Case hardening steel

Maximum hardness is achieved by heating the carbon steel slightly above it's critical point and quenching it using one of three popular methods. The proper method of cooling is chosen based on the desired end result. Cooling methods are water, air, and molten metal bath. The faster steel is cooled, the harder the outside shell. This will also create the highest degree of brittleness. The slower it is cooled, the less hardness is created in the outside shell and the least amount of brittleness results as well.

Critical temps vary according to C content. Generally, the lower the C content the higher the critical temp.

Case hardening results are best if done in a furnace with a carbon rich atmosphere like CO2. Areas where hardness is undesirable are usually copper coated to prevent the absorption of C into the base metal. The part is heat treated via the quench and temper process. Final tempering improves toughness in the outer shell.

These are basic manufacturing processes that change the grain structure of the metal. For best results, guidelines should be followed in regard to maximum temps and C content.
HTH

Let this cool slowly after adding carbon, a bed of sand works best, stress reliving, then reheat to a dull orange color in the shade, then water quench, then reheat to straw color in the shade, looks like the color of straw hence the name straw and quench in oil.

You can use nitric acid and sulfuric acid to also control the degree of hardness. As the name implies these are VERY strong acids so great caution is advised.

T_Bone

Also check out US Alloys
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