Causes and effects of 304 stainless steel magnetism

Update:16-06-2020
Summary:

1 Introduction When a new batch of equipment (E-450, D- […]

1 Introduction
When a new batch of equipment (E-450, D-450, etc.) of the 1# ethylene glycol EO capacity expansion and transformation project was shipped to the site, we found that its austenitic stainless steel head (near the flange) had a larger magnetic. As we all know, austenitic stainless steel should not be magnetic. It was initially suspected that the stainless steel material used was unqualified. After spectroscopic inspection, no abnormality was found in the material. They are all qualified S30408 stainless steel. After communicating with the technical personnel of the manufacturer and consulting relevant materials, it is basically determined that the magnetic properties of the head are due to the change in the structure of the austenitic stainless steel during the cold working deformation, and some of the austenite is transformed into martensite. The body-centered cubic martensite is magnetic.
Then, the reasons for the martensite organization, which factors are related, whether it has an impact on the performance of the equipment, how to avoid the occurrence of martensite and other issues are the users and equipment management personnel need to pay attention.
2 The causes and influencing factors of martensite according to the composition of the structure, stainless steel can be divided into ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, duplex stainless steel and precipitation hardening phase stainless steel, of which The most used one. Due to the structure and structure, austenitic stainless steel is theoretically non-magnetic, but the commonly used 18-8 series (304, etc.) austenitic stainless steel after cold working often produces magnetism, especially the degree of processing such as heads and elbows. The larger parts are particularly noticeable. Some researches at home and abroad have shown that the main reason for the magnetic properties of these head parts is that austenitic stainless steel is formed by cold working, and some austenite will undergo martensite transformation.
2.1 Martensite transformation mechanism
Generally, the martensite structure can be obtained through the quenching process, that is, the steel is heated above the austenite transformation temperature, and the temperature is kept for a certain period of time, so that the steel is fully austenitized and quickly cooled. When the austenite drops below the Ms point of the martensite transformation temperature, its structure begins to transform into martensite until the temperature Mf point to stop the transformation. Experimental research shows that when austenitic stainless steel is formed by cold working, part of the austenite will undergo martensite transformation due to tensile and compressive stress, and the martensite and austenite share the lattice, which is sheared in the pole. Non-diffusion phase transformation occurs in a short time, this kind of martensite is also called deformed martensite.
2.2 Factors affecting the transformation of martensite
The main factors that affect the transformation of martensite are: the stability of austenitic stainless steel, the amount of processing deformation, the processing method, etc.
2.2.1 Influence of chemical composition
According to the stability of austenite, austenitic stainless steel can be divided into stable and metastable austenitic stainless steel. Metastable austenitic stainless steel is more likely to produce martensite under cold deformation, such as 304, 304L, 321 cold working is easier to produce martensite, while 316, 316L basically do not produce martensite.
The stability of austenitic stainless steel is determined by its chemical composition. The more austenitizing elements such as Ni, N, C, and Mn, the more stable the austenite; while the ferrite elements such as Cr, Mo, and Nb are in solid solution. It has a diffusion effect, which can prevent the transformation of austenite to martensite when the content is appropriate, but when it is excessive, it will promote the transformation of austenite to martensite and ferrite.
2.2.2 Influence of processing deformation amount Under the same conditions, the larger the processing deformation amount, the greater the amount of deformation martensite.
2.2.3 Influence of processing methods The forming process of austenitic stainless steel heads generally adopts cold stamping or cold spinning. Cold stamping adopts standard stamping to form at one time; cold spinning is formed by repeated extrusion of two dies. The degree of cold stamping is more intense (the deformation speed is faster), and the content of deformed martensite is higher under the same conditions. In addition, the generation 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 the performance of equipment
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 the volume will expand after transformation, which causes internal residual stress. The austenite structure has finer grains and better comprehensive mechanical properties such as strength and toughness, while the martensite structure has higher hardness and poor plasticity. When the martensite phase variable is large, the influence on the properties of the steel cannot be ignored.
1) Due to the change in volume, the martensite transformation will cause internal residual stress, which may cause defects such as cracks in the equipment.
2) The potential of martensite is lower than that of austenite. In a corrosive medium environment, martensite is an anode relative to austenite, which is corroded preferentially, causing 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 to be one of the important causes of stress corrosion of austenitic stainless steel in the CL ion environment.
4 Preventive measures According to the causes and influencing factors of martensite, the main preventive measures are as follows:
1) When ordering head steel plates, increase the content of austenitizing elements within the standard allowable range.
2) Material upgrade, using materials with higher Ni content such as 316L and 310
3) Improve processing technology. If a manufacturer develops a new process, the head is cold stamped and pre-pressed, and then heated to about 250°C for spinning. Due to the use of pre-pressing, the repeated pressing is reduced to reduce the martensitic phase variable, and the spinning temperature is 250 ℃, which is higher than Md (the upper temperature limit of martensite transformation caused by processing), so it can avoid the austenite stainless steel cold processing Greater magnetism.
4) Solid solution heat treatment can completely eliminate the magnetic and work hardening phenomena. However, the solution treatment cost is higher, and it has a greater impact 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 technology regulations.

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