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.
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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