Viking Age SWORDS - part 3 - HEAT TREATING

Sorry there has been a delay here. I prepared this for NORSEFOLK - and forgot to post it here!

The next part of this overview is the mysterious part - Heat Treating. (As Kettil I would NEVER discuss this with you!)

From our earlier discussion, we can see that the sword maker, modern or ancient, has made decisions about what metals to incorporate into the blade. An understanding of the handling characteristics of the tool has been filtered through technical specifications and working methods to decide upon some 'ideal' alloy, or combination of alloys to form into the final profile of the sword.

There could be a long discussion (read argument) over traditional hot forging versus modern machine working methods. I'm going to completely side step that one. My only statement will be this: Understand that the differing alloys are best suited to certain fabrication methods. Some modern alloys are virtually impossible to hand forge, but still produce excellent results when machined to shape.

Now, any specific carbon content iron alloy can further be modified by just how you heat and cool it. One of the Ancient Mysteries is how a skilled smith can take the same piece of metal, and by cooling it one way make it soft enough to bend back on itself, yet treat it another way and it becomes so hard that it shatters when dropped on the floor. Physically what is happening is that specific sequences of temperature and cooling rates change the way the underlaying crystal structure develops at a microscopic (even atomic) level. There are three fundamental sequences to the overall heat treating of carbon alloys: Annealing, Hardening, and Tempering.

Annealing is the first step, done right after the blade has been rough formed. It is most important to be undertaken in any blade that has been forged (hammered - hot or cold). The metal must first be heated up to past 'critical' , at least to a dull red . At this point the crystals in the metal loose their definition, the component atoms start to loose their tight bonds with each other and become somewhat 'slippery'. If the metal is then allowed to cool very slowly, the end result is that the material will become as soft as it can get (dependent on the actual other elements in the specific alloy). Any stresses built up into the structure during the hammering process will ease away. Imagine giving yourself a long soak in a hot tub. A rough rule of thumb is that the longer the metal takes to cool down from critical, the softer it can become. (Of course remember that the exact alloy content determines the base level.)
As any forged blade will require considerable grinding to even up and clean the surface, softening the metal makes that next process easier. The blade blank is ground to shape, with any holes for things like hilt pins made. It will then be polished to 'close' to the final finish desired.

Hardening the material is almost the exact opposite. Again the blade is heated to critical, loosening the bonds and thus allowing changes to be made. Now the metal is quickly cooled. Imagine jumping from that hot tub into cold water. In rough terms, the faster the metal is cooled from critical, the harder it will become. Again remember there is a maximum possible as determined by the alloy chosen in the first place. Now differing quenching mediums will pull heat at different rates. As a rough order from slower to quicker - on liquids available to the Norse : oil / water / salt water. So it is possible to take the same carbon content metal and change its relative hardness depending on how you cool it.
Generally the tang is left in its original annealed (soft) state and only the blade is quenched.
It should be noted that modern alloys can be extremely exotic in the required quenching mediums. The simplest will actually harden in air. There are those that require slow liquid salt baths through to instant cooling in liquid nitrogen. (All too weird for me!)
A couple of interesting historical notes:
The ideal oil for quenching would heat quickly and evenly. It would have a very high vapor point - thats the temperature where the oil flashes to gas. (Bubbles of gas pull heat at a much lower rate than liquid does, so can give uneven cooling effects. Now it turns out just about the best oil in terms of its physical characteristics (so I have read) is * whale * oil. Not exactly available to modern blade makers, but modern quenching oils are very like a synthetic whale oil. Again I have it on good authority that rendered bacon fat yields good results. Any high quality cooking oil works effectively as well.
Consider salt water at roughly 5% salt content. Ocean water varies considerably from place to place and season to season. What does remain much more consistent in terms of salt content is blood. This is the science behind the myth of the blade 'quenched inside a human body'. (Actually using a living body to quench a red hot sword blade will ABSOLUTELY NOT WORK, by the way!)

It should be mentioned here that the process of heating to critical for hardening also creates a thin film of scale on the blade surface. Now you have to repeat that last polishing step to clean down to bare metal again. Only this time with extreme care. The blade has been made as hard all over as may be required. Hard almost always means brittle. If you drop it at this point (or more commonly the sander tries to grab it out of your hands!) it may shatter. More importantly, the last heat treating step is a LOW temperature one. If you over heat the blade even slightly during the required polish now, you have to go all the way back to the anneal step. (Thats repeat : heat / slow cool / POLISH / heat / quench/ POLISH)

TEMPERING is the last of the three heat treating steps. The common perception of this process is totally wrong. Tempering is actually REMOVING hardness in a selective fashion. Remember we have selected a possible range of hardness by picking a specific carbon content in the metal. We then have fixed the maximum hardness in the blade overall by the cooling speed of the quenching medium.
Consider that different parts of a blade perform different functions, and ideally should have different characteristics in a correctly designed weapon. In the perfect blade, the point / edge / back (or centre in double edged) and tang all undertake different stresses and thus should have different amounts of hardness against flexibility. Generally the point is extremely hard, the edge very hard, the back somewhat soft and the tang dead soft. Since hardening sets the maximum hardness, the blade maker has defaulted to what is required for the point. Now some of the hardness must be removed from the cutting edge (lest it remain too brittle) and especially the back / core (to allow flexibility).
Tempering is undertaken by carefully heating the blade in as controlled a fashion as possible. Changes in the component crystal structure will start to occur once the metal is heated to roughly 450 F. In practice, an extremely thin film of oxide starts to form on the polished metal, which can be seen as a progression of colours over the surface. The range from cool through to hottest is : Yellow / straw / brown / red / purple / blue / grey. At roughly 800 F the metal passes beyond the temper range and develops the solid grey iron oxide. Once the ideal combination of colours (thus temperatures) is observed, the blade is quickly quenched to 'freeze' the varied hardnesses in place. Depending on just what method is used to heat the blade when tempering, those temperature colours can develop extremely fast. If an error is made and the metal gets too hot, then the blade maker ** should repeat the entire heat treating process at the initial annealing phase **. One last surface polish remains to remove the thin oxide film before the blade goes on to be hilted.
Just what temperature / temper colour should be created just where on a given blade can vary considerably. Use of the blade, alloy selected, hardening degree, all will relate to the way the blade is finally tempered.

Most commercial knife makers will in fact avoid the skills involved and simply oven temper their blades. An average is determined, again based on the dynamics of the alloy and quenching. Then an electric kiln is set to a specific temper range and the entire blade is fixed at an average degree of hardness. Although use of more complex alloys can help give decent results, the simple truth is that an oven tempered blade will never perform as well a correctly eye tempered one.
(Note * correctly * !! Hand forging and eye tempering a blade requires a significant amount of acquired skills, knowledge and experience to create what can be a superior finished result. Its also easy to screw it up. All that accumulated practice comes with a cost. Machine ground and oven heat treated blades can expected to be uniform in quality and certainly much cheaper to produce.)

As I said when the topic of knives came up a while back - you get what you pay for...

Darrell

PS - I've intentionally left a lot of practical details out of this general description. Sorry, but I do earn a living from courses I teach on these subjects. I also recommend my DVD - "Historic Bladesmithing" - available on my web site.