Heat treatment characteristics and process system of stainless steel
It has been less than a hundred years since the invention of stainless steel in the early 20th century, but the momentum of its development and application is extremely rapid. Especially since the late 1960s, the output of stainless steel in the world has basically maintained an average annual growth rate of 4%, and the application scope of stainless steel has gradually expanded to various fields of the national economy. An important factor for the rapid development of stainless steel is its corrosion resistance and heat resistance. The quality of stainless steel heat treatment process has a great influence on the corrosion resistance and heat resistance of stainless steel, and plays a decisive role in the processing performance of stainless steel. Therefore, the heat treatment process of stainless steel has always been in a very important position in the production process of stainless steel.
1. Characteristics of stainless steel heat treatment
The heat treatment of stainless steel is to change its physical properties, mechanical properties, residual stress and restore the corrosion resistance that has been seriously affected by pre-processing and heating, so as to obtain the best performance of stainless steel or to enable further cold and hot processing of stainless steel. The so-called heat treatment is to carry out corresponding annealing, quenching and tempering, normalizing and other treatments for different structures and different types of stainless steel.
Stainless steel is a special kind of steel. The content of nickel and chromium in steel is very high. Due to the existence of alloying elements such as nickel and chromium, its heat treatment has the characteristics that ordinary steel heat treatment does not have:
The heating temperature is higher and the heating time is relatively longer.
Stainless steel has low thermal conductivity and poor temperature uniformity at low temperatures.
Austenitic stainless steel expands more seriously at high temperature.
Furnace atmosphere control is very important to prevent carburizing, nitriding and decarburization and overoxidation.
The surface gloss of stainless steel has a decisive influence on the use and price of the product, and the iron oxide scale produced during heat treatment will seriously affect the surface gloss.
Be sure to avoid scratches on the stainless steel surface and prevent II: deformation during heat treatment. Stainless steel can be divided into three types according to its structure: austenite, martensite and ferrite (in addition to precipitation hardening type, ferritic austenite type, etc.), the heat treatment of these three types of stainless steel is no matter the treatment method or purpose Not all are the same.
① Austenitic stainless steel
This type of stainless steel is the most widely used and used in the largest amount. It is characterized by an austenite structure at room temperature, which does not undergo phase transformation and cannot be hardened by heat treatment, but can be hardened by cold working. The commonly used heat treatment method is solution treatment.
② Ferritic stainless steel
This type of stainless steel generally has no ν-α transformation, and is a ferrite structure at high temperature and normal temperature, without phase transformation. However, when the steel contains a certain amount of austenite-forming elements such as carbon and nitrogen, austenite structure can also be formed at high temperature. Such steel cannot be strengthened by heat treatment, but can only be annealed to eliminate internal stress. for further processing.
③ Martensitic stainless steel
This type of stainless steel has an obvious transformation point, and it is austenite at high temperature. When cooling, martensitic transformation can occur, and it is transformed into martensite and hardened. Because of its high chromium content and good hardenability, various heat treatment methods such as quenching and tempering can be used.
