Stainless Steel: Characteristics and Applications

The development of stainless steel stems from its inherent properties, which in turn meet various demands. While its key characteristic is corrosion resistance, stainless steel is not solely defined by this trait. It also possesses unique mechanical properties (such as yield strength, tensile strength, creep strength, high-temperature strength, and low-temperature strength), physical properties (including density, specific heat capacity, linear expansion coefficient, thermal conductivity, electrical resistivity, magnetic permeability, and elastic modulus), processability (such as formability, weldability, and machinability), as well as metallographic characteristics (phase composition, microstructure, etc.). These properties collectively define stainless steel. Below, we will briefly introduce some of its fundamental characteristics.

(1) Strength (Tensile Strength, Yield Strength)

The strength of stainless steel is influenced by various factors, with the addition of different chemical elements, primarily metallic elements, being the most crucial and fundamental. Different types of stainless steel exhibit varying strength characteristics due to differences in their chemical compositions.

(a) Martensitic Stainless Steels

Martensitic stainless steels, like ordinary alloy steels, possess the ability to harden through quenching. Therefore, a wide range of mechanical properties can be achieved by selecting appropriate grades and heat treatment conditions.

Martensitic stainless steels can be broadly classified as iron-chromium-carbon stainless steels. They can be further divided into martensitic chromium stainless steels and martensitic chromium-nickel stainless steels. The trends in strength changes when adding elements such as chromium, carbon, and molybdenum to martensitic chromium stainless steels, as well as the strength characteristics when adding nickel to martensitic chromium-nickel stainless steels, are described below.

Under quenched and tempered conditions, increasing the chromium content in martensitic chromium stainless steels leads to an increase in ferrite content, thereby reducing hardness and tensile strength. For low-carbon martensitic chromium stainless steels in the annealed condition, an increase in chromium content results in a slight increase in hardness and a slight decrease in elongation. With a fixed chromium content, an increase in carbon content raises the hardness of the steel after quenching while reducing its plasticity. The primary purpose of adding molybdenum is to enhance the steel's strength, hardness, and secondary hardening effect. The effect of molybdenum addition becomes particularly pronounced after low-temperature quenching, with its content typically being less than 1%. In martensitic chromium-nickel stainless steels, the presence of a certain amount of nickel reduces the δ-ferrite content in the steel, allowing it to achieve higher hardness values.

The chemical composition of martensitic stainless steels is characterized by the addition of elements such as molybdenum, tungsten, vanadium, and niobium on the basis of different combinations of 0.1%-1.0% carbon and 12%-27% chromium. Due to their body-centered cubic structure, their strength decreases sharply at high temperatures. However, below 600°C, they exhibit high-temperature strength and creep strength that are among the highest among all types of stainless steels.

(b) Ferritic Stainless Steels

According to research findings, when the chromium content is less than 25%, the ferritic structure inhibits the formation of martensitic structure, causing the strength to decrease with increasing chromium content. When the chromium content exceeds 25%, the solid solution strengthening effect of the alloy slightly increases the strength. An increase in molybdenum content facilitates the formation of a ferritic structure, promotes the precipitation of α', β, and χ phases, and enhances strength through solid solution strengthening. However, it also increases notch sensitivity, thereby reducing toughness. The effect of molybdenum on enhancing the strength of ferritic stainless steels is greater than that of chromium.

The chemical composition of ferritic stainless steels is characterized by containing 11%-30% chromium, with the addition of niobium and titanium. Their high-temperature strength is the lowest among all types of stainless steels, but they exhibit strong resistance to thermal fatigue.

(c) Austenitic Stainless Steels