Low cycle fatigue behavior of a quenched and tempered niobium bearing HSLA steel
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atmosphere to completely dissolve the niobium in the austenite) After this they were water quenched. Subsequent tempering treatments at 400 and 550 ~ were performed in a salt bath controlled to • 2 ~ The tempering behavior was followed by R A hardness measurements and is shown in Fig. 1. The secondary hardening observed during tempering at 550 ~ may be attributed to fine N b C precipitates. 8,9 From these tempering curves two heat treatments were chosen to give the same hardness (R A 64) but different microstructures. In one heat treatment the steel was tempered at 400 ~ for 5 h (hereafter referred to as the 400-5H heat treatment). In the other, the steel was tempered at 550 ~ for 10 h (hereafter referred to as the 550-10H heat treatment). The 550-10H treatment resulted in a tempered martensitic microstructure presumably containing fine niobium carbides while the 400-5H treatment gave a less tempered structure presumably with no niobium carbides.
Low Cycle Fatigue Tests All low cycle fatigue tests except the lattice microstrain tests were performed on a 90 kN (20 kip) servohydraulic closed loop testing machine equipped with Wood's metal grips. The tests were run in axial strain control with constant strain rates of 4 • 10-3/s for strain amplitudes of ___0.01 or greater and 4 • 10-2/s for strain amplitudes of less than ___0.01 using longitudinal specimens having a 4.3 mm diam and a 7.6 mm gage length and a specimen geometry similar to that described by Raske and Morrow.L~ Gage length was parallel to the rolling direction. Cyclic stress-strain curves were obtained fi:om incremental step tests 1~with the size of the strain increments being chosen to produce 20 hysteresis loops with strain amplitudes ranging from 0 to ___0.02. Strain-life tests were performed at eight strain amplitudes ranging from ___0.004 to _ 0.045 with one specimen being used for each strain amplitude. Softening and hardening behavior during strain cycling was studied by stress relaxation tests at a
ISSN 0360-2133 / 80 /0811-1429500.75 / 0 METALLURGICAL TRANSACTIONS A 9 1980 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 11A, AUGUST 1980--1429
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radiation. Instrumental broadening effects were eliminated by using the method of Stokes ~5and a profile from a well annealed (austenitized at 950 ~ for 15 h and furnace cooled) specimen of the steel. Lattice microstrain and domain size were then calculated from the corrected profile. Column length for microstrain determination was taken to be 5 nm.
4 0 0 " C (I,250~
(5-- 550"C (I.250"C) =-
550"C (950%1
tu =1:
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