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Mechanical properties and strain hardening of low-alloyed and stainless steels for piping at elevated temperatures

Dr. Dmitry Pympyanskiy1, Dr. Igor Pyshmintcev2, Dr. Vladimir Khatkevich2

1- Ural Federal University (Ekaterinburg); 2- TMK Research LLC (Moscow)

Dr. Igor Yu. Pyshmintcev

General director of Russian Research Institute for the Tube and Pipe Industries

Director of TMK company

Abstract. Low temperature behavior of steels as well as other alloys is studied and described in detail due to critical importance of resistance to brittle fracture. High temperature properties of metals are normally given as rupture stress and/or creep resistance. Chemical composition, lattice type (BCC or FCC) with main parameters of microstructure such as grain size, secondary phases and fine precipitation are usually considered as key factors. Nevertheless, strength of materials at elevated temperature lower than critical creep temperature has high importance for design of machines and constructions for different applications. Maximum applied stress is main parameter for proper design of piping systems in many fields including oil refining, gas chemistry, energy generation, metallurgy, and others. Industrial design codes consider maximum applied stress at elevated temperatures as function of tensile strength or yield stress of selected material. Proper measurement and correct prediction of main properties at elevated temperatures has high importance. The goal of present study was to establish the influence of chemical composition, microstructure and strain rate on tensile strength or yield stress of low alloyed and stainless steels widely used for technological piping at room and elevated temperatures. Special attention was paid to phenomenon of dynamic strain hardening strongly dependent on strain rate and temperature.

The mechanical properties of 0.09 %C - 1,32 %Mn - 0,52 %Si steel are measured by tensile tests of samples in various structural states at temperatures of 20 – 400°C and strain rates from 5 × 10–2 to 5 × 10–5 1/s. The test results are shown to be significantly determined by dynamic strain aging. A correlation between the dynamic strain aging intensity and the temperature, the strain rate, and the structural state of the material is found. The yield strength of steel in all structural states was shown to decrease monotonically as the temperature increases. The tensile strength has a minimum at 150°C; then, it increases and reaches a maximum at 300–350°C. This result is related to dynamic strain aging, which peculiarity is an unstable plastic flow observed at the stage of uniform deformation at temperatures of 150–300°C. The influence of dynamic strain aging is dependent not only on the temperature but also on the strain rate that shall defined rigorously since tensile test method standards enable it to be varied over wide limits. The influence of dynamic strain aging on tensile strength is shown to be dependent only on the ferrite grain size. Fine microstructure is more effective for strengthening at 150–200°C, which enables one to smooth the decrease in tensile strength.

The microstructure, phase composition, and mechanical properties of austenitic chromium–nickel Fe–18Cr–10Ni steels in the temperature range 20–650°C are measured in the framework of the studies on developing a new standard for welded and seamless pipes made of stainless steel. The mechanical characteristics are shown to have a substantially scatter, and the contributions of various hardening factors are estimated quantitatively. The grain size and solid-solution hardening with carbon, nitrogen, or titanium was found to influence the mechanical properties most significantly, providing up to 45% of the yield strength of steel. The temperature dependence of the ultimate strength is found to be determined by the stability to the formation of strain-induced martensite and the dynamic strain aging intensity.

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