What is the difference between ferritic stainless steel (SUS430) and martensitic stainless steel?
Ferritic system stainless steel is a chromium system stainless steel with iron and chromium as its main chemical components. It is an alloy that is corrosion-resistant, heat-resistant, has excellent processability, and often has magnetic properties. Compared with austenitic system stainless steel, it has corrosion resistance, processability and low strength material, but because it does not contain nickel, it is cheap and is sometimes used as a substitute material for austenitic system stainless steel. However, compared with Martensitic stainless steel, it has corrosion resistance, heat resistance and better processability. There are various types of steel with a wide range of characteristics. As a result, highly corrosion-resistant components used in harsh corrosive environments are used for a variety of purposes, from indoor household items and kitchen machines to outdoor building components.
Ferritic system stainless steel consists of metal structure "ferrite phase" stainless steel. The ferrite phase is soft and deformable because it can hardly dissolve carbon.
Like martensitic stainless steel, chromium is divided into "chromium-based stainless steel" as its main component and contains almost no nickel. Representative steel types. The chromium content in SUS430 is about 18%. SUS, a representative steel type of the Martensitic system, has a higher chromium content than 410, which is about 13%. But depending on the steel type, there are low steel types with a chromium content of about 11% and high steel types with a chromium content of about 32%.
The chemical composition of SUS, a representative steel type of ferrite system, is based on 430, and the JIS specifications (JISG4303:2012) are as shown in the table above. In the ferritic system, take this as an example. SUS is based on 430, and has many steel types that change the chromium, carbon content and various alloying elements.
The physical properties of ferritic stainless steel (SUS430) are shown in the table above. For comparison, the austenitic system is also recorded. (SUS304) and Martensitic system (SUS410) physical properties. Compared with the austenitic system, the ferritic system has high thermal conductivity but low thermal expansion coefficient. Therefore, the dimensional change from normal temperature to high temperature is small and the partial expansion change is small, so the thermal fatigue characteristics are excellent. Furthermore, unlike the Austenitic system, it always displays magnetic properties. This is due to the crystal structure, the "body-body cubic structure" ferrite system and martensite system are paramagnetic, and the "face-centered cubic structure" austenitic system is non-magnetic.
The mechanical properties of ferritic stainless steel (SUS430) and the mechanical properties JIS standard (JISG4303:2012) are as shown in the table above. For comparison, austenite is also introduced. (SUS304) and martensite (SUS410) mechanical properties. Compared with austenitic systems, ferritic systems do not have much difference in endurance and hardness, but their tensile strength and elongation are poor. This means it deforms easily and breaks with minimal deformation. However, since ferrite is difficult to machine and harden, it may not be worse than austenite. In addition, ferritic systems are materials used in an annealed state and are hardly solidified by heat treatment. Therefore, the mechanical properties of the annealed state are basically maintained after processing. On the other hand, the Austenitic and Martensitic systems can improve their strength through machining and heat treatment. In other words, ferritic systems are not suitable for high-intensity or load-bearing applications.
Ferritic stainless steels can embrittle in high and low temperature environments.
Embrittlement at 475℃
Ferrite systems will embrittle when exposed to temperatures ranging from 4000°C to 540°C within a few hours to dozens of hours. This phenomenon is caused by the separation of iron multi-structure and chromium multi-structure, which proceeds rapidly at 475°C, so it is called "475°C embrittlement". When embrittled at 475°C, the hardness increases, but the ductility increases. The toughness decreases, it is easy to be damaged, and the corrosion resistance also decreases. This embrittlement can be maintained at 600°C for a certain period of time at the above temperature to eliminate chromium in solid solution again.
σ phase embrittlement
In addition, ferrite systems will also experience embrittlement if kept in the temperature range of 550°C to 800°C for more than hundreds of hours. Embrittlement consists of compounds between iron and chromium metals. σ phase embrittlement caused by the precipitation of "σ phase". σ becomes the cause of cracks and cracks due to the brittleness of hard objects. In order to eliminate phase embrittlement, it is necessary to maintain 800°C at the above temperature for a certain period of time. σ phase embrittlement It occurs not only in the ferritic system but also in the austenitic system.
Low temperature brittleness
In ferrite systems, due to the sharp drop in impact resistance below a certain temperature, the "ductile-brittle transition temperature", there is a risk of brittle failure when used at low temperatures. This property is called brittle failure. "Low temperature brittleness" is common in martensitic systems and other metals with a physical and mental cubic structure. In order to improve the low-temperature brittleness of the ferrite system and reduce the carbon and nitrogen content, it is effective to add titanium and niobium. In addition, ferritic stainless steel with lower carbon and nitrogen content than before is called "high-purity ferritic stainless steel".
Ferritic stainless steel processing
Although ferritic stainless steel is not like the austenitic system, it is as easy to process as ordinary steel. At the same time, it has better processing performance than the Markovian system. However, the ferritic system is better than the austenitic system. In addition, the ferrite system is different from the austenite system and is difficult to process. Processing abnormalities (austenite changes to martensite) do not occur and the processing difficulty is low. In addition, in order to improve the workability of the ferrite system, it is effective to reduce the carbon and nitrogen content and add titanium and niobium. Regarding machinability, such as machinability, such as machinability. SUS430F is improved by adding sulfur.
Regarding weldability, attention should be paid to the occurrence of embrittlement at 475°C caused by heating. The grains in the heat-affected zone will become coarse. Embrittlement at 475°C can lead to ductility, toughness, and reduced corrosion resistance, but can be avoided by increasing the cooling rate after welding. On the other hand, the coarsening of grains will significantly reduce the ductility and toughness of the heat-affected part. Heat treatment can eliminate the reduction in ductility, but the toughness will not be restored. Adding titanium and zirconium maximizes, adding titanium and zirconium is effective. In addition, in ferritic systems, grain boundary corrosion is less likely to occur during austenitic welding. The intergranular corrosion resistance of ferritic systems can be further improved by reducing the carbon content and adding titanium and niobium.
Ferritic stainless steels vary greatly depending on the characteristics of the steel type, and therefore have different uses depending on the steel type. Therefore, ferritic systems are divided into the following five groups for listed uses.
The chromium content is 10%~14%, including SUS405.SUS409.SUS410L and other combinations. The chromium content is small and the price is the lowest. How much rust is allowed due to low corrosion resistance. For use in containers. . Buses, non-corrosive parts of automobiles, etc.
The group with a chromium content of 14% to 18% corresponding to SUS430 is the most widely used in ferrite systems. Because of proportion. SUS304 is very cheap, so it is often used as a partial replacement material for SUS304. Mainly used for indoor panels, household products, washing machine drums, pots and other indoor purposes.
The chromium content is 14%~18%Ti.Nb and other stabilizing elements include SUS430LX·SUS430F and other groups. The processability and weldability are improved by adding stabilizing elements. Many steel grades show proximity. The characteristics of SUS304 are used in flow tables, exhaust devices, welding parts of washing machines, etc.
Chromium content exceeding 18% Mo includes groups such as SUS434, SUS436, SUS444, etc., which exhibit high corrosion resistance due to the presence of molybdenum. Primary uses include outdoor panels. .Various water tanks.Microwave oven parts, etc.
The chromium content of 18% to 30% includes SUS445·SUSXM27·SUS447 and other groups, increasing the chromium content, adding molybdenum, etc. Among ferritic systems, it is the most corrosion-resistant group. Mainly used in chemical plants with harsh corrosive environments such as sea water and contact with pharmaceuticals.
