Temper too long???

Phil Dwyer

Phil Dwyer

Joined
Feb 28, 2006
Messages
3,494
Hi Friends,

I feel quite anxious asking any HT questions, but am compelled to, as I just muffed up a bit.

I am doing my first reasonably controlled HT of a 5160 blade. It was heated to 1525 (the blade was brought up to temp along with the oven), held at1525 for 5 minutes and quenched into Parks 50. It went straight from the quench to another oven preheated to 400. It stayed at 400 for an hour. The next day it was put in the preheated 400 temper oven a second time, but forgotten. It was at 400 for three hours before pulling the plug.

I know I should understand all this stuff, considering all the reading I've done of all you good people's posts, but I'm not sure if the blade is alright or not. My understanding says it is, but my understanding is not as confident as I'd like it to be.

With gratitude and trepidation, Phil

PS - This is my extremely late Christmas exchange blade, so I have to get it right!
 
phil i have no idea.....

stephen fowler is going to quench a blade in a turkey this weekend tho.... i hope that helps!

good luck

jake
 
I would say you're fine, based on that a lot of people temper two or three cycles that are two hours long, so what you did couldn't be that bad (if bad at all, besides raising carbon emissions and causing global warming)
 
From my understanding the reason for tempering for 1-2 hours is to make sure the steel was tempered all the way through and that all stresses were relieved. I do not know what tempering longer could harm... it seems it would only make you more sure that the steel was tempered all the way through because the temp never raised but, then again, maybe being in that state for that long is bad for the temper. This is just my guess but, I'd like to hear from someone more knowledgable. I would say do a brass rod edge test and see if it's satisfactory or too soft.
 
Dude, I will fully admit that my comp cutter that ive used more than any other knife sat in the oven tempering for about 12 hours after I fell asleep drinking beers. I didnt even remember it was in the oven until I went to make breakfast.

I think you are fine....just test it like normal.
 
The conversion of structure in tempering is a function of time and temperature. Time is far inferior to temperature. You could temper for many extra hours and not affect the hardness more than a point or two. Temper at 10-15 degrees higher, and the hardness can be greatly lowered in two hours.
BTW, I recommend tempering for two ,two hour cycles.
Stacy
 
I have walked out of the shop and left die sections in the temper all night long. The only thing you would ever have to worry about is the oven could go berzerk and overheat. 400 is 400 all day long. Rule of thumb is two, two hour cycles. You should be fine. You can always take it to your local tool and die shop and have it rockwell tested.
 
I misspoke, or mistyped...I actually did the first temper cycle for 2 hours. The second one went for three (due to absent mindedness and preoccupation with an unscheduled class running over time). Sounds like it will be alright. Thanks guys!
 
I have left knives in the oven all night long with no advise affects.
 
Yes Phil,you should be OK.
Two - three -four hours... it won't matter if the temperature is controlled. A kitchen oven may be more of a problem as they swing a lot above the set temp when the heat comes on. Even if it was a kitchen oven, it probably is fine. If anything ,you may have increased the toughness of the blade.

A little more explanation of what is going on during temper may help some folks.
In the initial quench ( if quenched properly) the austenite ( if soaked properly ) is converted to Martensite. This conversion starts about 400F. The trip to that temperature has a little scary spot around 1000F ,when the steel wants to go into pearlite, but with the proper quenchant and technique, it passes that turnoff and heads down to the Martensitic start point. We then cool the blade the rest of the way to room temperature. This allows most of the Austenite to change into Martensite. At this point the steel is not good for a blade at all. It is too hard, too brittle, and probably still has some austenite in it ( The austenite may not finish converting unless cooled to nearly -200F= cryo treatment, or left at room temp for months= age tempering ).
So we need to make this unstable Martensite stable and give the austenite a chance to finish. We heat the steel up to about the Ms ( Martensitic start temp) and hold it there for a long enough time for the Martensite to change to a mixed structure and loose its brittleness. This mostly happens by changes in the grain boundaries. The austenite becomes more active at the Ms and can finish converting to Martensite. These conversions are a factor of time and temperature. It will take at least an hour at 400F, so we use 2 hours to allow for heat up time and to give a little extra wiggle room. We then cool the blade to room temperature, and have steel consisting of tempered Martensite (good) and a little new untempered Martensite ( not so good). We need to give it another temper cycle to temper the new Martensite. We will convert any left over Austenite in the second temper, but with the exception of very complex stainless steel there is no need to deal with more than two temper cycles. After the second two hour temper, we should have a blade with nearly 100% tempered Martensite. The steel will be hard, tough, and ready to finish into a knife. Failure to do the quench and tempering right can lead to improper hardness, excessive blade breaking, or severe edge chipping.

The 400F is a start area, and the temper has to be worked out to give the hardness/toughness desired. If you don't have the experience to tell if it is OK or not - temper most carbon steels at 400F and you will most likely be OK. Below 375F is not going to do much good, and above 450F is for specific tempers, and should be avoided without some background in why you want that.

Hope this helps,
Stacy
 
Thanks Stacy!

I used a toaster oven with ceramic kiln shelving cut to fit on the rack over the bottom elements. I placed the blade on a rack in the middle. I used a PID and thermocouple (placed horizontally near the blade) to monitor the temp. I found that the temp held with a 3 degree various once the little oven had been on for a couple of hours. I'm going to look into some quilted plumbers insulation to help hold the heat even better.

I also used the PID with the jeweler's burnout oven I got from you to monitor the quenching temp.

I have a couple of SSRs, heat sinks and fans on their way so we'll actually be able to control the temp. All I'll have to do is figure out how to program the little PID.

I've got to get all this worked out for the 11th graders who are making various knives (with 1084) in their mixed media class with me. They've finished the forging and are now grinding and filing. Their HT is just around the corner. :thumbup:

All the best, Phil
 
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