Heat treatment is where a gear gains its working strength. A gear cut from steel may have the right geometry, but it will wear, pit or break quickly if the surface is too soft for the load. Most industrial gears are therefore case-hardened: a hard, wear-resistant surface over a tough, shock-absorbing core. Choosing the right hardening process and specifying it clearly on the drawing is one of the most important decisions in gear procurement.
Carburizing, also called case hardening, is the most common process for heavily loaded gears. The gear is heated in a carbon-rich atmosphere so that carbon diffuses into the surface layer, then quenched to form a hard martensitic case. Typical case depths range from 0.3 mm to 2.0 mm depending on module and duty. Materials such as 20CrMnTi are designed for this process, and carburized gears are the default choice for automotive and heavy industrial transmission gears.
Induction hardening heats only the tooth surface locally with an electromagnetic coil, followed by a quench. It suits medium-carbon steels such as 45 steel or 40Cr, produces a hard surface while keeping the core tough, and is efficient for large gears or for parts that need hardened zones in specific areas. Because only the surface is heated, distortion is usually lower and the process is fast, but the achievable case depth and pattern depend on tooth geometry and coil design.
Nitriding introduces nitrogen into the surface at a relatively low temperature, typically 480 to 580 °C, with no subsequent quench. This produces a very hard, thin case with minimal distortion, which makes it attractive for precision gears that must not change shape during treatment. Nitriding suits gears running at moderate loads with high precision requirements, where the low distortion compensates for the thinner case depth.
Hardness is specified as surface hardness and core hardness, usually measured on the Rockwell or Vickers scale. Typical targets are 58 to 62 HRC on the tooth flank for carburized gears, with a core hardness around 30 to 40 HRC for toughness. Buyers should state the hardness range, the case depth, and whether hardness testing on a test piece or on the actual gear is required, because these choices affect cost and lead time.
For a quote, include the material grade, the hardening process you expect, the surface and core hardness range, and the case depth if the application demands it. If you are unsure, a capable supplier can recommend a process based on load, speed, tooth module and operating environment. A clear heat treatment specification on the drawing prevents rework and shortens the whole project cycle.