The purpose of heat treatment to steel is to obtain the desired microstructure of metals and alloys by heating and cooling, in order to change the processing performance of the material or improve the service performance of the workpiece, thereby extending its service life. The mechanical properties of heat treated workpiece fail to meet the design technical requirements, which is a common quality defect of heat treatment. The reasons are improper selection of materials, inherent defects of materials, improper heat treatment process, improper heating or cooling methods, and lax execution of heat treatment process.
1.the hardness of steel is not qualified
The hardness of steel metal materials has a certain empirical relationship with its static tensile strength and fatigue strength, and has a certain degree of relationship with the cold formability, machinability and weldability of metal processing properties.
Hardness failure is one of the most common defects in heat treatment. It is mainly caused by insufficient hardness, insufficient quenching cooling speed, surface decarburization, insufficient hardenability of steel, excessive residual austenite after quenching, insufficient tempering and other factors. The phenomenon of low hardness in the local area of the quenched workpiece is called soft point. Most of the onlookers in the soft spot area are martensite and tostenite mixed structures distributed along the grain boundary of protoaustenite. Uneven soft point or hardness is usually caused by uneven quenching heating or uneven quenching cooling. The main cause of uneven heating is uneven furnace temperature and insufficient heating temperature or holding time. The uneven cooling is mainly caused by the bubbles attached to the quenching medium on the surface of the workpiece during quenching cold, the quenching medium is polluted (such as oil suspended beads in the water) or the quenching medium is not stirred sufficiently. In addition, the steel structure is too thick, there is serious segregation, large carbides or large free ferrite will also cause uneven hardening to form soft points.
1.1 steel Soft Points
The purpose of quenching heating is to make the workpiece complete the microstructure transformation during quenching. For this purpose, it must be heated to the appropriate temperature and have sufficient holding time. Due to the low heating temperature and insufficient holding time, the original pearlite structure can not be completely transformed into austenite and the austenite composition is uneven, and the martensitic structure can not be completely obtained after quenching, resulting in the formation of soft points after quenching.
When the quenching medium is not stirred enough, the workpiece does not move enough in the quenching medium or the workpiece enters the medium in the wrong direction, it often delays the steam film rupture on some parts of the workpiece surface, resulting in a reduction in the cooling rate, resulting in high temperature decomposition products, forming soft points or local hardness reduction.
1.2 Insufficient hardness of steel
Insufficient heating often leads to insufficient hardness of quenched parts. However, improper cooling is a common cause of insufficient hardness of the workpiece. Too long pre-cooling time from the workpiece out of the oven to before quenching, improper selection of cooling medium or high temperature control of cooling medium, resulting in insufficient cooling capacity, oxide skin on the surface of the workpiece or attached salt solution, and too high temperature when the workpiece is removed from the quenching medium after quenching may lead to the decomposition of supercooled austenite in the pearlite transition area of the C-curve. The formation of non-martensitic structures such as sotensite and totensite makes the hardness of the workpiece insufficient.
The existence of a large amount of residual austenite in the quenched structure is an important reason for the insufficient hardness of the quenched workpiece. The residual austenite volume is related to the chemical composition of austenite, when the carbon content is greater than 0.5%~0.6%, the existence of residual austenite can be obviously observed in the quenched structure, and the residual austenite volume rises sharply, when the mass fraction of carbon is 1.4%, the residual austenite volume (volume fraction) reaches 30%. All alloying elements dissolved in austenite by replacement will cause the increase of residual austenite volume. When the volume of residual austenite is small, it has no obvious effect on the hardness. When the volume of residual austenite is large, the hardness will decrease. The quenching hardness of residual austenite with 20% volume fraction will decrease by about 6.5HRC.
1.3 The soft point and hardness of high-frequency quenching and carburizing steel pipes are insufficient
The soft points of high-frequency hardening steel workpiece include two kinds of residual soft points which are not partially hardened on the surface surface and the depth of the hardening layer is not uniform. These hardness defects are caused by improper material selection, poor original structure, improper electrical parameters of high-frequency quenching heating, sensors and cooling devices.
The lack of hardness and soft points of carburized steel pipes are mostly caused by insufficient carburizing, decarburization during quenching, too low quenching temperature, insufficient quenching cooling speed, excessive residual austenite volume on the surface, excessive tempering, unclean surface of the workpiece, uneven carburizing or uneven cooling.
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