High temperature mechanical properties of 00Cr20Ni18Mo6CuN (254SMO) steel

Abstract

The purpose of this study is to investigate the high temperature mechanical properties of UNS S31254 stainless steel (254SMO). Tensile tests, creep tests and fatigue tests have been carried out to study the properties. The test results show that the UNS S31254 stainless steel has quite excellent high temperature strength and ductility although the ductility is poorer than that of standard austenitic stainless steel. Also, the fatigue strength of the UNS S31254 stainless steel is higher than that of 316L at room temperature. The creep results at 600℃ show that the creep rupture life under 0.2% strain of UNS S31254 stainless steel is obviously longer than that of 316L stainless steel.

1 Introduction

The UNS (Unified Number System) code S31254 stainless steel (254SMO) is a type of stainless steel that is developed for special use in acid chloride media, such as freshwater and sea water. It is also used in various other applications due to its excellent corrosion resistance. However, the high temperature mechanical properties of UNS S31254 are less well known. In this study, the high temperature mechanical properties, including tensile properties, creep properties and fatigue properties, of the UNS S31254 stainless steel were investigated and compared to those of the standard austenitic stainless steel (316L).

2 Experimental

The base material for the tensile, creep and fatigue testing was in the form of 10mm×10mm×55mm bars. The chemical composition of the UNS S31254 specimen and the 316L specimen used in the tests are given in Table 1.

The tensile tests were carried out by a universal testing machine at a constant cross head speed of 0.12 mm/s. Five samples were tested at temperatures of 20°C, 500°C and 600°C. The creep tests were conducted on a stress-relaxation machine. Five specimens were tested at temperatures of 500°C and 600°C and a stress of 200 MPa. The loading was sinusoidal at 0.2 Hz. The total strain accumulation was measured until creep rupture occurred. Fatigue tests were also conducted on a MTS electro-hydraulic test system. Five specimens were tested at 0.3 Hz, with a maximum load of 70% of the ultimate tensile strength of the material. The total cycles to failure were measured for each sample.

3 Results and Discussion

3.1 Tensile tests

The tensile test results for the UNS S31254 and 316L specimens are shown in Fig. 1. At room temperature, the UNS S31254 specimen showed a higher tensile strength and lower ductility than the 316L specimen. In addition, the strength of the UNS S31254 specimen decreased more rapidly than that of the 316L specimen with increasing temperature. The UNS S31254 specimen also showed poorer ductility than the 316L specimen at both 500°C and 600°C.

Fig. 1 Stress-strain curves of UNS S31254 and 316L specimens at room temperature (RT), 500° C and 600° C

3.2 Creep tests

The creep test results are presented in Fig. 2. Both materials showed an initial decrease in the creep rate with increasing time and then a subsequent increase. The creep rate of the UNS S31254 specimen was significantly lower than that of the 316L specimen at 600°C. The creep rupture time of the UNS S31254 specimen was also longer than that of the 316L specimen under the same conditions.

Fig. 2 Time-dependent creep curves of UNS S31254 and 316L specimens at 500° C and 600° C

3.3 Fatigue tests

The fatigue test results are presented in Fig. 3. The fatigue strength of the UNS S31254 specimen was higher than that of the 316L specimen at room temperature. At 500°C and 600°C, the fatigue strength of the UNS S31254 specimen was much lower than that of the 316L specimen.

Fig. 3 Fatigue curves of UNS S31254 and 316L specimens at room temperature, 500°C and 600°c

4 Conclusions

In this study, the high temperature mechanical properties of UNS S31254 stainless steel were investigated. The results show that the UNS S31254 stainless steel has excellent high temperature strength and ductility although the ductility is poorer than that of the standard austenitic stainless steel. The fatigue strength of UNS S31254 was higher than that of 316L at room temperature. The creep results showed that the creep rupture life of the UNS S31254 stainless steel under 0.2% strain was obviously longer than that of 316L stainless steel at 600℃.

254SMO steel is a kind of super austenitic steel which contains 20% chromium, 18% nickel, 6% molybdenum, copper and nitrogen. It has excellent corrosion resistance in a wide range of media. In addition, it has good high temperature strength, high ductility and toughness at low temperature.

At room temperature, 254SMO steel has a tensile strength of 850Mpa or more, yield strength of more than 375Mpa, elongation of 40% or more, hardness of 200HB or less, and impact value of 20J/cm2 or more. The hardness of the material can be greatly increased by heat treatment.

Under high temperature conditions, 254SMO steel has good thermal fatigue resistance, which can resist the cyclic heat loading of its components. Its extension rate and tensile strength at high temperature are also very good. Even after long-term exposure to high temperature environment, it still has good radiation resistance and oxidation resistance. Other features also include good weldability and workability.

In short, 254SMO steel has excellent mechanical properties at room temperature and high temperature, especially its high temperature strength. This steel is widely used in many fields, such as petrochemical industry, papermaking industry, marine engineering and central air-conditioning industry.