Grain Growth and Thermal Cycles

[Laredo7mm]

Hey all, I have been looking around for some info on grain growth of steel. More specifically, how to reduce the size of the grain once it has grown.

Everything I make (so far) is damascus steel. My two steels are 15N20 and C1095. I tend to work at welding temps when drawing, twisting, flattening, and forging the billet to the general shape of the knife. Needless to say, the steel is above critical for long periods of time. After the blade is forged to shape, I normalize it.

On my last two knives (numbers 4 and 5) I have triple quenched the blade based on some info I got about reducing grain size.


Based on that brief description here are my questions:

Does the triple quench do anything?

Will a series of thermal cycles (bring upto critical and air cool rapidly to 900 deg F) work better?

Does the use of a power hammer when drawing/forging break down the grain structure?

How do you all reduce grain size?


I am going to do a series of tests to try to figure out the best way to go. I would just like to get your comments/experience before I figure out how many processes I want to test. I plan on doing these while I am doing some "pattern Development". I just need to get mom and dads x mas gift done first. Hopefully I can come up with an orginal pattern that looks good.

Also, are there any good technical books (college text books) or references that go into this subject? It has been a long time since my college classes on material science, so I can't remember if they went into how to reduce grain size or not. Heck I felt pretty good about remembering that I even went to college.

Thanks for your help, and work is tollerable today, but the weahter is way too cold. Man I always have something to B about don't I?

 

[Joe Renner]

Multiple quenching does it. the martensite when austenized forms carbides that will keep the grain from growing(or slow it down anyway).
I still go with normalizing, works just as good if you use more cycles.
If you get 150 above critical, grain will grow, if it is heated too slowly or held at at a temp(for too long), grain will grow.
This is where molten salt baths help, I normalize once at 1500, another at 1475 another at 1450 and another at 1400. I do it like this with variations multiple times. Again 3 is not good enough.
I get damn near invisible grain structures, by the way.

 

[Epsilon]

Laredo,
Start with this...

- Grain size is primarily determined by prior austenite size.

- Forging will reduce the grain size depending upon the area of effect and force of the blow and the temperature it was at. However, if you continue to heat and reheat (usually above Ac1) you negate the effect. Example: If you have hot steel, then hit, then let it be, chances are surrounding heat will effect the area and again negate the effect. One scenario.

Many variables effect your good question. I'd recommend picking up "Metallurgy Fundamentals" by Daniel A. Brandt & J.C. Warner. I found it at Borders a few years ago, I think they still carry it. It will clarify the basics for you. Then you can go from there. Good luck.

-Jason

 

[Joe Renner]

"Epsilon Carbides" are formed from austenized martensite. Your name was a good reminder.