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.