Knowledge

Introduction

Metal materials is a discipline full of mysteries. Whether it is the materials needed for tool manufacturing itself or the processing of parts, one will encounter material issues. If you are engaged in the steel industry, have you ever noticed what the chemical components listed in the steel test report actually mean? You may only know that different grades of steel have many different chemical components and different contents of elements. In this article, we have sorted out and listed 21 chemical elements and their influence on the performance of steel.

#1 Carbon (C)

Carbon is the most important element in steel. It is crucial for steels that need to be hardened through quenching. The carbon content controls the material's hardness and strength, as well as its response to heat treatment (quenchability). As the carbon content increases, the steel's ductility, malleability, and workability decrease, and its weldability also declines.

#2 Manganese (Mn)

Manganese may be the second most important element after carbon. Its function is similar to that of carbon, and steel producers combine these two elements to obtain materials with the desired properties. Manganese is essential for the hot rolling process of steel by binding with oxygen and sulfur.

Its existence has the following main functions:

 

It is a mild deoxidizer and acts as a purifying agent to carry sulfur and oxygen from the melt into the slag. It enhances hardenability and tensile strength but reduces ductility. It combines with sulfur to form spherical manganese sulfide, which is crucial for the good machinability of free-cutting steels. Steels typically contain at least 0.30% manganese, but in some carbon steels, contents as high as 1.5% can be found.

Manganese also tends to increase the carbon permeability during the carburizing process and acts as a mild deoxidizer. However, when both carbon and manganese contents are too high, embrittlement occurs. Manganese can form manganese sulfide (MnS) with sulfur, which is beneficial for mechanical processing. At the same time, it can counteract the brittleness brought by sulfur and is conducive to the surface finish of carbon steel.

In terms of welding, the ratio of manganese to sulfur should be at least 10:1. A manganese content lower than 0.30% may cause porosity and cracks inside the weld seam, and a content exceeding 0.80% may also lead to cracks. Steels with a low manganese-sulfur ratio may contain sulfur in the form of iron sulfide (FeS), which can cause the weld seam to crack.

#3 Phosphorus (P)

Although it can enhance the tensile strength of steel and improve its machinability, it is generally regarded as an undesirable impurity due to its embrittlement effect.

The influence of phosphorus on steel varies with its concentration. Due to its harmfulness, the maximum phosphorus content in high-grade steel is between 0.03% and 0.05%. In low-alloy high-strength steel, up to 0.10% phosphorus can enhance strength and improve the steel's corrosion resistance. When the content is too high in hardened steel, the possibility of embrittlement increases. Although strength and hardness are improved, ductility and toughness decline.

Phosphorus improves the machinability of free-cutting steel, but if the phosphorus content exceeds 0.04%, welding brittleness and/or weld cracks may occur during welding. Phosphorus also affects the thickness of the zinc coating during galvanizing.

#4 Sulfur (S)

Sulfur is generally regarded as an impurity. When the sulfur content in steel is high and the manganese content is low, it will have an adverse effect on the impact performance. Sulfur improves machinability but reduces transverse ductility and notch impact toughness, with a relatively small impact on longitudinal mechanical properties. The sulfur content in steel is limited to 0.05%, but in free-cutting steels, the addition can reach up to 0.35%, while increasing the manganese content to counteract any adverse effects, as sulfur alloy additions of 0.10% to 0.30% can improve the machinability of steel. Such steels can be called "resulfurized" or "free-cutting" steels. Free-cutting steels add sulfur to improve machinability, typically up to 0.35%.

Although sulfur has a negative impact on steel at certain stages, sulfur content below 0.05% has a positive effect on the grade of steel.

#5 Silicon (Si)

Silicon is one of the main deoxidizers for steel. It helps to remove oxygen bubbles in molten steel. It is the most commonly used element in the production of semi-deoxidized and fully deoxidized steel, usually with a content of less than 0.40%. When used as a deoxidizer, it typically contains only a small amount (0.20%) in rolled steel. However, in steel castings, it usually contains 0.35% to 1.00%.

Silicon dissolves in iron and tends to strengthen it. Some filler metals may contain up to 1% silicon to provide better cleaning and deoxidation effects when welding on contaminated surfaces. When these filler metals are used for welding on clean surfaces, the strength of the resulting weld metal will be significantly enhanced. Silicon increases strength and hardness, but to a lesser extent than manganese. The resulting decrease in ductility may cause cracking problems.

When it comes to galvanizing, steel with a silicon content exceeding 0.04% will significantly affect the thickness and appearance of the galvanized coating. This will result in a thick coating mainly composed of zinc-iron alloys, with a dull and unattractive surface. However, it provides the same anti-corrosion protection as the bright galvanized coating with a pure zinc outer layer.

#6 Chromium (Cr)

Chromium is a powerful alloying element in steel. A small amount of Cr exists in some structural steels. It is mainly used to enhance the hardenability of steel, increase corrosion resistance and the yield strength of steel. Therefore, it is often combined with nickel and copper. Stainless steel may contain more than 12% chromium. The well-known "18-8" stainless steel contains 8% nickel and 18% chromium.

When the chromium content in steel exceeds 1.1%, a surface layer that helps protect the steel from oxidation is formed.

#7 Vanadium (V)

The role of vanadium as a chemical element is similar to that of manganese, molybdenum and niobium. When used in combination with other alloying elements, it restricts grain growth, refines grain size, enhances hardenability, fracture toughness and resistance to impact loads. It also improves high-temperature softening, fatigue stress and wear resistance. When the content exceeds 0.05%, there may be a tendency to embrittle during heat stress relief treatment.

Vanadium is used in nitriding, heat-resistant, tool and spring steels together with other alloying elements.

#8 Tungsten (W)

It is used together with chromium, vanadium, molybdenum or manganese to produce high-speed steel for cutting tools. Tungsten steel is known as "red hard", meaning it remains hard enough for cutting even after being heated to red-hot. After heat treatment, the steel retains its hardness at high temperatures, making it particularly suitable for cutting tools.

Tungsten in the form of tungsten carbide:

It can impart high hardness to steel even at red-hot temperatures. Promotes fine-grain formation, enhances thermal resistance, and boosts high-temperature strength.

#9 Molybdenum (Mo)

The role of molybdenum is similar to that of manganese and vanadium, and it is often used in combination with one or both of them. This element is a strong carbide former, and its content in alloy steel is usually less than 1%. It enhances hardenability and high-temperature strength, while improving corrosion resistance and increasing creep strength. It is added to stainless steel to increase its corrosion resistance and is also used in high-speed tool steel.

#10 Cobalt (Co)

Cobalt improves high-temperature strength and magnetic permeability. It increases hardness while allowing for higher quenching temperatures (during heat treatment). In more complex steels, it enhances the individual effects of other elements. Cobalt is not a carbide former, but adding cobalt to alloys can achieve higher attainable hardness and higher red hardness.

#11 Nickel (Ni)

In addition to being beneficial to the corrosion resistance of steel, the addition of nickel can also enhance the hardenability. Nickel improves the low-temperature performance of the material by enhancing the fracture toughness. The presence of this element does not reduce the weldability of steel. Nickel significantly increases the notch toughness of steel.

Nickel is often combined with other alloying elements, especially chromium and molybdenum. It is a key component of stainless steel, but its content in carbon steel is relatively low. Stainless steel contains 8% to 14% nickel.

Another reason for adding nickel to the alloy is that it can create brighter parts in Damascus steel.

#12 Copper (Cu)

Copper is another major corrosion-resistant element. It also has a minor effect on hardenability. Usually, its content is no less than 0.20%, and it is the main anti-corrosion component in steel grades such as A242 and A441.

The most common in steel is residual agent, and copper is also added to produce precipitation hardening properties and improve corrosion resistance.

#13 Aluminium (Al)

Aluminum is one of the most important deoxidizers in materials, with a very low content. It helps form a finer grain structure and enhances the toughness of steel grades. It is usually used together with silicon to obtain semi-deoxidized or fully deoxidized steel.

#14 Titanium (Ti)

Titanium is used to control grain growth, which enhances toughness. It also transforms sulfide inclusions from elongated to spherical shapes, improving strength, corrosion resistance, as well as toughness and ductility.

Titanium is a very strong and very light metal that can be used alone or alloyed with steel. It is added to steel to give it high strength at high temperatures. Modern jet engines use titanium steel.

Prevent the depletion of chromium in local areas of stainless steel during long-term heating, prevent the formation of austenite in high-chromium steel, and reduce the martensitic hardness and quenchability of medium-chromium steel.

#15 Niobium (Nb)

Niobium is a key grain-refining element and also a strength-enhancing element in steel production. It is a strong carbide former, forming very hard and very small simple carbides. It improves ductility, hardness, wear resistance and corrosion resistance. At the same time, it refines the grain structure. Formerly known as columbium.

#16 Boron (B)

The most important role and purpose of boron in steel is to significantly enhance hardenability.

The greatest advantage of boron is that only a small amount is needed to achieve the same hardenability effect that other elements require a large amount to reach. The typical range in steel alloys is 0.0005% to 0.003%.

During the heat treatment process, boron is added as a substitute for other elements to enhance the hardenability of medium carbon steel. The cutting performance of high-speed steel is improved, but at the expense of forging quality. Excessive boron content may also reduce hardenability, toughness and cause embrittlement. The percentage of carbon in steel also plays a role in the hardenability effect of boron. As the influence of boron on hardenability increases, the carbon content should be reduced accordingly.

When boron is added to steel, precautions must be taken to ensure that it does not react with oxygen or nitrogen, as the combination of boron with either of these elements will render it ineffective.

#17 Lead (Pb)

A small amount of lead, up to 0.30%, is added to improve workability. As long as it is evenly distributed, it has little effect on the physical properties of the steel. Contrary to the common belief, it does not affect weldability.

#18 Zirconium (Zr)

Zirconium is added to steel to change the shape of inclusions. It is usually added to low-alloy and low-carbon steels. When the shape changes from elongated to spherical, the toughness and ductility are improved.

#19 Tantalum (Ta)

It is very similar to niobium (Nb) in chemical properties, thus having a similar effect on alloys - forming very hard and very small simple carbides. It improves ductility, hardness, wear resistance and corrosion resistance. At the same time, it refines the grain.

#20 Nitrogen (N)

The role of nitrogen in alloys is very similar to that of carbon. Nitrogen can replace carbon in small amounts (or even large amounts in modern technology) to increase hardness. Obviously, nitrogen forms nitrides instead of carbides. INFI contains nitrogen, as do some others, among which Sandvik is the champion, with 3% nitrogen in the alloy, completely replacing carbon. Unfortunately, tool manufacturers cannot obtain it. Because nitrogen has a smaller tendency to form chromium nitrides than carbon to form chromium carbides, its presence improves corrosion resistance and retains more free chromium in the alloy. Due to the lower reactivity of nitrogen when forming nitrides, it can be used to increase hardness without increasing carbide size and volume, such as in Sandvik 14C28N steel.

#21 Selenium (Se)

It is usually not favored in tool steel. It is added to improve machinability. Similar to sulfur, it belongs to the same chalcogen group.

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