Thick Welded Steel Pipe Low Temperature Toughness
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Solution Overview
Problem
Thickened steel pipes for transporting crude oil and natural gas face challenges in maintaining low temperature toughness, particularly in the weld heat-affected zone (HAZ), due to increased heat input during submerged arc welding, which leads to coarsening of the structure and formation of martensite-austenite constituents, making them prone to fracture.
Innovation Solution
Optimizing the chemical composition of the base material steel plate and welding conditions, specifically controlling the contents of Mn and Mo, and adjusting heat input to ensure a balanced metal structure with adequate grain boundary ferrite and intragranular ferrite, while limiting the area ratio of martensite-austenite constituents, to enhance the low temperature toughness of the HAZ.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the thickness of the steel pipe is increased to improve transport efficiency, then the strength and transport capacity are improved, but the low temperature toughness of the weld heat affected zone decreases due to coarsening of structure and formation of martensite-austenite constituents
Solution Approach 1:
The patent applies parameter changes by optimizing the chemical composition parameters (C: 0.03-0.085%, Si: ≤0.15%, Mn: 1.45-1.85%, P: ≤0.02%, S: ≤0.005%, Al: ≤0.015%, Ti: 0.005-0.020%, Nb: 0.005-0.050%, O: 0.0005-0.005%, Mo: ≤0.20%) and controlling the microstructure parameters (grain boundary ferrite area ratio ≥1.5%, total ferrite area ratio 11-90%, MA area ratio ≤10%) of the base material steel plate to achieve both high strength and excellent low temperature toughness in the HAZ
Solution Approach 2:
The patent applies local quality by creating a specific microstructure distribution in the HAZ with grain boundary ferrite and intragranular ferrite phases strategically positioned to prevent brittle fracture, while maintaining the required strength through controlled bainite and MA constituents in specific proportions
2Strength
If the alloy adding amount is increased to achieve higher strength in thick steel pipes, then the strength is improved, but the low temperature toughness of the HAZ decreases due to increased hardenability leading to more MA formation
Solution Approach 1:
The patent applies parameter changes by precisely controlling the alloying element contents within specific ranges - particularly C (0.03-0.085%), Mn (1.45-1.85%), and Mo (≤0.20%) - to achieve the optimal balance between strength and toughness, avoiding excessive alloying that would increase hardenability and MA formation
Solution Approach 2:
The patent applies copying by using Ti and Nb microalloying elements that form fine precipitates to replicate the strengthening effect of higher carbon content without increasing hardenability, thus maintaining toughness while achieving the required strength level
3Ease of manufacture
If submerged arc welding with large heat input is used to seam weld thick plates, then the welding process is feasible for thick materials, but the low temperature toughness decreases due to coarsening of HAZ structure and formation of mixed martensite-austenite structure
Solution Approach 1:
The patent applies preliminary action by pre-optimizing the base material steel plate composition and microstructure before welding, creating a material that is inherently resistant to HAZ coarsening and MA formation even under high heat input conditions, thus preventing toughness degradation before it occurs
Solution Approach 2:
The patent applies blessing in disguise by utilizing the high heat input of submerged arc welding to create a coarse-grained HAZ structure, but then converting this potential harm into benefit by controlling the microstructure to contain grain boundary ferrite and intragranular ferrite phases that prevent brittle fracture despite the coarse grain size
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 results in a thick welded steel pipe with improved low temperature toughness at -60°C, achieving a Charpy absorbed energy of 60 J or more in the HAZ, thereby enhancing the pipe's durability and resistance to brittle fracture.
Implementation Method 1
for improving the toughness of the HAZ, there has been proposed a method of utilizing intragranular transformation to make a structure fine
Implementation Method 2
submerged arc welding with a large amount of heat input is employed
Implementation Method 3
heat input of the welding to be performed later
Data Source
Figure 1~2
Figure 3~4
Figure 5~6
AI summary
The present invention provides a thick welded steel pipe excellent in low temperature toughness in which contents of Mn and Mo satisfy (Expression 1) below, Pcm obtained by (Expression 2) below is 0.16 to 0.19, and a metal structure of a base material steel plate consists of ferrite being 30 to 95% in an area ratio and a low temperature transformation structure, and in a metal structure of a coarse-grained HAZ, an area ratio of grain boundary ferrite is 1.5% or more, the total area ratio of the grain boundary ferrite and intragranular ferrite is not less than 11 % nor more than 90%, an area ratio of MA is 10% or less, and its balance is composed of bainite. and