Carburizing

Carburizing diffuses carbon into steel surfaces to create a hard, wear‑resistant case over a tough, ductile core for fatigue‑resistant parts.

Overview

Carburizing, also called case hardening, is a heat treatment that enriches the surface of low-carbon steels with carbon, followed by quenching. This produces a very hard, wear-resistant outer layer (case) while keeping the core relatively soft and tough. It is commonly done in gas or vacuum furnaces and specified by effective case depth and surface hardness.

Use carburizing when you need high contact fatigue strength, wear resistance, and impact toughness in the same part—typical for gears, shafts, and cam components. It suits medium to high volumes and parts that can tolerate post-heat-treat grinding on critical features. Tradeoffs include distortion risk, limited material range (steels only), and longer cycle times compared to simple through-hardening. Good results depend on choosing a suitable steel grade, defining case depth and hardness clearly, and planning for machining or grinding after heat treatment on tight-tolerance surfaces.

Common Materials

  • AISI 1018
  • AISI 1020
  • AISI 8620
  • AISI 9310
  • AISI 5120
  • AISI 4140

Tolerances

Case depth typically held to ±0.005"; dimensional change from heat treatment often in the ±0.001"–±0.003" range depending on geometry and fixturing.

Applications

  • Gear teeth and gear blanks
  • Spline shafts and transmission shafts
  • Camshafts and rocker arms
  • Chain sprockets and drive components
  • Bearing races and sleeves
  • Pins, followers, and wear pads

When to Choose Carburizing

Use carburizing when you need a very hard, wear- and fatigue-resistant surface on a steel part while keeping the core tough and impact-resistant. It is ideal for loaded contact surfaces such as gears, splines, cams, and bearing interfaces, especially in medium to high production where tooling and process control can be optimized. Choose it when you can allow for some post-heat-treat machining or grinding on tightly toleranced features.

vs Annealing

Choose carburizing over annealing when the goal is a hard, wear-resistant surface with a tough core, not soft, ductile material for machining or stress relief. Annealing improves machinability and formability; carburizing sacrifices machinability at the surface to gain hardness and contact fatigue strength for fully finished parts.

vs Normalizing

Use carburizing instead of normalizing when you need a controlled hard case for wear and fatigue, not just refined grain structure and uniform mechanical properties. Normalizing is a conditioning step; carburizing is a final performance-driven treatment tailored to case depth and surface hardness on high-load surfaces.

vs Quenching and Tempering

Pick carburizing over quench-and-temper when you require a hard surface with a relatively soft, ductile core, such as in heavily loaded gears and splined shafts. Through hardening (quench and temper) gives uniform hardness, which can increase brittleness in the core, whereas carburizing concentrates hardness at the surface for better impact and bending fatigue behavior.

vs Nitriding

Choose carburizing instead of nitriding when you want deeper case depths, can accept higher processing temperatures, and plan on significant stock removal after heat treat. Nitriding is lower temperature with low distortion but shallower cases and stricter material requirements; carburizing offers deeper, very hard cases and is often better suited for heavy-duty gears and thick sections.

Design Considerations

  • Specify steel grade, required effective case depth, and surface/core hardness ranges explicitly on the drawing or specification
  • Keep tight-tolerance or critical geometry surfaces accessible for post-carburizing grinding or honing and allow sufficient grind stock
  • Avoid very sharp corners, thin walls, and abrupt section changes in high-case areas to reduce distortion and case-depth variability
  • Call out masked or non-carburized regions clearly, including threads, sealing surfaces, or bores that must remain soft or machinable
  • Define allowable distortion and flatness/roundness limits after heat treatment so the shop can plan fixturing and straightening
  • Coordinate geometry and batch size with furnace dimensions and hanging/fixturing method to ensure uniform case depth and minimize shadowing