Tempering reduces hardness, increases toughness

The blades are reheated, held at about 1000 degrees F for about 2 hours, and air cooled; reheated to about 950 degrees for another 2 hours and allowed to cool again. The tempering process relieves stresses created by hardening so that the steel will not crack. Although tempering increases toughness, it lowers hardness, as can be seen in the tempering graph. Tempering also stabilizes the internal structure and increases the volume of the steel. Toughness is the ability of the steel to absorb energy (such as from impact) without breaking. For example, glass is hard but lack toughness. Chipper blades must be both hard and tough. Tempered blade blanks are ready for the final process of shaping and sharpening by grinding.
The black line in the tempering graph shows that steel which has undergone an austenitizing temperature of 1850ºF will have a hardness of about 57 after being held for one hour at 1000ºF. This temperature is a compromise between reducing retained austenite (desirable) and decreasing hardness (undesirable).
Suppose the steel is tempered for one hour at 600º F instead of 1000º F ?
The after-temper hardness is the same, but the amount austenite ( left over from the hardening process) has not been reduced. We want the retained austenite to be as low as possible without sacrificing hardness.
The steel manufacturer recommends a second tempering at 950º to make a more uniform microstructure and increase toughness.
The graph below shows reduction in hardness resulting from longer tempering times. For example a 2 hour temper at 1000º F results in a hardness of 56+.
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