Causes and Influencing Factors of Martensite Production According to the composition of the different components, stainless steel can be divided into ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, duplex stainless steel, and precipitation hardening stainless steel. Among them, austenitic stainless steel is used. The largest amount. Due to the structure of the structure, austenitic stainless steel is theoretically non-magnetic, but commonly used 18-8 series (304, etc.) austenitic stainless steels often produce magnetic properties after cold working, especially the degree of processing of the head, elbow, etc. Larger parts are particularly noticeable. Some studies at home and abroad have shown that the magnetic properties of the parts of these heads are mainly due to the cold forming of austenitic stainless steels and the transformation of some martensite to austenite.
1. Martensitic transformation mechanism
Usually the martensite structure can be obtained through the quenching process, that is to say, the steel is heated to the austenite transformation temperature above, held for a certain period of time, the steel is austenitized, and then rapidly cooled. When austenite falls below the Ms point of the martensitic transformation temperature, its microstructure begins to transform into martensite until the temperature Mf stops. Experimental studies have shown that when austenitic stainless steels are cold-formed, some austenite may undergo martensite transformation due to tensile and compressive stresses, and martensite and austenite share a lattice, which is sheared at the poles. Diffusion-free phase change occurs in a short time, and this martensite is also called deformed martensite.
2. Factors influencing martensitic transformation
The main factors influencing the martensitic transformation are: the stability of austenitic stainless steel, the amount of processing deformation, processing methods, etc.
2.1 The influence of chemical composition
According to the stability of austenite, austenitic stainless steel can be divided into steady state and metastable austenitic stainless steel. Metastable austenitic stainless steels are more likely to produce martensite under cold deformation. For example, 304, 304L, and 321 are easier to produce martensite in cold working, while 316 and 316L do not produce martensite.
The stability of austenitic stainless steel is determined by its chemical composition. The more austenite elements such as Ni, N, C, and Mn are, the more stable austenite is, and the ferrite elements such as Cr, Mo, and Nb are in solid solutions. The medium has a diffusion effect, and when the content is appropriate, it can prevent austenite from transforming into martensite, but when it is excessive, it will promote the transformation of austenite to martensite and ferrite.
2.2 The effect of processing deformation In the same conditions, the greater the processing deformation, the greater the amount of deformation martensite.
2.2 Influence of Processing Methods The forming process of austenitic stainless steel heads generally adopts cold stamping or cold spinning. Cold stamping uses a standard mold for stamping and forming. Cold spinning is formed by repeated extrusion of two molds. The degree of cold stamping is relatively intense (fast deformation), and the martensite content of deformation is higher under the same conditions. In addition, the production of martensite is also related to the processing temperature. The higher the processing temperature, the lower the content of deformed martensite.
3 The effect of martensite transformation on equipment performance
Austenite is a face-centered cubic structure, while martensite is a body-centered cubic structure; the density of martensite is lower than that of austenite, so after the transformation, the volume expands, causing internal residual stress. The grain size of austenite microstructure is fine, and the mechanical properties such as strength and toughness are good, while the martensite microstructure has high hardness and poor plasticity. When the martensite phase change is large, the influence on the performance of the steel can not be ignored.
1) Due to the volume change, the martensitic transformation will cause internal residual stress, which may cause cracks and other defects in the equipment.
2) The potential of martensite is lower than that of austenite. In the corrosive medium environment, martensite is an anode relative to austenite, and it is preferentially corroded, resulting in electrochemical corrosion of stainless steel.
3) Some scholars believe that there is a certain relationship between the local corrosion of metastable stainless steel and the amount of deformed martensite.
4) Due to the existence of residual stress and electrochemical corrosion conditions, deformation induced martensite is considered as one of the important causes of stress corrosion in austenitic stainless steels in CL ion environments.
4 Preventive measures Based on the causes and influencing factors of martensite production, the following are the main preventive measures:
1) Increase the content of austenitizing elements within the allowable range of the standard when ordering the head plate.
2) Material upgrades using materials with higher Ni contents such as 316L and 310
3) Improve the processing technology. If a manufacturer develops a new process, the head is cold-pressed and pre-pressed, and then heated to approximately 250°C. Due to the use of precompression, repeated compression is reduced to reduce the martensitic phase change, and the spinning temperature is 250°C, which is higher than Md (the upper temperature limit of the martensitic transformation caused by processing), thus avoiding the cold working of austenitic stainless steels. Larger magnetic.
4)Solid-melt heat treatment completely eliminates magnetism and work hardening. However, the cost of solid solution treatment is high, and it has a great influence on the deformation of the head size.
5) Strengthen the quality management of each link, strictly control the quality of raw materials, and strictly abide by the processing procedures.