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.