Steel variability effects on low cycle fatigue behavior of a single grade of high strength low alloy steel
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Low
cycle fatigue (LCF) properties of materials are becoming increasingly important t o d e s i g n e n g i n e e r s interested in applications susceptible to a few cycles of s t r e s s or s t r a i n in the plastic r e g i o n (viz, in power plants, n u c l e a r reactors, aerospace vehicles, automobiles, and so forth). ¢*-4) High strength low alloy (HSLA) steels are a relatively new group of alloys having h i g h e r strength as a r e s u l t of composition and processing variations. 5-n In recent y e a r s , the automotive industry's weight savings and safety performance g o a l s are putting g r e a t e r and g r e a t e r emphasis on low cycle fatigue properties of these n e w e r steels, n-~7 As a result, a knowledge of the extent of the effects of s e v e r a l s t e e l variables (viz, thickness, composition, processing variables, and so forth) and cold-work (as encountered in a f o r m e d or stamped p a r t ) on the LCF properties of a single s t e e l g r a d e is very desirable from design safety considerations. The present work attempts t o determine the influence of t h e s e variations on the LCF characteristics of a comm e r c i a l g r a d e of hot-rolled, pickled and oiled, Nbb e a r i n g (Cb) fine grained HSLA steel. EXPERIMENTAL
PROCEDURE
Hot-rolled, pickled and oiled, Nb-bearing HSLA s t e e l sample pieces (full coil width)were obtained from the front, center and back of each of four coils covering t h r e e thicknesses and two heats (two coils from each heat). The chemical analyses and the thickness of each coil are shown in Table I. The coil locations from Which sample materials were obtained are shown in Fig. 1. Some of the sample pieces obtained from the center-center location of coil n u m b e r 2 were given the following cold-work treatments before machining of test specimens:
i) Two levels (five and ten pct, respectively) of coldwork were imparted by tensile stretching in the transv e r s e direction. The tensile stretching was performed on 10.1 × 50.8 cm (4 × 20 in.) s t r i p s which had previously been m a r k e d with 6.35 mm sq (0.25 in. sq) g r i d s by electrochemical m e a n s . Test specimens were cut from a r e a s of the s t r i p s having uniform elongation as indicated by the g r i d s . ii) Twenty pct cold-work was imparted by c r o s s r o l l i n g sample pieces in a laboratory-type cold-rolling mill with 21.6 cm (8.5 in.) diam work r o i l s at a speed of approximately'4.6 m / m i n (15 ft/min). Lines were scribed on the surfaces of the specimens perpendicular to the cold-rolling direction and at pres c r i b e d distances apart t o allow accurate determination of the amount of deformation. One single pass was used for this deformation. Monotonic and cyclic properties of all longitudinal test specimens obtained from the front-center, centere d g e , center- c e n t e r and b a c k - center locations of each coil as well as all longitudinal and transverse test specimens obtained from the cold-worked pieces were characterized as per the prescribed procedures. 18-21
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