IPC2010-31153
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EFFECTS OF M/A ISLANDS MICROSTRUCTURE CHARACTERISTIC ON MECHANIC PROPERTIES OF HIGH GRADE PIPELINE STEEL
Tong Ke Tubular Goods Resarch Center of CNPC, Xi’an, China Shao Xiaodong Tubular Goods Resarch Center of CNPC, Xi’an, China
Zhu Lixia Tubular Goods Resarch Center of CNPC, Xi’an, China
Liu Qiang Tubular Goods Resarch Center of CNPC, Xi’an, China Li Jinfeng Tubular Goods Resarch Center of CNPC, Xi’an, China
ABSTRACT The microstructure characteristic of martensite/austenite(M/A) islands of X80-1#, X80-2#, X100-3#, X100-4# steel pipelines come from different manufacturer were investigated by OM and SEM contrastively, and the influence of M/A islands microstructure characteristics of pipelines were studied combining with tests of mechanic properties. The result shows that the effect of the M/A islands volume fraction, average dimension, form and distribution on the material strength, yield ratio and toughness were distinct. When the M/A islands distribute uniformly with small size and spherical or crossed strip form, under the certain volume fraction, the pipeline steel exhibits excellent strength-toughness matching. INRODUCTION The adjustment of energy structure and the increasing demand of the energy promoted the development of large caliber, high pressure and long-distance gas transmission pipeline. In order to reduce pipeline construction and operating costs, enhance pipeline safety and reliability, high pressure and
large caliber pipeline steel is required to have not only higher strength but also higher toughness. The practice shows that pipeline construction cost can be saved by 7% -10% for the increase of steel grade every once. Therefore, the pipeline is commonly required to be made of the steel with grade of X80 or above. At present, high-grade pipeline steel has become a hotspot in the field of low alloy high strength steel and micro-alloying [1,2]. The strength-toughness performance of the pipeline steel is closely related to the microstructure. Along with the improvement in the strength-toughness performance of the pipeline steel, the corresponding organization is gradually evolved into granular bainite (GB) type from the initial ferrite-pearlite (PF-P) type[3]. The notable characteristics of granular bainite are that martensite/austenite （M/A）components are distributed on the high-density dislocation ferritic matrix. M/A components, which are the important composition of granular bainite, usually present as island shape, so called M/A islands[4,5]. Researchers have conducted
Copyright © 2010 by ASME 1
Proceedings of the 8th International Pipeline Conference IPC2010 September 27-October 1, 2010, Calgary, Alberta, Canada
IPC2010- active studies on the M/A islands. The effects of the cooling rate, deformation temperature and other rolling process on M/A islands were reported in the literatures[6,7], while those of the M/A island shape on the properties of welding heat-affected zone were studied in the literatures[8]. However, there is little systematic analysis on the relationship between the microcosmic characteristics of M/A components and the strength-toughness performance of the pipeline steel till now. The effect of size, shape, distribution and other microcosmic characteristic factors of M/A islands on the strength-toughness performance of the pipeline steel is systematically studied, which is of great significance in optimizing structural design of the pipeline steel and improving material properties. Therefore, representative X80-1, X80-2, X100-3 and X100-4 steel pipelines were investigated in this paper, aiming at exploring the relationship between microcosmic characteristics of M/A islands and the strength-toughness performance of material. SPECIMEN PREPARATION AND TEST METHOD The pipeline steel material for testing comes from UOE LSAW pipes produced by 4 manufacturers, while the steel plates for testing are produced using Thermo Mechanical Control Process (TMCP) technology. Specifications, steel grade and production place of specimen material are shown in Table 1. Chemical compositions are shown in Table 2. A pipeline cross-sectional specimen which is 180° from welded seam was taken and processed through rough grinding, fine grinding and polishing, and then etched using 2% alcohol solution of nitric acid. Subsequently, its metallographic structure was observed on MEF4M metallographical microscope and analysis system for analyzing distribution of M/A islands; the shape of M/A components was observed on VEGAⅡ scanning electronic microscope; and then content and size of M/A islands were quantitatively analyzed using “Phase Content Measurement” and “Size Measurement” of SISC IAS V8.0 image analysis system. Mechanical properties specimen was taken and processed according to API Spec 5L：2007 standard: transverse round bar tensile specimen of pipeline which is 180° from welded seam was taken, in which the
