The hardness of metal materials has a certain empirical relationship with their static tensile strength and fatigue strength, and is to some extent related to the processing performance of metals such as cold formability, cutting ability, and welding performance; Hardness testing does not damage the workpiece, is simple, and the data is intuitive, so it is widely used as the most important quality inspection indicator for heat treated workpieces, and many workpieces are still their only technical requirements.
Unqualified hardness is one of the most common heat treatment defects. Mainly manifested as insufficient hardness, insufficient quenching cooling rate, surface decarburization, insufficient steel hardenability, excessive residual austenite after quenching, insufficient tempering, and other factors. The phenomenon of low hardness in local areas of quenched workpieces is called soft spots.
The surrounding microstructure in the soft spot area is mostly a mixture of martensite and troostite distributed along the grain boundaries of the original austenite. Soft spots or uneven hardness are usually caused by uneven quenching heating or uneven quenching cooling. Uneven furnace temperature during heating, insufficient heating temperature or insulation time are the main reasons for uneven heating.
Uneven cooling is mainly caused by the attachment of quenching medium bubbles on the surface of the workpiece during quenching, contamination of the quenching medium (such as oil suspended floating beads in the water), or insufficient stirring of the quenching medium. In addition, the steel structure is too coarse, with severe segregation, and large carbides or free ferrites can also cause uneven quenching and the formation of soft spots.
The purpose of quenching and heating is to complete the structural transformation of the workpiece during the quenching process. To achieve this, it is necessary to heat to an appropriate temperature and have sufficient insulation time. The low heating temperature and insufficient holding time result in the incomplete transformation of the original pearlite structure into austenite and the uneven composition of the transformed austenite. After quenching, a complete martensitic structure cannot be obtained, resulting in the formation of soft spots in the workpiece after quenching.
Figure 1 shows the microstructure of a hand tap made of T12 steel due to insufficient heating: fine needle martensite+quenched martensite+pearlite. The performance is characterized by uneven hardness.
Insufficient stirring of the quenching medium, insufficient movement of the workpiece in the quenching medium, or incorrect direction of the workpiece entering the medium often delays the rupture of the steam film on certain parts of the workpiece surface, leading to a decrease in cooling rate at that location, resulting in high-temperature decomposition products, forming soft spots or local hardness decreases. Water vapor film is more stable than salt water, so soft spots are more likely to form on water quenched workpieces. The higher the temperature of water and aqueous solution, the easier it is to produce soft spots.
Carbon steel with poor hardenability is prone to soft spots when the cross-section of the workpiece is large. The surface of the workpiece is not clean, such as rust, carbon black, etc., which can also cause low hardness after quenching.
