Reasons for brittle fracture of Heat-resistant Steel Castings
Heat-resistant steel castings brittle fracture characteristics: fracture surface is smooth, bright, slightly curved small plane, looks like rock sugar fracture.
In 1947 Lorig and Elsear first proposed AIN is the reason for the brittle fracture of the cast steel. Many researchers believe that the reason for the brittle fracture of the cast steel is due to the precipitation of AIN at the primary austenite grain boundary and the embrittlement of the grain boundaries.
The solubility of N and Al in the heat-resistant steel castings depends on their concentration product:
KAIN = [A1%] [N%]. When the equilibrium value of KAIN is exceeded, AIN inclusions are precipitated at a temperature of 800 to 1100 ° C, and if N is lower than the KAIN balance, N and Al in the steel do not interact. The change in N and Al solubility in steel depends on the chemical composition of the steel. AIN in the formation of austenite grain, it will be coarse film-like precipitation in the grain boundary, resulting in macroscopic tissue defects, reduce the plasticity and toughness of cast steel, steel casting to increase the tendency to crack.
Many studies have confirmed the adverse effect of coarse film-like AIN on the mechanical properties of carbon steel and low alloy steels. The film-like AIN is different from other diffused inclusions. When the thickness of the AIN film is 0.05um, its size is generally no more than 5um. The inclusions of this form are distributed along the austenite grain boundary, thus weakening the intergranular binding force The
Tests show that the change in nitrogen content in steel is related to smelting equipment. For example, the amount of nitrogen in the oxygen converter steel is 0.002% to 0.004%, the furnace steel is 0.005% to 0.007%, and the furnace steel is 0.008% to 0.01%.
It can be seen, the use of electric arc furnace smelting steel, steel nitrogen in the highest, in the production should be given special attention.
There have been professionals to study the SCMnZA and SC42 large steel castings brittle fracture causes. Tests show that the larger the size and weight of the casting, the more prone to brittle fracture. The mold has a cooling rate that is prone to brittle fracture, and when the cooling rate is very fast and very slow, no brittle fracture occurs.
This is because the cooling rate is fast, can prevent the precipitation of steel AIN, or just evenly out of the salute inside, and grain boundary precipitation is very small. And the cooling rate is very slow, only grow into a small amount of large particles AIN, and produce condensation, so grain boundary is not brittle.
A.H.Rabphllhh has studied the Cr-Ni-Mo steel brittle fracture of the reasons, also considered that the film-like AIN weakened the austenite grain boundary. However, in another study, the opposite of the argument, that Cr ~ Ni-Mo steel brittle fracture of the reasons, mainly due to precipitation along the austenite grain boundary iron nitride.
In summary, according to the results of most researchers, the main reason for the brittle fracture of heat-resistant castings is due to precipitation of film-like AIN along the austenite grain boundary.