Ferritic Steel Welded Joint Root Pass Control Against Solidification Cracking
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Solution Overview
Problem
Ferritic heat-resistant steel welded joints using Ni-based welding materials for heat-resistant alloys face solidification cracking issues during multi-layer welding due to the high boron content in the base material.
Innovation Solution
A method involving multi-layer welding of ferritic heat-resistant steel base materials with 0.006% to 0.023% boron, where the ratio of melted base material area to weld metal area in the transverse cross-section after root pass welding is controlled within the range of 0.1 to −50×[% BBM] + 1.3, using a Ni-based welding material, to suppress solidification cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Ni-based welding material is used for multi-layer welding of ferritic heat-resistant steel with high boron content, then excellent creep strength can be obtained, but solidification cracking occurs in the weld metal
Solution Approach 1:
The invention changes the chemical composition parameters of the welding material by strictly controlling boron content to 0.0005-0.006% (lower than conventional materials) and optimizing the (Mo+W)/(Ni+Co) ratio to 0.05-0.50. This parameter adjustment prevents excessive boron from causing solidification cracking while maintaining creep strength through optimized alloying elements.
Solution Approach 2:
The invention applies local quality by creating a welding material with non-uniform element distribution characteristics - specifically limiting boron to very low levels while concentrating creep-strengthening elements (Mo, W, Co, Ni) in optimized ratios. This localized compositional control ensures crack prevention in the weld metal while maintaining overall creep performance.
2Strength
If boron content is increased to achieve both creep strength and toughness, then mechanical properties improve, but solidification cracking susceptibility increases
Solution Approach 1:
The invention transforms the boron parameter from a high-content approach (conventional 0.007-0.015% or higher) to a low-content approach (0.0005-0.006%). This parameter inversion resolves the contradiction by finding that minimal boron combined with optimized (Mo+W)/(Ni+Co) ratio achieves both mechanical properties and crack resistance.
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
This method effectively stabilizes the suppression of solidification cracking in ferritic heat-resistant steel welded joints, ensuring the quality and integrity of the welds while preventing other defects like lack of fusion or incomplete penetration.
Implementation Method 1
a multi-layer welding step in which a ferritic heat-resistant steel base material including B at 0.006% by mass to 0.023% by mass is multi-layer welded using a Ni-based welding material for heat-resistant alloy
Data Source
AI summary
A method of producing a ferritic heat-resistant steel welded joint, the method including: a multi-layer welding step in which a ferritic heat-resistant steel base material including B at 0.006% by mass to 0.023% by mass is multi-layer welded using a Ni-based welding material for heat-resistant alloy, wherein root pass welding is performed under a welding condition such that a ratio of an area [SBM] that has been melted of the ferritic heat-resistant steel base material to an area [SWM] of a weld metal, in a transverse cross-section of a weldment after the root pass welding but before second pass welding in the multi-layer welding step, satisfies the following formula (1): 0.1≤[SBM]/[SWM]≤−50×[% BBM]+1.3, with respect to a mass percent of B, [% BBM], which is included in the ferritic heat-resistant steel base material.