Creep deformation and fracture of a Cr/Mo/V bolting steel containing selected trace-element additions
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I. INTRODUCTION
DUREHETE D1055 is a CrMoV low-alloy steel commonly used for bolting material in electrical power stations, under conditions of elevated temperatures up to 600 8C at low stress, where creep is the major deformation mode. The D1055 alloy shows a large scatter in creep properties[1,2] and is susceptible to intergranular failure. For these reasons, research has been focused on the longterm reliability of this steel, in terms of both creep strength and creep ductility. Furthermore, since a major application of D1055 is as a bolting steel,[3] it is appropriate to examine the sensitivity of this material to high stress concentrations and to assess its resistance to cracking under creep conditions. Service problems were encountered with early 1CrMoV steels which had good creep strength accompanied by low ductility and sensitivity to high stress concentrations.[4] This problem was reduced in 1CrMoV steel by additions of boron and titanium.[5] These elements apparently prevented the formation of weak grain-boundary precipitate-free zones, thus improving the creep strength of grain-boundary regions. An improvement of ductility without loss in creep strength was also observed in a modified 1CrMo0.75V steel.[5] However, the presence of trace elements in other CrMoV steels has been found to be a significant factor in reducing the creep rupture properties.[6] It is well known that trace elements segregate to grain boundaries under creep conditions, altering their strength and inducing cavity nucleation and growth, which results in premature intergranular failure.[7,8,9] The effect of trace elements, individual or combined, in CrMoV steels also shows contradictory results and varies from one steel to another.[6,10–12] The present work is a further attempt to clarify the effect of trace elements on creep properties, by
Z. LAROUK is with the Institute of Physics, University of Constantine, Constantine, 25000 Algerie. R. PILKINGTON is with the Materials Science Centre, University of Manchester, Manchester, M1 7HS, United Kingdom. Manuscript submitted July 22, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
identifying the individual effects of P, Sn, and As on the Ti-containing steel Durehete D1055.
II. EXPERIMENTAL Four casts of D1055 steel were produced by vacuum melting. These casts are referred to as P, As, and Sn for the P-, As-, and Sn-doped casts, respectively, and their chemical compositions are given in Table I. The Ti cast, without the trace additions, was used as a reference cast, hence the designation “Ref”. The material of each cast was received in two forms of hot-rolled bar, of either 12 3 12 or 10 3 25 mm cross section. The first type was used for producing smooth creep-test specimens, and the second was used for producing double-edge notched tension specimens (DENT). The material was subjected to one of two heat treatments. First, the material was heat treated at 980 8C for 2 hours, water quenched, and tempered at 690 8C for 4 hours; this material is referred to the fine-grained material. Second,
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