Development of 110-ksi Grade OCTG With Good Resistance to Sulfide-Stress-Corrosion Cracking
Authors: Motoda K. , Masuda T. 
Published: December 2007

Seven trial heats that were quenched and tempered into the C-110 strength range were tested for sulfide-stress-corrosion cracking (SSCC) resistance with three popular test procedures: the NACE tension, the Shell bent-beam, and the double-cantilever-beam (DCB) methods.

One of the heats, containing 0.45% C, 0.6% Mn, 1.0% Cr, 0.8% Mo, 0.04% Nb, 0.05% V, and 0.03% Ti, was tempered at temperatures above 1.292 degrees F [700 degrees C] and satisfied the requirements for C-110, where the NACE threshold stress was set at a minimum of 80% of its specified minimum yield strength (SMYS).

Introduction

In 1984, Grade C-90 oil-country tubular goods (OCTG) for sour services was registered to API Spec, 5AC. In this specification, manufacturers are required to guarantee the minimum threshold stress, which is determined according to NACE Standard TM-01-77, of 72 ksi [496 MPa], 80% of the SMYS for C-90.

It has been reported that Grade C-90 might have the highest threshold stress for SSCC. Some researchers, however, showed that it was possible to obtain a steel with higher SMYS and better SSCC resistance than C-90.

Grade C-110 material, which is superior both in strength and in anti-SSCC property to C-90, has been tried. The tensile requirements for C-110 were basically taken from P-110, and the yield strength was restricted within the range of values from 110 to 125 ksi [758 to 862 MPa]. Because of the API requirement for C-90, the goat of SSCC resistance of C-110 was decided to be a threshold stress of 88 ksi [607 MPa], 80% of the SMYS for C-110, in the NACE test method.

Microstructure and strength level are two of the most important factors that determine the cracking resistance of a steel. At a given strength level, a microstructure consisting of fine spheroidized carbides dispersed uniformly in a ferrite matrix-a typical microstructure of martensite tempered at high temperatures-gives steel the best resistance to SSCC.

The tempering temperatures of the C-90 materials, which are made from 0.25 % C, 1 % Cr, 0.6% Mo, and 0.03 % Nb steel, are higher than 1,292 degrees F [700 degrees C]. The first step was to find the alloy elements that allow the steel to be tempered at temperatures above 1,292 degrees F [700 degrees C] for obtaining the yield strength level of C-110.

Cr, Mo, Nb, and C were increased and V and Ti added to modify the C-90 material.

Laboratory Work

Materials and Test Procedures. The main chemical compositions of steels used in the study were as follows. Steel A: 0.25% C/1% Cr/0.6% Mo/0.03% Nb. Steel B: 0.25% C/2% Cr/0.6% Mo/0.05% Nb/0.08% V/0.02% Ti. Steel C: 0.25% C/3% Cr/0.6% Mo/0.05% Nb/0.08% V/0.02% Ti. Steel D: 0.25% C/2% Cr/1.0% Mo/0.05% Nb/0.08% V/0.02% Ti. Steel E: 0.45% C/1% Cr/0.8% Mo/0.03% Nb. Steel F: 0.45% C/1 % Cr/0.8% Mo/0.04% Nb/0.05% V/0.03% Ti. Steel G: 0.45% C/1.5% Cr/1.0% Mo/0.06% Nb.

Steel A is the same composition as our steel for Grade C-90. All the steels were melted with a 66-lbm [30-kg] vacuum induction furnace. Actual chemical compositions of steels are shown in Table 1. The heats were forged and rolled to plate 0.6 in. [15 mm] thick and then heat-treated.

The heat treatment for 0.25 % C steels (Steels A through D) consisted of austenitizing at 1,616 degrees F [920 degrees C] for 30 minutes, followed by water quenching and tempering for 30 minutes at various tem-peratures. The 0.45 % C steels (Steels E through G) were austenitized at 1,616 degrees F[880 degrees C] for 30 minutes, oil quenched, and then were tempered for 1 hour at various temperatures. Relationships between tempering conditions and tensile properties were examined.

SSCC resistances of the tempered steels were studied according to the NACE Standard Test Procedure TM-01-77. In addition, the Shell bent-beam tests and the DCB test were carried out for 0.45% C steels. The SSCC test conditions are listed in Table 2, and the geometries of the specimens are illustrated in Fig. 1.

Results and Discussion. Relationships linking tempering temperature and yield strength are plotted in Fig. 2. This figure indicates that increasing or adding carbide-forming elements, such as Cr, Mo, Nb, V, and Ti, is not very effective in raising the tempering temperature, and none of the 0. 25 % C steels can be tempered at temperatures above 1,292 degrees F [700 degrees C] to obtain the yield strength level of C-110. Only two 0.45% C steels, Steels F and G, are able to be tempered at higher temperatures than 1,292 degrees F [700 degreees C].

The tempering conditions and the tensile properties of the steels for SSCC tests are shown in Table 3, as are the results of SSCC tests. The NACE threshold stress, the critical stress deter-mined by the bent-beam test, and the SCC fracture toughness, Kjscc, determined by DCB test are plotted in Fig. 3 as a function of yield strength.

Steel F exhibits a good SSCC resistance in the NACE test, with a threshold stress of approximately 102 ksi [700 MPa], 92.5% SMYS of C-110, and is the only steel that reached the goal. Considering the difference in strength among the three 0.45 % C steels, the difference of the S, values is negligible. On the other hand, the Kjscc values have a noticeable difference. Steel F also exhibited high SSCC resistance in the DCB test, with a Klscc of 35.8 ksi square root in.. [39.4 MPa square root m]. Fig. 4 shows a scanning electron microscope (SEM) image of Steel F tempered at 1,320 degrees F [715 degrees C]. The carbides in this steel are well spheroidized and are uniformly dispersed.

Steel G also can be tempered at the same temperature as Steel F to obtain the strength level of C-110. This steel, however, exhibits rather low SSCC resistance in the NACE and the DCB tests. Steel G contained higher percentages of Cr and Mo than Steels E and F.

JPT

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