Effect of elements in stainless steel

At present, there are more than 100 known chemical elements, and more than 20 kinds of chemical elements can be encountered in steel materials commonly used in industry. For the special series of stainless steel formed by people’s long-term struggle against corrosion, there are more than ten kinds of elements most commonly used. In addition to the basic element iron, the most influential elements on the properties and microstructure of stainless steel are carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, titanium, manganese, nitrogen, copper, cobalt, etc. In addition to carbon, silicon and nitrogen, these elements are in the transition group in the periodic table of chemical elements.
In fact, there are several or even more than ten kinds of elements in industrial stainless steel at the same time. When several elements coexist in the unity of stainless steel, their influence is much more complex than when they exist alone. In this case, we should not only consider the role of each element itself, but also pay attention to their mutual influence. Therefore, the microstructure of stainless steel depends on various elements The sum of the effects of elements.

1、 Effects of various elements on properties and microstructure of stainless steel
1-1. The decisive role of chromium in stainless steel: there is only one element that determines the nature of stainless steel, which is chromium. Each 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 properties of stainless steel is that after adding chromium as an alloy element to the steel, the contradictory movement inside the steel will develop to the aspect of corrosion resistance. This change can be explained in the following aspects:
① Chromium increases the electrode potential of iron-based solid solution
② Chromium passivates iron by absorbing its electrons
Passivation is a phenomenon that the corrosion resistance of metals and alloys is improved due to the inhibition of anodic reaction. There are many theories about passivation of metals and alloys, including film theory, adsorption theory and electron arrangement theory.

1-2. Duality of carbon in stainless steel
Carbon is one of the main elements of industrial steel. The properties and microstructure of steel are largely determined by the content and distribution of carbon in steel, especially in stainless steel. The influence of carbon on the microstructure of stainless steel is mainly manifested in two aspects. On the one hand, carbon is an element that stabilizes austenite and plays a great role (about 30 times of nickel); on the other hand, due to the great affinity between carbon and chromium, it forms a series of complex carbides with chromium. Therefore, the role of carbon in stainless steel is contradictory in terms of strength and corrosion resistance.

We can choose stainless steel with different carbon content according to different requirements.
For example, the standard chromium content of 0crl3-4cr13, the most widely used stainless steel in industry, is 12-14%, which is determined after taking into account the factors of forming chromium carbide from carbon and chromium. The purpose is to make the chromium content in solid solution not less than 11.7%, the minimum chromium content. Spring
Because of the different carbon content, the strength and corrosion resistance of the five steel grades are different. 0Cr13 ~ 2Crl3 steel has better corrosion resistance, but the strength is lower than 3Crl3 and 4Cr13 steel, which are mostly used to manufacture structural parts. The latter two grades can obtain high strength due to their high carbon content. They are mainly used to make springs, cutting tools and other parts that require high strength and wear resistance. For example, in order to overcome the intergranular corrosion of 18-8 Cr Ni stainless steel, the carbon content of the steel can be reduced to less than 0.03%, or the elements (titanium or niobium) with greater affinity than chromium and carbon can be added to prevent chromium carbide formation. For example, when high hardness and wear resistance become the main requirements, we can increase the chromium content properly while increasing the carbon content of the steel to meet the hardness and wear resistance In industry, 9Cr18 and 9cr17movco steels are used as bearings, measuring tools and cutting edges. Although the carbon content of 9Cr18 and 9cr17movco steels is as high as 0.85-0.95%, the requirements of corrosion resistance are still guaranteed due to the corresponding increase of chromium content.

Generally speaking, the carbon content of stainless steel used in industry is relatively low. The carbon content of most stainless steel is between 0.1% and 0.4%, while that of acid resistant steel is 0.1-0.2%. Stainless steel with carbon content greater than 0.4% only accounts for a small part of the total steel grades, because in most service conditions, stainless steel is always corrosion-resistant. In addition, the low carbon content is also due to some process requirements, such as easy welding and cold deformation.
1-3. The role of nickel in stainless steel comes into play after it is combined with chromium
Nickel is an excellent corrosion resistant material and an important alloying element of alloy steel. Nickel is the element forming austenite in steel, but the content of nickel in low carbon nickel steel should reach 24% to obtain pure austenite structure, and the corrosion resistance of steel in some media will be changed significantly only when the nickel content is 27%. So nickel can’t be made of stainless steel alone. However, when nickel and chromium exist in stainless steel at the same time, nickel containing stainless steel has many valuable properties.
Based on the above situation, the role of nickel as an alloy element in stainless steel is that it changes the microstructure of high chromium steel, thus improving the corrosion resistance and process performance of stainless steel.

 

1-4. 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 development and application of nickel based heat-resistant alloy and heat-resistant steel containing less than 20% nickel, as well as the increasing development of chemical industry, the demand for stainless steel is increasing, and the nickel mineral reserves are less and concentrated in a few areas, so there is a contradiction between the supply and demand of nickel in the world. Therefore, in the fields of stainless steel and many other alloys (such as steel for large casting and forging, tool steel, hot strength steel, etc.), especially in the countries where nickel resources are relatively scarce, the scientific research and production practice of nickel saving and substituting other elements for nickel are widely carried out. In this regard, manganese and nitrogen are used 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 more exact, the role of manganese is not to form austenite, but to reduce the critical quenching speed of steel, increase the stability of austenite during cooling, inhibit the decomposition of austenite, and keep the austenite formed at high temperature to normal temperature. In the aspect of improving the corrosion resistance of steel, manganese has little effect. For example, the manganese content in steel changes from 0 to 10.4%, and the corrosion resistance of steel in air and acid does not change significantly. This is because manganese has little effect on improving the electrode potential of iron-based solid solution, and the protective effect of oxide film formed is also very low. Therefore, although there are austenitic steels (such as 40mn18cr4, 50mn18cr4wn, ZGMn13 steel, etc.) alloyed with manganese in industry, they can not be used as stainless steel springs. The role of manganese in stabilizing austenite in steel is about half of that of nickel, that is, the effect of 2% nitrogen on stabilizing austenite in steel is also greater than that of nickel. For example, in order to obtain austenite structure of steel containing 18% chromium at room temperature, low nickel stainless steel with manganese and nitrogen instead of nickel and chromium manganese nitrogen free steel with elemental nickel have been applied in industry, some of which have successfully replaced the classic 18-8 chromium nickel stainless steel.
1-5. Adding titanium or niobium in stainless steel is to prevent intergranular corrosion.1-6. Molybdenum and copper can improve the corrosion resistance of some stainless steels.

1-7. Effect of other elements on properties and microstructure of stainless steel
The influence of the above nine elements on the properties and structure of stainless steel, in addition to these elements which have a greater impact on the properties and structure of stainless steel, there are some other elements in stainless steel. Some of them are common impurity elements, such as silicon, sulfur, phosphorus and so on, and some are added for some specific purposes, such as cobalt, boron, selenium, rare earth elements, etc. From the main property of corrosion resistance of stainless steel, these elements are not the main aspects compared with the nine elements discussed. However, they can not be completely ignored, because they also affect the properties and microstructure of stainless steel.
Silicon is an element that forms ferrite, and is a common impurity element in stainless steel.

As an alloying element, cobalt is seldom used in steel because of its high price and its more important use in other fields (such as high speed steel, cemented carbide, cobalt based heat-resistant alloy, magnetic steel or hard magnetic alloy, etc.). Cobalt is not used as alloy element in common stainless steel, such as 9crl7movco steel (containing 1.2-1.8% cobalt). The purpose of adding cobalt is not to improve the corrosion resistance, but to improve the hardness, because the main purpose of this stainless steel is to make slicing machine tools, scissors and surgical blades.

Boron: adding 0.005% boron to crl7mo2ti steel can improve the corrosion resistance in boiling 65% acetic acid. The hot ductility of austenitic stainless steel can be improved by adding a small amount of boron (0.0006-0.0007%). A small amount of boron forms eutectic with low melting point, which increases the tendency of hot crack in austenitic steel welding. However, when there is more boron (0.5-0.6%), it can prevent hot crack. Because when 0.5-0.6% boron is contained, the austenite boride two-phase structure is formed and the melting point of the weld is reduced. When the solidification temperature of the molten pool is lower than that of the semi melting zone, the tensile stress produced by the base metal during cooling is borne by the weld metal in the liquid or solid state, which will not cause cracks at this time. Even if cracks are formed near the weld pool, they can be filled by the molten pool metal in the liquid solid state. Chromium nickel austenitic stainless steel containing boron has special application in atomic energy industry.

Phosphorus: it is an impurity element in general stainless steel, but its harmfulness in austenitic stainless steel is not as obvious as that in ordinary steel, so the content can be allowed to be higher, if some data put forward that it can reach 0.06%, so as to facilitate smelting control. The phosphorus content of individual manganese bearing austenitic steels can reach 0.06% (for example, 2crl3nimn9 steel) to 0.08% (such as cr14mnl4ni steel). Because of the strengthening effect of phosphorus on steel, there are also adding phosphorus as alloy element of age hardening stainless steel. Ph17-10p steel (containing 0.25% phosphorus) is ph-hnm steel (containing 0.30 phosphorus).

Sulfur and selenium: in general stainless steel, there are often impurity elements. However, adding 0.2-0.4% sulfur to stainless steel can improve the cutting performance of stainless steel, and selenium has the same effect. Sulfur and selenium improve the machinability of stainless steel because they reduce the toughness of stainless steel. For example, the impact value of 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 kg / cm2, and the impact value is 0. The impact value of 22% se 18-8 steel (0.094% C, 18.4% Cr, 9% Ni) is 3.24 kg / cm2. Both sulfur and selenium decrease the corrosion resistance of stainless steel, so they are used as the combination of stainless steel in practice