Science of Alloying
|What Can be Done|
|What, exactly, do you expect from the "science" of alloying? Well - it should at least be able to answer all questions coming up in this context. If you think about for this a bit you realize that the issue boils down to two basic question:|
do the properties of my system change if I
add an amount x of element y and process with recipe z.
2. How do I have to compose a system and find the right processing to get the properties I want?
|The first question asks for analysis, the second one for synthesis. The second one is much harder to answer then the first one. Let's look at some examples.|
| Example for the first
My first example concerns you, the ancient smith. Your system may be "ideal" plain carbon steel, say iron with 0.4 % carbon (C). Your process is to forge it around
Now you wonder how properties like hardness and so on change when you add 0.2 % phosphorus or 0.15 % sulfur (S). That's a rather good question of the type 1 that you, the ancient smith, could have asked because it relates very much to what you were doing then. You probably would have been satisfied if someone would have told you what would happen with respect to just a few properties like hardness and cold / hot shortness.
Of course you, the modern reader of this hyperscript and fully conscientious of the claim I made early on, want to know more. You want to know what would happen with respect to a lot of properties and in particular why that happens.
Can I answer these questions? No, I can't. I'm not a steel expert, remember? I'm fairly confident, however, that there are scientists out there who could tell you what is going to happen because they can actually calculate it. They don't have to know from experience or try it experimentally. They can calculate it because the theory is there. Maybe the computer power isn't quite up to the job right now but it will be there in the fullness of time.
The question has a definite answer, and we are going to produce it - now or later.
|Example for the second
My second example concerns a modern engineer. Working for Audi, she might ask what kind of aluminum (Al) based composition would keep all the good properties of the aluminium alloy she is using right now for the body of the more expensive Audi cars, but would add easy weldability to the list of desirable properties. Her present aluminum alloy can't be welded, causing major cost issues.
A definite answer to that question would be the death knell for that part of the steel industry that makes steel for car bodies. So her colleague over at the steel plant might ask a question along a similar line: what composition (plus process recipe) would make a better steel that could compete against the new aluminum alloy.
Both engineers supply a long list of detailed properties they need, plus some requirements as to costs to you, the scientist in charge.
|Can you or anybody else give them a definite answer? No you can't. There are no easy positive answers to their questions. But maybe the answer will be "Can't be done, ever!" Let's see why.|
|It is rather simple to come up with a wish list
of properties, where the scientist answers right away: "can't be
done". For example, for rather well-known reasons, a single material from
which you can make a solar cell with an efficiency of 50 % does not exist.
Never ever. Theory is clear on this.
But how about a material with 18 % efficiency at a cost of 2 $ per kilogram? This is a question that no scientist can really answer because money and costs are not part of science. There is no way to compute scientifically, for example, what the cost of tellurium (Te), will be in 15 years. That is big topic in solar cell business, by the way. Maybe it is possible (theory doesn't forbid it), maybe it's not. No way of telling. At best you can give probabilities.
|It is one thing to calculate the properties of
something from first principle. It is perfectly possible for example to
calculate what kind of X-ray diffraction pattern would result from a given
arrangement of the building blocks of DNA. Given the structure, you can
calculate the diffraction pattern.
It's a completely different thing to calculate what you have to assemble to obtain a certain property. The reverse of the example above, calculating the structure from a given diffraction pattern, is more or less impossible. In strict math terms: it is always possible to calculate the value of a definite integral for some function, even if it is a "difficult" one. The result is a number. It is impossible, however, to calculate backwards and derive this function from a given number. The reason is simply that an infinitely large number of functions, if integrated, could give the same number.
Questions of the second type fall into this category to some extent. Sorry. But see the positive side of that: Materials Science will never come to an end. Computers somewhere and sometime will be able to give answer to all your questions of the first type but not to questions of the second type. Materials scientists, however, will find answers on occasion because they can do more than just pure math.
|The long and short of this is simple.
If you supply a list of desirable properties for the steel you want, the answer
might be that it can't be done. But even if your list does not include the
impossible, nobody will be able to sit down and calculate the necessary
composition and the processing that will do the trick.
Now that we know what we can't do, let's see what is possible
|Classification by Influence on the Iron-Carbon Phase Diagram|
|The science of iron alloying must start with binary phase diagrams for all sensible binary iron - element x compositions. Non-sensible compositions for example, are iron - noble gases for all concentrations and temperatures or the x-rich end of the iron - element x phase diagram.|
|The good news is that we can actually
calculate all those phase diagrams pretty
well and in the not too distant future we will be able to do this really well
(partially due to the efforts of your's truly and colleagues with silicon and
microelectronics, giving us computers with ever increasing power).
Now that we know what kind of phases we are going to encounter with binary alloys (e.g. Fe3P, Fe2P for iron - phosphorus systems; Fe2N for iron - nitrogen), we need to know about moving atoms around in those phases. How often does an atom /vacancy jump per second? That's harder to calculate - but we will get there.
|But let's not get carried away. Even
if we can do all there is to do for the binary system iron with just one kind
of alloying element, we will not get very far with complex systems, e.g. iron
with 5 or more alloying elements if we try to calculate everything possible
from first principles. There is simply too much that could happen. Most of what
could happen will not happen, but you have to do very precise math on occasion
just to figure that out. It is far easier to figure it out some other way, even
if that involves some "intelligent" guessing plus experience. Rules
then can be derived that may not always be 100 % correct but at least most of
the time. One such set of rules comes from the insight that the phase diagrams
of most alloy elements and iron fall into just 2 major categories with two
Keep in mind for all comparisons that for very small concentrations you have almost pure iron with its two phase changes from an a bcc structure to a g fcc structure at
|There are two major ways to do that,
and two sub-groups for each basic possibiltiy. The two major ways are.
|Let's look at some examples:|
Expanded g-field systems.
That's the kind of phase diagram we know because carbon (C) and nitrogen (N) are the most important elements in this group. The g-phase field is expanded, but its range of existence is cut short by compound formation.
The expansion of the g-field by carbon (or nitrogen) is at the core of heat treatments of steels since it allows the formation of a homogeneous solid solution (austenite) containing up to 2.0 wt % of carbon (or 2.8 wt % of nitrogen).
Copper (Cu), zinc (Zn) and gold (Au) have a similar influence.
| Class 2:
Open g-field systems
To this group belong the important steel alloying elements nickel (Ni) and manganese (Mn), as well as cobalt (Co) and the inert (noble) metals ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) and platinum (Pt). These alloy elements are also known as g stabilizers.
Both nickel and manganese, if added in sufficiently high concentration, completely eliminate the bcc -iron phase and replace it, down to room temperature, with the g-phase. So nickel and manganese depress the phase transformation from g to a to lower temperatures. It is thus much easier to obtain metastable austenite by quenching from the g-region to room temperature, consequently nickel and manganese are useful elements in the formulation of austenitic steels.
|The paradigmatic g stabilizers producing open g-field systems are nickel (Ni) and manganese (Mn). Their phase diagrams with iron are practically identical for the iron-rich side:|
|Next we have the two subgroups of the a-stabilizers:|
Closed g-field systems
Many elements restrict the formation of g-iron, causing the g-area of the diagram to contract to a small area referred to as the "gamma loop". This means that the relevant elements are encouraging the formation of the a phase or bcc iron (ferrite). One result is that the d and g-phase fields merge and become continuous, completely enveloping the g region.
Closed g-field systems are therefore not amenable to the normal heat treatments involving cooling through the g/a-phase. transformation.
Silicon (Si), aluminium (Al), beryllium (Be) and especially phosphorus (P) fall into this category (phase diagram here), together with the strong carbide forming elements like titanium (Ti), vanadium (V), molybdenum (Mo) and chromium (Cr).
| Class 4:
Contracted g-field systems.
Boron (B) is the most significant element of this group, together with the carbide forming elements tantalum (Ta), niobium (Nb) and zirconium (Zr).
The g-loop is strongly contracted, but is accompanied by compound formation (the white unspecified fields).
|Now for a few rules and examples.|
| Although only the binary systems have been considered so far, we can
predict to some extent what will happen if we now add carbon. We're looking now
at a ternary system where the same general
principles usually apply. For a fixed carbon content, as the alloying clement
is added the g-field is either expanded or
contracted depending on the particular solute. Some examples:
|Normally, elements with opposing tendencies will cancel each other at appropriate concentrations. In some cases, however, anomalies occur. For example, chromium (Cr) added to nickel (Ni) in a steel in concentrations around 18% helps to stabilize the g-phase, even so it is listed under the "closed g phase elements". This "Ni-Cr anomaly" is used in the class of (18% Cr 8 % Ni) austenitic steels.|
|This is all pre-war stuff. Even then it was clear in principle how one could go beyond qualitative rules and get quantitative results. The only problem was that doing the necessary math was impossible. Present-day computer-based methods overcome these difficulties and the science of alloying is becoming a true quantitative science, able to make precise predictions.|
|Classification of alloying Elements by "Stabilities"|
|Here we look on the effect of an element on the stability of the important phases in steels containing some carbon. This involves in particular:|
which tend to form carbides. In other words, elements that produce
stable carbides rather than compounds with iron. Of course, this implies that
there is some carbon around.
We have essentially chromium (Cr), tungsten (W), titanium (Ti), niobium (Nb; sometimes also called columbium), vanadium (V), molybdenum (Mo) and manganese (Mn) as carbide formers. The mixture of the complex carbides formed whenever those elements are present is often - wrongly - referred to as "cementite". The reason is that the carbides may contain both metals - iron and the alloying element - e.g. (FeCr)3C, (CrFe)3C2 (CrFe)7C3 or (CrFe)4C, to name some iron-chromium carbides.
If we have several metals in the mix, there is a competition for which one gets the carbon.
which tend to graphitise the carbide.
We have Silicon (Si), cobalt (Co), aluminium (Al) and nickel (Ni). Only a small proportion of these elements can be added to the steel before graphite forms during processing, with attendant ruin of the properties of the steel, unless elements from group 1 are added to counteract the effect.
|3. Elements which tend to stabilize austenite.
Prominent are manganese (Mn), nickel (Ni), cobalt (Co) and copper (Cu). These elements alter the critical points of iron in a similar way to carbon by raising the A4 point and lowering the A3 point, thus increasing the range in which austenite is stable, see above. This will lead to austenitic steels in the end. They also tend to retard the precipitation of carbides.
Most of these elements have a fcc crystal lattice like g-iron, and thus tend to be more soluble than in g-austenite than in a-ferrite. Sitting as a substitutional foreign atom in austenite at least preserves the kind of environment the atom is accustomed to.
| 4. Elements which tend to stabilize ferrite.
Chromium (Cr), tungsten (W), molybdenum (Mo), vanadium (V) and silicon (Si) belong to this group. Most have a bcc lattice and thus tend to be more soluble in a-ferrite than in g-austenite.
They diminish the amount of carbon dissolved in the austenite (because there is now less austenite) and thus tend to increase the volume of carbides in the steel for a given carbon content. They lower the A4 point and raise A3 (although it may be lowered initially), until the two points merge to form a
No phase transformation occurs and such steels (e.g. 18% chromium irons) thus cannot be hardened in the usual way. You harden by adding austenite stabilizers from group 3 above.
For example, 2 % of nickel added to a 18 % chromium stainless steel allows "refinement" by normal heat-treatment; carbon has the same effect.
|You realized, I assume, that in parts the information given is the same as in the first system, just expressed in different words.|
|A few Illustrations, Examples and Graphs|
|Let's look at few illustrations now and at an ingenious way to deal with multicomponent steels without resorting to (typically non-existent and otherwise completely unpractical) multi-dimensional phase diagrams.|
|Here is a figure that should be familiar. It gives the hardness of a steel alloyed with the elements shown. It is assumed that the elements are atomically dissolved, so we have solid solution hardening. In the link a similar picture was shown; this one just gives a bit more information.|
|In all cases the transformation from
austenite to ferrite takes place; the measurements were done in the ferrite
phase at room temperature. Obviously you want to go for manganese (Mn) if
hardness is all that counts.
A picture like this, based on extensive experiments, often tells you more than what is visible "at the surface". For example, the silicon (Si) and molybdenum (Mo) lines end between 1 % and 2 %. That simply tells you that it is not possible to keep larger concentrations in solid solution. It also tells you that for all those elements and concentrations, you still have the g ® a phase transformation. Noticing that, natural questions are coming up:
|Some answers to these questions are given in the next figure. It shows how the temperature A1 (upper figure) and the composition (lower figure) of the eutectoid point changes if various elements are alloyed to a carbon steel.|
|To give examples on how to read these
curves: Alloying 4 % molybdenum (Mo) increases A1 from its
"normal" carbon-steel value of 723 oC (1.333
oF) to almost 900 oC (1.670oF), and decreases
the eutectoid composition from 0.8 % carbon to about 0.2 %. Adding
|There is a lot of information in
these curves. Titanium and others (Mo, Si, W) seem to stabilize the ferrite
since the transition to austenite occurs at higher temperatures if these
elements are present.
Manganese (Mn) and nickel (Ni) stabilize the the austenite.
That's exactly what has been stated above - now we have data. There is much more information in the details of the curves but I will stop at this point.
|Now let's look at a rather ingenious
(if approximate) way to deal with steel that contains many different alloy elements. Anton L.
Schäffler published something like the following diagram in
Nov. 1949; W.T. De-Long improved on it in 1974; that's why it is
sometime called the Schäffler -
De Long diagram. There are two essential insights:
|Three kinds of steel are drawn in
either as as red cross or red lines. The percentages at the black lines
indicate the level of ferrite formation.
Let's look at the examples:
|There are other diagrams like the Schaeffler diagram; they all are helpful but must be treated with a bit of caution. For example, the austenite-stabilizing effect of manganese is nowadays considered to be smaller than that predicted in the Schaeffler-Delong diagram. Its effect is also dependent on temperature.|
|It's time for a few more insights and
rules. If we restrict ourselves to steels with some carbon and process in such
a way that the austenite transforms to ferrite and carbide (i.e. slow
cooling), so we leave enough time for alloying atoms to move around, the
alloying elements can be divided into three categories:
|In the first category there are elements such as nickel (Ni), copper (Cu), phosphorus (P), and silicon (Si). These elements are normally found in solid solution in the ferrite phase, their solubility in cementite or in alloy carbides being quite low.|
|The majority of alloying elements
used in steels, however, falls into the second category. Those elements are essentially
carbide formers. At low concentrations they form solid solutions in ferrite but
also go into solid solution in
cementite. At higher concentrations most will form alloy carbides,
which are thermodynamically more stable than cementite. Typical examples are
manganese (Mn), chromium (Cr), molybdenum (Mo), vanadium (V).
The carbide-forming elements are usually present in concentration far in excess of carbon. When the available carbon is used up for carbide formation, the remainder enters into solid solution in the ferrite, together with the non-carbide forming elements like nickel (Ni) and silicon (Si), if those are around. Some of these elements, notably titanium (Ti) , tungsten (W), and molybdenum (Mo), then produce substantial solid solution hardening of ferrite, see above.
|The third category contains only a few elements, which enter predominantly the carbide phase. Nitrogen (N) is the most important element and it forms carbo-nitrides with iron and many alloying elements. However, in the presence of certain very strong nitride forming elements, e.g. titanium (Ti) and aluminum (Al), separate alloy nitride phases can occur.|
|Finally, a few words to the structures one can expect:|
|Adding elements like manganese (Mn),
silicon (Si) or nickel (Ni) that do not
form carbides (or if they do cannot win against the competition of cementite
formation), does not basically alter the microstructure of plain-carbon steel
as formed after transformation.
In the case of strong carbide-forming elements such as molybdenum (Mo), chromium (Cr) and tungsten (W), cementite will be replaced by the appropriate alloy carbides, often at relatively low alloying element concentrations. The microstructure is thus quite different from that of plain carbon steel.
Very strong carbide forming elements, such as niobium (Nb), titanium (Ti) and vanadium (V), always form alloy carbides, preferentially at alloying concentrations less than
|Cementite is the only carbide in
plain carbon steel. The wealth of
structures found in plain carbon steel results exclusively from the high
solubility of carbon in the g-austenite and its low
solubility on the a-ferrite. If we now have many possible carbides to play with, we first need
to know if they form already in the austenite or later in the ferrite.
What was found more or less empirically but is more and more based on calculations, is:
|An Application to "Wootz" Steel?|
|I could go on like this for quite a
bit but will stop now, except for a last remark that may explain some of the
mystery about wootz steel.
The question we ask ourselves is: "Where do all those metal carbides nucleate when they first start to form?". The enticing quote from the "key to metals" article cited above is:
|"Interphase precipitation has been shown to nucleate periodically at the g/ a interface during the transformation. The precipitate particles form in bands which are closely parallel to the interface, and which follow the general direction of the interface even when it changes direction sharply."|
|Now consider a slowly solidifying piece of wootz steel in its crucible. It is an hypereutectoid steel with an extremely high carbon concentration around 2 %, and it contains a bit of strong carbide formers like vanadium (V). When it cools down and crystallizes, the g / a interphase is somewhere between the still liquid part and the final pearlite / cementite mixture. If the geometry is as shown below, you might get carbide nuclei arranged in sheets as shown.|
|So we might find Vanadium-carbide
particles in "bands", i.e. in a
more or less parallel to the surface of the final wootz ingot. This is
schematically illustrated above. These banded or striated tiny metal-carbide
particles will supply the
nuclei for forming
cementite particles later.
If the smith is very careful in preserving the striated structure in the blade he forges. he can obtain the characteristic "water" pattern of a wootz sword, essentially showing a cut through the still striated but "wobbly" metal carbide position now expressed in "large" cementite precipitates. This is outlined in detail in chapter 11.5
Far more could be said to this (especially to the "why" questions coming up now), and I have actually said something rather speculative to the issue. The geometry shown is rather unlikely to occur without special measures.
General Remarks to Literature and Sources
Science of Welding Steel
Overview of Major Steels
9.1.1 Things are Complicated
Phosphorous Steel; 9.4.1 General Remarks
9.2.2 Designing Low Alloy Steels
Alloying Elements in Detail
9.5.2 Kinds of Cast Iron
Jominy Test and Hardness Depth
Smelting Science - 1. Furnaces
Overview of Major Steels: Scientific Steels
11.5 Wootz Swords; 11.5.1 The Winner is....
11.5.2 Structure by Dendrites?
Segregation at Room Temperature
8.2.2 Tempering and Ostwald Ripening
Overview of Major Steels: 1. Classifying Steels
9.2.1 A Closer Look at Low Alloy Steels
Overview of Major Steels
High Alloy steels; 9.3.1 Stainless Steel
The Iron Carbon Phase Diagram
Segregation at Room Temperature
High Alloy Steels and Schäffler Diagram
Microsegregation and "Current Burst" Theory
Alloying Elements and Properties of Steel
Phase Diagram of Fe-P and Fe-N
Ghost Structures in Phosphorous Steel
© H. Föll (Iron, Steel and Swords script)