How much do you know about white spots and hydrogen embrittlement in large forgings?

White spots are a defect caused by excessive hydrogen content in steel. White spots are internal cracks in steel that appear as round or oval silver-white spots with clear edges on the longitudinal fracture surface of the forging. There are small hairline-like cracks on the transverse low-magnification test piece, with a length of several millimeters and a maximum of tens of millimeters. The micromorphology of the white spots is transgranular quasi-cleavage consisting of tearing ridges and cleavage facets. The white spots of alloy steel are bright in color, while those of carbon steel are darker.

White spots are produced under the combined action of hydrogen and stress in steel. The formation temperature of white spots is about 200 to 50 degrees, which basically does not change with the chemical composition of the steel. The formation of white spots requires an incubation period to allow the hydrogen in the steel to form a sufficient degree of segregation and embrittle the metal. White spots are polymorphically nucleated at grain boundaries, sub-grain boundaries, inclusion surfaces and other crystal defects.

The appearance of white spots will lead to a sharp decrease in the lateral properties of forgings (mainly plasticity and toughness) and become the most dangerous source of fracture, seriously reducing the performance and life of parts. Therefore, once white spots are found, the forging should be scrapped or forged into a smaller size forging.

In order to prevent the formation of white spots, the residual hydrogen in the steel must be limited to below the white spot-free limit hydrogen content of the steel. The white point-free limit hydrogen content of steel is controlled by the white point sensitivity of the steel and is related to the chemical composition, organizational state and other factors of the steel. Alloying elements such as Ni, Mn, Ni-Cr, etc. can increase the white point sensitivity of steel. Zr, Nb, Mo, W, V, Ti, Cr alone and the rare earth element Ce can increase the white point sensitivity of steel. decline. Among various structures, the order of decrease in white point sensitivity is: pearlite, bainite, and martensite. White points are more likely to appear in mixed structures than in single structures. Factors such as refined grains, refinement and flaking of carbide material points, and increased dislocation density can increase the capture of hydrogen by structural defects and reduce the white point sensitivity of steel.

According to the different white point sensitivities, the steel grades commonly used in production can be divided into the following four groups:

First group. For high-nickel alloy steels with high white point sensitivity, such as 12CrNi3MoV, 18Cr2Ni4WA, 34CrNi3Mo, 26Cr2Ni4MoV, etc., the limit hydrogen content without white points can be taken as 1.8×lO-6.

Second Group. Medium and high carbon Ni-Cr alloy steels with high white point sensitivity, such as 40CrNi, 34CrNilMo, 5CrNiMo, 70Cr3Mo and 9Cr2Mo, etc., have a white point-free limit hydrogen content of 2. 7 × 1O-6.

The third group . For medium-carbon low-alloy steels with moderate white point sensitivity, such as 40Cr, _3sCrMo, 34CrMolA, etc., the limit hydrogen content without white points can be taken as 3×10-6.

Fourth group. For carbon structural steel and low carbon low alloy steel with low white point sensitivity, such as 25, 15CrMo, 20CrMo, 20MnMo, etc., the white point-free limit hydrogen content can be taken as 3.5×10-6.

Special attention should also be paid to the fact that the presence of small amounts of retained austenite can drastically increase the white point sensitivity of the steel. Because retained austenite not only hinders the diffusion and escape of hydrogen, but also attracts and stores hydrogen, making hydrogen highly enriched in local areas of the steel. Subsequently, when the retained austenite transforms into martensite, the highly concentrated hydrogen combined with the huge phase transformation stress increases the risk of white spots forming.

The brittleness caused by white spots increases sharply as the strain rate of the steel increases during loading. This phenomenon is called the first type of hydrogen embrittlement of steel.

When the hydrogen in the steel is not enough to form white spots, the plasticity and toughness of the steel also decrease with the increase of the hydrogen content in the steel, but the degree of decrease decreases with the increase of the strain rate during loading. This phenomenon is called steel Type II hydrogen embrittlement. With the emergence of the second type of hydrogen embrittlement, the plasticity index of the steel can be reduced by more than half, causing delayed fracture of the steel during sustained loading. Therefore, the hazards caused by the second type of hydrogen embrittlement must be considered for a variety of important large forgings. In order to avoid the second type of hydrogen embrittlement, the remaining hydrogen in important large forgings should be reduced to less than 1-1.5×10-6.

Regarding the white spots and hydrogen embrittlement of forgings, I will briefly introduce these today. Zhengzhou Gou's electromagnetic induction heating equipment manufacturer specializes in high-frequency induction heating equipment, high-frequency quenching equipment, medium frequency melting furnaces, ultrasonic induction heating equipment and other induction heating As a manufacturer of equipment manufacturing and sales, we have been engaged in induction heating equipment for many years, and we have accumulated considerable experience. Today we only talk about the knowledge about white spots and hydrogen embrittlement in forgings. For more knowledge, you are welcome to call the hotline to learn more.