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What happens when we harden and temper steel.... - richardkell
What happens when we harden and temper steel....
First sentence of the first google result couldn't put it better, I quote ....

"A study of the constitution and structure of all steels and irons must first start with the iron-carbon equilibrium diagram."


from http://steel.keytometals.com/articles/art153.htm

It cannot be done any other way.

For City and Guilds Prod Eng Technician second year with Mr Clarke, these being my first ever sessions at Ashington Technical College it was required to learn, retain and draw from memory with explanations for the T2 exam the above diagram tho thankfully in simplified form as per below; it was what Mr Clarke termed a 'chestnut',  you got it in the exam every year.

I've used a handwritten and simplified sketch as that is the way you work, its not just something set apart in books or on the blackboard, your hand and memory must to able to draw it, to understand it. I would prefer to have Mr Clarkes actual version for this as I trust him, though dear knows where my Tech files are and the above is very close as to how I remember it. 

Its shows us how carbon content and temperature affect the chemical composition of steel, by hardening and tempering we are attempting to catch a certain changed part of it, it shows why not all iron-carbon combinations can be hardened by simple techniques or hardened at all, it is Austenite that interests us, hence the max point of 1.7% carbon if we are to obtain a hardened steel.

To 'freeze it in time' (this chemical change at elevated temperature) would be a good analogy, as in fact we seem to be doing by quenching.  As the books tell us, we are studying the 'allotropy of iron' ie a substance that can exist in more than one physical form. There are chemical changes occurring within steel that have been mapped and revealed by scientists and metallurgists certainly more than a hundred years ago; some are named after actual people ie Martensite; all of this to me is a fascinating subject.

There are some very interesting points to realise, for instance what happens at elevated temperatures and the incredible transformations happening therein, chemical and atomic;  pure iron (ie near zero carbon content) at room temp is termed a 'body-centred cubic lattice' ie the atoms are in the imaginary centre of the atomic lattice structure (termed alpha iron)  whereas above the magical 900 degrees C they become 'face-centred cubic lattice' (ie gamma iron) with an atom at each corner and at the centre of each face.  But its not necessary we know this to perform heat treatment, merely what chemical transformations take place, the names of the bits we are trying to capture and how best to make the better functioning, more durable and appropriate tool.

There is too much needed to provide a full and proper explanation on a blog post and here we only deal with the most simple plain carbon steel, something never actually used as alloy steels are near universal, so I'll just throw bits in and hopefully provide some insight and encourage others to learn more.

In some ways theres a story attached to this........

Once I had finished school in 1974 I enrolled myself into Ashington Technical College, my Dad had feet of clay on this but my Headmaster was adamant I grasp this opportunity. Part of our problem is that we were totally new to all this, not only day release technical education but the world of organised work even, how factories are organised, people trained, qualifications; we lived in a very rural and remote backwater, a tremendous handicap in many ways.

I was actually on the dole, unable to get digs for the first firm I contacted for a toolmaking apprenticeship here in the North East of England after being let down by a one-man band in Suffolk and after three months persuaded Hardy Bros of Alnwick to take me on. It was a bit of a longwinded trek each week for day release, those that lived nearby had no idea how lucky they were or those people with cars. From my experience on the six year day release City and Guilds Prod Eng Technicians course I have nothing but praise for C&G and the Technical College itself. We had top-notch lecturers, men like Mr Varah, Mr McCormack and for this topic here Mr Clarke took us for I think 'workshop technology' and being thrown in at the second year I suddenly found myself faced with maths and science I had never encountered before; also it was quite an education in other respects, all the other students were older.

I live in a house here where I am interested in information, books, to learn, to see wider. I am a light hearted person to live with, but I cannot stand to have my natural sense of 'being alive' to be throttled by dull or sullen people around me. I remember returning back say 9pm from the first day at Tech and I cannot remember if it was the Iron-Carbon Equilibrium diagram we had studied all morning or the Jominy End Quench hardenability test with Mr Clarke but I do remember having the most ferocious heated row with my dad over whatever we had spent three hours on that morning, I probably got that crap every time I returned. I thought and still do that this information is invaluable, its a window onto a wider and better way of seeing, I suppose its called education. But particularly for a kid whose dad had a home workshop for wood and metal, Myford bench lathe, moulding planes, taps and dies etc to reject this more advanced tho quite basic information was pure madness.

I remember the first day on vector diagrams in the Maths session, with a lecturer I cannot remember the name of, I'd missed the first week and been put in at the second year and was utterly bamboozled by these lines and directions of force, which is all they really are, resolving into a triangle of forces. This was probably compounded by the metric usage of newton metres when all I knew was pound feet, the latter preferable even now for getting a grasp of what really is there. Vector analysis should be more widely known, certainly has application in practical woodworking.

You must realise, if I had a child I would love to sit and see what they were doing, can I learn too ... but not my dad, going home with these lecture notes brought nothing but derision, anger and ridicule. Anything pertaining to algebraic notation, equations and incorporating letters and numbers was akin to the devils work! The real world of work to his mind was arse deep in shit and diesel and brake fluid, he loved it!!  I'm not sure what we would do the first week with Mr Clarke, possibly the iron carbon equilibrium diagram as I can see in my minds eye it being first in my written notes, certainly the iron-carbon and the jominy would draw equal venom, shouting, screaming, throwing things; thirty eight years later I can see it must have been good stuff, wow he couldn't handle it, pure jealousy, I rattled his cage!  I remember trying to get the point across that in big manufacturing concerns, say a car plant and all its satellite suppliers there has to be this more scientific and controlled approach, steel is more than just bright mild, silver and high carbon with wrought iron and cast iron on the far ends of carbon content.  Engineering production is all about control and this is part of it.

Brief points.....

Looking at several books here from the shelves its a real walk down memory lane, if there were enough hours in the day I should like to re-read; in fact I find I'm re-reading as I write this post, cannot help it, as well as finding accurate material to draw from. 

Mis-information and sloppy writing really irritate me, for instance it has always 'pipped my noodle' when in the amateur press 'annealing' is used when in actual fact its normalising that is being performed, annealing in its correct sense is a very slow reduction from an elevated temperature.  

Note in the first diagram at the top of the page, in the iron-carbon equilibrium diagram the importance of 0.83%  carbon, at this point in simple terms steel becomes hardenable at a more 'accessible' more easily achieved temperature, becoming a high carbon steel that is hardenable, this has a limit though, note the limit of Austenite at 1.7% and above this we are moving towards cast iron. Note where wrought iron (at left) and cast iron occur along with carbon content ie at the extreme ends of the carbon scale.

Its useful too to have knowledge of the method of manufacture of the steel,  high quality edged tool makers of say the eighteenth and nineteenth century from what i remember reading would probably use 'cast steel' and in fact demanded it, this is a term or brand that would persist into the twentieth century. Crucible steel is another term, Rollason tells us the Huntsman crucible process now being superceded by the high frequency induction furnace; to learn about steel you have to get into the detail.

Here I copy and paste from American 'The Household Cyclopedia' of 1881......

The finest of steel, called English cast-steel, is prepared by breaking to pieces blistered steel, and then melting it in a crucible with a flux composed of carbonaceous and vitrifiable ingredients. The vitrifiable ingredient is used only inasmuch as it is a fusible body, which flows over the surface of the metal in the crucibles, and prevents the access of the oxygen of the atmosphere. Broken glass is sometimes used for this purpose.

When thoroughly fused it is cast into ingots, which. by gentle heating and careful hammering, are tilted into bars. By this process the steel becomes more highly carbonized in proportion to the quantity of flux, and in consequence is more brittle and fusible than before. Hence it surpasses all other steel in uniformity of texture, hardness, and closeness of grain, and is the material employed in all the finest articles of English cutlery.

end of copy and paste.

You cannot go back to that, modern day steels are far superior, but for low and middle market items its a cheaper material altogether they use for the worst of the soft cutting edges sold to woodworkers, the ones that cause all the upset, the complaints from woodworkers. From that we can assume it is material selection at fault due to pressure of price. I do a lot of hand turning with metals, using the graver and suchlike and it looks to me all my handtools are nineteenth century, but that is because many dozens of hand tools came to me with the Edward Hines ornamental turning lathe I bought at the age of just turned eighteen, it was the lathe of Jamie Maxtone Grahams grandfather, tho I was not to learn of that connection till years later.  With the graver you learn what a really sharp edge is and is periodically touched up on the Arkansas stones.

So, it seems for early workers the term cast and crucible steel may well have been interchangeable. You must realise for our crappy little low performance low and middle market modernday chisels, planes and irons that have started this discussion thirty, forty or fifty years ago there have been terrific, overwhelming constraints and pressures on price. I do think though at its worst some manufacturers lost sight of what it was they were making. What you pay your tool supplier is only a fraction of what is available to manufacture the item; so therefore its inevitable that over many successive occasions engineers and technicians have been given the job of lowering costs, to utilise lower cost (cheaper) materials, tho one is not the same as the other. Manufacturing companies sell their product by means of reps and commission agents, they work in a world of extreme commercial pressure.  So bang goes your laminated Norris iron with its crucible steel and softer steel lamination to a pressed and sheared thin plate with alloying elements to make the heat treatment as quick and low cost as possible, the technician merely having to prove that it will 'shave wood'.  A thin iron is actually preferable for the Stanley type plane, the cap iron fuctioning correctly to bed the blade to the body.

 In the rush to China, you cannot blame China, the western companies perceive that the customer wants a fixed or lower cost product therefore etc etc. There seems often hardly ever any philanthropic desire to retain a hardworking faithful workforce, a lot of companies seem now to operate as a brand with probably even R&D outsourced, its not only handtools but many products. What bugs me tho is the additional profits made by some firms that seem not to pass on the cost saving, this must be extraordinary to the point of embarrassing for them, all that money!!  The downside is that we lose the knack of making things, the insight and the slow build-up of knowledge and ability, firms pretend to design but are more often a little office outsourcing to their latest stylist ideas. Drawing from a smaller labour pool they will have hard problems finding the right men that at one time they would have been able to see them grow and develop on their own premises. 
At the lower sketch diagrams at left is illustration of points in the heating and cooling of plain carbon steel where arrest or critical points occur, the steel does not increase at a uniform rate even tho the furnace rise is uniform, similarly on cooling; the heat is used to bring about the change from alpha to gamma iron hence the 'joggle' in the temp rise at Ac1, at the Ac3 points the original temp rise is now back on track, the original gradient. After this recovery in temperature rise it will be passing thro the austenitic phase as per the iron carbon equilibium diagram at top of page.   Ac1 is fixed at 725 C and as per the iron carbon diagram and Ac3 depends upon carbon content. Interestingly 'A' stands for arrest and 'c' for chauffage ie French for heating, similarly the lower case 'r' is refroidissement, French for cooling. Between Ac1 and Ac3 is termed the critical or transformation range. There are also Curie or Ac2 points when the steel becomes non-magnetic.

From years ago it still remains in my mind as to the importance and usefullness of TTT curves, ie time, temperature and transformation; its these three factors that create the correct result, words to bear in your mind when one is performing heat treatment at whatever level, in my case its firebricks, propane flame and craft skill. I remember receiving a letter from the 'Model Engineer' contributor Tubal Cain say late 1988, he had read an article of mine in 'Woodworker' magazine re making my 'three tray hinged' container and where I mention making my own tapered reamers and cutters he picked me up on my supposed desire for too great a 'let down' from the hardened state ie tempered to medium straw whereas he himself would hold the temper to a more brittle palest straw. Perhaps fair comment but inappropriate to context, as I was working with silver steel which is a quite basic unsophisticated material (which I oil quench anyway) and when I make something as with hundreds of cutters and tooling detail (ie press tools, lathe form cutters etc) as per my own workshop and Hardys before it, once something is made I never want to see it again, ie it must go out onto the shop floor and function without failure, too light a temper would invite failure; so from that as LBSC would say 'nuff sed'.  My article in 'Woodworker' was aimed at the cleverer amateur worker and I wanted the most surefire way of getting him workable results, fancy tempering tricks or running risks come later. With a better quality material ie gauge plate and a light duty application a paler temper colour perhaps, but I don't think he had the experience I had or the amount of practical factory work I had, in fact I'm certain.

As E.C.Rollason says in the excellent  'Metallurgy for Engineers'  ..... "Hardening of steel is obtained by a suitable quench from within or above the critical range. This critical range is not uniform, varies according to among other things carbon content." Realise my quoting is a very over simplified treatment of the subject, I'm trying to make some salient points for those that are new to this.  To further quote "The quenching is necessary to suppress the normal breakdown of austenite into ferrite and cementite and to cause a partial decomposition at such a low temp that martensite is produced. For this to occur each steel requires a critical cooling velocity which is greatly reduced by the presence of alloying elements."  Note martensite from autenite being the key objective here.

Water is a dire quench, quenching oil being much preferable; old books will mention mercury (and even urine) for an even faster cooling, I remember when first starting at Hardys I was put to work for a retired man that came in part-time to keep active, a very good man indeed, Tommy Goodfellow; a highly knowledgable and skilled man. In our conversations mixed in with our work he showed me his lighter that he had made himself on night shift during the war, he had quenched the steel striking wheel in mercury, he would be tickled by my knowing this trick already, I had been reading Britten, Saunier and Holtzapffel and suchlike before I started at Hardys. I always tried to keep my learning quiet, no one likes to be upstaged by a kid. And of course 'book lernin' is not all there is to it, character and life experience are needed too.

So once we have achieved our martensite which is brittle and highly stressed it needs to be made tougher, and as put by Rollason .. "Retained austenite is unstable, and as it changes with time dimensions may alter" ... realise we are dealing with big maufacturing here, possibly high tolerance sophisticated components, big stuff .... " It is neccessary therefore to warm the steel below the critical range in order to relieve stresses and to allow the arrested reaction of cementite precipitation to take place. This is known as tempering." .... "The reactions of tempering occur slowly and time as well as temperature of heating is important"

I remember having one hell of an arguament with a specailist steel supplier about twenty years ago. I always stuck with two particular brands of gauge plate as I knew from experience how they responded to my heat treatment technique, but this new rep insisted I buy some material from him, which I reluctantly did and came to regret it badly. I was occasionally making air operated pre-form tooling for electronic components ie say bend one or more legs and crop all to length, on short order often with only a week to make and deliver; often ingenious work and a high craft level. This different version of gauge plate tho on the accompanying analysis label seemed the same, actually gave me strange and unpredictable temper colours as if in two sets, I wondered if it contained traces of alloying alements I was unaware of, it was weird material.  Perhaps there was a glitch with the hardening too? I showed the rep the results, he would not budge or accept my views and this would be on components that already embodied valuable man hours, often with little time to spare. I had another rep from another steel suplier that was so bloody ignorant as well as not being able to take all signals of  'go away'  he insisted basic 'silver steel' was whats termed 'O1' ie an oil quenching (to me ) low distortion gauge plate and all during this he is wasting my time while I'm engaged on price-work; add your own expletives ..... 

What a lot of this book-gleaned information gives us is an insight and realisation what is happening with these bits of metal when we are instructed to 'heat to cherry red and quench, temper to straw' .....to realise what is occuring within the bits of metal we intend as tools and the transformations that must occur. Without this naming of chemical constituents, changes occurring and without these diagrams and importantly knowledge of austenite into martensite we are totally and utterly in the dark.

I am giving all this information which is really quite elementary as there seems such a paucity (lack) of knowledge in some of the writing I read out there aimed at the amateur.  I really need a lot more space and time to do this basic intro justice, but theres sufficient here to have offered an insight.

Also consulted here from my bookshelves and used for all the diagrams, a very good metallurgy book, slightly more elementary than the Rollason but a good text all the same .... 'Materials for Engineers' by M.H.A Kempster, he lectured the preface tells us at Rolls Royce Technical College. I also have Kempsters 'Principles of Jig and Tool Design' an utter classic. Even the way it is illustrated, makes it very easy to memorise and regurgitate with pen and paper. Drawing and sketching is so very important, it trains the memory, develops minds eye visualisation and hand-eye co-ordination, factors seemingly lost in the more 'modern' education offered at schools these days. I always used to judge an engineer as to his sketching, if he couldn't get his ideas across on paper then he wasn't worth very much, after all we are sometimes dealing with ideas more complex than a few mere words can describe in the noise and pace of the workplace. Funnilly enough when I turned up at Hardys in Dec 1974 for my interview and to be shown around I had with me in my coat pocket a copy of Kempsters 'Principles of Jig and Tool Design'  .... but no-one wanted to see it.

Heres wiki for the great Adolf Martens, Kempster describing as one of the founders of modern metallurgy ... http://en.wikipedia.org/wiki/Martensite

http://en.wikipedia.org/wiki/Benjamin_Huntsman .. (born 1704) read how the starving beggar conned his way into his Works.


These images as usually with all pics I post can be clicked three times to get a higher definition.

These are all bits and pieces I have made as and when required, I've lots more scatterd about my workshops. Form tools and cutters to make particular components and current products I keep in small self seal plastic bags along with small sketch notes and a sample of the component, a system that helps beat fading memory. Lots for development type work and odd custom specials are pictured above. Pictured at right are close-ups of interesting examples of craftsmanship, starting at left ...

a tapered reamer for the tapered holes in my 'three three hinged' container with its three internal boxwood stacking trays.

next a broach to make a brass washer with an internal protruding tab.

a tiny broach to produce a square hole from a prior drilled hole; hole and produced AF of square is 0.125 inches, sharp corners.

another broach to produce two internal flats on a quarter hole, broach for 0.312 to a .375 internal square, rounded corners.

extreme right a coarse acme tap, to tap brass. 

All are silver steel hardened and tempered with propane and firebricks. The broaches particularly can be severely stressed as they are forced through the component in the flypress, in my case brass or aluminium; points to note are minimum tooth form to create max strength yet sufficient tooth space not to jam or clog with swarf and all teeth have a clearance angle as per any milling cutter or lathe tool. The finish these things can achieve is both accurate often remarkable. Measuring them with a micrometer for writing this text I see I am a more accurate worker nowadays than when I was younger, all of these were made many years ago.

The broaches are quite interesting as I remember I conciously used Kempsters 'Jig and Tool design' as guidance for this, there would be utterly nothing in the amateur press aiming at this level. They are made by first turning a tapered blank with spaced cutting teeth (even on the eighth square version) and feeding in a tool at say five degrees from ninety on the lathe topslide, thus giving each tooth a positive rake. From there the form is machined and filed as required. Its all with hand files, no way would you ever allow an abrasive paper near these to create a 'smooth finish' all genuine accuracy of tooth form and sharpness would be obliterated, shiney and rounded does not mean accurate. Its a bit like good quality gun work, its all handfiles and integrity.

The other items in the wooden blocks are various forms of pin drill, counterbore, milling cutter, form drill, rivetting and forming punches, drifts hex and otherwise and 'd'-bits all for specific sizes and applications. I've other blocks elsewhere.

Here is a link to a film on my youtube channel describing and showing these custom cutters...... http://youtu.be/BM670mOlbw0

To clarify, I use this blog to post articles and information that I hope are of interest to others, anything to do with tools, craftsmanship and workshop things, my main selling website with paypal facility is managed by my web-man Alan Culpitt ie for my honing guides and woodworkers bevel gauge, dovetail markers, etc ... http://richardkell.co.uk/honingECom.htm

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