Iron and steel – part IV (Bessemer steel)

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By the 1850s the best crucible steel was the standard choice for makers of edge tools and no other steel had been found that could better it at any price.   However, the steel making industry – driven by the increased demand for the large castings needed to satisfy rapidly expanding railway, armaments and ship building projects – was about to undergo a huge change.

Ultimately Sheffield was unable to produce the quantities of crucible steel for all the wheels, rails, axels, plates and machine parts that these industries needed but the initial attempts to scale the crucible steel process delivered some impressive results.

The Limits of Crucible Steel

There are a couple of famous examples where the old crucible process was used to create very large castings.  The first was a steel bell made by Naylor, Vickers and co at their River Don Works, Brightside, Sheffield in 1860.  The bell was ordered by the city of San Francisco to act as a fire warning alarm and it was about 5 ft tall, weighing over 2 tons.  A casting on this scale was clearly an event of some note and it was attended by the general public and reported in the illustrated London News:

 

The Illustrated London News – January 7th 1860

As can be seen in the first etching above, creating large castings required many crucibles to be poured into a reservoir which, once enough steel had been added, was unplugged to allow an uninterrupted flow of molten metal to a mould suspended in a pit below.  A Frenchman visiting the River Don works a few years later in 1867 reported in Métallurgie du fer et de l’acier1)Metallurgy of iron and steel: practical and comprehensive studies … by Samson Jordan that Vickers were by then able to make casting weighing 20,000kg (over seven times the weight of the San Fransisco bell) which required around 500 crucibles to be poured in succession.

Vickers had 288 melting holes at the time2)Engineer, Oct 25th 1867 and a casting of this size would have needed nearly all of them to be heated simultaneously.

Castings of this immense size were exceptional however, and remained newsworthy even a decade later.   A royal visit to Firths was reported in August 1875:

The second day was chiefly occupied with the inspection of the famous Sheffield manufactures of iron and steel. The Prince and Princess were accompanied by the Duke of Norfolk and the Ladies Howard Earl and Countess Fitzwilliam and their family, Lord and Lady Wharmcliffe, Lord and Lady John Manners, Lord Auckland the Earl and Countess of Galway and other ladies and gentlemen of rank.

The Norfolk Works belonging to the firm of Messrs Thomas Firth and Sons of which the Mayor is the head, were the first visited by the Prince and Princess.  Here they saw the old process of melting which yields from the Swedish iron the finest steel. The rows of clay crucibles at once attracted their notice. Each crucible lasts only three castings so that the consumption of these vessels is incessant and they are made by wholesale in these works, the material used being a mixture of pot clay and china clay. The crucibles are in holes in the floor which seems honeycombed for the purpose In vaults below the fire is kindled, the crucible is covered with hard coke and left till its contents are melted. The dexterous workmen drag it from its cavity by tongs and pour the contents into iron moulds These are of different sizes and vary from one ton up to sixteen or even eighteen tons the largest steel ingot ever cast.

There are 400 of these melting holes in which the tools the steel must be of a different quality and temper a result secured by a certain mixture of the molten steel.The Illustrated London News, August 25 1875

 

The Prince and Princes of Wales also visited Firth and Sons – etching fromThe Illustrated London News –August 25 1875
Possibly the largest crucible steel casting ever made – again by Firth’s – was reported in 1874:
NOTES FROM SOUTH YORKSHIRE

Sheffield, Wednesday. The Woolwich 81 ton Gun

Messrs Thomas Firth Sons of the Norfolk Works Sheffield are now proceeding with the production of the crucible steel ingot required for the 81 ton gun which is about to be built at Woolwich.  This ingot will be used for the internal tube which lines the gun.

The rough casting was made at Messrs Firth’s works Friday last and is in all probability one of the largest castings ever made.  In its production 628 crucibles, each containing 70 lb of steel were used, making the total weight about 20 tons. One hundred and ninety four men were engaged in the work which occupied about forty minutes.

The ingot is 42 in in diameter and 13 ft in length.Engineering May 1st 1874.

 

although this was created decades later, here is an example of the size of steel casting that Vickers and Firth’s were capable of creating during the 1860s.
Despite these impressive achievements,  the process to make large crucible steel castings was unwieldily and extremely lengthy.  To recap:
  • The process began with the mining and smelting of iron ore in Sweden.
  • The iron was then “puddled”3)c.f Edge tools – iron and steel – part I (Wrought Iron) and rolled into bars.
  • Once shipped to Sheffield these bars were carbonised in a cementation furnace that had to be continually heated for 10-14 days.
  • The furnace took another 1 or 2  weeks to cool down to a point where the steel could be safely removed.
  • Finally, the best part of a day was needed to melt and teem the steel, and this stage required a lot of manpower (the largest castings needing a couple of hundred men to run the melting and casting work)
In addition to taking many weeks to complete, the equipment needed occupied many large buildings and consumed huge amounts of fuel at each stage.  As a result it was simply impractical to rely on crucible steel to meet the burgeoning demand for large steel parts in the long term – a better way was needed.

Bessemer

Henry Bessemer had no connection to the iron or steel trade but he was a prolific inventor and convinced that a fresh perspective, untrammelled by worries about details of the existing day-to-day steel-making processes, might make possible the change needed to meet the growing demand for steel.

In the early 1850s he set about studying metallurgy and, after months of consultation and experimentation, set on the possibility of decarbonising cast iron by blowing air into the molten metal.  His early experiments were encouraging and in 1856 he applied for a patent to cover his new invention and announced it in a paper – Manufacture of Malleable Iron and Steel Without Fuel – read to the British Association in Cheltnham on August the 11th 18564)the paper was also published in The Times a few days later – The creators of the age of steel, W T Jeans, 1884.   At this time, the chemical analysis of steel was still a new science, but Bessemer had gleaned that when pig iron, which typically contained around 5%  carbon, was heated to a white heat the carbon in the metal would combine with oxygen in the atmosphere and boil of as a gas.  His great insight was that if he blew air, and thus oxygen, through the molten metal then the carbon and oxygen would have more opportunities to come into contact and the reaction would accelerate.

His provisional solution was to create a circular vessel about 5 feet high by 3 ft wide and to use an engine to blast air under high pressure through the melted metal from underneath.   Reports of the initial experiment in 1856 were rather dramatic:

The primitive apparatus being ready, the engine was made to force streams of air under high-pressure through  the bottom of the vessel, which was lined with fire-clay, and the stoker was told to pour the metal when it was sufficiently melted in at the top of it.  A cast iron plate one of those lids which commonly cover the coal-holes in the pavement  was hung over the converter; and all being got ready, the stoker in some bewilderment poured in the metal.

Instantly out came a volcanic eruption of such dazzling coruscations as had never been seen before. The dangling pot-lid dissolved in the gleaming volume of flame, and the chain by which it hung grew red and then white as the various stages of the process were unfolded to the gaze of the wondering spectators. The air-cock to regulate the blast was beside the converting vessel, and no one dared to go near it, much less to deliberately shut it. In this dilemma, however, they were soon relieved by finding that the process of decarburisation or combustion had expended all its fury; and, most wonderful of all, the result was steel.The creators of the age of steel, W T Jeans 1884

At this point in the development of steel there was no firm agreement on the chemical difference between iron and steel, but using todays terminology we would say that Bessemer had succeeded in creating a purified form of iron, since his process was very effective at removing the carbon from the melted pig iron and, by doing, so he imparted the metal with some of the malleability and toughness of wrought iron (which as we saw earlier is also a low carbon form of iron, having been decarbonised in a puddling furnace).  This was a important step forward, making possible the rapid creation of larger volumes of malleable iron compared to the previous technology, but even so he faced great difficulties getting the technology accepted.

In additional to the inevitable scepticism and conservatism of the incumbent steelmakers,  initial attempts by third parties to repeat Bessemer’s experiment were disappointing, creating metal that was either brittle when cold or when hot, or both.  With no one – including Bessemer – able to explain the failures, optimism for the prospects of the new technology quickly turned to despondency.

Bessemer eventually discovered that the poor results were caused by the use pig iron – sourced locally – which was high in phosphorus, an impurity that had been absent in the pig iron he used in the original experiment.   Happily his investigations coincided with the time when Swedish government lifted the ban on the export on their pig iron – which was naturally very low in phosphorus – and he was able to prove the point by repeating his demonstrations using this new cleaner raw material.

The creation of a cheaper alternative to wrought iron was a critical step forwards but Bessemer had a bigger goal – he wanted to produce inexpensive steel.

Bessemer persisted, no doubt encouraged by the enormous size of the opportunity ahead of him:  the average price of steel in Sheffield at the time was £60 per ton, but he could buy Swedish pig iron for £7 a ton and, if he could find away from making steel from this raw material with a 20 minute ‘blow’ in his convertor, then the rewards would be enormous5)op cit p55.

Years of further experimentation and some IP-related controversy later,  he eventually settled upon  a combination of iron, carbon and manganese that, when added to his ‘converter’, imparted sufficient carbon to make a mild steel with the properties of elasticity and toughness that were advantageous in the making of plates for ships, forgings, ordnance, rail tracks and the like.

Four years had elapsed since his first announcement and, needless to say, he had an uphill battle to convince the public that a process previously condemned by all and sundry as a failure was now capable of reliably converting cheap iron, costing only £6 or £7, per ton into to steel by blowing air through it his ‘converter’ for 20 minutes.   The claim was all the more startling given that the average price of Sheffield steel was £60 per ton and each ton of material took several weeks to make.

Such was the reluctance to take him seriously that even the handful of companies who had licensed his original patent neglecting to pursue the work any further6)op cit p66.

In desperation, Bessemer bought back the licenses and acquired land in Sheffield so he could begin making steel himself.   Over the next couple of years he released his new steel to the market at a significant discount to the products of the other steelmakers and gradually began to build a reputation for his invention.

in 1861 he persuaded the London and North Western Railway to try his steel rails instead of the wrought iron they had been using previously.  The tests were a great success, with the steel rails lasting many time longer than the wrought iron equivalents and thus Bessemer at a stroke had created a huge opportunity (there were 25k miles of wrought iron rails that needed replacing).

By 1863 Bessemer was targeting the next largest consumer of wrought iron and steel after the railways: the ship builders, and the first Bessemer steel ship set sail in 1864.  In the midst of the developments in shipping and railways, large makers of ordnance and boilers also started to use the new steel.

Before long it was difficult for the traditional steel makers to ignore Bessemer’s success, and they began to license his technology.  Bessemer, clearly a canny businessman, was delighted to do so on condition they paid him a royalty of £2 for every ton of Bessemer steel they produced.

By 1865s there were 17 bessemer steel works, producing 1200 tons per week.  From this point onwards Bessemer’s invention went from strength to strength and was an unequivocal success, the scale of which is captured nicely in an this illustration:

 

The picture shows the annual production of Bessemer converters by 1892 – 4m tons (80x the entire steel production of Sheffield in 18517)Sir Henry Bessemer, F.R.S. : an autobiography

This was to prove to be the high point for Bessemer steel, with competing technologies for large-scale steel production invented by Siemens coming to Sheffield before the turn of the century, but he had succeeded in almost entirely displacing the needed for large scale production of crucible steel.

However,  despite the huge success of this much cheaper alternative, the crucible process continued to dominate the tool steel market and, surprisingly, demand in other areas actually increased during the latter half of the century.   Henry Seebohm in a controversial paper presented to the Iron and Steel Institute of Great Britain in1884 offers a robust explanation for this rather surprising situation:

The accumulated experience of a century has convinced the manufacturers of crucible cast steel that the finest qualities can only be made from bar steel which has been converted from iron made from Dannemora ore. This iron is expensive, its average cost for the last forty years has been at least £25 a ton, the process of converting it into steel is slow and costly the process of melting in small crucibles is extravagant both in labor and fuel and subsequently the best qualities of crucible cast steel can only be sold at a high price.

So called best crucible cast steel is sold at low prices by unscrupulous manufacturers and bought by credulous consumers but though it is quite possible for high priced steel to be bad it is absolutely impossible that low priced steel can be good.  The finest quantity of steel cannot be made of cheap material or by a cheap process. Every year the attempt is made and every year it signally fails.  No one ever made a better try than Sir Henry Bessemer but his failure was as complete as that of his predecessors.  He attempted to produce an article at £6 a ton to compete with one at £60 a ton and failed absolutely.  It is true that his steel was a success perhaps the most brilliant success of the century.

I am not quite sure that he himself believes in his failure.  In his lecture before the Cutlers company of London in 1880 he chaffed the steel manufacturers of Sheffield on their antiquated attachment to the rule of thumb and twitted them with the assertion that the high price of crucible cast steel arose from a combination of trade interest on their part and of prejudice on the part of their customers.  Sir Henry Bessemer may have half ruined the wrought iron trade and revolutionised the pig iron trade but the crucible cast steel trade holds its own in spite of his great discoveries.  When railways were first introduced and waggons and coaches to a large extent driven off the road many people thought that the price of horses would permanently fall but exactly the contrary took place.  Similar fears were entertained that the demand for crucible cast steel would seriously decline when Bessemer and Siemens steel came into the market. This has not been the case.  The commoner qualities of crucible cast steel have been to a large extent suppressed by Bessemer’s and Siemens steel but the enormous quantities made by the latter processes have required for their manipulation directly or indirectly such a large quantity of the better qualities of crucible cast steel that the total amount of the latter now produced in various parts of the world is probably double that which was required before the birth of its rivals steel iron. Henry Seebohm. The Engineer, Volume 58, September 26 1884

 

During the subsequent lively discussion Bessemer provided a stout defence arguing again that it was the traditional steel makers resistance to change that hindered adoption of his steel for requirements like tool making but, despite his protestations, it was apparent that Sheffield makers had experimented with his metal and simply got better results from crucible steel8) for example Jessops and son report that they had  experimented with melting bessemer iron in their crucibles but found it was ‘a false economy’ compared with Swedish bar: The engineering and Mining journal

It is hard but not to sympathise with Bessemer, fighting as he was with an intrenched industry, but it is also clear that crucible steel was genuinely superior for certain applications, including edge-tools.  The debate about why this was so was not satisfactorily resolved in Bessemer’s lifetime, largely because at the time chemists lacked the tools to accurately analyse all aspects of the two metals and were forced to resorting instead to vague terms like ‘body’ which went some way to explain the prejudices of the traditional steelmakers but did little to advance the understanding.

Thus the 150 year old crucible steel lived on, and as we shall see, it lived on surprisingly long time.

Bessemer Convertor at the Kelham Island museum, Sheffied.  Donated by the Workington Iron company and last used in 1974 , By cooldudeandy01 [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons


References   [ + ]

1. Metallurgy of iron and steel: practical and comprehensive studies … by Samson Jordan
2. Engineer, Oct 25th 1867
3. c.f Edge tools – iron and steel – part I (Wrought Iron
4. the paper was also published in The Times a few days later – The creators of the age of steel, W T Jeans, 1884
5. op cit p55
6. op cit p66
7. Sir Henry Bessemer, F.R.S. : an autobiography
8. for example Jessops and son report that they had  experimented with melting bessemer iron in their crucibles but found it was ‘a false economy’ compared with Swedish bar: The engineering and Mining journal

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