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What are the Differences between Carbon Steel and Stainless Steel?

28-January-2024

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Building industrial equipment is all about stacking, fixing and constructing metal parts and some auxiliary materials like plastics and aluminium parts altogether, despite the fact that it is actually not as easy as what we describe here now that certain level of engineering skills is indispensable.

Opting for the right one between carbon steel and stainless steel sometimes comes as prioritized task when designing an industrial machine, not only for the structural reason in terms of engineering, however as well as to comply with the specifications forced by the client and then the regulations to be complied with, for example, in case that the machines are supposed to be used in specific industries like food, dairy products, and sometimes pharmaceuticals and medical products, especially when it comes to primary packaging equipment that has direct contact with the products.

Supposing that you are a project engineer or purchase manager hunting for an industrial packaging equipment for your company, with this article we will cast the light into the pros and cons, differences and characteristics of carbon steel and stainless steel so that you can take the right decision when negotiating with your range of options of packaging manufacturers and their equipment.

Why Does It Matter to Select the Right Steel?

Carbon steel and stainless steel basically differ in their alloying elements that result in their pros and cons in terms of chemical and physical properties, now that they will have impact on the steel’s characteristics through their effects on the matrix.[1] To put it in a more intelligible way, the use of carbon steel or stainless-steel in constructing industrial equipment determine various factors of the machine among which including the cost, stability, hygiene, corrosion resistance, and various concepts.

Stainless Steel Cartoner
Cartoner Machines by ÉLITER Packaging Machinery
are built with robust stainless steel construction
Credit: ÉLITER Packaging Machinery

What are the Factors that Set Different in Nature Carbon Steel and Stainless Steel?

Both carbon steel and stainless steel are in fact steel alloys but vary in how they are composed. The composition of steel alloy is determined basically by how Iron (Fe) is combined with, Carbon (C), accompanying elements and alloying elements. As mentioned hereby,

Accompany elements refer to those chemical elements that are not intentionally added for the purpose of alloying during the manufacturing processes of steel but are generated inherently from the processes and are considered impurities, for example, the most commonly discussed accompanying elements are Sulphur (S) and Phosphorus (P).
Alloying elements, on the other hand, are those chemical elements intentionally added to alter the properties and characteristics of the steel during its manufacturing processes. Typical alloying elements are Chromium (Cr), Molybdenum (Mo), Nickel (Ni). The baseline to call a metal alloy stainless is that the Chromium (Cr) containment should be at least 10.5% by weight, which is the condition based on which stainless steel is distinguished from carbon steel.

Basically to put it in a simpler way for explanation, carbon steel is the steel alloy of iron, carbon and accompanying elements, while stainless steel is that of iron, carbon, accompanying elements and alloying elements.

What is Carbon Steel?

Carbon steel refers to a range of metal alloy of Iron (Fe) and Carbon (C) and the portion of the latter of which ranges from 0.021% to 2.11%. Carbon steel sometimes comes with also accompanying elements of Sulphur (S), Phosphorus (P) and Manganese(Mn).

In accordance to the proportion of carbon context, carbon steel is also categorized into low, medium and high-carbon steel, for respectively each of them the carbon elements contained should be less than 0.25%, between 0.25% and 0.6% or more than 0.6%. The level to which the carbon proportion is contained in carbon steel will affect properties including strength, hardness, toughness, and ductility. Basically, with the portion of carbon elements increasing in the carbon steel’s context, the metal alloy will acquire better strength and hardness, yet with the ductility compromised.

The use of carbon steel on industrial equipment, especially on packaging machines, is frequently combined with finishing techniques including painting and coating, given the fact the carbon steel is in nature susceptible and vulnerable to rust and corrosion. However, galvanized carbon steel will eventually still rust and the use of such components will lead to more concerns around hygiene and migration issues in certain circumstances that it should be not taken into consideration along the metals used for building the equipment.

How Does Carbon Steel Rust Eventually?

While high-carbon steel provides a better degree of strength, the higher the portion of carbon elements, the more vulnerable is carbon steel to rust. To explain why does carbon steel rust eventually, by all means, one should look into the process of rust from an electrochemical point of view.

For example, a thin water film forms on the surface of steel, containing small amounts of H+ and OH- ions, and dissolves gases such as oxygen. Following the step of which, an electrolyte solution forms on the steel surface, creating countless microscopic electrochemical cells with the iron and a small amount of carbon within the steel. Over a sufficient period, with an abundant supply of oxygen and water, ordinary carbon steel develops a reddish-brown substance known as rust.

The rust is primarily composed of hydrated ferric oxide compounds, such as Fe2O3·nH2O (trioxide) and iron hydroxides (FeO(OH), Fe(OH)3). This electrochemical corrosion process occurs due to the interaction of iron with oxygen and water, leading to the formation of rust on the surface of carbon steel.

galvanized metal parts are not food safe serving
Typical galvanized carbon steel used for low configuration packaging machines
Credit: ELITER Packaging Machinery

What are the Pros and Cons of Carbon Steel?

Talking about the advantages or disadvantages without considering the specific situations where carbon steel is used is in fact not an objective way to give a conclusion to such kind of metal alloy though, carbon steel can be an ideal option in the case that the project or construction attaches importance to the affordability while the industrial equipment built is not supposed to be used in sensitive environments including low or high temperature and then high humidity or dealing with the automated manufacturing processes of corrosive products.

Yet basically, the primary advantage of carbon steel is all about its affordability: A same part or component, if made by carbon steel, and not to consider the necessary finishing techniques, will be basically some 5 times more affordable in price than being made from stainless steel, such is basically the price difference between stainless steel and carbon steel. [2]
Carbon steel is stronger against bending and denting than stainless steel due to the higher content of carbon that it would be a better option for construction project and in some cases the building of industrial equipment that does not have to be excellent in cleanability or hygiene.

The downside of carbon steel, as we have mentioned sporadically in above content, are basically about those as follows:

Carbon steel is not resistant to corrosion or rust: All carbon steels are vulnerable to rust and thus are not ideal for some applications like direct contact with food or where the mechanical components should be rust-free. Even though carbon steel can acquire better corrosion resistance by processing like coating and painting, they are still not any kind of hygienic of materials, for example, for food application.

What is Stainless Steel?

Stainless steel refers to a range of metal alloy specifically meets the standards that contains a minimum 10.5% of Chromium by weight and carbon context that does not surpass 2.1%. Stainless steel is actually not 100% rust free but can withstand the process of rust in most common situations thanks to the protective passive layer formed by the oxidation of Chromium with oxygen in the air.

Stainless steel can be categorized by its microstructures into types including Austenitic, Martensitic, Ferrite, and among others, cementite and pearlite. In the general sense, austenitic stainless steel has better property in terms of corrosion resistance and is non-magnetic, typically with those commonly used ones including SUS 201, SUS 303, SUS 304 and SUS 316. On the contrary, martensitic stainless steel is magnetic but less corrosion resistant, typically including SUS 403, SUS 410, SUS 420 and SUS 440.

A table as follows shows the properties of differences between the most commonly used Austenitic and Martensitic stainless steels.

$$
\begin{array}{c|lcr}
\text{Category} & \text{Type} & \text{Application} & \text{Characteristics} & \text{Magnetic} \\
\hline
\text{Austenitic}& \text{SUS303} & \text{For rust-free components} & \text{Better machinability than SUS 304} & \text{No}^* \\
\text{Austenitic}& \text{SUS304} & \text{For rust-free components} & \text{Most commonly used rust-free steel}& \text{No}^* \\
\text{Austenitic}&\text{SUS316} & \text{For rust-free components} & \text{Improved resistance against seawater}& \text{No}^*\\
\text{Martensitic}& \text{SUS404C} & \text{Worse corrosion resistance} & \text{Available for quenching} & \text{Yes}\\
\text{Martensitic}& \text{SUS410} & \text{Worse corrosion resistance} & \text{Available for quenching, better processability} & \text{Yes}
\end{array}
$$

(*) Note: Austenitic stainless steel can be processed to be magnetic by certain methods.

How Do Different Stainless Steels Come into Being?

All kinds of stainless steels, no matter austensitic or martensitic, stem from the processing of adding carbon content along with other alloying elemtns into iron. Iron, in its case, takes form into different forms of microstructures or crystal structures when it is cooling down from molten staus and within different ranges of temeprature.

To start with the explanation, one should understand in the firsly place the different forms of crystal structures formed under different temperature ranges. Iron (Fe) is an important metallic element that exhibits multiple crystal structures, and undergoes phase transformations based on temperature and alloy composition. Under solid-state conditions, iron can exist in three different phases: δ-Fe, γ-Fe, and α-Fe,

Body-Centered Cubiod (bcc), or $ \delta – \text{Fe} $ takes from between the melting point of iron 1538 ℃ and 1394 ℃.
Face-Centered Cuboid (fcc), or $ \gamma – \text{Fe} $ takes from between 1394℃ and 912℃.
When the temperature is below 912℃, again the mircrostructure takes form into Body-Centered Cubiod (bcc), referred to as $ \alpha- \text{Fe} $

fcc and bcc
Crystal Structure Diagram of Molten Iron
Credit: ELITER Packaging Machinery

The three kinds of crystal structure vary in their capability to have carbon dissolved wihtin.

$ \gamma – \text{Fe} $ has larger interstitial space in the crystal lattice than can have more carbon content dissolved, in which case, referred to as $ \text{Austenitte = Au} $ that maintaines its Face-Centered Cuboid (FCC) structure, and has a maximum carbon content until 2.11% by weight.
On the other hand, $ \alpha- \text{Fe} $, when have carbon content dissolved, is referred to as $ \text{Ferrite=F} $, which keeps Body-Centered Cubiod (BCC) and has only a maximum carbon content until 0.0218% by weight.
When the solid solution of carbon exceeds the limites of both above, the compound formed will be cementite (Fe3C) with a carbon content until 6.69% by weight.

However, as we have noticed that Austentie only takes form when the temperature is between 1394℃ and 912℃, and when we try to get the compond by cooling down the temperature under 912℃, $ \gamma – \text{Fe} $ will transform into $ \alpha- \text{Fe} $ that what we acquired will be a mix of Austentite and Ferrite, and again in the case we try to get the stainless steel we would use in normal temperature, the material will again get mixed with other compounds. The question is how can we get Austenitic Stainless Steel finally?

To answer this quesion we are once back to the topic of carbon steel and transformations that take place when it is molten and cooled down.

Phase Diagram of Fe-Fe3C
Credit:Suriasnyah Sabarudin, P. Pratikto [3]

At room temperature, when carbon steel with different mass fractions of carbon is heated above a critical temperature, it forms austenite. Austenite exhibits a specific characteristic where it forms different structures when held at a constant temperature or cooled at different rates.

The critical temperature refers to the A3, Acm, and A1 lines in the Fe-Fe3C phase diagram, representing the temperature at which iron starts to transform into austenite during heating for different mass fractions of carbon. For instance, if the room temperature structure is pearlite in the case of a carbon steel with 0.77% carbon content, when heated to 727 ℃, it begins to form austenite.

fe fe3c microstructure phase diagram
Fe-Fe3C Microstructural Aspects Phase Diagram
Credit: Suriasnyah Sabarudin, P. Pratikto [3]

For instance, in the case of eutectoid steel with a carbon content of 0.77%, during isothermal holding between 727 ℃ and 560 ℃, pearlite forms. During isothermal holding between 560 ℃ and the Ms point, bainite forms, and between Ms and Mf temperatures, martensite forms. When austenite is held at temperatures between 727 and 560 ℃, cementite forms at the austenite grain boundaries and slowly grows, causing the surrounding austenite to become depleted in carbon, leading to the formation of ferrite on both sides. This process forms a pearlite unit, and numerous units diffuse, intertwine, and overlap, ultimately causing the entire austenite to transform into pearlite, making pearlite a mixture of ferrite and cementite.

When austenite is held at temperatures between 560 and Ms, supersaturated ferrite precipitates at the austenite grain boundaries, followed by the precipitation of fine cementite within the ferrite. Hence, bainite is a mixture of supersaturated ferrite and cementite.

Ms refers to the martensite start temperature, where the transformation of austenite to martensite begins. This temperature varies for different mass fractions of carbon, typically between 150 and 310 ℃. Mf represents the martensite finish temperature, which also varies based on the carbon mass fraction, typically ranging from -100 to 50 ℃.

Martensite forms as a result of the rapid transformation of austenite at low temperatures between Ms and Mf, where only the crystal structure of the ferrite changes, and the carbon atoms do not have time to redistribute, becoming retained in the martensite. The carbon mass fraction in martensite is the same as that in the parent austenite. Thus, martensite is a supersaturated solid solution of carbon in α-Fe.

The last step then to acquire Martensinitic or Autenistic Steel is add alloy elements, for example, to alter phase region of the Austentite by adding Chromium by 9% by weight or Manganese at 13% by weight so that one can have the Austentite stable in room temperature and to get austentic steel.

What are Austentic and Martensinitic Stainless Steels and Why Is It Free from Rust?

Austenitic stainless steel is based on low-carbon steel with the addition of 17%-25% chromium and 8%-29% nickel. The addition of nickel allows the steel to exhibit a single-phase austenitic structure at room temperature, which reduces the number of microcells formed within the metal due to different microstructures and as a result of which it enhances the steel’s resistance to electrochemical corrosion.

Electrochemical corrosion occurs when different microstructures in the metal create numerous microcells in a solution. To look into the details, for example, the pearlite consists of layers of ferrite (α-iron) and cementite (Fe3C), which form countless microcells in a nitric acid-alcohol solution. The α-iron has a low electric potential, acting as the anode in the microcell, leading to continuous dissolution of iron ions, which is a form of corrosion. On the other hand, the Fe3C has a higher electric potential and acts as the cathode, allowing the transfer of electrons to hydrogen ions in the solution, forming hydrogen gas.

The addition of chromium raises the electric potential of the matrix and forms a dense layer of chromium oxide (Cr2O3) on the surface of the steel, making it less prone to rusting in certain environments. This is why it’s called austenitic stainless steel.

Similarly, by adding 12%-18% chromium to carbon steel with a carbon content of 0.1%-1% and then with air cooling one can finally achieve the formation of martensitic stainless steel. Due to the singular alloying elements, martensitic stainless steel exhibits excellent corrosion resistance in non-oxidizing environments such as atmospheric air and steam, however, its corrosion resistance is significantly lower in oxidizing environments such as hydrochloric acid solutions.

What are the Pros and Cons of Stainless Steel?

The edge that stainless steel has over carbon steel is no doubt its excellent resistance to corrosion and hygiene, allowing this kind of metal materials to be used for those tools, equipment and machines to be used for even direct food contact and has minimized impact on human health from metal elements, for example, kitchen utensiles, primary packaging machines like VFFS and HFFS, form fill seal machine for dairy products, etc. To put it in a workd, stainless steel is the ideal option for hygienic construction of tools and machines.

However, the use of stainless steel will have the object’s raw cost rockets to such a extent that it can be 5 or even 7 times more expensive than being made with carbon steel. Stainless steel is neither totally free from rust and corrosion especially if it used for extreme environments or hazard products like chemicals.

Carbon Steel vs. Stainless Steel: Which One is the Better Option for Industrial Equipment?

There is not something definitely correct to tell stainless steel is absolutely better than carbon steel or vice versa, however, one should take into consideration various factors to decide which one should be used for building an industrial equipment.

Consider the application of the industrial equipment: For example, if it were a tertiary packaging equipment such as a case packer or palletizer, painted carbon steel can be an adequate option for the machine’s frame or structrual components now that it would only deal with corrugated carton boxes through which the contained products still have more layers of packaging between that the downside of carbon steel will not entail hygiene issues with the processes.
Consider the project budget: A packaging machine will be almostly 10 times more expensive if made by full stainless steel construction compared to being built with carbon steel. In the case that the client has very limited budget, it is not reasonsble to force a SUS machine proposal but may have to consider carbon steel.
Consider the compliacne and regulations: The use of galvanized carbon steel or painted parts should be prohibited for sections on the industrial packaging equipment where food materials or ingredients have direct contact with these parts, in which case, they must be replaced by stainless steel.
Consider the environment: A low temperature workplace with high degree of humidity where, for example, seafood products are being manufactured and processed for automate packaging processes, should have their industrial packaging equipment made with stainless steel and especially with SUS 316 to withstand the corrosion of seawater.

Bibliography

[1] K.B. Rundman, F. Iacoviello, Cast Irons, Reference Module in Materials Science and Materials Engineering, Elsevier, 2016, ISBN 9780128035818, https://doi.org/10.1016/B978-0-12-803581-8.09803-9.
[2] MATERIAL Type Cost, Massachusetts Institute of Technology, https://web.mit.edu/course/3/3.11/www/modules/props.pdf
[3] Sabarudin, Suriasnyah et al. “Effect of heat treatment and cryogenics on hardness of ductile cast iron microstructure (FCD50).” Eastern-European Journal of Enterprise Technologies 2 (2018): 20-26.