NAK55 vs NAK80: Selecting Prehardened Mold Steel for Precision Machining

NAK55 and NAK80 are Daido Steel prehardened mold-steel grades supplied after solution treatment and age hardening. Daido lists a typical supply-hardness range of 37–43 HRC, so the stock can normally proceed to mold machining without an assumed full through-hardening cycle. Both are Ni–Al–Cu precipitation-hardening steels with relatively uniform hardness from surface to core. This common condition can remove one source of distortion compared with machining soft stock followed by hardening, but it does not guarantee zero movement. Unbalanced stock removal, thin sections, welding, EDM and surface processing still require engineering review. The specified grade, producer, supply condition and material certificate should agree across the drawing and purchase documents.

Two steels near 40 HRC can still differ in composition, microstructure, toughness, polishing response and EDM behavior. A hardness check alone does not identify the grade. Do not replace NAK80 with NAK55 merely because both are prehardened and have similar HRC values. A substitution requires functional review and controlled approval from the design authority. Incoming material should be checked against the lot certificate, stock size, supply condition and identification. Brochure values are representative rather than guaranteed for every section and source.

Prehardened steel needs a rigid tool–holder–machine system, short overhang and a toolpath that avoids sudden load changes. Tool grade, coating, speed, feed and depth of cut must follow the actual cutter maker's guidance and the setup; parameters copied from annealed P20 or SKD are not a sound basis. Deep pockets require chip evacuation and controlled corner stock. Separate roughing and finishing tools help protect texture and dimensional consistency. VietTech reviews geometry, certified hardness, depth, corner radii, roughness and quantity for each RFQ rather than applying a universal cutting recipe.

A note such as polished surface is not enough for quotation or acceptance. Identify the area, finish grade or master sample, viewing condition and unacceptable defects. For EDM, define whether the EDM texture remains, which layer must be removed and the final dimensional requirement. Daido notes that NAK80 can produce a finer EDM surface, while NAK55 may show stripe patterns in some conditions. Actual results still depend on the machine, electrode, dielectric, settings and finishing. Supplier examples should never be converted into a guaranteed Ra or mirror grade without an agreed acceptance method.

Daido provides build-up welding guidance, but mold repair is specialist work because the heat-affected zone, hardness, texture color and dimensional movement can affect the part. Approved filler and procedure are required. Surface processes such as ferritic nitrocarburizing may improve wear, but treatment temperature and dimensional change must be evaluated. VietTech performs CNC machining and inspection within confirmed capability; specialist welding, heat treatment and surface processing are coordinated with suitable partners when specified.

Submit the controlled 2D PDF and STEP model, identify the exact grade and state whether substitution is allowed. Mark mirror areas, EDM regions, textures, thin ribs, sharp corners, threads and deep holes. Define certificate, hardness, final inspection, rust protection and process ownership. For molds, resin, expected production and product-surface needs can be relevant, but confidential details should be limited to what is necessary under the applicable agreement. Avoid a vague note such as NAK steel or approximately 40 HRC.

• NAK55 or NAK80 and producer/purchasing specification
• Controlled 2D drawing and STEP revision
• Mirror, texture and EDM zones
• Hardness, certificate and substitution policy
• Heavy stock removal, ribs, corners, holes and threads
• Quantity, inspection, rust protection and delivery

This article uses Daido Steel's NAK55/NAK80 datasheet, including the published 37–43 HRC supply condition, precipitation-hardening system, relative machining, polishing and EDM behavior, and its toughness caution for NAK55. Mechanical data, tool-wear graphs and welding parameters are representative conditions, not universal guarantees. Final selection must follow the current certificate, stock size, design, life requirement, finishing route and design-owner approval.