Thread Forming Screw Surface Hardening
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Solution Overview
Problem
Conventional thread forming screws are not hard enough to successfully cold form threads into workpieces with a hardness exceeding Rockwell C23, leading to thread collapse and joint failure, necessitating additional operations and hardware.
Innovation Solution
A method involving a heat treating process to achieve a surface hardness of at least HRC 56 on the lead threads and first three to four full threads of the screw, combined with induction hardening, to ensure the screw can cold form threads in workpieces with higher hardness levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional thread forming screws with core hardness of HRC 33-39 are used, then the screw maintains good ductility and toughness, but the surface hardness is insufficient to cold form threads in workpieces exceeding HRC 23
Solution Approach 1:
The patent applies induction hardening to create a localized hardened zone on the screw threads. The surface layer is hardened to HRC 56 while the core remains at HRC 33-39, providing different properties in different regions: hard surface for thread formation and soft core for ductility.
Solution Approach 2:
The patent changes the physical-chemical state of the screw surface through induction hardening. The surface hardness is increased from HRC 33-39 to HRC 56 by heating and rapid cooling, fundamentally altering the material properties of the surface layer to enable thread formation in hard workpieces.
2Strength
If the entire screw is hardened to increase surface hardness, then thread formation capability improves, but the screw becomes too brittle and loses ductility
Solution Approach 1:
The patent creates a non-uniform hardness distribution where only the surface layer (0.005-0.020 inches deep) is hardened to HRC 56, while the core remains at HRC 33-39. This localized treatment provides hard surface for thread formation while maintaining ductile core properties.
Solution Approach 2:
The patent segments the screw into two distinct zones with different properties: a hardened surface zone for thread formation and a soft core zone for ductility. This segmentation allows each zone to perform its specific function optimally without compromising the other.
3Strength
If conventional heat treating processes are used, then the manufacturing cost is lower, but the surface hardness does not reach HRC 56 required for hard workpieces
Solution Approach 1:
The patent replaces conventional slow heat treating processes with induction hardening, which uses electromagnetic induction to rapidly heat and harden the surface. This substitution achieves HRC 56 surface hardness more efficiently and cost-effectively than traditional methods.
Solution Approach 2:
The induction hardening process uses periodic electromagnetic cycles to rapidly heat the surface layer. The alternating current generates eddy currents that cyclically heat the material, achieving the desired hardness through controlled periodic thermal action.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables consistent thread formation in materials with hardness exceeding HRC 23, such as HSLA, without thread collapse, reducing the need for additional operations and hardware, and providing a more cost-effective and efficient process.
Implementation Method 1
second induction hardening the lead threads and at least three of the full threads of the screw that are provided closest to the lead threads
Implementation Method 2
carbon enriching, quenching and tempering the screw to provide that a surface hardness and a core hardness of the screw is at a Rockwell C33 to 39 hardness
Data Source
Figure 1~2
Figure 3
AI summary
A thread forming screw (10) having a minimum surface hardness of HRC 56. The screw (10) can form threads in a workpiece having a surface hardness which exceeds HRC 23. A method of surface hardening a screw (10) includes carbon enriching the screw (10) to at least a 0.48 carbon level, and then quenching the screw (10). Then, the screw (10) is tempered such that the surface hardness does not exceed the core hardness by more than 3 Rockwell C points, and both the surface and core are at a Rockwell C33-C39 hardness. Subsequently, the point (16) is induction hardened, and the screw is quenched again. The screw (10) is again tempered such that the lead threads (18) and the first 3-4 full threads (22, 24) are at a Rockwell C56 minimum hardness, preferably to a depth (26) of at least 0.008 inches, and the core of the fastener (10) is at Rockwell C33-C39 hardness. Finally, a finish is applied.