Alloys in steels.

What are steel alloys?

In any type of steel in which one or more elements are added to obtain a desired characteristic or physical property, for example, thermal resistance or corrosion resistance, etc., they are called steel alloys.

The most common elements that are added to these steel alloys are molybdenum, manganese, nickel, silicon, boron, chromium and vanadium.

This type of post, although they may seem long and boring to read, are actually fundamental to better understand how the steels that are used behave, it is an aspect of the culture of a knife maker that I think are an important aspect to understand how steels behave and the various influences of alloys.

Clearly you can still build a knife but this type of knowledge makes you more aware of how what you are using behaves.

How many times have you read on a knife site or heard from a friend that he bought a knife made with vanadium steel, or more simply when you were in a brico or a hardware store to buy a drill bit and you read about the wall bits at widia, Or those with 8% cobalt to cut steel?

Here in this post if you read it to the end calmly and patiently and it will be easier to understand what it means for a steel to contain these alloying elements.

Each steel has its own “recipe” (chemical composition) made with its own ingredients (alloying elements), which mixed in the right doses give it unique characteristics.

There are basically two methods to create these recipes:

• using pure raw materials (precise and high quality recipes are created);
• or using elements derived from the recycling of existing steels (dirty recipes are created containing quantities, albeit small, of unwanted alloying elements).

How many times have you heard from a friend that he bought a knife made with vanadium steel, or hear him talk about the widia wall tips, or those with 8% cobalt to cut steel?

I will now explain to you what it means for a steel to contain these alloying elements.

Each steel has its own recipe (chemical composition) made with its own ingredients (alloying elements), which mixed in the right doses give it unique characteristics.

There are basically two methods to create these recipes:

• using pure raw materials (precise and high quality recipes are created);
• or using elements derived from the recycling of existing steels (dirty recipes are created containing quantities, albeit small, of unwanted alloying elements).

Alloy elements of steel and their function in steels used for cutlery

How they influence the alloying elements on the characteristics of steel is a question that is often asked by those who are looking for the right steel to make their handmade knife but often the same makers use some types of steels a little for fashion, a little because if you use certain steels “you are not wrong” and a little because they are used to using a certain steel and know it well and continue to use it.

Characteristics of Steel

Steel can be considered, as a first approximation, as a binary alloy consisting of iron and a percentage of carbon not exceeding 2.11%, beyond which the alloys obtained are called cast iron.

It is important to consider that in steels carbon does not appear as an independent constituent, but is always combined with iron in the form of carbide (Fe₃C) also called cementite.

Therefore it is more correct to consider steels as iron alloys and cementite.

This characteristic is of fundamental importance during the transformation processes of steels and in fact different transformation conditions imply a different decomposition of cementite, with consequent variation in the carbon content, the molecular structure and, in general, the chemical-physical characteristics of the steel alloy.

Heat treatments

The transformation of steel alloys takes place through heat treatments that can be divided into three fundamental phases:

• preheating,
• maintenance in temperature,
• controlled cooling.

Bringing the fluid to temperatures above 723 ° C, the steel alloy assumes a stable conformation called Austenite in which the carbon is uniformly dispersed in solid solution.

At temperatures above 912°C, iron molecules are arranged in such a way as to dissolve a greater amount of carbon (gamma iron with face-centered cubic lattice).

In the cooling phase, iron assumes the alpha configuration (body-centered cubic lattice) which has lower solubility power than carbon.

This implies that amounts of insoluble carbon migrate to form other molecular compounds.

The resulting alloy therefore depends on the percentage of carbon and can consist of:

• ferrite+perlite in steels (hypoeutectoids) with carbon percentages 0.77<%.
• perlite in steels (eutectoids) with carbon percentages = 0.77%.
• Perilite+Secondary cementite in steels (hypereutectoids) with a carbon percentage of >0.77%.

These three elements have very different mechanical characteristics:

ferrite has low hardness, about 90 Brinell, low tensile strength (280-300 N/mm²) and great plasticity;
perlite has a hardness of about 250 Brinell and a high tensile strength, about 900 N/mm²;
Cementite has a very high hardness and a high fragility.

It can be deduced that the mechanical characteristics of the resulting alloy is closely linked to the presence of these three elements.

Cooling time

The cooling time also greatly affects the composition of the alloy.

Take for example a steel with 0.44% C (hypoeutectoid).

After a slow cooling from the austenitic state to room temperature, it will consist of approximately 50% ferrite and 50% perlite.

By accelerating the cooling process, the percentage of ferrite tends to decrease, while that of perlite to increase.

This is explained by the fact that in a rapid cooling the carbon atoms do not have time to migrate to collect in cementite sheets, remaining incorporated in the ferrite molecules and increasing the volume of perlite.

Therefore, a steel cooled with significantly high speed shows mechanical characteristics similar to those of a steel with a higher carbon content, but cooled slowly.

If the cooling rate exceeds a certain value (characteristic of the type of steel) called the critical hardening speed, the alloy assumes a different structure from the normal structure of ferrite and perlite.

It is made up of martensite and has a much higher hardness than normal.

Cooling performed with sufficient speed to bring the steel to the martensitic state is called quenching.

By subjecting the hardened steel to a new heating phase at moderate temperatures (300-500 ° C) the tempering process starts, in which the alpha iron begins the expulsion of the carbon atoms that were forcibly incorporated during the hardening phase.

These atoms are concentrated in tiny iron carbide granules that help to form alloys with lower hardness and brittleness than martensite.

Dimensional analysis

The size of the casting is an additional aspect that influences the structural composition of the steel alloy.

In principle, during the cooling phase carbon molecules tend to migrate to higher temperature areas, so peripheral sections will tend to have a lower carbon content.

Moreover, for large castings it is unthinkable to respect slow cooling times because this could take even years.

Therefore you are forced to force the cooling rate with the consequences we have just seen.

It follows that in the design phase of the alloy this phenomenon must be taken into account in order to optimally dose the carbon content according to the characteristics required of the alloy itself.

Classification of steels by carbon content

Depending on the carbon content, steels are divided into:

• extra sweet: carbon between 0.05% and 0.15%;
• sweet seeds: carbon between 0.15% and 0.25%
• sweets: carbon between 0.25% and 0.40%;
• hard seeds: carbon between 0.40% and 0.46%;
• hard: carbon between 0.60% and 0.70%;
• very hard: carbon between 0.70% and 0.80%;
• extra hard: carbon between 0.80% and 0.85%.

Although there is a tendency to consider mild steels as less valuable, it can be said more precisely that the quality of a steel alloy is all the better the more its characteristics meet the requirements of the applications for which it is intended.

So if you make knives you have to think about the steel to be used for that specific use and linked to the characteristics that a knife must have and in its field of application.

A diving knife, a survival knife, a kitchen knife, an everyday knife to carry around on many occasions, a heavy duty knife, etc.

As you can imagine the choice of steel changes because corrosion resistance, wire tightness, flexural strength of steel, etc. are one of the things to look at, which then connect with the choice of design, steel thickness, quality of heat treatment execution, cost of knife, etc. but as is clear it cannot be a random choice.

It is also clear that if you want a top knife there are some steels that are a guarantee, let’s say that you are never wrong but that clearly automatically bring the price of the knife to more important values and that for some applications are excessive.

How do alloying elements affect the characteristics of steel

The physical-mechanical characteristics of a steel alloy can be modified by adding secondary alloying elements , in addition to carbon, such as to give it specific properties suitable for particular uses.

The makers who build knives, the most attentive ones, look at these characteristics to choose the most suitable steel for the realization of the knife considering the intended use, the type of finish of the blade, the value for money, etc.

Here is a list of alloying elements that are normally used in the production of steels:

C – Carbon

It is the main element in the structuring of steel alloys, it is the element that cannot be missing when it comes to steel.

• It increases its hardness and hardenability.
• It strongly increases the hardness in all treatment states and especially in alloys subjected to hardening.
• In the form of secondary cementite, in hypereutectoid steels it also determines its fragility.

It is the main element of all steel.

Carbon is used in all steels.

Al – Aluminium

It is mainly used in fine-grained steels.

Being one of the main alloying elements of iron and steel, aluminum (Al) plays the role of grain deoxidation and refinement, which can improve the impact resistance of steel and reduce the tendency to cold and aging.

• Aluminum can also improve the corrosion resistance of steel, especially when used with molybdenum, copper, silicon, chromium and other elements will produce better results.
• The addition of Al in Cr-Mo or Cr steel can improve its wear resistance, and the presence of Al in high-carbon tool steel can cause hardening to be brittle.
• But Al will affect the hot work property, welding property and cutting property of steel.

It is widely used in special alloys, including nitriding steel, stainless acid resistant steel, heat-resistant steel, electrothermal alloy, hard magnetic alloy and soft magnetic and so on.

• It has an energetic deoxidizing effect.
• It forms very hard nitrides with nitrogen.
• Combined with Molybdenum and Chromium from remarkable surface hardness.
• Gives resistance to hot oxidation.
• Worsens weldability.
• In fact, in addition to having a high deoxidizing power, it greatly refines the grain of steel.
• Used together with molybdenum and chromium increases surface hardness.
• Mixed with nitrogen, on the other hand, it allows to create very hard nitrirs.

However, its use also brings negative effects, in fact it decreases resilience, weldability, shrinkage (i.e. the reduction of the cross section suffered by a body subjected to traction), forgeability and resistance to oxidation.

Used in steels type: not used in steel for cutlery.

B – Boron

In very small percentages the aptitude for heat treatments increases.

• It is used in bass alloys to increase hardenability and therefore its aptitude for heat treatments in doses between 0.0005 and 0.003%.

Used in steels type: 30 MN B5.

Co-Cobalt

Used a lot in fast and super fast hot working steels.

• It does not form carbides by itself, but is a multiplier of effects of the other elements.
• It prevents oxidation, and makes martensite more stable, but decreases hardening penetration.

Used in steels type: CPM S110V, N690Co, VG10, ATS-55, COS-Laminated.

Cr – Chrome

It is one of the most used elements in carburizing and tempering steels, normally accompanied by nickel and molybdenum.

• It strongly increases hardenability because it strongly reduces the critical cooling rate.
• Increases wear resistance.
• Increases tempering stability.
• Reduces cold brittleness.
• With percentages greater than 12% percent it is used in ferritic and martensitic stainless steels to improve corrosion resistance.

It is the element that distinguishes the so-called carbon steels from stainless steels, in fact they become stainless when its presence is greater than or equal to 12%.

As a rule, it is used together with nickel and molybdenum.

It increases corrosion resistance, wear resistance and temper stability.

It also increases hardness, elasticity limit and the formation of wear resistance carbides, improves tensile strength and reduces cold brittleness.

Used in all stainless steels and in some carbonaceous in small quantities.

Cu – Copper

Improves resistance to atmospheric corrosion.

May cause structural failures as a result of hot working.

• It is often used in CORTEN (low alloying element) steels and improves its resistance to atmospheric corrosion and fatigue resistance.

Used in steels type: COR-TEN, 125 SC, C70.

Mn – Manganese

It is present in small tenors in all types of steel.

• Reduces the heat brittleness caused by sulphides of other elements.
• Reduces cooling rate by increasing steel hardenability.
• Increases mechanical strength.
• In high percentages it increases wear resistance, but makes the steel very susceptible to tempering fragility.

It is probably one of the most used alloying elements in steel after carbon.

It behaves a bit like aluminum as a deoxidizer, but also as a desulfurizer, in fact it reduces the hot fragility caused by the sulphides of other elements.

It manages to reduce the cooling speed thus increasing the hardenability, it also increases mechanical resistance and hardness.

Used in almost all steels but in quantity degrees in these: O1, 5160, A2, 440, N690.

Mo – Molybdenum

It is one of the most frequently used elements in the treatment of steels, often combined with nickel and chromium.

• It greatly affects the increase in hardenability, tempering stability and decreased sensitivity to overheating.
• Increases hardness, toughness and wear resistance.
• It greatly increases the mechanical resistance to heat.
• It strongly reduces the fragility of tempering in steels that are susceptible to it.
• Also widely used in all steels often combined with nickel and chromium.
• It significantly affects the hardenability and stability at tempering.

It manages to increase hardness, wear resistance, toughness, corrosion resistance and fatigue resistance.

Used in almost all steels but in quantitative degrees in these: ATS-34, CPM 3V, CPM S90V, CPM M4, Niolox, Becut, Sleipner, A2, Vanadis 4.

N – Nitrogen

Mainly used in stainless steels to increase mechanical strength and stabilize the state of austenite.

• It is used in stainless steels as it greatly increases the resistance to corrosion (pitting), also increases the mechanical resistance and stabilizes the state of austenite.
• It is also used for the production of sintered and annealing processes.

Used in steels type: NITRO B, 14C28N, 420MOD.

Ni – Nickel

Nickel is also widely used in the treatment of steels.

• Combined with chromium and molybdenum increases the aptitude for heat treatment.
• Even at low temperatures it improves the complex resistance-tenacity.
• It causes the lowering of critical points, reduces the critical cooling speed increasing hardenability.
• Increases hardness and mechanical strength.
• Reduces sensitivity to overheating.
• It increases hardness, mechanical strength, corrosion resistance and hardenability.
• It does not form carbides.

Used in steels type: AUS 8, D2, N695, O1, CPM M4.

Nb – Niobium (Nb)

It is a hard metal, with the ability to refine grain.

• Its use prevents chipping
• It increases wear resistance, thus creating a steel capable of having a great resistance to edge nailing.

Used in steels type: CPM S35VN, CPM S110V, Niolox, Cos, CTS-XHP.

P – Phosphorus

Decreases brittleness when used in high concentrations.

• It also increases hardness, corrosion resistance and machinability.
• If a concentration of 0.2% is exceeded, resilience decays to 0.

Used in steels type: 125 SC, Shirogami 1, Shirogami 2.

Pb – Lead

Facilitates machining to machine tools.

• It increases the machinability of steel but has no known effect on the mechanical properties of steel when used in specific concentration ranges.

Used in steels type: 125 SC, Shirogami 1, Shirogami 2.

Yes – Silicon

Like manganese it is present in all types of steel.

• Increases hardness, strength, hardenability, tempering stability and wear resistance.
• It increases the elasticity limit, therefore it is used in spring steels and magnetic laminations.
• Silicon steels have a tendency to brittleness, fibrousness and grain enlargement.
• Together with manganese it is present in all types of steel and like manganese it is used to deoxidize.
• It increases in hardenability, hardness, strength, tempering stability and wear resistance.
• It is often used in spring steels as it is able to increase the elasticity limit.
• On the other hand, silicon steels have a tendency to brittleness and swelling of grain.

Used in large percentages in steels type: M390, N690, N695, 420, 440C, AUS 8, Sleipner, Becut.

Ti – Titanium

• Increases corrosion resistance in stainless steels.
• Reduces the size of the grain.
• Inhibits the fragility of welded stainless steel structures.
• It belongs to the group of chromium and is the element that has the greatest tendency to form carbides.
• It counteracts the formation of austenite in steels with high chromium content, increases corrosion resistance in stainless steels and reduces grain size.
• It reduces hardness and hardenability in steels with medium chromium content and forms highly abrasive carbides, thus reducing the machinability of the tool, thus subtracting carbon from the die.

Used in steels type: NITINOL, 316 Ti

V – Vanadium

It has a very strong tendency to the formation of carbides, therefore it increases the hardness even when hot, the stability to tempering and reduces the sensitivity to overheating.

• It is not normally used in construction steel.
• Like titanium, he also belongs to the chromium group.
• It tends to form many carbides but subtract carbon from the matrix, so many sintered steels with a high V content have huge amounts of carbon in their alloy recipe.
• Together with tungsten, it gives steels extreme hardness even at high temperatures.
• They are divided into fast steels (vanadium-tungsten) and super fast steels (vanadium-tungsten-cobalt).

In cutlery it confers characteristics such as high impact resistance and hardness, while inhibiting the growth of wheat.

Used in steels type: CPM S90V, CPM M4, D2, Vanadis 23, Elmax, M390, Niolox, Becut, SG2, Aogami.

W – Tungsten

• Gives hardness and wear resistance.
• It is used in high-speed steels and hot work steels.
• It limits the thermal conductivity of steel.
• Its effects are very similar to those of molybdenum giving hardness and wear resistance.

An alloy with the presence of tungsten is extremely hard and heat resistant and has excellent hardenability.

Thanks to its thermal conductivity it is widely used in fast steel, as it does not lose quenching up to 600 °, and in self-hardening steel, as it spontaneously hardens in air after reaching the austenitic temperature.

Used in steels type: VG7, W1, W2, CPM M4, Aogami, Cos, Vanadis 23, M390.

S – Sulphur

It is also an element to be considered harmful but, if used in small quantities, it helps to increase workability.

Used in steels type: 125SC, C70, C100.

Conclusions

At this point the situation seems to have become simple but it is not so.

What I wrote to you is just a smattering to get you a little closer to the world of metallurgy.

Now, however, if you want to understand something more, try to see the table below and try to understand what one steel is for rather than another.

Finally, we summarize the effects of alloying elements by making explicit reference to variations in the characteristics of steel:

• Increased hardness: [Cr,Mn, Mo, Ni, Si, V].
• Increase in hot hardness: [Cr, Mo, V]
• Decrease in critical cooling rate and consequent increase in hardenability: [Cr,Mn, Mo, Ni, Si, V].
• Improvement of the complex strength-toughness: [Cr, Mn, Mo, Ni].
• Decreased sensitivity to overheating: [Cr, Mo, Ni, V].
• Increased temper stability: [Cr, Mo, V].
• Reduction of the tendency to fragility of the temper: Mn.
• Increased elasticity limit: [Mo, Si, V].

The effect exerted by a single alloying agent is not necessarily cumulative with the simultaneous application of several elements.

The resulting effect depends on the mutual interactions between the various elements.