bands and a corresponding cooling period, steel part hardness drops while its ductility quotient rises. However, certain low temperatures can actually diminish these effects and leave the part brittle. It’s the same with quenching and other cooling operations. Hypothetically, perhaps the downside ramp of the graph sees the quenching phase take place in an unsuitable medium. Water, as one example, can induce part’s expansion and contraction, a response that could weaken or even deform the steel part.
Wielded by seasoned exports, a heat treatment processes always yields predictably strengthened parts. However, if the equipment has a vacuum leak, if the process is quenched improperly, or if it’s heated unevenly, then alloy weakness is a likely upshot. That’s a lot of “ifs,” but they all fall before an expertly conducted heat treatment service.
While medium carbon steel clearly isn’t as hard as a high-carbon alloy, those lenient mechanical qualities can be enriched. By exposing the steel to flame hardening equipment, a heat treatment technique that uses a focused cone of combusted gas, we fortify the material and reinforce its generally weaker microcrystalline bonds. A mild steel workpiece is approaching the flame now, so let’s do a before and after comparison.
When Oxy-Gas Meets Medium Carbon Steel
As mentioned in several past articles, flame hardening technology is a targeted high-temperature heat treatment process, one that uses a focused, unwavering open flame to selectively surface harden different types of metal. In this case, we’re transporting a medium carbon steel workpiece towards the flame. Locked in place, the procedure begins. The oxy-gas head traverses the length of the part, the metal reaches its critical transformative temperature, and then it’s rapidly quenched. A hardening depth of around 3 – 5mm is certainly feasible here, but it’s difficult to go any deeper without introducing a secondary source of carbon.
Flame Hardened Qualities
For medium carbon steel, the open flame surface is harder and stronger than the “before” material, but the hardness depth is inflexible. Transformed until the lower carbon content gifts the alloy with moderate strength and a reasonable amount of wear resistance, the heat treated parts processed here tend to end up in the light-to-medium industrial sector. Axles and transmission rods exit the equipment ready to endure substantial loading factors, as do shafts, spindles and gear tooth areas. As long as the flame head covers every square millimetre of the medium carbon workpiece, it will gain all of the qualities required to satisfy these parts taxing applications.
Supported by Numerical Data
The first relevant number here is the amount of carbon incorporated into the steel. Medium carbon workpieces typically add between 0.3 and 0.65 percent of this heat-transformative element, so the formerly soft metal receives a hard but possibly slender shell. Somewhere in the region of HRC 54 to HRC 60 (Rockwell Hardness) is entirely possible when that flame gets to work. As for varying that fixed value, quenching fluid types are known to enhance the process. Furthermore, specially adjusted twin flame heads have an advantage. Spaced just-so, the twin flame heads introduce more carbon into the operation, so the heat treated hardening depth deepens.
An investigation into flame head geometries yields positive results. Although the lower carbon content in that medium grade steel alloy forms a thinner case depth, that quality does become controllable when the head array is adjusted. Last of all, additional manganese and alternative quenching fluids are known to improve the qualities of medium grade steels. At the end of the day, this heat treatment procedure equips medium carbon steel components with a moderately hardened surface, with exteriors that are also relatively wear resistant.
Much has been covered in past posts. The basics of heat treatment technology formed a foundation, then that groundwork received new and interesting additional levels. They dealt with surface hardening issues, quenching anomalies, and all manner of tricky alloy manipulating techniques. Now we’re reinforcing the groundwork and reviewing the fundamentals of heat treating science, plus many of the physical traits that characterize this essential post-processing phase.
Property Manipulating Essentials
Heat treatment engineers master many alloy altering disciplines, yet they never lose track of one basic truth. Qualified to wield all sorts of advanced engineering equipment, the goal is still to develop a raw metal workpiece until it exhibits the desired material profile. On balance, pure metals don’t slide into a furnace. No, it’s a specially amalgamated alloy that undergoes the process. Put it this way, thanks to added carbon, to property-altering trace metals, that selected material comes to the heat treatment area with a unique fingerprint. Faced with this specially tailored alloy, it’s the job of the engineering team to process that workpiece until it’s imbued with a nominated set of mechanical and physical properties.
In-Process Stress and Desirable End Properties
After the attached ASME or SAE-AISA labelling codifiers are interpreted, the project gets underway. The goal is to convert the microcrystalline structure of the alloy, to change it so that it becomes corrosion-resistant and harder. Other essential material properties include ductility and tensile strength. Take heed, processing environments and machine shops can also inadvertently alter the structure of a worked metal part. They cut and bend, drill and mill components until cold-worked stress is trapped inside the part. Therefore, not only must the heat treatment equipment add the desired end-process material characteristics, it must also dissolve in-process work stress.
The Different Heat Treatment Options
At the most fundamental level, there are furnaces and quenching pools. The furnace hardens or softens the part. Alternatively, the outer surface is subjected to a case hardening procedure. Next, the alloy part is rapidly cooled. The quench operation locks in the desired material grain, and thus the sought after metallurgical traits are achieved. Of course, a hard component isn’t much use if it’s going to fracture. To avoid this brittle condition, the alloy needs to be tempered. As the furnace imparts an element of toughness to the tempered material, it ends up malleable and workable.
Reviews of fundamental processes are always helpful. Still, that groundwork can’t hide the complex issues that come into play when the furnaces are lit. There’s numerous alloy grain types, heat treatment systems, localized flames, all-covering induction ovens, and vacuum or atmospheric gases employed during a working heat treatment operation. Driving away below these complex elements, however, the fundamentals keep the process in check.