5160 heat treat

[Steve Sells]

8 hours ago, George N. M. said:

Similarly, I am somewhat confused by multiple tempering cycles.

the first temper is done immediately after the quench  to prevent cracking from the stress of the quench.  We temper again later, as in waiting until the next day or so, to temper the retained austenite that converted into martensite on its own (which is brittle ) during the wait between cycles, I see no reason to quench more than once, nor tempering more than twice

[pnut]

18 hours ago, bigfootnampa said:

.  It is time consuming to produce a razor sharp polished edge on a shape like a hook knife that cannot be fully worked with power equipment!

I know this thread is about heat treating the steel but since you mentioned it, I would suggest using a rod after you get it sharpened up the first time. We used a hook shaped blade while working on the mills at the rubber refinery and the only way I could get a good edge was with a rod. There was a whetstone mounted on a rail between the two mills but I had no luck using it to sharpen the hawkbill shaped mill knifes. 

Pnut

[Buzzkill]

12 hours ago, George N. M. said:

Still no explanation of why or how triple quenching benefits 5160 steel

In addition to the article I remembered, I did a little digging over at Blade Forums.  The conclusion/consensus seemed to be that multiple quenches do indeed reduce grain size, but you can accomplish nearly the same reduction by normalization cycles before quenching.  It didn't seem like anyone was a big proponent of the multiple quench idea when taking all factors into account.

[ThomasPowers]

Pretty much what I got back too: " By multiple cycling up and down from austenite (900 C) to ferrite (600 C) you can reduce (refine) the grain size, which would increase the toughness and the hardness on the final quench."   So Normalization over Hardening.  

Next to hit up a professional metallurgist working in Industry who may have a different slant on things.  Patrick?

[Buzzkill]

We've probably just about beat this to death, but here's another relevant tidbit:

Excerpt from Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel by John D. Verhoeven  (Page 69 if anyone is interested)

"When heat treating to form martensite, toughness is also enhanced by fine grained austenite because it results in a finer lath or plate size in the martensite. Again the same ideas apply. Rapid heating and repeated cycling produce smaller martensite microstructures. Grange [8.3] has presented a study showing the beneficial effect of small austenite grain size on the mechanical properties of 8640 steel. He achieved grain sizes in the ultrafine range of ASTM No. 13 to 15 by a 4 cycle process where the steel was austenitized in molten lead for around 10 s, cooled to room temperature, cold worked and then cycled again. A series of similar experiments was performed here on 3 steels to examine the effectiveness of thermal cycling alone, no cold working was employed. The steels were heated by immersion in a salt pot. Initially the steels were austenitized for 15 min. at 1650 oF and oil quenched in rapidly stirred oil. Then the steels were given 3 thermal cycles consisting of a 4 minute austenitization in 1450 oF salt and a quench in rapidly stirred oil. The grain sizes were measured with the same technique described by Grange [8.3] and the ASTM numbers before and after the 3 cycle treatment are given in Table 8.2. It is seen that ultrafine grain sizes were obtained."