The Influence and Function of Various Elements in Stainless Steel

We often think of stainless steel as a type of steel when we see it, but in fact, the term 'stainless steel' does not simply refer to a single type of stainless steel, but rather represents over a hundred types of industrial stainless steel, each of which has excellent performance in its specific application field. There are about twenty chemical elements that can be encountered in commonly used steel materials in industry. For the special steel series of stainless steel formed by people's long-term struggle against corrosion, there are more than ten commonly used elements. In addition to the basic element iron that makes up steel, the elements that affect the properties and structure of stainless steel are: carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, titanium, manganese, nitrogen, copper, cobalt, etc.

In practical industrial applications, stainless steel contains several or even a dozen elements simultaneously. When several elements coexist in a single entity of stainless steel, their effects are much more complex than when they exist alone. In this case, not only the effects of each element itself should be considered, but also their mutual effects should be noted. Therefore, the structure of stainless steel depends on the sum of the effects of various elements.

1. The decisive role of chromium in stainless steel

There is only one element that determines the nature of stainless steel, which is chromium. Each type of stainless steel contains a certain amount of chromium. So far, there is no stainless steel without chromium. The fundamental reason why chromium has become the main element determining the performance of stainless steel is that by adding chromium as an alloying element to the steel, it promotes the internal contradiction movement to develop in favor of resisting corrosion damage. This change can be explained from the following aspects:

① Chromium increases the electrode potential of iron based solid solution

② Chromium absorbs electrons from iron to passivate it

Passivation is a phenomenon where the corrosion resistance of metals and alloys is improved due to the inhibition of anodic reactions. There are many theories that constitute passivation of metals and alloys, mainly including thin film theory, adsorption theory, and electron arrangement theory.

2. The duality of carbon in stainless steel

Carbon is one of the main elements in industrial steel, and the properties and microstructure of steel largely depend on the content and distribution of carbon in the steel, with carbon having a particularly significant impact in stainless steel. The influence of carbon on the structure of stainless steel is mainly manifested in two aspects. On the one hand, carbon is a stable element of austenite and has a significant effect (about 30 times that of nickel), and on the other hand, due to the high affinity between carbon and chromium, it forms a series of complex carbides with chromium. So, from the perspectives of strength and corrosion resistance, the role of carbon in stainless steel is contradictory.

By understanding the laws of this impact, we can choose stainless steel with different carbon contents based on different usage requirements.

For example, the standard chromium content of the five grades of stainless steel, 0Crl3~4Cr13, which is widely used in industry and is the minimum, is 12~14%, which is determined after taking into account the factors that carbon and chromium form chromium carbide. The purpose is to make the chromium content in solid solution not lower than the low limit of 11.7% after carbon and chromium combine to form chromium carbide.

As for these five steel grades, due to their different carbon content, their strength and corrosion resistance are also different. 0Cr13~2Crl3 steel has good corrosion resistance but lower strength than 3Crl3 and 4Cr13 steel, and is mostly used to manufacture structural parts. The latter two steel grades can obtain high strength due to their high carbon content, and are mostly used to manufacture parts requiring high strength and wear resistance, such as springs and cutters. For example, in order to overcome the intergranular corrosion of 18-8 chromium nickel stainless steel, the carbon content of the steel can be reduced to below 0.03%, or elements (titanium or niobium) with a greater affinity than chromium and carbon can be added to prevent the formation of chromium carbide. For example, when high hardness and wear resistance become the main requirements, we can increase the carbon content of the steel while appropriately increasing the chromium content, achieving both hardness and wear resistance requirements while also taking into account certain corrosion resistance functions, In industry, stainless steel 9Cr18 and 9Cr17MoVCo steels are used for bearings, measuring tools, and cutting edges. Although their carbon content is as high as 0.85-0.95%, their chromium content has also been correspondingly increased, ensuring the requirements for corrosion resistance.

Overall, the carbon content of stainless steel currently used in industry is relatively low, with most stainless steels having carbon content ranging from 0.1 to 0.4%, while acid resistant steels have a majority of carbon content ranging from 0.1 to 0.2%. Stainless steel with a carbon content greater than 0.4% only accounts for a small portion of the total number of steel grades, because under most usage conditions, stainless steel always focuses on corrosion resistance. In addition, the lower carbon content is also due to certain process requirements, such as ease of welding and cold deformation.

3. The role of nickel in stainless steel is only realized after being combined with chromium

Nickel is an excellent corrosion-resistant material and an important alloying element in alloy steel. Nickel is an element that forms austenite in steel, but for low-carbon nickel steel to obtain pure austenite structure, the nickel content must reach 24%; Only when nickel content is 27% can the corrosion resistance of steel in certain media be significantly changed. So nickel cannot form stainless steel alone. However, when nickel and chromium coexist in stainless steel, nickel containing stainless steel has many valuable properties.

Based on the above situation, it can be seen that the role of nickel as an alloying element in stainless steel is that it causes changes in the structure of high chromium steel, thereby improving the corrosion resistance and process performance of stainless steel.

4. Manganese and nitrogen can replace nickel in chromium nickel stainless steel

Although chromium nickel austenitic steel has many advantages, in recent decades, due to the extensive development and application of nickel based heat-resistant alloys and heat strength steels containing less than 20% nickel, as well as the increasing demand for stainless steel in the chemical industry, the nickel reserves are relatively small and concentrated in a few regions, resulting in a contradiction between nickel supply and demand worldwide. Therefore, in the fields of stainless steel and many other alloys (such as steel for large castings and forgings, tool steel, hot strength steel, etc.), especially in countries where nickel resources are relatively scarce, scientific research and production practices have been widely carried out to save nickel and replace nickel with other elements. In this regard, research and application are more focused on using manganese and nitrogen to replace nickel in stainless steel and heat-resistant steel.

The effect of manganese on austenite is similar to that of nickel. But to be precise, the function of manganese is not to form austenite, but to reduce the critical quenching rate of steel, increase the stability of austenite during cooling, inhibit its decomposition, and maintain the austenite formed at high temperatures to room temperature. In terms of improving the corrosion resistance of steel, the role of manganese is not significant. For example, the change in manganese content in steel from 0 to 10.4% does not significantly alter the corrosion resistance of steel in air and acid. This is because manganese has little effect on improving the electrode potential of iron based solid solution, and the protective effect of the formed oxide film is also very low. Therefore, although there are manganese alloyed austenitic steels in industry (such as 40Mn18Cr4, 50Mn18Cr4WN, ZGMn13 steels, etc.), they cannot be used as stainless steel. The role of manganese in stabilizing austenite in steel is about half that of nickel, which means that 2% of nitrogen also stabilizes austenite in steel, and the degree of action is greater than that of nickel. For example, in order to obtain austenite structure in steel containing 18% chromium at room temperature, low nickel stainless steel with manganese and nitrogen replacing nickel and chromium manganese nitrogen non inducing steel with elemental nickel have been applied in industry, and some have successfully replaced the classic 18-8 chromium nickel stainless steel.

5. Adding titanium or niobium to stainless steel is to prevent intergranular corrosion.

6. Molybdenum and copper can improve the corrosion resistance of certain stainless steels.

The above specifically discusses the main factors that affect the performance and microstructure of stainless steel. In addition, there are other elements that play a role in stainless steel. Some are common impurities such as silicon, sulfur, phosphorus, etc., just like ordinary steel. Some are also added for specific purposes, such as cobalt, boron, selenium, rare earth elements, etc. From the perspective of the main property of corrosion resistance of stainless steel, these elements are not the main aspects compared to the nine elements discussed. However, they cannot be completely ignored as they also have an impact on the properties and structure of stainless steel.

Silicon is an element that forms ferrite and is a common impurity element in general stainless steel.

Cobalt, as an alloying element, is not widely used in steel due to its high price and more important applications in other areas such as high-speed steel, hard alloys, cobalt based heat resistant alloys, magnetic steels, or hard magnetic alloys. There are few common stainless steels that add cobalt as an alloying element. Common stainless steels, such as 9Crl7MoVCo steel (containing 1.2-1.8% cobalt), add cobalt not to improve corrosion resistance but to improve hardness, because the main purpose of this stainless steel is to manufacture cutting tools, scissors and surgical blades for slicing machines.

Adding 0.005% boron to Crl7Mo2Ti steel with boron high chromium ferrite can improve its corrosion resistance in boiling 65% acetic acid. Adding trace amounts of boron (0.0006~0.0007%) can improve the hot ductility of austenitic stainless steel. A small amount of boron, due to the formation of low melting point eutectic, increases the tendency for hot cracks to occur during welding of austenitic steel. However, when it contains a large amount of boron (0.5-0.6%), it can actually prevent the occurrence of hot cracks. Because when containing 0.5~0.6% boron, the austenite boride two-phase structure is formed, which reduces the melting point of the weld. When the solidification temperature of the molten pool is lower than that of the semi melted zone, the tensile stress generated by the base metal during cooling is borne by the weld metal in the liquid or solid state, which does not cause cracks. Even if cracks are formed near the crack zone, they can still be filled by the molten pool metal in the liquid solid state. Chromium nickel austenitic stainless steel containing boron has special applications in the atomic energy industry.

Phosphorus is an impurity element in general stainless steel, but its harm in austenitic stainless steel is not as significant as in general steel, so the content can be allowed to be higher. If some data suggests that it can reach 0.06%, which is beneficial for smelting control. The phosphorus content of individual manganese containing austenitic steels can reach 0.06% (such as 2Crl3NiMn9 steel) to 0.08% (such as Cr14Mnl4Ni steel). Utilizing the strengthening effect of phosphorus on steel, phosphorus is also added as an alloying element for age hardening stainless steel, such as PH17-10P steel (containing 0.25% phosphorus) and PH-HNM steel (containing 0.30 phosphorus).

Sulfur and selenium are also common impurities in general stainless steel. But adding 0.2~0.4% sulfur to stainless steel can improve its cutting performance, and selenium also has the same effect. Sulfur and selenium improve the cutting performance of stainless steel because they reduce its toughness. For example, the impact value of typical 18-8 chromium nickel stainless steel can reach 30 kg/cm2. The impact value of 18-8 steel containing 0.31% sulfur (0.084% C, 18.15% Cr, 9.25% Ni) is 1.8 kilograms per square centimeter; Including 0. The impact value of 18 8 steel with 22% selenium (0.094% C, 18.4% Cr, 9% Ni) is 3.24 kilograms per square centimeter. Both sulfur and selenium reduce the corrosion resistance of stainless steel, so their practical application as alloying elements for stainless steel is rare.

The application of rare earth elements in stainless steel mainly focuses on improving process performance. Adding a small amount of rare earth elements to Crl7Ti steel and Cr17Mo2Ti steel can eliminate bubbles caused by hydrogen gas in the ingot and reduce cracks in the billet. Adding 0.02-0.5% rare earth elements (cerium lanthanum alloy) to austenitic and austenitic ferritic stainless steels can significantly improve forging performance. There was once an austenitic steel containing 19.5% chromium, 23% nickel, molybdenum, copper and manganese. In the past, only castings could be produced due to hot working process properties. After adding rare earth elements, various profiles could be rolled.