Strain aging kinetics of vanadium or titanium strengthened high-strength low-alloy steels
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, whereT1 < T2 < 4 7 8 K
can be used to predict the time tl n e c e s s a r y at temperature TI for producing strain aging identical to that observed in a s h o r t e r time at a higher temperature. STRAIN-AGING is observed in plain carbon steels and results in an increase in strength and decrease in ductility. 1-4 The extent of property changes and the kinetics of strain aging depend on steel chemistry, mechanical history and aging temperature. The basic chemistry of high strength low alloy (HSLA) steels is similar to that of plain carbon steel, but HSLA steels are considerably stronger. The added strength is often obtained by controlled hot-rolling and rapid, controlled cooling which results in a very small grain size. Further, by minor additions of appropriate alloying elements, V, Nb or Ti, additional strength is developed by both solution and precipitation hardening, s Precipitation hardening is one of the principal strengthening mechanisms in HSLA steels. These precipitates are reported to be small, finely dispersed, and coherent. They have been exceedingly difficult to observe, however, unless they are overaged to a l a r g e r size. Of course, aging makes the precipitates noncoherent and reduces material strength. As reported earlier, 6 HSLA steels are susceptible to strain aging by interstitial solutes very much like plain carbon steel. However, a brief examination revealed that the kinetics of strain aging were slower in the HSLA steels, and this prompted the additional work which is the subject of this paper. F u r t h e r m o r e , because strain aging is both time and temperature dependent, it would be desirable to be able to predict mechanical property changes which occur at low temperatures, such as room temperature, without having to conduct long term tests. A method of doing this for HSLA steels is described subsequently. The o c c u r r e n c e of strain aging can be determined by a simple tension test. F o r a plain carbon or HSLA
steel, the stress//strain curve takes the form of curve (a) in Fig. 1. If the specimen is strained to point B, beyond the lower yield extension A, unloaded and immediately retested, the stress//strain curve rejoins and follows the same curve (a). If the material is susceptible to strain aging, unloading at B, followed by aging at room temperature or above, results in the return of the yield point elongation (ype) and the stress//strain curve follows (b) in Fig. 1. The yield A Y : change in yield stress due to strain aging ~U
: change in UTS due to s t r a i n aging
A e = change in total elongation due to strain aging
strain aged
o ~q
n i t i a l lower
yield extension
uders strain
--Prestrain
Residual Ductility Elongation
- - T o t a l
M. S. RASHID is Associate Senior Research Engineer, Research Laboratories, Metallurgy Department, General Motors Technical Center, Warren, MI 48090. Manuscript submitted August 19, 1975. METALLURGICAL
TRANSACTIONS
A
Strain
Fig. 1--Schematic r e p r e s e n t a t i o n of the e[fects of s t r a i n - a g i n g on the
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