Stainless Steel Background Research

VTT Building Technology and Helsinki University of Technology in Finland have carried out a series of extensive experimental research investigating mechanical properties of various structural steels at elevated temperatures from 1993 to 1997 (Makelaine and Outinen 1998; Kouhi et al. 2000). One research project concentrated on an austenitic stainless steel, named Polarit 725 (grade EN 1.4301). The research investigated the mechanical properties of this stainless steel material at elevated temperatures by using transient-state and steady-state tensile tests. The heating rate in the transient-state tests was 10°C/min. The tests were carried out on both the base material taken from a virgin cold-rolled steel sheet and a strongly cold-formed material taken from a rectangular hollow section (RHS).

Figure 1 compares the test results of the thermal elongation of stainless steel EN 1.4301 with the design values provided by EN1993-1-2. The test results were obtained from transient-state tests at stress level of 3N / mm² (Makelaine & Outinen 1998). There is no significant discrepancy between the test results and the design values.

Figure 2 shows the test results for the reduction factor of proof strength f_0.2p,θ, relative to yield strength at 20°C f_y, of stainless steel EN 1.4301 and the design values by EN1993-1-2. The proof strength f_0.2p,θ is based on 0.2% non-proportional strain. The test results showed that the strongly cold-formed materials from RHS retained greater strength than the base materials. The steady tests generated smaller reduction factors for the material proof stress compared to the transient tests. It can be seen that the design values in EN1993-1-2 correspond approximately to the mean of the values of the transient-state and steady-state tests of the base material.

Figure 3 shows the test results for the reduction factor of ultimate strength f_u,θ, relative to ultimate strength at 20°C f_u, of stainless steel EN 1.4301 and the design values by EN1993-1-2. The ultimate strength f_u,θ is based on 2% total strain. Obviously, the design curve forms the upper bound of the test results obtained from transient-state tests. The test results also showed that the strongly cold-formed materials from RHS retained greater tensile strength than the base materials up to 700°C.

Figure 4 shows the test results for the reduction factor of elastic modulus E_a,θ of stainless steel EN 1.4301 and the design values by EN1993-1-2. The value of elastic modulus at elevated temperatures was difficult to determine because the stress-strain curves of stainless steels do not have clear proportional limit. The test results from different kinds of tests varied significantly (Makelaine & Outinen 1998). Curve fitting method was, therefore, performed to generate the test curve (Kouchi et al. 2000). As a result, the discrepancy between the test and design values for elastic modulus is greater.

Baddoo and Burgan (1998) reported a test programme carried out by The Steel Construction Institute on austenitic stainless steel of EN 1.4301 at elevated temperatures. Both stainless steel beams and columns made of RHS and I-sections, which were fabricated from cold-formed C channel, were tested. The tensile tests on the material coupons cut from the C channels were also carried out under transient-state and steady-state heating conditions. The specimen heating rate was 10°C/min. Figure 5 shows the test results of the reduction factors for the 0.2% proof strength f_p (relative to yield strength at 20°C) and 2% ultimate strength f_u (relative to tensile strength at 20°C). The difference between the two reduction factors is not significant. Relatively, the proof strength reduction factor recommended by EN1993-1-2 tends to be conservative, whereas the reduction factor for ultimate strength closely follows the test results.