Failure behavior of 304l stainless steel in torsion

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INTRODUCTION

AUSTENITIC stainless steels such as type 304 are among the most formable of common engineering alloys. A large measure of this formability at cold-working temperatures derives from relatively high rates of strain hardening, a characteristic which promotes uniform strain distribution and strong resistance to flow localization. At hot-working temperatures, the occurrence of dynamic recrystallization enhances workability through the ability to annihilate or heal microcracks which may form during deformation. Despite the acceptance that the austenitic stainless steels enjoy as high-formability alloys, however, little quantitative work documenting the formability of these materials in general, or 304 in particular, over wide ranges of temperature exists in the literature. For example, at cold- and warm-working temperatures (200 ~ 20 (a) 200 ~ 20 ---->200!b~

I 5mmj

RESULTS

The torsional ductility of 304L was found to be a strong function of both strain rate and temperature (Table IlI). The initially-straight axial scribe lines on the surfaces of the torsion specimens were used to test the uniform-strain assumption of Eq. [4]. This assumption was violated only for the 20 °C and 200 °C tests at the high strain rate because flow localization in the form of shear bands had occurred (Figure 1). The occurrence of nonuniform deformation in those tests was also supported through examination of the microstructure along the gage length (Figure 2). Specimens tested at the same temperatures, but at the low strain rate, exhibited straight scribe lines at failure, indicating a fracture-controlled failure (Figure 3). The angle, 05, between these lines and the torsion axis was used to calculate Us using Eq. [5]. These values of F, correlated very well with the values of Fs calculated from the measured twists to failure (Eq. [4]). Similar straight lines (Figure 4) and F, correlations were found for specimens tested at either strain rate at 400 °C, 600 °C, 800 °C, 1000 °C, and 1200 °C, indiTable III.

(a)

Torsional Ductility Data Surface Effective Strain Rate (s-') 0.01 10.00 0.01 10.00 0.01 10.00 0,01 10.00 0.01 10.00 0.01 10.00 0.01 10.00 0.01 0.01

~52.5 pct of total deformation applied at 20 °C (b~80 pct of total deformation applied at 200 °C

METALLURGICALTRANSACTIONSA

Average F, at Failure 4.02 1.58 5.74 3.30 5.17 4.31 4.59 4.02 5.89 5.31 8.33 10.77 >8.04 13.21 5.02 5.74

(b) Fig. 1 - - F a i l e d torsion specimens, from ~ = 10 s t tests, showing evidence of flow localization: (a) 20 °C, average F, = 1.3, (b) 200 °C, average Fs = 2.9.

cating that these latter specimens also failed without flow localization. Lastly, failure-strain data for the low-strainrate temperature-change specimens, which also failed by fracture-controlled processes, are extremely consistent with data from the isothermal tests (Table III). IV.

DISCUSSION

A. Flow-Localization-Controlled Failure Analysis. The initiation and growth of flow localizations in the 304L torsion tests can be analyzed in the context of previous work in this area, t'1

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