Tempering Temperature
Is tempering temperature below the critical temperature?
Yes. Tempering reheats hardened steel to a temperature below the lower critical range, unlike hardening or austenitizing, which occurs above critical temperatures. This keeps the desired hardened structure while relieving stress and adjusting toughness.
Why does higher tempering temperature reduce hardness?
Higher tempering promotes further decomposition of martensite and greater stress relief, which increases toughness and ductility while reducing as-quenched hardness. The higher the temper, the more the original hard martensite softens.
What tempering range is commonly used for tool steels in CNC work?
Common references place many tool steels around 200–300°C (roughly 350–600°F), but the exact value must come from the alloy data sheet and the required hardness callout. Lower temps preserve hardness; higher temps favor toughness.
Tempering temperature is the controlled reheating temperature applied after hardening or quenching to reduce brittleness, relieve quench stress, and tune the hardness–toughness balance of steel. It sits below the lower critical temperature and typically ranges from 150°C to 650°C depending on alloy and required hardness. This post-quench step determines whether a part stays hard for wear or becomes tough for impact loading.
On the shop floor, tempering temperature is not a generic heat-it-up value; it is the alloy-specific setpoint that locks in final microstructure. The operator selects the temper range based on whether the part must hold a hard cutting edge, survive shock loading, or remain stable after finish machining. Low tempering around 150–300°C preserves as-quenched hardness for tool steels and wear surfaces. Medium tempering around 300–500°C balances hardness and toughness for gears and shafts. High tempering around 500–650°C maximizes toughness and impact resistance for structural parts. In a CNC cell, timing matters: tempering is done after quench hardening and before final finish because it relieves residual stress and can shift thin walls, long shafts, or asymmetric features. Critical dimensions are often re-verified after tempering, and rework stock may be left for post-heat-treat machining. The chosen temperature ultimately governs edge retention versus brittleness, so data sheets and required hardness callouts drive the setpoint.
Wrong Temper Range: Tempering too low leaves brittle martensite that chips or cracks under shock load, while tempering too high softens the part past spec, causing premature wear or rolled edges. The duty cycle must drive the setpoint.
Ignoring Alloy-Specific Temper Response: The same numeric temperature does not produce the same hardness across different steel grades; a generic shop recipe can miss the target by several HRC points, leaving the part under-tempered and brittle or over-tempered and too soft.
Not Planning for Dimensional Movement: Tempering relieves residual stress and shifts critical features on thin walls, deep pockets, or slender shafts; a part within tolerance before heat treat can drift out of size afterward, causing scrap, extra grinding, or unstable final fits.