When the steel is quenched fast enough to pass the pearlite nose, the structure remains austenite ( too slow and it becomes pearlite at 900F). As it cools on down and reaches around 400F, it suddenly starts converting to martensite ( the Ms). When the temp has dropped to about 100-200F this conversion is finished on carbon steels (Mf). The structure is now an untempered martensite called "brittle martensite". Anyone who has tried to straighten a blade at room temp right after the quench knows why it is called that
The first temper tempers this into "tempered martensite", which has a structure a tad like pearlite...and thus is much tougher. When the first temper was done, there was also a small amount of retained austenite that never converted in the initial quench. During the rise to 400F and cooling in the first temper, it converts to martensite. The cooling from temper should be rapid and done in room temp or cooler water. This assures the retained austenite converts (slow cooling from 400 between tempers may help stabilize the RA). This new martensite is still brittle, and the second temper converts and toughens it. A blade with only one temper won't snap in half, but the edge may be chippier. Tempers should be at 400F or higher, and there needs to be two. One hour each is sufficient for carbon steels.
During temper, the steel needs to rise back to the Ms to make the changes it needs. Tempering below that will relieve most of the stress, but the blade isn't fully tempered....and thus still has some brittleness. Some people think they are getting a harder blade by staying below the Ms, and tempering at 325-350, even 375F. The thing they don't know is that the hardness isn't appreciably dropped until the Ms is reached. A temper at 350 and one at 425 may only be one or two Rc points apart.
A hamon is the junction of structures mixing pearlite and martensite. There will be wisps or one or the other going both ways, and pockets or dots of hard martensite crystals in the pearlite matrix. The old name for this mix was troosite. A hamon forms when the entire blade is heated to the austenitization point, and the the entire blade is cooled in quench. What forms the hamon is the cooling is varied in some areas due to clay and/or blade thickness.
A temper line is more properly called a quench line, and is a line of demarcation where the heating and cooling has separated the hardened area from the unhardened area. Temper lines will be pretty much straight, as they form along the radiated heat border between two temperature ranges. A temper line forms when the pearlitic blade is heated to austenitization only along the edge. It gets its name from the older method of tempering where the "temper line" was walked down the blade from the spine by gently heating the spine with a torch or hot piece of steel. This line was stopped when it got about 1/3 away from the edge. The resulting temper colors on the blade had a similar look as the line formed in edge quenching. However, a quench line is deep into the steel, and the temper colors are only on the surface.
A hamon will vary a lot depending on how fast the martensite forms and how much rapid cooling happens above the hamon. A temper line varies almost none....just a darker line.
