Precautions for heat treatment of screw fasteners(1)

Update:06 Dec 2019

First, decarburization and carburization In the product […]

First, decarburization and carburization
In the production process of mass heat treatment, whether it is metallographic method or micro-hardness method, it can only be a random sampling, because if it is all measured, the time required is too long and the cost is high. Therefore, in order to judge the carbon control of the furnace, the heat treatment manufacturers can use spark detection and Rockwell hardness test to decarburize and carburize as a preliminary judgment. The spark detection is to grind the hardened parts on the grinding machine by grinding the sparks from the surface and the inside to judge whether the non-quantity of the surface and the heart is consistent. This requires that the operating technicians have skilled experience and spark identification capabilities.
Carbon steel screws also have a Rockwell hardness test, which is performed on one side of the hexagonal bolt. Generally, a hexagonal plane of the quenched part is first lightly polished with sandpaper, and the first Rockwell hardness is measured. Then grind this surface on the grinder for about 0.5MM. Measure the Rockwell hardness again. If the two hardness values are almost the same, it means that the screw fastener is neither decarburized nor carburized. When the previous carbonity is lower than the next carbonity, the surface decarburization is explained. Carbon, if the previous hardness is higher than the next, it means carburizing, but in general, when the hardness difference between the two times is within 5HRC, the decarburization or carburization of the part is basically qualified when the metallographic method or microhardness method is used Within range.
Second, hardness and strength
In the inspection of threaded fasteners, you can't simply check the relevant manuals based on the hardness value to convert the strength value. Among them is the influence of a hardenability factor. Because the standard GB3098.1 and standard GB3098.3 stipulate that the arbitration hardness is measured at 1/2 the radius of the cross section of the part. Tensile specimens are also taken from a 1/2 radius. Because it is not excluded that the central part of the part has low hardness and low strength.
In general, the hardenability of the material is good, and the hardness of the cross section of the screw can be evenly distributed. As long as the hardness is qualified, the strength and guaranteed stress can also meet the requirements. However, when the hardenability of the material is poor, although the hardness is qualified according to the specified parts, the strength and guaranteed stress often fail to meet the requirements. Especially when the surface hardness tends to the lower limit.
In order to control the strength and guaranteed stress within the acceptable range, the lower limit of hardness is often increased. For example, the hardness control range of grade 8.8 is 26 ~ 31HRC for specifications below M16, 28 ~ 34HRC for specifications above M16 is appropriate; 10.36 is within 39 ~ 39HRC. Above 10.9 is another matter.