LATEST NEWS
Press releases & Product news
Metallographic Analysis | Microstructural Analysis and Performance Evaluation of Induction Hardened Camshaft
2025-12-26
Induction hardening is a heat treatment process that locally or entirely hardens the surface of metal workpieces through the principle of electromagnetic induction. This process features fast heating speed, minimal deformation, low energy consumption, and strong controllability, and is widely used for surface strengthening of components such as gears, shafts, and guide rails.
Today, we will observe and analyze a sample of a high-horsepower engine camshaft whose surface has undergone induction hardening. The material is 80B. Induction hardening on the surface of steel with eutectoid carbon content is relatively uncommon. So, will the microstructure of this camshaft sample have distinctive characteristics?
Metallographic Sample Preparation
The sample was taken from the cross-section of the camshaft to observe the complete hardened layer and the microstructure at different locations. It was then embedded with NaiBao edge-retaining resin. The high-hardness resin material protects the integrity of the induction hardened layer on the sample surface.
Next, using a NaiBao semi-automatic grinding and polishing machine, the sample was sequentially ground with 240#, 600#, and 800# water sandpaper to remove cutting marks. After grinding, silk polishing cloth was selected, and 5μm and 1μm diamond sprays were used for rough and fine polishing until the surface achieved a mirror finish.
Subsequently, the sample was etched with a 4% nital solution, rinsed with water, sprayed with alcohol, and quickly dried with a blower. After etching, a crescent-shaped dark area was visible to the naked eye on the sample surface. This area is the induction hardened zone.
Microscopic Observation
The sample was first observed under a NaiBao metallographic microscope at 500X magnification. The surface induction hardened layer, after quenching, exhibits an acicular martensitic structure. The size of the acicular martensite affects part performance to some extent. Therefore, there are inspection standards for rating the coarseness and structure of martensite needles. Some companies use internal standards, while others refer to the GBT 38720-2020 standard for inspecting the quenched martensitic microstructure of medium-carbon and medium-carbon alloy structural steels.
The core of the sample cross-section is the area not subjected to induction hardening, unaffected by heating and cooling. Its microstructure is pearlite. As the steel composition is eutectoid, the material of the part can be confirmed to meet the drawing requirement of 80B based on the pearlite content in the microstructure, without the need for chemical analysis.
One of the most important inspection items for induction hardened parts is measuring the effective hardened layer depth and hardness to verify compliance with technical requirements.
After completing the metallographic observation, the sample was polished again to a mirror finish. The effective hardened layer depth was measured using a NaiBao micro-Vickers hardness tester. The testing method follows the provisions of GB-T 5617-2005 for determining the effective hardness depth after induction or flame hardening of steel. Generally, the boundary hardness value for the effective hardened layer of induction hardening is determined based on material composition. For this camshaft, the hardness boundary is 400 HV.
Hardness was measured from the surface to the point where the hardness value reached 400 HV. The distance from this point to the surface is the effective hardened layer thickness of the induction hardening treatment, denoted as DS. Using this micro-Vickers hardness tester, we can not only detect the depth of the induction hardened layer but also generate a hardness distribution curve to observe the hardness gradient. A smooth and gradual transition in hardness from the hardened layer to the core is desirable. Conversely, a sharp drop in hardness significantly impacts the fatigue life of the part.
