AMS 6308 is a secondary-hardening alloy supplied as plate and bar for aircraft gearing, splines and connectors that must serve at temperature. Its wear resistance comes from a controlled carbon and molybdenum balance that precipitates fine M6C carbides during heat treatment, not from bulk hardness alone. Because the plate is normally produced by forging, the metallurgical state left by forging is the starting point for every subsequent operation.
Material Background and the Role of M6C Carbides
AMS 6308 belongs to the second generation of aviation wear-resistant alloys. The alloy is designed to work at elevated temperature as well as under sliding and rolling contact, so its microstructure must combine a tough, fine-grained matrix with a dense dispersion of hard carbides. The M6C type carbides deliver wear resistance, while the matrix supplies the toughness required by gear teeth and connectors.
Two conditions are known to spoil that balance during production and trial production. When the amount of deformation applied to the plate is insufficient, coarse grains and an uneven structure survive into the finished component. When the plate is forged at high temperature and the final forging temperature is not properly controlled, coarse M6C carbides precipitate along the grain boundaries and the mechanical properties fall.
What Preliminary Heat Treatment Must Achieve
A preliminary heat treatment is applied before the final hardening sequence. Its purpose is to remove the structural inheritance of forging: to dissolve a coarse grain-boundary carbide network, to refine the austenite grain size, and to leave the plate in a condition where quenching produces uniform fine martensite with carbides dispersed as spheroids or ellipsoids rather than as continuous boundary films.
Normalizing followed by tempering is the standard route. Normalizing temperature is the controlling variable, because it governs carbide dissolution and austenite grain growth at the same time, and those two effects pull in opposite directions.
The influence of preliminary heat treatment on structure and properties is studied by combining microstructural examination with mechanical testing. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterise carbide morphology, size and distribution, together with martensite lath size, carbide volume fraction and dislocation density. Tensile strength and impact values are then measured on specimens given the same preliminary treatment, so the structural observations can be linked directly to the property response.
SEM: carbide shape, size and distribution at lath boundaries and grain boundaries
TEM: martensite lath and lath bundle dimensions, dislocation density
Mechanical testing: strength and impact value after quenching
Normalizing Below 980 °C
When the normalizing temperature is held below 980 °C, raising the temperature produces a progressive improvement. M6C carbides dissolve gradually, the austenite grain size becomes finer, and after quenching the martensite laths are uniform and fine. Carbides appear as spheroids or ellipsoids dispersed along lath boundaries and grain boundaries. Carbide volume fraction and dislocation density are relatively high, and both strength and impact value increase as the normalizing temperature rises toward 980 °C.
Normalizing Above 980 °C
Above 980 °C the trend reverses. Carbide dissolution continues, but the driving force for grain growth takes over and the austenite grains begin to coarsen. After quenching, the martensite lath bundles are larger, the carbide volume fraction and dislocation density are lower, and both strength and impact value decrease. The loss of toughness is the more serious effect for gearing service, because coarse lath bundles and a sparse carbide population provide an easy path for crack propagation. The 980 °C level therefore behaves as a practical threshold that separates a refining regime from a coarsening regime.
Recommended Preliminary Heat Treatment Schedule
| Parameter | Recommended value | Purpose |
|---|---|---|
| Normalizing temperature | 980–1010 °C | Dissolve M6C carbides while limiting austenite grain growth |
| Tempering temperature | 680–700 °C | Relieve stress and condition the structure for the final hardening step |
| Structure after final heat treatment | Uniform fine martensite, dispersed carbides | Good match of strength and toughness |
The recommended normalizing window sits at and just above the 980 °C threshold. That window is a compromise: it is high enough to dissolve the coarse M6C network that causes embrittlement, yet tight enough that grain growth and lath bundle coarsening stay under control. What must be avoided is uncontrolled overheating well above the window, which pushes the alloy fully into the coarsening regime and cannot be recovered by later tempering.
After the recommended preliminary treatment and the subsequent performance heat treatment, AMS 6308 wear-resistant plate shows a good match of strength and toughness. For plate users, the practical points are to control furnace uniformity around the normalizing set point, to record the final forging temperature of the incoming plate, and to verify carbide morphology and grain size on a test coupon before committing a production batch.
Frequently Asked Questions
Q: What is the role of M6C carbides in AMS 6308 wear-resistant plate?
M6C carbides are the hard phase that provides wear resistance. They must be fine and evenly dispersed; when they form a coarse network along grain boundaries the plate loses toughness.
Q: Why does normalizing below 980 °C improve properties?
In that range M6C carbides dissolve gradually while the austenite grain size becomes finer, so quenching produces uniform fine martensite laths with a high carbide volume fraction and dislocation density.
Q: What happens if the normalizing temperature goes far above 980 °C?
Carbides continue to dissolve, but the austenite grains coarsen. Martensite lath bundles grow, carbide volume fraction and dislocation density fall, and both strength and impact value decrease.
Q: What preliminary heat treatment schedule is recommended?
A normalizing temperature of 980–1010 °C followed by tempering at 680–700 °C. After the performance heat treatment this gives a good strength and toughness match.
Q: Can preliminary heat treatment compensate for insufficient forging deformation?
Partially. Normalizing refines grains and redistributes carbides, but a structure that is still uneven after insufficient deformation must be assessed on a coupon before release.
Q: How should the structure be verified before production?
Use SEM and TEM to check carbide morphology and distribution, martensite lath dimensions and dislocation density, and confirm the result with tensile and impact testing on the same coupon.
