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triple quenching?

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I have seen some posts here were people say to triple harden and quench when hardening. There is no need to harden something three times. Each time you re-heat to harden the piece your are just defeating the initial hardening process and possibly causing excessive grain growth. To harden steel properly it only needs to be brought up to the proper temp. to bring it to the austenitic state then quenched in an appropriate solution (Air, Oil or water) depending on the grade and type of steel. After hardening (Quenching) the part should be tempered immediately at least one time at the appropriate temp. to obtain the level of hardness OR toughness wanted in the finished product.
When dealing with certain tool steels (such as A2 or D2) you should not cool the steel form hardening below 150 deg F and they should be tempered immediately after hardening to prevent cracking. They also should be given a second tempering after cooling to room temp. after the first temper.
Tool steels such as High Speed Steel (Lathe turning bits for example) should be triple tempered to achieve the best results.
There are a lot of factors that come into play when heat treating those tool steels and the makers of those steels will supply appropriate hardening and tempering temperature for them.

However, as to the steels that most people are dealing with on this site, they are not tool steels for the most part. They are generally just medium to high carbon steels with limited alloys.Most only require just a single hardening and tempering cycle. The high carbon steels can benefit some with a double tempering cycle but the most important factors are still a proper tempering temperature and held for an appropriate length of time at temperature.

I have done a LOT of heat treating over the years and dealt primarily with the tool steels and never had anything fail due to the wrong heat treat process.

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the triple hardening is a way of refining the grain size of certain steels. To refine the grain size and thus get the best edge nd toughness possible the steel must get from above to below criticle. Normalizing is the usual way, but hardening also shrinks the grains (though it does impart stress as well). Steels containing certain carbides need to be quenched in order for the comlex carbides to form, so if you triple quench you get a finer matirx of carbides; thus a better blade. Simple steels (10 series in the US for example) don't need the extra stress and buggering about, but 52100 (lots of chrome and vanadium) really benefits from it.

I was dubious myself until I ran some experiments last year and it did make a difference to the overall edge retention on some of the steels I use, but not others. My rate of warpages went up, bt then so did the quality of the blades that made it through intact ;) The simple steels that I use I only single quench, relying on triple normalizing ot do the refinement of the grain size (there are no complex carbides to form, only Iron Carbides that form easily when in solution)

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The following is an excerpt from "Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel"
John D. Verhoeven Emeritus Professor Iowa State University March 2005

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. Figure 8.5 presents photomicrographs of the martensite structures found in the 1086 steel before and after the cycling. The composition of this steel is in the range where we expect the martensite to be a mixture of lath and plate morphologies, and in the uncycled coarser grained sample Fig. 8.5 (A), one can see dark plates in a matrix of
the lath structure. However, in the finer grained austenite produced by thermal cycling, Fig. 7(B), the martensite structure is clearly finer and the plates are not easily identified.

Table 8.2 ASTM grain size no. of austenite before
and after the 3 cycle treatment done here.
Steel ASTM Number Initial After cycling
1045 9 14
1086 11 15
5150 8.5 14

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