Development of Sour Service Corrosion Resistant High Strength Steel Pipe Grades at TMK

Abstract

API Specification 5CT [1] and NACE MR0175 [2] and TM0177 [3] provide guidelines for the use of carbon steels with desired characteristics in oxygen-free environment containing H2S and CO2. TMK IPSCO has developed a range of suitable grades to meet such service requirements; this paper describes in detail the necessary processing tools that have been developed and implemented to achieve the desired pipe properties and consequently ensure their performance in the well. It is well known that low alloy carbon steels of higher strengths are susceptible to SSC (Sulphide Stress Cracking) in such sour service conditions. As the strength (and hardness) of steel increases, controlling impurities within the steel matrix becomes critical in the development of pipes for such applications. Development of technologies to reduce the inclusion content and ensure "clean" steel for the production of these grades at our steel production facility in Koppel is presented. Additionally, the use of rigorous heat treatment practices aimed at controlling the micro-structure and mechanical properties of the steel, which are achieved by monitoring and controlling critical parameters such as the furnace temperatures, are discussed. To meet the stringent quality requirements for these grades, improve process efficiency and reduce operational error, the use of automatic hardness testing machines have been implemented to batch process the hardness tests on such grades. Furthermore, a novel use of automated microhardness machine to predict the disposition of test samples to failure during SSC testing is presented. Investigations of steel specimens have shown that application of "clean" steel practices, like the implementation of TruStir® technology has resulted in a marked and consistent enhancement in the cleanliness of such sour service steel grades. Heat treatment studies point to enhancements in hardness control and distribution resulting from heat treatment practices involving double quench and tempering. Such heat treatment routes are believed to result in a better control of prior austenite grain size and prevent formation of localized "hard" spots in the material, which could become sites susceptible to SSC. Recent developments of the control of furnace operating parameters at our Baytown Heat Treating facility and resulting improvements in the performance of our sour service grades are discussed. Production of these sour service grades require stringent quality checks as mandated by API specification 5CT and ISO specification 15156. Apart from extensive hardness testing, these grades require prolonged corrosion testing. Aside from standardized testing methods for these grade, our internal microhardness testing of these grades have revealed a relation between the frequency of microhardness distribution and the corrosion performance of these grades. Based on these results, a quick methodology of predicting the corrosion performance of a material is being explored.