Apex Dynamics utilizes an in-house plasma Nitriding Heat treatment process which controls the gear tooth surface hardness at 840 Hv (68 Rc) for excellent wear resistance and a core hardness of 30 Rc for toughness and protection from shearing. The Nitriding process has large advantages over every other type of heat-treating when applied to precision gearing.
The cost of the actual process is less expensive than Case hardening or Carburizing, but the upfront investment costs of the Hot wall pulse power plasma furnaces force many manufacturers away from Nitriding due to the long term pay back schedule. Basically, Nitriding is typically too expensive for the start up manufacturer and becomes even more expensive to change over once successful. The Nitriding process eliminates the standard gear tooth distortion you find in other heat-treating processes and guarantees a perfect and harder case depth.
This is one of the concerns with Carburizing and other Case hardening processes being used. The Case depth is dependent on heating and cooling times, which are hard to control. The Depth of hardness can vary greatly and cause what is called Case-core separation which developes premature wear and failure of the gearing itself.
Other processes are dependent on post-process machining, such as grinding, honing & skiving (hard hobbing) gear teeth. This can relieve the distortion in the gear tooth to a degree and is an acceptable practice in lower precision gearboxes. The larger issue is the re-grinding of the smooth gear face diameter to re-create the perpendicularity prior to this post machining. Most manufacturers skip this process due to economic reasons, dis-allowing for precision gear finishing.
What is Plasma Nitriding?
Plasma, or ion, nitriding is a method of surface hardening using an electrical discharge to introduce elemental nitrogen to the gear surface. In a vacuum, high voltage electrical energy is used to form a plasma, or process gas (a mixture of nitrogen and hydrogen). In the presence of this process gas, the load is maintained at a high DC potential with respect to the ion-nitriding vessel. Under the influence of this voltage, the nitrogen gas is dissociated and accelerated to impinge the work piece, which acts as a cathode. Within a short distance of the work piece, the positively charged nitrogen ion then acquires an electron from the cathode and emits a photon. The photon emission during the return of the nitrogen ions to their atomic state results in a visible glow (Figure 4). As the nitrogen concentration increases towards the surface, very fine precipitates are formed when the solubility limit of nitrogen is exceeded. These precipitates distort the lattice structure and thereby increase the hardness of the material. The nitriding current, temperature and process time determine the depth of the nitride case. This process can precisely control the gear material's chemical composition. The advantages provided by this process are:
Harder gear case hardness
Improved control of case thickness and uniformity
Elimination of component distortion
Increased tensile strength of the surface of the gear.
A gear's life rate is directly related to the case hardness. The harder the gear surface, the longer the gear will survive before wearing. Typical gear manufacturers rate their gears for a hardness of approximately 55 Rc. Our plasma nitrided gears have a surface hardness greater than 68 Rc for excellent wear resistance, and consequently longer life. (20,000 hr rated life)
In addition to hardness and wear resistance, the fatigue strength of the gear tooth is significantly increased. The formation of the precipitates on the case results in lattice expansion. The core, in order to maintain its original dimensions, keeps the nitrided case in compression. This compressive stress lowers the applied tensile stress on the material, increasing the fatigue strength.
Another feature of the plasma nitriding process is the gears inherent lubricity. During the latter phase of the heat-treating cycle, the excess nitrides are diffused into the metal, leaving the "white layer". This layer is approximately 0.05 mm thick. The white layer composition formed on the gear provides natural lubricity. Also, the white layer is relatively inert, which provides for corrosion resistance in a variety of environments.
Plasma Nitriding versus Carburizing
Carburizing is the most widely used method of heat-treating gears. The gear is placed in the furnace and heated above the critical, or transformation, range temperature. At this point free carbon is introduced into the furnace and is allowed to soak into the case of the gear material. Typically, a low carbon steel of 0.1% - 0.2% carbon is allowed to reach 0.8% - 0.9% range during the carburizing process, providing a soft core of 24 Rc. After achieving the desired case depth, the gear is quenched in a water or oil medium (The carbon content may sometimes go as high as 1.5%, but it is then tempered back to 0.8% - 0.9%). The case depth of the carburized gear is directly proportional to the time it is in the furnace and the temperature at which it is being soaked. The higher the temperature the faster the soaking and deeper the case, but the drawback of that is that quenching from a higher temperature may cause higher distortion. Small parts and fine pitch parts may be difficult to carburize, and a 55 Rc case may be the highest hardness attainable.
During carburizing the gear is red hot. Distortion is caused when the rate of cooling is uneven in the gear as the outside of the gear cools down faster than the inner part. In addition, the carburized case tends to be larger than before as additional carbon atoms are now embedded in the surface. The net result of this distortion is a tendency to end up with a slightly larger pressure angle and the helix angle tends to unwind. Also, the bore shrinks, the outside diameter becomes slightly coned and the part may develop radial and axial run out. For these reasons carburized parts may need post treatment processing such as grinding or hard hobbing with a carbide hob.
Case depth is usually considered the depth to which the hardness is still above 50 rc. It is typically 75% - 90% of the total case. Case depth is a function of the pitch. In general, the coarser the pitch the deeper the case. Too deep a case will cause the teeth tips to become too brittle and possibly break. This condition is called case-core separation. Too thin a case will reduce teeth strength and cause premature pitting or lead to case crushing.
Unlike carburization, plasma nitrided gears require no rework. The ion nitriding process can be performed at relatively low temperatures, usually between 930 Deg. F to 1,000 Deg. F., way below the transformation temperature. The part is first drawn and tempered to relieve any internal stresses and brittleness, allowing the core to retain its original hardness value of approximately 36 Rc. The ductile core exhibits very high shear strength, allowing the tooth to handle high shock loads. Due to the low temperature, as well as the gears being gas cooled after nitriding, there is no requirement for post-process machining, such as grinding, after treatment. |
