How to make high-quality screws when fasteners are heat-treated Some special quality inspection and control

Update:19 May 2020
Summary:

In addition to general quality inspection and control, […]

In addition to general quality inspection and control, the heat treatment of fasteners also has some special quality inspection and control.
1. Decarburization and carburization
In the production process of mass heat treatment, the metallographic method or the micro-hardness method can only be selected at regular intervals. Because of its long inspection time and high cost.
In order to judge the carbon control situation of the furnace in time, spark detection and Rockwell hardness test can be used to make a preliminary judgment on decarburization and carburization. Spark detection is to harden the hardened parts and grind the sparks on the grinder from the front and back to determine whether the carbon content of the surface and the core is the same. Of course, this requires operators to have skilled skills and spark identification capabilities.
Rockwell hardness testing is performed on one side of the hexagon bolt. First, lightly polish a hexagonal plane of the quenched parts with sandpaper, and measure the first Rockwell hardness. Then grind the surface to about 0.5mm on the grinder, and measure the Rockwell hardness again. If the hardness values ​​of the two are basically the same, it means neither decarburization nor carburization. When the previous hardness is lower than the latter hardness, it indicates that the surface is decarburized. When the hardness of the previous time is higher than that of the latter time, it indicates that the surface is carburized. Under normal circumstances, when the hardness difference between two times is within 5HRC, when the metallographic method or microhardness method is used to check, the decarburization or carburization of the part is basically within the acceptable range.

 


2. Hardness and strength
In the detection of threaded fasteners, it is not easy to check the relevant manuals based on the hardness value and convert it to the strength value. There is an influence of hardenability factors. Because the national standard GB3098.1 and national standard GB3098.3 stipulate that the arbitration hardness is measured at 1/2 radius of the cross section of the part. The tensile sample is also taken from the 1/2 radius. Because it is not excluded that there are low hardness and low strength parts in the center of the part.
In general, the hardenability of the material is good, and the hardness of the screw section 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 inspection of 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 ensure the stress within the acceptable range, the lower limit of hardness is often increased. For example, the hardness control range of grade 8.8: 26 ~ 31HRC for the specifications below M16, 28 ~ 34HRC for the specifications above M16; the control of 36 ~ 39HRC for 10.9 grade. Level 10.9 or higher is another matter.
3. Re-tempering test
8.8 ~ 12.9 grade bolts, high-strength screws and studs should be tempered according to the actual production of the lowest tempering temperature 10 ℃ lower 30 ℃ heat preservation test. On the same sample, the difference between the average values ​​of the three points of hardness before and after the test shall not exceed 20HV.
The re-tempering test can check the incorrect operation due to insufficient quenching hardness and tempering with too low temperature to barely reach the specified hardness range to ensure the comprehensive mechanical properties of the parts. Especially for threaded fasteners made of low carbon martensitic steel, low temperature tempering is used. Although other mechanical properties can meet the requirements, the residual elongation fluctuates greatly when measuring the guaranteed stress, which is much greater than 12.5um. Moreover, under certain conditions of use, sudden fractures can occur. In some automotive and construction bolts, sudden fractures have occurred. When the lowest tempering temperature is adopted for tempering, the above phenomenon can be reduced. However, when making 10.9 bolts with low-carbon martensitic steel, special care should be taken.
4. Inspection of hydrogen embrittlement
The sensitivity of hydrogen embrittlement increases with the strength of the fastener. The external threaded fasteners of grade 10.9 or above or surface hardened self-tapping screws and combination screws with hardened steel washers shall be subjected to dehydrogenation treatment after electroplating.
Hydrogen removal treatment is generally carried out in an oven or tempering furnace at 190 ~ 230 ℃ for more than 4h to allow hydrogen to diffuse out.
Threaded fasteners can be tightened by screwing them on a special fixture to a tension that allows the screw to bear a considerable amount of guaranteed stress for 48 hours. After loosening, the threaded fasteners will not break. This method is used as an inspection method for hydrogen embrittlement, and is also the hydrogen embrittlement test we call daily.