What is thermal brittleness

What is thermal brittleness

The plasticity of metal materials increases under the action of high temperature and short load. However, after the metal material is cooled under the action of high temperature and long time load, its plasticity will be significantly reduced, notch sensitivity will be increased, often showing brittle fracture phenomenon. This property of metallic materials is called thermal brittleness.

Sulfur has very little solubility in solid iron, and it can form FeS with low melting point (1190℃). FeS+Fe eutectic has a lower melting point (989℃). Such eutectic crystals with low melting point are generally distributed in the form of divorced eutectic at grain boundaries. When the steel is hot working (forging, rolling), the heating temperature is usually above 1000℃, then the FeS+Fe eutectic on the grain boundary melts, leading to the cracking of the steel during the hot working. This phenomenon is called "hot brittleness" or "red brittleness" of steel.

Factors affecting thermal brittleness

Some steel stays at 400~550℃ for a long time, and its impact value will decrease significantly after cooling to room temperature. This phenomenon is called thermal brittleness of steel. Almost all steels are prone to thermal brittleness. It should be noted that steel with thermal brittleness does not present embrittlement at high temperature, but still has high impact toughness. Only when cooled to room temperature does it show embrittlement phenomenon. The thermal brittleness of steel only through the impact test will be obviously shown, generally than the normal impact toughness decreased by 50%~60%, or even 80%~90%. The microstructure of hot brittle steel has no obvious change. The main factors affecting the hot brittleness of steel are:

1. Chemical composition

Pure iron itself does not show thermal brittleness. If the element C is added, the steel will have thermal brittleness if it is subjected to plastic deformation at high temperature. If plastic deformation is not sustained at high temperature, the thermal brittleness of steel is not obvious. The elements Cr and Mn are the most easy to promote the formation of thermal brittleness, such as low alloy Chrome-nickel steel (Cr content is 0.5%~1.0% and nickel content is 1.0%~4.0%), manganese steel (Mn content is 1.0%~2). 0%) the thermal brittleness is obvious; The thermal brittleness of a small amount of Cu is not obvious, but the thermal brittleness is obvious when the mass content of Cu exceeds 0.4%. P increases the tendency of thermal brittleness; W and V elements can reduce thermal brittleness.

2. Insulation time

Holding time is an important factor of thermal brittleness. The thermal brittle time of different steels is different. The impact toughness of low-alloy chrome-nickel steel, manganese steel and chrome-manganese steel decreases obviously at room temperature after heat preservation for 100~200h. If Mo element is added to these steels, the holding time required for obvious thermal brittleness can be delayed to 800~1000h.