Seamless Steel Pipe Surface Hardness Control
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Solution Overview
Problem
Existing seamless steel pipes for line pipes face challenges in maintaining low surface hardness while achieving high strength and toughness, particularly in environments with high H2S levels, due to variations in cooling rates and microstructure differences between the surface and interior.
Innovation Solution
Incorporating a predetermined amount of Ni or Cu into the steel pipe to disperse metal particles in the scale, ensuring uniform scale adhesion and reducing cooling rates, along with controlled chemical composition and heat treatment processes to maintain low surface hardness and high strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If quenching followed by tempering is performed to assure high strength, then the strength of the steel pipe is improved, but the surface hardness becomes excessively high due to high cooling rate at the surface
Solution Approach 1:
The steel pipe surface is subjected to preliminary heating to form austenite before quenching. This preliminary thermal treatment ensures uniform microstructure transformation during subsequent cooling, preventing excessive surface hardness while maintaining high strength through controlled phase transformation.
Solution Approach 2:
The invention changes the thermal parameters by performing quenching from the austenite region (above Ac3 transformation point) rather than from two-phase region. By controlling the heating temperature to ensure complete austenite formation and then applying rapid cooling, the microstructure transforms uniformly, reducing surface hardness while maintaining high strength.
2Strength
If alloying elements are increased to improve strength, then the strength of the steel pipe is improved, but the surface hardness increases remarkably
Solution Approach 1:
The invention changes the thermal processing parameters to quench from complete austenite region, which transforms the relationship between alloying elements and hardness. This parameter change allows the alloying elements to contribute to strength through solid solution strengthening and precipitation hardening in the bainitic microstructure, while the uniform transformation reduces excessive surface hardness.
Solution Approach 2:
The invention creates a composite microstructure consisting of bainite as the primary phase with dispersed carbides and alloy element precipitates. This composite structure achieves high strength through the combined effects of bainitic matrix and dispersed strengthening phases, while the uniform distribution prevents localized hardness spikes at the surface.
3Strength
If rapid cooling is applied to the surface to achieve high strength, then the strength is improved, but the microstructure differs between surface and interior, causing non-uniform hardness distribution
Solution Approach 1:
The steel pipe is preliminarily heated to the austenite region (Ac3 + 50°C or higher) to ensure complete and uniform austenite transformation throughout the cross-section before quenching. This preliminary action eliminates microstructure differences between surface and interior, enabling uniform transformation during rapid cooling and achieving both high strength and homogeneous microstructure.
Solution Approach 2:
The invention utilizes the phase transition from austenite to bainite through controlled quenching. By ensuring complete austenite formation beforehand and then applying rapid cooling, the phase transition occurs uniformly throughout the steel pipe, transforming the unstable austenite into a stable bainitic microstructure with consistent properties throughout the cross-section.
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 results in a seamless steel pipe with a yield strength of 448 MPa or higher and surface hardness of 250 HV10 or lower, enhancing sulfide stress cracking resistance and suitability for transmitting H2S-containing crude oil and natural gas.
Implementation Method 1
a scale formed on a surface of the steel pipe
Implementation Method 2
reducing cooling rates
Data Source
Figure 1(a)~2
AI summary
There is provided a seamless steel pipe for line pipe, wherein a chemical composition consists, by mass percent, of C: 0.03-0.10%, Si: ≤0.50%, Mn: 1.0-2.0%, P: ≤0.050%, S: ≤0.005%, Cr: 0.05-1.0%, Mo: 0.01-0.30%, Al: 0.001-0.10%, N: ≤0.01%, Ni: 0.04-2.0%, Ca: 0.0005-0.0050%, Cu: 0-2.0%, Ti: 0-0.05%, Nb: 0-0.05%, V: 0-0.10%, the balance: Fe and impurities, and satisfies the conditions of Cu + Ni: ≥0.10%, and Mo + V: ≤0.30%, wherein in a scale formed on the surface of the steel pipe, metal particles consisting mainly of Ni or Cu having an average circle-equivalent diameter of 0.1-5 µm exist, and a distance from a boundary between the base metal of the steel pipe and the scale to a region in which the metal particles do not exist is 20 µm or longer.