QPQ Treatment for Steel Parts: Benefits, Tolerance Effects, and RFQ Requirements

QPQ is often described simply as a black finish for steel components, but that description is incomplete. It is a surface-engineering sequence based on ferritic nitrocarburizing, followed by polishing and post-oxidation. The objective is to create a functional surface with improved wear, scuffing, and seizure resistance, together with corrosion performance suitable for the specified environment. QPQ is not black paint, black oxide, conventional plating, or through hardening. Diffusion and chemical reactions modify the near-surface region. The substrate, prior heat treatment, pre-process roughness, dimensional requirements, and inspection method must therefore be defined before an order is placed.

QPQ is a diffusion process rather than a thick build-up coating. Nevertheless, the compound layer, oxide, intermediate polish, and original surface condition may change the measured size. Bores, shaft diameters, threads, and narrow fits should therefore have tolerances defined after treatment. Intermediate polishing reduces roughness peaks and prepares the surface for final oxidation. The polishing method, removal amount, and accessibility differ between external surfaces, deep bores, and narrow grooves. One Ra value should not be imposed on every surface without reviewing process and measurement feasibility.

QPQ can be applied to many carbon steels, alloy steels, and cast irons, but they do not produce identical results. Alloy content affects surface hardness, diffusion depth, and compound-layer response. Stainless steels or substrates with a passive film require specialized processing and should not automatically use the same cycle as carbon steel. If the component needs core strength, that property must come from material selection and prior heat treatment. QPQ primarily modifies the surface and does not replace through-section mechanical requirements. The process temperature must also be compatible with the previous tempering condition to avoid unintended changes in base-material properties.

Checking only the black appearance is insufficient. Depending on service requirements, an inspection plan may include: Destructive examinations normally require a witness coupon or sample component. The RFQ should define frequency, section location, and acceptance criteria before production.

• Visual condition, color consistency, and scratches.
• Dimensions, GD&T, and functional gauge checks after treatment.
• Roughness on functional surfaces.
• Surface hardness or microhardness using an agreed method.
• Compound-layer thickness and diffusion depth on a representative specimen.
• Corrosion or wear testing when required by the specification.

A typical route may include rough machining, stress relief when justified, near-final machining, cleaning and QPQ, followed by dimensional and visual inspection. If grinding or polishing is required after QPQ, the permitted removal must be defined because excessive stock removal can reduce or remove the functional surface zone. VietTech provides CNC turning, CNC milling, precision grinding, wire EDM, and dimensional inspection. QPQ is coordinated with a suitable surface-treatment partner to the agreed order requirements. VietTech does not apply one default QPQ condition to every material or component.

• Material grade, standard, and incoming heat-treatment condition.
• 2D drawing, 3D model, quantity, and service environment.
• Mandatory process/specification or target performance.
• Surface hardness, case depth, and test method when required.
• Treatment and masking areas, including threads and precision bores.
• Post-treatment dimensions, GD&T, and roughness.
• Appearance, color, and approved sample requirements.
• Process certificate, inspection report, and lot traceability.