Fuel Injection Steel Pipe Composition for Ultra-High Cycle Fatigue
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Solution Overview
Problem
Existing fuel injection pipes face challenges in achieving high tensile strength and critical internal pressure, particularly in the ultra-high cycle region, leading to fatigue issues and reduced reliability under high internal pressures.
Innovation Solution
A steel pipe with a specific chemical composition and microstructure, including tempered martensite and tempered bainite, controlled hardness, and reduced inclusion sizes, combined with optimized production methods to enhance fatigue strength and critical internal pressure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the injection pressure of fuel is increased to reduce black smoke, then the CO2 emissions are reduced, but the steel pipe requires high fatigue strength which is difficult to achieve
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.25%, Si: 0.03-0.30%, Mn: 0.50-2.00%, P: 0.020% or less, S: 0.010% or less, Al: 0.005-0.060%, Ti: 0.005-0.015%, Nb: 0.015-0.045%, Cr: 0.03-1.00%, Mo: 0.03-1.00%, B: 0.0005-0.0050%) and microstructure parameters (prior-austenite grain size number ≥9.0, tempered martensite area fraction ≥90%, hardness 350-460 HV1) to achieve both high strength and high fatigue resistance, enabling the steel pipe to withstand high injection pressures required for reducing black smoke
Solution Approach 2:
The patent creates a composite microstructure consisting of tempered martensite (≥90% area fraction) with controlled inclusions (maximum diameter ≤20 μm, average diameter ≤10 μm). This composite microstructure combines the high strength of martensite with the fatigue resistance achieved through controlled inclusion distribution and refined grain structure, resolving the contradiction between strength and fatigue strength
2Stress or pressure
If the tensile strength of the steel pipe is increased to withstand high internal pressure, then the critical internal pressure is improved, but the fatigue strength in the ultra-high cycle region is reduced
Solution Approach 1:
The patent achieves the balance between critical internal pressure and ultra-high cycle fatigue strength by optimizing multiple parameters simultaneously: chemical composition (particularly low S ≤0.010% and P ≤0.020%, controlled Al and Ti for inclusion control), microstructure (prior-austenite grain size number ≥9.0, tempered martensite ≥90%, hardness 350-460 HV1), and inclusion characteristics (maximum diameter ≤20 μm, average diameter ≤10 μm, uniform distribution). This multi-parameter optimization enables the steel pipe to achieve both high critical internal pressure and high fatigue strength in the ultra-high cycle region
Solution Approach 2:
The patent applies local quality by ensuring uniform distribution of small inclusions (average diameter ≤10 μm) throughout the steel pipe microstructure and maintaining consistent tempered martensite structure (≥90% area fraction) and hardness (350-460 HV1) across the material. This uniform local quality prevents stress concentration and fatigue crack initiation, enabling high fatigue strength in the ultra-high cycle region while maintaining high critical internal pressure
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 solution achieves a tensile strength of 1100 MPa or more and improved fatigue strength in the ultra-high cycle region, ensuring reliability and safety under high internal pressures.
Implementation Method 1
a metal micro-structure at a center portion of a wall thickness of the steel pipe includes tempered martensite, or tempered martensite and tempered bainite
Implementation Method 2
the steel pipe having a tensile strength of 1100 MPa or more, and also having high fatigue strength in an ultra-high cycle region
Data Source
AI summary
A steel pipe for a fuel injection pipe has a chemical composition consisting of, by mass %: C: 0.17 to 0.27%, Si: 0.05 to 0.40%, Mn: 0.30 to 2.00%, P: 0.020% or less, S: 0.0100% or less, O: 0.0040% or less, Ca: 0.0010% or less, Al: 0.005 to 0.060%, N: 0.0020 to 0.0080%, Ti: 0.005 to 0.015%, Nb: 0.015 to 0.045%, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, V: 0 to 0.15%, and the balance: Fe and impurities. The metal micro-structure consists substantially of tempered martensite, or tempered martensite and tempered bainite. A prior-austenite grain size number is 9.0 or more. The hardness is within the range of 350 to 460 HV1. When a maximum value of a square root of an area of inclusions observed in a cross section perpendicular to a longitudinal direction of the steel pipe is taken as an (n=1 to 20), a maximum value amax of an is 30.0 μm or less, and an average value aav of an is 40% or more of amax.