A comparative study of the mechanical properties and the behavior of carbon and boron in stainless steel cladding tubes fabricated by PM HIP and traditional technologies

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Abstract

The ring tensile test method was optimized and successfully used to obtain precise data for specimens of the cladding tubes of AISI type 316 austenitic stainless steels and ferritic–martensitic stainless steel. The positive modifications in the tensile properties of the stainless steel cladding tubes fabricated by powder metallurgy and hot isostatic pressing of melt atomized powders (PM HIP) when compared with the cladding tubes produced by traditional technology were found. Presently, PM HIP is also used in the fabrication of oxide dispersion strengthened (ODS) ferritic–martensitic steels. The high degree of homogeneity of the distribution of carbon and boron as well the high dispersivity of the phase-structure elements in the specimens manufactured via PM HIP were determined by direct autoradiography methods. These results correlate well with the increase of the tensile properties of the specimens produced by PM HIP technology.

Highlights

► The ring tensile test method was optimized and successfully used. ► The cladding tubes fabricated by PM HIP and traditional technologies were tested. ► Improvement of the cladding tubes properties fabricated by PM HIP was found. ► Correlation of the homogeneity of carbon, boron with the properties was revealed.

Introduction

Austenitic and ferritic/martensitic stainless steels are the materials predominantly used as structural materials in fast breeder reactors as well as for various designs of D–T fusion reactors [1], [2], [3], [4]. To improve the mechanical properties of AISI type 316 austenitic stainless steels and ferritic/martensitic steels, the effects of variations in the composition and fabrication technology on the structure and properties of these materials is necessary to study. The well-known main problems of the traditional technology of high temperature alloys, specifically high alloying stainless steels and Ni-based superalloys, are the high degree of phase-structure and composition heterogeneity, particularly involving carbon and boron, and the presence of macro- and microstructure defects in massive ingots. Rapidly quenched Ni-base superalloys produced by PM HIP technology using fine melt atomized powders have real advantages as high temperature materials of critical components for gas-turbine [5], [6]. High temperature materials used in the critical components of a gas-turbine must possess phase-structure stability during the manufacture of the products as well as during operating conditions. The forementioned stability has been obtained for a set of the Ni-based commercial alloys, such as EP741, EP962, Rene95, and Rene88, with carbon and boron doping, which possess the unique mechanical properties as the materials of the gas-turbine components [5], [6].

Currently, only the duplex stainless steels manufactured by PM HIP technology using rapidly quenched powders are sufficient for use in all-round applications [7].

The oxide dispersion strengthened stainless steels such as ODS-EUROFER, used in the fabrication of blanket structural components, are presently the object of intensive study [8], [9], [10]. These high performance steels are composed of compacted melt-atomized powders and are manufactured by the PM HIP method. Therefore, the comparison of the mechanical properties, as well as the features of the distribution of carbon and boron in the microstructure of the stainless steels fabricated by both PM HIP and traditional technologies, is desirable for estimating the role of the matrix in the mechanical properties of ODS stainless steels.

In this work, a comparative study of the tensile properties at room temperature of cladding tubes composed AISI type 316 austenitic stainless steel and ferritic/martensitic stainless steel, which were produced by PM HIP using melt atomized powders and by traditional metallurgy (by casting and forging), was performed. The optimized ring tensile test method was used to obtain data for specimens of the cladding tubes of the stainless steels. The essential feature of this work is an investigation of the behavior of carbon and boron, as these interstitial elements undergo the highest degree of liquation and composition heterogeneity in these stainless steels as well as in other high temperature alloys.

Section snippets

Experimental procedures

The compositions of the investigated austenitic and ferritic/martensitic stainless steels are listed in Table 1.

All compositions are given in mass percent.

In this work, the ring tensile test method was optimized similar to what was reported in Ref. [11] and was successfully used to obtain precise data. This method consists of placing the ring tensile test specimens into tension via two semi-cylindrical mandrels, with a radius of 2.7 mm. Ring specimens, with a length of 3 mm, were cut from the

Results and discussion

Fig. 4 shows the stress–strain curves of the austenitic stainless steel EP172 cladding tube specimens fabricated by powder metallurgy using the rapidly quenched powder and by the traditional technology. Two typical stress–strain curves which illustrate the test results for each technology are presented. The ultimate tensile and yield strength of the cladding tube specimens produced by powder metallurgy using the rapidly quenched powder (740 MPa and 650 MPa, respectively) are considerably higher

Conclusion

In this work, tensile tests, conducted at room temperature and at a strain rate of 1 · 10−4 s−1, of the ring specimens cut from cladding tubes of austenitic and ferritic/martensitic stainless steels fabricated by PM HIP and traditional technologies were performed. An autoradiography study of the behavior of carbon and boron in the tested specimens was conducted simultaneously with metallography, Scanning Electron Microscopy (SEM), and X-ray Spectral MicroAnalysis (XRSMA). Modifications of the

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