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Study of High Strength Low Alloy Steel OCTG with High Carbon Contents for Mildly Sour Service

For development of further high-strength low-alloy steel OCTG for sour service, effects of carbon content on the strength, microstructure, and sulfide stress cracking (SSC) resistance of low-alloy steel were fundamentally investigated.

Product Number: 51317--9051-SG
ISBN: 9051 2017 CP
Author: Yuji Arai
Publication Date: 2017
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High strength OCTG is required for high pressure / high temperature (HPHT) well applications in order to withstand the expected extreme partial pressures and depths. For development of further high strength low alloy steel OCTG for sour service effects of carbon content on strength microstructure and sulfide stress cracking (SSC) resistance of low alloy steel were investigated fundamentally.Modified AISI 4130 (1.0wt%Cr - 0.7wt%Mo) steel with varying carbon content from 0.25 to 0.60wt% quenched and tempered were analyzed with respect to martensite sub-structure (such as block size) precipitation of carbide dislocation density and hydrogen absorption behavior. By increasing carbon content the sizes of martensite blocks were refined (become smaller). It resulted in increasing strength of the steel even after tempering at high temperature. The tempering at high temperature led to reducing dislocation density and spheroidizing carbides. Hydrogen absorption in the steel with higher carbon content was reduced in a sour environment due to the reduction in dislocation density. SSC resistance was also evaluated using tensile tests according to NACE Standard TM0177 Method A. The steel with higher carbon content had higher threshold strength of SSC resistance which is likely to be due to the above mentioned microstructural improvements.From the result a development of further high strength low alloy steel OCTG such as 140 ksi Grade for sour service could be expected by using higher carbon materials.

Key words: Sulfide Stress Cracking, high-strength low-alloy steel, hydrogen.

High strength OCTG is required for high pressure / high temperature (HPHT) well applications in order to withstand the expected extreme partial pressures and depths. For development of further high strength low alloy steel OCTG for sour service effects of carbon content on strength microstructure and sulfide stress cracking (SSC) resistance of low alloy steel were investigated fundamentally.Modified AISI 4130 (1.0wt%Cr - 0.7wt%Mo) steel with varying carbon content from 0.25 to 0.60wt% quenched and tempered were analyzed with respect to martensite sub-structure (such as block size) precipitation of carbide dislocation density and hydrogen absorption behavior. By increasing carbon content the sizes of martensite blocks were refined (become smaller). It resulted in increasing strength of the steel even after tempering at high temperature. The tempering at high temperature led to reducing dislocation density and spheroidizing carbides. Hydrogen absorption in the steel with higher carbon content was reduced in a sour environment due to the reduction in dislocation density. SSC resistance was also evaluated using tensile tests according to NACE Standard TM0177 Method A. The steel with higher carbon content had higher threshold strength of SSC resistance which is likely to be due to the above mentioned microstructural improvements.From the result a development of further high strength low alloy steel OCTG such as 140 ksi Grade for sour service could be expected by using higher carbon materials.

Key words: Sulfide Stress Cracking, high-strength low-alloy steel, hydrogen.

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