first knife: opinions?

[chewbroccoli]

I finally got around to making my first knife. A friend of mine has been working with me in Canada for 10 months and had to go back to france this week and I wanted to give him a nice canadian going away present. I figured a little knife would be perfect. I had a 5160 leaf spring that I got to make knifes out of a while ago so I cut it up, heated it and bashed it flat. Then I put it through the rolling mill at work to get it down to about 4mm thick. the mill wouldn't cold work it anymore than that and I didn't want to decarb it anymore than I had so I left it at 4mm even though I would have preferred to get it thinner. I drew out a bunch of knives and decided to work on two in tandem and give him the one that worked out better. I ended up only having a day in my shop at my parents to get the shaping and heat treating done so the fit and finish isn't quite where I'd like it to be, but I think it's ok for a first go.

here's the strip of 5160 with the two knife outlines:

outline cut using an angle grinder:

bevels ground with a 4x36" belt sander and pin holes drilled:

I used a little kiln my dad got to do enameling to do the heat treat. Heated them to 850C and checked that they were non-magnetic then water quenched them. Tempered them at 550C for 15 minutes. I didn't hardness test them, hopefully I'll get a chance to test the second one and see where it's at.
the two knives quenched:

I made the scales out of some lilac branch I got from my uncles backyard, i used some nails as pins:

scales and pins glued to the blade:

that's all I got done on in the shop I had to do the rest just in my basement with hand tools. I also made a little sheath for it out of some leather I had.

I would definately go further on my polish before heat treating next time. I only went to 120 on my belt so i had to do everything past that by hand on hardened steel. I'd also do more polishing before I glued the scales on. any criticism, thoughts and/or advice on it would be greatly appreciated!

thanks!

 

[Stacy E. Apelt - Bladesmith]

I am sorry to tell you that there is a BIG problem in your HT.

560C is the temperature you temper a stainless steel at. 5160 would be dead soft after tempering that high.
5160 tempers at around 200-225C.

The 850C for austenitization won't ruin the blade, but is high. 800C would be much better.

 

[chewbroccoli]

the 850C austenitizing temp came from the 1550F used in the data sheet I have. I've done quenches at 850C for 5160 before but never really cared about grain size, just 100% austenite transformation. is 800C enough to get 100% austenite? or is that not critical because of geometry? would you end up with more austenite formed at the edge than spine?

My goal was to heat treat at 200C for half an hour, but since I was so rushed I went higher for less time. I think I remembered 550C wrong, my kiln displays temp in F and there's no way I would have done it above 1000F. looking at the datasheet I think I went with 750F for 15mins. I'll definitely find a rockwell C tip and test the second blade I did to see what hardness I got them to.

 

[Stacy E. Apelt - Bladesmith]

830C (1525F ) is the top target temp for 5160. In thin section like a knife blade, that can be lower, down to about 800C ( 1475F).
The amount of steel structure converting to austenite once the blade gets above 768C (1414F) and reaches either of those target temps with a 10-15 minute hold/soak should be 100%.

The conversion to martensite in a proper quenchant should be nearly 100% martensite. Tempering time should be two hours and done twice, with a cooling to about 25C (80F) by dunking in water between the two temper cycles. Tempering 5160 for a good knife hardness is done at 230C (425F). 200C (390F) is a bit low for 5160. The toughness will be better with almost no drop in hardness at 230C.

The final hardness when done at those temps should be about Rockwell C 57-58.

 

[chewbroccoli]

looking at the phase diagram you're right 850C is way above where it needs to be. if it were plain carbon steel at .6% C it would be way down closer to 750C where you'd be at 100% austenite.

why the double temper cycle? what's the reasoning vs. just one go? also why so long? what advantage is that vs a higher temperature for much less time?

 

[Bo T]

I just looked at a table that indicated a HRc in the low 50's for a blade tempered at 750 F. I think the temperature is more important than time for the hardness.

 

[Stacy E. Apelt - Bladesmith]

In tempering we are doing two things, The first is tempering the brittle martensite into tempered, or stable, martensite. Even this newly tempered martensite should get a second cycle to assure that the process is complete.
The second is converting retained austenite into martensite. The newly converted martensite is in the brittle state, so it needs a second temper to make it tempered martensite.

Rapid cooling ( water quench) between the temper cycles, to cool from the 200°C temper area to ambient ( 25°C), is needed to avoid the structure formed in tempering the martensite from having things happen that we don't want in a blade. If you want more info on that, here is a thread I made on the subject: http://www.bladeforums.com/forums/showthread.php/1009018-Cooling-from-Temper

Concerning the length of the temper, time is an important function of tempering. Temperature and time are both axes on the diagram. Temperature weights about ten times what time does, so you need as accurate a temperature control as possible. Secondly, the conversion of structure from brittle to stable ( tempered) martensite take considerable time. At temps above 700°C changes in structure happen in minutes, at 200C it happens in hours. Two hours per cycle is the standard time period. Less time means an incomplete tempering and potential blade failure or lower edge retention. +10° error on the temperature will have more effect on the blade hardness than an extra 2 hours ( four total) would. Even a ten hour temper would probably only affect the blade by a point or two in hardness.

My mnemonic device for this subject is - "The "T" in tempering stands for TIME=TOUGHNESS."