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.
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