Classification of stainless steel and knowledge of forging and heat treatment
Stainless steel refers to steel that does not rust in weakly corrosive media such as air and fresh water. It is mainly achieved by adding a certain amount of alloying elements such as chromium (Cr) to ordinary steel to form a dense oxide film (mainly Cr₂O₃) on the surface of the steel. This film can prevent further corrosion of the steel from the external medium. Stainless steel generally has good corrosion resistance, high strength and toughness, and is bright in appearance and easy to clean.
1. Classification of stainless steel
1.1 Classification by chemical composition
(1) Chromium stainless steel
Chromium (Cr) is the main alloying element, and the chromium content is generally between 12% and 30%. For example, 430 stainless steel in ferritic stainless steel has a chromium content of about 16-18%. It has good corrosion resistance, especially in oxidizing environments. This is because chromium can form a dense protective film of chromium oxide (Cr₂O₃) on the surface of stainless steel, preventing oxygen and other corrosive media from further corroding the metal matrix. This type of stainless steel is relatively low in cost and is often used in the manufacture of kitchen utensils such as pots and pans.
(2) Chromium-nickel stainless steel
In addition to chromium, it also contains nickel (Ni), and the nickel content is usually around 8%-25%. A typical representative is austenitic stainless steel 304, which contains about 18% chromium and 8-10% nickel. The addition of nickel changes the crystal structure of stainless steel from body-centered cubic (ferrite structure) to face-centered cubic (austenite structure). This structure gives it good toughness, machinability and corrosion resistance, and is widely used in many fields such as architectural decoration (such as stainless steel curtain walls) and chemical equipment (such as reactors).
(3) Chromium-manganese-nitrogen stainless steel
Manganese (Mn) and nitrogen (N) are used to partially replace nickel to stabilize the austenite structure. For example, 201 stainless steel has a lower cost than chromium-nickel stainless steel, a higher manganese content of about 7.5%, a nitrogen content of about 0.25%, and a chromium content of about 16-18%. This type of stainless steel is used in some occasions where corrosion resistance is not extremely high but cost is more sensitive, such as interior decorative strips.
1.2 Classification by metallographic structure
(1) Ferritic stainless steel
The metallographic structure is mainly ferrite, and no phase change occurs during heating and cooling. This type of stainless steel has good corrosion resistance, thermal conductivity and oxidation resistance. For example, 409L stainless steel is widely used in automobile exhaust systems because it can withstand corrosion and oxidation from high-temperature exhaust gas and has a relatively low cost. Its toughness is slightly worse than that of austenitic stainless steel, and its strength is also lower, but it performs well in some environments that do not require large external forces.
(2) Austenitic stainless steel
The metallographic structure is austenite, non-magnetic or weakly magnetic. It has very good corrosion resistance and can remain stable in a variety of chemical media. For example, 316L stainless steel, in addition to chromium and nickel, also contains molybdenum (Mo). The addition of molybdenum further improves its corrosion resistance in highly corrosive media such as chloride ions. This type of stainless steel is often used in medical devices (such as surgical instruments) and food processing equipment (such as milk pipes) and other fields that have extremely high requirements for hygiene and corrosion resistance.
(3) Martensitic stainless steel
The martensitic structure is obtained mainly through heat treatment such as quenching and tempering. It has high strength and hardness, and also has certain corrosion resistance. For example, 420 stainless steel, with a chromium content of about 12-14%, can achieve high hardness through heat treatment and is often used to manufacture knives, valves and other parts that require high strength and certain corrosion resistance.
(4) Duplex stainless steel
Its structure is composed of austenite and ferrite. Generally, the ratio of austenite phase to ferrite phase is close to 1:1. Duplex stainless steel combines the advantages of austenitic stainless steel and ferritic stainless steel, with good strength and toughness, as well as excellent corrosion resistance. For example, 2205 duplex stainless steel is widely used in seawater treatment equipment, chemical pipelines and other fields because it can resist chloride ion stress corrosion cracking and has higher strength than austenitic stainless steel.
1.3 Classification by use
(1) Atmospheric corrosion resistant stainless steel
This type of stainless steel is mainly used in outdoor environments and can resist corrosive media such as humidity, oxygen, and sulfur dioxide in the atmosphere. For example, 304 stainless steel is used for building facade decoration. It can maintain its surface gloss and structural integrity after long-term exposure to the atmosphere.
(2) Acid and alkali resistant stainless steel
It is used in places where it is exposed to various acids, alkalis and other chemical substances. For example, 316L stainless steel is used in the chemical industry to process equipment containing acidic media such as sulfuric acid and hydrochloric acid. Its molybdenum content makes it more resistant to corrosive ions such as chloride ions.
(3) High temperature resistant stainless steel
It can maintain stable performance in high temperature environments. For example, 310S stainless steel has a chromium content of about 24-26% and a nickel content of about 19-22%. It can be used at temperatures above 1000°C and is often used in the manufacture of high temperature furnace components, such as furnace tubes.
2. Knowledge of stainless steel forging
2.1 Preparation before forging
(1) Billet selection
Select the appropriate billet according to the final product shape and performance requirements of the stainless steel. The billet can be a bar, plate or ingot. For parts with complex shapes, it may be necessary to select ingots for forging to ensure the uniformity of the material. At the same time, the size of the blank should be considered, and sufficient processing allowance should be left. Generally, the allowance ranges from a few millimeters to more than ten millimeters, depending on the size and precision requirements of the parts.
(2) Heating equipment
Stainless steel forging requires suitable heating equipment. Commonly used ones are resistance furnaces and gas furnaces. The resistance furnace has accurate heating temperature control and high thermal efficiency, which can make the stainless steel blank evenly heated. The gas furnace has a fast heating speed, but the temperature control is relatively complicated. The control of heating temperature is crucial for stainless steel forging, because different types of stainless steel have different initial forging temperature and final forging temperature ranges. For example, the initial forging temperature of austenitic stainless steel is generally around 1150-1200℃, and the final forging temperature is not less than 900℃.
2.2 Forging process
(1) Forging method
Including free forging and die forging. Free forging has high flexibility and is suitable for single-piece small batch production or large parts with simple shapes. Forging workers gradually shape the blank by operating forging tools such as air hammers, steam-air hammers, etc. Die forging uses a die to shape the blank. It has high production efficiency and is suitable for mass production of parts with complex shapes and high dimensional accuracy requirements. For example, stainless steel connecting rods for automobile engines are usually produced by die forging.
(2) Selection of forging ratio
The forging ratio is an indicator of the degree of metal deformation during the forging process. For stainless steel forging, a suitable forging ratio can refine the grains and improve the mechanical properties of the material. Generally, a forging ratio between 3 and 5 is more appropriate. For example, through a forging process with a forging ratio of 4, the grains of stainless steel can be effectively refined, and its strength and toughness can be improved. However, a forging ratio that is too high may cause defects such as surface cracks.
(3) Post-forging treatment
① Surface cleaning
After forging, the surface of stainless steel will have impurities such as oxide scale and lubricant residue. It needs to be cleaned by pickling, shot peening, etc. Pickling can remove oxide scale. Generally, a nitric acid-hydrofluoric acid mixed solution is used. This solution can effectively dissolve the oxide layer on the surface of stainless steel. Shot peening uses high-speed projectiles to impact the surface, remove impurities on the surface and generate a certain amount of compressive stress on the surface, thereby improving the fatigue life of the parts.
② Quality inspection
Quality inspection should be carried out on forged stainless steel parts. It mainly includes dimensional accuracy inspection, surface quality inspection and internal defect inspection. Dimensional accuracy can be measured using tools such as calipers and micrometers; surface quality can be checked by visual observation or using flaw detection equipment to check whether there are defects such as cracks and folds; internal defects can be checked by ultrasonic flaw detection, radiographic flaw detection and other methods to ensure that there are no holes, inclusions and other problems inside the parts.
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Knowledge of heat treatment of stainless steel
3.1 Solution treatment (for austenitic stainless steel)
(1) Purpose
The main purpose of solution treatment is to fully dissolve precipitated phases such as carbides in the austenite matrix to obtain a single-phase austenite structure, thereby improving the corrosion resistance and toughness of stainless steel. For example, after solution treatment, the resistance of 304 stainless steel to intergranular corrosion is significantly enhanced.
(2) Process
The stainless steel is heated to a high temperature (usually 1050-1150℃), kept at a certain temperature for a certain period of time (depending on the size of the parts and the amount of furnace loading, generally kept at a temperature of 1-2 hours) to fully dissolve the alloying elements, and then cooled rapidly (usually by water cooling). The purpose of rapid cooling is to prevent carbides from re-precipitating during the cooling process, thereby maintaining a single-phase austenite structure.
3.2 Quenching and tempering (for martensitic stainless steel)
(1) Purpose and process of quenching
The purpose is to obtain a martensitic structure and improve the hardness and strength of the stainless steel. During quenching, the martensitic stainless steel is heated to the quenching temperature (such as the quenching temperature of 420 stainless steel is about 1000-1050℃), kept at a temperature for a suitable period of time, and then cooled by oil cooling or air cooling. Oil cooling can achieve a better quenching effect, but attention should be paid to fire safety during operation; air cooling is relatively slow and is suitable for some situations where the quenching stress requirements are low.
(2) Purpose and process of tempering
Tempering is to eliminate quenching stress, stabilize the structure and improve toughness. The tempering temperature is generally between 200 and 600°C. The appropriate tempering temperature is selected according to the specific performance requirements. For example, when the tempering temperature is around 400°C, the quenching stress can be effectively eliminated while maintaining a high hardness and strength, making it less likely for martensitic stainless steel parts to crack during use.
3.3 Stabilization treatment (for austenitic stainless steel containing titanium or niobium)
(1) Purpose
For austenitic stainless steel containing titanium (Ti) or niobium (Nb), the purpose of stabilization treatment is to prevent intergranular corrosion. In stainless steel, titanium and niobium can combine with carbon to form stable carbides, thereby preventing chromium from combining with carbon to form chromium carbide (Cr₂₃C₆), because the formation of chromium carbide consumes chromium, resulting in intergranular chromium depletion and reducing the corrosion resistance of stainless steel.
(2) Process
Heat the stainless steel to 850-900°C, keep warm for 2-4 hours, and then air cool. Through this treatment, titanium or niobium can be fully combined with carbon, making the stainless steel less prone to intergranular corrosion during subsequent use.
