When designing a robust joint, engineers focus heavily on thread geometry, material selection, and clamp load. However, the performance of a high-performance fastener is ultimately dependent on its metallurgy, and specifically how it is heat-treated. A "one-size-fits-all" approach to hardening simply doesn’t work across different materials and applications.

To maximize structural integrity, ductility, and corrosion concerns, REMINC specifies three distinct heat treatment processes across the TAPTITE®, POWERLOK®, and REMFORM® fastener families. Understanding when and why each is used is critical for a reliable assembly.

1. Case Hardening:
The Standard for Smaller Thread-Forming Fasteners – In Steel Applications

For standard thread-rolling and thread-cutting screws in sizes M5 (#12) and smaller, case hardening is the industry norm.

  • The Process: A hard outer "skin" (approx. 450 HV / HRC 45) surrounds a softer, tougher inner core (Typically 270 – 370HV / HRC 26–38).
  • The Advantage: The hardened surface provides the strength required to displace nut member material during thread rolling without deforming the screw threads.
  • The Limitation: Case-hardened fasteners have lower overall ductility compared to neutral-hardened (through-hardened) screws; case-hardened fasteners are typically restricted to smaller diameters or non-structural applications. Furthermore, case-hardened fasteners should be strictly avoided in certain environments, as detailed below.

2. Neutral-Hardening / CORFLEX®-N™ Heat Treatment:
Through-Hardened Reliability for Non-Ferrous Applications

When fasteners require a constant, uniform hardness throughout their entire structure—such as Property Class 8.8, 9.8 or 10.9 (standard Grade 5 or Grade 8 for imperial sizes) —they are neutral-hardened (through-hardened). REMINC utilizes a specialized modification of this process known as CORFLEX®-N™ for thread-rolling fasteners in non-ferrous applications.

  • Where It’s Used:
    • All locking screws within the POWERLOK® family.
    • Fasteners designed specifically for plastic assemblies, such as the REMFORM® family.
    • The TAPTITE® Family of thread-rolling fasteners used in non-ferrous materials, such as aluminum, zinc and magnesium.
  • The Engineering Rationale: When a steel fastener is driven into an aluminum component in the presence of an electrolyte like moisture, a galvanic couple forms. High surface hardness (such as the 450 HV / HRC 45 skin on case-hardened screws) makes fasteners highly susceptible to stress corrosion—also known as environmentally induced hydrogen embrittlement—which can cause delayed structural failure. Neutral-hardened CORFLEX®-N™ fasteners minimize this risk, providing safe, reliable performance even in thermally active multi-material joints.

3. Induction Hardening / CORFLEX®-I™: Selective Strength for Structural Steel

While neutral hardening provides excellent ductility, standard grades (such as Property Class 8.8, 9.8, or 10.9 / Grade 5 or 8) are not hard enough to form threads directly into steel components. Conversely, case-hardening large fasteners create unacceptable brittleness. To solve this, REMINC developed the CORFLEX®-I™ heat treatment.

  • The Process: This specialized induction-hardening method selectively hardens only the thread-forming threads on the tip of the fastener, while leaving the remainder of the body neutral-hardened to the desired structural property class or grade strength.

  • Where It’s Used: Applied primarily to fasteners M6 (1/4”) and larger for structural steel applications.

  • The Automotive Standard: Major automotive manufacturers—including Ford and General Motors—maintain strict internal policies requiring all TAPTITE® fastener designs for steel in M6 and larger diameters have CORFLEX®-I™ heat treatment, regardless of the specific application. This engineering safeguard ensures that no large, case-hardened parts can accidentally enter an assembly plant's inventory and find their way into a structural application where case hardening would have detrimental effects.

  • CORFLEX® I™ heat treated TAPTITE® products can also be used in Aluminum with enough engagement depth. Many Automotive engineers will specify an existing CORFLEX® I™ product in aluminum commonize parts.

(Note: The CORFLEX®-I™ process can only be applied to screws M5 / #12 and larger; smaller screws threading into steel must rely on case hardening).

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Matching the Heat Treatment to the System

Selecting the correct heat treatment is just as important as selecting the right thread profile. By aligning your application's material (for example; aluminum vs. steel), fastener size, and structural loads with the appropriate hardening method—whether standard case hardening, CORFLEX®-N™, or CORFLEX®-I™

heat treatment you ensure long-term joint reliability and prevent costly field failures.