Bent Steel Pipe Heat Treatment for Uniform Hardness and X65 Strength
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Solution Overview
Problem
Conventional techniques fail to produce steel bend pipes with sufficient strength and corrosion resistance, particularly for thick-wall bend pipes with wall thicknesses of 30 mm or more, due to uneven hardness distribution and inadequate cooling rates during quenching and tempering processes.
Innovation Solution
The steel bend pipe is produced by heating the steel to a temperature of 1050°C or more for bending and then cooling at an average rate of 3°C/s or more, followed by heat treatment at 510°C or more to achieve balanced hardness across the pipe, ensuring high strength and HIC resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional quenching and tempering processes are used on steel bend pipes, then the pipe achieves high strength, but the hardness distribution becomes uneven (U-shaped) with high surface hardness and low mid-wall hardness
Solution Approach 1:
The invention changes the thermal parameters by heating to a higher temperature range (Ac3 point or more to 1050°C or less) before bending, and controls the cooling rate (5°C/s or more in the temperature range of 700 to 500°C) to achieve a balanced hardness distribution while maintaining high strength. This parameter optimization resolves the U-shaped hardness distribution problem.
2Strength
If the cooling rate is increased during quenching to achieve higher strength, then the surface hardness increases, but the mid-wall hardness remains low due to slower cooling at the center
Solution Approach 1:
The invention optimizes the cooling rate parameter to 5°C/s or more in the critical temperature range of 700 to 500°C, which ensures sufficient cooling throughout the pipe wall thickness to achieve uniform hardness distribution while maintaining high strength properties.
3Volume of moving object
If thick-wall bend pipes (30 mm or more) are produced with conventional methods, then the pipe achieves required wall thickness, but the cooling rate becomes insufficient leading to low strength
Solution Approach 1:
The invention maintains a cooling rate of 5°C/s or more even in thick-wall pipes (30 mm or more) by controlling the cooling process in the temperature range of 700 to 500°C, which overcomes the natural cooling rate reduction in thick sections and achieves both required wall thickness and high strength (X65 Grade).
4Strength
If the pipe is cooled rapidly to achieve high strength, then the mechanical strength increases, but the HIC resistance decreases due to high surface hardness
Solution Approach 1:
The invention optimizes the cooling rate to 5°C/s or more in the temperature range of 700 to 500°C, which achieves a balanced hardness distribution that provides both high mechanical strength (X65 Grade) and excellent HIC resistance by avoiding excessive surface hardness.
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 results in a steel bend pipe with a high strength of X65 Grade (450 to 600 MPa) and excellent HIC resistance, suitable for pipelines and flowlines, by maintaining a balanced hardness distribution and enhancing mechanical properties.
Implementation Method 1
subjected to heating to a temperature of 1050°C or more
Implementation Method 2
cooled at an average rate of 3°C/s or more
Implementation Method 3
heat treatment at 510°C or more
Data Source
Figure 1
Figure 2
AI summary
A steel bend pipe (1) includes a straight pipe portion (2) and a bent portion (3) having a bending radius r not less than three times an outer diameter OD of the straight pipe portion (2), and has a chemical composition containing, in mass percent, C: 0.04 to 0.08%, Si: 0.05 to 0.50%, Mn: 1.00 to 1.70%, P: 0.015% or less, S: 0.002% or less, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Mo: 0 to 0.50%, Sol.Al: 0 to 0.10%, Ca: 0 to 0.0050%, Nb: 0 to 0.050%, V: 0 to 0.10%, and Ti: 0 to 0.030%, with the balance being Fe and impurities, wherein Ceq (= C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15) is 0.35% or more, yield stresses of the straight pipe portion (2) and the bent portion (3) are 450 to 600 MPa, and Vickers hardnesses (HV10) of the straight pipe portion (2) and the bent portion (3) are 230 or less.