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It’s well known that quenching most alloyed steels starting from AISI 4140 to high speed steels in water, brine , or sometimes oil will cause cracking and distortions.

in industry oil quenching sometimes replaced by molten salt, which considered better than oil in decreasing crack probability and this advantage attributed to uniform cooling provided by salt bath compared to other quenching mediums.

cracks and distortions caused by water or brine quenching usually attributed to rapid cooling , but sometimes they attributed to non-uniform cooling caused by vapor blanket. that’s so confusing!!

it is expected to obtain uniform cooling by quenching steel in high concentration salt solution "nearly saturated solution" thanks to large amounts of ion particles which disturb vapor blanket, but these type of solutions will also have high cooling rate compared to oil. such rapid cooling usually attributed to be the of quench cracks in water quenching !

O1 steel can’t fully hardened in parts larger than 1 inch!

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so my problem if it is safe to quench thick parts of such low hardenability steels in very high concentration salt solution which are expected to combining uniform cooling with relatively high speed cooling to fully harden thick parts of such low hardenability steel ?

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    $\begingroup$ What about cracks that develop through faults in the material... $\endgroup$ – Solar Mike Jan 3 '19 at 13:09
  • $\begingroup$ "Hardenability of steels" formerly was about a one year /6 semester hour class ; I doubt it can be explained in a couple paragraphs. $\endgroup$ – blacksmith37 Jan 3 '19 at 16:19
  • $\begingroup$ It’s not about hardenability it’s about quenching. I have no issues with hardenability . $\endgroup$ – Johnny Jan 3 '19 at 22:06
  • $\begingroup$ Are they not related? $\endgroup$ – Solar Mike Jan 4 '19 at 7:30
  • $\begingroup$ If I wouldn’t change the grade to be used of course they are not related . $\endgroup$ – Johnny Jan 5 '19 at 23:08
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To answer your specific question ; maybe. O-1 tool steel has good hardenability ; the standard Jominy test showing HRC 60 two inches from the end ( austenitized at 1525 F). From experience with O-1 , cracking occurred in small parts ( less than 1/2" thick) when oil quenched from 1475 F , but when quenched into oil from 1600 F no cracking occurred. This was because the higher temperature produced retained austinite ; being relatively soft and ductile it prevented cracking. The same affect would occur with water quenching ( retained austenite). One would need to experiment with the best austenitizing temperature for each part configuration using a water quench. Double tempering will be needed as the retained austenite will mostly transform during the first temper. Heat-treated steels tend to crack and distort because of the stresses caused by the contraction and expansion: As the steel cools ( contraction) and transforms to martensite ( expansion), then continues cooling (contraction).

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  • $\begingroup$ sorry but I can’t consider 60 Rc full hardening , full hardening meaning that the hardness both in the surface and the core are nearly equal. in our case we are talking about 64-67 Rc in the core of 4 inch O1 steel punch. $\endgroup$ – Johnny Jan 5 '19 at 23:15
  • $\begingroup$ I read an article about causes of quench cracks and they say : "fine-needle or latent-leaf martensite formed from fine-grained (no. 11) and ultrafine-grained (no. 12–14) austenite has significantly higher potential of brittle strength of steel than the level of internal stresses of any character (local tensile, compressive stresses and so on). In this case, cracks did not appear both immediately during quenching and upon infinitely prolonged holding of parts after quenching." they mentioned also quenching fine grained U8A carbon steel in brine with intense mixing without cracks !! $\endgroup$ – Johnny Jan 5 '19 at 23:22
  • $\begingroup$ Quench cracking is caused primarily in steels by the martensite expansion occurring at different times at different depths .To learn about quench severity. harden-ability, tool steels , etc. I recommend American Society for Metals books. $\endgroup$ – blacksmith37 Jan 7 '19 at 3:01
  • $\begingroup$ "Intensive Quenching Systems: Engineering and Design" this the book I will read , it seems very non-conventional and really interesting. they claims that very high cooling rate creating large enough compressive stress counteract tensile stresses resulted from martensite expansion so it actually suppressing cracks ! $\endgroup$ – Johnny Jan 7 '19 at 18:24
  • $\begingroup$ Like preventing water from expanding as it freezes into ice; It is going to require a lot of strength to contain the martensite expansion. $\endgroup$ – blacksmith37 Jan 30 '20 at 15:23

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