QPQ

QPQ is a three-stage surface hardening process for steel that cycles through quenching, polishing, and quenching again. The first quench hardens a thin case layer; the polish removes the soft oxidized skin; the second quench re-hardens that fresh surface. The result is a case typically 0.3 to 0.8 mm deep with high hardness, excellent corrosion resistance, and retained toughness in the core. It is widely used for punches, dies, gauges, and fasteners where wear and rust are both problems.

The process uses molten salt baths, usually potassium nitrite or a nitrite-nitrate eutectic mix at 570 to 620°C for the first quench. This temperature is deliberately lower than full hardening temperatures because QPQ relies on nitriding chemistry as much as thermal hardening. Nitrogen from the salt diffuses into the steel surface, forming hard iron nitride compounds. The part is then cooled, removed, and mechanically polished to expose fresh metal. This removes the brittle oxide layer (which would flake under stress) and reveals a sound surface for the second quench.

The second quench is often done in the same salt bath or sometimes in oil, depending on the steel grade and desired final hardness. The re-hardening step seals the nitrogen-enriched layer and locks in its hardness. Because the core steel has never been heated above tempering temperature, it retains its original toughness and impact resistance. This combination, a hard, corrosion-resistant skin on a tough, shock-resistant body, is why QPQ is preferred to simple case hardening or carburizing for many precision tools.

QPQ works best on steels with 0.15% to 0.5% carbon content and is most effective on alloy steels containing chromium, molybdenum, or vanadium. Stainless steels and high-carbon tool steels can be treated but require careful control of timing and temperature to avoid distortion. Part geometry matters: blind holes and thin sections are prone to salt entrapment, which causes corrosion later. Residual salt must be thoroughly removed by washing in hot water and ethanol. The process produces minimal dimensional change compared to conventional hardening, making it ideal for finished precision parts.

Common defects include uneven case depth from poor circulation in salt, excessive oxide formation if polishing is incomplete, and soft spots if the second quench is too slow. The name reflects the literal sequence: the term gained currency in the 1970s and 1980s as industrial hardening shops sought alternatives to case carburizing for high-performance punches and gauges. Modern QPQ is still a niche process, not as common as case hardening or nitriding alone, but it remains the specification of choice in tool and die work where corrosion, wear, and core toughness must all be controlled.