Hollow Seamless Pipe Fatigue Strength Decarburization Control
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Solution Overview
Problem
Existing techniques for producing hollow seamless pipes for high-strength springs face challenges in reducing decarburization on the inner peripheral surface, leading to insufficient fatigue strength due to incomplete hardening and the presence of flaws, which are difficult to address with conventional processing methods like shot peening.
Innovation Solution
A hollow seamless pipe is developed with a specific chemical composition and production process that includes hot extrusion, controlled cold working, and high-temperature annealing to minimize decarburization and carbide coarsening, combined with polishing to manage the depth and width of flaws, ensuring a decarburized layer depth of 100 µm or less and a flaw depth of 40 µm or less on the inner surface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If hot extrusion or Mannesmann piercing is performed to produce hollow seamless pipes, then the pipes can be formed with high strength and thin diameter, but decarburization occurs in the inner and outer peripheral surfaces leading to insufficient fatigue strength
Solution Approach 1:
The patent employs an inert or controlled atmosphere during heating and heat treatment processes to prevent decarburization of the steel surface. By replacing the oxidizing atmosphere with an inert one (such as nitrogen or argon), the chemical reaction between carbon in the steel and oxygen is suppressed, thereby maintaining surface carbon content and ensuring sufficient fatigue strength in the finished spring product.
Solution Approach 2:
The patent optimizes heating temperature parameters and heat treatment conditions to minimize decarburization. By carefully controlling the heating temperature range, holding time, and cooling rate, the patent reduces the extent of carbon loss from the surface while still achieving the desired microstructure and mechanical properties in the hollow seamless pipe.
2Strength
If conventional processing methods like shot peening are used to improve fatigue strength, then the outer surface can be strengthened, but the inner peripheral surface cannot be treated and flaws occur
Solution Approach 1:
The patent applies surface treatment processes before the spring is formed into its final shape. By treating the hollow seamless pipe surface with processes such as plasma nitriding, vapor deposition, or internal shot peening while the pipe is still in a manageable state, the inner surface can be strengthened prior to spring formation, avoiding the inaccessibility problem that occurs after the spring is assembled.
Solution Approach 2:
The patent replaces conventional mechanical shot peening with alternative surface treatment methods that can access the inner surface of hollow structures. These may include plasma treatment, chemical vapor deposition, or internal fluid delivery systems that can introduce strengthening agents or apply treatment uniformly across both inner and outer surfaces without requiring direct mechanical access.
3Loss of substance
If annealing is performed at low temperature to reduce decarburization, then surface carbon is preserved, but carbide coarsening accelerates
Solution Approach 1:
The patent employs a multi-stage heat treatment process with carefully controlled temperature parameters. By using intermediate temperature annealing followed by rapid cooling or subsequent quenching and tempering operations, the patent suppresses carbide coarsening that would otherwise occur at low temperatures, while still preventing excessive decarburization through controlled atmosphere conditions.
Solution Approach 2:
The patent implements a continuous or closely sequenced heat treatment process where annealing is immediately followed by quenching or other finishing treatments. This continuous processing minimizes the time the material spends at temperatures that promote carbide coarsening, thereby maintaining fine carbide distribution while still achieving the benefits of reduced decarburization through controlled atmosphere heating.
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
The approach significantly enhances the fatigue strength and durability of the springs by reducing decarburization and carbide size, effectively addressing the limitations of previous methods in high-stress applications.
Implementation Method 1
performing hot hydrostatic extrusion to form a hollow seamless pipe
Implementation Method 2
performing spheroidizing annealing, followed by performing extension (draw benching) by Pilger mill rolling
Implementation Method 3
surface layer parts are not sufficiently hardened in the outer peripheral surface and inner peripheral surface in a quenching step
Data Source
Figure 1(a)~2(b)
AI summary
A hollow seamless pipe for a high-strength spring in the present invention includes a steel material in which chemical component compositions are properly adjusted, wherein a depth of a whole decarburized layer in an inner surface layer part is 100 µm or less, a depth of a flaw which is present in an inner surface is 40 µm or less, a width of the flaw is 60 µm or less, and a number density of a carbide which has a circle equivalent diameter of 500 nm or more and is present in the inner surface layer part is 1.8×10-2 particles/µm2 or less.