Reasons and effects of magnetic properties of 304 stainless steel

1 Introduction

When a new batch of equipment (E-450, D-450, etc.) was added to the site of the 1# glycol EO expansion project, we found that the austenitic stainless steel head (near the hem) has a larger magnetic. It is well known that austenitic stainless steels should not be magnetic. Initially suspected that the stainless steel material used was unqualified, after spectral detection, no material was found to be abnormal, all qualified S30408 ​​stainless steel. After communicating with the technicians of the manufacturer and consulting the relevant materials, it is basically determined that the magnetism of the head is due to the change of the microstructure of the austenitic stainless steel during cold forming deformation, and some austenite is transformed into martensite. The martensite of the body-centered cubic structure is magnetic.

Then, the reasons for the martensite structure are related to which factors, whether it has an impact on the performance of the equipment, and how to avoid the problem of martensite is a concern of the users and equipment managers.

2 Causes and influencing factors of martensite According to different composition, stainless steel can be divided into ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, duplex stainless steel and precipitation hardened phase stainless steel, of which austenitic stainless steel is The one that uses the most. Due to the structure of the structure, austenitic stainless steel is theoretically non-magnetic, but the 18-8 series (304, etc.) austenitic stainless steel commonly used after cold working often produces magnetism, especially the degree of processing such as heads and bends. Larger parts are especially noticeable. Some researches at home and abroad have shown that the magnetism of these parts is mainly due to the cold forming of austenitic stainless steel and the martensite transformation of some austenite.

2.1 Martensitic transformation mechanism

Generally, the martensite structure can be obtained by a quenching process, that is, the steel is heated to above the austenite transformation temperature, and is kept for a certain period of time, so that the steel is sufficiently austenitized and rapidly cooled. When austenite falls below the Ms point of the martensite transformation temperature, its structure begins to transform into martensite until the temperature Mf stops. Experimental studies have shown that when austenitic stainless steel is cold-formed, part of austenite undergoes martensite transformation due to tensile and compressive stress, and martensite and austenite share a lattice, which is shear-induced. A non-diffusion phase transition occurs in a short period of time, and this martensite is also called deformed martensite.

2.2 Factors affecting the transformation of martensite

The main factors affecting the transformation of martensite are: the stability of austenitic stainless steel, the amount of processing deformation, processing methods and so on.

2.2.1 Effects 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 steel is more prone to martensite under cold deformation. For example, 304, 304L, and 321 are more likely to produce martensite during 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, the more stable the austenite is; and the ferrite elements such as Cr, Mo and Nb are in solid solution. It has a diffusion effect and can prevent the transformation of austenite to martensite when the content is appropriate, but it will promote the transformation of austenite to martensite and ferrite.

2.2.2 Influence of processing deformation Under the same conditions, the greater the amount of deformation, the greater the amount of deformed martensite produced.

2.2.3 Influence of processing method The forming process of austenitic stainless steel head is generally cold stamping or cold spinning, and cold stamping is performed by standard stamping; cold spinning is formed by repeated extrusion of two molds. The degree of cold stamping is relatively intense (fast deformation speed), and the deformation martensite content is higher under the same conditions. In addition, the production of martensite is also related to the processing temperature, and the higher the processing temperature, the lower the deformed martensite content.

3 Effect of martensitic transformation on equipment performance

Austenite is a face-centered cubic structure, and martensite is a body-centered cubic structure; martensite has a lower density than austenite, so the volume expands after the transformation, causing internal residual stress. The austenite microstructure is finer, and the comprehensive mechanical properties such as strength and toughness are better, 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 martensitic transformation causes 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 the corrosive medium environment, martensite is the anode relative to austenite, which is preferentially corroded, 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) Deformation-induced martensite is considered to be one of the important reasons for the stress corrosion of austenitic stainless steel in the CL ion environment due to residual stress and electrochemical corrosion conditions.

4 Preventive measures According to the causes and influencing factors of martensite production, the main preventive measures are as follows:

1) When ordering the head steel plate, increase the content of austenitizing elements within the allowable range of the standard.

2) Material upgrade, using materials with high Ni content such as 316L and 310

3) Improve the processing technology. If a manufacturer develops a new process, the head is cold stamped and pre-pressed, and then heated to a temperature of about 250 ° C. Due to the preloading, the repeated pressing is reduced to reduce the martensite phase variation, and the spinning temperature is 250 ° C, which is higher than Md (the upper temperature limit for processing martensite transformation), so that the cold working of austenitic stainless steel can be avoided. Larger magnetic.

4) Solid solution heat treatment can completely eliminate the magnetic and work hardening phenomenon. However, the cost of solid solution treatment is high, and the deformation of the head is greatly affected.
5) Strengthen the quality management of all links, strictly control the quality of raw materials, and strictly abide by the processing procedures.

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