Screw Tip Hardening via Local Quenching
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Solution Overview
Problem
Existing methods for producing screws are complex and cost-intensive due to partial carburizing, which makes it difficult to achieve a high degree of hardness in the tip while maintaining a lower hardness in the shaft to prevent hydrogen embrittlement.
Innovation Solution
A method involving forming a screw from low-alloy carbon steel wire, heating to austenitizing temperature, quenching to bainitizing temperature to create a bainitic structure, and then locally reheating and quenching the tip to form an ultra-hard martensitic structure, with optional case-hardening to increase carbon content in the edge zone.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tip of the screw is hardened to achieve high degree of hardness, then the tip becomes ultra-hard for tapping high-strength materials, but the shaft becomes more prone to hydrogen embrittlement
Solution Approach 1:
The patent applies different heat treatment conditions to different regions of the screw. The tip region undergoes full austenitizing and quenching to achieve ultra-hard martensitic structure for tapping high-strength materials, while the shaft region is kept at lower hardness through controlled cooling and bainitic structure formation, reducing its proneness to hydrogen embrittlement. This spatial differentiation of material properties directly resolves the contradiction between tip hardness and shaft reliability.
2Strength
If partial carburizing is performed to increase carbon content in the tip, then the tip hardness is improved, but the manufacturing process becomes complex and cost-intensive
Solution Approach 1:
The patent changes the thermal parameters (temperature and time) during heat treatment to achieve differential carbon distribution and microstructure formation. By controlling the austenitizing temperature and holding time, combined with specific quenching conditions, the process achieves tip hardening without requiring separate carburizing operations. This parameter optimization simplifies the manufacturing process while maintaining the desired hardness gradient.
Solution Approach 2:
The patent combines multiple functions into a single integrated heat treatment process. Instead of performing separate carburizing, hardening, and tempering operations, the invention achieves tip hardening and shaft protection simultaneously through one controlled heating and cooling cycle. This merging of process steps reduces manufacturing complexity and cost while achieving the desired differential hardness profile.
3Strength
If the entire screw is hardened to achieve high strength, then the tip becomes ultra-hard, but the shaft becomes more susceptible to hydrogen embrittlement and loss of ductility
Solution Approach 1:
The patent creates local quality differentiation by controlling the heat treatment process to affect only specific regions. The tip region is exposed to conditions that produce ultra-hard martensite for tapping operations, while the shaft region maintains a softer bainitic or martensitic structure with retained ductility. This localized property assignment resolves the contradiction between tip strength and shaft manufacturability.
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
Results in a screw with an ultra-hard tip and a shaft less prone to hydrogen embrittlement, offering high ductility and reduced risk of embrittlement, while maintaining efficiency and cost-effectiveness.
Implementation Method 1
The screw is then heated to an austenitizing temperature, with the austenitizing temperature being a temperature at which the respective wire material used is in the austenite phase field of its TTT diagram
Implementation Method 2
After heating the screw to the austenitizing temperature, the screw is quenched to a bainitizing temperature, which temperature is maintained until the screw has a bainitic structure
Implementation Method 3
the quenching time is selected so as to prevent both ferrite and pearlite formation during the quenching process
Implementation Method 4
after which the tip of the screw is heated again locally to an austenitizing temperature
Implementation Method 5
At least the tip of the screw is then quenched again to below the martensite starting temperature, with the quenching time being selected such that ferrite, pearlite and bainite formation is largely prevented. This results in the tip being hardened again locally, particularly in its edge zone, which means that an ultra-hard tip can be provided
Implementation Method 6
heating of the screw to an austenitizing temperature before quenching the screw to a bainitizing temperature can be carried out in a carbon atmosphere having a carbon content higher than the carbon content of the screw, so that a layer is formed in the edge zone of the screw that has a higher carbon content than the core
Data Source
AI summary
The invention relates to a method for producing a screw, having the following steps: (a) rolling a screw wire made of low-alloy carbon steel to produce screw (10) having a thread; (b) heating the entire screw (10) to an austenitizing temperature under a carbon atmosphere and/or nitrogen atmosphere and maintaining the temperature; (c) quenching the entire screw (10) to a bainitizing temperature and maintaining the bainitizing temperature until the screw has a bainitic structure over its cross-section. The invention is characterized in that the screw (10) is subsequently hardened locally at its tip (22), by the tip (22) being heated to an austenitizing temperature and the screw (10) being subsequently quenched to a temperature below the martensite starting temperature (MS).