Pack Carburizing
What steels are best suited for pack carburizing?
Low-carbon steels are the usual choice because carburizing is intended to add carbon to a carbon-poor surface. The process is commonly applied to low-carbon steel or iron components to build a hard, wear-resistant case while keeping the core tough.
Why is austenitizing required during carburizing?
Carbon diffuses effectively into steel only when the surface is in the austenitic phase, which occurs at carburizing temperatures around 850–950°C. This phase allows carbon atoms to enter the iron lattice and form the enriched case.
What hardness can be achieved with pack carburizing?
Depending on carbon level and post-quench treatment, carburized cases typically reach 55–65 HRC. The exact hardness depends on the steel grade, case depth, and tempering cycle after quenching.
Pack carburizing, also known as box carburizing, is a solid-state case-hardening process used for low-carbon steel parts. They are packed in a sealed container with a carbon-rich material and heated to around 900–950°C in the austenitic range. Carbon diffuses into the surface, creating a hard, wear-resistant case over a tougher, lower-carbon core.
On the shop floor, pack carburizing is a practical choice for low-volume work and legacy heat-treat lines. A machinist starts with cleaned low-carbon steel parts—small gears, pins, fasteners, hand tools, or wear components—and packs them evenly in a steel box filled with charcoal, coke, or commercial carburizing compound. The box is sealed with refractory clay to keep out oxygen, then placed in a furnace and soaked at 900–950°C. The carbon-rich pack generates carbon monoxide, which transfers carbon to the part surfaces. Operators must monitor packing density, seal integrity, soak time, and quench response. Uneven packing or a poor seal leads to inconsistent case depth, distortion, or oxidation. Because diffusion controls the depth, process times run from hours to tens of hours. This method tolerates slower turnaround and less precise carbon-potential control than gas or vacuum carburizing, so it is reserved for parts that can be finished by grinding after hardening.
Leaking box seal: If the container is not fully sealed with refractory clay, oxygen enters and oxidizes the charge, reducing carbon transfer. The result is a shallow, uneven case that fails wear requirements.
Overpacked box: Tightly crowded parts block contact between the carburizing medium and steel surfaces. This creates inconsistent carbon pickup and hardness variation across the batch, forcing rework or rejection.
Wrong thermal cycle: Insufficient soak time leaves the case shallow and soft, while overheating or an improper quench promotes grain growth, distortion, or cracking. Each failure weakens the part or forces rework.