Properties and Microstructure of Stainless Steel
At present,there are more than 100 known chemical elements,and about 20 kinds of chemical elements can be found in commonly used steel materials in industry.Stainless steel is a special steel system formed by people's long-term corrosion resistance,with a total of more than ten common elements.Except that iron is the basic element of steel,the elements that have a great impact on the properties and structure of stainless steel are carbon,chromium,nickel,manganese,silicon,molybdenum,titanium,niobium,titanium,manganese,nitrogen,copper and cobalt.In addition to carbon,silicon and nitrogen,these elements are transition elements in the periodic table of chemical elements.
In fact,stainless steel used in industry contains many elements at the same time.When several elements coexist in stainless steel as a whole,their influence is much more complex than when they exist alone.In this case,we should consider not only the influence of each element itself,but also the influence of each other.Therefore,the structure of stainless steel depends on various elements.The sum of element effects.
1.Effect of different elements on properties and microstructure of stainless steel
1-1.Decisive role of chromium in stainless steel
Only one element determines the properties of chromium in stainless steel.Every kind 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 the addition of chromium as an alloy element in steel promotes the internal contradiction movement of chromium and is conducive to corrosion resistance.This change can be explained from the following aspects:
(1)Chromium increases the electrode potential of iron-based solid solution
(2)Chromium absorption iron electron passivation iron
Passivation is a phenomenon that improves the corrosion resistance of metals and alloys due to blocked anodic reaction.There are many theories about passivation of metals and alloys,such as film theory,adsorption theory and electron arrangement theory.
Duality of carbon in stainless steel
Carbon is one of the main elements in industrial steel.The performance and structure of steel largely depend on the content and distribution of carbon in steel,especially 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 austenitic element,and its role is very large(about 30 times that of nickel).On the other hand,due to the affinity between carbon and chromium,it forms a series of complex carbides with chromium.Therefore,in terms of the strength and corrosion resistance of candles,the role of carbon in stainless steel is contradictory.
To understand the law of this effect,stainless steel with different carbon content can be selected from different requirements.
For example,the standard chromium content of five stainless steel grades 0crl3-4cr13,which are widely used and less used in industry,is set at 12-14%,which is determined after considering the factors of carbon and chromium forming chromium carbide.The purpose is that after the synthesis of chromium carbide from carbon and chromium,the chromium content in the solid solution should not be less than 11.7%of the chromium content.
For these five kinds of steel,due to different carbon content,the strength and corrosion resistance are also different.0cr13-2cr3 steel has better corrosion resistance than 3cr3 and 4Cr13 steel,but its strength is lower.The latter is mainly used to manufacture structural parts.The latter two steels can obtain higher strength due to their high carbon content,and are used to manufacture parts requiring high strength and wear resistance,such as springs and knives.For example,in order to overcome the intergranular corrosion of 18-8 chromium nickel stainless steel,the carbon content in the steel can be reduced to less than 0.03%,or elements with more affinity than chromium and carbon(titanium or niobium)can be added to prevent the formation of chromium carbide.When high hardness and wear resistance become the main requirements,we can increase the carbon content of steel while appropriately increasing the chromium content to meet the requirements of hardness and wear resistance.The requirements of wear resistance and corrosion resistance are also considered.Industrial bearings,measuring tools and blades are made of 9Cr18 and 9cr17movco stainless steel.Although the carbon content is as high as 0.85-0.95%,due to the corresponding increase in chromium content,it can still meet the requirements of corrosion resistance.
Generally speaking,the carbon content of industrial stainless steel is relatively low.Most stainless steels have a carbon content between 0.1%and 0.4%,white.