While in the U.K. on metallurgical consulting assignments, the writer was presented with a problem of varying surface hardness due to inconsistent formations of retained austenite on repeated batches of carburized and hardened heat-treated components.

Material Analysis and Component Geometry

The steel grade was a European specification known as 21CrCrMo₂, which is almost the same analysis as is for AISI 8620. The principle alloying elements are shown in Figure 1.

Heat-Treatment Process

The products were treated in an integral-quench furnace atmosphere that was electrically heated. The primary atmosphere endothermic-gas generator was produced by blending (and balancing) with natural gas.

• Carburize = 1760°F
• Soak time at temperature = 5 hours
• Dewpoint = 16°F
• Diffusion cycle = Maintaining process temperature of 1760°F with natural gas enrichment (for carbon enrichment during process), turned off once the cycle begins to approach a temperature of 1600°F
• Furnace cool-down to 1510°F in preparation for the oil-quench sequence.
• Oil-quench temperature variation from initial oil temperature of 150°F up to 230°F
• Soak at process temperature to austenitize the carburized case with a dew-point observation of 28-36°F.

More next time.