difficulty in heat treating

[Ed Tipton]

I am having difficulty in understanding heat-treating. I understand that drawing the steel from the full hardened condition produces less hardness and more toughness. I have been told that I should draw the steel three times in order to assure there are no "hard spots" remaining in the blade. I have also been told that it is not necessary to "re-harden" the steel prior to tempering for the second and third times. It seems to me that once tempered, the steel is no longer at max hardness. If I begin at that point and temper for a second, and possibly a third time, am I not reducing the hardness each time, and thereby arriving at a steel that is too soft to produce a good edge? What am I missing here? I feel there is some part of this process that is escaping me, and I need to get a better handle on this subject.

[Quenchcrack]

Ed, join the crowd. Lots of confusion on this. When you harden the blade the first time, you hope you have achieved a fully hardened structure: martensite. However, in high carbon steels there is a tendency for some of the austenite (the stuff you form when you heat to non-magnetic) not to transform to martensite. This becomes retained austenite. The first temper after you quench will soften the martensite AND cause most of the retained austenite to transform to martensite. The martensite formed during the first temper needs to be tempered. So we temper a second time. I prefer to temper about 50 degrees colder on a second temper so I don't over-temper the steel but this is really hard to gage if you temper in your forge. If your wife lets you use the oven, it can be done pretty easily. You can temper a third time but this follows the law of diminishing returns and I doubt that for most of the alloys we use a third temper is of any real benefit.

[TASMITH]

When you harden your steel, using the appropriate temperature and quenching medium, the steel will be at the maximum point of hardness and also be the most brittle stage. This is because of a transformation in the structure of the steel. When you heat the steel and then rapidly quench it , the steel structure remains in this transformed state. There are several technical terms for each of the states of the steel but I will not use them here to try and simplify things.

In the hardened condition the steel you heat treated is at maximum hardness. For a knife blade this means the it would theoretically maintain its edge for the longest period of use without getting dull. However the steel is also in the most brittle stage meaning it is subject to cracking or breaking with little effort. Any edge ground on the blade would simply break off.

If you re-heat the steel to a lower temperature the steel again under goes a change in structure. This change in the internal structure makes the steel somewhat softer but at the same time tougher. The steel is able to flex somewhat without breaking. These conditions are very easily seen when you harden spring steels such as 5160. Fully hardened and not temepered, spring steel will not flex if you try to bend it. It will not move at all and if enough pressure is applied it simply snaps. After tempering however it will easily flex and will in fact return to its' original position if not over flexed. This shows the difference between hard and tough.

Double and triple tempering of steel is generally done with alloy steels. Plain carbon steels and silicon based spring steels don't generally benefit from a second tempering and a third tempering is not required at all. In plain carbon steels the transformations from the hardened condition structure to the tempered condition occur in the first temper. The greater the tempering temperature or the longer it is held at temperature does however have a great effect on the steel. The higher the tempering temperature the less hardness and greater toughness is produced in the steel and vice- versa.

Also the longer the steel is held at the tempering temperature the less hardness and greater toughness is induced in the steel. This would be the same effect as double or triple tempering the steel. The difference between plain carbon steels and alloy steels is the alloys will raise the initial temperature required to harden the steel and the alloys go into solution with carbon and iron in the steel producing a different type of structure. These alloys cause a different type of transformation when the steel is tempered leaving a mixed structure in the steel. They therefore must be double or triple tempered to change all of the mixed structures into the same structure required in the final product.

Heat treating steels, especially the alloys requires a good understanding of what happens to the steel when it under goes these various transformations and in the case of alloys, you must know what change each alloy has on the steel both individually and in combination with other alloys.

That is why most individuals work mainly with plain carbon or low alloy steels. You only need to be aware of the carbon content, proper quenching medium, hardening temp. (which remains relatively constant with just carbon and minor amounts of alloys), and the correct tempering temperature to achieve the level of hardness/toughness required in the final product.

There are also two simple rules when heat treating plain carbon steels.

When heating to harden: Soak steel for 10 min/in of thickness once steel has reached hardening temperature.

When tempering: Soak steel for 1 hr/in thickness once steel has reached tempering temperature then air cool to room temperature.

This is a VERY simplified explanation of heat treating. There are a number of good sites on the web that explain what happens when heat treating steel along with the proper terminology for each of the conditions and many of them are written in easy to understand language. A lot of the knife maker websites in particular have written simple to understand basic heat treating articles and are well worth reading for anyone interested in basic heat treating.

Terry

[Steve Sells]

We have 5 sticky's that not only explane everything you have asked, but include references to over 50 other resourced for further reading.

[Rich Hale]

If yoiu do some testing in your own shop it will become clear/ take several samples on one iece of steel harden one and bend it and see what happens..beware of saftey issues..it will break then harden one and temper it once. test again then test with tempering twice. and lastly at three times..if yoiu are pleade with any of the blade you tempered the grind to and edge and performance test them. See what happens
And when you change to another steel it may all change..............

[markb]

Quote:
It seems to me that once tempered, the steel is no longer at max hardness. If I begin at that point and temper for a second, and possibly a third time, am I not reducing the hardness each time, and thereby arriving at a steel that is too soft to produce a good edge? Quote



Think of the steel as paint, made up of a number of different colors.( austenite, cementite, martensite etc.
If they are not mixed well you see each individual color. ( first temper)
If you want them to combine to make a new color you have to mix well.( repeat tempering)

I've been tring to understand this also.
Hope this isn't over simplified or off base.
Please feel free to correct if I'm to far off base.

Edited March 3, 2009 by markb