In response to a request for the why of this discussion, here is a simplified answer:
5160 is considered a high alloy steel. The alloy ingredients give it certain properties. The two that matter are the Chromium (the "5" in the name means that it is a high alloy chromium steel) and the manganese. There is approx. 1% of each, along with .60% carbon. These alloys allow the steel to be deep hardening and tough. When a steel is deep hardening the hardness goes deep into a large block or thickness of the steel. In a knife blade thickness,the hardness goes all the way through anyway, so it does not matter in this aspect one way or the other. What does matter is the rate at which the steel must be hardend, which is slower with deep hardening steels.
1084 is a simple steel, having just .84% carbon and 1% manganese. It is shallow hardening ( the manganese gives it just a bit of hardenability to make it a better steel). It hardens fairly fast. 1084 is considered the eutectoid steel. At .83% carbon, steel reaches the eutectic point. This is the lowest spot on the hardening curve. If the carbon is lower or higher, the temperature at which the steel converts to austenite gets higher.
Now, when steel is heated to its austenitization point (1520F for 5160 and 1500F for 1084), the arrangement of the atoms changes. Certain alloy ingredients go into solution ,too. This takes a little time, depending on how much alloy and what type. 5160 must be held at 1520F for about 5-10 minutes to allow this to happen. With 1084, there are few alloys, so there is no need to wait for them to go into solution. Furthermore, there is no excess of carbon or shortage of carbon. 83% is exactly the amount that is needed to tie up all the iron into nice little grains. Since you don't need to wait for excess carbon to form iron carbides, or a shortage of carbon to be evenly distributed, 1084 can be quenched as soon as it fully reaches 1500F.
Here is where it gets a little harder to understand:
If you look at the IT (isothermal transformation chart), TTT (time -temperature transformation chart) and the CCC (continuous cooling curve chart), you will see that 1084 crosses the pearlite nose ( around 1000F) in about one second. 5160 has five seconds to do it in. If either steel cools fast enough, it misses the nose and goes to martensite at the Ms (martensitic start point) which is between 400F and 500F). If it takes too long it becomes pearlite, or a mixture of structures. Martensite is the hard, but brittle structure that bladesmiths want for knives. Pearlite is the soft structure you want when sanding and grinding prior to HT. The martensite is tempered after HT to make it tougher, but it always is somewhat brittle. This is where certain of the alloy ingredients help out by adding toughness to the steel.
When doing a differentially hardened blade ,the blade spine is coated with a refractory clay. The part that is coated,cools slower. In 10XX steels the edge (uncoated) goes to martensite by cooling fast enough to miss the pearlite nose. The spine cools a few seconds slower, and becomes pearlite. The point where one stops and the other starts is the beautiful hamon.It takes a fast quenchant, like Parks #50 or water to cool 10XX steel fast enough to get the best from it.The problem with water is that it is almost too fast, and cracking may be a problem.
The cooling curve for 5160 is too slow to get a martensite/pearlite blade done this way. It all turns to martensite. Because of the much more leisurely rate of cooling, any quench oil will work for 5160.Specifically Parks AAA or a medium rate oil.
When you do a differential temper, the fully hardened (all martensite) blade is given a normal temper to the desired edge hardness and temper. Then the edge is set in a shallow pan of water (1/4" of water in a cookie sheet works perfect), and the spine is heated with a torch. The edge is kept in the water and rocked from tip to ricasso to keep the entire edge from going above 212F ( actually you just need to keep it below 450F). The spine is slowly heated to a higher temperature (650-700F) to make the martensite in that area have a much higher spring temper.This will create a softer springier spine and a harder edge. It will show a difference in the polish, but it is not anything as distinct as a hamon. What you see is called a temper line, and shows the difference in hardness mostly. A true hamon shows two different steel structures.
Another method of getting a dual structure blade is to edge quench the blade after austenitization. It involves just quenching the edge in a quenchant and letting the spine cool in air. This gets a martensitic edge and a pearlitic spine.....but at a cost of great stresses and unpredictable hardness results. Many old timers swear by this method. Many metallurgists swear at this method. While it still persists as an option, it is a more advanced technique than a beginner should try....and to be frank, gets mediocre results anyway.
So, the breakdown :
5160 makes a good and very tough blade that can be differentially tempered to make a superb camp knife.
1084 makes a great and fairly tough blade that can be clay coated to attain a lovely hamon.
5160 is very forgiving in HT and can be quenched in most anything that resembles oil.
1084 requires a fast oil or water.
5160 finishes to a somewhat blah appearance.
1084 finishes (with hamon) to a stunning effect (just look at any good katana).
5160 forges fairly easily, but must be kept between 1600F and 2200F
1084 forges very well and is kept between 1500F and 2100F
5160 and 1084 are both cheap and readily available.
5160 is an excellent choice for differential temper
1084 is a good choice for differential hardening
Hope this short course helped.
Stacy
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