Seamless Steel Pipe Sour Service Cracking Resistance
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Solution Overview
Problem
Seamless steel pipes with X80 grade or higher yield strength face challenges in achieving reliable sour service cracking resistance due to surface hardness exceeding the API 5L standards, and existing manufacturing processes are either costly or inefficient in controlling surface layer hardness.
Innovation Solution
A seamless steel pipe with a carbon equivalent (Ceq) between 0.430% and 0.500% and a Larson-Miller parameter of 18800 or higher is produced using direct or in-line quenching after hot forming, ensuring a yield strength of 555 MPa or higher and reduced surface hardness, without the need for reheating-and-quenching, resulting in improved SSC resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the yield strength of seamless steel pipe is increased to X80 grade or higher, then the strength and pressure resistance are improved, but the surface hardness exceeds API 5L standards and sour service cracking resistance deteriorates
Solution Approach 1:
The invention changes the chemical composition parameters (Ceq controlled at 0.38-0.48%, specific ranges of alloying elements) and heat treatment parameters (tempering temperature 500-700°C, cooling rate control) to achieve a balance between high yield strength (≥555 MPa) and acceptable surface hardness for sour service cracking resistance
Solution Approach 2:
The invention creates different microstructural zones within the pipe wall - the surface layer with tempered martensite structure for strength, and the central portion with controlled hardness through specific cooling rates, achieving local optimization of both strength and cracking resistance
2Strength
If conventional quenching and tempering processes are used to achieve high strength, then yield strength is improved, but the manufacturing cost increases due to complex multi-step processes
Solution Approach 1:
The invention merges the quenching and tempering operations into a more integrated process where the pipe is quenched after hot forming and then tempered in a controlled manner, reducing the number of separate heating cycles and associated costs while maintaining high strength properties
Solution Approach 2:
The invention implements continuous cooling after quenching to room temperature without intermediate reheating steps, and maintains continuous tempering action, eliminating idle time and reducing energy consumption compared to batch processing methods
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 process ensures a seamless steel pipe with a yield strength of 555 MPa or higher and good SSC resistance while maintaining reasonable manufacturing costs and avoiding excessive surface hardness, thus enhancing sour resistance.
Implementation Method 1
a seamless steel pipe which has a carbon equivalent (Ceq) of 0.38 to 0.48%, a microstructure mainly consisting of tempered martensite or tempered bainite from the surface layer to the in-the-wall portions
Implementation Method 2
the seamless steel pipe has a microstructure mainly consisting of tempered martensite or tempered bainite from the surface layer to the in-the-wall portions
Data Source
Figure 1
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AI summary
A seamless steel pipe is provided that provides a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner. A seamless steel pipe contains, in mass %, C: 0.02 to 0.15 %; Si: 0.05 to 0.5 %; Mn: 0.30 to 2.5 %; Al: 0.01 to 0.10 %; Ti: 0.001 to 0.010 %; N: up to 0.007 %; Cr: 0.05 to 1.0 %; Mo: not less than 0.02 % and less than 0.5 %; Ni: 0.03 to 1.0 %; Cu: 0.02 to 1.0 %; V: 0.020 to 0.20 %; Ca: 0.0005 to 0.005 %; and Nb: 0 to 0.05 %, among others, where the carbon equivalent Ceq is not less than 0.430 % and less than 0.500 %, the main phase of the microstructure from the surface layer to an in-the-wall portion is tempered martensite or tempered bainite, the size of prior austenite grains is lower than 6.0 in crystal grain size number according to ASTM E112-10, a portion between a position at 1 mm from the inner surface and a position at 1 mm from the outer surface has a Vickers hardness of 250 Hv or lower, and the yield strength is 555 MPa or higher.