ERW Steel Pipe Heat Treatment for Weld Toughness Retention
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Solution Overview
Problem
Thick-walled electric resistance welded steel pipes face challenges in achieving uniform heating during heat treatment, leading to temperature deviations between the outer and inner surfaces, which results in coarsening of the microstructure and deterioration of toughness, particularly at the welded portion.
Innovation Solution
A method involving multiple stages of induction heating and controlled cooling to transform the outer surface microstructure to bainitic ferrite and/or bainite phases, with specific grain size and area ratios, to prevent coarsening and enhance toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the heating temperature for the outer surface of the steel pipe is increased to transfer more heat to the inner surface, then the inner surface reaches the target temperature, but the outer surface is overheated causing coarsening of the steel microstructure and deterioration in toughness
Solution Approach 1:
The heat treatment process is divided into multiple sequential stages: first heating the outer surface to a high temperature (1000-1400°C) to rapidly heat the inner surface, then cooling to Ac1 transformation temperature or below to transform the microstructure to bainitic ferrite and/or bainite phases, and finally reheating to 900-1120°C to achieve the desired microstructure. This segmentation allows the inner surface to reach target temperature while controlling outer surface microstructure coarsening through intermediate cooling and controlled reheating.
Solution Approach 2:
The patent applies preliminary anti-action by cooling the outer surface to Ac1 transformation temperature or below after the initial high-temperature heating, which transforms the microstructure to bainitic ferrite and/or bainite phases before the final reheating. This preliminary cooling and phase transformation counteracts the potential microstructure coarsening that would occur if the outer surface were simply held at high temperature for extended periods.
2Strength
If multiple stages of heating and cooling are applied to control microstructure, then toughness is enhanced, but the process complexity increases
Solution Approach 1:
The heat treatment process maintains continuity of useful action by seamlessly transitioning between heating and cooling stages without interrupting the pipe's movement through the production line. The induction heating devices and water cooling devices are arranged in sequence to provide continuous heat treatment as the pipe moves through the system, eliminating the need for stopping or complex intermittent operations.
Solution Approach 2:
The patent controls microstructure and toughness by dynamically changing key parameters including heating temperature (1000-1400°C initial heating, then 900-1120°C final heating), cooling rate (to achieve Ac1 transformation temperature or below), and holding time at each stage. These parameter changes enable precise microstructure control while maintaining a relatively simple continuous process.
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 method produces a steel pipe with excellent toughness, as evidenced by a Charpy impact test of 100 J or more at 0°C, by suppressing microstructure coarsening in the heat-affected zone, ensuring consistent performance across the pipe thickness.
Implementation Method 1
heating the outer surface of a welded portion to a temperature of 1000° C. or above and 1400° C. or below
Implementation Method 2
heating and cooling of the inner surface side of the welded portion is done by thermal conduction from the outer surface side by the heat treatment of the welded portion as described above
Implementation Method 3
heating and cooling of the inner surface side of the welded portion is done by thermal conduction from the outer surface side
Implementation Method 4
locally melting end faces on both sides in the width direction of the steel strip by electric resistance heating
Data Source
AI summary
An electric resistance welded steel pipe, and a method for manufacturing the same are provided. An electric resistance welded steel pipe has a welded portion that includes a heat-affected zone having a steel microstructure principally including a bainitic ferrite phase and/or a bainite phase. The steel microstructure at half the wall thickness includes a bainitic ferrite phase and/or a bainite phase in a total area ratio of 90% or more. In the steel microstructure located 1 mm in the wall thickness direction, the bainitic ferrite phase and/or the bainite phase has an average grain size of 20 μm or less. The average grain size of the bainitic ferrite phase and/or the bainite phase located 1 mm in the wall thickness direction is 0.5 times or more and 2 times or less the average grain size of the bainitic ferrite phase and/or the bainite phase at half the wall thickness.