An investigation of the validity of certain tempering parameters
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s t e e l s m a k e s it a p p e a r u n l i k e l y that any s i m p l e r a t e t h e o r y e x p r e s s i o n involving a s i n g l e a c t i v a t i o n e n e r g y can a d e q u a t e l y d e s c r i b e the e f f e c t s of t e m p e r i n g o v e r a wide r a n g e of t e m p e r a t u r e , and H y a m and Nutting 7 have shown that d i f f e r e n t a c t i v a t i o n e n e r g i e s a r e r e q u i r e d to d e s c r i b e the p r o c e s s . T h e p r e s e n t w o r k w a s u n d e r t a k e n with the o b j e c t of e s t a b l i s h i n g the extent to which a v a r i e t y of p a r a m e t e r s fit a s e t of c a r e f u l l y c o n t r o l l e d e x p e r i m e n t a l r e s u l t s a n d of t h r o w i n g light on t h e v a l i d i t y of the e x p e r i m e n t a l b a s e s of the p a r a m e t e r s . EXPERIMENTAL
PROCEDURE
A 0.39 pct C steel containing 0.ql pct M n and 0.22 pct Si was used. The steel also contained 0.02 pct Cr and not m o r e than 0.01 pct Ni, Mo, and V. The material was m a d e by the Kaldo process and had low sulfur and phosphorus, though their amounts were not determined. The specimens used were disks having a diam1 eter of ~ in. and thickness ~in. Before quenching the specimens were given a grain-refining treatment by heating to 830 ~ • 5~ in nitrogen, holding for half an hour and air cooling. They were then reheated to the s a m e temperature for a further half an hour in batches of five and quenched in water. This treatment gave satisfactory martensific structures without quench cracking. In a few cases a very small amount of fine nodular pearlife was observed but the quantity was so small that it was considered safe to use the specimens. Immediately after the quench, the specimens were i m m e r s e d in liquid nitrogen to eliminate any retained austenite although there was little possibility, and no evidence, of the presence of this phase. There was very little decarburization around the edges of the specimens. Specimens were tempered for ten different, logarithmically-spaced times, at each of ten temperatures between 250 ~ and 700~ Three specimens were chosen at random from the quenched batches for each treatment. The tempering treatments were carried out in a lead bath for temperatures above 450~ and in a lead-bismuth bath for the lower temperatures. Temperatures were controlled in general to within •176 although on long time treatments there was a tendency for the temperature to vary gradually by up to •176 Checks were m a d e frequently and any tendency to "wander" was corrected as soon as it was observed. Consequently the larger variations only occurred for tempering
VOLUME 3, MARCH 1972-727
t i m e s l o n g e r t h a n 1 d a y a n d it i s t h o u g h t t h a t t h e i r e f fects were largely self-compensating. H a r d n e s s m e a s u r e m e n t s w e r e m a d e on a V i c k e r s machine after the specimens had been wet ground to a d e p t h of 0.03 in. a n d p o l i s h e d w e t on s i l i c o n c a r b i d e p a p e r t o t h e 600 g r a d e . A
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