What's the real story on "Through-Hardened" cylinder barrel hardness?

Cylinder barrel hardness is critical to controlling wear. But let's examine the phrase "through-hardened barrel." Cylinder barrel manufacturing typically starts with an Aircraft Quality SAE 4140 steel forging which is heat treated to raise the base metal hardness by a process commonly known as "through-hardening."

For 65 years TCM barrels have been made from through hardened steal forgings. Certainly not a novel concept. These barrels worked well when properly machined and honed in lower power engines. With turbocharging, higher output and higher duty cycle engines, the additional wear resistance process of nitriding was added to TCM's cylinder production. For additional information on this feature click on the Nitride Tough button.