Posted by Sarup Chopra on July 03, 2008 at 16:29:59:
In Reply to: Effect of grain size on hardness posted by Sanjay Deshmukh on July 03, 2008 at 09:27:13:
52100 is a high Carbon steel with high hardenability, mainly attributable to a Chromiun content of 1.30. to 1.60 wt.%. Now, a fine grain also enhances the hardenability somewhat but not the hardness per se. In order for a steel to harden, it needs to reach a critical cooling speed at the time of quenching from the austenitizing temperature, in this case around 1550 degrees Fahrenheit (850 degrees Centigrade). Spots which do not reach the critical cooling speed, will not experience transformation from Austenite to Martensite, hence they will not harden. An end quench hardenability curve, a.k.a. Jominy test, for 52100 steel in "HEAT TREATER'S GUIDE" published by ASM, is showing an as quenched hardness of 60HRC all the down to a depth of half an inch which translates into 12.7 mm which in turn is 3.3mm shy of your sample's half way mark. Did you soak the piece for long enough prior to quenching? A sample of this size should be soaked at the austenitizing temperature for at least half an hour.