Problems that may occur during annealing and normalizing of workpieces
When workpieces undergo high-frequency quenching or annealing, problems such as overheating, excessively high hardness after annealing, the appearance of large Widmanstätten structures, and black quenching may occur. Below is a detailed introduction to various defects that may appear during high-frequency quenching or annealing:
1. Overheating: When the heating temperature for annealing or normalizing is too high, austenite grain boundaries may suffer severe oxidation or even partial melting, leading to the scrapping of the workpiece. The main measures to prevent this defect include educating operators to avoid negligence, regularly calibrating temperature measurement devices and temperature control instruments.
2. High Hardness After Annealing: This defect may occur when high-carbon tool steel or structural steel with high stability of overcooled austenite is annealed. The main reasons are excessive cooling speed, or too low temperature or too short holding time during isothermal annealing. An excessively high heating temperature can make the overcooled austenite more stable. In some alloy steels, this may also be one of the reasons for high hardness after annealing. Poor spheroidization after spheroidizing annealing often results from excessively high heating temperatures, too fast cooling speeds, or too short isothermal holding times.
3. Appearance of Coarse Widmanstätten Structures: This defect commonly occurs during the normalization or annealing of hypo-eutectoid steel. The characteristic of Widmanstätten structures in hypo-eutectoid steel is that ferrite forms needle-like structures extending from austenite grain boundaries into the grains, while pearlite fills the spaces between the needle-like ferrite. The presence of Widmanstätten structures reduces the ductility and toughness of steel, especially when austenite grains are coarse. Such structures are commonly found in cast steel components and forging blanks. Eliminating Widmanstätten structures is one of the purposes of annealing and normalizing these materials. However, improper operation can cause this structural defect to reappear. Different chemical compositions result in different risks of forming Widmanstätten structures during normalization or annealing. Regardless of the type of steel, coarser austenite grains increase the tendency to form Widmanstätten structures. The effect of cooling speed is unique: there is a certain range of cooling speeds within which Widmanstätten structures form; cooling speeds above or below this range will not lead to the formation of Widmanstätten structures. The upper and lower limits of this cooling speed range depend on the chemical composition of the steel and the austenite grain size. The coarser the austenite grains, the wider this cooling speed range becomes. When this defect appears during annealing or normalizing, reheating to a temperature slightly higher than Ac3 can dissolve the needle-like ferrite into austenite while keeping the austenite relatively fine-grained. Cooling at an appropriate cooling speed can then eliminate the Widmanstätten structures.
4. Black Brittleness: Black brittleness is one of the possible defects that can occur during the annealing of carbon tool steel or low-alloy industrial steel. Its characteristic is that the fracture surface of the annealed component appears dark gray, and graphite appears in the microstructure. At this point, the hardness of the steel is not particularly high, but its toughness is very low. The main causes of this defect are excessively high annealing heating temperatures, overly long holding times, and overly slow cooling speeds. Steel with a higher carbon content, lower manganese content, or higher content of elements promoting graphitization is more prone to this defect.
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