Restoration of elevated temperature tensile strength in 2.25Cr-1Mo steel
- PDF / 4,694,431 Bytes
- 11 Pages / 603.28 x 788 pts Page_size
- 48 Downloads / 185 Views
INTRODUCTION
THE2.25Cr-lMo steel has been widely used for service at temperatures of 400 to 600 ~ in pressure vessels and piping because of its excellent elevated temperature strength and resistance to hydrogen attack. Recently it has been reported that prolonged exposure to service conditions at elevated temperature can result in some deterioration of mechanical properties. One such case has been documented by Watanabe and Sato ~ who observed a loss of tensile strength and impact toughness in 2.25Cr-lMo steel after long term (80,000 to 150,000 hours) service at 510 to 650 ~ The reduction of the tensile strength was found to be higher than would be expected due to continued tempering during service. An equivalent loss of strength was observed after furnace cooling I or step-cooling2 of the steel from the tempering temperature. The loss of strength was more pronounced in elevated temperature (300 to 500 ~ tensile strengths than in the room temperature tensile strength, but it was less significant in terms of yield strength. Furthermore, the reduced strength was found to be reversible to its original value by a short time retempering at 700 ~ a temperature higher than the service temperature. These observations suggest that the loss in the tensile strength was caused mainly by a reduction in the solid solution hardening, which is strongly affected by the concentration of the interstitial elements, nitrogen and carbon, in the matrix. The present paper summarizes the results of metallographic studies conducted on 2 . 2 5 C r - l M o steel in various heat treatment and service conditions to determine the mechanism of the reduction and restoration of tensile properties after long term service exposure at elevated temperature. II.
EXPERIMENTAL P R O C E D U R E S
A. Test Steels The materials employed in this study were three lots of 2.25Cr-lMo steel with compositions given in Table I. T. WADAand V.A. BISS are Staff Metallurgist and Senior Research Associate, respectively,for the ClimaxMolybdenumCompanyof Michigan, Ann Arbor, MI 48106, a subsidiaryof AMAX Inc. Manuscript submittedApril 12, 1982. METALLURGICALTRANSACTIONSA
Specimens from Steel No. 1 were taken from one tube which had not been exposed to any service and from another furnace tube which had been operated at 500 to 570 ~ for approximately 100,000 hours. The mechanical properties of Steel 1 were reported by Watanabe and Sato I as "Steel A"; Sato, Okubo, and Watanabe 2 also reported test results from this steel. The original heat treatment of the steel by the manufacturer consisted of annealing by heating to 920 ~ holding for approximately 10 minutes at temperature, furnace-cooling to 730 ~ holding for 45 minutes at temperature, and furnace-cooling to room temperature. Steel No. 2 was taken from a seamless pipe with 355 mm I. D. and wall thickness of 70 mm. The pipe was originally normalized at 950 ~ for six hours, air cooled, tempered at 760 ~ for six hours and air cooled. A section of the pipe was reheat treated and used for several experiments as described
Data Loading...
