Grains, Carbides, and You

[Doug Lester]

Ok, next question. What explains a hypoeuticoid steel like 6150 or S5 acting like a hypereuticoid steel in that it need a soak at temperature, 10 minutes according to a person who has used 6150 and 1/2 hour according to the data sheet for S5. Are the alloying elements like the molybdenum, vanadium, and tungsten forming carbides that compete with the cementite for the carbon atoms as they escape the austinite upon cooling at room temperature causing a need to soak those alloys to release the carbon when simpler hypoeutecoid steels release carbon to iron atoms, forming cementite which releases it's carbon more easily when the steel is austinized? Thanks.

Doug

 

[me2]

To answer that, lets start at the beginning. For a plain carbon steel, any amount of carbon over 0.02% will form some carbide (cementite) in the annealed condition. In the slow cooled, annealed condition, something like 1050 will have pearlite surrounded by ferrite. Pearlite is a mixture of ferrite and cementite. The less carbon the steel has, the thicker the ferrite plates and thinner the cementite platea will be. In hypoeutectoid steel, 1050, the grains of pearlite will be surrounded by ferrite also. There is a lot of ferrite to cover. It has a maximum solubility of 0.02% carbon, practically none. The soak is to get all the carbon from the cementite, where it concentrates, to evenly distribute through the ferrite. Its a lot of area to cover and takes some time. Alloying elements will slow the process down, or, in the case of tungsten and vanadium, may not give up the carbon at all at the temperatures used to austenize lower alloy steels.

 

[One]

Since carbides are chemically bonded they do not break up and move as easy as the free carbon atoms so we need to throw more heat at them to dissolve them and disperse their carbon evenly, that is why we have soak times and higher temperatures for alloyed steels. When we have things like chrome or vanadium the bonds are very tight and much greater heat is involved in breaking the carbides and that is why you will see temps in excess of 1800F for more richly alloyed steels. If you were to add .5 or .6 percent vanadium to 1084 you could easily reduce its maximum hardness to that of 1040 steel because all of its carbon would be locked in very tough carbides with none left over for proper hardening without extreme overheating.

The other thing to remember about carbide is that it is extremely hard, much harder than the surrounding steel, and this makes it brittle. So having large clusters of carbide is seldom a good thing. It makes machining impossible and can lead to overall embrittlement. Since it is so much harder to abrade than the surrounding material knife edges can only get as fine as the carbides dictate as they will either resist sharpening efforts or pull out and continuously leave large voids in the edge. You can wear away a portion of a grain but no so with a carbide. The worst place to have carbide gather en mass is in the grain boundaries. The preferred path of travel for fracture is often the grain boundaries and if you fill them with the most brittle component of steel you have a weak blade even if it is relatively soft. The ideal carbide condition is as fine as possible and spread and evenly as possible throughout the grains.

How do we control this? It is actually quite easy once we understand the temperature-diffusion relationships. My image of the 1095 with the with the with grain boundaries that me2 linked to is bad but something I did intentionally. I regularly use carbide to highlight grain boundaries when doing metallography. I have had visitors to the shop scratch their heads and ask “you mean you can put carbides exactly where you want inside the steel?” Yes I can, and so can you!

The trick with carbide is tow things, how hot you get it and how fast you cool it. Going hotter dissolves carbides and puts them into solution. Cooling slowly makes carbides, and the slower you cool the larger they get. But be careful since where they will want to grow is seldom what you want, e.g. the grain boundaries. This is why you should avoid slow cooling steels with more than .85% carbon from above “critical”; air cooling is as slow as you want to go. A cooling forge, or a bucket of vermiculite is no place for a hot piece of 1095 or W2!

Just to add that, the high temperatures used in hot forging helps break up and dissolve large carbides and the mechanical aspect of forging helps keep grain growth in check at higher temps.,... It kills multiple birds with one stone. This along with the other possible advantages makes it an extremely efficient forming method whenever applicable, which to this day is virtually unrivaled by any other.

 

[Kevin R. Cashen]

Just to add that, the high temperatures used in hot forging helps break up and dissolve large carbides and the mechanical aspect of forging helps keep grain growth in check at higher temps.,... It kills multiple birds with one stone. This along with the other possible advantages makes it an extremely efficient forming method whenever applicable, which to this day is virtually unrivaled by any other.

Tai, I have tried to be tolerant. I have explained my comment that you took exception with and demonstrated how it was merely a critique of the accuracy of the terms used by many people, myself included in the past, and I simply ignored your much more personal attack on me in this thread. I have also explained why this constant disruptive carping on forging is not even on topic in this forum, and you are just pressing the issue to cause trouble now. When your last post didn’t get a reaction you edited it to be more abrasive, making your intent clear. You have now been addressed by two moderators in this thread alone, so enough already. Either remove the forging chip from your shoulder or take it to the Hot Metal forum, I am done being tolerant.

 

[One]

I'm sorry if that's how you take it Kevin, but I think I've tried to show how it does pertain to this thread along with just about every other aspect of the steel etc. I edit many of my posts, simply to clarify, elaborate and try to weed out any semantic errors or points of confusion.

 

[Kevin R. Cashen]

Sorry, I read both posts and know what they said. The well meaning folks should not have their conversations ended by the behavior of single individuals, I will find a way to deal with that in the future but for now my apologies to me2 that I have to lock this one.