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advanced Heat Treating

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Advanced ideas for thermal treatments for blade steels.

The previous installment covered the conversion of perlite into martensite for hardening steel. In addition, there are a few other ways we can use heat to make changes to our steels internal structure.

Excessive heat can cause the grains to grow to a point where an otherwise fine blade steel is unusable, If it has not been burned, we can use various techniques to get our grains small again.

In the past there was an erroneous term for a mythical process called “edge packing”, which was a method used to get the grain smaller. The theory was that hammering steel colder than normal forging temperatures would shatter the grains to get them smaller, this is partially a a false assumption. Steel grains do not pack. And most anything we do as to grain size in our forging, is mostly wiped out during the final heat treating anyway.

We have already touched on triple normalizing to refine grain. This is a variation for refinement is using temperature control to get even finer grains in our steel. In this process, we start at a point about 125F above the curie temp of the steel in question for our normalizing/ After allowing to cool, the next step is taking the steel to a point about 75F above the curie point, and for the third time about 25F above. This thermal cycling by 50F steps is to cause the grain boundaries to reform again, as with our other triple normalizing process. But by reducing the target temperature by 50 degrees F each time, we are not allowing the time for the same grain growth before being cycled again. So therefore gets us somewhat smaller grains than we would have if using the curie temperature alone.

Another thermal process is called Sub Zero freezing, the science behind this process is that because when hardening, not all of the austenite is converted to martensite during the quench, this left over is referred to as “retained austenite”. But this will usually at least partly convert given a long enough time. The sub-zero Quench is to force this conversion in a more reasonable amount of time, allowing us to get it tempered, rather than allowing it to remain as the brittle untempered martensite. This is mostly applicable to High alloy steels, but some low alloy's may benefit from a home shop process, Only testing will prove if this is true or false. A home method involving crushed dry ice and acetone, can result in a temperature drop to about -170F, while a true Sub-zero quench with liquid Nitrogen will result in a drop to -270F, and is used for many stainless steels. In the Home method, dry ice is crushed and placed in a container, along with the blade, then acetone is poured into this, and blade left to get cold, and convert some of the retained austenite into martensite. This process can be volatile, so take proper precautions. Either method requires a follow up with a tempering cycle to address the newly formed martensite.

Many alloy steels we use have other issues that may at times work in our favor. Carbide precipitation can be used with good effect in the the form of alloy banding or carbide banding, While most metallurgists work very hard to avoid this in their steels, as blade makers one can exploit this for a 'Wootz' type effect on the surface of our blades. A mild acid bath, such as used with pattern welding, can assist in accentuating this effect.

Next is a suggestion on how to adjust our techniques to our steels. No matter what we buy there can be unknown additions due to the modern manufacturing process, as well as the substitution some suppliers do when out of a given product. Add to this, we all have our own equipment, not as accurate as the testing labs that make the charts. So our temperatures can be off.

Getting to know the properties of our chosen steel can aid us in getting better blades. Remember that the spec sheets temps and soak times are for 1 inch thicknesses, in blade work we are dealing with much thinner sections. First, harden using the manufacturers recommended temperature. For this example I will use 1550F. After hardening test this steel, a blade is not needed, only thickness matters. Now break and examine the grain. Then harden another the exact same way at 1500F, then another at 1600F. You may have to do a few more at various temps, but compare what hardening temp changes alone does to this thickness of steel. Also feel free to allow a soak to this thin steel, and compare. Testing like this or similar testing methods is the only way to decide if you should soak your steel or not at the hardening temp before your quench. I think you will find most simple steels do not need a soak. In fact many will have pronounced grain growth from this extra heat. This way you can discover the exact perfect temperature for this batch of steel, with your methods of making them.

Also common are techniques known as “Differential Hardening” where as only a portion of the blade is hardened. As in a Japanese Katana, creating a line of demarcation, know as Hamon. Usually clay is used to mask off the spine of the blade, which slows the cooling upon quenching. Also one can “edge quench” With this method only the cutting edge and a small fraction of the blade in immersed into the quenchant. Both of these methods still need to be tempered.

Along these same line is a process called “Differential tempering” while we still temper the entire blade, in this procedure we draw the spine even more. As an example if we draw the temper color to gold (380F) on the cutting edge, but want more flex in our blade, we can use a heated iron, plate, or even small tip torch; to draw the spine only, to a blue(500F). This is much easier to do if we place the cutting edge in water while heating the back. This is very nice as it gives the option of a blade that is tough as a blue temper would be, but the cutting edge is as a gold temper of the same blade. Because of course a blue temper for the entire blade would not hold the same type of edge for long because it would be softer.

Salt tanks are becoming popular lately, due in part to Prof Bain, and Howard Clark's work. I will not go into detail here, as that is a subject for advanced makers topic of its own. It also has higher risk of danger if mistakes are made. I will state that special salts are heated to heat or cool the steel being treated. These salts are NOT table salt, and any moisture on the blade upon entering the salt can result in an explosion as water turns into steam expanding about 150 times. But these are perfect for creating “bainite”, or even “austempering”, “marquenching”, and various other methods that are far beyond the scope of a simple Sticky at a blacksmith site.

Bainite at http://www.msm.cam.ac.uk/phase-trans/newbainite.html
Or read "Metallurgy Therory and Practice" by Dell Allen
See the blacksmithing lessons section at http://www.iforgeiron.com/index.php?option=com_content&view=article&id=1047:lb00130005-heat-treating-information-&catid=47:lb-000-100&Itemid=45 for more detailed information.

Edited by steve sells
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