Posted 06 November 2011 - 07:42 PM
I could be wrong (thankfully there are a lot of smart folks here that can correct me if so) but I think the answer to your question depends upon a few factors. One of which is the temp that the two cycles are at. Two cycles of two hours for 5160 at 450 degrees is going to give you a different result than the same cycles at 400 degrees. This would result in a different amount of hardness lost after the quench. I know some makers trouble shoot hardness problems by using the tempering cycles; if the blade edge tests too hard, back in the oven for more tampering cycles at a slightly higher temp to soften it just a bit. Hopefully that's all correct and makes sense. Good question, though-all the facets of the heat treating process sometimes makes my head spin .
Posted 06 November 2011 - 09:51 PM
Perhaps the biggest factor is the success of the hardening operation. If a full solution via appropriate soak times was achieved and the quench was fully successful, you can expect the HRC values to stay surprisingly high for a given temperature, but not surprising if you follow the spec. sheets which are based on optimal heat treatments. I used to think, like many others, that the books and spec sheets are for industry and don’t apply to knifemaking since the HRC values never matched the tempering temperatures in the books. But then when I asked around about what temperatures smiths were using for hardening, and for how long, I was most often told that a torch or a forge was used so they have no idea what the exact temperature was. When I started following the recommended soak times and tempering temperatures more closely it was no surprise that suddenly the other numbers started matching up as well. So then I had to ask why we expect to be able to ignore all the other numbers on the spec sheets and yet have just the ones we choose match up? Sorry it is a bit of a rant on another tangent but is quite relevant in that there is no way to really predict the exact HRC value for the tempering temperature without a complete set of numbers to put it all together.
In helping others troubleshoot heat treating I can often get an idea of how effective a smith’s hardening operation is by the tempering temperatures they use. I am often told of tempering temperatures on larger choppers in the range of 350F to 375F. If the hardening was a thorough as it could be for most of the steels we work with those temperatures shouldn’t be getting things much lower than 62-63HRC, and the fact that those blades are drawn back to the desired working hardness tells me that full hardness was probably not achieved to begin with.
On my web site I have working specs for many of the common steels, including modified charts for HRC from tempering temperatures which you will find at these links:
I have kept records for years on everything I have ever hardness tested, including each blade after quenching and after each subsequent tempering at different temperatures and I have entire databases filled with numbers. In all of this data I have noticed certain trends that seem to be quite reliable. One is that the most marked drop in hardness occurs in the first hour of the initial tempering cycle. It is here that say 1084 for example will drop from as quenched 65-66 HRC down to around 62 HRC from tempering from 375F to 400F. Each point of Rockwell after that will be a struggle requiring as much as 25 degrees or greater to accomplish. 5160 will only give you 62-63 HRC maximum right from the quench so you want to ease into the tempering with a slightly lower temperature as it can drop faster than expected. While 1095 in the other hand will scare you at how hot you may have to heat it just to get it down to HRC 60-61 (like 450F-500F degrees!).
Tempering, while often just covered as an afterthought in HT discussions may actually be the most complex of all the HT procedures as far as what is happening in the steel. There are sub microscopic formations of carbides allowing shifting of atomic stacking, transformations not complete from the quench, complex secondary carbide formation resulting it various embrittlement phenomena and secondary hardening and eventual formation of large spheroidal carbides as the heat rises to the higher end of tempering.