Mechanical properties, wear, and corrosion of boronized N80 tube steel
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MECHANICAL PROPERTIES, WEAR, AND CORROSION OF BORONIZED N80 TUBE STEEL D. S. Xu,1 X. Xu,2 Z. G. Su,1 J. An,1, 3 and Y. Lu1 The effect of boronizing on the mechanical properties, wear, and corrosion of boronized N80 tube steel is studied. A dual-phase boride layer consisting of the FeB and F2 B phases is formed on the surface of steel substrate within the hardness range 1220–1340 HV. A special setup was designed to use smaller amounts of the boriding agent and accelerate the process of cooling of the pipe. In order to meet the tensile properties of N80 steel required by the API SPEC 5L, four cooling methods were employed. The procedure of fan cooling with a graphite bar inside the boriding agent resulted in the highest mechanical properties in accordance with the mechanical properties of the API SPEC 5L. The boronized N80 steel reveal a high wear resistance under the conditions of dry sliding and excellent corrosion resistance in the as-received oil-field water from the Jilin oil field, Northeast China. Keywords: boronizing, mechanical properties, wear, corrosion.
Boronizing is a thermochemical surface treatment by thermodiffusion followed by the chemical reaction into the surface. Its use strongly increases the surface hardness (about 1200–2000 HV), wear resistance, and anticorrosion properties of the boride layers [1]. Boronizing of carbon steels usually leads to the formation of two different iron-boride phases: FeB on the surface and F2 B between the matrix and FeB [2]. Industrial boronizing can be carried out for most ferrous materials, such as structural steels, cast steel, Armco iron, gray iron, and ductile iron [3–5]. Boronized steels consistently and significantly outperform the nitrided and carburized steels because iron boride formed in these steels exhibits a substantially higher hardness as compared with carburized or nitrided steels (650–900 HV). In particular, boronized steels exhibit excellent resistance to a variety of tribological wear mechanisms. In addition, the resistance of boronized steels to the attack of nonoxidizing dilute acids, alkalis, and melted metals is also outstanding. N80 steel is a relatively inexpensive material widely used as material of the tubes for oil fields. However, it displays unsatisfactory performance under severe conditions of high corrosion and heavy wear, especially in acid environments containing CO 2 and H 2 S according to the investigations on the degradation of tubing metals in hydrogen-sulfide environments [6, 7]. Boronizing is expected to efficiently improve the corrosion and wear resistance of N80 tube steel. However, to ensure that the procedure of boronizing of N80 tube steel is practical for the oil industry, two problems associated with this procedure must be addressed. First, the minimum usage of the boriding agent should be realized. Second, an appropriate cooling manner must be chosen to ensure that the tube will meet the mechanical properties demanded by the API SPEC 5L. We designed a special boronizing setup to use less pack powder and chose one of the
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