Steel Pipe Girth Weld Composition for High-Strength Crack Resistance
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Solution Overview
Problem
Girth welded joints in large-sized mechanical structures, such as crane booms, require high strength and resistance to cold cracking to ensure safety and weight reduction, but existing methods often compromise on strength or suffer from weld cracking issues.
Innovation Solution
A girth welded joint with specific chemical compositions for the base metal and weld metal, including elements like C, Si, Mn, Ni, Cr, Mo, and B, optimized to achieve tensile strengths of 980 MPa or more, tempered martensite microstructure, and controlled weld heat affected zone properties to prevent cold cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If seamless steel pipes with high tensile strength (980 MPa or more) are used for crane booms, then weight reduction is achieved, but weld cracking easily occurs in the welded zone
Solution Approach 1:
The invention changes the chemical composition parameters of the base metal by strictly controlling the content of alloying elements (C: 0.10-0.20%, Si: 0.05-1.00%, Mn: 0.05-1.20%, Cr: 0.30-0.50%, Mo: 0.30-0.50%, B: 0.0005-0.0020%, etc.) and limiting harmful impurities (P: 0.025% or less, S: 0.005% or less). This parameter optimization resolves the contradiction by achieving tensile strength of 980 MPa or more while maintaining Pcm value of 0.30 or less, which prevents cold cracking in the welded zone.
Solution Approach 2:
The invention creates a composite material system by combining multiple alloying elements (Cr, Mo, Ni, B, Ti, V, Nb) in specific proportions to achieve both high strength and good weldability. The synergistic effect of these elements produces a steel composition that simultaneously delivers tensile strength of 980 MPa or more and excellent cold cracking resistance, resolving the contradiction between strength and weldability.
2Weight of moving object
If high strength seamless steel pipes are used to reduce weight of mechanical structures, then weight reduction is achieved, but the welded joint requires sufficient strength which is difficult to obtain
Solution Approach 1:
The invention changes the chemical composition parameters to optimize both base metal strength and weldability. By controlling C content at 0.10-0.20% and limiting Pcm to 0.30 or less, the base metal achieves tensile strength of 980 MPa or more while maintaining good weldability, ensuring that the welded joint can also achieve sufficient strength without requiring excessive weight.
3Strength
If alloying elements are added to increase strength of steel pipes, then tensile strength is improved, but weldability deteriorates due to increased Pcm value
Solution Approach 1:
The invention optimizes the parameters of alloying elements by specifying precise content ranges: Cr (0.30-0.50%), Mo (0.30-0.50%), Ni (0.20-0.50%), B (0.0005-0.0020%), Ti (0.003-0.050%), V (0.01-0.20%), and Nb (0.01-0.05%). This balanced composition achieves tensile strength of 980 MPa or more while keeping Pcm value at 0.30 or less, thereby maintaining good weldability despite the high strength requirement.
Solution Approach 2:
The invention uses a composite material approach by combining multiple alloying elements in specific proportions. The synergistic interaction between Cr, Mo, Ni, B, Ti, V, and Nb produces a steel composition that simultaneously achieves high tensile strength (980 MPa or more) and good weldability (Pcm ≤ 0.30), resolving the contradiction between strength enhancement and weldability maintenance.
Data Source
AI summary
A girth welded joint of steel pipe 10 includes base metal portions 1a, 1b and a girth welded portion 2, wherein the girth welded portion 2 is formed of a weld metal portion 2a and weld heat affected zones 2b, 2c, the base metal portions 1a, 1b have a predetermined chemical composition, Pcm is within a range of 0.25 to 0.30, the weld metal portion 2a has a predetermined chemical composition, a content of B is 0.0010% or less, tensile strength of both base metal portions 1a, 1b and tensile strength of the girth welded portion 2 in a cross weld tensile test is 980 MPa or more, average hardness of the base metal portions 1a, 1b is 300 HV10 or more, an average softening width of the weld heat affected zones 2b, 2c is 4.0 mm or less, and an average softening degree is 80 HV10 or less.