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).
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.