STEEL BEFORE SCIENCE



Steel Before Science

AIM

THE SCIENCE OF STEEL MAKING IS NOW COMMONPLACE BUT FOR THE FIRST 3000 YEARS OF ITS USE, AND MOST NOTABLY DURING THE INDUSTRIAL REVOLUTION SPANNING THE 18TH AND 19TH CENTURIES, THERE WAS NO REAL UNDERSTANDING OF THE UNDERLYING CHEMISTRY AND PHYSICS OF STEEL. STEEL-MAKING COULD ONLY HAVE PROGRESSED BY LABORIOUS TRIAL AND ERROR.

cracked pinion

This is an arbor with integral pinion from a English 30hr long case clock from about 1750 . This crack goes right through and you can see daylight through the pivot. This is clearly the result of some non uniformity in the steel from which the item was made. So, what material was the steel-maker of 1750 able to supply? And for that matter, we might ask, What material could the steel-maker of any era supply?

Tonight I’m going to present some snapshots in the development of steel as a reliable product, in the period before the metallurgy of steel making was known.

Introduction

CALIPERS

This period encompasses the period of the classic scientific instruments. The metallurgy of steel production was not even partially known until 1770 and it was not until 1860 that science explained how and why our methods worked. So that will set our time frame.

Steel has been around for thousands of years, but, the long and the short of it is that, while the ancient iron founders and blacksmiths provided the tools of civilisation, their work began so long ago, and became so accepted as commonplace, that we have only the most meagre record of their methods. I have read that it is not possible to assemble more than forty pages of pre-Renaissance text on metallurgical processes.

Of all the technology which we take for granted today, good quality steel must be at the top of the list. It’s everywhere - roofs, railways, bridges, bolts, nails, ships and tools of science. But no one notices - unless it breaks.

214ST

The history of steel exemplifies a general characteristic of industrial development that lasted well into the Industrial Revolution. The theory came after the practice. Perhaps it is more correct to say that it was the best of the artisans who advanced the science.

art/science

(Which reminds me to say that, as distinct from science, there is such a thing as engineering art. Don’t ask me to give you an example because if I could it wouldn’t be art it would be science.)

Outline

First of all tonight I want to sort out some jargon. Then look at what is steel. After that I’ll go down this route. There’ll be some snapshots of the state of the art. I’ll talk about samurai and Toledo swords, Damascus steel and Wootz steel, that some of you will have heard of.

Show steel sample

In the books of traditional English recipes for making clocks this type of steel is specified for everything that is steel - screws, springs, cutting tools, pinions, pivots, points and punches. Now as the very model of a modern engineer, I know that is not good practice. (For a start it is a bugger to machine.) But it works. By the end of the night you will know the curious reason why this stuff is called silver steel.

jargon

LET’S BE CLEAR ABOUT SOME JARGON.

cast

Casting - manufacturing by pouring molten metal

forge

Forging - shaping by hammering

ductile

Ductile - able to be drawn into thin wires

malleable

Malleable - able to be beaten into thin sheets

elastic

Elasticity - tendency to return to shape after deformation and therefore to resist deformation

yield

This is closely associated with the term yield stress the stress at which the material begins to permanently deform.

tensile

Tensile strength

Compress

Compressive strength

Hard

Hardness - resistance to indentation as measured for example by Brinell hardness tester.

Tough

Toughness - ability to absorb energy without fracture.

Wear

Wear resistance resistance to abrasion is not the same as hardness.

Grains

Grains - Iron and steel are crystalline materials. As they solidify from molten metal, crystals growing from thousands of sites bump into one another. The result looks granular. The crystal lattices of material at the boundary of the grains are severely distorted. This contributes to the strength and toughness of the material. At ordinary temperatures the more grain boundary material, ie the finer the grains, the better.

All the properties I have just defined depend on the composition and arrangement of these grains. This microstructure is the key to the behaviour of steel.

The grains cannot be seen with the naked eye, and they are invisible ,even under the microscope, unless the steel is specially polished and etched.

Next definition is Quenching - cooling rapidly by immersion in water or oil .

If we do this with steel it becomes especially hard. It is a very simple process. This is our piece of silver steel in my work shop the other day.

Heat

quench

The end of this piece is now as hard as steel ever gets. It will cut other steels. The entire Industrial Revolution was based on this stuff.

However, it is also very brittle.

hammer

broken

We can improve upon this by Tempering - which means reheating after quenching to improve toughness - albeit at the expense of some hardness.

This is what happens on the other tempered end of the same piece of steel

bend

result

what is steel

SO WHAT IS STEEL ?

Steel is a mixture of iron and carbon. More specifically it is a mixture of the intermetallic compound iron carbide with a solid solution of carbon in iron.

Commercially pure iron is soft and ductile and has useable tensile strength. Iron carbide is very hard and brittle and is weak in tension.

Pearlite.

Iron carbide is this white stuff, and the black stuff is a solid solution of carbon in iron.

Solid sol

Some of you will be familiar with the concept of a solid solution.

Iron is a crystalline substance from room temperature up to 1200 Celcius. It is allotropic The two important allotropes are these:

fccbcc

They are able to absorb carbon atoms to form interstitial solutions in the solid state.

austenite and ferrite.

We will meet these names ferrite and austenite again..

The way the whole mixture comes together is described by an equilibrium phase diagram.

Phasediag

This describes the proportions of each phase present in the mixture at various temperatures. It is predicated on very slow changes of temperature.

Total percentage of carbon in the mix shown on horizontaal axis. Only showing left hand six percent of diagram.

At far left pure iron . At far right off the page pure carbon. At 6.6 % carbon the mixture is all iron carbide.

Mixtures above 1.6% carbon are called cast iron. The name steel is given to mixtures with up to 1.7% carbon.

At high temperatures, above this line the mixture, is liquid.

If we now look at the steel corner there is room to write some labels.

colourphasediag

All I really want you to note is that at 1333F the iron changes its allotropic form. This transformation is not instantaneous. It only takes place in a small temperature range and requires time.

So, if you quench the steel from above 1333F the transformation from austenite to ferrite is suppressed. On arriving at room temperature there is an entirely new phase present. This is a highly stressed, unstable and very hard material called martensite.

Martensite

This is in fact is what was produced by the quenching exercise in the earlier pictures. Note that you can just see the grain boundaries of what was austenite and there is a bad crack caused by the volumetric changes during quenching.

There is another phase which I must mention. At exactly 0.8% carbon a quite special micro structure is formed. It consists of very fine plates of ferrite and iron carbide. It has a very good combination of hardness, toughness, and tensile strength. It forms naturally as the mixture slowly solidifies . This iswhat it looks like.

pearlite

This is where it fits on the phase diagram.

Phasepearl

So here now are the phases present

finalphases

Almost none of what I have just told you was known before about 1780; and there was no knowledge of the contribution of the microstructure before 1863.

As I have said, the properties of steel, its hardness, toughness, strength and ductility, are in the first instance determined by the quantity of iron carbide present in the mixture and the way in which the iron carbide is dispersed. Steel with less than 0.3% carbon will not significantly respond to quenching.

So, the properties of the steel available to our ancestors will have depended upon how well they could produce and control iron carbide.

smelting iron

SMELTING

No less a person than the eminent civil engineer and President of the United States, Herbert Hoover researched the history of metallurgy at length and tells us that people were smelting iron ore before 3500BC. Hoover says that the pyramids of Egypt were built with steel tools. Bronze would have to have been hardened to cut stone and no hardened bronze tools have been found. Hoover puts a cogent argument that the Iron Age in fact preceded the Bronze Age [i].

In the absence of records, we are bound to assume that the ancients treated iron ore by the general methods they used for other metallic ores, and that they used early versions of the methods recorded as being used by iron-masters in the 1500s.

buildbenification

Thus we assume that the first process was "benefication" of the ore. This involves breaking and crushing the ore, concentrating it by removing trash, and then "calcining" ie roasting the ore to remove deleterious sulphides and volatiles. The useful part of the ore at that point consisted of fairly finely divided metallic oxide mixed with minerals from the rock - predominantly silica.

The second process was to use charcoal to take up the oxygen, to "reduce" the metallic oxide to metal.

buildreduction

This was done by mixing the ore with charcoal and a flux in a furnace and setting the lot on fire. The usual flux was limestone and it helped with separation of the trash as a slag and with coagulation of the metal.

Charcoal was the fuel of choice. Unlike wood, charcoal does not contain cellulose or water, either of which would have retarded the reduction of the ore and would also have limited the rate of heat production. Charcoal has a calorific value of about 12,000 BThU per pound about the same as black coal.

Early furnaces were shallow pits or holes in the side of a hill. Later they became above ground structures shaped like the traditional beehives and then they increased in size and sophistication. These early iron furnaces are now called "bloomeries".

Time line

Egyptian inscriptions show above ground furnaces by 2000 BC

build2000

with forced draught from bellows by 1500 BC..

draughtbuild

By the time of the Biblical prophets and the first Greek literature, say 800 BC, there are frequent references to bellows and there are archeological remains of smelting furnaces which were still fairly primitive. The writings of the Greeks around 50 AD reveal considerable advances and we may conclude that their furnaces were fair sized structures of some sophistication - perhaps comparable to those of 1500 AD for which we have written descriptions.

furnacebuild

We know that the ancients were able to achieve temperatures between 900°C and 1100°C in their furnaces at an early date. For example, pottery furnaces dating from around 4000 BC were able to reach temperatures of 1000°C

potterybuild

and we can see the later evidence of smelted copper and cast silver and gold which also require such temperatures.

Modern reproductions of Roman furnaces have produced temperatures as high as 1300C.

1300build

Of course, the larger the furnace the very much greater the volume of forced draught that was required to achieve those temperatures.

Crucibles for secondary melting were used by the Egyptians around 2000 BC

Build crucible

The implication of this will become evident to you as I go on..

What then came out of these furnaces when used for iron? Well, it depended on the temperature at which they were actually run.

Many complicated reactions take place in the furnace and most of them are reversible depending on the concentration and temperature.

If the operating temperature is about 500°C:

reduction500

The iron oxide is reduced by carbon monoxide from the burning charcoal, and at the same time the iron reduces the carbon monoxide. The limestone is decomposed. The iron coagulates into a spongy mass.

At this temperature the reduction process is very slow.

If the operating temperature is about 900°C:

reduction 900

Carbon monoxide is decomposed to carbon dioxide and carbon. Any remaining iron oxide is reduced by the carbon. The iron, however, is still well below its melting point and remains a spongy coagulate. The silicate trash is picked up as a calcium silicate slag is formed and this slag is molten. It can be run off

Note now the surplus of carbon monoxide.

buildarrow

This has an important consequence.

Experiments have shown that no carbon is absorbed by the iron if all the air or oxygen is removed. According to metallurgists,[ii] it is probable that the carbon absorption at the surface occurs by a reaction between iron and carbon monoxide.

2CO + 3Fe « Fe3C + CO2

The carbon dioxide produced by this reaction breaks down to form more carbon monoxide and the process continues at the expense of the charcoal[iii]

Thus in the furnace at 900°C the ancients were able to get iron carbide into their mixture. The higher the temperature the more carbon monoxide and the faster the carbon was absorbed and this had a further interesting effect.

phasediag

As the proportion of carbon dissolved in the mixture increased, the melting point went down. At about 2% dissolved carbon and 1100°C the mixture was a slush, and carbon diffused much more rapidly and evenly through it. On reaching 4% and 1200°C the mixture was a pourable fluid.

Now we can see what these furnaces would produce.

products

If as soon as the slag was molten it was run off and the furnace was opened, the product would be a lump of spongy iron that could be hammered, that is “wrought” into solid metal bars - "wrought iron". It would be better than 971/2 % pure iron, contain less than 1% of dissolved iron carbide, and would be intermixed with about the same amount of slag. Producing wrought iron was not an easy business. The slag would be run off several times as the bloom of iron accumulated. When the bloom was removed it had to be strenuously hammered while hot to remove more slag and consolidate the iron. A 5kg bloom would reduce to about 3kg of wrought iron bar.

If the furnace charge was allowed to soak at 900°C before the slag was run off, the yield would be greater and there would be less included slag, but the iron sponge would not be particularly malleable and would, at first, have been difficult to use. Parts of it would have up to 15% or more iron carbide.

If the iron-master allowed the furnace to soak at around 1100°C, or forced the temperature above 1250°C to achieve faster reduction of the ore, the iron would run to the bottom as a slushy liquid and freeze into a hard brittle useless mass and ruin the furnace to boot. It would contain about 50% iron carbide uniformly distributed. “We would now call this cast iron”

Wrought iron was the preferred product for many many years. It was a valuable product. Bars of wrought iron were traded in Greece during the 6th century BC and they may also have been used in the manner of money.

But wrought iron is not steel

2800words

table wrought vs steel vs bronze

Steeling

From the evidence of their furnaces which had soaked at 900C, the ancients would have been aware that a very desirable harder material than wrought iron was available. They would have tried to obtain it by

steeling

direct recovery from the furnace - because this carburised material was hard and could only be worked when red hot this was not an easy process. We might call this “Natural Steel

mixing the wrought iron with the hard material from the furnace. This could be done by twisting strips of wrought iron and carburised iron together and consolidating them by hammering while red hot. We call this piling.

There was also the prospect of re-melting the wrought iron and hard useless iron from the overheated furnace. This is called cofusion.

As it was obviously the fire that caused the hardeness, returning the wrought iron to the charcoal fire was bound to be tried - with a bit of hammering for good measure. This resulted in carbon being adsorbed by the surface of the iron in what we would call superficial hardening.

Steel in Antiquity

THERE IS LITTLE DOUBT THAT STEEL WAS BEING MADE AND USED BEFORE 1000 BC.

The Chalybes, a sub tribe of the Hittites who lived in what we would now call Armenia around 1300BC, were renowned for having the secret of making iron that was harder than hammered bronze.

Iron articles dating from about 1200 BC have be found with higher carbon content on the outside surfaces than the interior. Hardened tools made around 800 BC have been found in both Assyria and Egypt.

Homer's Odyssey of around 800 BC contains, as translated, the lines:

homer

"And as when armourers temper in the ford

The keen-edg'd pole-axe, or the shining sword,

The red hot metal hisses in the lake"

(This was on the occasion of Ulysses bunging a cyclops in the eye with a stake.)

So quench hardening, a characteristic of steel, was well known in classical Greece. It was also known to the Romans who it seems also knew about tempering.

In pre Roman Britain billets of steel known as sword bars have been found.

The Romans had established a trade with Syria for so called Noricum iron which some authors suspect was a high quality natural steel. The Romans later imported lustrous silky “Seric” iron from India. No one seems to know what this material was, but perhaps by the end of my story we will have a clue.

When the Roman legion was pulled out of Inchtuthill near Perth in Scotland in 76 BC it was instructed to leave nothing that could help the enemy. Timber was taken away, pottery smashed and wattle was burned. They buried 875,320 nails and spikes ranging from 2" to 16" long. Sir Ian Richmond dug them up in 1961. Metallurgists reported that the composition of the nails varied from pure iron to high carbon steel. This exemplifies the metallurgical problem of the day - quality control. The steel one produced was largely a matter of luck.

The place where one would use the best iron product available would be in swords and there is a variety of legends about very special swords.

The sword that was so sharp that if held in a stream it would slice in two a lock of wool or a lily floating on the current against its edge. The Saracen's magic Damascus sword that would cut through a silken veil which fell through the air against its edge. Excalibur and Durandel, the swords of Arthur and Roland, and those of Siegfried and Godfried and Charlemaigne

swordfairy

Our records of these legends come from medieval times or later and it is very hard to establish when the owners of these swords were actually in business. The legendary swords may have existed as the very special, perhaps fortuitous, products of the iron-masters. They may have been an attempt to medievalize ancient mythological concepts, or they may simply represent another unattainable ideal of the romantics - like perfect chivalry and chastity, and no more real than fire breathing dragons.

On the other hand, Charlemaigne and his famous sword Joyeuse were quite real. He was King of the Franks and Lombards in 742 AD.

But, the fact that there are legends about especial swords suggest ordinary swords were probably not made by methods giving invariably high quality.

We do not know how Roman swords were made. The quality varied as it did in the nails. Thus the swords were probably made by piling strips of soft wrought iron and natural steel. Harder steel strip may then have been scarf welded to the edges under the hammer.

romanswordweld

renaissance steel

WE MUST NOW STEP FORWARD ABOUT A THOUSAND YEARS.

If the ancient Europeans had a technique for making fine steel consistently then it was lost during the Dark Ages 450 to 1000 AD. Because for the next seven hundred years they persisted with the method we believe were used by the ancients.

In Europe the late Medieval methods of producing steel fitted the categorization I showed you earlier

Natural steels were still being used.

Refiring and superficial-hardening were the earliest recorded method of steeling iron. Here is a contemporary illustration.

agricforge

The technique was to take the iron as it first came from the furnace and run it through again. It seems the iron mass was broken into small pieces and again mixed with limestone. This was re heated with charcoal in a forced draught furnace and when the iron coagulated it was quenched. The resulting lump was broken into pieces on the anvil and each piece checked for hardness. The soft pieces were returned to the furnace and the steeled pieces were hot forged into bars.[iv]

Blister Steel and Cementation

In a later refinement of the process, wrought iron bars were packed with charcoal in a crucible and reheated. At first the crucible was fired internally. Later the crucible was made air tight and heated externally. The wrought iron could not be melted but it was made hot enough to react with carbon monoxide, as I previously described, to give an iron and iron carbide mixture. The carbon diffused about 1/8th inch into the steel per day. Steel produced by this method contained from 5%% to 30% iron carbide and could be hardened and tempered - though the properties of each piece were different. The process was called "cementation" and iron carbide became known as cementite.

shearsteel

This type of steel, as we would expect, appeared burnt on the outside and was called "blister steel". It was invaluable for making tools and weapons. It was easily welded to iron so that axes, plane irons, draw knives, scissors and shears were made of the cheaper wrought iron with blister steel edges welded on. For this reason it was also called "shear steel". Shear steel quality was uneven and unreliable. Each piece had to be tested. Even the best pieces still contained impurities, usually in the form of non metallic inclusions

We do not know when this process began, but Hoover says it was a method used in primitive Japan and India. The process grew in scale and persisted in England until the 1750s

cementation furnace

Despite the quality control problems, iron and steel were engineering materials in the fourteenth century. In the Archives of Aragon in Barcelona there are the financial records of the construction of a tower clock for the palace of King Pere IV of Aragon at Perpignon.[v] The work, which began in 1356, was administered and recorded by Ramon Sans. The total project cost was some $A8million in today’s values. The clock was fairly large. The cost of the 970 kg of forged wrought iron components was about $A40/kg The steel used for some parts of the clock, and especially for files to finish and adjust the parts, was very much more expensive at about $A160/kg[vi]

salisbury

Nor should we forget the clock of Salisbury Cathedral which was made entirely of iron.

The best metallurgical practices were used by the armourers. In the thirteenth century the Germans devised a machine for drawing wrought iron through hardened steel dies to make wire. The wire was then used for making chain mail. Techniques for forging armour plate were refined and both chain mail and plate armour were probably superficially hardened.

mail

clock.

Some quite remarkable things were done with wrought iron. This clock dates from the late C15th and is made entirely of iron.

Now, some time around 500AD another technique of making steel had emerged. This was the famous Wootz steel.

Wootz steel

It seems that at first, Wootz steel was made by mixing soft wrought iron with the hard brittle high carbon waste product of the overheated furnace. The hard overcarburised material, which today we would call cast iron, was broken up, re-melted in a crucible, and a faggot of thin wrought iron strips or plates was then immersed in the liquid. Capilliary action drew the liquid between the plates and fused them together.

Wootz

This was an example of the cofusion I previously mentioned.

On cooling, the ingot had a fine lamellar structure of alternating iron and high carbon steel that was further refined by hammering at red heat as it was forged into the final article. It produced particularly superior swords.

When polished and etched they had surface patterns like this.

laminatedblade

But wait! There’s more. For a period of about 600 years to 1750, a small proportion of Wootz steel was a much higher grade material. This produced swords with surface patterns like this. An almost silky moire like pattern.

moire blade

This was the steel of the famous Damascus blades of the Saracens.

European cutlers attempted to emulate the product by twisting and hammer-welding strips of blister steel and wrought iron - often enough with good enough results. The popularly romanticised methods of Japanese sword manufacturers, involving repetitive piling strips of wrought iron and steeled iron, also seem to date from the fourteenth century.

But these were faux Damascus blades - not the real thing. There was more to it than piling.

During this period Wootz steel was a product of India. It reached Europe along the caravan route via Damascus . It does seem possible though that Wootz steel may have originated with the Hittites. Wootz steel may have been available to the Romans and was perhaps the the seric or silky iron they purchased from India.

Europeans who used this high grade Wootz steel centuries later did not know how it was produced.

There was however a similar European technique. In the late medieval Brescian process, a wrought iron billet was immersed in molten cast iron and the billet absorbed enough carbon to be steeled someway through its thickness.

steelingiron

Following the Renaissance, those Europeans, who could afford the cost, had their sword blades made entirely of steel - natural steel, shear steel or high grade Wootz steel imported from India. In the next 300 years a trade in Wootz steel developed between Europe and India, but the methods of producing it were not publicised until the nineteenth century.

Manufacturing centres, such as Toledo and Solingen, grew in the areas where the local iron contained fewer impurities and trace elements that produced cleaner natural steel with better hardenability.

Quality Steel

SWORDSINTO SHARES

Turn your swords into ploughshares was some one or other’s exhortation to give the fighting a miss. But I’m prepared to bet no one ever literally made such a transition. On the other hand they may well have done this.

buildswordsinto springs

Swords are remarkably like springs. I have seen an C18th sword bent double and still spring back. If you were making a sword today you would make it out of what’s loosely called spring steel.

The first spring driven clock arrived in about 1500. The spring would have looked rather like this one which comes from 1630 . It looks much the same as any modern clock- spring.

spring

It is tied up with wire beause when a clock spring of this size is wound up, it has enough explosive power to do you a very nasty injury. Actually this one looks rather tired and soggy. That is a direct consequence of the inadequate properties of the material. Almost all clock springs are stressed just beyond their yield point every time they are wound up. After five hundred or a thousand windings no wonder they decide to stay bent.

Any clockmaker or gunsmith will tell you that springs are the most difficult things to make. To produce a successful spring you have to have everything going for you.

Now wrought iron was still the basis of the production of steel. Remember, the shear steel process and Wootz steel process both began with wrought iron. Between 1450 and 1750 there were vast improvements in the production of wrought iron.

They arose firstly from the harnessing of water power to provide air-blast for the furnaces. The smelting furnaces could then be run hot enough to continuously produce cast iron and methods were found for converting cast iron into wrought iron.

blastand puddle

First through the finery and then by puddling in a coal fired reverbatory furnace - both of which were intended to remove carbon from the cast iron.

Wrought iron fostered some heroic engineering. It brought in the era of the big bridges.

bridge

But unfortunately we don’t have time to go down that road.

Some of the so called wrought iron still had enough carbon in it to be regarded as medium carbon steel. But I don’t know if it was ever used as such.

However, wrought iron continued to contain 2 - 3% slag. That was said to be one of its virtues. But these impurites were quite unwelcome when they found their way into small pieces of shear steel being used for say clock springs.

This clock was made in about 1680. It is all steel.

Clock1+ springsections

There are also two photographs of the microstructure of the clock spring.

The upper photo was taken before the section was etched. The slag inclusions are quite obvious. The lower photo is taken after etching and it shows a heavily distorted pearlite indicating a carbon content of at least 0.8% and considerable cold forging. There is no evidence of quenching or tempering. The clock is German and the spring was probably made from natural steel. Or possibly even Wootz steel.

Here are some illustrations from an English treatise on spring making in about 1720.

Makespring1

Discuss process

makespring2

makespring3

Shear steel lacked uniformity and so, despite all this care, the fitness for purpose, the quality, still varied.

The history books will tell you that the next step was taken in 1740, when Benjamin Huntsman actually managed to melt shear steel and thus allow the iron carbide on its surface to disperse throughout the steel. This made steel with fewer inclusions, a higher yield strength and a more uniform surface hardness.

Huntsman was a clockmaker in Attersea, Lincolnshire, who, it is said, was dissatisfied with the quality of the shear steel he was using for clock springs. He used a coke-fired air-blown furnace and made special crucibles for the purpose.

Huntsmansshop

The steel was cast into ingots of about 40lb which were then forged. The product became known as crucible steel and was the foundation of the subsequently famous spring-steel based industry of Sheffield.

I think the popular history books give Huntsman too much credit.

In 1677 Englishman Joseph Moxon wrote about Wootz steel in the following way.[vii]

Quotes

"It is the most dificult of any steel to work at the Forge, for you shall scarce be able to strike upon a blood heat but it will Redsear; in so much that these Symeters [scimitars] are by many Workmen thought to be cast steel; but when it is wrought it takes the finest and keeps the strongest edge of any other Steel. Workmen set an almost inestimable value on it, to make Punches, Cold Punches, &c of."

In the diary of Robert Hoooke, the entry for 21 November 1675 includes this observation

" ... bringing [blister steel] soe as to melt made the best steel after it had been wrought over again"

These references to cast and molten steel leave little doubt that cast steel was commonly known seventy years before Robert Huntsman's efforts. It was a fairly wide spread process. Huntsman did not invent crucible steel - he made a business out of it.

Crucible steel was made from blister steel, a deeply carburised material. When the carbon content was averaged out by melting, the result was not a high carbon steel. It was more what we would call medium carbon steel. As cast and forged it would be a good machinery or cutlery steel and could be hardened by direct quenching without cracking or becoming brittle too readily.

As an aside, the dual process of quenching and then tempering was quite late on the scene. High carbon cast steel, which would have been easier to melt, would have almost impossible to harden without inducing brittleness.

By contrast Wootz steel was given a special slow quench (mythically, this was rather tough on personnel since it involved quenching the blade within the intestines of a local slave/virgin/etc)

Crucible steel definitely lead to higher quality clock springs as Huntsman had itended. This is the micro structure of a spring from a chronometer made by John Roger Arnold in 1804.

tempmartensite

Crucible steel came to be used for casting quite large components in steel. The Engineer of 29 January 1858 carried an account of the pouring of a 5,000lb steel casting at Sheffield. It needed ninety two crucibles each containing 55lb of molten metal. The casting was poured manually in only eight minutes. One white hot crucible every five seconds. All quite unnecessarily exciting .

An Art becomes Science

Art becomes Science

Did our early steel makers really know what they were doing? Well, yes and no. Whilst ever engineering art could be considered separately from science, the art preceded the science.[viii]

Because of the appearance of the superior Damascus swords, the European sword-smith and his customers related a certain visible surface texture to the serviceability of the weapon, Erroneously thinking that the texture came from the forge, rather than from the raw material, the sword-smith adjusted his manufacturing technique to reproduce the right surface texture. The knowledge was science. The technique was art.

Early European metallurgists sought to produce steel with the damascene or moire surface patterns characteristic of Wootz steel because they thought that when they had the patterns they had the steel. They also spent time examining fracture surfaces, because when they saw a fracture surface like that of wrought iron, which they called fibrous, they thought they had a tough material. We now know they were only half right - but it was a good beginning.

Build mysticism

Mysticism was ever present. Perhaps as late as 1720, steel was generally regarded as the purest form of iron. No doubt there was some religious background to this - virtue being the consequence of purity. But it seems to have come from Aristotle and the view that fire is purifying and the observation that steel comes from heating iron in a fire.

buildChemistry

By the seventeenth century the natural philosophers understood quite clearly that the hardening and softening of metals depended upon some variation in the arrangement of particles in the metal and on the interaction of neighbouring particles.

buildHooke

Robert Hooke, in the 1670s, was one of those who did not accept the purity story. He thought that vitreous (ie non crystalline) matter mixed with the iron was responsible for the properties of steel. He examined razor steel under his microscope, correctly explained temper colours, and set out a reasonable theory of hardening.

The iron metallurgists began to recognise that there was something in the mixture that altered its properties and they called it "sulphurs and salts".

buildReamur

In 1722 Reamur postulated that there was, in Wootz and cast steel, an interdiffusing and interaction and segregation of particles in solid solution. His hypothesis would be almost completely acceptable today, but it never escaped the walls of academia.

Improved chemical methods of assay or analysis began to give a very useful degree of control over raw materials, impurities and products.

buildRinman

Chemical analysis by the Swede Rinman in 1774, reinforced by his countryman Bergman in 1781, finally established that it was carbon which caused the significant difference between wrought iron, steel, and cast iron. [ix] Up until about 1780 there was no word for the element carbon or graphite.. Rinman had no option but to call carbon "iron earth overcharged with phlogiston". Incidentally, it was probably Bergman who ensured the demise of the phlogiston theory.

buildB,VandM

In 1786, the French chemist Claude Berthollet in collaboration with Vandermonde and Monge wrote a book on the nature of steel saying that it was simply a compound of carbon and iron. The concept of carbon in solid solution and as precipitated carbide in steel was well accepted early in the nineteenth century.

But, these newly emergent theories in chemistry were concentrated on the composition of molecules of compounds and they ignored the possibilities of variation in the arrangement of the molecules in a solid.

buildMicrostructureSorby

The microscope is the proper tool for this job, but it was not successfully applied to metallography. until 1863 - for which Henry Sorby gets the credit. It was his work , and subsequent similar work, which gave a view of the micro structure and a far deeper understanding of the roles played by all the factors.

Silver Steel

One of the more curious results from the faux Damascus period was the work of James Stoddard and Michael Faraday In 1819 Faraday analysed Wootz steel and got it wrong. In 1820, he and Stoddard published descriptions of alloys they had made of steel with silver, nickel, platinum, chromium, and other metals. Many of these showed a good Wootz-like moire. "They observed the extrusion and retention of globules of silver in castings, and of fibres in forgings, made from their alloy of silver in steel. Their "silver steel" was the first alloy steel to become popular."[x] The quality probably resulted from Faraday’s remelting processes. The silver just happened to make it look right.

What we now call silver steel contains no silver at all and is certainly not what Faraday produced. I am very much inclined to believe that Faraday's silver steel was taken up for mechanical parts by clock makers in 1820 or thereabouts and the name stuck for the steel later supplied to satisfy their requirements. We have continued to use “silver steel”, even though it is a different material from Faraday's 1820 alloy. Silver steel today is a plain carbon steel of about 1% carbon with traces of manganese, chromium, tungsten and vanadium.

Wootz steel

So what was the secret of that high grade Wootz steel. It turns out that what was happening was that from time to time the wrought iron dipped in the molten cast iron actually completely melted. This resulted in a very uniform fairly high carbon steel that was mostly pearlite and iron carbide.

pearlite

Good tough hard steel. There was some excess iron carbide and this was the secret of the pattern. It so happened, that the ores that were being used, naturally contained small amounts of vanadium. About 40 parts per million of vanadium splits these carbide lumps into bands of clustered iron carbide particles . Experiments during 1998 showed that it was these bands that produced the moire patterns. The finely dispersed carbides also increased the effective hardness of the steel.

So there it was 1% carbon steel with a trace of vanadium. After all these years, so little difference between our materials of choice silver steel and Wootz steel

FIN

In closing I really must acknowledge my use of the work of Wayman, Leopold and Evans of the British Museum who analysed the metallurgy of the early clock springs.

and the steel and cutlery specialists Verhoeven ,Pendray, and Dauksch who uncovered the secret of Wootz steel

( A.J. Emmerson, Brisbane 2004

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[i] Georg Bauer, nom de plume Georgius Agricola (Hoover H.C. and Hoover L.H. translators) De Re Metallica 1556, The Mining Magazine London 1912, reprinted Dover Publications New York 1950

[ii] For example, Clark D.S. and Varney W.R. Physical Metallurgy For Engineers, van Nostrand Princeton New Jersey 1962

[iii] Actually at these temperatures the carbon goes into a solid solution of carbon in austenite at the metal surface and then migrates towards the centre of the metal by diffusion. It forms iron carbide on cooling.

[iv] Agricola op cit

[v] vide Beeson C.F.C. Perpignan 1356 and the Earliest Clocks, Antiquarian Horology 7, June 1970 pp 408-414

and Beeson C.F.C. Perpignan 1356: The making of a Tower Clock and Bell for the King’s Castle, Antiquarian Horological Society London 1983

[vi] Landes D.S. Revolution in Time, Clocks and the Making of the Modern World. 2nd ed, Penguin Books Ltd London 2000

[vii] Moxon J. Mechanik Exercises, or the Doctrine of Handiwork Vol 1 No3, London 1677

[viii] Science is what you can write down and prove. Art is what you can't.

[ix] Rinman had no option but to call carbon "iron earth overcharged with phlogiston". Bergman ensured the demise of the phlogiston theory.

[x] Smith op cit

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