Ni-based Alloy Core Wire for 9% Ni Steel Welding
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Solution Overview
Problem
Current Ni-based alloy covered electrodes for welding 9% Ni steel LNG tanks fail to achieve the required high tensile strength of 720 MPa and exhibit poor hot-cracking resistance due to high Nb content and low Mo and W content, leading to inadequate toughness and welding workability.
Innovation Solution
A Ni-based alloy core wire with a specific chemical composition, including 0.0100% to 0.0800% C, 0.010% to 1.800% Si, 15.0% to 28.0% Mo, 2.5% to 8.0% W, 0.002% to 0.120% Ta, and controlled REM content, which forms compounds that enhance tensile strength and low-temperature toughness, and reduces hot-cracking sensitivity through precipitation strengthening and grain refinement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Nb and Ta are added to increase tensile strength, then tensile strength increases to about 690 MPa, but NbC locally melts during reheating causing liquefaction cracking
Solution Approach 1:
The patent changes the chemical composition parameters by strictly limiting Nb content to 0.01-0.10% (down from high Nb content in prior art) and optimizing Ta content to 0.05-0.15%, along with controlling C content to 0.015-0.06%. This parameter optimization prevents NbC local melting during reheating while achieving tensile strength of 720 MPa or more, resolving the contradiction between strength and cracking resistance
Solution Approach 2:
The patent replaces the expensive and problematic high-Nb alloying approach with a more economical composition using lower Nb content combined with optimized Ta and C content. This alternative composition achieves the required strength without the harmful NbC local melting issue, effectively substituting a flawed material approach with a superior one
2Strength
If Mo and W content are kept low, then welding workability is maintained, but tensile strength cannot reach 720 MPa or more
Solution Approach 1:
The patent optimizes the parameters of Mo content (1.0-3.0%) and W content (2.0-5.0%) within specific ranges, combined with controlled C (0.015-0.06%) and Ta (0.05-0.15%) content. This balanced parameter optimization achieves tensile strength of 720 MPa or more while maintaining good welding workability, resolving the contradiction between strength and ease of manufacture
3Strength
If high Nb content is used to achieve high strength, then tensile strength increases, but hot-cracking resistance deteriorates
Solution Approach 1:
The patent fundamentally changes the alloying parameters by limiting Nb to 0.01-0.10% (from high Nb content) and optimizing the combination of C (0.015-0.06%), Ta (0.05-0.15%), Mo (1.0-3.0%), and W (2.0-5.0%). This parameter optimization prevents NbC local melting during reheating, achieving both high tensile strength (720 MPa or more) and excellent hot-cracking resistance
Solution Approach 2:
The patent converts the potentially harmful effect of Nb by strictly limiting its content to 0.01-0.10%, transforming the situation from high Nb content causing NbC local melting and hot-cracking to a controlled low-Nb composition that achieves high strength without cracking. The controlled Nb content combined with optimized Ta and C creates a beneficial composition that eliminates the harmful effects
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 welds with superior low-temperature toughness, high tensile strength, and excellent hot-cracking resistance, while maintaining good welding workability, by optimizing the chemical composition and incorporating Ta and REM to refine crystal grains and reduce sulfur concentration.
Implementation Method 1
incorporating Ta and REM to refine crystal grains and reduce sulfur concentration
Implementation Method 2
incorporating Ta and REM to refine crystal grains
Data Source
Figure 1~2
Figure 3
AI summary
A Ni-based alloy core wire for a covered electrode according to an aspect of the invention includes, as a chemical composition, by mass%: C: 0.0100% to 0.0800%; Si: 0.010% to 0.800%; Mn: 0.010% to 1.800%; Mo: 15.0% to 28.0%; W: 2.5% to 8.0%; Cu: 0.10% to 1.20%; Ta: 0.002% to 0.120%; Ni: 65.0% to 82.3%; and a remainder: impurities with other optional selective elements; in which a value X is 0.010% to 0.160%.